Bond Dissociation Energies of Organic Molecules STEPHEN J. BLANKSBY* ,† AND G. BARNEY ELLISON* ,‡ Department of Chemistry, University of Wollongong, NSW, 2522, Australia, and Depar tment of Chemi stry &Biochemistry, University of Colorado, Boulder, Colorado 80309-0215Receiv ed Augu st 6 , 2 0 0 2 ABSTRACT In this Account we have compiled a list of reliable bond energies that are based on a set of critically evaluated experiments. A briefdescription of the three most important experimental techniques for measuring bond energies is provided. We demonstrate howthe se exp eri men tal data can be applie d to yie ld the hea ts offormation of organic radicals and the bond enthalpies of more than 100 representative organic molecules. Introduction The making and breaking of bonds is the basis of all che mic al trans for mat ion . A sou nd kno wle dge of the energies required to break bonds and the energies released upon their formation is fundamental to understandingchemical processes. 1 The energy required for homolyticbond cleavage at 298 K corresponds to the enthalpy ofreaction 1, ∆ rxn H298 (1), which is by definition 2 the bond dissociation enthalpy of the molecule AB, DH298 (AB): Conversely, if the radicals A and B recombine to form the molecule AB, then thermal energy equivalent to the bond dissociation enthalpy is released, according to the first lawof thermodynamics. Using these ideas, it is possible to determine the energetics of a wide range of simple but important reactions involving the exchange of a single bond. This is achieved by subtracting the energy gained from the bond formed from the energy required to breakthe initial bond. For example, consider the energetics ofrea ction 2, where a sin gle car bon-hydr ogen bond is broken and a hydrogen -chlorine bond is formed: Table 1 provides a set of experimentally determined bond entha lpies: typic al values rang e from 60 to 130 kcal mol -1 . Using the tabulated values for the carbon -hydrogen bond enthalpy of ethane, DH298 (CH 3 CH 2 -H) ) 101.1 ( 0.4 kcal mol -1 , and the bon d ent hal py of hyd rochl ori c aci d, DH298 (HCl) ) 103.15 ( 0.0 3 kc al mol -1 , the for war d reaction is determined to be exothermic since ∆ rxn H298 (2) ) DH298 (CH 3 CH 2 -H) - DH298 (HCl) ) -2.1 ( 0.4 kca l mol -1 ; the reverse reaction is endothermic, ∆ rxn H298 (-2) ) +2.1 (0.4 kcal mol -1 . This simple scheme allows the elucidation of precise thermochemistry for a broad range of chemical reactions for which experimental bond en- thalpies are available. One must be cautious, however. In reac tion s wher e multi ple bond s are broken, the bond enthalpy of a particular bond can be changed dramaticallyby the cleavage of ancillary bonds within the molecule. That is, once the first bond is broken, the remaining bond ent hal pie s are oft en alt ere d. Sev era l exampl es of thi s behavior will be discussed. Thermochemistry The bond dissociation energy for a species, AB, at room temperature is the bond enthalpy,DH298 (AB). By definition, it is the reaction enthalpy of the bond homolysis reaction 1, ∆ rxn H298 (1), and thus depends exclusively on the relative enthalpies of formation of reactant and product states: By computing heat capacity corrections, one can adjust the bond enth alpie s to any reaction tempera ture. For example, correctingDH298 (AB) to 0 K using eq 4 yields D0 (AB), which is strictly defined by spectroscopist s 3 as the bond dissociation energyand is shown schematically in Figure 1. The bond dissociation energy is related to the dept h of the pote ntia l well, De (AB), by the zero-poi nt energy, D0 (AB) )De (AB) -ZPE (Figure 1): AdjustingDH298 (AB ) to obt ain D0 (AB), or vice ve rsa, requires the evaluation of the integrated heat capacityterm shown in eq 4. It should be noted that the integrated heat capacity of a molecule, AB, from 0 to 298 K is equal to the heat content, H298 (AB) -H0 (AB) )∫ 0 298 Cp (H) dT. For some common molec ules, values for heat conten t have been ev al uated and tabu late d over a range of temperatures. 4 In most cases, however, heat content is calculated using equilibrium statistical mechanics with harmonic oscillator and rigid rotor approximations. 3 In such calculations, the contributions to the heat content arising from translation, rotation, and vibration (electronic contributions are generally zero) are treated as indepen- dent and are calcu late d usin g vibra tional frequ encies obta ined from exper iment or ab init io calcu lati on (for examples of such calculations, see refs 5 and 6). * Address correspondenc e to either auth or. E-ma il: blanksb y@ uow.edu.au and [email protected]. † University of Wollongong. ‡ University of Colorado. Steph en Blanksb y obtained his Ph.D. from the Unive rsity of Adelaide in 19 99 un de r th e supe rv isio n of J oh n Bowie . He th en work ed as a po st do cto ral researcher at the Unive rsity of Colorado from 20 00 to 20 0 2, and has recent ly ta ke n up a facu lt y po sit ion in th e De pa rt m en t of Ch em ist ry at th e Un iv er sit y ofWollongong. Barn ey Ellison st ud ied at Tri ni ty Co llege (H art for d, CT ) an d then earned a Ph.D. in chemistry at Yale University under the supervision of K. B. Wiberg. Ellison te ache s org ani c chem ist ryat th e Un ive rsit y of Col orad o an d stu die s th e chem ical physics of organic molecules. ∆ rxn H298 (1) ) ∆ fH298 (A) + ∆ fH298 (B) - ∆ fH298 (AB) ) DH298 (AB) (3) D0 (AB) ) DH298 (AB) - ∫ 0 298 [Cp (A) + Cp (B) - Cp (AB)] dT(4) AB fA+ B (1) Cl + CH 3 CH 3 aHCl + CH 3 CH 2 (2) Acc. Chem. Res. 2003, 3 6 ,255 -263 10.1021/ar020230d CCC: $25.00 2003 Ameri can Ch emi cal S oci et y VOL. 36, NO. 4, 2003 / ACCOUNTS OF CHEMICAL RESEARCH2 5 5 Published on Web 01/28/2003
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892019 2003 - Bond Dissociation Energies of Organic Molecules
Bond Dissociation Energies ofOrganic MoleculesSTEPHEN J BLANKSBYdagger ANDG BARNEY ELLISONDagger
Department of Chemistry University of WollongongNSW 2522 Australia and Department of Chemistry amp
Biochemistry University of ColoradoBoulder Colorado 80309-0215
Received August 6 2002
ABSTRACTIn this Account we have compiled a list of reliable bond energiesthat are based on a set of critically evaluated experiments A brief description of the three most important experimental techniquesfor measuring bond energies is provided We demonstrate how these experimental data can be applied to yield the heats of formation of organic radicals and the bond enthalpies of more than100 representative organic molecules
Introduct ionThe making and breaking of bonds is the basis of all
chemical transformation A sound knowledge of the
energies required to break bonds and the energies released
upon their formation is fundamental to understanding
chemical processes1 The energy required for homolytic
bond cleavage at 298 K corresponds to the enthalpy of
reaction 1 ∆rxnH 298(1) which is by definition2 the bond
dissociation enthalpy of the molecule AB DH 298(AB)
Conversely if the radicals A and B recombine to form the
molecule AB then thermal energy equivalent to the bond
dissociation enthalpy is released according to the first law of thermodynamics Using these ideas it is possible to
determine the energetics of a wide range of simple but
important reactions involving the exchange of a single
bond This is achieved by subtracting the energy gained
from the bond formed from the energy required to break
the initial bond For example consider the energetics of
reaction 2 where a single carbon-hydrogen bond is
broken and a hydrogen-chlorine bond is formed
Table 1 provides a set of experimentally determined bond
enthalpies typical values range from 60 to 130 kcal mol-1
Using the tabulated values for the carbon-hydrogen bond
enthalpy of ethane DH 298(CH3CH2-H) ) 1011 ( 04 kcal
mol-1 and the bond enthalpy of hydrochloric acid
DH 298(HCl) ) 10315 ( 003 kcal mol-1 the forward
reaction is determined to be exothermic since ∆rxnH 298(2)
) DH 298(CH3CH2-H) - DH 298(HCl) ) -21 ( 04 kcal
mol-1 the reverse reaction is endothermic ∆rxnH 298(-2)
) +21 ( 04 kcal mol-1 This simple scheme allows the
elucidation of precise thermochemistry for a broad range
of chemical reactions for which experimental bond en-thalpies are available One must be cautious however In
reactions where multiple bonds are broken the bond
enthalpy of a particular bond can be changed dramatically
by the cleavage of ancillary bonds within the molecule
That is once the first bond is broken the remaining bond
enthalpies are often altered Several examples of this
behavior will be discussed
ThermochemistryThe bond dissociation energy for a species AB at room
temperature is the bond enthalpy DH 298(AB) By definition
it is the reaction enthalpy of the bond homolysis reaction1 ∆rxnH 298(1) and thus depends exclusively on the relative
enthalpies of formation of reactant and product states
By computing heat capacity corrections one can adjust
the bond enthalpies to any reaction temperature For
example correcting DH 298(AB) to 0 K using eq 4 yields
D 0(AB) which is strictly defined by spectroscopists3 as the
bond dissociation energy and is shown schematically in
Figure 1 The bond dissociation energy is related to the
depth of the potential well D e(AB) by the zero-point
energy D 0(AB) ) D e(AB) - ZPE (Figure 1)
Adjusting DH 298(AB) to obtain D 0(AB) or vice versa
requires the evaluation of the integrated heat capacity
term shown in eq 4 It should be noted that the integrated
heat capacity of a molecule AB from 0 to 298 K is equal
to the heat content H 298(AB) - H 0(AB) ) int0298 C p(H) dT
For some common molecules values for heat content
have been evaluated and tabulated over a range of
temperatures4 In most cases however heat content is
calculated using equilibrium statistical mechanics with
harmonic oscillator and rigid rotor approximations3 Insuch calculations the contributions to the heat content
arising from translation rotation and vibration (electronic
contributions are generally zero) are treated as indepen-
dent and are calculated using vibrational frequencies
obtained from experiment or ab initio calculation (for
examples of such calculations see refs 5 and 6)
Address correspondence to either author E-mail blanksbyuoweduau and barneyjilacoloradoedu
dagger University of WollongongDagger University of Colorado
Stephen Blanksby obtained his PhD from the University of Adelaide in 1999under the supervision of J ohn Bowie He then worked as a postdoctoralresearcher at the University of Colorado from 2000 to 2002 and has recentlytaken up a faculty position in the Department of Chemistry at the University of Wollongong
Barney Ellisonstudiedat TrinityCollege (Hartford CT) and then earned a PhDin chemistry at Yale University under the supervision of K B Wiberg Ellisonteaches organic chemistryatthe Universityof Colorado and studies the chemicalphysics of organic molecules
∆rxnH 298(1) ) ∆f H 298(A) + ∆f H 298(B) - ∆f H 298(AB) ) DH 298(AB) (3)
D 0(AB) ) DH 298(AB) - int0
298[C p(A) + C p(B) - C p(AB)] dT (4)
AB f A + B (1)
Cl + CH3CH3 a HCl + CH3CH2 (2)
Acc Chem Res 200336 255-263
101021ar020230d CCC $2500 983209 2003 American Chemical Society VOL 36 NO 4 2003 ACCOUNTS OF CHEMICAL RESEARCH 255Published on Web 01282003
892019 2003 - Bond Dissociation Energies of Organic Molecules
H where R isan organic radical R-Hf R + H For large RH molecules
it is common that intC p(R) dT = intC p(RH) dT Thus one can
simplify eq 4 to D 0(RH) = DH 298(RH) - intC p(H) dT At 298
K the heat capacity integral for an atom can be calculated
exactly int0298 C p(H) dT ) 148 kcal mol-1 so D 0(RH) =
DH 298(RH) - 15 kcal mol-1 For many organics D 0(RH)
and DH 298(RH) are almost numerically equivalent and as
a consequence the terms ldquobond dissociation energyrdquo
(BDE) and ldquobond dissociation enthalpyrdquo often appear
interchangeably in the literature (and indeed in this
Account) For the most part this is innocuous but if a
precision of better than (3 kcal mol-1 is required then it
is crucial to specify the appropriate D 0(RH) or DH T (RH)
By definition3 the enthalpy of a system (H ) is equal
to its internal energy (U ) and its ability to do pressure-
volume work (pV ) H ) U + pV For 1 mol of an idealgas pV ) RT therefore as an approximation as T f 0 K
H f U From statistical mechanics it is possible to
approximate the internal energy of a system as indepen-
dent contributions from electronic (E elec) vibrational (E vib)
rotational (E rot) and translational (E trans) energy At T ) 0
K all molecules will be in their ground vibrational and
rotational states (written as |v primeprime ) 0 J primeprime ) 0rang where the
double primes indicate the ground electronic state) and
will have ceased translation thus U = E 0 ) E elec + ZPE
(Figure 1) Therefore at 0 K the homolytic bond dissocia-
tion energy can be given in terms of eq 5
Electronic structure calculations (such as those pro-
vided by the GAUSSIAN suite of codes)7 can derive E 0 by
a range of ab initio and semiempirical methods Thus
D 0(AB) values provide a useful comparison between
experimental and theoretical thermochemistry While this
is outside the scope of this Account these theoretical
methods show excellent agreement with experimental
bond dissociation energies8
Table 1 Molecular Bond Dissociation Energies for RH f R + H Experimental Bond E nthalpies and RadicalHeats of Formation at 298 K
DH 298
(kcal mol-1)∆fH 298(R)
(kcal mol-1) r efDH 298
(kcal mol-1)∆fH 298(R)
(kcal mol-1) r ef
I norga nicsH 2 104206 ( 0 0 03 5 2 10 3 ( 0003 4 OH - f O - + H 11021 ( 007 -3323 ( 007 34H F 13625 ( 001 1883 ( 017 6 OH + f O + H + 1152 ( 01 5955 ( 002 34H C l 10315 ( 003 2903 ( 004 9 H 2S 912 ( 01 342 ( 02 6H B r 8754 ( 005 2862 ( 006 9 S H 841 ( 02 662 ( 03 6H I 7132 ( 006 2604 ( 008 9 H -NO 495 ( 07 218 ( 01 4
H -C N 1263 ( 02 1050 ( 07 6 H -ONO (t r a ns) 791 ( 02 82 ( 01 4NH 3 1076 ( 01 445 ( 01 6 H -ON O2 1017 ( 04 176 ( 03 35H 2O 11882 ( 007 886 ( 007 34 S iH 4 917 ( 05 479 ( 06 9OH 10176 ( 007 5955 ( 002 34 G eH 4 83 ( 2 53 ( 2 9
Hydroca rbonsC H 4 10499 ( 003 3505 ( 007 31 C H 2C H -H 1107 ( 06 711 ( 07 6C H 3 1104 ( 02 933 ( 02 31 H C C -H 13332 ( 007 1356 ( 02 36C H 2 1013 ( 03 1425 ( 02 9 C 6H 5-H 1129 ( 05 805 ( 05 6C H 809 ( 02 1713 ( 01 9 C 6H 5 f o -C6H 4 + H 78 ( 3 106 ( 3 37C H 3C H 2-H 1011 ( 04 290 ( 04 10 C 6H 5 f m -C6H 4 + H 94 ( 3 122 ( 3 37(C H 3)2C H -H 986 ( 04 215 ( 04 10 C 6H 5 f p -C6H 4 + H 109 ( 3 138 ( 3 37C H 3C H 2(C H 3)C H -H 982 ( 05 161 ( 05 10 C H 2C H C H 2-H 888 ( 04 414 ( 04 38(C H 3)3C -H 965 ( 04 123 ( 04 10 C 6H 5C H 2-H 898 ( 06 497 ( 06 38
AlcoholsH -C H 2OH 961 ( 02 -408 ( 02 16 C H 3C H 2O-H 1047 ( 08 -36 ( 08 6C H 3O-H 1046 ( 07 43 ( 07 6 (C H 3)2C H O-H 1057 ( 07 -115 ( 07 6
C H 3S-
H 874(
05 298(
04 3940 (C H 3)3C O-
H 1063(
07 -
205(
07 6H -C H 2S H 94 ( 2 36 ( 2 39 40 C 6H 5O-H 90 ( 3 -58 ( 3 41
PeroxidesH OO -H 878 ( 05 32 ( 05 14 C H 3C H 2OO -H 85 ( 2 -68 ( 23 42C H 3OO -H 88 ( 1 48 ( 12 42 (C H 3)3C OO -H 84 ( 2 -252 ( 23 42
Ca rbonylsH -C H O 88144 ( 0008 101 ( 01 6 H -COOH is g 96 ( 1 -465 ( 07 45C H 3C (O)-H 894 ( 03 -24 ( 03 43 C H 3C OO -H 112 ( 3 -43 ( 3 44H -C H 2C H O 94 ( 2 25 ( 22 9 C 6H 5C OO -H 111 ( 4 -12 ( 4 44H C O O -H 112 ( 3 -30 ( 3 44
FIGURE1 The energy requiredtofragment the moleculeAB in itsground state (ie the lowest energy electronic vibrational androtational eigenstate) to ground-state fragments A and B is bydefinition the bond dissociation energy D 0(AB)
D 0(AB) ) E 0(A) + E 0(B) - E 0(AB) (5)
BondDissociationEnergiesofOrganic Molecules Blanksby and Ellison
256 ACCOUNTS OF CHEMICAL RESEARCH VOL 36 NO 4 2003
892019 2003 - Bond Dissociation Energies of Organic Molecules
C H 3C H 2 R 87 9(0 6) 8 7 1(0 5) 85 6(0 6) 100 0(0 8) 75 4(0 6) 102 2(0 7) 76 7(0 7) R 83 3(0 5) 8 3 5(0 5)
(CH 3)2C H R R 856(05) 827(06) 992(08) 752(07) 1010(07) 764(08) R 831(05) 819(05)
(CH 3)3C R R R 786(07) 978(08) 732(07) 983(08) - R - 794(06)
C H 2C H R R R R 116(1) 873(08) 116(1) - - - 41(3)a
C H 2C H C H 2 R R R R R 627(06) - - - - -
H C C 1 26 5(0 3 ) 1 25 1(0 5 ) 1 24 5(0 6 )a 1223(05)a - - - - - - -
H C C -C H 2 78(3) 77(3)a - - - - - - - - -
C 6H 5 R R R R R - 118(1) R R 993(09) 988(08)
C 6H 5C H 2 R R R - - - 97(1) 65 2(0 9) - - 714(09)
a B ond ent ha lpies of s t a ble orga nic molecules a re t a bula t ed a long w it h t h eir uncert a int ies For exa mple D H 298(C 6H 5-OH ) ) 1124 (
06 kca l mol-1 T hese bond ent ha lpies a re ca lcula t ed from t he ra dica l hea t s of forma t ion from T a ble 1 a nd t he pa rent ∆fH 298 va luest a bula t ed by Pedley et a l 1920 There are a few entries (such as CH 3F or C 6H 5C H 2F) where ∆ fH 298(par ent) is not provided by Pedley et a lso we ha ve a dopt ed t he va lue recommended by t he NI S T Web sit e46 (htt pwebbooknistgov) w e ha ve ma rked t hese pa rent compounds
wit h a n ldquo a rdquo I n some ca ses t he hea t of forma t ion of t he pa rent species is not a va ila ble (eg vinylmet hyl et her CH 2C H -OC H 3) so t h ebond entha lpy cann ot be computed and this is ma rked with a dash There are a n umber of redundant entr ies in this table [eg DH 298(C 6H 5-
C H 3) ) D H 298(C H 3-C 6H 5)] so t he second ent ry is ma rked wit h a n ldquo Rrdquo T he uncert a int ies ha ve been a dded in qua dra t ure
X + CH3OH a HX + CH
2OH (17)
CH3OH + h νthresh f +[CH2OH] + H (18)
∆f H 298(R) ) DH 298(R-H) + ∆f H 298(RH) - ∆f H 298(H) (19)
CH3-CH2CH3 f CH3 + CH2CH3 (20)
BondDissociationEnergiesofOrganic Molecules Blanksby and Ellison
258 ACCOUNTS OF CHEMICAL RESEARCH VOL 36 NO 4 2003
892019 2003 - Bond Dissociation Energies of Organic Molecules
unavailable in the dissociation of the hydroperoxide
(CH3)3C-OOH
Chart 3 shows a comparison of the C-H bond energies
of benzene and the phenyl radical These experimentalvalues are available from the ingenious negative ion
studies of Squires and co-workers2728 and demonstrate the
increasing bond strengths at the ortho lt meta lt para
positions quantifying the increasing stability of the result-
ing benzynes as o - gt m - gt p -benzyne
What Are Bond Strengths A common term which often appears in the literature is
the average bond energy This is defined as the energy
required to break all the bonds in a given molecule at 298
K divided by the number of bonds For example consider
the atomization of methane which is the dissociation of CH4 to carbon and four hydrogens (CH4 f C + 4H) The
enthalpy of atomization is known from the experimental
heats of formation of methane carbon and hydrogen
∆atomH 298(CH4) ) ∆f H 298(C) + 4∆f H 298(H) - ∆f H 298(CH4) )
3975 kcal mol-1 This implies that the methane molecule
contains 3975 kcal mol-1 of energy that is partitioned
among four bonds thus D av H 298 ) 994 kcal mol-1 In
Table 1 we list the separate experimental bond enthalpies
of methane and its constituent radicals CH3 CH2 and
CH DH 298(CH4 f CH3 + H) ) 10499 kcal mol-1
DH 298(CH3 f CH2 + H) ) 1104 kcal mol-1 DH 298(CH2 f
CH + H) ) 1013 kcal mol-1 and DH 298(CH f C + H) )
809 kcal mol-1 Notice that not one of these bond
enthalpies is equal to the average bond energy of methane
Therefore one should be cautious when interpreting thesignificance of the average bond enthalpy of a molecule
It is important to note that if these four independently
measured bond enthalpies are combined they give
∆rxnH 298(CH4 f C + 4H) ) 3975 ( 06 kcal mol-1 as
required by the first law of thermodynamics
One should also be careful about the term bond
strength and the tendency to treat these energetic quanti-
ties as transferable objects between different molecules
Equation 3 shows that the bond dissociation energy is the
energy of a fragmentation reaction rather than any intrin-
sic property of a chemical bond Acetylene is a good
example (Chart 1) One could say that the HCtCH bond
strength is 231 kcal mol
-1
[HCt
CHf
HC+
CH] and thatboth CsH bonds are equivalent with bond strengths of
81 kcal mol-1 [HC f C + H see Table 1] However one
could equally well claim that the first CH bond strength
is 133 kcal mol-1 [HCtCH f HCtC + H] which is
different from the second CH bond strength of 117 kcal
mol-1 [HCtC f CtC + H] while the CC bond strength
[CtC f C + C] is only 142 kcal mol-1 Or alternatively
the first CH bond strength is 133 kcal mol-1 while the
CC bond strength [HCtC f HC + C] is 178 kcal mol-1
and the second CH bond strength [HC f H + C] is only
81 kcal mol-1 Clearly one arrives at different bond
Chart 1 Experimental Bond Enthalpies DH 29 8 for Several ImportantHydrocarbons and Hydrocarbon Radicalsa
a Values are in kcal mol-1 and represent the energy required to break only the single bond indicated Thus DH 298(HCtCHf HC + CH) ) 2307( 02 kcal mol-1 but DH 298(HCCsH f HCC + H) ) 13332 ( 007 kcalmol-1
Chart 2 Experimental Bond Enthalpies DH 29 8 for Several ImportantOxycarbons and Oxycarbon Radicals a
a Values are in kcal mol-1 and represent the energy required to break only the single bond indicated Thus DH 298(CH3OsH f CH3O + H) )
1046 ( 07 kcal mol-1 but DH 298(HsCH2OH f H + CH2OH) ) 961 ( 03kcal mol-1 and DH 298(CH3sOH f CH3 + OH) ) 921 ( 01 kcal mol-1
BondDissociationEnergiesofOrganic Molecules Blanksby and Ellison
260 ACCOUNTS OF CHEMICAL RESEARCH VOL 36 NO 4 2003
892019 2003 - Bond Dissociation Energies of Organic Molecules
strengths by breaking the bonds in different orders
Rigorous quantum mechanical discussions of these trends
are available elsewhere26 along with other provocative
discussions of bond strengths232429
The example of HCCH serves to reiterate that the bond
enthalpy is the enthalpy of a homolysis reaction (eq 3)
and thus depends exclusively on the relative stability of
reactant and product states More generally creation of
new bonds in the products or otherwise stabilized
products always decreases the bond enthalpy As an
example consider ketene in Chart 2 The carbon-carbon
bond enthalpy 3031 of ketene DH 298(CH2dCO) ) 787 ( 02
kcal mol-1 is almost 100 kcal mol-1 less than that of
ethylene DH 298(CH2dCH2) ) 174 kcal mol-1 because one
of the products of the ketene fragmentation is an ex-
tremely stable molecule namely carbon monoxide This
example illustrates that not all double bonds are createdequal2426 and that extrapolations of bond energies from
one molecular species to another must be conducted
carefully
Bond Enthalpies in Solut ionThe bond enthalpies tabulated in this Account are exclu-
sively gas-phase values This raises the question of how
to relate gas-phase bond enthalpies to chemical problems
occurring in solution The difference between a gas-phase
bond enthalpy and that in solution D solnH 298(R-H)
depends on the difference in the enthalpy of solvation
∆solnH 298 of the two radicals and the parent compound
as expressed in terms of eq 23 There are not many
accurate measurements available for the enthalpy of
solvation of radical species and we can only estimate the
effects of solvation on organic BDEs The solvation energy
of a hydrogen atom is likely to be negligible in most
solvents and therefore the correction to an R-H gas-
phase bond enthalpy can be can be approximated as[∆solnH 298(R) - ∆solnH 298(RH)] As both the radical and the
parent are neutral the solvation energies are likely to be
small and similar in most cases particularly in nonpolar
solvents The small effect on bond enthalpy is likely to be
most noticeable is protic polar solvents where hydrogen
bonding may play a key role in preferentially stabilizing
the radical or parent in solution Consider for example
the C-H and O-H bond enthalpies of CH3OH in a polar
protic solvent (reactions 24)
While in reaction 24a both methanol and the hydroxy-
methyl radical can hydrogen bond to the surrounding
solvent such stabilization may be less prevalent for the
methoxyl radical in reaction 24b due to the absence of
the highly polarized O-H bond Therefore one might
surmise that the H-CH2OH bond energy of methanol in
a protic solvent would be largely unchanged from the gas
phase while the CH3O-H value could be slightly higher
in solution There is clearly much work to be done in this
area
Chart 3 Experimental Bond Enthalpies DH 29 8 for Benzene andPhenyl Radicala
a Values are in kcal mol-1 and represent the energy required to break only the single bond indicated Thus DH 298(C6H5sHf C6H5 + H) ) 1129( 05 kcal mol-1 but DH 298(C6H4sH f H + o -C6H4) ) 78 ( 3 kcal mol-1DH 298(C6H4sH f H + m -C6H4) ) 94 ( 3 kcal mol-1 and DH 298(C6H4sHf H + p -C6H4) ) 109 ( 3 kcal mol-1
88 kcal mol-1 gt DH 298(bisallylicC-H) Given these esti-
mates the hydrogens most susceptible to radical abstrac-
tion will be those in the allylic positions While there is
at present no direct data for doubly allylic C-H bond
enthalpies one might conjecture that the bond energy
will be roughly 80 kcal mol-1 given that the dif-
ference between a typical methylenic Cs
H (egDH 298((CH3)2CHsH) = 986 kcal mol-1) and a singly allylic
CsH (eg DH 298(H2CdCHCH2sH) = 888 kcal mol-1) is
about 10 kcal mol-1 Thus the bisallylic hydrogens at
carbons 7 10 and 13 represent the most labile hydrogens
in the molecule Once a carbon-centered radical is pro-
duced and rearranged to its most stable form it will
readily add O2 to form a peroxyl radical Chart 2 indicates
that the OO-CMe3 bond enthalpy is 38 kcal mol-1 Given
that the O-H bond enthalpy of a hydroperoxide is
approximately 85 kcal mol-1 a radical chain reaction will
be exothermic
The biological activity of the enediyne anticancer
antibiotic agents is thought to be due to their ability toform reactive diradicals in situ 33 Molecules such as
calicheamicin γ1I possess an extended sugar residue which
serves to deliver the enediyne moiety the active part of
the molecule to a sequence-specific position on the DNA
double-helix Upon delivery to the target the enediyne
functionality is activated to undergo a Bergman cycloaro-
matization which yields a substituted p -benzyne (Scheme
2) From Chart 3 we can estimate that the diradical can
abstract all hydrogens bound by e109 kcal mol-1 This
makes the p -benzyne a quite powerful hydrogen abstrac-
tion reagent and further once one hydrogen has been
abstracted a substituted phenyl radical results which can
abstract all hydrogens bound by e113 kcal mol-1 Thus
two exothermic hydrogen abstractions by the reactive
p -benzyne moiety can lead to selective cutting of double-
stranded DNA
SummaryThe critically evaluated bond enthalpies listed in Tables
1 and 2 should serve as an important resource for the
organic chemist The values listed may be used to calcu-late rigorous experimental thermochemistry for many
common reactions and further with appropriate care
instructive estimations of reaction thermochemistry can
be made for complex chemical problems
This work was supported by grants from the Chemical Physics
Program United States Department of Energy (DE-FG02-
87ER13695) and the National Science Foundation (CHE-0201848)
We are grateful for the sustained advice and criticism from our
Colorado colleagues Carl Lineberger Veronica Bierbaum Shuji
Kato Mark Nimlos Xu Zhang Bob Damrauer Charles H DePuy
Geoff Tyndall and Veronica Vaida We are also continually
educated by our friends Emily Carter George Petersson Larry
Harding Kent Ervin Richard OrsquoHair and Branko Ruscic Finally GBE would like to thank Joseph Berkowitz now retired for 25
years of friendship and physics
References(1) Benson S W Thermochemical Kinetics 2nd ed Wiley-Inter-
science New Y ork 1976(2) MillsI Cvitas T Homann KKallay NKuchitsuK Quantities
Units and Symbols in Physical Chemistry Blackwell ScientificPublications Oxford 1988 This reference lists the IUPA Crsquosguidelines concerning thermochemical symbols which we adoptInstead of the more common expressions ∆G rxn 298(1) ∆H f 0deg(R)or ∆H f 298deg(RH) the use of ∆rxnG 298(1) ∆f H 0(R) or ∆f H 298(RH) isrecommended See p 46
(3) Herzberg G H M olecular Spectra and Molecular StructureInfrared and Raman S pectra of Polyatomic Molecules D Van
Nostrand Princeton NJ 1945 Vol II see Chapter V(4) Gurvich L V Veyts I V Alcock C B Iorish V S Thermody-
namic Properties of Individual Substances 4th ed HemisphereNew York 1991 Vol 2
(5) Ervin K M Gronert SBarlow S E Gilles M K Harrison AGBierbaumV M Charles H DePuy Lineberger W CEllisonG B Bond Strengths of Ethylene and Acetylene J Am ChemSoc 1990 112 5750-5759
(6) Ervin K M DeTuri V F Anchoring the Gas-Phase AcidityScale Experiment and Theory J Phys Chem A 2002 106 9947-
9956(7) Frisch M J Trucks G WSchlegel H B Scuseria G E Robb
M A Cheeseman J R Zakrzewski V G Montgomery J A J r Stratmann R E Burant J C Dapprich S M illam J MDaniels A D Kudin K N Strain M C Farkas O Tomasi J Barone V Cossi M Cammi R M ennucci B Pomelli C
Scheme 2
BondDissociationEnergiesofOrganic Molecules Blanksby and Ellison
262 ACCOUNTS OF CHEMICAL RESEARCH VOL 36 NO 4 2003
892019 2003 - Bond Dissociation Energies of Organic Molecules
Adamo C Clifford S Ochterski J Petersson G A Ayala P Y Cui Q Morokuma K Malick D K Rabuck A D Raghava-chari K Foresman J B Cioslowski J Ortiz J VStefanov BB LiuG Liashenko A Piskorz PKomaromiI Gomperts RMartin R L Fox D J Keith T Al-Laham M A Peng C YNanayakkara A Gonzalez C Challacombe M Gill P M W J ohnson B G Chen W Wong M W Andres J L Head-Gordon M Replogle E S Pople J A Gaussian 98 GaussianInc Pittsburgh PA 1998
(8) Petersson G A In Computational Thermochemistry Irikura KK Frurip D J Eds ACS Symposium Series 677 AmericanChemical Society Washington DC 1998 pp 237-266
(9) Berkowitz J Ellison G B Gutman D Three Methods toMeasure RH Bond Energies J Phys Chem1994 98 2744-2765(10) Seakins P W Pilling M J Niiranen J T Gutman D
Krasnoperov L N Kinetics and Thermochemistry of R + HBrT RH + Br ReactionssDeterminations of the Heat of Formation of C2H5 i -C3H7 sec -C4H9 and tert -C4H9 J Phys Chem 1992 96 9847-9855
(11) Ruscic B Berkowitz J Curtiss L A Pople J A The EthylRadicalsPhotoionization and Theoretical Studies J Chem Phys1989 91 114-121
(12) Ervin K M Experimental techniques in gas-phase ion thermo-chemistry Chem Rev 2001 101 391-444
(13) Rienstra-Kiracofe J C Tschumper G S Schaefer H F IIINandi S Ellison G B Atomic and Molecular Electron Affini-ties Photoelectron Experiments and Theoretical ComputationsChem Rev 2002 102 231-282
(14) Ramond T M Blanksby S J Kato SBierbaum V M DavicoG E Schwartz R L Lineberger W C Ellison G B The Heatof Formation of the Hydroperoxyl Radical HOO via Negative Ion
Studies J Phys Chem A 2002 106 9641-9647(15) Seetula J AGutman DKineticsof the CH2OH + HBr and CH2OH
+ HI Reactions and Determination of the Heat of Formation of CH2OH J Phys Chem 1992 96 5401-5405
(16) Ruscic B Berkowitz J Heat of Formation of CH2OH andD0(H-CH2OH) J Phys Chem 1993 97 11451-11455
(17) Lias S G Bartmess J E Liebman J F Holmes J L LevinR D MallardW G Gas Phase Ion and Neutral Thermochemistry J Phys Chem Ref Data1988 17 (Suppl 1) 1
(18) Ramond T M Davico G E Schwartz R L Lineberger W CVibronic structure of alkoxy radicals via photoelectron spectros-copy J Chem Phys 2000 112 1158-1169
(19) Pedley J B Naylor R D Kirby S P Thermochemistry of Organic Compounds 2nd ed Chapman and Hall New Y ork1986
(20) Pedley J B Thermochemical Data and Structures of Organic Compounds Thermodynamics Research Center College Station TX 1994
(21) Thehuge value of DH 298(H2O) is the reason that the OH radical issuch an important species in atmospheric chemistry Hydroxylradicals result from solar photodissociation of O3 and they reactwith all organic species pumped into the atmosphere
(22) Ingold K U Wright J S U nderstanding trends in C-H N -Hand O-H bond dissociation enthalpies J Chem Educ 2000 77 1062-1064
(23) Goddard W A III Harding L B The Description of ChemicalBonding from Ab Initio Calculations Annu Rev Phys Chem1978 29 363-396
(24) Carter E A Goddard W A Relation between S inglet TripletGaps and Bond-Energies J Phys Chem 1986 90 998-1001
(25) Dunning T H J r A Road Map for the Calculation of M olecularBinding Energies J Phys Chem A 2000 104 9062-9080
(26) WuC J Carter E A Ab Initio Thermochemistry for UnsaturatedC2 Hydrocarbons J Phys Chem 1991 95 8352-8363
(27) Wenthold P G Squires R R Gas-phase acidities of o- m- andp-dehydrobenzoic acid radicals Determination of the substituentconstants for a phenyl radical site Int J Mass Spectrom 1998
175 215-224(28) Wenthold P G S quires R R Lineberger W C Ultraviolet
photoelectron spectroscopy of theo - m - and p -benzynenegativeions Electron affinities and singlet-triplet splittings for o - m -and p -benzyne J Am Chem Soc 1998 120 5279-5290
(29) Chen P In Unimolecular and Bimolecular Reaction Dynamics NgC YBaer T PowisIEds J ohnWiley amp Sons New York1994 Vol 3 pp 372-425
(30) Oakes J M J ones M E Bierbaum V M Ellison G BPhotoelectron Spectroscopy of CCO- and HCCO- J Phys Chem1983 87 4810-4815
(31) Ruscic B Litorja M Asher R L Ionization energy of methylenerevisited Improved values for the enthalpy of formation of CH2
and the bond dissociation energy of CH3 via simultaneoussolution of the local thermochemical network J Phys Chem A1999 103 8625-8633
(32) Halliwell B G utteridge J M C Free Radicals in Biology and
Medicine 3rd ed Oxford University Press Inc New York1999(33) Nicolaou K C Smith A L Yue E W Chemistry and Biology
of Natural and Designed Enediynes Proc Natl Acad Sci USA1993 90 5881-5888
(34) Ruscic B Feller D Dixon D A Peterson K A Harding L BAsher R L Wagner A F Evidence for a lower enthalpy of formation of hydroxyl radical and a lower gas-phase bonddissociation energy of water J Phys Chem A 2001 105 1-4
(35) Davis H F Kim B S J ohnston H S Lee Y T Dissociation-Energy and Photochemistry of NO3 J Phys Chem 1993 97 2172-2180
(36) M ordaunt D H Ashfold M N R Near-UltravioletPhotolysis of C2H2sa Precise Determination of D0(HCC-H) J Chem Phys1994 101 2630-2631
(37) Wenthold P G Squires R R Biradical Thermochemistry fromCollision-Induced Dissociation Threshold Energy MeasurementsAbsolute Heats of Formation of ortho - meta- and para-Benzyne J Am Chem S oc 1994 116 6401-6412
(38) Ellison G B Davico G E Bierbaum V M DePuy C H The Thermochemistry of the Benzyl and A llyl Radicals and Ions Int J Mass Spectrom Ion Processes 1996 156 109-131
(39) Nicovich J M Kreutter K D Vandijk C A Wine P H Temperature-Dependent Kinetics Studies of the Reactions Br(2P32)+ H 2S Reversible SH + HBr and Br(2P32) + CH3SH ReversibleCH3S + HBrsHeats of Formation of SH and CH3S Radicals J Phys Chem 1992 96 2518-2528
(40) Ruscic B Berkowitz J Photoionization Mass-SpectrometricStudies of the Isomeric Transient Species CH2SH and CH3S J Chem Phys 1992 97 1818-1823
(41) DeTuri V F Ervin K M Proton transfer between Cl- andC6H5OH O-H bond energy of phenol Int J Mass Spectrom1998 175 123-132
(42) Blanksby S J Ramond T M Davico G E Nimlos M R KatoS Bierbaum V M Lineberger W C Ellison G B OkumuraM Negative Ion Photoelectron Spectroscopy Gas-Phase Acidityand Thermochemistry of the Peroxyl Radicals CH3OO andCH3CH2OO J Am Chem Soc 2001 123 9585-9596
(43) Niiranen J T Gutman D Krasnoperov L N Kinetics and Thermochemistry of the CH3CO RadicalsStudy of the CH3CO +
(45) Ruscic B Litorja M Photoionization of HOCO revisited a newupper limit to the adiabatic ionization energy and lower limit tothe enthalpy of formation Chem Phys Lett 2000 316 45-50
(46) Linstrom P J Mallard W G NIST Chemistry WebBook NISTStandard Reference Database No 69 National Institute of
Standards and Technology Gaithersburg MD 2001 httpwebbooknistgov
AR020230D
BondDissociationEnergiesofOrganic Molecules Blanksby and Ellison
VOL 36 NO 4 2003 ACCOUNTS OF CHEMICAL RESEARCH 263
892019 2003 - Bond Dissociation Energies of Organic Molecules
H where R isan organic radical R-Hf R + H For large RH molecules
it is common that intC p(R) dT = intC p(RH) dT Thus one can
simplify eq 4 to D 0(RH) = DH 298(RH) - intC p(H) dT At 298
K the heat capacity integral for an atom can be calculated
exactly int0298 C p(H) dT ) 148 kcal mol-1 so D 0(RH) =
DH 298(RH) - 15 kcal mol-1 For many organics D 0(RH)
and DH 298(RH) are almost numerically equivalent and as
a consequence the terms ldquobond dissociation energyrdquo
(BDE) and ldquobond dissociation enthalpyrdquo often appear
interchangeably in the literature (and indeed in this
Account) For the most part this is innocuous but if a
precision of better than (3 kcal mol-1 is required then it
is crucial to specify the appropriate D 0(RH) or DH T (RH)
By definition3 the enthalpy of a system (H ) is equal
to its internal energy (U ) and its ability to do pressure-
volume work (pV ) H ) U + pV For 1 mol of an idealgas pV ) RT therefore as an approximation as T f 0 K
H f U From statistical mechanics it is possible to
approximate the internal energy of a system as indepen-
dent contributions from electronic (E elec) vibrational (E vib)
rotational (E rot) and translational (E trans) energy At T ) 0
K all molecules will be in their ground vibrational and
rotational states (written as |v primeprime ) 0 J primeprime ) 0rang where the
double primes indicate the ground electronic state) and
will have ceased translation thus U = E 0 ) E elec + ZPE
(Figure 1) Therefore at 0 K the homolytic bond dissocia-
tion energy can be given in terms of eq 5
Electronic structure calculations (such as those pro-
vided by the GAUSSIAN suite of codes)7 can derive E 0 by
a range of ab initio and semiempirical methods Thus
D 0(AB) values provide a useful comparison between
experimental and theoretical thermochemistry While this
is outside the scope of this Account these theoretical
methods show excellent agreement with experimental
bond dissociation energies8
Table 1 Molecular Bond Dissociation Energies for RH f R + H Experimental Bond E nthalpies and RadicalHeats of Formation at 298 K
DH 298
(kcal mol-1)∆fH 298(R)
(kcal mol-1) r efDH 298
(kcal mol-1)∆fH 298(R)
(kcal mol-1) r ef
I norga nicsH 2 104206 ( 0 0 03 5 2 10 3 ( 0003 4 OH - f O - + H 11021 ( 007 -3323 ( 007 34H F 13625 ( 001 1883 ( 017 6 OH + f O + H + 1152 ( 01 5955 ( 002 34H C l 10315 ( 003 2903 ( 004 9 H 2S 912 ( 01 342 ( 02 6H B r 8754 ( 005 2862 ( 006 9 S H 841 ( 02 662 ( 03 6H I 7132 ( 006 2604 ( 008 9 H -NO 495 ( 07 218 ( 01 4
H -C N 1263 ( 02 1050 ( 07 6 H -ONO (t r a ns) 791 ( 02 82 ( 01 4NH 3 1076 ( 01 445 ( 01 6 H -ON O2 1017 ( 04 176 ( 03 35H 2O 11882 ( 007 886 ( 007 34 S iH 4 917 ( 05 479 ( 06 9OH 10176 ( 007 5955 ( 002 34 G eH 4 83 ( 2 53 ( 2 9
Hydroca rbonsC H 4 10499 ( 003 3505 ( 007 31 C H 2C H -H 1107 ( 06 711 ( 07 6C H 3 1104 ( 02 933 ( 02 31 H C C -H 13332 ( 007 1356 ( 02 36C H 2 1013 ( 03 1425 ( 02 9 C 6H 5-H 1129 ( 05 805 ( 05 6C H 809 ( 02 1713 ( 01 9 C 6H 5 f o -C6H 4 + H 78 ( 3 106 ( 3 37C H 3C H 2-H 1011 ( 04 290 ( 04 10 C 6H 5 f m -C6H 4 + H 94 ( 3 122 ( 3 37(C H 3)2C H -H 986 ( 04 215 ( 04 10 C 6H 5 f p -C6H 4 + H 109 ( 3 138 ( 3 37C H 3C H 2(C H 3)C H -H 982 ( 05 161 ( 05 10 C H 2C H C H 2-H 888 ( 04 414 ( 04 38(C H 3)3C -H 965 ( 04 123 ( 04 10 C 6H 5C H 2-H 898 ( 06 497 ( 06 38
AlcoholsH -C H 2OH 961 ( 02 -408 ( 02 16 C H 3C H 2O-H 1047 ( 08 -36 ( 08 6C H 3O-H 1046 ( 07 43 ( 07 6 (C H 3)2C H O-H 1057 ( 07 -115 ( 07 6
C H 3S-
H 874(
05 298(
04 3940 (C H 3)3C O-
H 1063(
07 -
205(
07 6H -C H 2S H 94 ( 2 36 ( 2 39 40 C 6H 5O-H 90 ( 3 -58 ( 3 41
PeroxidesH OO -H 878 ( 05 32 ( 05 14 C H 3C H 2OO -H 85 ( 2 -68 ( 23 42C H 3OO -H 88 ( 1 48 ( 12 42 (C H 3)3C OO -H 84 ( 2 -252 ( 23 42
Ca rbonylsH -C H O 88144 ( 0008 101 ( 01 6 H -COOH is g 96 ( 1 -465 ( 07 45C H 3C (O)-H 894 ( 03 -24 ( 03 43 C H 3C OO -H 112 ( 3 -43 ( 3 44H -C H 2C H O 94 ( 2 25 ( 22 9 C 6H 5C OO -H 111 ( 4 -12 ( 4 44H C O O -H 112 ( 3 -30 ( 3 44
FIGURE1 The energy requiredtofragment the moleculeAB in itsground state (ie the lowest energy electronic vibrational androtational eigenstate) to ground-state fragments A and B is bydefinition the bond dissociation energy D 0(AB)
D 0(AB) ) E 0(A) + E 0(B) - E 0(AB) (5)
BondDissociationEnergiesofOrganic Molecules Blanksby and Ellison
256 ACCOUNTS OF CHEMICAL RESEARCH VOL 36 NO 4 2003
892019 2003 - Bond Dissociation Energies of Organic Molecules
C H 3C H 2 R 87 9(0 6) 8 7 1(0 5) 85 6(0 6) 100 0(0 8) 75 4(0 6) 102 2(0 7) 76 7(0 7) R 83 3(0 5) 8 3 5(0 5)
(CH 3)2C H R R 856(05) 827(06) 992(08) 752(07) 1010(07) 764(08) R 831(05) 819(05)
(CH 3)3C R R R 786(07) 978(08) 732(07) 983(08) - R - 794(06)
C H 2C H R R R R 116(1) 873(08) 116(1) - - - 41(3)a
C H 2C H C H 2 R R R R R 627(06) - - - - -
H C C 1 26 5(0 3 ) 1 25 1(0 5 ) 1 24 5(0 6 )a 1223(05)a - - - - - - -
H C C -C H 2 78(3) 77(3)a - - - - - - - - -
C 6H 5 R R R R R - 118(1) R R 993(09) 988(08)
C 6H 5C H 2 R R R - - - 97(1) 65 2(0 9) - - 714(09)
a B ond ent ha lpies of s t a ble orga nic molecules a re t a bula t ed a long w it h t h eir uncert a int ies For exa mple D H 298(C 6H 5-OH ) ) 1124 (
06 kca l mol-1 T hese bond ent ha lpies a re ca lcula t ed from t he ra dica l hea t s of forma t ion from T a ble 1 a nd t he pa rent ∆fH 298 va luest a bula t ed by Pedley et a l 1920 There are a few entries (such as CH 3F or C 6H 5C H 2F) where ∆ fH 298(par ent) is not provided by Pedley et a lso we ha ve a dopt ed t he va lue recommended by t he NI S T Web sit e46 (htt pwebbooknistgov) w e ha ve ma rked t hese pa rent compounds
wit h a n ldquo a rdquo I n some ca ses t he hea t of forma t ion of t he pa rent species is not a va ila ble (eg vinylmet hyl et her CH 2C H -OC H 3) so t h ebond entha lpy cann ot be computed and this is ma rked with a dash There are a n umber of redundant entr ies in this table [eg DH 298(C 6H 5-
C H 3) ) D H 298(C H 3-C 6H 5)] so t he second ent ry is ma rked wit h a n ldquo Rrdquo T he uncert a int ies ha ve been a dded in qua dra t ure
X + CH3OH a HX + CH
2OH (17)
CH3OH + h νthresh f +[CH2OH] + H (18)
∆f H 298(R) ) DH 298(R-H) + ∆f H 298(RH) - ∆f H 298(H) (19)
CH3-CH2CH3 f CH3 + CH2CH3 (20)
BondDissociationEnergiesofOrganic Molecules Blanksby and Ellison
258 ACCOUNTS OF CHEMICAL RESEARCH VOL 36 NO 4 2003
892019 2003 - Bond Dissociation Energies of Organic Molecules
unavailable in the dissociation of the hydroperoxide
(CH3)3C-OOH
Chart 3 shows a comparison of the C-H bond energies
of benzene and the phenyl radical These experimentalvalues are available from the ingenious negative ion
studies of Squires and co-workers2728 and demonstrate the
increasing bond strengths at the ortho lt meta lt para
positions quantifying the increasing stability of the result-
ing benzynes as o - gt m - gt p -benzyne
What Are Bond Strengths A common term which often appears in the literature is
the average bond energy This is defined as the energy
required to break all the bonds in a given molecule at 298
K divided by the number of bonds For example consider
the atomization of methane which is the dissociation of CH4 to carbon and four hydrogens (CH4 f C + 4H) The
enthalpy of atomization is known from the experimental
heats of formation of methane carbon and hydrogen
∆atomH 298(CH4) ) ∆f H 298(C) + 4∆f H 298(H) - ∆f H 298(CH4) )
3975 kcal mol-1 This implies that the methane molecule
contains 3975 kcal mol-1 of energy that is partitioned
among four bonds thus D av H 298 ) 994 kcal mol-1 In
Table 1 we list the separate experimental bond enthalpies
of methane and its constituent radicals CH3 CH2 and
CH DH 298(CH4 f CH3 + H) ) 10499 kcal mol-1
DH 298(CH3 f CH2 + H) ) 1104 kcal mol-1 DH 298(CH2 f
CH + H) ) 1013 kcal mol-1 and DH 298(CH f C + H) )
809 kcal mol-1 Notice that not one of these bond
enthalpies is equal to the average bond energy of methane
Therefore one should be cautious when interpreting thesignificance of the average bond enthalpy of a molecule
It is important to note that if these four independently
measured bond enthalpies are combined they give
∆rxnH 298(CH4 f C + 4H) ) 3975 ( 06 kcal mol-1 as
required by the first law of thermodynamics
One should also be careful about the term bond
strength and the tendency to treat these energetic quanti-
ties as transferable objects between different molecules
Equation 3 shows that the bond dissociation energy is the
energy of a fragmentation reaction rather than any intrin-
sic property of a chemical bond Acetylene is a good
example (Chart 1) One could say that the HCtCH bond
strength is 231 kcal mol
-1
[HCt
CHf
HC+
CH] and thatboth CsH bonds are equivalent with bond strengths of
81 kcal mol-1 [HC f C + H see Table 1] However one
could equally well claim that the first CH bond strength
is 133 kcal mol-1 [HCtCH f HCtC + H] which is
different from the second CH bond strength of 117 kcal
mol-1 [HCtC f CtC + H] while the CC bond strength
[CtC f C + C] is only 142 kcal mol-1 Or alternatively
the first CH bond strength is 133 kcal mol-1 while the
CC bond strength [HCtC f HC + C] is 178 kcal mol-1
and the second CH bond strength [HC f H + C] is only
81 kcal mol-1 Clearly one arrives at different bond
Chart 1 Experimental Bond Enthalpies DH 29 8 for Several ImportantHydrocarbons and Hydrocarbon Radicalsa
a Values are in kcal mol-1 and represent the energy required to break only the single bond indicated Thus DH 298(HCtCHf HC + CH) ) 2307( 02 kcal mol-1 but DH 298(HCCsH f HCC + H) ) 13332 ( 007 kcalmol-1
Chart 2 Experimental Bond Enthalpies DH 29 8 for Several ImportantOxycarbons and Oxycarbon Radicals a
a Values are in kcal mol-1 and represent the energy required to break only the single bond indicated Thus DH 298(CH3OsH f CH3O + H) )
1046 ( 07 kcal mol-1 but DH 298(HsCH2OH f H + CH2OH) ) 961 ( 03kcal mol-1 and DH 298(CH3sOH f CH3 + OH) ) 921 ( 01 kcal mol-1
BondDissociationEnergiesofOrganic Molecules Blanksby and Ellison
260 ACCOUNTS OF CHEMICAL RESEARCH VOL 36 NO 4 2003
892019 2003 - Bond Dissociation Energies of Organic Molecules
strengths by breaking the bonds in different orders
Rigorous quantum mechanical discussions of these trends
are available elsewhere26 along with other provocative
discussions of bond strengths232429
The example of HCCH serves to reiterate that the bond
enthalpy is the enthalpy of a homolysis reaction (eq 3)
and thus depends exclusively on the relative stability of
reactant and product states More generally creation of
new bonds in the products or otherwise stabilized
products always decreases the bond enthalpy As an
example consider ketene in Chart 2 The carbon-carbon
bond enthalpy 3031 of ketene DH 298(CH2dCO) ) 787 ( 02
kcal mol-1 is almost 100 kcal mol-1 less than that of
ethylene DH 298(CH2dCH2) ) 174 kcal mol-1 because one
of the products of the ketene fragmentation is an ex-
tremely stable molecule namely carbon monoxide This
example illustrates that not all double bonds are createdequal2426 and that extrapolations of bond energies from
one molecular species to another must be conducted
carefully
Bond Enthalpies in Solut ionThe bond enthalpies tabulated in this Account are exclu-
sively gas-phase values This raises the question of how
to relate gas-phase bond enthalpies to chemical problems
occurring in solution The difference between a gas-phase
bond enthalpy and that in solution D solnH 298(R-H)
depends on the difference in the enthalpy of solvation
∆solnH 298 of the two radicals and the parent compound
as expressed in terms of eq 23 There are not many
accurate measurements available for the enthalpy of
solvation of radical species and we can only estimate the
effects of solvation on organic BDEs The solvation energy
of a hydrogen atom is likely to be negligible in most
solvents and therefore the correction to an R-H gas-
phase bond enthalpy can be can be approximated as[∆solnH 298(R) - ∆solnH 298(RH)] As both the radical and the
parent are neutral the solvation energies are likely to be
small and similar in most cases particularly in nonpolar
solvents The small effect on bond enthalpy is likely to be
most noticeable is protic polar solvents where hydrogen
bonding may play a key role in preferentially stabilizing
the radical or parent in solution Consider for example
the C-H and O-H bond enthalpies of CH3OH in a polar
protic solvent (reactions 24)
While in reaction 24a both methanol and the hydroxy-
methyl radical can hydrogen bond to the surrounding
solvent such stabilization may be less prevalent for the
methoxyl radical in reaction 24b due to the absence of
the highly polarized O-H bond Therefore one might
surmise that the H-CH2OH bond energy of methanol in
a protic solvent would be largely unchanged from the gas
phase while the CH3O-H value could be slightly higher
in solution There is clearly much work to be done in this
area
Chart 3 Experimental Bond Enthalpies DH 29 8 for Benzene andPhenyl Radicala
a Values are in kcal mol-1 and represent the energy required to break only the single bond indicated Thus DH 298(C6H5sHf C6H5 + H) ) 1129( 05 kcal mol-1 but DH 298(C6H4sH f H + o -C6H4) ) 78 ( 3 kcal mol-1DH 298(C6H4sH f H + m -C6H4) ) 94 ( 3 kcal mol-1 and DH 298(C6H4sHf H + p -C6H4) ) 109 ( 3 kcal mol-1
88 kcal mol-1 gt DH 298(bisallylicC-H) Given these esti-
mates the hydrogens most susceptible to radical abstrac-
tion will be those in the allylic positions While there is
at present no direct data for doubly allylic C-H bond
enthalpies one might conjecture that the bond energy
will be roughly 80 kcal mol-1 given that the dif-
ference between a typical methylenic Cs
H (egDH 298((CH3)2CHsH) = 986 kcal mol-1) and a singly allylic
CsH (eg DH 298(H2CdCHCH2sH) = 888 kcal mol-1) is
about 10 kcal mol-1 Thus the bisallylic hydrogens at
carbons 7 10 and 13 represent the most labile hydrogens
in the molecule Once a carbon-centered radical is pro-
duced and rearranged to its most stable form it will
readily add O2 to form a peroxyl radical Chart 2 indicates
that the OO-CMe3 bond enthalpy is 38 kcal mol-1 Given
that the O-H bond enthalpy of a hydroperoxide is
approximately 85 kcal mol-1 a radical chain reaction will
be exothermic
The biological activity of the enediyne anticancer
antibiotic agents is thought to be due to their ability toform reactive diradicals in situ 33 Molecules such as
calicheamicin γ1I possess an extended sugar residue which
serves to deliver the enediyne moiety the active part of
the molecule to a sequence-specific position on the DNA
double-helix Upon delivery to the target the enediyne
functionality is activated to undergo a Bergman cycloaro-
matization which yields a substituted p -benzyne (Scheme
2) From Chart 3 we can estimate that the diradical can
abstract all hydrogens bound by e109 kcal mol-1 This
makes the p -benzyne a quite powerful hydrogen abstrac-
tion reagent and further once one hydrogen has been
abstracted a substituted phenyl radical results which can
abstract all hydrogens bound by e113 kcal mol-1 Thus
two exothermic hydrogen abstractions by the reactive
p -benzyne moiety can lead to selective cutting of double-
stranded DNA
SummaryThe critically evaluated bond enthalpies listed in Tables
1 and 2 should serve as an important resource for the
organic chemist The values listed may be used to calcu-late rigorous experimental thermochemistry for many
common reactions and further with appropriate care
instructive estimations of reaction thermochemistry can
be made for complex chemical problems
This work was supported by grants from the Chemical Physics
Program United States Department of Energy (DE-FG02-
87ER13695) and the National Science Foundation (CHE-0201848)
We are grateful for the sustained advice and criticism from our
Colorado colleagues Carl Lineberger Veronica Bierbaum Shuji
Kato Mark Nimlos Xu Zhang Bob Damrauer Charles H DePuy
Geoff Tyndall and Veronica Vaida We are also continually
educated by our friends Emily Carter George Petersson Larry
Harding Kent Ervin Richard OrsquoHair and Branko Ruscic Finally GBE would like to thank Joseph Berkowitz now retired for 25
years of friendship and physics
References(1) Benson S W Thermochemical Kinetics 2nd ed Wiley-Inter-
science New Y ork 1976(2) MillsI Cvitas T Homann KKallay NKuchitsuK Quantities
Units and Symbols in Physical Chemistry Blackwell ScientificPublications Oxford 1988 This reference lists the IUPA Crsquosguidelines concerning thermochemical symbols which we adoptInstead of the more common expressions ∆G rxn 298(1) ∆H f 0deg(R)or ∆H f 298deg(RH) the use of ∆rxnG 298(1) ∆f H 0(R) or ∆f H 298(RH) isrecommended See p 46
(3) Herzberg G H M olecular Spectra and Molecular StructureInfrared and Raman S pectra of Polyatomic Molecules D Van
Nostrand Princeton NJ 1945 Vol II see Chapter V(4) Gurvich L V Veyts I V Alcock C B Iorish V S Thermody-
namic Properties of Individual Substances 4th ed HemisphereNew York 1991 Vol 2
(5) Ervin K M Gronert SBarlow S E Gilles M K Harrison AGBierbaumV M Charles H DePuy Lineberger W CEllisonG B Bond Strengths of Ethylene and Acetylene J Am ChemSoc 1990 112 5750-5759
(6) Ervin K M DeTuri V F Anchoring the Gas-Phase AcidityScale Experiment and Theory J Phys Chem A 2002 106 9947-
9956(7) Frisch M J Trucks G WSchlegel H B Scuseria G E Robb
M A Cheeseman J R Zakrzewski V G Montgomery J A J r Stratmann R E Burant J C Dapprich S M illam J MDaniels A D Kudin K N Strain M C Farkas O Tomasi J Barone V Cossi M Cammi R M ennucci B Pomelli C
Scheme 2
BondDissociationEnergiesofOrganic Molecules Blanksby and Ellison
262 ACCOUNTS OF CHEMICAL RESEARCH VOL 36 NO 4 2003
892019 2003 - Bond Dissociation Energies of Organic Molecules
Adamo C Clifford S Ochterski J Petersson G A Ayala P Y Cui Q Morokuma K Malick D K Rabuck A D Raghava-chari K Foresman J B Cioslowski J Ortiz J VStefanov BB LiuG Liashenko A Piskorz PKomaromiI Gomperts RMartin R L Fox D J Keith T Al-Laham M A Peng C YNanayakkara A Gonzalez C Challacombe M Gill P M W J ohnson B G Chen W Wong M W Andres J L Head-Gordon M Replogle E S Pople J A Gaussian 98 GaussianInc Pittsburgh PA 1998
(8) Petersson G A In Computational Thermochemistry Irikura KK Frurip D J Eds ACS Symposium Series 677 AmericanChemical Society Washington DC 1998 pp 237-266
(9) Berkowitz J Ellison G B Gutman D Three Methods toMeasure RH Bond Energies J Phys Chem1994 98 2744-2765(10) Seakins P W Pilling M J Niiranen J T Gutman D
Krasnoperov L N Kinetics and Thermochemistry of R + HBrT RH + Br ReactionssDeterminations of the Heat of Formation of C2H5 i -C3H7 sec -C4H9 and tert -C4H9 J Phys Chem 1992 96 9847-9855
(11) Ruscic B Berkowitz J Curtiss L A Pople J A The EthylRadicalsPhotoionization and Theoretical Studies J Chem Phys1989 91 114-121
(12) Ervin K M Experimental techniques in gas-phase ion thermo-chemistry Chem Rev 2001 101 391-444
(13) Rienstra-Kiracofe J C Tschumper G S Schaefer H F IIINandi S Ellison G B Atomic and Molecular Electron Affini-ties Photoelectron Experiments and Theoretical ComputationsChem Rev 2002 102 231-282
(14) Ramond T M Blanksby S J Kato SBierbaum V M DavicoG E Schwartz R L Lineberger W C Ellison G B The Heatof Formation of the Hydroperoxyl Radical HOO via Negative Ion
Studies J Phys Chem A 2002 106 9641-9647(15) Seetula J AGutman DKineticsof the CH2OH + HBr and CH2OH
+ HI Reactions and Determination of the Heat of Formation of CH2OH J Phys Chem 1992 96 5401-5405
(16) Ruscic B Berkowitz J Heat of Formation of CH2OH andD0(H-CH2OH) J Phys Chem 1993 97 11451-11455
(17) Lias S G Bartmess J E Liebman J F Holmes J L LevinR D MallardW G Gas Phase Ion and Neutral Thermochemistry J Phys Chem Ref Data1988 17 (Suppl 1) 1
(18) Ramond T M Davico G E Schwartz R L Lineberger W CVibronic structure of alkoxy radicals via photoelectron spectros-copy J Chem Phys 2000 112 1158-1169
(19) Pedley J B Naylor R D Kirby S P Thermochemistry of Organic Compounds 2nd ed Chapman and Hall New Y ork1986
(20) Pedley J B Thermochemical Data and Structures of Organic Compounds Thermodynamics Research Center College Station TX 1994
(21) Thehuge value of DH 298(H2O) is the reason that the OH radical issuch an important species in atmospheric chemistry Hydroxylradicals result from solar photodissociation of O3 and they reactwith all organic species pumped into the atmosphere
(22) Ingold K U Wright J S U nderstanding trends in C-H N -Hand O-H bond dissociation enthalpies J Chem Educ 2000 77 1062-1064
(23) Goddard W A III Harding L B The Description of ChemicalBonding from Ab Initio Calculations Annu Rev Phys Chem1978 29 363-396
(24) Carter E A Goddard W A Relation between S inglet TripletGaps and Bond-Energies J Phys Chem 1986 90 998-1001
(25) Dunning T H J r A Road Map for the Calculation of M olecularBinding Energies J Phys Chem A 2000 104 9062-9080
(26) WuC J Carter E A Ab Initio Thermochemistry for UnsaturatedC2 Hydrocarbons J Phys Chem 1991 95 8352-8363
(27) Wenthold P G Squires R R Gas-phase acidities of o- m- andp-dehydrobenzoic acid radicals Determination of the substituentconstants for a phenyl radical site Int J Mass Spectrom 1998
175 215-224(28) Wenthold P G S quires R R Lineberger W C Ultraviolet
photoelectron spectroscopy of theo - m - and p -benzynenegativeions Electron affinities and singlet-triplet splittings for o - m -and p -benzyne J Am Chem Soc 1998 120 5279-5290
(29) Chen P In Unimolecular and Bimolecular Reaction Dynamics NgC YBaer T PowisIEds J ohnWiley amp Sons New York1994 Vol 3 pp 372-425
(30) Oakes J M J ones M E Bierbaum V M Ellison G BPhotoelectron Spectroscopy of CCO- and HCCO- J Phys Chem1983 87 4810-4815
(31) Ruscic B Litorja M Asher R L Ionization energy of methylenerevisited Improved values for the enthalpy of formation of CH2
and the bond dissociation energy of CH3 via simultaneoussolution of the local thermochemical network J Phys Chem A1999 103 8625-8633
(32) Halliwell B G utteridge J M C Free Radicals in Biology and
Medicine 3rd ed Oxford University Press Inc New York1999(33) Nicolaou K C Smith A L Yue E W Chemistry and Biology
of Natural and Designed Enediynes Proc Natl Acad Sci USA1993 90 5881-5888
(34) Ruscic B Feller D Dixon D A Peterson K A Harding L BAsher R L Wagner A F Evidence for a lower enthalpy of formation of hydroxyl radical and a lower gas-phase bonddissociation energy of water J Phys Chem A 2001 105 1-4
(35) Davis H F Kim B S J ohnston H S Lee Y T Dissociation-Energy and Photochemistry of NO3 J Phys Chem 1993 97 2172-2180
(36) M ordaunt D H Ashfold M N R Near-UltravioletPhotolysis of C2H2sa Precise Determination of D0(HCC-H) J Chem Phys1994 101 2630-2631
(37) Wenthold P G Squires R R Biradical Thermochemistry fromCollision-Induced Dissociation Threshold Energy MeasurementsAbsolute Heats of Formation of ortho - meta- and para-Benzyne J Am Chem S oc 1994 116 6401-6412
(38) Ellison G B Davico G E Bierbaum V M DePuy C H The Thermochemistry of the Benzyl and A llyl Radicals and Ions Int J Mass Spectrom Ion Processes 1996 156 109-131
(39) Nicovich J M Kreutter K D Vandijk C A Wine P H Temperature-Dependent Kinetics Studies of the Reactions Br(2P32)+ H 2S Reversible SH + HBr and Br(2P32) + CH3SH ReversibleCH3S + HBrsHeats of Formation of SH and CH3S Radicals J Phys Chem 1992 96 2518-2528
(40) Ruscic B Berkowitz J Photoionization Mass-SpectrometricStudies of the Isomeric Transient Species CH2SH and CH3S J Chem Phys 1992 97 1818-1823
(41) DeTuri V F Ervin K M Proton transfer between Cl- andC6H5OH O-H bond energy of phenol Int J Mass Spectrom1998 175 123-132
(42) Blanksby S J Ramond T M Davico G E Nimlos M R KatoS Bierbaum V M Lineberger W C Ellison G B OkumuraM Negative Ion Photoelectron Spectroscopy Gas-Phase Acidityand Thermochemistry of the Peroxyl Radicals CH3OO andCH3CH2OO J Am Chem Soc 2001 123 9585-9596
(43) Niiranen J T Gutman D Krasnoperov L N Kinetics and Thermochemistry of the CH3CO RadicalsStudy of the CH3CO +
(45) Ruscic B Litorja M Photoionization of HOCO revisited a newupper limit to the adiabatic ionization energy and lower limit tothe enthalpy of formation Chem Phys Lett 2000 316 45-50
(46) Linstrom P J Mallard W G NIST Chemistry WebBook NISTStandard Reference Database No 69 National Institute of
Standards and Technology Gaithersburg MD 2001 httpwebbooknistgov
AR020230D
BondDissociationEnergiesofOrganic Molecules Blanksby and Ellison
VOL 36 NO 4 2003 ACCOUNTS OF CHEMICAL RESEARCH 263
892019 2003 - Bond Dissociation Energies of Organic Molecules
C H 3C H 2 R 87 9(0 6) 8 7 1(0 5) 85 6(0 6) 100 0(0 8) 75 4(0 6) 102 2(0 7) 76 7(0 7) R 83 3(0 5) 8 3 5(0 5)
(CH 3)2C H R R 856(05) 827(06) 992(08) 752(07) 1010(07) 764(08) R 831(05) 819(05)
(CH 3)3C R R R 786(07) 978(08) 732(07) 983(08) - R - 794(06)
C H 2C H R R R R 116(1) 873(08) 116(1) - - - 41(3)a
C H 2C H C H 2 R R R R R 627(06) - - - - -
H C C 1 26 5(0 3 ) 1 25 1(0 5 ) 1 24 5(0 6 )a 1223(05)a - - - - - - -
H C C -C H 2 78(3) 77(3)a - - - - - - - - -
C 6H 5 R R R R R - 118(1) R R 993(09) 988(08)
C 6H 5C H 2 R R R - - - 97(1) 65 2(0 9) - - 714(09)
a B ond ent ha lpies of s t a ble orga nic molecules a re t a bula t ed a long w it h t h eir uncert a int ies For exa mple D H 298(C 6H 5-OH ) ) 1124 (
06 kca l mol-1 T hese bond ent ha lpies a re ca lcula t ed from t he ra dica l hea t s of forma t ion from T a ble 1 a nd t he pa rent ∆fH 298 va luest a bula t ed by Pedley et a l 1920 There are a few entries (such as CH 3F or C 6H 5C H 2F) where ∆ fH 298(par ent) is not provided by Pedley et a lso we ha ve a dopt ed t he va lue recommended by t he NI S T Web sit e46 (htt pwebbooknistgov) w e ha ve ma rked t hese pa rent compounds
wit h a n ldquo a rdquo I n some ca ses t he hea t of forma t ion of t he pa rent species is not a va ila ble (eg vinylmet hyl et her CH 2C H -OC H 3) so t h ebond entha lpy cann ot be computed and this is ma rked with a dash There are a n umber of redundant entr ies in this table [eg DH 298(C 6H 5-
C H 3) ) D H 298(C H 3-C 6H 5)] so t he second ent ry is ma rked wit h a n ldquo Rrdquo T he uncert a int ies ha ve been a dded in qua dra t ure
X + CH3OH a HX + CH
2OH (17)
CH3OH + h νthresh f +[CH2OH] + H (18)
∆f H 298(R) ) DH 298(R-H) + ∆f H 298(RH) - ∆f H 298(H) (19)
CH3-CH2CH3 f CH3 + CH2CH3 (20)
BondDissociationEnergiesofOrganic Molecules Blanksby and Ellison
258 ACCOUNTS OF CHEMICAL RESEARCH VOL 36 NO 4 2003
892019 2003 - Bond Dissociation Energies of Organic Molecules
unavailable in the dissociation of the hydroperoxide
(CH3)3C-OOH
Chart 3 shows a comparison of the C-H bond energies
of benzene and the phenyl radical These experimentalvalues are available from the ingenious negative ion
studies of Squires and co-workers2728 and demonstrate the
increasing bond strengths at the ortho lt meta lt para
positions quantifying the increasing stability of the result-
ing benzynes as o - gt m - gt p -benzyne
What Are Bond Strengths A common term which often appears in the literature is
the average bond energy This is defined as the energy
required to break all the bonds in a given molecule at 298
K divided by the number of bonds For example consider
the atomization of methane which is the dissociation of CH4 to carbon and four hydrogens (CH4 f C + 4H) The
enthalpy of atomization is known from the experimental
heats of formation of methane carbon and hydrogen
∆atomH 298(CH4) ) ∆f H 298(C) + 4∆f H 298(H) - ∆f H 298(CH4) )
3975 kcal mol-1 This implies that the methane molecule
contains 3975 kcal mol-1 of energy that is partitioned
among four bonds thus D av H 298 ) 994 kcal mol-1 In
Table 1 we list the separate experimental bond enthalpies
of methane and its constituent radicals CH3 CH2 and
CH DH 298(CH4 f CH3 + H) ) 10499 kcal mol-1
DH 298(CH3 f CH2 + H) ) 1104 kcal mol-1 DH 298(CH2 f
CH + H) ) 1013 kcal mol-1 and DH 298(CH f C + H) )
809 kcal mol-1 Notice that not one of these bond
enthalpies is equal to the average bond energy of methane
Therefore one should be cautious when interpreting thesignificance of the average bond enthalpy of a molecule
It is important to note that if these four independently
measured bond enthalpies are combined they give
∆rxnH 298(CH4 f C + 4H) ) 3975 ( 06 kcal mol-1 as
required by the first law of thermodynamics
One should also be careful about the term bond
strength and the tendency to treat these energetic quanti-
ties as transferable objects between different molecules
Equation 3 shows that the bond dissociation energy is the
energy of a fragmentation reaction rather than any intrin-
sic property of a chemical bond Acetylene is a good
example (Chart 1) One could say that the HCtCH bond
strength is 231 kcal mol
-1
[HCt
CHf
HC+
CH] and thatboth CsH bonds are equivalent with bond strengths of
81 kcal mol-1 [HC f C + H see Table 1] However one
could equally well claim that the first CH bond strength
is 133 kcal mol-1 [HCtCH f HCtC + H] which is
different from the second CH bond strength of 117 kcal
mol-1 [HCtC f CtC + H] while the CC bond strength
[CtC f C + C] is only 142 kcal mol-1 Or alternatively
the first CH bond strength is 133 kcal mol-1 while the
CC bond strength [HCtC f HC + C] is 178 kcal mol-1
and the second CH bond strength [HC f H + C] is only
81 kcal mol-1 Clearly one arrives at different bond
Chart 1 Experimental Bond Enthalpies DH 29 8 for Several ImportantHydrocarbons and Hydrocarbon Radicalsa
a Values are in kcal mol-1 and represent the energy required to break only the single bond indicated Thus DH 298(HCtCHf HC + CH) ) 2307( 02 kcal mol-1 but DH 298(HCCsH f HCC + H) ) 13332 ( 007 kcalmol-1
Chart 2 Experimental Bond Enthalpies DH 29 8 for Several ImportantOxycarbons and Oxycarbon Radicals a
a Values are in kcal mol-1 and represent the energy required to break only the single bond indicated Thus DH 298(CH3OsH f CH3O + H) )
1046 ( 07 kcal mol-1 but DH 298(HsCH2OH f H + CH2OH) ) 961 ( 03kcal mol-1 and DH 298(CH3sOH f CH3 + OH) ) 921 ( 01 kcal mol-1
BondDissociationEnergiesofOrganic Molecules Blanksby and Ellison
260 ACCOUNTS OF CHEMICAL RESEARCH VOL 36 NO 4 2003
892019 2003 - Bond Dissociation Energies of Organic Molecules
strengths by breaking the bonds in different orders
Rigorous quantum mechanical discussions of these trends
are available elsewhere26 along with other provocative
discussions of bond strengths232429
The example of HCCH serves to reiterate that the bond
enthalpy is the enthalpy of a homolysis reaction (eq 3)
and thus depends exclusively on the relative stability of
reactant and product states More generally creation of
new bonds in the products or otherwise stabilized
products always decreases the bond enthalpy As an
example consider ketene in Chart 2 The carbon-carbon
bond enthalpy 3031 of ketene DH 298(CH2dCO) ) 787 ( 02
kcal mol-1 is almost 100 kcal mol-1 less than that of
ethylene DH 298(CH2dCH2) ) 174 kcal mol-1 because one
of the products of the ketene fragmentation is an ex-
tremely stable molecule namely carbon monoxide This
example illustrates that not all double bonds are createdequal2426 and that extrapolations of bond energies from
one molecular species to another must be conducted
carefully
Bond Enthalpies in Solut ionThe bond enthalpies tabulated in this Account are exclu-
sively gas-phase values This raises the question of how
to relate gas-phase bond enthalpies to chemical problems
occurring in solution The difference between a gas-phase
bond enthalpy and that in solution D solnH 298(R-H)
depends on the difference in the enthalpy of solvation
∆solnH 298 of the two radicals and the parent compound
as expressed in terms of eq 23 There are not many
accurate measurements available for the enthalpy of
solvation of radical species and we can only estimate the
effects of solvation on organic BDEs The solvation energy
of a hydrogen atom is likely to be negligible in most
solvents and therefore the correction to an R-H gas-
phase bond enthalpy can be can be approximated as[∆solnH 298(R) - ∆solnH 298(RH)] As both the radical and the
parent are neutral the solvation energies are likely to be
small and similar in most cases particularly in nonpolar
solvents The small effect on bond enthalpy is likely to be
most noticeable is protic polar solvents where hydrogen
bonding may play a key role in preferentially stabilizing
the radical or parent in solution Consider for example
the C-H and O-H bond enthalpies of CH3OH in a polar
protic solvent (reactions 24)
While in reaction 24a both methanol and the hydroxy-
methyl radical can hydrogen bond to the surrounding
solvent such stabilization may be less prevalent for the
methoxyl radical in reaction 24b due to the absence of
the highly polarized O-H bond Therefore one might
surmise that the H-CH2OH bond energy of methanol in
a protic solvent would be largely unchanged from the gas
phase while the CH3O-H value could be slightly higher
in solution There is clearly much work to be done in this
area
Chart 3 Experimental Bond Enthalpies DH 29 8 for Benzene andPhenyl Radicala
a Values are in kcal mol-1 and represent the energy required to break only the single bond indicated Thus DH 298(C6H5sHf C6H5 + H) ) 1129( 05 kcal mol-1 but DH 298(C6H4sH f H + o -C6H4) ) 78 ( 3 kcal mol-1DH 298(C6H4sH f H + m -C6H4) ) 94 ( 3 kcal mol-1 and DH 298(C6H4sHf H + p -C6H4) ) 109 ( 3 kcal mol-1
88 kcal mol-1 gt DH 298(bisallylicC-H) Given these esti-
mates the hydrogens most susceptible to radical abstrac-
tion will be those in the allylic positions While there is
at present no direct data for doubly allylic C-H bond
enthalpies one might conjecture that the bond energy
will be roughly 80 kcal mol-1 given that the dif-
ference between a typical methylenic Cs
H (egDH 298((CH3)2CHsH) = 986 kcal mol-1) and a singly allylic
CsH (eg DH 298(H2CdCHCH2sH) = 888 kcal mol-1) is
about 10 kcal mol-1 Thus the bisallylic hydrogens at
carbons 7 10 and 13 represent the most labile hydrogens
in the molecule Once a carbon-centered radical is pro-
duced and rearranged to its most stable form it will
readily add O2 to form a peroxyl radical Chart 2 indicates
that the OO-CMe3 bond enthalpy is 38 kcal mol-1 Given
that the O-H bond enthalpy of a hydroperoxide is
approximately 85 kcal mol-1 a radical chain reaction will
be exothermic
The biological activity of the enediyne anticancer
antibiotic agents is thought to be due to their ability toform reactive diradicals in situ 33 Molecules such as
calicheamicin γ1I possess an extended sugar residue which
serves to deliver the enediyne moiety the active part of
the molecule to a sequence-specific position on the DNA
double-helix Upon delivery to the target the enediyne
functionality is activated to undergo a Bergman cycloaro-
matization which yields a substituted p -benzyne (Scheme
2) From Chart 3 we can estimate that the diradical can
abstract all hydrogens bound by e109 kcal mol-1 This
makes the p -benzyne a quite powerful hydrogen abstrac-
tion reagent and further once one hydrogen has been
abstracted a substituted phenyl radical results which can
abstract all hydrogens bound by e113 kcal mol-1 Thus
two exothermic hydrogen abstractions by the reactive
p -benzyne moiety can lead to selective cutting of double-
stranded DNA
SummaryThe critically evaluated bond enthalpies listed in Tables
1 and 2 should serve as an important resource for the
organic chemist The values listed may be used to calcu-late rigorous experimental thermochemistry for many
common reactions and further with appropriate care
instructive estimations of reaction thermochemistry can
be made for complex chemical problems
This work was supported by grants from the Chemical Physics
Program United States Department of Energy (DE-FG02-
87ER13695) and the National Science Foundation (CHE-0201848)
We are grateful for the sustained advice and criticism from our
Colorado colleagues Carl Lineberger Veronica Bierbaum Shuji
Kato Mark Nimlos Xu Zhang Bob Damrauer Charles H DePuy
Geoff Tyndall and Veronica Vaida We are also continually
educated by our friends Emily Carter George Petersson Larry
Harding Kent Ervin Richard OrsquoHair and Branko Ruscic Finally GBE would like to thank Joseph Berkowitz now retired for 25
years of friendship and physics
References(1) Benson S W Thermochemical Kinetics 2nd ed Wiley-Inter-
science New Y ork 1976(2) MillsI Cvitas T Homann KKallay NKuchitsuK Quantities
Units and Symbols in Physical Chemistry Blackwell ScientificPublications Oxford 1988 This reference lists the IUPA Crsquosguidelines concerning thermochemical symbols which we adoptInstead of the more common expressions ∆G rxn 298(1) ∆H f 0deg(R)or ∆H f 298deg(RH) the use of ∆rxnG 298(1) ∆f H 0(R) or ∆f H 298(RH) isrecommended See p 46
(3) Herzberg G H M olecular Spectra and Molecular StructureInfrared and Raman S pectra of Polyatomic Molecules D Van
Nostrand Princeton NJ 1945 Vol II see Chapter V(4) Gurvich L V Veyts I V Alcock C B Iorish V S Thermody-
namic Properties of Individual Substances 4th ed HemisphereNew York 1991 Vol 2
(5) Ervin K M Gronert SBarlow S E Gilles M K Harrison AGBierbaumV M Charles H DePuy Lineberger W CEllisonG B Bond Strengths of Ethylene and Acetylene J Am ChemSoc 1990 112 5750-5759
(6) Ervin K M DeTuri V F Anchoring the Gas-Phase AcidityScale Experiment and Theory J Phys Chem A 2002 106 9947-
9956(7) Frisch M J Trucks G WSchlegel H B Scuseria G E Robb
M A Cheeseman J R Zakrzewski V G Montgomery J A J r Stratmann R E Burant J C Dapprich S M illam J MDaniels A D Kudin K N Strain M C Farkas O Tomasi J Barone V Cossi M Cammi R M ennucci B Pomelli C
Scheme 2
BondDissociationEnergiesofOrganic Molecules Blanksby and Ellison
262 ACCOUNTS OF CHEMICAL RESEARCH VOL 36 NO 4 2003
892019 2003 - Bond Dissociation Energies of Organic Molecules
Adamo C Clifford S Ochterski J Petersson G A Ayala P Y Cui Q Morokuma K Malick D K Rabuck A D Raghava-chari K Foresman J B Cioslowski J Ortiz J VStefanov BB LiuG Liashenko A Piskorz PKomaromiI Gomperts RMartin R L Fox D J Keith T Al-Laham M A Peng C YNanayakkara A Gonzalez C Challacombe M Gill P M W J ohnson B G Chen W Wong M W Andres J L Head-Gordon M Replogle E S Pople J A Gaussian 98 GaussianInc Pittsburgh PA 1998
(8) Petersson G A In Computational Thermochemistry Irikura KK Frurip D J Eds ACS Symposium Series 677 AmericanChemical Society Washington DC 1998 pp 237-266
(9) Berkowitz J Ellison G B Gutman D Three Methods toMeasure RH Bond Energies J Phys Chem1994 98 2744-2765(10) Seakins P W Pilling M J Niiranen J T Gutman D
Krasnoperov L N Kinetics and Thermochemistry of R + HBrT RH + Br ReactionssDeterminations of the Heat of Formation of C2H5 i -C3H7 sec -C4H9 and tert -C4H9 J Phys Chem 1992 96 9847-9855
(11) Ruscic B Berkowitz J Curtiss L A Pople J A The EthylRadicalsPhotoionization and Theoretical Studies J Chem Phys1989 91 114-121
(12) Ervin K M Experimental techniques in gas-phase ion thermo-chemistry Chem Rev 2001 101 391-444
(13) Rienstra-Kiracofe J C Tschumper G S Schaefer H F IIINandi S Ellison G B Atomic and Molecular Electron Affini-ties Photoelectron Experiments and Theoretical ComputationsChem Rev 2002 102 231-282
(14) Ramond T M Blanksby S J Kato SBierbaum V M DavicoG E Schwartz R L Lineberger W C Ellison G B The Heatof Formation of the Hydroperoxyl Radical HOO via Negative Ion
Studies J Phys Chem A 2002 106 9641-9647(15) Seetula J AGutman DKineticsof the CH2OH + HBr and CH2OH
+ HI Reactions and Determination of the Heat of Formation of CH2OH J Phys Chem 1992 96 5401-5405
(16) Ruscic B Berkowitz J Heat of Formation of CH2OH andD0(H-CH2OH) J Phys Chem 1993 97 11451-11455
(17) Lias S G Bartmess J E Liebman J F Holmes J L LevinR D MallardW G Gas Phase Ion and Neutral Thermochemistry J Phys Chem Ref Data1988 17 (Suppl 1) 1
(18) Ramond T M Davico G E Schwartz R L Lineberger W CVibronic structure of alkoxy radicals via photoelectron spectros-copy J Chem Phys 2000 112 1158-1169
(19) Pedley J B Naylor R D Kirby S P Thermochemistry of Organic Compounds 2nd ed Chapman and Hall New Y ork1986
(20) Pedley J B Thermochemical Data and Structures of Organic Compounds Thermodynamics Research Center College Station TX 1994
(21) Thehuge value of DH 298(H2O) is the reason that the OH radical issuch an important species in atmospheric chemistry Hydroxylradicals result from solar photodissociation of O3 and they reactwith all organic species pumped into the atmosphere
(22) Ingold K U Wright J S U nderstanding trends in C-H N -Hand O-H bond dissociation enthalpies J Chem Educ 2000 77 1062-1064
(23) Goddard W A III Harding L B The Description of ChemicalBonding from Ab Initio Calculations Annu Rev Phys Chem1978 29 363-396
(24) Carter E A Goddard W A Relation between S inglet TripletGaps and Bond-Energies J Phys Chem 1986 90 998-1001
(25) Dunning T H J r A Road Map for the Calculation of M olecularBinding Energies J Phys Chem A 2000 104 9062-9080
(26) WuC J Carter E A Ab Initio Thermochemistry for UnsaturatedC2 Hydrocarbons J Phys Chem 1991 95 8352-8363
(27) Wenthold P G Squires R R Gas-phase acidities of o- m- andp-dehydrobenzoic acid radicals Determination of the substituentconstants for a phenyl radical site Int J Mass Spectrom 1998
175 215-224(28) Wenthold P G S quires R R Lineberger W C Ultraviolet
photoelectron spectroscopy of theo - m - and p -benzynenegativeions Electron affinities and singlet-triplet splittings for o - m -and p -benzyne J Am Chem Soc 1998 120 5279-5290
(29) Chen P In Unimolecular and Bimolecular Reaction Dynamics NgC YBaer T PowisIEds J ohnWiley amp Sons New York1994 Vol 3 pp 372-425
(30) Oakes J M J ones M E Bierbaum V M Ellison G BPhotoelectron Spectroscopy of CCO- and HCCO- J Phys Chem1983 87 4810-4815
(31) Ruscic B Litorja M Asher R L Ionization energy of methylenerevisited Improved values for the enthalpy of formation of CH2
and the bond dissociation energy of CH3 via simultaneoussolution of the local thermochemical network J Phys Chem A1999 103 8625-8633
(32) Halliwell B G utteridge J M C Free Radicals in Biology and
Medicine 3rd ed Oxford University Press Inc New York1999(33) Nicolaou K C Smith A L Yue E W Chemistry and Biology
of Natural and Designed Enediynes Proc Natl Acad Sci USA1993 90 5881-5888
(34) Ruscic B Feller D Dixon D A Peterson K A Harding L BAsher R L Wagner A F Evidence for a lower enthalpy of formation of hydroxyl radical and a lower gas-phase bonddissociation energy of water J Phys Chem A 2001 105 1-4
(35) Davis H F Kim B S J ohnston H S Lee Y T Dissociation-Energy and Photochemistry of NO3 J Phys Chem 1993 97 2172-2180
(36) M ordaunt D H Ashfold M N R Near-UltravioletPhotolysis of C2H2sa Precise Determination of D0(HCC-H) J Chem Phys1994 101 2630-2631
(37) Wenthold P G Squires R R Biradical Thermochemistry fromCollision-Induced Dissociation Threshold Energy MeasurementsAbsolute Heats of Formation of ortho - meta- and para-Benzyne J Am Chem S oc 1994 116 6401-6412
(38) Ellison G B Davico G E Bierbaum V M DePuy C H The Thermochemistry of the Benzyl and A llyl Radicals and Ions Int J Mass Spectrom Ion Processes 1996 156 109-131
(39) Nicovich J M Kreutter K D Vandijk C A Wine P H Temperature-Dependent Kinetics Studies of the Reactions Br(2P32)+ H 2S Reversible SH + HBr and Br(2P32) + CH3SH ReversibleCH3S + HBrsHeats of Formation of SH and CH3S Radicals J Phys Chem 1992 96 2518-2528
(40) Ruscic B Berkowitz J Photoionization Mass-SpectrometricStudies of the Isomeric Transient Species CH2SH and CH3S J Chem Phys 1992 97 1818-1823
(41) DeTuri V F Ervin K M Proton transfer between Cl- andC6H5OH O-H bond energy of phenol Int J Mass Spectrom1998 175 123-132
(42) Blanksby S J Ramond T M Davico G E Nimlos M R KatoS Bierbaum V M Lineberger W C Ellison G B OkumuraM Negative Ion Photoelectron Spectroscopy Gas-Phase Acidityand Thermochemistry of the Peroxyl Radicals CH3OO andCH3CH2OO J Am Chem Soc 2001 123 9585-9596
(43) Niiranen J T Gutman D Krasnoperov L N Kinetics and Thermochemistry of the CH3CO RadicalsStudy of the CH3CO +
(45) Ruscic B Litorja M Photoionization of HOCO revisited a newupper limit to the adiabatic ionization energy and lower limit tothe enthalpy of formation Chem Phys Lett 2000 316 45-50
(46) Linstrom P J Mallard W G NIST Chemistry WebBook NISTStandard Reference Database No 69 National Institute of
Standards and Technology Gaithersburg MD 2001 httpwebbooknistgov
AR020230D
BondDissociationEnergiesofOrganic Molecules Blanksby and Ellison
VOL 36 NO 4 2003 ACCOUNTS OF CHEMICAL RESEARCH 263
892019 2003 - Bond Dissociation Energies of Organic Molecules
C H 3C H 2 R 87 9(0 6) 8 7 1(0 5) 85 6(0 6) 100 0(0 8) 75 4(0 6) 102 2(0 7) 76 7(0 7) R 83 3(0 5) 8 3 5(0 5)
(CH 3)2C H R R 856(05) 827(06) 992(08) 752(07) 1010(07) 764(08) R 831(05) 819(05)
(CH 3)3C R R R 786(07) 978(08) 732(07) 983(08) - R - 794(06)
C H 2C H R R R R 116(1) 873(08) 116(1) - - - 41(3)a
C H 2C H C H 2 R R R R R 627(06) - - - - -
H C C 1 26 5(0 3 ) 1 25 1(0 5 ) 1 24 5(0 6 )a 1223(05)a - - - - - - -
H C C -C H 2 78(3) 77(3)a - - - - - - - - -
C 6H 5 R R R R R - 118(1) R R 993(09) 988(08)
C 6H 5C H 2 R R R - - - 97(1) 65 2(0 9) - - 714(09)
a B ond ent ha lpies of s t a ble orga nic molecules a re t a bula t ed a long w it h t h eir uncert a int ies For exa mple D H 298(C 6H 5-OH ) ) 1124 (
06 kca l mol-1 T hese bond ent ha lpies a re ca lcula t ed from t he ra dica l hea t s of forma t ion from T a ble 1 a nd t he pa rent ∆fH 298 va luest a bula t ed by Pedley et a l 1920 There are a few entries (such as CH 3F or C 6H 5C H 2F) where ∆ fH 298(par ent) is not provided by Pedley et a lso we ha ve a dopt ed t he va lue recommended by t he NI S T Web sit e46 (htt pwebbooknistgov) w e ha ve ma rked t hese pa rent compounds
wit h a n ldquo a rdquo I n some ca ses t he hea t of forma t ion of t he pa rent species is not a va ila ble (eg vinylmet hyl et her CH 2C H -OC H 3) so t h ebond entha lpy cann ot be computed and this is ma rked with a dash There are a n umber of redundant entr ies in this table [eg DH 298(C 6H 5-
C H 3) ) D H 298(C H 3-C 6H 5)] so t he second ent ry is ma rked wit h a n ldquo Rrdquo T he uncert a int ies ha ve been a dded in qua dra t ure
X + CH3OH a HX + CH
2OH (17)
CH3OH + h νthresh f +[CH2OH] + H (18)
∆f H 298(R) ) DH 298(R-H) + ∆f H 298(RH) - ∆f H 298(H) (19)
CH3-CH2CH3 f CH3 + CH2CH3 (20)
BondDissociationEnergiesofOrganic Molecules Blanksby and Ellison
258 ACCOUNTS OF CHEMICAL RESEARCH VOL 36 NO 4 2003
892019 2003 - Bond Dissociation Energies of Organic Molecules
unavailable in the dissociation of the hydroperoxide
(CH3)3C-OOH
Chart 3 shows a comparison of the C-H bond energies
of benzene and the phenyl radical These experimentalvalues are available from the ingenious negative ion
studies of Squires and co-workers2728 and demonstrate the
increasing bond strengths at the ortho lt meta lt para
positions quantifying the increasing stability of the result-
ing benzynes as o - gt m - gt p -benzyne
What Are Bond Strengths A common term which often appears in the literature is
the average bond energy This is defined as the energy
required to break all the bonds in a given molecule at 298
K divided by the number of bonds For example consider
the atomization of methane which is the dissociation of CH4 to carbon and four hydrogens (CH4 f C + 4H) The
enthalpy of atomization is known from the experimental
heats of formation of methane carbon and hydrogen
∆atomH 298(CH4) ) ∆f H 298(C) + 4∆f H 298(H) - ∆f H 298(CH4) )
3975 kcal mol-1 This implies that the methane molecule
contains 3975 kcal mol-1 of energy that is partitioned
among four bonds thus D av H 298 ) 994 kcal mol-1 In
Table 1 we list the separate experimental bond enthalpies
of methane and its constituent radicals CH3 CH2 and
CH DH 298(CH4 f CH3 + H) ) 10499 kcal mol-1
DH 298(CH3 f CH2 + H) ) 1104 kcal mol-1 DH 298(CH2 f
CH + H) ) 1013 kcal mol-1 and DH 298(CH f C + H) )
809 kcal mol-1 Notice that not one of these bond
enthalpies is equal to the average bond energy of methane
Therefore one should be cautious when interpreting thesignificance of the average bond enthalpy of a molecule
It is important to note that if these four independently
measured bond enthalpies are combined they give
∆rxnH 298(CH4 f C + 4H) ) 3975 ( 06 kcal mol-1 as
required by the first law of thermodynamics
One should also be careful about the term bond
strength and the tendency to treat these energetic quanti-
ties as transferable objects between different molecules
Equation 3 shows that the bond dissociation energy is the
energy of a fragmentation reaction rather than any intrin-
sic property of a chemical bond Acetylene is a good
example (Chart 1) One could say that the HCtCH bond
strength is 231 kcal mol
-1
[HCt
CHf
HC+
CH] and thatboth CsH bonds are equivalent with bond strengths of
81 kcal mol-1 [HC f C + H see Table 1] However one
could equally well claim that the first CH bond strength
is 133 kcal mol-1 [HCtCH f HCtC + H] which is
different from the second CH bond strength of 117 kcal
mol-1 [HCtC f CtC + H] while the CC bond strength
[CtC f C + C] is only 142 kcal mol-1 Or alternatively
the first CH bond strength is 133 kcal mol-1 while the
CC bond strength [HCtC f HC + C] is 178 kcal mol-1
and the second CH bond strength [HC f H + C] is only
81 kcal mol-1 Clearly one arrives at different bond
Chart 1 Experimental Bond Enthalpies DH 29 8 for Several ImportantHydrocarbons and Hydrocarbon Radicalsa
a Values are in kcal mol-1 and represent the energy required to break only the single bond indicated Thus DH 298(HCtCHf HC + CH) ) 2307( 02 kcal mol-1 but DH 298(HCCsH f HCC + H) ) 13332 ( 007 kcalmol-1
Chart 2 Experimental Bond Enthalpies DH 29 8 for Several ImportantOxycarbons and Oxycarbon Radicals a
a Values are in kcal mol-1 and represent the energy required to break only the single bond indicated Thus DH 298(CH3OsH f CH3O + H) )
1046 ( 07 kcal mol-1 but DH 298(HsCH2OH f H + CH2OH) ) 961 ( 03kcal mol-1 and DH 298(CH3sOH f CH3 + OH) ) 921 ( 01 kcal mol-1
BondDissociationEnergiesofOrganic Molecules Blanksby and Ellison
260 ACCOUNTS OF CHEMICAL RESEARCH VOL 36 NO 4 2003
892019 2003 - Bond Dissociation Energies of Organic Molecules
strengths by breaking the bonds in different orders
Rigorous quantum mechanical discussions of these trends
are available elsewhere26 along with other provocative
discussions of bond strengths232429
The example of HCCH serves to reiterate that the bond
enthalpy is the enthalpy of a homolysis reaction (eq 3)
and thus depends exclusively on the relative stability of
reactant and product states More generally creation of
new bonds in the products or otherwise stabilized
products always decreases the bond enthalpy As an
example consider ketene in Chart 2 The carbon-carbon
bond enthalpy 3031 of ketene DH 298(CH2dCO) ) 787 ( 02
kcal mol-1 is almost 100 kcal mol-1 less than that of
ethylene DH 298(CH2dCH2) ) 174 kcal mol-1 because one
of the products of the ketene fragmentation is an ex-
tremely stable molecule namely carbon monoxide This
example illustrates that not all double bonds are createdequal2426 and that extrapolations of bond energies from
one molecular species to another must be conducted
carefully
Bond Enthalpies in Solut ionThe bond enthalpies tabulated in this Account are exclu-
sively gas-phase values This raises the question of how
to relate gas-phase bond enthalpies to chemical problems
occurring in solution The difference between a gas-phase
bond enthalpy and that in solution D solnH 298(R-H)
depends on the difference in the enthalpy of solvation
∆solnH 298 of the two radicals and the parent compound
as expressed in terms of eq 23 There are not many
accurate measurements available for the enthalpy of
solvation of radical species and we can only estimate the
effects of solvation on organic BDEs The solvation energy
of a hydrogen atom is likely to be negligible in most
solvents and therefore the correction to an R-H gas-
phase bond enthalpy can be can be approximated as[∆solnH 298(R) - ∆solnH 298(RH)] As both the radical and the
parent are neutral the solvation energies are likely to be
small and similar in most cases particularly in nonpolar
solvents The small effect on bond enthalpy is likely to be
most noticeable is protic polar solvents where hydrogen
bonding may play a key role in preferentially stabilizing
the radical or parent in solution Consider for example
the C-H and O-H bond enthalpies of CH3OH in a polar
protic solvent (reactions 24)
While in reaction 24a both methanol and the hydroxy-
methyl radical can hydrogen bond to the surrounding
solvent such stabilization may be less prevalent for the
methoxyl radical in reaction 24b due to the absence of
the highly polarized O-H bond Therefore one might
surmise that the H-CH2OH bond energy of methanol in
a protic solvent would be largely unchanged from the gas
phase while the CH3O-H value could be slightly higher
in solution There is clearly much work to be done in this
area
Chart 3 Experimental Bond Enthalpies DH 29 8 for Benzene andPhenyl Radicala
a Values are in kcal mol-1 and represent the energy required to break only the single bond indicated Thus DH 298(C6H5sHf C6H5 + H) ) 1129( 05 kcal mol-1 but DH 298(C6H4sH f H + o -C6H4) ) 78 ( 3 kcal mol-1DH 298(C6H4sH f H + m -C6H4) ) 94 ( 3 kcal mol-1 and DH 298(C6H4sHf H + p -C6H4) ) 109 ( 3 kcal mol-1
88 kcal mol-1 gt DH 298(bisallylicC-H) Given these esti-
mates the hydrogens most susceptible to radical abstrac-
tion will be those in the allylic positions While there is
at present no direct data for doubly allylic C-H bond
enthalpies one might conjecture that the bond energy
will be roughly 80 kcal mol-1 given that the dif-
ference between a typical methylenic Cs
H (egDH 298((CH3)2CHsH) = 986 kcal mol-1) and a singly allylic
CsH (eg DH 298(H2CdCHCH2sH) = 888 kcal mol-1) is
about 10 kcal mol-1 Thus the bisallylic hydrogens at
carbons 7 10 and 13 represent the most labile hydrogens
in the molecule Once a carbon-centered radical is pro-
duced and rearranged to its most stable form it will
readily add O2 to form a peroxyl radical Chart 2 indicates
that the OO-CMe3 bond enthalpy is 38 kcal mol-1 Given
that the O-H bond enthalpy of a hydroperoxide is
approximately 85 kcal mol-1 a radical chain reaction will
be exothermic
The biological activity of the enediyne anticancer
antibiotic agents is thought to be due to their ability toform reactive diradicals in situ 33 Molecules such as
calicheamicin γ1I possess an extended sugar residue which
serves to deliver the enediyne moiety the active part of
the molecule to a sequence-specific position on the DNA
double-helix Upon delivery to the target the enediyne
functionality is activated to undergo a Bergman cycloaro-
matization which yields a substituted p -benzyne (Scheme
2) From Chart 3 we can estimate that the diradical can
abstract all hydrogens bound by e109 kcal mol-1 This
makes the p -benzyne a quite powerful hydrogen abstrac-
tion reagent and further once one hydrogen has been
abstracted a substituted phenyl radical results which can
abstract all hydrogens bound by e113 kcal mol-1 Thus
two exothermic hydrogen abstractions by the reactive
p -benzyne moiety can lead to selective cutting of double-
stranded DNA
SummaryThe critically evaluated bond enthalpies listed in Tables
1 and 2 should serve as an important resource for the
organic chemist The values listed may be used to calcu-late rigorous experimental thermochemistry for many
common reactions and further with appropriate care
instructive estimations of reaction thermochemistry can
be made for complex chemical problems
This work was supported by grants from the Chemical Physics
Program United States Department of Energy (DE-FG02-
87ER13695) and the National Science Foundation (CHE-0201848)
We are grateful for the sustained advice and criticism from our
Colorado colleagues Carl Lineberger Veronica Bierbaum Shuji
Kato Mark Nimlos Xu Zhang Bob Damrauer Charles H DePuy
Geoff Tyndall and Veronica Vaida We are also continually
educated by our friends Emily Carter George Petersson Larry
Harding Kent Ervin Richard OrsquoHair and Branko Ruscic Finally GBE would like to thank Joseph Berkowitz now retired for 25
years of friendship and physics
References(1) Benson S W Thermochemical Kinetics 2nd ed Wiley-Inter-
science New Y ork 1976(2) MillsI Cvitas T Homann KKallay NKuchitsuK Quantities
Units and Symbols in Physical Chemistry Blackwell ScientificPublications Oxford 1988 This reference lists the IUPA Crsquosguidelines concerning thermochemical symbols which we adoptInstead of the more common expressions ∆G rxn 298(1) ∆H f 0deg(R)or ∆H f 298deg(RH) the use of ∆rxnG 298(1) ∆f H 0(R) or ∆f H 298(RH) isrecommended See p 46
(3) Herzberg G H M olecular Spectra and Molecular StructureInfrared and Raman S pectra of Polyatomic Molecules D Van
Nostrand Princeton NJ 1945 Vol II see Chapter V(4) Gurvich L V Veyts I V Alcock C B Iorish V S Thermody-
namic Properties of Individual Substances 4th ed HemisphereNew York 1991 Vol 2
(5) Ervin K M Gronert SBarlow S E Gilles M K Harrison AGBierbaumV M Charles H DePuy Lineberger W CEllisonG B Bond Strengths of Ethylene and Acetylene J Am ChemSoc 1990 112 5750-5759
(6) Ervin K M DeTuri V F Anchoring the Gas-Phase AcidityScale Experiment and Theory J Phys Chem A 2002 106 9947-
9956(7) Frisch M J Trucks G WSchlegel H B Scuseria G E Robb
M A Cheeseman J R Zakrzewski V G Montgomery J A J r Stratmann R E Burant J C Dapprich S M illam J MDaniels A D Kudin K N Strain M C Farkas O Tomasi J Barone V Cossi M Cammi R M ennucci B Pomelli C
Scheme 2
BondDissociationEnergiesofOrganic Molecules Blanksby and Ellison
262 ACCOUNTS OF CHEMICAL RESEARCH VOL 36 NO 4 2003
892019 2003 - Bond Dissociation Energies of Organic Molecules
Adamo C Clifford S Ochterski J Petersson G A Ayala P Y Cui Q Morokuma K Malick D K Rabuck A D Raghava-chari K Foresman J B Cioslowski J Ortiz J VStefanov BB LiuG Liashenko A Piskorz PKomaromiI Gomperts RMartin R L Fox D J Keith T Al-Laham M A Peng C YNanayakkara A Gonzalez C Challacombe M Gill P M W J ohnson B G Chen W Wong M W Andres J L Head-Gordon M Replogle E S Pople J A Gaussian 98 GaussianInc Pittsburgh PA 1998
(8) Petersson G A In Computational Thermochemistry Irikura KK Frurip D J Eds ACS Symposium Series 677 AmericanChemical Society Washington DC 1998 pp 237-266
(9) Berkowitz J Ellison G B Gutman D Three Methods toMeasure RH Bond Energies J Phys Chem1994 98 2744-2765(10) Seakins P W Pilling M J Niiranen J T Gutman D
Krasnoperov L N Kinetics and Thermochemistry of R + HBrT RH + Br ReactionssDeterminations of the Heat of Formation of C2H5 i -C3H7 sec -C4H9 and tert -C4H9 J Phys Chem 1992 96 9847-9855
(11) Ruscic B Berkowitz J Curtiss L A Pople J A The EthylRadicalsPhotoionization and Theoretical Studies J Chem Phys1989 91 114-121
(12) Ervin K M Experimental techniques in gas-phase ion thermo-chemistry Chem Rev 2001 101 391-444
(13) Rienstra-Kiracofe J C Tschumper G S Schaefer H F IIINandi S Ellison G B Atomic and Molecular Electron Affini-ties Photoelectron Experiments and Theoretical ComputationsChem Rev 2002 102 231-282
(14) Ramond T M Blanksby S J Kato SBierbaum V M DavicoG E Schwartz R L Lineberger W C Ellison G B The Heatof Formation of the Hydroperoxyl Radical HOO via Negative Ion
Studies J Phys Chem A 2002 106 9641-9647(15) Seetula J AGutman DKineticsof the CH2OH + HBr and CH2OH
+ HI Reactions and Determination of the Heat of Formation of CH2OH J Phys Chem 1992 96 5401-5405
(16) Ruscic B Berkowitz J Heat of Formation of CH2OH andD0(H-CH2OH) J Phys Chem 1993 97 11451-11455
(17) Lias S G Bartmess J E Liebman J F Holmes J L LevinR D MallardW G Gas Phase Ion and Neutral Thermochemistry J Phys Chem Ref Data1988 17 (Suppl 1) 1
(18) Ramond T M Davico G E Schwartz R L Lineberger W CVibronic structure of alkoxy radicals via photoelectron spectros-copy J Chem Phys 2000 112 1158-1169
(19) Pedley J B Naylor R D Kirby S P Thermochemistry of Organic Compounds 2nd ed Chapman and Hall New Y ork1986
(20) Pedley J B Thermochemical Data and Structures of Organic Compounds Thermodynamics Research Center College Station TX 1994
(21) Thehuge value of DH 298(H2O) is the reason that the OH radical issuch an important species in atmospheric chemistry Hydroxylradicals result from solar photodissociation of O3 and they reactwith all organic species pumped into the atmosphere
(22) Ingold K U Wright J S U nderstanding trends in C-H N -Hand O-H bond dissociation enthalpies J Chem Educ 2000 77 1062-1064
(23) Goddard W A III Harding L B The Description of ChemicalBonding from Ab Initio Calculations Annu Rev Phys Chem1978 29 363-396
(24) Carter E A Goddard W A Relation between S inglet TripletGaps and Bond-Energies J Phys Chem 1986 90 998-1001
(25) Dunning T H J r A Road Map for the Calculation of M olecularBinding Energies J Phys Chem A 2000 104 9062-9080
(26) WuC J Carter E A Ab Initio Thermochemistry for UnsaturatedC2 Hydrocarbons J Phys Chem 1991 95 8352-8363
(27) Wenthold P G Squires R R Gas-phase acidities of o- m- andp-dehydrobenzoic acid radicals Determination of the substituentconstants for a phenyl radical site Int J Mass Spectrom 1998
175 215-224(28) Wenthold P G S quires R R Lineberger W C Ultraviolet
photoelectron spectroscopy of theo - m - and p -benzynenegativeions Electron affinities and singlet-triplet splittings for o - m -and p -benzyne J Am Chem Soc 1998 120 5279-5290
(29) Chen P In Unimolecular and Bimolecular Reaction Dynamics NgC YBaer T PowisIEds J ohnWiley amp Sons New York1994 Vol 3 pp 372-425
(30) Oakes J M J ones M E Bierbaum V M Ellison G BPhotoelectron Spectroscopy of CCO- and HCCO- J Phys Chem1983 87 4810-4815
(31) Ruscic B Litorja M Asher R L Ionization energy of methylenerevisited Improved values for the enthalpy of formation of CH2
and the bond dissociation energy of CH3 via simultaneoussolution of the local thermochemical network J Phys Chem A1999 103 8625-8633
(32) Halliwell B G utteridge J M C Free Radicals in Biology and
Medicine 3rd ed Oxford University Press Inc New York1999(33) Nicolaou K C Smith A L Yue E W Chemistry and Biology
of Natural and Designed Enediynes Proc Natl Acad Sci USA1993 90 5881-5888
(34) Ruscic B Feller D Dixon D A Peterson K A Harding L BAsher R L Wagner A F Evidence for a lower enthalpy of formation of hydroxyl radical and a lower gas-phase bonddissociation energy of water J Phys Chem A 2001 105 1-4
(35) Davis H F Kim B S J ohnston H S Lee Y T Dissociation-Energy and Photochemistry of NO3 J Phys Chem 1993 97 2172-2180
(36) M ordaunt D H Ashfold M N R Near-UltravioletPhotolysis of C2H2sa Precise Determination of D0(HCC-H) J Chem Phys1994 101 2630-2631
(37) Wenthold P G Squires R R Biradical Thermochemistry fromCollision-Induced Dissociation Threshold Energy MeasurementsAbsolute Heats of Formation of ortho - meta- and para-Benzyne J Am Chem S oc 1994 116 6401-6412
(38) Ellison G B Davico G E Bierbaum V M DePuy C H The Thermochemistry of the Benzyl and A llyl Radicals and Ions Int J Mass Spectrom Ion Processes 1996 156 109-131
(39) Nicovich J M Kreutter K D Vandijk C A Wine P H Temperature-Dependent Kinetics Studies of the Reactions Br(2P32)+ H 2S Reversible SH + HBr and Br(2P32) + CH3SH ReversibleCH3S + HBrsHeats of Formation of SH and CH3S Radicals J Phys Chem 1992 96 2518-2528
(40) Ruscic B Berkowitz J Photoionization Mass-SpectrometricStudies of the Isomeric Transient Species CH2SH and CH3S J Chem Phys 1992 97 1818-1823
(41) DeTuri V F Ervin K M Proton transfer between Cl- andC6H5OH O-H bond energy of phenol Int J Mass Spectrom1998 175 123-132
(42) Blanksby S J Ramond T M Davico G E Nimlos M R KatoS Bierbaum V M Lineberger W C Ellison G B OkumuraM Negative Ion Photoelectron Spectroscopy Gas-Phase Acidityand Thermochemistry of the Peroxyl Radicals CH3OO andCH3CH2OO J Am Chem Soc 2001 123 9585-9596
(43) Niiranen J T Gutman D Krasnoperov L N Kinetics and Thermochemistry of the CH3CO RadicalsStudy of the CH3CO +
(45) Ruscic B Litorja M Photoionization of HOCO revisited a newupper limit to the adiabatic ionization energy and lower limit tothe enthalpy of formation Chem Phys Lett 2000 316 45-50
(46) Linstrom P J Mallard W G NIST Chemistry WebBook NISTStandard Reference Database No 69 National Institute of
Standards and Technology Gaithersburg MD 2001 httpwebbooknistgov
AR020230D
BondDissociationEnergiesofOrganic Molecules Blanksby and Ellison
VOL 36 NO 4 2003 ACCOUNTS OF CHEMICAL RESEARCH 263
892019 2003 - Bond Dissociation Energies of Organic Molecules
unavailable in the dissociation of the hydroperoxide
(CH3)3C-OOH
Chart 3 shows a comparison of the C-H bond energies
of benzene and the phenyl radical These experimentalvalues are available from the ingenious negative ion
studies of Squires and co-workers2728 and demonstrate the
increasing bond strengths at the ortho lt meta lt para
positions quantifying the increasing stability of the result-
ing benzynes as o - gt m - gt p -benzyne
What Are Bond Strengths A common term which often appears in the literature is
the average bond energy This is defined as the energy
required to break all the bonds in a given molecule at 298
K divided by the number of bonds For example consider
the atomization of methane which is the dissociation of CH4 to carbon and four hydrogens (CH4 f C + 4H) The
enthalpy of atomization is known from the experimental
heats of formation of methane carbon and hydrogen
∆atomH 298(CH4) ) ∆f H 298(C) + 4∆f H 298(H) - ∆f H 298(CH4) )
3975 kcal mol-1 This implies that the methane molecule
contains 3975 kcal mol-1 of energy that is partitioned
among four bonds thus D av H 298 ) 994 kcal mol-1 In
Table 1 we list the separate experimental bond enthalpies
of methane and its constituent radicals CH3 CH2 and
CH DH 298(CH4 f CH3 + H) ) 10499 kcal mol-1
DH 298(CH3 f CH2 + H) ) 1104 kcal mol-1 DH 298(CH2 f
CH + H) ) 1013 kcal mol-1 and DH 298(CH f C + H) )
809 kcal mol-1 Notice that not one of these bond
enthalpies is equal to the average bond energy of methane
Therefore one should be cautious when interpreting thesignificance of the average bond enthalpy of a molecule
It is important to note that if these four independently
measured bond enthalpies are combined they give
∆rxnH 298(CH4 f C + 4H) ) 3975 ( 06 kcal mol-1 as
required by the first law of thermodynamics
One should also be careful about the term bond
strength and the tendency to treat these energetic quanti-
ties as transferable objects between different molecules
Equation 3 shows that the bond dissociation energy is the
energy of a fragmentation reaction rather than any intrin-
sic property of a chemical bond Acetylene is a good
example (Chart 1) One could say that the HCtCH bond
strength is 231 kcal mol
-1
[HCt
CHf
HC+
CH] and thatboth CsH bonds are equivalent with bond strengths of
81 kcal mol-1 [HC f C + H see Table 1] However one
could equally well claim that the first CH bond strength
is 133 kcal mol-1 [HCtCH f HCtC + H] which is
different from the second CH bond strength of 117 kcal
mol-1 [HCtC f CtC + H] while the CC bond strength
[CtC f C + C] is only 142 kcal mol-1 Or alternatively
the first CH bond strength is 133 kcal mol-1 while the
CC bond strength [HCtC f HC + C] is 178 kcal mol-1
and the second CH bond strength [HC f H + C] is only
81 kcal mol-1 Clearly one arrives at different bond
Chart 1 Experimental Bond Enthalpies DH 29 8 for Several ImportantHydrocarbons and Hydrocarbon Radicalsa
a Values are in kcal mol-1 and represent the energy required to break only the single bond indicated Thus DH 298(HCtCHf HC + CH) ) 2307( 02 kcal mol-1 but DH 298(HCCsH f HCC + H) ) 13332 ( 007 kcalmol-1
Chart 2 Experimental Bond Enthalpies DH 29 8 for Several ImportantOxycarbons and Oxycarbon Radicals a
a Values are in kcal mol-1 and represent the energy required to break only the single bond indicated Thus DH 298(CH3OsH f CH3O + H) )
1046 ( 07 kcal mol-1 but DH 298(HsCH2OH f H + CH2OH) ) 961 ( 03kcal mol-1 and DH 298(CH3sOH f CH3 + OH) ) 921 ( 01 kcal mol-1
BondDissociationEnergiesofOrganic Molecules Blanksby and Ellison
260 ACCOUNTS OF CHEMICAL RESEARCH VOL 36 NO 4 2003
892019 2003 - Bond Dissociation Energies of Organic Molecules
strengths by breaking the bonds in different orders
Rigorous quantum mechanical discussions of these trends
are available elsewhere26 along with other provocative
discussions of bond strengths232429
The example of HCCH serves to reiterate that the bond
enthalpy is the enthalpy of a homolysis reaction (eq 3)
and thus depends exclusively on the relative stability of
reactant and product states More generally creation of
new bonds in the products or otherwise stabilized
products always decreases the bond enthalpy As an
example consider ketene in Chart 2 The carbon-carbon
bond enthalpy 3031 of ketene DH 298(CH2dCO) ) 787 ( 02
kcal mol-1 is almost 100 kcal mol-1 less than that of
ethylene DH 298(CH2dCH2) ) 174 kcal mol-1 because one
of the products of the ketene fragmentation is an ex-
tremely stable molecule namely carbon monoxide This
example illustrates that not all double bonds are createdequal2426 and that extrapolations of bond energies from
one molecular species to another must be conducted
carefully
Bond Enthalpies in Solut ionThe bond enthalpies tabulated in this Account are exclu-
sively gas-phase values This raises the question of how
to relate gas-phase bond enthalpies to chemical problems
occurring in solution The difference between a gas-phase
bond enthalpy and that in solution D solnH 298(R-H)
depends on the difference in the enthalpy of solvation
∆solnH 298 of the two radicals and the parent compound
as expressed in terms of eq 23 There are not many
accurate measurements available for the enthalpy of
solvation of radical species and we can only estimate the
effects of solvation on organic BDEs The solvation energy
of a hydrogen atom is likely to be negligible in most
solvents and therefore the correction to an R-H gas-
phase bond enthalpy can be can be approximated as[∆solnH 298(R) - ∆solnH 298(RH)] As both the radical and the
parent are neutral the solvation energies are likely to be
small and similar in most cases particularly in nonpolar
solvents The small effect on bond enthalpy is likely to be
most noticeable is protic polar solvents where hydrogen
bonding may play a key role in preferentially stabilizing
the radical or parent in solution Consider for example
the C-H and O-H bond enthalpies of CH3OH in a polar
protic solvent (reactions 24)
While in reaction 24a both methanol and the hydroxy-
methyl radical can hydrogen bond to the surrounding
solvent such stabilization may be less prevalent for the
methoxyl radical in reaction 24b due to the absence of
the highly polarized O-H bond Therefore one might
surmise that the H-CH2OH bond energy of methanol in
a protic solvent would be largely unchanged from the gas
phase while the CH3O-H value could be slightly higher
in solution There is clearly much work to be done in this
area
Chart 3 Experimental Bond Enthalpies DH 29 8 for Benzene andPhenyl Radicala
a Values are in kcal mol-1 and represent the energy required to break only the single bond indicated Thus DH 298(C6H5sHf C6H5 + H) ) 1129( 05 kcal mol-1 but DH 298(C6H4sH f H + o -C6H4) ) 78 ( 3 kcal mol-1DH 298(C6H4sH f H + m -C6H4) ) 94 ( 3 kcal mol-1 and DH 298(C6H4sHf H + p -C6H4) ) 109 ( 3 kcal mol-1
88 kcal mol-1 gt DH 298(bisallylicC-H) Given these esti-
mates the hydrogens most susceptible to radical abstrac-
tion will be those in the allylic positions While there is
at present no direct data for doubly allylic C-H bond
enthalpies one might conjecture that the bond energy
will be roughly 80 kcal mol-1 given that the dif-
ference between a typical methylenic Cs
H (egDH 298((CH3)2CHsH) = 986 kcal mol-1) and a singly allylic
CsH (eg DH 298(H2CdCHCH2sH) = 888 kcal mol-1) is
about 10 kcal mol-1 Thus the bisallylic hydrogens at
carbons 7 10 and 13 represent the most labile hydrogens
in the molecule Once a carbon-centered radical is pro-
duced and rearranged to its most stable form it will
readily add O2 to form a peroxyl radical Chart 2 indicates
that the OO-CMe3 bond enthalpy is 38 kcal mol-1 Given
that the O-H bond enthalpy of a hydroperoxide is
approximately 85 kcal mol-1 a radical chain reaction will
be exothermic
The biological activity of the enediyne anticancer
antibiotic agents is thought to be due to their ability toform reactive diradicals in situ 33 Molecules such as
calicheamicin γ1I possess an extended sugar residue which
serves to deliver the enediyne moiety the active part of
the molecule to a sequence-specific position on the DNA
double-helix Upon delivery to the target the enediyne
functionality is activated to undergo a Bergman cycloaro-
matization which yields a substituted p -benzyne (Scheme
2) From Chart 3 we can estimate that the diradical can
abstract all hydrogens bound by e109 kcal mol-1 This
makes the p -benzyne a quite powerful hydrogen abstrac-
tion reagent and further once one hydrogen has been
abstracted a substituted phenyl radical results which can
abstract all hydrogens bound by e113 kcal mol-1 Thus
two exothermic hydrogen abstractions by the reactive
p -benzyne moiety can lead to selective cutting of double-
stranded DNA
SummaryThe critically evaluated bond enthalpies listed in Tables
1 and 2 should serve as an important resource for the
organic chemist The values listed may be used to calcu-late rigorous experimental thermochemistry for many
common reactions and further with appropriate care
instructive estimations of reaction thermochemistry can
be made for complex chemical problems
This work was supported by grants from the Chemical Physics
Program United States Department of Energy (DE-FG02-
87ER13695) and the National Science Foundation (CHE-0201848)
We are grateful for the sustained advice and criticism from our
Colorado colleagues Carl Lineberger Veronica Bierbaum Shuji
Kato Mark Nimlos Xu Zhang Bob Damrauer Charles H DePuy
Geoff Tyndall and Veronica Vaida We are also continually
educated by our friends Emily Carter George Petersson Larry
Harding Kent Ervin Richard OrsquoHair and Branko Ruscic Finally GBE would like to thank Joseph Berkowitz now retired for 25
years of friendship and physics
References(1) Benson S W Thermochemical Kinetics 2nd ed Wiley-Inter-
science New Y ork 1976(2) MillsI Cvitas T Homann KKallay NKuchitsuK Quantities
Units and Symbols in Physical Chemistry Blackwell ScientificPublications Oxford 1988 This reference lists the IUPA Crsquosguidelines concerning thermochemical symbols which we adoptInstead of the more common expressions ∆G rxn 298(1) ∆H f 0deg(R)or ∆H f 298deg(RH) the use of ∆rxnG 298(1) ∆f H 0(R) or ∆f H 298(RH) isrecommended See p 46
(3) Herzberg G H M olecular Spectra and Molecular StructureInfrared and Raman S pectra of Polyatomic Molecules D Van
Nostrand Princeton NJ 1945 Vol II see Chapter V(4) Gurvich L V Veyts I V Alcock C B Iorish V S Thermody-
namic Properties of Individual Substances 4th ed HemisphereNew York 1991 Vol 2
(5) Ervin K M Gronert SBarlow S E Gilles M K Harrison AGBierbaumV M Charles H DePuy Lineberger W CEllisonG B Bond Strengths of Ethylene and Acetylene J Am ChemSoc 1990 112 5750-5759
(6) Ervin K M DeTuri V F Anchoring the Gas-Phase AcidityScale Experiment and Theory J Phys Chem A 2002 106 9947-
9956(7) Frisch M J Trucks G WSchlegel H B Scuseria G E Robb
M A Cheeseman J R Zakrzewski V G Montgomery J A J r Stratmann R E Burant J C Dapprich S M illam J MDaniels A D Kudin K N Strain M C Farkas O Tomasi J Barone V Cossi M Cammi R M ennucci B Pomelli C
Scheme 2
BondDissociationEnergiesofOrganic Molecules Blanksby and Ellison
262 ACCOUNTS OF CHEMICAL RESEARCH VOL 36 NO 4 2003
892019 2003 - Bond Dissociation Energies of Organic Molecules
Adamo C Clifford S Ochterski J Petersson G A Ayala P Y Cui Q Morokuma K Malick D K Rabuck A D Raghava-chari K Foresman J B Cioslowski J Ortiz J VStefanov BB LiuG Liashenko A Piskorz PKomaromiI Gomperts RMartin R L Fox D J Keith T Al-Laham M A Peng C YNanayakkara A Gonzalez C Challacombe M Gill P M W J ohnson B G Chen W Wong M W Andres J L Head-Gordon M Replogle E S Pople J A Gaussian 98 GaussianInc Pittsburgh PA 1998
(8) Petersson G A In Computational Thermochemistry Irikura KK Frurip D J Eds ACS Symposium Series 677 AmericanChemical Society Washington DC 1998 pp 237-266
(9) Berkowitz J Ellison G B Gutman D Three Methods toMeasure RH Bond Energies J Phys Chem1994 98 2744-2765(10) Seakins P W Pilling M J Niiranen J T Gutman D
Krasnoperov L N Kinetics and Thermochemistry of R + HBrT RH + Br ReactionssDeterminations of the Heat of Formation of C2H5 i -C3H7 sec -C4H9 and tert -C4H9 J Phys Chem 1992 96 9847-9855
(11) Ruscic B Berkowitz J Curtiss L A Pople J A The EthylRadicalsPhotoionization and Theoretical Studies J Chem Phys1989 91 114-121
(12) Ervin K M Experimental techniques in gas-phase ion thermo-chemistry Chem Rev 2001 101 391-444
(13) Rienstra-Kiracofe J C Tschumper G S Schaefer H F IIINandi S Ellison G B Atomic and Molecular Electron Affini-ties Photoelectron Experiments and Theoretical ComputationsChem Rev 2002 102 231-282
(14) Ramond T M Blanksby S J Kato SBierbaum V M DavicoG E Schwartz R L Lineberger W C Ellison G B The Heatof Formation of the Hydroperoxyl Radical HOO via Negative Ion
Studies J Phys Chem A 2002 106 9641-9647(15) Seetula J AGutman DKineticsof the CH2OH + HBr and CH2OH
+ HI Reactions and Determination of the Heat of Formation of CH2OH J Phys Chem 1992 96 5401-5405
(16) Ruscic B Berkowitz J Heat of Formation of CH2OH andD0(H-CH2OH) J Phys Chem 1993 97 11451-11455
(17) Lias S G Bartmess J E Liebman J F Holmes J L LevinR D MallardW G Gas Phase Ion and Neutral Thermochemistry J Phys Chem Ref Data1988 17 (Suppl 1) 1
(18) Ramond T M Davico G E Schwartz R L Lineberger W CVibronic structure of alkoxy radicals via photoelectron spectros-copy J Chem Phys 2000 112 1158-1169
(19) Pedley J B Naylor R D Kirby S P Thermochemistry of Organic Compounds 2nd ed Chapman and Hall New Y ork1986
(20) Pedley J B Thermochemical Data and Structures of Organic Compounds Thermodynamics Research Center College Station TX 1994
(21) Thehuge value of DH 298(H2O) is the reason that the OH radical issuch an important species in atmospheric chemistry Hydroxylradicals result from solar photodissociation of O3 and they reactwith all organic species pumped into the atmosphere
(22) Ingold K U Wright J S U nderstanding trends in C-H N -Hand O-H bond dissociation enthalpies J Chem Educ 2000 77 1062-1064
(23) Goddard W A III Harding L B The Description of ChemicalBonding from Ab Initio Calculations Annu Rev Phys Chem1978 29 363-396
(24) Carter E A Goddard W A Relation between S inglet TripletGaps and Bond-Energies J Phys Chem 1986 90 998-1001
(25) Dunning T H J r A Road Map for the Calculation of M olecularBinding Energies J Phys Chem A 2000 104 9062-9080
(26) WuC J Carter E A Ab Initio Thermochemistry for UnsaturatedC2 Hydrocarbons J Phys Chem 1991 95 8352-8363
(27) Wenthold P G Squires R R Gas-phase acidities of o- m- andp-dehydrobenzoic acid radicals Determination of the substituentconstants for a phenyl radical site Int J Mass Spectrom 1998
175 215-224(28) Wenthold P G S quires R R Lineberger W C Ultraviolet
photoelectron spectroscopy of theo - m - and p -benzynenegativeions Electron affinities and singlet-triplet splittings for o - m -and p -benzyne J Am Chem Soc 1998 120 5279-5290
(29) Chen P In Unimolecular and Bimolecular Reaction Dynamics NgC YBaer T PowisIEds J ohnWiley amp Sons New York1994 Vol 3 pp 372-425
(30) Oakes J M J ones M E Bierbaum V M Ellison G BPhotoelectron Spectroscopy of CCO- and HCCO- J Phys Chem1983 87 4810-4815
(31) Ruscic B Litorja M Asher R L Ionization energy of methylenerevisited Improved values for the enthalpy of formation of CH2
and the bond dissociation energy of CH3 via simultaneoussolution of the local thermochemical network J Phys Chem A1999 103 8625-8633
(32) Halliwell B G utteridge J M C Free Radicals in Biology and
Medicine 3rd ed Oxford University Press Inc New York1999(33) Nicolaou K C Smith A L Yue E W Chemistry and Biology
of Natural and Designed Enediynes Proc Natl Acad Sci USA1993 90 5881-5888
(34) Ruscic B Feller D Dixon D A Peterson K A Harding L BAsher R L Wagner A F Evidence for a lower enthalpy of formation of hydroxyl radical and a lower gas-phase bonddissociation energy of water J Phys Chem A 2001 105 1-4
(35) Davis H F Kim B S J ohnston H S Lee Y T Dissociation-Energy and Photochemistry of NO3 J Phys Chem 1993 97 2172-2180
(36) M ordaunt D H Ashfold M N R Near-UltravioletPhotolysis of C2H2sa Precise Determination of D0(HCC-H) J Chem Phys1994 101 2630-2631
(37) Wenthold P G Squires R R Biradical Thermochemistry fromCollision-Induced Dissociation Threshold Energy MeasurementsAbsolute Heats of Formation of ortho - meta- and para-Benzyne J Am Chem S oc 1994 116 6401-6412
(38) Ellison G B Davico G E Bierbaum V M DePuy C H The Thermochemistry of the Benzyl and A llyl Radicals and Ions Int J Mass Spectrom Ion Processes 1996 156 109-131
(39) Nicovich J M Kreutter K D Vandijk C A Wine P H Temperature-Dependent Kinetics Studies of the Reactions Br(2P32)+ H 2S Reversible SH + HBr and Br(2P32) + CH3SH ReversibleCH3S + HBrsHeats of Formation of SH and CH3S Radicals J Phys Chem 1992 96 2518-2528
(40) Ruscic B Berkowitz J Photoionization Mass-SpectrometricStudies of the Isomeric Transient Species CH2SH and CH3S J Chem Phys 1992 97 1818-1823
(41) DeTuri V F Ervin K M Proton transfer between Cl- andC6H5OH O-H bond energy of phenol Int J Mass Spectrom1998 175 123-132
(42) Blanksby S J Ramond T M Davico G E Nimlos M R KatoS Bierbaum V M Lineberger W C Ellison G B OkumuraM Negative Ion Photoelectron Spectroscopy Gas-Phase Acidityand Thermochemistry of the Peroxyl Radicals CH3OO andCH3CH2OO J Am Chem Soc 2001 123 9585-9596
(43) Niiranen J T Gutman D Krasnoperov L N Kinetics and Thermochemistry of the CH3CO RadicalsStudy of the CH3CO +
(45) Ruscic B Litorja M Photoionization of HOCO revisited a newupper limit to the adiabatic ionization energy and lower limit tothe enthalpy of formation Chem Phys Lett 2000 316 45-50
(46) Linstrom P J Mallard W G NIST Chemistry WebBook NISTStandard Reference Database No 69 National Institute of
Standards and Technology Gaithersburg MD 2001 httpwebbooknistgov
AR020230D
BondDissociationEnergiesofOrganic Molecules Blanksby and Ellison
VOL 36 NO 4 2003 ACCOUNTS OF CHEMICAL RESEARCH 263
892019 2003 - Bond Dissociation Energies of Organic Molecules
unavailable in the dissociation of the hydroperoxide
(CH3)3C-OOH
Chart 3 shows a comparison of the C-H bond energies
of benzene and the phenyl radical These experimentalvalues are available from the ingenious negative ion
studies of Squires and co-workers2728 and demonstrate the
increasing bond strengths at the ortho lt meta lt para
positions quantifying the increasing stability of the result-
ing benzynes as o - gt m - gt p -benzyne
What Are Bond Strengths A common term which often appears in the literature is
the average bond energy This is defined as the energy
required to break all the bonds in a given molecule at 298
K divided by the number of bonds For example consider
the atomization of methane which is the dissociation of CH4 to carbon and four hydrogens (CH4 f C + 4H) The
enthalpy of atomization is known from the experimental
heats of formation of methane carbon and hydrogen
∆atomH 298(CH4) ) ∆f H 298(C) + 4∆f H 298(H) - ∆f H 298(CH4) )
3975 kcal mol-1 This implies that the methane molecule
contains 3975 kcal mol-1 of energy that is partitioned
among four bonds thus D av H 298 ) 994 kcal mol-1 In
Table 1 we list the separate experimental bond enthalpies
of methane and its constituent radicals CH3 CH2 and
CH DH 298(CH4 f CH3 + H) ) 10499 kcal mol-1
DH 298(CH3 f CH2 + H) ) 1104 kcal mol-1 DH 298(CH2 f
CH + H) ) 1013 kcal mol-1 and DH 298(CH f C + H) )
809 kcal mol-1 Notice that not one of these bond
enthalpies is equal to the average bond energy of methane
Therefore one should be cautious when interpreting thesignificance of the average bond enthalpy of a molecule
It is important to note that if these four independently
measured bond enthalpies are combined they give
∆rxnH 298(CH4 f C + 4H) ) 3975 ( 06 kcal mol-1 as
required by the first law of thermodynamics
One should also be careful about the term bond
strength and the tendency to treat these energetic quanti-
ties as transferable objects between different molecules
Equation 3 shows that the bond dissociation energy is the
energy of a fragmentation reaction rather than any intrin-
sic property of a chemical bond Acetylene is a good
example (Chart 1) One could say that the HCtCH bond
strength is 231 kcal mol
-1
[HCt
CHf
HC+
CH] and thatboth CsH bonds are equivalent with bond strengths of
81 kcal mol-1 [HC f C + H see Table 1] However one
could equally well claim that the first CH bond strength
is 133 kcal mol-1 [HCtCH f HCtC + H] which is
different from the second CH bond strength of 117 kcal
mol-1 [HCtC f CtC + H] while the CC bond strength
[CtC f C + C] is only 142 kcal mol-1 Or alternatively
the first CH bond strength is 133 kcal mol-1 while the
CC bond strength [HCtC f HC + C] is 178 kcal mol-1
and the second CH bond strength [HC f H + C] is only
81 kcal mol-1 Clearly one arrives at different bond
Chart 1 Experimental Bond Enthalpies DH 29 8 for Several ImportantHydrocarbons and Hydrocarbon Radicalsa
a Values are in kcal mol-1 and represent the energy required to break only the single bond indicated Thus DH 298(HCtCHf HC + CH) ) 2307( 02 kcal mol-1 but DH 298(HCCsH f HCC + H) ) 13332 ( 007 kcalmol-1
Chart 2 Experimental Bond Enthalpies DH 29 8 for Several ImportantOxycarbons and Oxycarbon Radicals a
a Values are in kcal mol-1 and represent the energy required to break only the single bond indicated Thus DH 298(CH3OsH f CH3O + H) )
1046 ( 07 kcal mol-1 but DH 298(HsCH2OH f H + CH2OH) ) 961 ( 03kcal mol-1 and DH 298(CH3sOH f CH3 + OH) ) 921 ( 01 kcal mol-1
BondDissociationEnergiesofOrganic Molecules Blanksby and Ellison
260 ACCOUNTS OF CHEMICAL RESEARCH VOL 36 NO 4 2003
892019 2003 - Bond Dissociation Energies of Organic Molecules
strengths by breaking the bonds in different orders
Rigorous quantum mechanical discussions of these trends
are available elsewhere26 along with other provocative
discussions of bond strengths232429
The example of HCCH serves to reiterate that the bond
enthalpy is the enthalpy of a homolysis reaction (eq 3)
and thus depends exclusively on the relative stability of
reactant and product states More generally creation of
new bonds in the products or otherwise stabilized
products always decreases the bond enthalpy As an
example consider ketene in Chart 2 The carbon-carbon
bond enthalpy 3031 of ketene DH 298(CH2dCO) ) 787 ( 02
kcal mol-1 is almost 100 kcal mol-1 less than that of
ethylene DH 298(CH2dCH2) ) 174 kcal mol-1 because one
of the products of the ketene fragmentation is an ex-
tremely stable molecule namely carbon monoxide This
example illustrates that not all double bonds are createdequal2426 and that extrapolations of bond energies from
one molecular species to another must be conducted
carefully
Bond Enthalpies in Solut ionThe bond enthalpies tabulated in this Account are exclu-
sively gas-phase values This raises the question of how
to relate gas-phase bond enthalpies to chemical problems
occurring in solution The difference between a gas-phase
bond enthalpy and that in solution D solnH 298(R-H)
depends on the difference in the enthalpy of solvation
∆solnH 298 of the two radicals and the parent compound
as expressed in terms of eq 23 There are not many
accurate measurements available for the enthalpy of
solvation of radical species and we can only estimate the
effects of solvation on organic BDEs The solvation energy
of a hydrogen atom is likely to be negligible in most
solvents and therefore the correction to an R-H gas-
phase bond enthalpy can be can be approximated as[∆solnH 298(R) - ∆solnH 298(RH)] As both the radical and the
parent are neutral the solvation energies are likely to be
small and similar in most cases particularly in nonpolar
solvents The small effect on bond enthalpy is likely to be
most noticeable is protic polar solvents where hydrogen
bonding may play a key role in preferentially stabilizing
the radical or parent in solution Consider for example
the C-H and O-H bond enthalpies of CH3OH in a polar
protic solvent (reactions 24)
While in reaction 24a both methanol and the hydroxy-
methyl radical can hydrogen bond to the surrounding
solvent such stabilization may be less prevalent for the
methoxyl radical in reaction 24b due to the absence of
the highly polarized O-H bond Therefore one might
surmise that the H-CH2OH bond energy of methanol in
a protic solvent would be largely unchanged from the gas
phase while the CH3O-H value could be slightly higher
in solution There is clearly much work to be done in this
area
Chart 3 Experimental Bond Enthalpies DH 29 8 for Benzene andPhenyl Radicala
a Values are in kcal mol-1 and represent the energy required to break only the single bond indicated Thus DH 298(C6H5sHf C6H5 + H) ) 1129( 05 kcal mol-1 but DH 298(C6H4sH f H + o -C6H4) ) 78 ( 3 kcal mol-1DH 298(C6H4sH f H + m -C6H4) ) 94 ( 3 kcal mol-1 and DH 298(C6H4sHf H + p -C6H4) ) 109 ( 3 kcal mol-1
88 kcal mol-1 gt DH 298(bisallylicC-H) Given these esti-
mates the hydrogens most susceptible to radical abstrac-
tion will be those in the allylic positions While there is
at present no direct data for doubly allylic C-H bond
enthalpies one might conjecture that the bond energy
will be roughly 80 kcal mol-1 given that the dif-
ference between a typical methylenic Cs
H (egDH 298((CH3)2CHsH) = 986 kcal mol-1) and a singly allylic
CsH (eg DH 298(H2CdCHCH2sH) = 888 kcal mol-1) is
about 10 kcal mol-1 Thus the bisallylic hydrogens at
carbons 7 10 and 13 represent the most labile hydrogens
in the molecule Once a carbon-centered radical is pro-
duced and rearranged to its most stable form it will
readily add O2 to form a peroxyl radical Chart 2 indicates
that the OO-CMe3 bond enthalpy is 38 kcal mol-1 Given
that the O-H bond enthalpy of a hydroperoxide is
approximately 85 kcal mol-1 a radical chain reaction will
be exothermic
The biological activity of the enediyne anticancer
antibiotic agents is thought to be due to their ability toform reactive diradicals in situ 33 Molecules such as
calicheamicin γ1I possess an extended sugar residue which
serves to deliver the enediyne moiety the active part of
the molecule to a sequence-specific position on the DNA
double-helix Upon delivery to the target the enediyne
functionality is activated to undergo a Bergman cycloaro-
matization which yields a substituted p -benzyne (Scheme
2) From Chart 3 we can estimate that the diradical can
abstract all hydrogens bound by e109 kcal mol-1 This
makes the p -benzyne a quite powerful hydrogen abstrac-
tion reagent and further once one hydrogen has been
abstracted a substituted phenyl radical results which can
abstract all hydrogens bound by e113 kcal mol-1 Thus
two exothermic hydrogen abstractions by the reactive
p -benzyne moiety can lead to selective cutting of double-
stranded DNA
SummaryThe critically evaluated bond enthalpies listed in Tables
1 and 2 should serve as an important resource for the
organic chemist The values listed may be used to calcu-late rigorous experimental thermochemistry for many
common reactions and further with appropriate care
instructive estimations of reaction thermochemistry can
be made for complex chemical problems
This work was supported by grants from the Chemical Physics
Program United States Department of Energy (DE-FG02-
87ER13695) and the National Science Foundation (CHE-0201848)
We are grateful for the sustained advice and criticism from our
Colorado colleagues Carl Lineberger Veronica Bierbaum Shuji
Kato Mark Nimlos Xu Zhang Bob Damrauer Charles H DePuy
Geoff Tyndall and Veronica Vaida We are also continually
educated by our friends Emily Carter George Petersson Larry
Harding Kent Ervin Richard OrsquoHair and Branko Ruscic Finally GBE would like to thank Joseph Berkowitz now retired for 25
years of friendship and physics
References(1) Benson S W Thermochemical Kinetics 2nd ed Wiley-Inter-
science New Y ork 1976(2) MillsI Cvitas T Homann KKallay NKuchitsuK Quantities
Units and Symbols in Physical Chemistry Blackwell ScientificPublications Oxford 1988 This reference lists the IUPA Crsquosguidelines concerning thermochemical symbols which we adoptInstead of the more common expressions ∆G rxn 298(1) ∆H f 0deg(R)or ∆H f 298deg(RH) the use of ∆rxnG 298(1) ∆f H 0(R) or ∆f H 298(RH) isrecommended See p 46
(3) Herzberg G H M olecular Spectra and Molecular StructureInfrared and Raman S pectra of Polyatomic Molecules D Van
Nostrand Princeton NJ 1945 Vol II see Chapter V(4) Gurvich L V Veyts I V Alcock C B Iorish V S Thermody-
namic Properties of Individual Substances 4th ed HemisphereNew York 1991 Vol 2
(5) Ervin K M Gronert SBarlow S E Gilles M K Harrison AGBierbaumV M Charles H DePuy Lineberger W CEllisonG B Bond Strengths of Ethylene and Acetylene J Am ChemSoc 1990 112 5750-5759
(6) Ervin K M DeTuri V F Anchoring the Gas-Phase AcidityScale Experiment and Theory J Phys Chem A 2002 106 9947-
9956(7) Frisch M J Trucks G WSchlegel H B Scuseria G E Robb
M A Cheeseman J R Zakrzewski V G Montgomery J A J r Stratmann R E Burant J C Dapprich S M illam J MDaniels A D Kudin K N Strain M C Farkas O Tomasi J Barone V Cossi M Cammi R M ennucci B Pomelli C
Scheme 2
BondDissociationEnergiesofOrganic Molecules Blanksby and Ellison
262 ACCOUNTS OF CHEMICAL RESEARCH VOL 36 NO 4 2003
892019 2003 - Bond Dissociation Energies of Organic Molecules
Adamo C Clifford S Ochterski J Petersson G A Ayala P Y Cui Q Morokuma K Malick D K Rabuck A D Raghava-chari K Foresman J B Cioslowski J Ortiz J VStefanov BB LiuG Liashenko A Piskorz PKomaromiI Gomperts RMartin R L Fox D J Keith T Al-Laham M A Peng C YNanayakkara A Gonzalez C Challacombe M Gill P M W J ohnson B G Chen W Wong M W Andres J L Head-Gordon M Replogle E S Pople J A Gaussian 98 GaussianInc Pittsburgh PA 1998
(8) Petersson G A In Computational Thermochemistry Irikura KK Frurip D J Eds ACS Symposium Series 677 AmericanChemical Society Washington DC 1998 pp 237-266
(9) Berkowitz J Ellison G B Gutman D Three Methods toMeasure RH Bond Energies J Phys Chem1994 98 2744-2765(10) Seakins P W Pilling M J Niiranen J T Gutman D
Krasnoperov L N Kinetics and Thermochemistry of R + HBrT RH + Br ReactionssDeterminations of the Heat of Formation of C2H5 i -C3H7 sec -C4H9 and tert -C4H9 J Phys Chem 1992 96 9847-9855
(11) Ruscic B Berkowitz J Curtiss L A Pople J A The EthylRadicalsPhotoionization and Theoretical Studies J Chem Phys1989 91 114-121
(12) Ervin K M Experimental techniques in gas-phase ion thermo-chemistry Chem Rev 2001 101 391-444
(13) Rienstra-Kiracofe J C Tschumper G S Schaefer H F IIINandi S Ellison G B Atomic and Molecular Electron Affini-ties Photoelectron Experiments and Theoretical ComputationsChem Rev 2002 102 231-282
(14) Ramond T M Blanksby S J Kato SBierbaum V M DavicoG E Schwartz R L Lineberger W C Ellison G B The Heatof Formation of the Hydroperoxyl Radical HOO via Negative Ion
Studies J Phys Chem A 2002 106 9641-9647(15) Seetula J AGutman DKineticsof the CH2OH + HBr and CH2OH
+ HI Reactions and Determination of the Heat of Formation of CH2OH J Phys Chem 1992 96 5401-5405
(16) Ruscic B Berkowitz J Heat of Formation of CH2OH andD0(H-CH2OH) J Phys Chem 1993 97 11451-11455
(17) Lias S G Bartmess J E Liebman J F Holmes J L LevinR D MallardW G Gas Phase Ion and Neutral Thermochemistry J Phys Chem Ref Data1988 17 (Suppl 1) 1
(18) Ramond T M Davico G E Schwartz R L Lineberger W CVibronic structure of alkoxy radicals via photoelectron spectros-copy J Chem Phys 2000 112 1158-1169
(19) Pedley J B Naylor R D Kirby S P Thermochemistry of Organic Compounds 2nd ed Chapman and Hall New Y ork1986
(20) Pedley J B Thermochemical Data and Structures of Organic Compounds Thermodynamics Research Center College Station TX 1994
(21) Thehuge value of DH 298(H2O) is the reason that the OH radical issuch an important species in atmospheric chemistry Hydroxylradicals result from solar photodissociation of O3 and they reactwith all organic species pumped into the atmosphere
(22) Ingold K U Wright J S U nderstanding trends in C-H N -Hand O-H bond dissociation enthalpies J Chem Educ 2000 77 1062-1064
(23) Goddard W A III Harding L B The Description of ChemicalBonding from Ab Initio Calculations Annu Rev Phys Chem1978 29 363-396
(24) Carter E A Goddard W A Relation between S inglet TripletGaps and Bond-Energies J Phys Chem 1986 90 998-1001
(25) Dunning T H J r A Road Map for the Calculation of M olecularBinding Energies J Phys Chem A 2000 104 9062-9080
(26) WuC J Carter E A Ab Initio Thermochemistry for UnsaturatedC2 Hydrocarbons J Phys Chem 1991 95 8352-8363
(27) Wenthold P G Squires R R Gas-phase acidities of o- m- andp-dehydrobenzoic acid radicals Determination of the substituentconstants for a phenyl radical site Int J Mass Spectrom 1998
175 215-224(28) Wenthold P G S quires R R Lineberger W C Ultraviolet
photoelectron spectroscopy of theo - m - and p -benzynenegativeions Electron affinities and singlet-triplet splittings for o - m -and p -benzyne J Am Chem Soc 1998 120 5279-5290
(29) Chen P In Unimolecular and Bimolecular Reaction Dynamics NgC YBaer T PowisIEds J ohnWiley amp Sons New York1994 Vol 3 pp 372-425
(30) Oakes J M J ones M E Bierbaum V M Ellison G BPhotoelectron Spectroscopy of CCO- and HCCO- J Phys Chem1983 87 4810-4815
(31) Ruscic B Litorja M Asher R L Ionization energy of methylenerevisited Improved values for the enthalpy of formation of CH2
and the bond dissociation energy of CH3 via simultaneoussolution of the local thermochemical network J Phys Chem A1999 103 8625-8633
(32) Halliwell B G utteridge J M C Free Radicals in Biology and
Medicine 3rd ed Oxford University Press Inc New York1999(33) Nicolaou K C Smith A L Yue E W Chemistry and Biology
of Natural and Designed Enediynes Proc Natl Acad Sci USA1993 90 5881-5888
(34) Ruscic B Feller D Dixon D A Peterson K A Harding L BAsher R L Wagner A F Evidence for a lower enthalpy of formation of hydroxyl radical and a lower gas-phase bonddissociation energy of water J Phys Chem A 2001 105 1-4
(35) Davis H F Kim B S J ohnston H S Lee Y T Dissociation-Energy and Photochemistry of NO3 J Phys Chem 1993 97 2172-2180
(36) M ordaunt D H Ashfold M N R Near-UltravioletPhotolysis of C2H2sa Precise Determination of D0(HCC-H) J Chem Phys1994 101 2630-2631
(37) Wenthold P G Squires R R Biradical Thermochemistry fromCollision-Induced Dissociation Threshold Energy MeasurementsAbsolute Heats of Formation of ortho - meta- and para-Benzyne J Am Chem S oc 1994 116 6401-6412
(38) Ellison G B Davico G E Bierbaum V M DePuy C H The Thermochemistry of the Benzyl and A llyl Radicals and Ions Int J Mass Spectrom Ion Processes 1996 156 109-131
(39) Nicovich J M Kreutter K D Vandijk C A Wine P H Temperature-Dependent Kinetics Studies of the Reactions Br(2P32)+ H 2S Reversible SH + HBr and Br(2P32) + CH3SH ReversibleCH3S + HBrsHeats of Formation of SH and CH3S Radicals J Phys Chem 1992 96 2518-2528
(40) Ruscic B Berkowitz J Photoionization Mass-SpectrometricStudies of the Isomeric Transient Species CH2SH and CH3S J Chem Phys 1992 97 1818-1823
(41) DeTuri V F Ervin K M Proton transfer between Cl- andC6H5OH O-H bond energy of phenol Int J Mass Spectrom1998 175 123-132
(42) Blanksby S J Ramond T M Davico G E Nimlos M R KatoS Bierbaum V M Lineberger W C Ellison G B OkumuraM Negative Ion Photoelectron Spectroscopy Gas-Phase Acidityand Thermochemistry of the Peroxyl Radicals CH3OO andCH3CH2OO J Am Chem Soc 2001 123 9585-9596
(43) Niiranen J T Gutman D Krasnoperov L N Kinetics and Thermochemistry of the CH3CO RadicalsStudy of the CH3CO +
(45) Ruscic B Litorja M Photoionization of HOCO revisited a newupper limit to the adiabatic ionization energy and lower limit tothe enthalpy of formation Chem Phys Lett 2000 316 45-50
(46) Linstrom P J Mallard W G NIST Chemistry WebBook NISTStandard Reference Database No 69 National Institute of
Standards and Technology Gaithersburg MD 2001 httpwebbooknistgov
AR020230D
BondDissociationEnergiesofOrganic Molecules Blanksby and Ellison
VOL 36 NO 4 2003 ACCOUNTS OF CHEMICAL RESEARCH 263
892019 2003 - Bond Dissociation Energies of Organic Molecules
strengths by breaking the bonds in different orders
Rigorous quantum mechanical discussions of these trends
are available elsewhere26 along with other provocative
discussions of bond strengths232429
The example of HCCH serves to reiterate that the bond
enthalpy is the enthalpy of a homolysis reaction (eq 3)
and thus depends exclusively on the relative stability of
reactant and product states More generally creation of
new bonds in the products or otherwise stabilized
products always decreases the bond enthalpy As an
example consider ketene in Chart 2 The carbon-carbon
bond enthalpy 3031 of ketene DH 298(CH2dCO) ) 787 ( 02
kcal mol-1 is almost 100 kcal mol-1 less than that of
ethylene DH 298(CH2dCH2) ) 174 kcal mol-1 because one
of the products of the ketene fragmentation is an ex-
tremely stable molecule namely carbon monoxide This
example illustrates that not all double bonds are createdequal2426 and that extrapolations of bond energies from
one molecular species to another must be conducted
carefully
Bond Enthalpies in Solut ionThe bond enthalpies tabulated in this Account are exclu-
sively gas-phase values This raises the question of how
to relate gas-phase bond enthalpies to chemical problems
occurring in solution The difference between a gas-phase
bond enthalpy and that in solution D solnH 298(R-H)
depends on the difference in the enthalpy of solvation
∆solnH 298 of the two radicals and the parent compound
as expressed in terms of eq 23 There are not many
accurate measurements available for the enthalpy of
solvation of radical species and we can only estimate the
effects of solvation on organic BDEs The solvation energy
of a hydrogen atom is likely to be negligible in most
solvents and therefore the correction to an R-H gas-
phase bond enthalpy can be can be approximated as[∆solnH 298(R) - ∆solnH 298(RH)] As both the radical and the
parent are neutral the solvation energies are likely to be
small and similar in most cases particularly in nonpolar
solvents The small effect on bond enthalpy is likely to be
most noticeable is protic polar solvents where hydrogen
bonding may play a key role in preferentially stabilizing
the radical or parent in solution Consider for example
the C-H and O-H bond enthalpies of CH3OH in a polar
protic solvent (reactions 24)
While in reaction 24a both methanol and the hydroxy-
methyl radical can hydrogen bond to the surrounding
solvent such stabilization may be less prevalent for the
methoxyl radical in reaction 24b due to the absence of
the highly polarized O-H bond Therefore one might
surmise that the H-CH2OH bond energy of methanol in
a protic solvent would be largely unchanged from the gas
phase while the CH3O-H value could be slightly higher
in solution There is clearly much work to be done in this
area
Chart 3 Experimental Bond Enthalpies DH 29 8 for Benzene andPhenyl Radicala
a Values are in kcal mol-1 and represent the energy required to break only the single bond indicated Thus DH 298(C6H5sHf C6H5 + H) ) 1129( 05 kcal mol-1 but DH 298(C6H4sH f H + o -C6H4) ) 78 ( 3 kcal mol-1DH 298(C6H4sH f H + m -C6H4) ) 94 ( 3 kcal mol-1 and DH 298(C6H4sHf H + p -C6H4) ) 109 ( 3 kcal mol-1
88 kcal mol-1 gt DH 298(bisallylicC-H) Given these esti-
mates the hydrogens most susceptible to radical abstrac-
tion will be those in the allylic positions While there is
at present no direct data for doubly allylic C-H bond
enthalpies one might conjecture that the bond energy
will be roughly 80 kcal mol-1 given that the dif-
ference between a typical methylenic Cs
H (egDH 298((CH3)2CHsH) = 986 kcal mol-1) and a singly allylic
CsH (eg DH 298(H2CdCHCH2sH) = 888 kcal mol-1) is
about 10 kcal mol-1 Thus the bisallylic hydrogens at
carbons 7 10 and 13 represent the most labile hydrogens
in the molecule Once a carbon-centered radical is pro-
duced and rearranged to its most stable form it will
readily add O2 to form a peroxyl radical Chart 2 indicates
that the OO-CMe3 bond enthalpy is 38 kcal mol-1 Given
that the O-H bond enthalpy of a hydroperoxide is
approximately 85 kcal mol-1 a radical chain reaction will
be exothermic
The biological activity of the enediyne anticancer
antibiotic agents is thought to be due to their ability toform reactive diradicals in situ 33 Molecules such as
calicheamicin γ1I possess an extended sugar residue which
serves to deliver the enediyne moiety the active part of
the molecule to a sequence-specific position on the DNA
double-helix Upon delivery to the target the enediyne
functionality is activated to undergo a Bergman cycloaro-
matization which yields a substituted p -benzyne (Scheme
2) From Chart 3 we can estimate that the diradical can
abstract all hydrogens bound by e109 kcal mol-1 This
makes the p -benzyne a quite powerful hydrogen abstrac-
tion reagent and further once one hydrogen has been
abstracted a substituted phenyl radical results which can
abstract all hydrogens bound by e113 kcal mol-1 Thus
two exothermic hydrogen abstractions by the reactive
p -benzyne moiety can lead to selective cutting of double-
stranded DNA
SummaryThe critically evaluated bond enthalpies listed in Tables
1 and 2 should serve as an important resource for the
organic chemist The values listed may be used to calcu-late rigorous experimental thermochemistry for many
common reactions and further with appropriate care
instructive estimations of reaction thermochemistry can
be made for complex chemical problems
This work was supported by grants from the Chemical Physics
Program United States Department of Energy (DE-FG02-
87ER13695) and the National Science Foundation (CHE-0201848)
We are grateful for the sustained advice and criticism from our
Colorado colleagues Carl Lineberger Veronica Bierbaum Shuji
Kato Mark Nimlos Xu Zhang Bob Damrauer Charles H DePuy
Geoff Tyndall and Veronica Vaida We are also continually
educated by our friends Emily Carter George Petersson Larry
Harding Kent Ervin Richard OrsquoHair and Branko Ruscic Finally GBE would like to thank Joseph Berkowitz now retired for 25
years of friendship and physics
References(1) Benson S W Thermochemical Kinetics 2nd ed Wiley-Inter-
science New Y ork 1976(2) MillsI Cvitas T Homann KKallay NKuchitsuK Quantities
Units and Symbols in Physical Chemistry Blackwell ScientificPublications Oxford 1988 This reference lists the IUPA Crsquosguidelines concerning thermochemical symbols which we adoptInstead of the more common expressions ∆G rxn 298(1) ∆H f 0deg(R)or ∆H f 298deg(RH) the use of ∆rxnG 298(1) ∆f H 0(R) or ∆f H 298(RH) isrecommended See p 46
(3) Herzberg G H M olecular Spectra and Molecular StructureInfrared and Raman S pectra of Polyatomic Molecules D Van
Nostrand Princeton NJ 1945 Vol II see Chapter V(4) Gurvich L V Veyts I V Alcock C B Iorish V S Thermody-
namic Properties of Individual Substances 4th ed HemisphereNew York 1991 Vol 2
(5) Ervin K M Gronert SBarlow S E Gilles M K Harrison AGBierbaumV M Charles H DePuy Lineberger W CEllisonG B Bond Strengths of Ethylene and Acetylene J Am ChemSoc 1990 112 5750-5759
(6) Ervin K M DeTuri V F Anchoring the Gas-Phase AcidityScale Experiment and Theory J Phys Chem A 2002 106 9947-
9956(7) Frisch M J Trucks G WSchlegel H B Scuseria G E Robb
M A Cheeseman J R Zakrzewski V G Montgomery J A J r Stratmann R E Burant J C Dapprich S M illam J MDaniels A D Kudin K N Strain M C Farkas O Tomasi J Barone V Cossi M Cammi R M ennucci B Pomelli C
Scheme 2
BondDissociationEnergiesofOrganic Molecules Blanksby and Ellison
262 ACCOUNTS OF CHEMICAL RESEARCH VOL 36 NO 4 2003
892019 2003 - Bond Dissociation Energies of Organic Molecules
Adamo C Clifford S Ochterski J Petersson G A Ayala P Y Cui Q Morokuma K Malick D K Rabuck A D Raghava-chari K Foresman J B Cioslowski J Ortiz J VStefanov BB LiuG Liashenko A Piskorz PKomaromiI Gomperts RMartin R L Fox D J Keith T Al-Laham M A Peng C YNanayakkara A Gonzalez C Challacombe M Gill P M W J ohnson B G Chen W Wong M W Andres J L Head-Gordon M Replogle E S Pople J A Gaussian 98 GaussianInc Pittsburgh PA 1998
(8) Petersson G A In Computational Thermochemistry Irikura KK Frurip D J Eds ACS Symposium Series 677 AmericanChemical Society Washington DC 1998 pp 237-266
(9) Berkowitz J Ellison G B Gutman D Three Methods toMeasure RH Bond Energies J Phys Chem1994 98 2744-2765(10) Seakins P W Pilling M J Niiranen J T Gutman D
Krasnoperov L N Kinetics and Thermochemistry of R + HBrT RH + Br ReactionssDeterminations of the Heat of Formation of C2H5 i -C3H7 sec -C4H9 and tert -C4H9 J Phys Chem 1992 96 9847-9855
(11) Ruscic B Berkowitz J Curtiss L A Pople J A The EthylRadicalsPhotoionization and Theoretical Studies J Chem Phys1989 91 114-121
(12) Ervin K M Experimental techniques in gas-phase ion thermo-chemistry Chem Rev 2001 101 391-444
(13) Rienstra-Kiracofe J C Tschumper G S Schaefer H F IIINandi S Ellison G B Atomic and Molecular Electron Affini-ties Photoelectron Experiments and Theoretical ComputationsChem Rev 2002 102 231-282
(14) Ramond T M Blanksby S J Kato SBierbaum V M DavicoG E Schwartz R L Lineberger W C Ellison G B The Heatof Formation of the Hydroperoxyl Radical HOO via Negative Ion
Studies J Phys Chem A 2002 106 9641-9647(15) Seetula J AGutman DKineticsof the CH2OH + HBr and CH2OH
+ HI Reactions and Determination of the Heat of Formation of CH2OH J Phys Chem 1992 96 5401-5405
(16) Ruscic B Berkowitz J Heat of Formation of CH2OH andD0(H-CH2OH) J Phys Chem 1993 97 11451-11455
(17) Lias S G Bartmess J E Liebman J F Holmes J L LevinR D MallardW G Gas Phase Ion and Neutral Thermochemistry J Phys Chem Ref Data1988 17 (Suppl 1) 1
(18) Ramond T M Davico G E Schwartz R L Lineberger W CVibronic structure of alkoxy radicals via photoelectron spectros-copy J Chem Phys 2000 112 1158-1169
(19) Pedley J B Naylor R D Kirby S P Thermochemistry of Organic Compounds 2nd ed Chapman and Hall New Y ork1986
(20) Pedley J B Thermochemical Data and Structures of Organic Compounds Thermodynamics Research Center College Station TX 1994
(21) Thehuge value of DH 298(H2O) is the reason that the OH radical issuch an important species in atmospheric chemistry Hydroxylradicals result from solar photodissociation of O3 and they reactwith all organic species pumped into the atmosphere
(22) Ingold K U Wright J S U nderstanding trends in C-H N -Hand O-H bond dissociation enthalpies J Chem Educ 2000 77 1062-1064
(23) Goddard W A III Harding L B The Description of ChemicalBonding from Ab Initio Calculations Annu Rev Phys Chem1978 29 363-396
(24) Carter E A Goddard W A Relation between S inglet TripletGaps and Bond-Energies J Phys Chem 1986 90 998-1001
(25) Dunning T H J r A Road Map for the Calculation of M olecularBinding Energies J Phys Chem A 2000 104 9062-9080
(26) WuC J Carter E A Ab Initio Thermochemistry for UnsaturatedC2 Hydrocarbons J Phys Chem 1991 95 8352-8363
(27) Wenthold P G Squires R R Gas-phase acidities of o- m- andp-dehydrobenzoic acid radicals Determination of the substituentconstants for a phenyl radical site Int J Mass Spectrom 1998
175 215-224(28) Wenthold P G S quires R R Lineberger W C Ultraviolet
photoelectron spectroscopy of theo - m - and p -benzynenegativeions Electron affinities and singlet-triplet splittings for o - m -and p -benzyne J Am Chem Soc 1998 120 5279-5290
(29) Chen P In Unimolecular and Bimolecular Reaction Dynamics NgC YBaer T PowisIEds J ohnWiley amp Sons New York1994 Vol 3 pp 372-425
(30) Oakes J M J ones M E Bierbaum V M Ellison G BPhotoelectron Spectroscopy of CCO- and HCCO- J Phys Chem1983 87 4810-4815
(31) Ruscic B Litorja M Asher R L Ionization energy of methylenerevisited Improved values for the enthalpy of formation of CH2
and the bond dissociation energy of CH3 via simultaneoussolution of the local thermochemical network J Phys Chem A1999 103 8625-8633
(32) Halliwell B G utteridge J M C Free Radicals in Biology and
Medicine 3rd ed Oxford University Press Inc New York1999(33) Nicolaou K C Smith A L Yue E W Chemistry and Biology
of Natural and Designed Enediynes Proc Natl Acad Sci USA1993 90 5881-5888
(34) Ruscic B Feller D Dixon D A Peterson K A Harding L BAsher R L Wagner A F Evidence for a lower enthalpy of formation of hydroxyl radical and a lower gas-phase bonddissociation energy of water J Phys Chem A 2001 105 1-4
(35) Davis H F Kim B S J ohnston H S Lee Y T Dissociation-Energy and Photochemistry of NO3 J Phys Chem 1993 97 2172-2180
(36) M ordaunt D H Ashfold M N R Near-UltravioletPhotolysis of C2H2sa Precise Determination of D0(HCC-H) J Chem Phys1994 101 2630-2631
(37) Wenthold P G Squires R R Biradical Thermochemistry fromCollision-Induced Dissociation Threshold Energy MeasurementsAbsolute Heats of Formation of ortho - meta- and para-Benzyne J Am Chem S oc 1994 116 6401-6412
(38) Ellison G B Davico G E Bierbaum V M DePuy C H The Thermochemistry of the Benzyl and A llyl Radicals and Ions Int J Mass Spectrom Ion Processes 1996 156 109-131
(39) Nicovich J M Kreutter K D Vandijk C A Wine P H Temperature-Dependent Kinetics Studies of the Reactions Br(2P32)+ H 2S Reversible SH + HBr and Br(2P32) + CH3SH ReversibleCH3S + HBrsHeats of Formation of SH and CH3S Radicals J Phys Chem 1992 96 2518-2528
(40) Ruscic B Berkowitz J Photoionization Mass-SpectrometricStudies of the Isomeric Transient Species CH2SH and CH3S J Chem Phys 1992 97 1818-1823
(41) DeTuri V F Ervin K M Proton transfer between Cl- andC6H5OH O-H bond energy of phenol Int J Mass Spectrom1998 175 123-132
(42) Blanksby S J Ramond T M Davico G E Nimlos M R KatoS Bierbaum V M Lineberger W C Ellison G B OkumuraM Negative Ion Photoelectron Spectroscopy Gas-Phase Acidityand Thermochemistry of the Peroxyl Radicals CH3OO andCH3CH2OO J Am Chem Soc 2001 123 9585-9596
(43) Niiranen J T Gutman D Krasnoperov L N Kinetics and Thermochemistry of the CH3CO RadicalsStudy of the CH3CO +
(45) Ruscic B Litorja M Photoionization of HOCO revisited a newupper limit to the adiabatic ionization energy and lower limit tothe enthalpy of formation Chem Phys Lett 2000 316 45-50
(46) Linstrom P J Mallard W G NIST Chemistry WebBook NISTStandard Reference Database No 69 National Institute of
Standards and Technology Gaithersburg MD 2001 httpwebbooknistgov
AR020230D
BondDissociationEnergiesofOrganic Molecules Blanksby and Ellison
VOL 36 NO 4 2003 ACCOUNTS OF CHEMICAL RESEARCH 263
892019 2003 - Bond Dissociation Energies of Organic Molecules
88 kcal mol-1 gt DH 298(bisallylicC-H) Given these esti-
mates the hydrogens most susceptible to radical abstrac-
tion will be those in the allylic positions While there is
at present no direct data for doubly allylic C-H bond
enthalpies one might conjecture that the bond energy
will be roughly 80 kcal mol-1 given that the dif-
ference between a typical methylenic Cs
H (egDH 298((CH3)2CHsH) = 986 kcal mol-1) and a singly allylic
CsH (eg DH 298(H2CdCHCH2sH) = 888 kcal mol-1) is
about 10 kcal mol-1 Thus the bisallylic hydrogens at
carbons 7 10 and 13 represent the most labile hydrogens
in the molecule Once a carbon-centered radical is pro-
duced and rearranged to its most stable form it will
readily add O2 to form a peroxyl radical Chart 2 indicates
that the OO-CMe3 bond enthalpy is 38 kcal mol-1 Given
that the O-H bond enthalpy of a hydroperoxide is
approximately 85 kcal mol-1 a radical chain reaction will
be exothermic
The biological activity of the enediyne anticancer
antibiotic agents is thought to be due to their ability toform reactive diradicals in situ 33 Molecules such as
calicheamicin γ1I possess an extended sugar residue which
serves to deliver the enediyne moiety the active part of
the molecule to a sequence-specific position on the DNA
double-helix Upon delivery to the target the enediyne
functionality is activated to undergo a Bergman cycloaro-
matization which yields a substituted p -benzyne (Scheme
2) From Chart 3 we can estimate that the diradical can
abstract all hydrogens bound by e109 kcal mol-1 This
makes the p -benzyne a quite powerful hydrogen abstrac-
tion reagent and further once one hydrogen has been
abstracted a substituted phenyl radical results which can
abstract all hydrogens bound by e113 kcal mol-1 Thus
two exothermic hydrogen abstractions by the reactive
p -benzyne moiety can lead to selective cutting of double-
stranded DNA
SummaryThe critically evaluated bond enthalpies listed in Tables
1 and 2 should serve as an important resource for the
organic chemist The values listed may be used to calcu-late rigorous experimental thermochemistry for many
common reactions and further with appropriate care
instructive estimations of reaction thermochemistry can
be made for complex chemical problems
This work was supported by grants from the Chemical Physics
Program United States Department of Energy (DE-FG02-
87ER13695) and the National Science Foundation (CHE-0201848)
We are grateful for the sustained advice and criticism from our
Colorado colleagues Carl Lineberger Veronica Bierbaum Shuji
Kato Mark Nimlos Xu Zhang Bob Damrauer Charles H DePuy
Geoff Tyndall and Veronica Vaida We are also continually
educated by our friends Emily Carter George Petersson Larry
Harding Kent Ervin Richard OrsquoHair and Branko Ruscic Finally GBE would like to thank Joseph Berkowitz now retired for 25
years of friendship and physics
References(1) Benson S W Thermochemical Kinetics 2nd ed Wiley-Inter-
science New Y ork 1976(2) MillsI Cvitas T Homann KKallay NKuchitsuK Quantities
Units and Symbols in Physical Chemistry Blackwell ScientificPublications Oxford 1988 This reference lists the IUPA Crsquosguidelines concerning thermochemical symbols which we adoptInstead of the more common expressions ∆G rxn 298(1) ∆H f 0deg(R)or ∆H f 298deg(RH) the use of ∆rxnG 298(1) ∆f H 0(R) or ∆f H 298(RH) isrecommended See p 46
(3) Herzberg G H M olecular Spectra and Molecular StructureInfrared and Raman S pectra of Polyatomic Molecules D Van
Nostrand Princeton NJ 1945 Vol II see Chapter V(4) Gurvich L V Veyts I V Alcock C B Iorish V S Thermody-
namic Properties of Individual Substances 4th ed HemisphereNew York 1991 Vol 2
(5) Ervin K M Gronert SBarlow S E Gilles M K Harrison AGBierbaumV M Charles H DePuy Lineberger W CEllisonG B Bond Strengths of Ethylene and Acetylene J Am ChemSoc 1990 112 5750-5759
(6) Ervin K M DeTuri V F Anchoring the Gas-Phase AcidityScale Experiment and Theory J Phys Chem A 2002 106 9947-
9956(7) Frisch M J Trucks G WSchlegel H B Scuseria G E Robb
M A Cheeseman J R Zakrzewski V G Montgomery J A J r Stratmann R E Burant J C Dapprich S M illam J MDaniels A D Kudin K N Strain M C Farkas O Tomasi J Barone V Cossi M Cammi R M ennucci B Pomelli C
Scheme 2
BondDissociationEnergiesofOrganic Molecules Blanksby and Ellison
262 ACCOUNTS OF CHEMICAL RESEARCH VOL 36 NO 4 2003
892019 2003 - Bond Dissociation Energies of Organic Molecules
Adamo C Clifford S Ochterski J Petersson G A Ayala P Y Cui Q Morokuma K Malick D K Rabuck A D Raghava-chari K Foresman J B Cioslowski J Ortiz J VStefanov BB LiuG Liashenko A Piskorz PKomaromiI Gomperts RMartin R L Fox D J Keith T Al-Laham M A Peng C YNanayakkara A Gonzalez C Challacombe M Gill P M W J ohnson B G Chen W Wong M W Andres J L Head-Gordon M Replogle E S Pople J A Gaussian 98 GaussianInc Pittsburgh PA 1998
(8) Petersson G A In Computational Thermochemistry Irikura KK Frurip D J Eds ACS Symposium Series 677 AmericanChemical Society Washington DC 1998 pp 237-266
(9) Berkowitz J Ellison G B Gutman D Three Methods toMeasure RH Bond Energies J Phys Chem1994 98 2744-2765(10) Seakins P W Pilling M J Niiranen J T Gutman D
Krasnoperov L N Kinetics and Thermochemistry of R + HBrT RH + Br ReactionssDeterminations of the Heat of Formation of C2H5 i -C3H7 sec -C4H9 and tert -C4H9 J Phys Chem 1992 96 9847-9855
(11) Ruscic B Berkowitz J Curtiss L A Pople J A The EthylRadicalsPhotoionization and Theoretical Studies J Chem Phys1989 91 114-121
(12) Ervin K M Experimental techniques in gas-phase ion thermo-chemistry Chem Rev 2001 101 391-444
(13) Rienstra-Kiracofe J C Tschumper G S Schaefer H F IIINandi S Ellison G B Atomic and Molecular Electron Affini-ties Photoelectron Experiments and Theoretical ComputationsChem Rev 2002 102 231-282
(14) Ramond T M Blanksby S J Kato SBierbaum V M DavicoG E Schwartz R L Lineberger W C Ellison G B The Heatof Formation of the Hydroperoxyl Radical HOO via Negative Ion
Studies J Phys Chem A 2002 106 9641-9647(15) Seetula J AGutman DKineticsof the CH2OH + HBr and CH2OH
+ HI Reactions and Determination of the Heat of Formation of CH2OH J Phys Chem 1992 96 5401-5405
(16) Ruscic B Berkowitz J Heat of Formation of CH2OH andD0(H-CH2OH) J Phys Chem 1993 97 11451-11455
(17) Lias S G Bartmess J E Liebman J F Holmes J L LevinR D MallardW G Gas Phase Ion and Neutral Thermochemistry J Phys Chem Ref Data1988 17 (Suppl 1) 1
(18) Ramond T M Davico G E Schwartz R L Lineberger W CVibronic structure of alkoxy radicals via photoelectron spectros-copy J Chem Phys 2000 112 1158-1169
(19) Pedley J B Naylor R D Kirby S P Thermochemistry of Organic Compounds 2nd ed Chapman and Hall New Y ork1986
(20) Pedley J B Thermochemical Data and Structures of Organic Compounds Thermodynamics Research Center College Station TX 1994
(21) Thehuge value of DH 298(H2O) is the reason that the OH radical issuch an important species in atmospheric chemistry Hydroxylradicals result from solar photodissociation of O3 and they reactwith all organic species pumped into the atmosphere
(22) Ingold K U Wright J S U nderstanding trends in C-H N -Hand O-H bond dissociation enthalpies J Chem Educ 2000 77 1062-1064
(23) Goddard W A III Harding L B The Description of ChemicalBonding from Ab Initio Calculations Annu Rev Phys Chem1978 29 363-396
(24) Carter E A Goddard W A Relation between S inglet TripletGaps and Bond-Energies J Phys Chem 1986 90 998-1001
(25) Dunning T H J r A Road Map for the Calculation of M olecularBinding Energies J Phys Chem A 2000 104 9062-9080
(26) WuC J Carter E A Ab Initio Thermochemistry for UnsaturatedC2 Hydrocarbons J Phys Chem 1991 95 8352-8363
(27) Wenthold P G Squires R R Gas-phase acidities of o- m- andp-dehydrobenzoic acid radicals Determination of the substituentconstants for a phenyl radical site Int J Mass Spectrom 1998
175 215-224(28) Wenthold P G S quires R R Lineberger W C Ultraviolet
photoelectron spectroscopy of theo - m - and p -benzynenegativeions Electron affinities and singlet-triplet splittings for o - m -and p -benzyne J Am Chem Soc 1998 120 5279-5290
(29) Chen P In Unimolecular and Bimolecular Reaction Dynamics NgC YBaer T PowisIEds J ohnWiley amp Sons New York1994 Vol 3 pp 372-425
(30) Oakes J M J ones M E Bierbaum V M Ellison G BPhotoelectron Spectroscopy of CCO- and HCCO- J Phys Chem1983 87 4810-4815
(31) Ruscic B Litorja M Asher R L Ionization energy of methylenerevisited Improved values for the enthalpy of formation of CH2
and the bond dissociation energy of CH3 via simultaneoussolution of the local thermochemical network J Phys Chem A1999 103 8625-8633
(32) Halliwell B G utteridge J M C Free Radicals in Biology and
Medicine 3rd ed Oxford University Press Inc New York1999(33) Nicolaou K C Smith A L Yue E W Chemistry and Biology
of Natural and Designed Enediynes Proc Natl Acad Sci USA1993 90 5881-5888
(34) Ruscic B Feller D Dixon D A Peterson K A Harding L BAsher R L Wagner A F Evidence for a lower enthalpy of formation of hydroxyl radical and a lower gas-phase bonddissociation energy of water J Phys Chem A 2001 105 1-4
(35) Davis H F Kim B S J ohnston H S Lee Y T Dissociation-Energy and Photochemistry of NO3 J Phys Chem 1993 97 2172-2180
(36) M ordaunt D H Ashfold M N R Near-UltravioletPhotolysis of C2H2sa Precise Determination of D0(HCC-H) J Chem Phys1994 101 2630-2631
(37) Wenthold P G Squires R R Biradical Thermochemistry fromCollision-Induced Dissociation Threshold Energy MeasurementsAbsolute Heats of Formation of ortho - meta- and para-Benzyne J Am Chem S oc 1994 116 6401-6412
(38) Ellison G B Davico G E Bierbaum V M DePuy C H The Thermochemistry of the Benzyl and A llyl Radicals and Ions Int J Mass Spectrom Ion Processes 1996 156 109-131
(39) Nicovich J M Kreutter K D Vandijk C A Wine P H Temperature-Dependent Kinetics Studies of the Reactions Br(2P32)+ H 2S Reversible SH + HBr and Br(2P32) + CH3SH ReversibleCH3S + HBrsHeats of Formation of SH and CH3S Radicals J Phys Chem 1992 96 2518-2528
(40) Ruscic B Berkowitz J Photoionization Mass-SpectrometricStudies of the Isomeric Transient Species CH2SH and CH3S J Chem Phys 1992 97 1818-1823
(41) DeTuri V F Ervin K M Proton transfer between Cl- andC6H5OH O-H bond energy of phenol Int J Mass Spectrom1998 175 123-132
(42) Blanksby S J Ramond T M Davico G E Nimlos M R KatoS Bierbaum V M Lineberger W C Ellison G B OkumuraM Negative Ion Photoelectron Spectroscopy Gas-Phase Acidityand Thermochemistry of the Peroxyl Radicals CH3OO andCH3CH2OO J Am Chem Soc 2001 123 9585-9596
(43) Niiranen J T Gutman D Krasnoperov L N Kinetics and Thermochemistry of the CH3CO RadicalsStudy of the CH3CO +
(45) Ruscic B Litorja M Photoionization of HOCO revisited a newupper limit to the adiabatic ionization energy and lower limit tothe enthalpy of formation Chem Phys Lett 2000 316 45-50
(46) Linstrom P J Mallard W G NIST Chemistry WebBook NISTStandard Reference Database No 69 National Institute of
Standards and Technology Gaithersburg MD 2001 httpwebbooknistgov
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892019 2003 - Bond Dissociation Energies of Organic Molecules
Adamo C Clifford S Ochterski J Petersson G A Ayala P Y Cui Q Morokuma K Malick D K Rabuck A D Raghava-chari K Foresman J B Cioslowski J Ortiz J VStefanov BB LiuG Liashenko A Piskorz PKomaromiI Gomperts RMartin R L Fox D J Keith T Al-Laham M A Peng C YNanayakkara A Gonzalez C Challacombe M Gill P M W J ohnson B G Chen W Wong M W Andres J L Head-Gordon M Replogle E S Pople J A Gaussian 98 GaussianInc Pittsburgh PA 1998
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(9) Berkowitz J Ellison G B Gutman D Three Methods toMeasure RH Bond Energies J Phys Chem1994 98 2744-2765(10) Seakins P W Pilling M J Niiranen J T Gutman D
Krasnoperov L N Kinetics and Thermochemistry of R + HBrT RH + Br ReactionssDeterminations of the Heat of Formation of C2H5 i -C3H7 sec -C4H9 and tert -C4H9 J Phys Chem 1992 96 9847-9855
(11) Ruscic B Berkowitz J Curtiss L A Pople J A The EthylRadicalsPhotoionization and Theoretical Studies J Chem Phys1989 91 114-121
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(13) Rienstra-Kiracofe J C Tschumper G S Schaefer H F IIINandi S Ellison G B Atomic and Molecular Electron Affini-ties Photoelectron Experiments and Theoretical ComputationsChem Rev 2002 102 231-282
(14) Ramond T M Blanksby S J Kato SBierbaum V M DavicoG E Schwartz R L Lineberger W C Ellison G B The Heatof Formation of the Hydroperoxyl Radical HOO via Negative Ion
Studies J Phys Chem A 2002 106 9641-9647(15) Seetula J AGutman DKineticsof the CH2OH + HBr and CH2OH
+ HI Reactions and Determination of the Heat of Formation of CH2OH J Phys Chem 1992 96 5401-5405
(16) Ruscic B Berkowitz J Heat of Formation of CH2OH andD0(H-CH2OH) J Phys Chem 1993 97 11451-11455
(17) Lias S G Bartmess J E Liebman J F Holmes J L LevinR D MallardW G Gas Phase Ion and Neutral Thermochemistry J Phys Chem Ref Data1988 17 (Suppl 1) 1
(18) Ramond T M Davico G E Schwartz R L Lineberger W CVibronic structure of alkoxy radicals via photoelectron spectros-copy J Chem Phys 2000 112 1158-1169
(19) Pedley J B Naylor R D Kirby S P Thermochemistry of Organic Compounds 2nd ed Chapman and Hall New Y ork1986
(20) Pedley J B Thermochemical Data and Structures of Organic Compounds Thermodynamics Research Center College Station TX 1994
(21) Thehuge value of DH 298(H2O) is the reason that the OH radical issuch an important species in atmospheric chemistry Hydroxylradicals result from solar photodissociation of O3 and they reactwith all organic species pumped into the atmosphere
(22) Ingold K U Wright J S U nderstanding trends in C-H N -Hand O-H bond dissociation enthalpies J Chem Educ 2000 77 1062-1064
(23) Goddard W A III Harding L B The Description of ChemicalBonding from Ab Initio Calculations Annu Rev Phys Chem1978 29 363-396
(24) Carter E A Goddard W A Relation between S inglet TripletGaps and Bond-Energies J Phys Chem 1986 90 998-1001
(25) Dunning T H J r A Road Map for the Calculation of M olecularBinding Energies J Phys Chem A 2000 104 9062-9080
(26) WuC J Carter E A Ab Initio Thermochemistry for UnsaturatedC2 Hydrocarbons J Phys Chem 1991 95 8352-8363
(27) Wenthold P G Squires R R Gas-phase acidities of o- m- andp-dehydrobenzoic acid radicals Determination of the substituentconstants for a phenyl radical site Int J Mass Spectrom 1998
175 215-224(28) Wenthold P G S quires R R Lineberger W C Ultraviolet
photoelectron spectroscopy of theo - m - and p -benzynenegativeions Electron affinities and singlet-triplet splittings for o - m -and p -benzyne J Am Chem Soc 1998 120 5279-5290
(29) Chen P In Unimolecular and Bimolecular Reaction Dynamics NgC YBaer T PowisIEds J ohnWiley amp Sons New York1994 Vol 3 pp 372-425
(30) Oakes J M J ones M E Bierbaum V M Ellison G BPhotoelectron Spectroscopy of CCO- and HCCO- J Phys Chem1983 87 4810-4815
(31) Ruscic B Litorja M Asher R L Ionization energy of methylenerevisited Improved values for the enthalpy of formation of CH2
and the bond dissociation energy of CH3 via simultaneoussolution of the local thermochemical network J Phys Chem A1999 103 8625-8633
(32) Halliwell B G utteridge J M C Free Radicals in Biology and
Medicine 3rd ed Oxford University Press Inc New York1999(33) Nicolaou K C Smith A L Yue E W Chemistry and Biology
of Natural and Designed Enediynes Proc Natl Acad Sci USA1993 90 5881-5888
(34) Ruscic B Feller D Dixon D A Peterson K A Harding L BAsher R L Wagner A F Evidence for a lower enthalpy of formation of hydroxyl radical and a lower gas-phase bonddissociation energy of water J Phys Chem A 2001 105 1-4
(35) Davis H F Kim B S J ohnston H S Lee Y T Dissociation-Energy and Photochemistry of NO3 J Phys Chem 1993 97 2172-2180
(36) M ordaunt D H Ashfold M N R Near-UltravioletPhotolysis of C2H2sa Precise Determination of D0(HCC-H) J Chem Phys1994 101 2630-2631
(37) Wenthold P G Squires R R Biradical Thermochemistry fromCollision-Induced Dissociation Threshold Energy MeasurementsAbsolute Heats of Formation of ortho - meta- and para-Benzyne J Am Chem S oc 1994 116 6401-6412
(38) Ellison G B Davico G E Bierbaum V M DePuy C H The Thermochemistry of the Benzyl and A llyl Radicals and Ions Int J Mass Spectrom Ion Processes 1996 156 109-131
(39) Nicovich J M Kreutter K D Vandijk C A Wine P H Temperature-Dependent Kinetics Studies of the Reactions Br(2P32)+ H 2S Reversible SH + HBr and Br(2P32) + CH3SH ReversibleCH3S + HBrsHeats of Formation of SH and CH3S Radicals J Phys Chem 1992 96 2518-2528
(40) Ruscic B Berkowitz J Photoionization Mass-SpectrometricStudies of the Isomeric Transient Species CH2SH and CH3S J Chem Phys 1992 97 1818-1823
(41) DeTuri V F Ervin K M Proton transfer between Cl- andC6H5OH O-H bond energy of phenol Int J Mass Spectrom1998 175 123-132
(42) Blanksby S J Ramond T M Davico G E Nimlos M R KatoS Bierbaum V M Lineberger W C Ellison G B OkumuraM Negative Ion Photoelectron Spectroscopy Gas-Phase Acidityand Thermochemistry of the Peroxyl Radicals CH3OO andCH3CH2OO J Am Chem Soc 2001 123 9585-9596
(43) Niiranen J T Gutman D Krasnoperov L N Kinetics and Thermochemistry of the CH3CO RadicalsStudy of the CH3CO +
(45) Ruscic B Litorja M Photoionization of HOCO revisited a newupper limit to the adiabatic ionization energy and lower limit tothe enthalpy of formation Chem Phys Lett 2000 316 45-50
(46) Linstrom P J Mallard W G NIST Chemistry WebBook NISTStandard Reference Database No 69 National Institute of
Standards and Technology Gaithersburg MD 2001 httpwebbooknistgov
AR020230D
BondDissociationEnergiesofOrganic Molecules Blanksby and Ellison
VOL 36 NO 4 2003 ACCOUNTS OF CHEMICAL RESEARCH 263