Study of the Halogen Bonding between Pyridine and Perfluoroalkyl ...

Hindawi Publishing CorporationInternational Journal of SpectroscopyVolume 2013 Article ID 216518 10 pageshttpdxdoiorg1011552013216518

Research ArticleStudy of the Halogen Bonding between Pyridine andPerfluoroalkyl Iodide in Solution Phase Using the Combinationof FTIR and 19F NMR

Briauna Hawthorne1 Haiyan Fan-Hagenstein1 Elizabeth Wood2

Jessica Smith2 and Timothy Hanks2

1 Chemistry Department Claflin University Orangeburg SC 29115 USA2Chemistry Department Furman University Greenville SC 29613 USA

Correspondence should be addressed to Haiyan Fan-Hagenstein hfan-hagensteinclaflinedu

Received 8 March 2013 Accepted 8 May 2013

Academic Editor Rolf W Berg

Copyright copy 2013 Briauna Hawthorne et al This is an open access article distributed under the Creative Commons AttributionLicense which permits unrestricted use distribution and reproduction in any medium provided the original work is properlycited

Halogen bonding between pyridine and heptafluoro-2-iodopropane (iso-C3F7I)heptafluoro-1-iodopropane (1-C

3F7I) was studied

using a combination of FTIR and 19F NMR The ring breathing vibration of pyridine underwent a blue shift upon the formationof halogen bonds with both iso-C

3F7I and 1-C

3F7I The magnitudes of the shifts and the equilibrium constants for the halogen-

bonded complex formation were found to depend not only on the structure of the halocarbon but also on the solventThe halogenbond also affected the C

120572-F (C-F bond on the center carbon) bending and stretching vibrations in iso-C

3F7I These spectroscopic

effects show some solvent dependence butmore importantly they suggest the possibility of intermolecular halogen bonding amongiso-C

3F7I molecules The systems were also examined by 19F NMR in various solvents (cyclohexane hexane chloroform acetone

and acetonitrile) NMR dilution experiments support the existence of the intermolecular self-halogen bonding in both iso-C3F7I

and 1-C3F7I The binding constants for the pyridineperfluoroalkyl iodide halogen bonding complexes formed in various solvents

were obtained through NMR titration experiments Quantum chemical calculations were used to support the FTIR and 19F NMRobservations

1 Introduction

Halogen bonding a noncovalent interaction between a halo-gen atom acting as an electron acceptor and an electron-rich Lewis base has been known for nearly 150 years [1ndash6]However only recently has halogen bond started to attractwide-spread attention among scientists in a diversity of fieldsMetrangolo and coworkers have reviewed the basic conceptsas well as the major applications of the halogen bond [7 8]Examples include a wide range of applications in separationscience synthesis of liquid crystals and electronic materialsand the assembly of functional super molecules [9ndash18]Recently Meyer and Dubois [19] highlighted the applicationof halogen bonding to the synthesis of functional materialssuch as liquid crystal nonlinear optical magnetic conductingmaterial and halogen bonding based surface modification Areview by Beale et al [20] provides an informative summary

of solution thermodynamics and applications of halogenbonding Of particular importance are their presentations ofdetailed thermodynamic parameters measured for the halo-gen bonding systems made of organic donors and halogenanions in the solution phase and which emphasized theirimportance in the anion recognition Erdelyi [21] reviewedthe nature of halogen bonding in solution phase and summa-rized the techniques that have been used to study the halogenbond in solution

Using a jet cooled molecular beam combined withthe Fourier-transform microwave spectroscopy Legon hasinvestigated the geometry and the stability of some halogenbonding complexes in the gas phase through the analysis oftheir rotational spectra [22 23] This research has allowedthe detailed comparison of halogen and hydrogen bondingIt was pointed out that halogen bonds can be as strongas or slightly weaker than hydrogen bonds depending on

2 International Journal of Spectroscopy

the structures of the participating component moleculesWhile hydrogen bonds can adopt a variety of angles halogenbonds are mostly linear relative to the halogen the donoratom and their covalently bonded substrates This is due tothe large electrostatic anisotropy of the halogen which ishighly localized along the extension of the carbon-halogenbond

Halogen bondwas first recognized in dihalide-containingcomplexes but it is now understood that other halogencontaining species can also act as electron acceptors [24ndash33]When the halogens are bound to an electron-withdrawingmoiety the positive potential on the halogen atom isenhanced and the potential for halogen bonding is increasedIn addition halogen bonding ability increaseswith increasingthe size of the halide because of the increase in the polar-izability Thus iodine forms the strongest halogen bondsFor these reasons perfluoroalkyl iodides have been oftenchosen to be the halogen donors in most halogen bondingstudies Fluorinated organoiodides have several scientific andindustrial uses including pharmaceuticals and nanoscienceapplications [34ndash39] For example halogen bond betweenperfluorobenezene iodide and pyridine containingmoleculeshas recently been used to assemble gold nanoparticles [40]

As it is experimentally challenging to directly measurethe strength of the halogen bond in a particular system mostresearch in the field has been focused on the geometry andstructural impact Like hydrogen bond [41 42] halogen bondoften leads to observable changes in the vibrational spectraof the participatingmolecules Focusing on the halogen bondimpact on the perfluoroalkyl iodideMessina et al [43] led thestudy of the systems between 120572 120596-diiodoperfluoroalkanesand other electron donors using FTIR and Raman spec-troscopy According to the noisy light Coherent Anti-StokesRaman Scattering Spectroscopy (I(2)CARS) [44] halogenbond between pyridine and perfluoroalkyl iodide causes thering breathing vibration of pyridine to be blue shifted Themagnitude of the shift was comparable with that observedin the hydrogen bonding system between pyridine andwater Using FTIR and Raman spectroscopy Van der Vekenet al [45ndash48] have studied the effect of halogen bondbetween CF

3X (X = Cl Br I) and various electron lone pair

donors on the vibrations of the participants The complex-ation enthalpies were determined based on the temperaturedependence of the bond association constants An ab initiocalculation at theMP2aug-cc-pVDZ(-PP) level was also per-formed to be compared with the spectroscopic observationIn addition to Raman and FTIR spectroscopy 19F NMR isanother sensitivemethod to detect the halogen bond betweenperfluoroalkyl halides (PFC-iodide or bromide) and Lewisbases [49ndash52]The change in the chemical shift upon halogenbonding was found to be the greatest on the alpha fluorinesignalsThus both vibrational spectroscopy and 19FNMR areuseful tools for the investigation of halogen bond in solution

Considering that halogen bond is one of the strongestnoncovalent intermolecular forces the thermodynamics inthe halogen bonding formation have also been receivedattention Fan and coworkers [44] studied the pyridine andperfluoroalkyl iodidesTheir observations led to a hypothesis

that there existed some self-intermolecular halogen bondamong iso-C

3F7I molecules in the liquid phase while this

self-intermolecular halogen bond did not exist or at least wasvery weak for 1-C

4F9I and 1-C

6F13I

It has been long believed that the magnitude of thevibrational frequency shift in the hydrogen bonded complexis proportional to the strength of the hydrogen-bond [53]In other words the magnitude of the vibrational frequencyshift is correlated with the complexation enthalpies It isreasonable to apply the same principle to the halogen bondTherefore FTIR results provide information about enthalpychange in the halogen bond formation On the other hand19F NMR titration experiments provide information aboutbinding constants and thereby changes in the Gibbrsquos freeenergy during halogen bond formation As a result thecomparison between the FTIR and 19F NMR results makes itpossible to derive the contribution of entropy to the halogenbond formation

Using the combination of 19FNMR and theoretical calcu-lations Sarwar and coworkers [52] examined the thermody-namics of halogen bonding in iodoperfluoroarenes involvedsolution system with respect to substituent structural andsolvent effects The experimental results were compared withthe density functional theory (DFT) calculations [52] in orderto be able to model the supermolecular interactions for drugdesign

Whether in a liquid phase binary system or in solutionhalogen bonding may play very important roles in definingthe structure of the liquid and solution Inspired by previouswork the present paper seeks to determine if halogen bondexists among molecules of perfluoroalkyl iodides Based onthe hypothesis by Fan et al [44] the intermolecular halogenbond among iso-C

3F7Imolecules a secondary iodide should

be stronger than those formed among molecules of primaryiodides (eg 1-C

4F9I 1-C6F13I) In order to test this argument

the present work uses a combination of FTIR and 19FNMR to quantify the halogen bonding between pyridineand iso-C

3F7I1-C3F7I in various solvents First the strength

of halogen bond is compared between pyridineiso-C3F7I

and pyridine1-C3F7I Second for the same halogen bonding

system the strength is compared in different solvents Thirda series of dilution experiments were performed using bothFTIR and 19F-NMR to gather evidence of intermolecular self-halogen bond Finally the thermodynamics of the halogenbond for the system between pyridine and perfluoroalkyliodides in solution phase was studied using the combinationof the results from FTIR and 19F NMR measurements

2 Experimental Section

21 Chemicals and Regents All reagents and solvents werepurchased from commercial houses All solvents were pur-chased as anhydrous or used immediately after dispensingfrom a solvent purification system

22 FTIR and 19F NMR Measurement FTIR spectra wererecorded on a PerkinElmer Paragon 500 FTIR spectrom-eter All the measurements were carried out at the room

International Journal of Spectroscopy 3

temperature and the corresponding solvents were used torecord the background spectra A liquid sample holder witha NaCl windows was used and the resolution of the FTIRspectrometer was set at 1 cmminus1 during experiments 19FNMR spectra were recorded on a Varian 400MR 400MHzHigh-Field Superconducting NMR spectrometer 100M KFsolution inD

2Owas used in eachmeasurement as an external

reference for 19F NMR

23 Theoretical Calculations All computations were per-formed with the Gaussian 03 or Gaussian 09 [54 55] suite ofprograms The interaction energies and geometrical parame-ters in solution phase were computed using the DFT methodwith the functional M052X and the aug-cc-pVDXZ basis setfor all atoms except iodine where a pseudopotential wasrequired (aug-cc-pVDXZ-PP) [56] Geometry optimizationsfor monomer and dimer of iso-C

3F7I 1-C

3F7I as well as

the hydrogen bonding complex between iso-C3F7I1-C3F7I

and CHCl3in vacuum phase were performed with density

function theory (DFT) using 3-21G-B3LYP CalcAll and thetight convergence criteria The calculations of 19F NMR wereperformed using the optimized geometry

3 Result and Discussion

31 Comparison of Halogen Bonding Strength and SolventEffect between PyridineIso-C

3F7I and Pyridine1-C

3F7I

According to previous work using I(2)CARS halogenbond between pyridine and perfluoroalkyl iodides in theliquid phase binary system led the ring breathing vibrationfrequency of pyridine blue shift by 78 76 and 97 cmminus1 for1-C4F9I 1-C

6F13I and iso-C

3F7I respectively [44] Using

FTIR the present work showed similar impact of halogenbond between pyridine and iso-C

3F7I1-C3F7I in the solution

phase The ring breathing vibration of pyridine was foundto be blue shifted on the formation of halogen bondingcomplex with iso-C

3F7I and 1-C

3F7I According to Table 1

the blue shift of 9plusmn1 cmminus1 observed for pyridineiso-C3F7I in

acetonitrile (04M) is comparable with the shift observed forthe same donorreceiver in the binary system The differencein the magnitude of blue shift between liquid binary systemand the solution phase is caused by the solvent rather thanthe concentration of the solute As shown in Table 1 for thesame solvent the ring breathing vibration of pyridine showsgreater blue shift for the pyridineiso-C

3F7I system than that

for the pyridine1-C3F7I system This indicates that pyridine

forms stronger halogen bond with iso-C3F7I than with

1-C3F7I regardless of the solvent used This is also consistent

with the 19F NMR results by Metrangolo et al [49 50]using perfluoroalkyl iodide involved in a halogen bondingsystem where secondary iodide invariably was found to givelarger change in the F chemical shift than a primary iodideIt was pointed out [57] that perfluorinated chains werestronger electron-withdrawing residues and more stericallydemanding than fluorine atoms Consequently the carbonconnected to iodine in iso-C

3F7I is more electropositive

than the carbon connected to iodine in 1-C3F7I The same

principle can be applied to the liquid phase binary systemdescribed by Fan et al [44]

Table 1 The magnitudes of the frequency blue shift in the ringbreathing vibration of pyridine on the formation of halogen bondingwith iso-C3F7I1-C3F7I in various solvents (the concentrations ofpyridine and perfluoroalkyl iodide are 02M in all the solutions)

Solvent Iso-C3F7I 1-C3F7ICyclohexane 8 plusmn 1 cmminus1 6 plusmn 1 cmminus1Hexane 8 plusmn 1 cmminus1 6 plusmn 1 cmminus1Chloroform 5 plusmn 1 cmminus1 4 plusmn 1 cmminus1Acetone 9 plusmn 1 cmminus1 7 plusmn 1 cmminus1Acetonitrile 9 plusmn 1 cmminus1 6 plusmn 1 cmminus1

For the same system the magnitude of the frequencyblue shift depended on the solvents used The magnitude ofshifts seems to be associated with the polarity of the solventas well as the potential competitive intermolecular interac-tions between solvent molecules and the halogen bondingprecursors (Table 1) Chloroform acetone and acetonitrilecan all provide an electron lone pairs for the potential halogenbond with perfluoroalkyl iodides On one hand chloroformhas been observed to form hydrogen bonds with pyridine[42] Moreover chloroform can potentially form hydrogenbonds with the fluorine in perfluoroalkyl iodide [58] Thecombination of these effects may explain the smallest blueshift of ring breathing vibration of pyridine in chloroformamong all the solvents used Except for chloroform thering breathing vibration of pyridine in its halogen bondingcomplexes with perfluoroalkyl iodides shows larger blueshifts in polar solvents indicating that the halogen bondingcomplexes between pyridine and perfluoroalkyl iodides canbe stabilized by the polar solvents

32 Impact of Halogen Bonding on Iso-C3F7I The halogen

bond between pyridine and a perfluoroalkyl iodide not onlyleads to the vibration frequency shift in pyridinemolecule butalso affects the vibrations of the perfluorocompounds Previ-ous research [43] indicated that the C-I stretching and C

120572-F

stretching vibration frequency experienced a red shift while aperfluoroalkyl iodide is engaged in halogen bonding Whenthe iso-C

3F7I was mixed with pyridine solution in cyclohex-

ane the infrared spectra were recorded Figure 1 shows that incyclohexane solution the vibration of iso-C

3F7I at 1113 cmminus1

and 1168 cmminus1 were both red shifted on the formation ofhalogen bond with pyridine Based upon quantum mechan-ical frequency calculations we assigned the vibration of iso-C3F7I observed at 1113 cmminus1 to the C

120572-F stretching involved

vibration (predicted to come sim1106 cmminus1) The vibration at1168 cmminus1 was assigned to the C

120573-F stretching involved vibra-

tion (predicted to come at 1165 cmminus1) Calculations (DFTand MP

2) performed by Valerio et al indicated that the

halogen bond between NH3and CF

3I led to the elongation

of C-F bondThis was attributed to charge-transfer involvingantibonding LUMO orbitals in the CF

3I molecule [57] This

explains why the frequency of C120572-F stretching exhibited

some red shift when the iodine in iso-C3F7I is engaged in

the halogen bond with pyridine The frequency red shiftobserved the for C

120573-F involving stretching indicated that

the fluorine connected to C120573might play certain roles in the

4 International Journal of Spectroscopy

0

1

2

1080 1105 1130 1155 1180

Abso

rban

ce (a

u)

Frequency (cmminus1)

Xpy Xpfc = 1 1

Xpy Xpfc = 1 2

Xpy Xpfc = 2 1

PyridineIso-C3F7I

Figure 1 FTIR absorbance of C120572-F stretching involved vibration in

iso-C3F7I upon the formation of halogen bonding with pyridine in

cyclohexane (Xpy and Xpfc denote the molar fractions of pyridineand that of iso-C

3F7I)

halogen bonding complexes Similar shift patterns about C120572-

F were observed for the systems acetoneiso-C3F7I triethyl-

amineiso-C3F7I and acetonitrileiso-C

3F7I in cyclohexane

indicating that acetone acetonitrile and triethylamine canform halogen bond with iso-C

3F7I However the mixture

of chloroform and iso-C3F7I did not show the red shift in

the C120572-F stretching vibration This suggests that the halogen

bond between iso-C3F7I and CHCl

3is weaker than both

halogen bond between iso-C3F7I and iso-C

3F7I and the

hydrogen bond between iso-C3F7I and CHCl

3

While the vibration of iso-C3F7I was significantly affected

by halogen bond the FTIR spectra of various 1-C3F7I halogen

bonding systems did not show similar shifts We suggest thatthe vibrations of 1-C

3F7I are not as sensitive as those of iso-

C3F7I to halogen bond formation as discussed below

33 Dilution of Iso-C3F7I with Various Solvents The effect

of halogen bonding on the C120572-F stretching involved vibra-

tion of iso-C3F7I does not provide direct evidence for the

intermolecular self-halogen bonding among the moleculesof iso-C

3F7I Therefore the dilution experiments with some

halogen bonding inactive solvents were performedThe intermolecular self-halogen bond between two iso-

C3F7Imolecules can be expressedwith the following equation

(1) where one molecule uses its iodine atom and the otheruses its alpha fluorine atom to form the halogen bond

iso-C3F7I + iso-C

3F7I 999445999468 iso-C

3F7I sdot sdot sdot iso-C

3F7I (1)

The dilution test will cause the equilibrium above to shiftto left that is the amount of iso-C

3F7I engaged in the self-

halogen bond will decrease and the amount disengaged fromthe self-halogen bond will increase upon dilution The dilu-tion test was performed first with cyclohexane and the resultswere summarized in Figure 2 According to the geometryoptimization calculations for iso-C

3F7I in vacuum (DFT3-

21GB3LYPCalcAll) the vibration absorption at sim900 cmminus1

0

002

004

006

008

880 890 900 910 920

Abso

rban

ce (a

u)

Frequency (cmminus1)02M01M008M

006MIso-C3F7I

Figure 2 FTIR absorbance of C120572-F bending involved vibration in

iso-C3F7I upon dilution with cyclohexane

shown in Figure 2 is a C120572-F bending involved vibration This

peak is very broad in pure iso-C3F7I resolving into two

components as the dilution was carried out Moreover thecomponent at higher frequency becomes more intense as thedilution proceeds A theoretical calculation was performedfor a dimer of iso-C

3F7I in vacuum The results showed that

the C120572-F bending involved vibration at sim900 cmminus1 split into

two (89995 cmminus1 and 90508 cmminus1) The one at the lowerfrequency corresponds to the C

120572-F engaged in the halogen

bonding and the one at the higher frequency is correlatedwith the C

120572-F free of the halogen bonding This is perfectly

consistent with the observation in Figure 2 As the dilutionproceeds the amount of nonhalogen bonded C

120572-F increases

and leads to a relative intensity increase of the component atthe higher frequency

Figure 3 shows dilution of iso-C3F7I in various solvents

Both acetone and acetonitrile form halogen bonds withiso-C3F7I consequently the spectra in these solvents show

primarily isolated fluorocarbon with little or no indicationof self-halogen bond Hexane on the other hand does nothave a significant influence on the equilibrium between theself-halogen bonded complex and the isolated iso-C

3F7I

As a result the dilution with hexane exhibited mainly theC120572-F bending vibration in an aggregation of iso-C

3F7I In

chloroform the C120572-F bending involved vibration peaked

at the same position with that in hexane This suggeststhat chloroform formed a hydrogen bond with the alphafluorine in iso-C

3F7I The hydrogen bond between iso-C

3F7I

and chloroform is supported by the geometry optimizationcalculation for the iso-C

3F7I-chloroform system in vacuum

The vibrational spectra of 1-C3F7I were not only insen-

sitive to the halogen bond between 1-C3F7I and various

halogen bonding acceptors they were also insensitive to thedilution tests Theoretical calculation suggests that the self-halogen bond in vacuum between two 1-C

3F7I molecules

International Journal of Spectroscopy 5

0

002

004

006

008

885 895 905 915

ChloroformHexaneCyclohexane

AcetoneAcetonitrile

Abso

rban

ce (a

u)

Frequency (cmminus1)

Figure 3 FTIR absorbance of C120572-F bending involved vibration in

iso-C3F7I upon dilution with various solvents (the concentrations

of iso-C3F7I in various solvents are 006M)

(368 kcalmol) is weaker than that between two iso-C3F7I

molecules (486 kcalmol) In addition while there is onlyone alpha fluorine in iso-C

3F7I there are two in 1-C

3F7I We

suggest that this further decreases any impact that the weakerhalogen bond might have on the C-F

120572bending frequency

rendering it undetectable in the FTIR spectrum at the presentresolution

34 19F NMRDilution Experiments The 19F NMR spectra ofboth iso-C

3F7I and 1-C

3F7I were collected in various solvents

A sealed capillary tube filled with KFD2O solution and

placed in the NMR sample tube was used as an internalreference for each measurement The 19F NMR spectrumof iso-C

3F7I is straightforward While a triplet at minus79 ppm

represents the resonance of fluorine in the CF3group the

pentet at minus153 ppm is the resonance of the alpha fluorineTwo triplets atminus83 ppmandminus122 ppmand nonets atminus67 ppmwere identified in the 19F NMR spectrum of 1-C

3F7I The

assignment of this spectrum is rather tricky Generally thecoupling constant 119869

119894119895decreases dramatically as the number

of chemical bonds separating nuclei 119894 and 119895 increases In thecase of 1-C

3F7I however the alpha fluorine shows a strongest

coupling with the gamma fluorine in CF3group [59] When

combinedwith the coupling of the beta fluorine in CF2group

the resulting nonet was assigned as the alpha fluorine (thefluorine germinal to iodine) resonance On the other handthe coupling constant 119869

120573120572is much larger than the coupling

constant 119869120573120574 Thus the beta fluorine appears as triplet at

minus122 ppm [59]The dilution of iso-C

3F7I and 1-C

3F7I was performed in

hexane cyclohexane chloroform acetone and acetonitrileThe 19F NMR spectra were collected over the concentrationrange of 004M to 4M The plot of Δ120575 = 120575solution minus120575pure iso-C3F7I1-C3F7I versus the concentration in hexanecyclohexane chloroform acetone and acetonitrile is shown

in Figures 4 5 6 7 and 8 respectively In allvspace04ptthesolvents the chemical shift of beta and gamma fluorineshowed a consistent upfield shift of NMR frequency upondilution In the dilution with hexane the chemical shift of allalpha beta and gamma fluorine displayed a similar trend Inthe dilutions with other solvents the change in the chemicalshift of alpha fluorine(s) exhibited a different trend from thatof beta and gamma fluorine While the upfield shift of betaand gamma fluorine upon dilution can be attributed to theimpact of screening constant 120590

0 determined mainly by the

regular solvent effect the trend in the chemical shift of alphafluorine indicates that the local diamagnetic screen constant120590119889is responsible for the shielding parameter change of the

alpha fluorine upon dilutionThe local diamagnetic screen constant 120590

119889depends on the

electronic structure in the immediate vicinity of the fluorineatom so the chemical shift in the 19F NMR signal shouldexperience the greatest change if the fluorine atom directlyparticipates in the halogen bond or is next to the halogenbonding donor iodine Previous work on 14NNMR indicatedthat the donation of the electron lone pair by nitrogenin the halogen bond relationship led to more shielding ofnitrogen [49] Similarly the alpha F when involved in self-halogen bond as an electron lone pair donor will be subjectto more shielding This is consistent with the results fromtheoretical calculations (NMR calculations for a dimer invacuum based on the optimized geometry) which showedthat the fluorine engaged in halogen bonding ismore shieldedthan the fluorine free of halogen bonding The difference inthe shielding parameters between the F involved in halogenbond and the F free of halogen bond is similar for iso-C

3F7I

and 1-C3F7I This also explains the similar results observed

in 19F NMR dilution tests between iso-C3F7I and 1-C

3F7I

The unified dilution trends among fluorine chemical shiftsin the dilution with hexane indicated that dispersion forceswere insufficient to break down the self-halogen bondedcomplex On the other hand cyclohexane provides strongerdispersion forces because the ring shape allows for a largerarea of contact Thus the dilution of both 1-C

3F7I and iso-

C3F7I using cyclohexane leads to the dissociation of self-

halogen bonded complex As the dilution proceeded morealpha fluorine disengaged the halogen bonding and becameless shielding leading to a downfield shift of the NMRfrequency This downfield shift because of the dissociationof self-halogen bonding complex counteracted the up-fieldshift from the general dilution effect As a result the Δ120575(Δ120575 = 120575solution minus 120575pure 1-C3F7I or Δ120575 = 120575solution minus 120575pure iso-C3F7I)values for the alpha fluorine in both 1-C3F7I and iso-C3F7Iare smaller than those of beta and gamma fluorine when theyare diluted with cyclohexane

As for the dilution with chloroform it was proposedearlier and by theoretical calculation that chloroform formedhydrogen bondwith alpha fluorine in iso-C

3F7I Additionally

our calculations showed that hydrogen bonds can also beformed between the alpha fluorine in 1-C

3F7I and chloro-

formMoreover formation of hydrogen bondmade the alphafluorine less shielding causing a downfield shift of the NMR

6 International Journal of Spectroscopy

0

04

08

12

16

0 2 4 6

CF2ICF3CF2

Concentration (M)

HexaneΔ120575

(ppm

)

(a)

0

04

08

12

16

0 2 4 6

CFICF3

Concentration (M)

Δ120575

(ppm

)(b)

Figure 4 Change in the chemical shift of 19F in the perfluoroalkyl iodides upon dilution with hexane (from Figure 4 to Figure 8 the upperpanel is for 1-C

3F7I the lower panel is for iso-C

3F7I Δ120575 = 120575solution minus 120575pure1-C3F7I for 1-C3F7I and for iso-C

3F7I Δ120575 = 120575solution minus 120575pure iso-C3F7I)

0

04

08

12

16

0 2 6Concentration (M)

4

Cyclohexane

Δ120575

(ppm

)

CF2ICF3CF2

(a)

0

04

08

12

16

0 2 4 6Concentration (M)

Δ120575

(ppm

)

CFICF3

(b)

Figure 5 Change in the chemical shift of 19F in the perfluoroalkyl iodides upon dilution with cyclohexane

frequency The same effect is brought by the dissociation ofself-halogen bonded complex As a result the 19F NMR dilu-tion tests in chloroform showed some similar alpha fluorinetrend to the ones in cyclohexane In the 19F NMR study of21015840-F-21015840-deoxyarabinoflavoproteins [60] the chemical shift offluorine moves to a higher frequency when the fluorine atomdonated its electron lone pair in hydrogen bond

While the dilution was performed using acetone andacetonitrile the solvent molecules are not only able todissociate the self-halogen boned complex but also to formhalogen bond with the iodine in both iso-C

3F7I and 1-C

3F7I

Both of these effects make the alpha fluorine less shieldingand a larger downfield shift of theNMR frequency As a resultthe Δ120575 value became negative upon dilution

International Journal of Spectroscopy 7

0

05

1

15

2

0 2 4 6Concentration (M)

Chloroform

CF2ICF3CF2

minus05

Δ120575

(ppm

)

(a)

0

05

1

15

2

25

0 2 4 6Concentration (M)

CFICF3

Δ120575

(ppm

)

(b)

Figure 6 Change in the chemical shift of 19F in the perfluoroalkyl iodides upon dilution with chloroform

CF2ICF3CF2

0

2

4

0 2 4 6

Acetone

Concentration (M)

minus6

minus4

minus2Δ120575

(ppm

)

(a)

CFICF3

0

2

4

0 2 4 6Concentration (M)

minus6

minus4

minus2Δ120575

(ppm

)

(b)

Figure 7 Change in the chemical shift of 19F in the perfluoroalkyl iodides upon dilution with acetone

35 19F NMR Titration and the Halogen Bonding AssociationConstants for PyridineIso-C

3F7I(1-C3F7I) in Various Solvents

19FNMR titration experiments were performed to determinethe equilibrium constants for the halogen bonding complexformed between pyridine and perfluoroalkyl iodides in vari-ous solvents (Table 2)These data indicate that the associationbetween pyridine and perfluoroalkyl iodides is more favor-able in nonpolar solvents such as hexane and cyclohexaneAccording to Table 1 the halogen bond formed between pyri-dine and perfluoroalkyl iodides caused the largest blue shiftof pyridine ring breathing vibration in acetone However thehalogen bonding association constants obtained in acetone

and acetonitrile are among the lowest values In Table 1the magnitudes of the frequency shift in the ring breathingvibration of pyridine are greater in polar solvents than thosein nonpolar solvents This indicates that the enthalpy changefor the halogen bonding formation is more negative in polarsolvents than in non-polar solvents On the other hand inTable 2 the halogen bonding association constants are greaterin nonpolar solvents than those in polar solvents In otherwords the changes in Gibbrsquos free energy are more negativein nonpolar solvents than those in polar solvents The onlyway to rationalize this observation is to consider the contri-bution of entropy change in the formation of halogen bonds

8 International Journal of Spectroscopy

CF2ICF3CF2

0 2 4 6Concentration (M)

Acetonitrile

0

2

4

minus4

minus2

Δ120575

(ppm

)

(a)

CFICF3

0

2

4

0 2 4 6Concentration (M)

minus4

minus2

Δ120575

(ppm

)(b)

Figure 8 Change in the chemical shift of 19F in the perfluoroalkyl iodides upon dilution with acetonitrile

The overall entropy change in the formation of halogen bondbetween pyridine and perfluoroalkyl iodides in differentsolvents depends on whether the molecules of perfluoroalkyliodides stay as an aggregation through self-halogen bondor isolated through hydrogenhalogen bonding with solventmolecules In the non-polar solvents like cyclohexane andhexane where perfluoroalkyl iodides mostly stay as anaggregation the formation of halogen bond between pyridineand perfluoroalkyl iodides results in a significant increasein entropy In polar solvents like chloroform acetone andacetonitrile where both perfluoroalkyl iodides and pyridinehave tendency to form halogen or hydrogen bond withsolvent molecules the formation of halogen bond betweenpyridine and perfluoroalkyl iodides does not necessarilylead to an increase in entropy and sometimes can evenlead to an entropy decrease After all in the nonpolarsolvents the contribution from the positive entropy changecompensates or even overrides the less negative enthalpychange in the formation of halogen bond between pyridineand perfluorocompound Thus the overall larger halogenbonding association constants were observed in the nonpolarsolvents than in the polar solvents

4 Conclusion

The combination of FTIR and 19F NMR indicated thatintermolecular self-halogen bond among the molecules ofiso-C3F7I and 1-C

3F7I is present in several solvents The

halogen bond between pyridine and perfluoroalkyl iodidenot only affects some of the vibrational frequencies ofpyridine but also impacts the vibrational frequencies of iso-C3F7I As an electron lone pair donor the alpha fluorine in

perfluoroalkyl iodides is more shielded when it is engagedin the halogen bond but less shielded when it is engaged

Table 2 Halogen-bonding association constants for pyridineiso-C3F7I and pyridine1-C3F7I systems in various solvents

Solvent Iso-C3F7I 1-C3F7ICyclohexane 215 plusmn 54 102 plusmn 20

Hexane 130 plusmn 27 84 plusmn 17

Chloroform 38 plusmn 09 26 plusmn 07

Acetone 31 plusmn 08 19 plusmn 06

Acetonitrile 36 plusmn 08 24 plusmn 06

in hydrogen bonding The halogen bonded complexes ofpyridine and perfluoroalkyl iodides though stronger in polarsolvents showed significantly lower association constants inacetone and acetonitrile than in non-polar solvents suchas hexane and cyclohexane The entropy change makes afavorite contribution to the halogen bond between pyridineand perfluoroalkyl iodides in the non-polar solvent such ashexane and cyclohexane

Acknowledgments

This material is based in part upon work supported bythe National Science Foundation under Grant no 0648964Any opinions findings and conclusions or recommendationsexpressed in this material are those of the author(s) anddo not necessarily reflect the views of the National ScienceFoundation

References

[1] F Guthrie ldquoOn the iodide of iodammoniumrdquo Journal of theChemical Society vol 16 pp 239ndash244 1863

International Journal of Spectroscopy 9

[2] R S Mulliken ldquoStructures of complexes formed by halogenmolecules with aromatic andwith oxygenated solventsrdquo Journalof the American Chemical Society vol 72 no 1 pp 600ndash6081950

[3] R S Mulliken ldquoMolecular compounds and their spectra IIrdquoJournal of the American Chemical Society vol 74 no 3 pp 811ndash824 1952

[4] R S Mulliken ldquoMolecular compounds and their spectra IIIThe interaction of electron donors and acceptorsrdquo The Journalof Physical Chemistry vol 56 no 7 pp 801ndash822 1952

[5] O Hassel J Hvoslef E Vihovde V Hadler and N A SorensenldquoThe structure of bromine 14-dioxanaterdquo Acta Chemica Scan-dinavica vol 8 p 873 1954

[6] O Hassel ldquoStructural aspects of interatomic charge-transferbondingrdquo Science vol 170 no 3957 pp 497ndash502 1970

[7] P Metrangolo and G Resnati Halogen Bonding Fundamentalsand Applications Springer Berlin Germany 2008

[8] P Metrangolo H Neukirch T Pilati and G Resnati ldquoHalogenbonding based recognition processes a world parallel to hydro-gen bondingrdquo Accounts of Chemical Research vol 38 no 5 pp386ndash395 2005

[9] P Metrangolo Y Carcenac M Lahtinen et al ldquoNonporousorganic solids capable of dynamically resolving mixtures ofdiiodoperfluoroalkanesrdquo Science vol 323 no 5920 pp 1461ndash1464 2009

[10] A GOrpen D Braga J SMiller G R Desiraju and S L PriceldquoInnovation in crystal engineeringrdquo CrystEngComm vol 4 no83 pp 500ndash509 2002

[11] G M J Schmidt ldquoPhotodimerization in the solid staterdquo Pureand Applied Chemistry vol 27 no 4 pp 647ndash678 1971

[12] P Metrangolo G Resnati T Pilati R Liantonio and F MeyerldquoEngineering functionalmaterials by halogen bondingrdquo Journalof Polymer Science A vol 45 no 1 pp 1ndash15 2007

[13] H L Nguyen M B Hursthouse A C Legon D W Bruce andP N Horton ldquoHalogen bonding a new interaction for liquidcrystal formationrdquo Journal of the American Chemical Societyvol 126 no 1 pp 16ndash17 2004

[14] P Metrangolo and G Resnati ldquoHalogen bonding a paradigmin supramolecular chemistryrdquoChemistry vol 7 no 12 pp 2511ndash2519 2001

[15] D W Bruce ldquoThe materials chemistry of alkoxystilbazoles andtheir metal complexesrdquo Advances in Inorganic Chemistry vol52 pp 151ndash159 2001

[16] A Sun J W Lauher and N S Goroff ldquoPreparation ofpoly(diiododiacetylene) an ordered conjugated polymer ofcarbon and iodinerdquo Science vol 312 no 5776 pp 1030ndash10342006

[17] C Wilhelm S A Boyd S Chawda et al ldquoPressure-inducedpolymerization of diiodobutadiyne in assembled cocrystalsrdquoJournal of the American Chemical Society vol 130 no 13 pp4415ndash4420 2008

[18] F C Pigge V R Vangala P P Kapadia D C Swenson andN P Rath ldquoHexagonal crystalline inclusion complexes of 4-iodophenoxy trimesoaterdquo Chemical Communications no 39pp 4726ndash4728 2008

[19] F Meyer and P Dubois ldquoHalogen bonding at work recentapplications in synthetic chemistry and materials sciencerdquoCrystEngComm vol 15 no 16 pp 3058ndash3071 2013

[20] T M Beale M G Chudzinski M G Sarwar and M STaylor ldquoHalogen bonding in solution thermodynamics andapplicationsrdquo Chemical Society Reviews vol 42 no 4 pp 1667ndash1680 2013

[21] M Erdelyi ldquoHalogen bonding in solutionrdquo Chemical SocietyReviews vol 41 no 9 pp 3547ndash3557 2012

[22] A C Legon ldquoThe halogen bond an interim perspectiverdquoPhysical Chemistry Chemical Physics vol 12 no 28 pp 7736ndash7747 2010

[23] A C Legon ldquoPrereactive complexes of dihalogens XY withLewis bases B in the gas phase a systematic case for the halogensnalogue Bsdot sdot sdotXY of the hydrogen bond Bsdot sdot sdotHXrdquo AngewandteChemie vol 38 no 18 pp 2686ndash2714 1999

[24] R N Hazeldine ldquoReactions of fluorocarbon radicals Part XIIThe synthesis of fluorocarbons and of fully fluorinated iodo-bromo- and chloroalkanessrdquo Journal of the Chemical Societyvol 75 pp 3761ndash3768 1953

[25] N F Cheetham and A D E Pullin ldquoInteraction betweentertiary amines and perfluoro-organo-halidesrdquo Chemical Com-munications no 18 pp 418ndash421 1965

[26] N F Cheetham and A D E Pullin ldquoA gas-phase donor-acceptor complexrdquo Chemical Communications no 5 pp 233ndash234 1967

[27] N F Cheetham and A D E Pullin ldquoDonor-acceptor com-plexes formed by perfluoro-organo bromides and iodides withnitrogenous andother bases I Vapour pressuremeasurementsrdquoAustralian Journal of Chemistry vol 24 pp 479ndash487 1971

[28] A Misha and A D E Pullin ldquoDonor-acceptor complexesformed by perfluoro-organo bromides and iodides withnitrogenous and other bases II Band shapes and widths inthe absorption spectrumof gaseous CF

3I-N(CH

3)3rdquoAustralian

Journal of Chemistry vol 24 no 12 pp 2493ndash2507 1971[29] N F Cheetham I J McNaught and A D E Pillin ldquoDonor-

acceptor complexes formed by perfluoro-organo bromides andiodides with nitrogenous and other bases III Qualitativeexamination of condensed phase spectra of CF

3I and CF

3Br

and of their complexes with trimethylamine and other basesrdquoAustralian Journal of Chemistry vol 27 no 5 pp 973ndash985 1974

[30] N F Cheetham I J McNaught and A D E Pullin ldquoDonor-acceptor complexes formed by perfluoro-organo bromides andiodides with nitrogenous and other bases IV Analysis of theinfrared spectra of CF

3IN(CH

3)3and CF

3BrN(CH

3)3and

related complexesrdquo Australian Journal of Chemistry vol 27 no5 pp 987ndash1007 1974

[31] J McNaught and A D E Pullin ldquoDonor-acceptor com-plexes formed by perfluoro-organo bromides and iodides wlthnitrogenous and other bases V Comparison of liquid phasecomplexes of CFI

3 C2F5I or C

3F7I with Nme

3 NEt3or NPr

3

infrared far-infrared and NMR spectrardquo Australian Journalof Chemistry vol 27 no 5 pp 1009ndash1015 1974

[32] A C Legon D J Millen and S C Rogers ldquoMicrowavespectrum of a gas-phase charge-transfer complexrdquo ChemicalCommunications no 14 pp 580ndash581 1975

[33] A L Allred and D W Larsen ldquoHalogen complexes III Theassociation of 246-trimethylpyridine and trifluoroiodometh-anerdquoThe Journal of Physical Chemistry vol 69 no 7 pp 2400ndash2401 1965

[34] P Auffinger F A Hays E Westhof and P S Ho ldquoHalogenbonds in biological moleculesrdquo Proceedings of the NationalAcademy of Sciences of the United States of America vol 101 no48 pp 16789ndash16794 2004

[35] L K Steinrauf J A Hamilton B C Braden J R Murrelland M D Benson ldquoX-ray crystal structure of the Ala-109 rarrThr variant of human transthyretin which produces euthyroidhyperthyroxinemiardquo The Journal of Biological Chemistry vol268 no 4 pp 2425ndash2430 1993

10 International Journal of Spectroscopy

[36] E I Howard R Sanishvili R E Cachau et al ldquoUltrahighresolution drug design I details of interactions in human aldosereductase-inhibitor complex at 066 Ardquo Proteins vol 55 no 4pp 792ndash804 2004

[37] C Yabe-Nishimura ldquoAldose reductase in glucose toxicity apotential target for the prevention of diabetic complicationsrdquoPharmacological Reviews vol 50 no 1 pp 21ndash34 1998

[38] F A Hays J M Vargason and P S Ho ldquoEffect of sequence onthe conformation ofDNAholliday junctionsrdquoBiochemistry vol42 no 32 pp 9586ndash9597 2003

[39] D A Kraut M J Churchill P E Dawson and D HerschlagldquoEvaluating the potential for halogen bonding in the oxyanionhole of ketosteroid isomerase using unnatural amino acidmutagenesisrdquo ACS Chemical Biology vol 4 no 4 pp 269ndash2732009

[40] T Shirman T Arad and M E van der Boom ldquoHalogenbonding a supramolecular entry for assembling nanoparticlesrdquoAngewandte Chemie vol 49 no 5 pp 926ndash929 2010

[41] E R Berg D D Green D C Moliva A B T Bjerke M WGealy and D J Ulness ldquoIon-pair interaction in pyridiniumcarboxylate solutionsrdquoThe Journal of Physical Chemistry A vol112 no 5 pp 833ndash838 2008

[42] H Fan D A Moliva J K Elison et al ldquoEffects of hydrogenbonding on the ring breathing mode of pyridine in pyri-dinechloroform and pyridinebromoform systemsrdquo ChemicalPhysics Letters vol 479 no 1ndash3 pp 43ndash46 2009

[43] M T Messina P Metrangolo W Navarrini S Radice GResnati and G Zerbi ldquoInfrared and Raman analyses ofthe halogen-bonded non-covalent adducts formed by 120572120596-diiodoperfluoroalkanes with DABCO and other electrondonorsrdquo Journal of Molecular Structure vol 524 no 1ndash3 pp 87ndash94 2000

[44] H Fan J K Eliason C D Moliva et al ldquoHalogen bondingin iodo-perfluoroalkanepyridine mixturesrdquo The Journal ofPhysical Chemistry A vol 113 no 51 pp 14052ndash14059 2009

[45] D Hauchecorne N Nagels B J van der Veken and W AHerrebout ldquoC-Xsdot sdot sdot 120587 halogen and C-Hsdot sdot sdot 120587 hydrogen bondinginteractions of CF

3X (X=Cl Br I or H) with ethene and

propenerdquo Physical Chemistry Chemical Physics vol 14 no 2 pp681ndash690 2012

[46] D Hauchecorne B J van der Veken A Moiana and W AHerrebout ldquoThe C-Clsdot sdot sdotN halogen bond the weaker relativeof the C-I and C-Brsdot sdot sdotN halogen bonds finally characterizedin solutionrdquo Chemical Physics vol 374 no 1ndash3 pp 30ndash36 2010

[47] D Hauchecorne A Moiana B J van der Veken and W AHerrebout ldquoHalogen bonding to a divalent sulfur atom anexperimental study of the interactions of CF

3X (X=Cl Br I)

with dimethyl sulfiderdquoPhysical Chemistry Chemical Physics vol13 no 21 pp 10204ndash10213 2011

[48] D Hauchecorne R Szostak W A Herrebout and B J vander Veken ldquoC-Xsdot sdot sdotO halogen bonding interactions of trifluo-romethyl halides with dimethyl etherrdquo ChemPhysChem vol 10no 12 pp 2105ndash2115 2009

[49] M T Messina P Metrangolo W Panzeri E Ragg and GResnati ldquoPerfluorocarbon-hydrocarbon self-assembly Part 3Liquid phase interactions between perfluoroalkylhalides andheteroactom containing hydrocarbonsrdquo Tetrahedron Lettersvol 39 no 49 pp 9069ndash9072 1998

[50] C Cavallotti P Metrangolo F Meyer F Recupero and GResnati ldquoBinding energies and 19F nuclear magnetic deshield-ing in paramagnetic halogen-bonded complexes of TEMPO

with haloperfluorocarbonsrdquo The Journal of Physical ChemistryA vol 112 no 40 pp 9911ndash9918 2008

[51] D Hauchecorne B J van der Veken W A Herrebout and PE Hansen ldquoA 19F NMR study of C-Isdot sdot sdot 120587 halogen bondingrdquoChemical Physics vol 381 no 1ndash3 pp 5ndash10 2011

[52] M G Sarwar B Dragisic L J Salsberg C Gouliaras and MS Taylor ldquoThermodynamics of halogen bonding in solutionsubstituent structural and solvent effectsrdquo Journal of theAmerican Chemical Society vol 132 no 5 pp 1646ndash1653 2010

[53] C Laurence M Queignec-Cabanetos T Dziembowska RQueignec and B Wojtkowiak ldquo1-iodoacetylenes 1 Spectro-scopic evidence of their complexes with Lewis bases A spectro-scopic scale of soft basicityrdquo Journal of the American ChemicalSociety vol 103 no 10 pp 2567ndash2573 1981

[54] M J Frisch G W Trucks H B Schlegel et al Gaussian 03Revision E01 Gaussian Wallingford Conn USA 2004

[55] M J Frisch G W Trucks H B Schlegel et al Gaussian 09Revision A1 Gaussian Wallingford Conn USA 2009

[56] K A Peterson D Figgen E Goll H Stoll and M DolgldquoSystematically convergent basis sets with relativistic pseu-dopotentials II Small-core pseudopotentials and correlationconsistent basis sets for the post-d group 16ndash18 elementsrdquoJournal of Chemical Physics vol 119 no 21 pp 11113ndash11123 2003

[57] G Valerio G Raos S V Meille P Metrangolo and G ResnatildquoHalogen bonding in fluoroalkylhalides a quantum chemicalstudy of increasing fluorine substitutionrdquoThe Journal of PhysicalChemistry A vol 104 no 8 pp 1617ndash1620 2000

[58] J A K Howard V J Hoy D OrsquoHagan and G T Smith ldquoHowgood is fluorine as a hydrogen bond acceptorrdquoTetrahedron vol52 no 38 pp 12613ndash12622 1996

[59] E Pitcher A D Buckingham and F G A Stone ldquoSpectroscopicstudies on organometallic compounds V Fluorine nuclearmagnetic resonance spectra of some perfluoroalkyl and perflu-oroacylmetal compoundsrdquoThe Journal of Chemical Physics vol36 no 1 pp 124ndash129 1962

[60] Y V S Murthy and V Massey ldquo19F NMR studies with 21015840-F-21015840-deoxyarabinoflavoproteinsrdquoThe Journal of Biological Chem-istry vol 271 no 33 pp 19915ndash19921 1996

Submit your manuscripts athttpwwwhindawicom

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

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