Chemistry 125: Lecture 14 October 4, 2010 Checking Hybridization Theory with XH 3 Infrafred and electron spin resonance experiments with three XH 3 molecules confirm our previous theoretical discussion of bonding in terms of hybridization of the central atom. The "Umbrella Vibration" and the associated rehybridization of the central atom is used to illustrate how competition between strong bonds and stable atoms creates differences in molecular structure and discriminates between bonding models. Previous examples of “pathological” bonding involving absent and bent bonds are shown to be consistent with expectations base on maximization of bonding overlap. For copyright notice see final page of this file
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Chemistry 125: Lecture 14 October 4, 2010 Checking Hybridization Theory with XH 3 Infrafred and electron spin resonance experiments with three XH 3 molecules.
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Chemistry 125: Lecture 14 October 4, 2010
Checking Hybridization Theory with XH3
Infrafred and electron spin resonance experiments with three XH3 molecules confirm our
previous theoretical discussion of bonding in terms of hybridization of the central atom. The
"Umbrella Vibration" and the associated rehybridization of the central atom is used to
illustrate how competition between strong bonds and stable atoms creates differences in
molecular structure and discriminates between bonding models. Previous examples of
“pathological” bonding involving absent and bent bonds are shown to be consistent with
expectations base on maximization of bonding overlap.
For copyright notice see final page of this file
Hybridization Reality Check:Structure and Dynamics of
XH3
BH3 CH3 NH3
Number of valence electrons of X
3 4 5
How to Optimize Hybridizationof the X Atom in XH3 ?
The X Atom says, “O.K. make 3 bonds, but Maximize s-orbital occupancy”B (3 e-) N (5 e-)C (4 e-)
(One X-electron in each of 3 bonding AOs; remainder in the 4th AO)
B (3 e-) N (5 e-)C (4 e-)
BH3
STRONGLY prefers sp2 bonds
(=> planar)
CH3
Less Stronglyprefers sp2 bonds
(=> planar)
NH3
must compromisesp
>2 bonds(=> pyramidal)
3 sp2
1 pWhatever
The three X-H Bonds say,“Use 3 sp2 to maximize overlap”
3 sp2
vacant p3 p2 s
Hybridization Reality Check:
Structure and Dynamics of
XH3BH3 CH3 NH3
valence electrons of X
3 4 5
• • •
Competes with bonds for s-character
BH3
STRONGLY prefers sp2 bonds
(planar)
CH3
less stronglyprefers sp2 bonds
(planar)
NH3
must compromisesp
>2 bonds(pyramidal)
Are these Predictions True?
Experiment:X-Ray?
Distortion from plane weakens bonds
and deprives electrons of s-character.
Distortion from plane weakens bonds
(shifts s-character to the lone electron thus not wasting it).
Distortion from plane weakens bonds
but shifts s-character from single electrons to a pair of electrons.
2 BH3 B2H6 2 CH3 C2H6 GasIR & ESR Spectroscopy
Infrared: Out-of-Plane Bend
X
H
H H
X
H
H H
X
H
HH
X
H
HH
X
H
HH
X
H
H
H
X
H
H
H
X
H
HH
X
H
H
H
This “umbrella” vibration may be treated as a 1-dimensional “Erwin” problem with a fictious “mass” that reflects the amount of motion of the four atoms.
Infrared: Out-of-Plane Bend
Weaker Planar Preference
Hooke’s Law potential energyadjusted to giveproper energy difference
Amount of deformation
34.2 Terahertz 18.7 THz
34,210,000,000,000vibrations per sec
C
HH
H
C
HH
H
C
H H
H
• • •
1141 cm-1 606 cm-1
B
HH
H
B
HH
H
B
H H
H
Strong Planar Preference
3.26 kcal/mole 1.73 kcal/moleH HAmount of deformationStiffer “Spring”
HH
HH
B B
x106
megax109
gigax1012
tera
Two closely-spaced absorptions
Infrared: Out-of-Plane Bend
932 cm-1968 cm-1
37 cm-1
“Tunnel”Splitting:
1 cm-1
Potential Energy“Inversion” Barrier
3 kcal/mole
2 cal/mole
Ground StateTunneling~1011/sec
UmbrellaClock!
Not a Hooke’s
Law pattern
Double Minimum potential energyadjusted to giveproper energies
510-14 sec0.002 (kcal)
0 & 1 node
2 & 3 nodes
N
HH
H
N
HH
H
N
H H
H
• • ••••Lect. 9frame 9
Electron Spin Resonance Spectrummeasures s-orbital character
of the SOMO electron in CH3.•
A line separation due to magnetic interaction between the unpaired
electron and the 13C nucleus occurs only if the electron
spends time ON the nucleus, which happens only for s-orbital.
•CH3
SOMOPlanar Bent
•CH3
SOMOPlanar Bent
Structural Isotope Effect:
CH3
spendsmore time more bent
than
CD3 (thus uses mores-character for
SOMO electron)
C
HH
H
C
HH
H
C
H H
H
• • •
CH3 38 Gauss 2% s
36 Gauss less sCD3
onaverage
CF3•
Repulsion between F atoms? Less Bent (flatter) than •CH3
Since Fluorine holds the lion's share of the bonding electron pair, Carbon has less reason to use its valuable s-character in the bonding orbitals. Uses more for the SOMO.
More Bent than •CH3
OR
•CF3
SOMO
271 Gauss!
20% s
(vs. 38 for •CH3)
sp4
Tension!
Differing Goals
ComputerChem 125
StudentMinimize kinetic
plus coulomb energies of
electrons and nuclei by
“settling down”
Minimize total energy using Schrödinger
equation with “realistic” constraints
Understand structure and reactivity with
the simplest“realistic” model
ExperimentalMolecule
e.g. limited set of AOs,SCF, some correlation,
delocalized MOs
e.g. localized bonds,lone pairs; hybridization
E-match/overlapHOMO/LUMO
Qualitative Insight
Useful Predictionsof Properties
Structure Total e-Density (X-Ray)
Energies (IR)
Nuclear e-Density (ESR)
Dipole Moment, etc.
Validationby Experiment
Validationby Experiment& Computer
Perspectives:
Molecule(Reality)
Computer(Approximate Schroedinger)
Chemist(Understand Bonds)
MissingBond ?
(e.g. 32nd of 33 occupied MOs)
Cf. Lecture 7 - Dunitz et al. (1981)
Experiment: Pathological Bonding
BentBonds ?
Would a Computer’s MOsProvide Understanding?
No! Far too complicated to answer “Why?”
But analysis in terms of pairwise bonding overlapof hybrid AOs provides
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