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

clear explanations.

Experiment: Pathological Bonding

MissingBond ?

BentBonds ?

BestOverlapPossiblefor 60°

C-C-C

Very PoorOverlap

>90°?

psp4.1sp1.4

Because sp4.1extendsto give best overlap

Why not p orbitals (90°)?

Rehybridizing to strengthen this

bond would weaken six others.

Three Views of BH3

2) Molecular Orbitals

1) Total Electron Density

3) Bonds from Hybrid AOs

(Nature)

(Computer)

(Student)Cf. Course Webpage

B

H

HH

ElectronCloud of

by "Spartan"

BH3

Total e-Density0.30 e/Å3

Mostly1s Core

of Boron

B

H

HH

BH3

Total e-Density0.15 e/Å3

BH3

Total e-Density (0.05 e/Å3)

Dimple

H atoms take e-densityfrom valence orbitals of B

BH+••H •

B•

BH3

Total e-Density0.02 e/Å3

BH3

Total e-Density0.002 e/Å3

van der Waalssurface

(definition)

BH3

Total e-Density0.002 e/Å3

ElectrostaticPotential

Energy of a+ probe onthe surface

low (-) high (+)

H

End of Lecture 14Oct. 4, 2010

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