Chem 125 Lecture 1410/6/08

This material is for the exclusive use of Chem 125 students at Yale and may

not be copied or distributed further.

It is not readily understood without

reference to notes from the lecture.

Hybridization Reality Check:

Structure and Dynamics of

XH3BH3 CH3 NH3

valence electrons of X3 4 5

sp1

sp3

There should be a relationship between

Hybridization and Structure

angle spm-spn = cos-1(mn)

1

m n angle

1 1

2 2

3 3

0

*

125.3°

* to avoid net overlap between different e-pairs (Pauli Principle)

1 3 125.3°

? 180° linear

120° trigonal

109.5°tetrahedral

90°

The three X-H Bonds say,“Use 3 sp2 to maximize overlap”

How to Optimize Hybridization

of 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-)3 p2 s

(One X-electron in each of 3 bonding AOs; remainder in the 4th AO)

3 sp2

vacant pWhatever

(all sets of four 1-electron valence AOs use same amount of 2s)

How to Optimize Hybridization

of 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 X3 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 and shifts s-character

to lone electron (not wasted).

Distortion from plane weakens

bonds but shifts s-character from single electrons to 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

H H

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

HH

H

• • •

1141 cm-1606 cm-1

B

HH

H

B

HH

H

B

HH

H

Strong Planar Preference

3.26 kcal/mole1.73 kcal/moleH HAmount of deformation

Stiffer “Spring”

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 energy

adjusted to giveproper energies

510-14 sec0.002 (kcal)

0 & 1 node

2 & 3 nodes

N

HH

H

N

HH

H

N

HH

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

electronspends 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 more

s-character for SOMO electron)

C

HH

H

C

HH

H

C

HH

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

Computer Chem 125Student

Minimize 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

Validationby

Experiment& Computer

Useful Predictionsof Properties

Validationby Experiment

Structure Total e-Density (X-Ray)

Energies (IR)Nuclear e-Density (ESR)Dipole Moment, etc.

End of Lecture 14Oct. 6, 2008