Chem 125 Lecture 1710/10/2005

Projected material

This material is for the exclusive use of Chem 125 students at Yale and may not

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It is not readily understood without reference to notes from the lecture.

•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

CD3

36 Gauss38 Gauss

CF3•

Repulsion between F atoms? 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. Use it for the SOMO.

More Bent than CH3•

•CF3

SOMO

271 Gauss!

20% s

(vs. 38 for •CH3)

sp4

Tension!

Goals

ComputerChem 125

StudentMinimize kinetic

plus coulomb energies of

electrons and nuclei by

“settling down”

Minimize kinetic plus coulomb 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

e.g. hybridization,bonds, lone pairs,E-match/overlapHOMO/LUMO

Validationwith

Experiment& Computer

Validationwith

Experiment

Structure e-Density (X-Ray)

Energies (IR)Nuclear Density (EPR)Dipole Moment, etc.

Dunitz et al. (1981)

Experiment: Pathological Bonding

MissingBond !

BentBonds !

BestOverlapPossiblefor 60°

Very PoorOverlap

Computer:

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

BH3

Total e-Density0.002 e/Å3

ElectrostaticPotential

Energy of a+ probe onthe surface

low (-) high (+)

H

Computer PartitionsTotal e-Density

intoSymmetrical MOs

(à la Chladni)

BH3

8 low-energy AOs 8 low-energy MOs

B : 1s , 2s , 2px , 2py , 2pz

3 H : 1s

noccupied

BH3

8 electrons / 4 pairs

B : 5 electrons3 H : 3 1 electron

••••

••

••

OMOs

UMOs

LUMO

HOMO(s) •• ••

ccupied

ighest

owest

1s

••••

••

••

1s ••Boron Core

2s ••••

••

••

Radial Node

2px ••••

••

••

2py••

••

••

••

2pz

••••

••

••

3s

••••

••

••

3dx2-y2

••••

••

••

3dxy

••••

••

••

Computer PartitionsTotal e-Density

intoSymmetrical MOs

(à la Chladni)

We PartitionTotal e-Density

intoAtom-Pair Bonds

(and anti-bonds)

& Lone Pairs(and vacant atomic orbitals)

(à la Lewis)

usually

When thisdoesn't work,and we must

use moresophisticatedorbitals, wesay there is

RESONANCE

2pz

••

••

••

•••••• ••BHHB

Same Total e-Density

Same Total Energy

BH

HB

For Many Purposes Localized Orbitals are Not Bad

Boron Core

Where are We?

MoleculesPlum-Pudding Molecules ("United Atom" Limit)

Understanding Bonds (Pairwise LCAO)"Energy-Match & Overlap"

Structure (and Dynamics) of XH3 MoleculesParsing Electron Density

Atoms3-Dimensional Reality (H-like Atoms)

HybridizationOrbitals for Many-Electron Atoms (Wrong!)Recovering from the Orbital Approximation

Recognizing Functional Groups

Payoff forOrganic

Chemistry!

ReactivitySOMOs, high HOMOs, and low LUMOs

The Localized Orbital Picture(Pairwise MOs and Isolated AOs)

Is Our Intermediate betweenH-like AOs and Computer MOs

When must we think more deeply?

When mixing of localized orbitals causes

Reactivityor

Resonance

Which MOMixings Matter

forReactivity?

••

••

••

••

etc.

••

••

••

etc.

UMOs

OMOsOMOs

B A

UMOs

Myr

iad

Pos

sibl

e P

airw

ise

Mix

ings

Which MOMixings Matter

forReactivity?

••

••

••

••

etc.

••

••

••

etc.

UMOs

OMOsOMOs

••SOMO SOMO••

B A

SOMO-SOMO(when they exist)

UMOs

many atoms"free radicals"

e.g. •H •Cl •CH3

not so common

ingly

Which MOMixings Matter

forReactivity?

••

••

••

••

etc.

UMOs

••

••

••

etc.

UMOs

OMOsOMOs

••

••

B A

Nothing

Weak NetRepulsion

••

Negligiblemixing

BadE-match

Which MOMixings Matter

forReactivity?

••

••

••

••

etc.

UMOs

••

••

••

etc.

UMOs

OMOsOMOs

B ABonding!

Unusually High

HOMOwith

Unusually Low

LUMO

••

Which MOMixings Matter

forReactivity?

••

••

••

••

etc.

UMOs

••

••

••

etc.

UMOs

OMOsOMOs

B A••

Bonding!

Unusually High

HOMOwith

Unusually Low

LUMO

BASE

ACID

Acid-Base Theories

Lavoisier(1789)

OxidizedSubstance

Substance tobe Oxidized

Arrhenius(1887)

H+ Source OH- Source

Incr

easi

ng G

ener

ality

THEORY ACID BASE

Brønsted/Lowry(1923)

H+ Donor H+ Acceptor

Lewis(1923)

e-Pair Acceptor"Electrophile"

e-Pair Donor"Nucleophile"

HOMO/LUMO(1960s)

unusually

High HOMOunusually

Low LUMO

Unusual:Compared to What?

End of Lecture 17