Chem 125 Lecture 1610/10/08

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.

Unusual:Compared to

What?

••

sp3C 1sH

••

*CH

CH

"usual"LUMO

"usual"HOMO

(or *CC )

(or CC )

I. Unmixed Valence-Shell AOs

III. Unusual AO Energy in MO

Sources of weirdness:

IV. Electrical Charge

II. Poor Overlap of AOs

I. Unmixed Valence-Shell

Atomic Orbitals

••

*CH

CH

••

sp3C 1sH

BH3

low LUMO

••

CH3

high HOMO

••

NH3

high HOMO

"usual"LUMO

"usual"HOMO

••

OH2

high HOMO

••

OHhigh HOMO

(or *CC )

(or CC )

H+

low “LUMO”

(energies qualitative only)

(Also IV: Electrical Charge)

Acid-Base Reactions

H+ OH H-OH

H+ :NH3

H3B OH

H3B :NH3

H-NH3+

H3B-OH

H3B-NH3+

Curved Arrows Designate e-Pair Shifts.

Start arrow at e-pair location in starting material.

End arrow at e-pair location in product.

(NOT atomic motion)

II. Poor-Overlap MOs

••

*C=O

C=O

•

•

pC

pO

••

*CH

CH

••

sp3C 1sH

••

*C=C

C=C

••

pC pC

high HOMO

low LUMO"usual"LUMO

"usual"HOMO

Bent

(energies qualitative only)

III. Unusual AO Energy in MO

low LUMO

••

*CH

CH

••

sp3C 1sH

••

*C-F

C-F

•

•

sp3C

pF

high HOMO

••

*H-B

H-B

•

•

1sH

sp2B

"usual"LUMO

"usual"HOMO

(energies qualitative only)

Is BH3 an Acid

or a Base?

HOMO

HBHH

and a Base

H B HH

LUMO

BH3 is an Acid

3-Center-2-Electron "Y" Bond

B BH

HHH

H H

LUMO

HBHH

HOMO

H B HH

s

Answer to the puzzle about

Lewis structures from Lecture 2

Compared to What?

Low-Energy MOs

Bonding

Lowered-Energy MOs

Bonding

Its Separated Components

True or False?

False

HOMO-LUMOmixing for

Reactivity&

Resonance

(between molecules)

(within molecule)

*LUMO

Low LUMO

H-F(4 valence e-pairs; 5 valence AOs)

2pFHOMOs

Shape of "Frontier" Orbitals

(ABN) Antibonding

Node

Atomic Orbital Node (AON)

1sH

2pF

1sH

2sF

1sH

2spFn

Alternative format froman early MO catalogueAn Organic Chemist’s Book of Orbitals

Salem & Jorgensen, 1973

Shape of "Frontier" Orbitals

F-CH3(7 valence pairs)

AON

AON

ABN

Comparetheir Energies

HOMOs

≈>&C-H pF

LUMOF-CH3

Low LUMO

(7 valence pairs)HOMOs

ABN

AON

AON

Why unusually low? Low LUMO

ABN

AON

Shape of "Frontier" Orbitals

F H

Direction of best overlap

:OH

F H OH

"Acid-Base"

ABN

Curved ArrowsDesignate

e-Pair ShiftsStart arrow at e-pair location in starting material.

End arrow at e-pair location in product.

LUMO Reaction Analogies

End of Lecture 16Oct. 10, 2008