Changes in the energy levels when 2 molecular fragments interact

The interacting orbitals have the same energy The interacting orbitals have different energies

Effect of the number of electrons4 electrons 2 electrons

E

NOTE: the antibonding orbital is destabilized more than the bonding orbital is stabilized

Net destabilization relative to noninteracting fragments Net stabilization relative to noninteracting fragments

1. The total energy is lowered only when a filled MO of one fragment interacts with an empty MO of the other fragment.2. The largest energy decrease occurs with HOMO-LUMO interactions - i.e., the Highest Occupied MO of one fragment with the Lowest Unoccupied MO of the other.

CONCLUSIONS

CONCLUSIONEnergy change is greatest when the interacting orbitals have the same energy.

Relative energies of substituted alkenes

H

H

H

HH2C

Plane of symmetry

Newman projectionCH3 in front

CH

H2C

Imagine propene in a conformation with a plane of symmetry

We consider the interaction of CH3 ! orbitals (filled) with the "# orbital (empty).These orbitals are either symmetric or antisymmetric with respect to the molecular plane of symmetry.

H

H

HH

Antisymmetric

"# orbital Symmetry-adapted C-H bonding MOs of CH3

" orbital

These have identical "-type symmetry and therefore have a nonzero interaction

SA

Symmetric S SAA

HH

H

H

H

CH H

CH CH3

CH H

CH HNet decrease in energy

Empty p orbital

CH3 orbital with A symmetry

Interaction of an empty "# orbital with antisymmetric ("-type) CH3 bonding MO

"-type bonding MO

"-type antibonding MO

CH3= H

H

H

Plane of symmetry

H2C

H2C

H2C

H2C

Application to carbocation energies

H

H

H

HH

Plane of symmetry

Newman projectionCH3 in front

CH3HH

Consider the ethyl cation in a geometry with a plane of symmetry

We consider the interaction of CH3 σ orbitals (filled) with the empty p orbital.These orbitals are either symmetric or antisymmetric with respect to the molecular plane of symmetry.

H

H

H H

Antisymmetric

p orbitalSymmetry-adapted C-H bonding MOs of CH3

p orbital

These have identical π-type symmetry and therefore have a nonzero interaction

SA

Symmetric

S S A A

H H

H

H

H

H2C H

H2C CH3

H2C H

H2C HNet decrease in energy

Empty p orbital

CH3 orbital with A symmetry

Interaction of an empty p orbital with antisymmetric (π-type) CH3 bonding MO

π-type bonding MO

π-type antibonding MO

Br

A

CB

Nu Nu

A

CB

+ Br

SN2 reaction

1. Bimolecular reaction in one step.2. Inversion of configuration (backside nucleophilic attack).

Why doesn't retention of configuration (frontside attack) compete with inversion?

Nu

A

C B

Br

HOMO σ* LUMO

Bonding interaction, good overlap

Transition state for backside attack Transition state for frontside attack

A

C B

Br

σ* LUMO

Bonding interaction

Nu Antibonding interaction

Bonding-antibonding cancellation!

C-Br σ*LUMO

C-Br σ*LUMO

Nu– HOMO Nu– HOMO

HOMO-LUMO interaction stabilizes the transition state

HOMO-LUMO interaction gives little or no stabilization