Orbital Interactions
Transcript of Orbital Interactions
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