3B, Frontier Molecular Orbital theory

Apart from electrostatic interactions, the overlap between orbitals may favour the reaction between an electron donor and an electron acceptor. A high (in energy) lying occupied orbital in the donor may overlap with a low lying empty one in the acceptor, leading to a net stabilization.

The strength of the interaction is determined by:

the energy difference between the two orbitals involved (the smaller the better).
the amount of overlap between the orbitals (the larger the better).

These effects can account for the subtle differences that are found in facial selectivity, caused by

steric effects which hinder overlap, or by
asymmetry of a pi orbital, which favours overlap on that side where the orbital is larger.

Example: Diels-Alder reaction of fluorocyclopentadiene, which takes place on the side where the F resides. Sterically the other side would be preferred, so an electronic effect must play a role. We can investigate the orbital concerned, the HOMO of fluorocyclopentadiene, and other cyclopentadienes.

See also the VRML representation of frontier orbitals (HOMO, LUMO) in a series of Diels-Alder reactions, at Imperial College, and a related paper.

This is paragraph B of chapter 3: More theory
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