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:

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

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