THE DELOCALIZATION OF PI ELECTRONIC SYSTEMS AS A DESTABILIZING CONSTRAINT IMPOSED BY THE SIGMA FRAME - ALLYL, BENZENE, CYCLOBUTADIENE AND RELATED HETEROANNULENES

A valence bond model of electronic delocalization, including aggregates of monovalent atoms as well as reaction transition states or pi systems, predicts that the pi bonding energies of benzene or allyl radical are weaker in the regular geometry than they are in a distorted geometry typical of a Kekuke structure. This prediction is verified by accurate ab initio calculations applied to allyl radical, benzene, cyclobutadiene and isoelectronic heteroannulenes, in which the driving force responsible for the regular geometry is decomposed into its sigma and pi components. It is found that the pi systems of these conjugated molecules are indeed unstable in the regular geometry, and stabilized by a kekulean distortion leading to alternate long and short bonds. On the other hand, the sigma frame always favor equal bond lengths.Thus, the regular geometry of benzene or allyl is the by-product of two opposing driving forces: a distortive pi system and a symmetrizing sigma frame. This latter driving force is the strongest of the two, and forces pi electronic delocalization. It is shown, through appropriate thermodynamic cycle, that this finding is not contradictory with the known empirical resonance energy of allyl, benzene and other aromatic molecules