The delocalization of π electronic systems as a destabilizing constraint imposed by the σ 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 π systems, predicts that the π bonding energies of benzene or allyl radical are weaker in the regular geometry than they are in a distorted geometry typical of a Kekuké 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 σ and π components. It is found that the π systems of these conjugated molecules are indeed unstable in the regular geometry, and stabilized by a kekuléan distortion leading to alternate long and short bonds. On the other hand, the σ 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 π system and a symmetrizing σ frame. This latter driving force is the strongest of the two, and forces π 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.