THE ROLE OF DELOCALIZATION IN BENZENE

The influence of pi delocalization on the geometry of benzene is examined at the ab initio SCF level of theory. We find that benzene favors a bond alternating geometry when its canonical pi MOs are replaced by three localized ethylenic orbitals, revealing that delocalization is in part responsible for the equilibrium symmetric structure. In apparent contrast, a sigma-pi energy partitioning analysis suggests that the benzene sigma framework is responsible for the symmetric structure, the pi system preferring a distorted geometry. Shaik et al. have therefore concluded that delocalization in not an important symmetrizing force in this molecule. We show, however, that the pi energy component contains a sizable and strongly geometry dependent contribution from the localized (Kekule) wave function. Thus, it appears to be misleading to judge the nature of delocalization based on a sigma-pi partition. We conclude that delocalization effects act to strongly stabilize symmetric benzene in essential accord with the concepts of classical resonance theory.