THE ROLE OF DELOCALIZATION IN BENZENE

The influence of pi delocalization on the geometry of benzene is exami ned 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 i n part responsible for the equilibrium symmetric structure. In apparen t contrast, a sigma-pi energy partitioning analysis suggests that the benzene sigma framework is responsible for the symmetric structure, th e pi system preferring a distorted geometry. Shaik et al. have therefo re concluded that delocalization in not an important symmetrizing forc e in this molecule. We show, however, that the pi energy component con tains 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 symmetr ic benzene in essential accord with the concepts of classical resonanc e theory.