DELOCALIZATION IN ALLYL CATION, RADICAL, AND ANION

Ab initio valence bond calculations are performed for the allyl cation , radical, and anion with 6-31G basis set. Delocalized and hypothetic ally localized structures of these systems are thoroughly optimized an d analyzed. The delocalization energies, defined as the energy differe nce between the delocalized structure and its hypothetically localized one, for the three allyl systems are -55.7, -28.4, and -52.3 kcal/mol , respectively. Our results clarify the recent debate on whether the a llyl anion has little or comparable resonance stabilization with the a llyl cation. The methylene rotation barriers of the allyl cation, radi cal, and anion are successfully explained in terms of resonance, hyper conjugation, and rehybridization. For the allyl radical, its resonance energy is only about half those of the allyl cation and anion; thus, it has the lowest rotation barrier. The twisted allyl cation, in which the rotating methylene group is perpendicular to the C-C-C plane, has the highest hyperconjugation energy (-6.8 kcal/mol), while, in its tw isted form, the allyl anion has a negligible hyperconjugation effect. As the allyl anion assumes its twisted form, the carbon atom in the ro tating methylene experiences a remarkable rehybridization from sp(2) m ode in the planar form to sp(3) mode. This process decreases the total energy of the twisted allyl anion as much as 14.3 kcal/mol and eventu ally makes its rotation barrier smaller.