VBSCF calculations on the bimolecular (E2) elimination reaction. The nature of the transition state

Valence bond calculations utilizing the Xiamen package have been carried out on the bimolecular (E2) elimination reaction X(-) + HCH(2)CH(2)Y --> XH + CH(2)=CH(2) + Y(-) where X,Y = F,F; F,Cl; Cl,F; Cl,Cl for anti and syn reactant complexes, transition states, and product complexes. The calculations were supplemented by MO-based calculations at MP2/6-311++G**//MP2/6-311++G**. The valence bond calculations give reasonable energies with eight contributors to the resonance hybrid. Charge-localized contributors dominate the transition states. NPA charges from the MO calculations confirm that the transition states possess a significant degree of localized charge and can be described by the key resonance structure X(-)-H(+)-CH(2)(-)-CH(2)(+)-Y(-). At the same time, the MO calculations show that electronically and geometrically the reactions are clearly concerted though not synchronous. Valence bond state correlation diagrams (VBSCD) show that a simple proton transfer such as that in the E1cB irreversible reaction is predicted to have a lower barrier than a synchronous concerted (E2) reaction. The E2 transition state evidently avoids this energetic disadvantage by becoming localized and nonsynchronous, though with important electronic and geometric changes at all of the reacting centers.