Origins of nonperfect synchronization in the lowest-energy path of identity proton transfer reactions leading to delocalized anions a vbscf study

Two proton-transfer reactions yielding delocalized conjugate bases, the identity reactions of allyl anion with propene (2a) and acetaldehyde enolate with acetaldehyde (2b), are examined by means of quantum-chemical calculations and compared with another proton-transfer yielding a localized anion, methide ion with methane (9). When both reactants and transition structures are constrained to conformations that prevent delocalization, barriers are lower, showing that delocalization stabilizes the anions more than the transition structures. Calculations utilizing valence bond self- consistent field (VBSCF) methods show that in all three cases the single most important contributing structure to the hybrid is a triple ion species R^- H⁺ R^-. This mixes well with localized covalent structures R-H R^- and R^- H-R, but poorly with delocalized covalent or triple ion structures. It is concluded that nonperfect synchronization in 2a and 2b results from a balance between maximizing resonance stabilization and maximizing covalent carbon- hydrogen bonding in the (R---H---R)^- transition structure.