Comparative study of identity proton-transfer reactions between simple atoms or groups by VB methods

Valence bond SCF calculations have been carried out on identity proton transfers of the type X-H + X^- → X^- + H-X and X-H⁺ + X → X + X-H⁺ for systems where X = F^-, Cl^-, Br^-, OH^-, SH^-, NH₂^-, PH₂^-, CH₃^-, SiH₃^-, OH₂, NH₃, and PH₃. A hybrid consisting of three contributing structures-reactantlike, productlike, and ionic (X^- H⁺ X^- or X H⁺ X)-gives reasonable results, though additional structures afford a slight further decrease in energy. Energies somewhat below Hartree-Fock and reasonable weights are obtained. Calculated barriers agree well with those obtained by high-level ab initio calculations with correlation. Insights into the factors determining barrier height are obtained.