High-level computational studies of nonidentity proton transfer reactions: Variations in their gas phase potential energy surfaces

High-level calculations (G2+) have been used to characterize the gas phase potential energy surfaces of the nonidentity proton transfer reactions of a series of simple anions (H-, CH3-, NH2-, OH-, F-, SiH3-, PH2-, SH-, Cl-, HCC-, and NC-) with their conjugate acids. Only the combinations that give reactions that are exothermic by less than 20 kcal/mol were considered. The surfaces for the nonidentity reactions appear to be hybridizations of those of the corresponding identity reactions, and a simple averaging approach provides a reasonable representation of the nonidentity surface. Measures based on the geometry or electron density indicate that the position of the transition state ("early" or "late") is better correlated with the properties of the proton donor/acceptor than the exothermicity of the proton transfer process in these systems.