Proton transfer in hydrogen-bonded acid–base complexes in polar solvents

Abstract

A theoretical approach exploiting molecular dynamics simulations to treat adiabatic proton transfer between an acid AH and a base B in a polar, aprotic solvent is presented. The dynamics of the proton transfer, which occurs on the electronic ground state surface of the reactive hydrogen-bonded complex AH...B, is strongly influenced by interaction of the reaction system with the solvent and by the AB stretch vibration. The approach fully incorporates the quantum character of the proton motion as well as that of the AB stretch vibration and yields a mechanistic picture for a thermally activated proton transfer reaction in a polar solvent. Rate constants are computed and solvent frictional effects are analyzed in application of the theory to a model of the system phenol-trimethylamine in methyl chloride solvent. In addition, it is shown how the excitation of the hydrogen bond symmetric stretch mode decelerates the reaction. The simulation results are also compared to a curve-crossing model. The impact of the solvent electronic polarization on the results is discussed.