QUANTUM SIMULATION STUDY OF PROTON-TRANSFER REACTIONS IN SOLUTION - FROM A MOLECULAR TO A STOCHASTIC DESCRIPTION

We present a quantum Molecular Dynamics (MD) study of a model for AH-B half arrow right over half arrow left A--H+B reactions in liquid chloromethane. The rate constant is first computed by using a quantum path-integral method, which connects it to the free energy surface of the proton quantum path centroid. The results allow us to discuss the role of the solvent, and the tunneling contribution to the rate. We then develop an influence functional approach, in which the influence of the solvent is reduced to that of the electric field along the AH-B bond and an effective dipolar interaction term is added to the intrinsic quantum action of the complex. The centroid density computed this way, and the associated rate constant, are shown to be in good agreement with the full MD results, whereas the computational effort is considerably diminished. Along the same lines, we introduce a semiclassical stochastic description, using the electric field acting on the complex as the reaction coordinate, which follows closely the view of Teresa Fonseca [J. Chem. Phys. 91, 2869 (1989)] for treating electron transfer reactions in non-Debye solvents.