SIMULATION OF PROTON-TRANSFER REACTION-RATES - THE ROLE OF SOLVENT ELECTRONIC POLARIZATION

A simulation method suitable for the prediction of the rate of a proto n transfer reaction of the form A-H-B -->A-H-Bi, where -denotes a hydr ogen bond, is presented. The method is based on a golden rule formulat ion, where the coupling between the two proton states is obtained by s olution of Schrodinger's equation for the proton states in a double-we ll potential whose shape is determined, in part, by the solvent's elec tronic polarization. The reaction activation energy is determined by s olvent fluctuations, as well as flanking group (A and B) vibrational m otion. The surfaces for the AB vibrational motion with the proton in i ts initial and final states are also modified by the coupling to the s olvent's electronic polarization. Consequently, the matrix elements of the proton coupling between the AB vibronic states with the proton in its initial and final state, as well as the reaction's activation ene rgy, are dependent upon the coupling to the solvent's electronic polar ization. The rate constant for proton transfer in a representative phe nol-amine hydrogen-bonded solute immersed in a polar/polarizable model for dichloromethane is simulated. The rate constant can be quite larg e, in the ps(-1) range, as the proton coupling can be large for smalle r AB distances, and the AB vibration provides a number of channels for proton transfer.