VIBRATIONALLY ENHANCED PROTON-TRANSFER

We report a computational study of the effects of vibrational excitati on of the hydrogen atom motion (i.e., excitation of the hydrogen bond asymmetric stretch mode) on proton transfer in solution. We use the me thod ''molecular dynamics with quantum transitions'' (MDQT) to properl y treat the quantum mechanical nature of the hydrogen motion. Previous ly we applied MDQT to a model for the proton transfer reaction AH-B-A reversible arrow A(-)-(HB)-H-+ in liquid methyl chloride, where the AH -B complex corresponds to a typical phenol-amine complex. In that appl ication, the hydrogen motion was treated quantum mechanically, and MDQ T was used to incorporate transitions among the hydrogen quantum state s into the molecular dynamics. It is a simple step to extend this appr oach to study the effects of vibrational excitation of the hydrogen mo tion. We show that, for this model system, the vibrational excitation significantly enhances the proton transfer rate for both hydrogen and deuterium, although the enhancement is much greater for-deuterium. Thu s, the proton transfer reaction is fast enough to couple with vibratio nal energy redistribution. We outline pictorially the competing pathwa ys for vibrational relaxation and vibrationally assisted tunneling tha t we observed in the simulations, Our demonstration of the feasibility of the application of MDQT to photoinduced and photoassisted reaction s should motivate further application of MDQT to such systems. More im portantly, we hope that our results will motivate experimental investi gations of vibrational excitation of the hydrogen bond asymmetric stre tch mode in proton transfer reactions.