Vibrationally enhanced proton transfer

We report a computational study of the effects of vibrational excitation of the hydrogen atom motion (i.e., excitation of the hydrogen bond asymmetric stretch mode) on proton transfer in solution. We use the method "molecular dynamics with quantum transitions" (MDQT) to properly treat the quantum mechanical nature of the hydrogen motion. Previously we applied MDQT to a model for the proton transfer reaction AH-B ⇄ A^--⁺HB in liquid methyl chloride, where the AH-B complex corresponds to a typical phenol-amine complex. In that application, the hydrogen motion was treated quantum mechanically, and MDQT was used to incorporate transitions among the hydrogen quantum states into the molecular dynamics. It is a simple step to extend this approach to study the effects of vibrational excitation of the hydrogen motion. 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. Thus, the proton transfer reaction is fast enough to couple with vibrational energy redistribution. We outline pictorially the competing pathways for vibrational relaxation and vibrationally assisted tunneling that we observed in the simulations. Our demonstration of the feasibility of the application of MDQT to photoinduced and photoassisted reactions should motivate further application of MDQT to such systems. More importantly, we hope that our results will motivate experimental investigations of vibrational excitation of the hydrogen bond asymmetric stretch mode in proton transfer reactions.