Simulation of excited state proton transfer reaction kinetics

Abstract

A simulation method suited to characterize excited state proton transfer reaction kinetics in a polar solvent is developed, and applied to an intramolecular reaction of the form A–HB*→AH–B*. The model is applicable to an exothermic electronically excited proton potential energy surface (pes). The solvent modulates this surface but does not have enough coupling strength to symmetrize the proton pes with any significant probability. The proton transfer mechanism then is tunneling through an asymmetric proton pes. As the proton is a fast, quantum object relative to the solvent degrees of freedom, the tunneling is solvent configuration dependent. For each configuration, a rate constant is evaluated by a Wentzel–Kramers–Brillouin (WKB) method. Excitation to the excited reactant state initiates a coupled process of solvent relaxation to equilibrate to the new solute charge state and proton transfer. Hence, the kinetics of the reaction may be inhomogeneous. A survival time formalism is introduced to carry out the average over the solvent fluctuations. The kinetics is roughly exponential. However, the long-time rate constant obtained from the survival probability (0.160 ps−1) is somewhat slower than the rate constant (0.260 ps−1) obtained by assuming that the proton transfer is slow compared to solvent relaxation. The kinetics is fast, in accord with that found in many experimental studies of excited state intramolecular proton transfer.