Self-consistent reaction field calculations of nonequilibrium solvent effects on proton transfer processes through low-barrier hydrogen bonds

We investigate dynamic solvent effects on proton transfer reactions in the strongly hydrogen-bonded hydroxyl-water model system by using a self-consis tent nonequilibrium reaction field method. The initial motivation for the p resent work lies in the results of a recently reported molecular dynamics s imulation for the same system in aqueous solution, carried out through comb ined density functional and molecular mechanics potentials. Such a study ha s confirmed that proton transfer occurs in an essentially frozen environmen t and that solvent fluctuations may play a crucial role in the reaction dyn amics. Nevertheless, owing to the use of effective charge water models in m olecular dynamics simulations, the effect of solvent electronic polarizatio n, which can be assumed to respond instantaneously to solute charge modific ations, cannot be accounted for explicitly. Our main goal in the present st udy is to analyze such an effect in the effective energy profile instantane ously experienced by the proton, using for this purpose ab initio methods a nd a dielectric continuum model of the solvent. Basically, the polarization of the solvent is divided into inertial and noninertial terms. The latter is assumed to be always in equilibrium with the solute whereas the former i s characterized by a finite relaxation time. The model allows us to estimat e the dependence of the activation energy and transition structure geometry on the solvent inertial polarization which is described by a fluctuating g lobal solvent coordinate related to solute internal parameters. In some cas es, the activation barrier may be lower than the equilibrium barrier. A det ailed analysis of the effect of electronic polarization on the solute is al so presented.