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

M. F. Ruiz-López, A. Oliva, Iñaki Tuñón, J. Bertrán

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Abstract

We investigate dynamic solvent effects on proton transfer reactions in the strongly hydrogen-bonded hydroxyl - water model system by using a self-consistent nonequilibrium reaction field method. The initial motivation for the present work lies in the results of a recently reported molecular dynamics simulation for the same system in aqueous solution, carried out through combined density functional and molecular mechanics potentials. Such a study has confirmed that proton transfer occurs in an essentially frozen environment and that solvent fluctuations may play a crucial role in the reaction dynamics. Nevertheless, owing to the use of effective charge water models in molecular dynamics simulations, the effect of solvent electronic polarization, which can be assumed to respond instantaneously to solute charge modifications, cannot be accounted for explicitly. Our main goal in the present study is to analyze such an effect in the effective energy profile instantaneously experienced by the proton, using for this purpose ab initio methods and 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 is characterized by a finite relaxation time. The model allows us to estimate the dependence of the activation energy and transition structure geometry on the solvent inertial polarization which is described by a fluctuating global solvent coordinate related to solute internal parameters. In some cases, the activation barrier may be lower than the equilibrium barrier. A detailed analysis of the effect of electronic polarization on the solute is also presented. © 1998 American Chemical Society.
Original languageEnglish
Pages (from-to)10728-10735
JournalJournal of Physical Chemistry A
Volume102
Issue number52
DOIs
Publication statusPublished - 24 Dec 1998

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