Simulation of solvent isotope effects on aqueous ferrous and ferric ions

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Abstract By appropriate averages over the molecular dynamics trajectories of solution models we find how the change in solvent from H 2 O to D 2 O affects both the solvation thermodynamics and the ferrous-ferric electron transfer rate constant k 23 for these models all at 298 K and a constant density of 1 gm cm −3 . The thermodynamic effects, generated by simulating systems comprising one ferrous or ferric ion and a hundred water molecules, are determined by the zero point energy differences. The kinetic effects are generated both from similar one-ion systems and from a ferrous-ferric pair of hexaaquo ions, a super molecule , immersed in 418 water molecules. The kinetic effects are determined by differences in zero point energies and nuclear tunneling, for which we adapt a semiclassical approximation due to Holstein. The quantitative conclusions from this study depend on the interpretation of the vibrations of the “bath” water molecules, those in the basic cell of the simulation, but outside the hexaaquo complexes. If we ignore the direct contributions of the bath molecules we attain satisfactory agreement between model calculation and laboratory experiment for the thermodynamic effect, while this extreme approximation leads to a value of the kinetic isotope effect somewhat lower than that based on experiment. These calculated results are compared with recent studies of a rather similar model of the same system by Bader et al . On balance, both models give kinetic solvent isotope effects large enough to account for the experimental data if contributions from beyond the hydration shell are included.