SIMULATION OF SOLVENT ISOTOPE EFFECTS ON AQUEOUS FERROUS AND FERRIC IONS

By appropriate averages over the molecular dynamics trajectories of so lution models we find how the change in solvent from H2O to D2O affect s both the solvation thermodynamics and the ferrous-ferric electron tr ansfer rate constant k(23) for these models all at 298 K and a constan t density of 1 gm cm(-3). The thermodynamic effects, generated by simu lating systems comprising one ferrous or ferric ion and a hundred wate r molecules, are determined by the zero point energy differences. The kinetic effects are generated both from similar one-ion systems and fr om a ferrous-ferric pair of hexaaquo ions, a super molecule, immersed in 418 water molecules. The kinetic effects are determined by differen ces in zero point energies and nuclear tunneling: for which we adapt a semiclassical approximation due to Holstein. The quantitative conclus ions from this study depend on the interpretation of the vibrations of the ''bath'' water molecules, those in the basic cell of the simulati on, but outside the hexaaquo complexes. If we ignore the direct contri butions of the bath molecules we attain satisfactory agreement between model calculation and laboratory experiment for the thermodynamic eff ect, while this extreme approximation leads to a value of the kinetic isotope effect somewhat lower than that based on experiment. These cal culated results are compared with recent studies of a rather similar m odel of the same system by Bader et al. On balance, both models give k inetic solvent isotope effects large enough to account for the experim ental data if contributions from beyond the hydration shell are includ ed.