Computations of solvation free energies for polyatomic ions in water in terms of a combined molecular-continuum approach

The combined molecular-continuum approach developed in the preceding paper was applied for calculations of equilibrium solvation energies for a large number of polyatomic ions. The structure and charge distribution of the given ion were computed using the restricted Hartree-Fock level with the 6-31G** basis set. The standard Lennard-Jones (LJ) parameters, which were not specially calibrated to fit the solvation energies, were used in molecular dynamics simulations. Water (the SPC model) was considered as a solvent. The computations show that the new scheme works satisfactorily for nitrogen cations in the frame of a standard parametrization and can be further improved for oxygen ions by tuning solute-solvent LJ parameters. The calculated relative change of the energies in families of similar cations-i.e., ammonium-type or oxonium-type cations-fits the experimental trends. The present approach is specially addressed to separate the inertial contribution to solvation free energies, which is important in view of further applications to electron transfer reactions. Computed values of the inertial contribution to solvation energies of the ions and reorganization energies for the model two-site dumbbell system are found to be systematically lower than those obtained in terms of the standard treatments (using the Pekar factor or the polarizable continuum model (PCM)). (C) 2003 American Institute of Physics.