Molecular dynamics simulation study of polarizable solute solvation in water .1. Equilibrium solvent structure and solute rotational dynamics

Equilibrium solvation for polarizable, polar, and nonpolar solutes in water is studied via molecular dynamics computer simulations. A valence-bond electronic description for the solute is employed to allow for the instantaneous solute dipole readjustment to the fluctuating solvent environment. The effects of the solute electronic structure variation with the solvent configuration on solvation-in particular, the solute-solvent structure and solute rotational dynamics-are examined. A general similarity between the effects of increasing polarizability and of growing dipole moment is noted. This is probably due to the preferential solvation of higher dipolar states of polarizable solutes, arising from the nonpairwise additivity. However, the details as to the way and extent polarizability influences a physical observable vary with the characteristics of the latter. Thus a mere increase of the solute dipole moment cannot reproduce many different aspects of diversified polarizability effects. As for the solute-solvent radial distribution functions, a polarizable solute tends to make a structure compared to that of a nonpolarizable one with the same average charge distribution. Most affected are the hydrogen radial distribution functions, The solute polarizability tends to enhance its hydrogen-bonding ability. Thus even a nonpolar solute can form weak hydrogen bonds with water molecules, depending on its polarizability and short-range repulsive terms, Solute orientational dynamics are also found to be significantly influenced by the solute electronic structure variation. Unlike the equilibrium radial distribution functions, however, it is the polarizability anisotropy that is responsible for altering the solute reorientational dynamics. With increasing polarizability anisotropy, the solute rotational dynamics become slower and the corresponding rotational friction grows. This increasing friction trend is attributed to coupling between the different components of the solvent electric field, induced by the solute polarizability anisotropy.