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 valenc e-bond electronic description for the solute is employed to allow for the instantaneous solute dipole readjustment to the fluctuating solven t environment. The effects of the solute electronic structure variatio n with the solvent configuration on solvation-in particular, the solut e-solvent structure and solute rotational dynamics-are examined. A gen eral similarity between the effects of increasing polarizability and o f growing dipole moment is noted. This is probably due to the preferen tial solvation of higher dipolar states of polarizable solutes, arisin g from the nonpairwise additivity. However, the details as to the way and extent polarizability influences a physical observable vary with t he characteristics of the latter. Thus a mere increase of the solute d ipole moment cannot reproduce many different aspects of diversified po larizability effects. As for the solute-solvent radial distribution fu nctions, a polarizable solute tends to make a structure compared to th at of a nonpolarizable one with the same average charge distribution. Most affected are the hydrogen radial distribution functions, The solu te polarizability tends to enhance its hydrogen-bonding ability. Thus even a nonpolar solute can form weak hydrogen bonds with water molecul es, depending on its polarizability and short-range repulsive terms, S olute orientational dynamics are also found to be significantly influe nced by the solute electronic structure variation. Unlike the equilibr ium radial distribution functions, however, it is the polarizability a nisotropy that is responsible for altering the solute reorientational dynamics. With increasing polarizability anisotropy, the solute rotati onal dynamics become slower and the corresponding rotational friction grows. This increasing friction trend is attributed to coupling betwee n the different components of the solvent electric field, induced by t he solute polarizability anisotropy.