Study of electrostatic interactions in condensed phase by using Brownian dynamics simulations.

摘要

A solvent exerts its influence on dissolved molecules through the process called "solvation". The dynamics of solvation have received much attention in the last decade mainly because of the increased availability of ultrafast lasers and its profound importance in chemical reactions. A true understanding of the experimental observations can be achieved by a fruitful combination of empirical information, analytical theories and computational studies linking the two and aiding in conceptualizing the important elements of the problem. Here, computer simulations involving a simple model often used in theoretical studies to represent polar fluids is utilized: a simple cubic lattice of point dipoles undergoing rotational Brownian motion and interacting with each other and the solute (a point charge or dipole) electrostatically. Therefore, the main focus is on generic polar solvation, rather than specific solute-solvent interactions. The important results are: (i) Orientational structure induced by a charge varies considerably with solute charge and solvent polarity. However, their effects on the reaction potential are separable and can be modeled simply. (ii) The solvation free energy well is anharmonic, but appears harmonic in a given simulation, because the thermally populated region of the solvation coordinate is neatly harmonic. (iii) Non-linear statics are reflected in the dynamics. (iv) Solvation becomes faster with increasing solvent polarity while individual solvent dipoles become slower. These two dynamics are intimately related through a simple relationship. For charge solvation it leads to the control of solvation dynamics by static fluctuation magnitudes. (v) Solvation time is determined by correlations among a relatively small number (