Molecular dynamics simulations of solvent reorganization in electron-transfer reactions

We present molecular dynamics simulations of solvent reorganization in electron-transfer reactions in water. Studying a series of solutes with the same core radius (typical for chlorine) but with varying charge from -3 to +3, the simulations show that the single-solute solvent reorganization energy depends quite strongly on the solute's charge, in contrast with the continuum Marcus theory. Due to the ion-dipole interactions, electrostriction plays an important role for charged species. The effective radius of a neutral species is comparatively larger, making the solvent reorganization energy small. Strong increases in the solvent reorganization energy occur when the solute is charged to either -1 to +1, due to the significantly smaller effective radius caused by the ion-dipole interactions. However, the effect is nonsymmetric because the center of the water dipole can approach closer to the negative species than to the positive species. Hence, the nonlinearity occurs mainly in the transition from 0 to -1. For higher charges (+3, +2, -2, -3), dielectric saturation causes a decrease in the reorganization energy with increasing charge. We also calculate the equilibrium activation energy for an outer-sphere electrochemical electron-transfer reaction of the X+e(-)reversible arrowX(-) type, with varying of the core radius of the X species. The deviations from Marcus theory are relatively small for large reactants, but get more significant for small reactants. This is mainly due to the fact that the changes in electrostriction have a comparatively large effect for small solutes. (C) 2001 American Institute of Physics.