MOLECULAR-DYNAMICS SIMULATION OF SOLVATION DYNAMICS IN METHANOL-WATERMIXTURES

The solvation dynamics following charge-transfer electronic excitation of diatomic solutes immersed in methanol-water mixtures is investigat ed through molecular dynamics simulations. The solvation response func tions associated with an instantaneous reversal of the solute's dipole moment for two different solute sizes in mixtures with methanol mole fractions, x(m) = 0.2, 0.5, and 0.8, are calculated and compared to th e corresponding ones in the pure liquids. The solvation response of th e mixtures is separated into methanol and water contributions in order to elucidate the role played by each molecular species on the solvati on dynamics. We find significantly different responses for the two sol utes and relate them to the fact that the solute with the smaller site diameters is a much better hydrogen (H)-bond acceptor than the larger diameter solute. For the small solute in methanol and in the mixed so lvents, we have also calculated H-bund response functions, which measu re the rate of solute-solvent H-bond formation after the solute's exci tation and find that, at longer times, the solvation and H-bond format ion response functions decay at similar rates. The implications of thi s finding for solvation dynamics of H-bonding solutes in H-bonding sol vents are discussed and related to recent experimental results for suc h systems.