Molecular dynamics simulation of solvation dynamics in methanol-water mixtures

The solvation dynamics following charge-transfer electronic excitation of diatomic solutes immersed in methanol-water mixtures is investigated through molecular dynamics simulations. The solvation response functions 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 the corresponding ones in the pure liquids. The solvation response of the mixtures is separated into methanol and water contributions in order to elucidate the role played by each molecular species on the solvation dynamics. We find significantly different responses for the two solutes 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 solvents, we have also calculated H-bund response functions, which measure the rate of solute-solvent H-bond formation after the solute's excitation and find that, at longer times, the solvation and H-bond formation response functions decay at similar rates. The implications of this finding for solvation dynamics of H-bonding solutes in H-bonding solvents are discussed and related to recent experimental results for such systems.