Solvation dynamics in methanol: Solute and perturbation dependence

Abstract

We present the results of a molecular dynamics (MD) simulation study of solvation dynamics in response to instantaneous changes in the charge distribution of neutral diatomic solutes in methanol at room temperature and atmospheric pressure. Our simulations are designed to model the time dependent Stokes shift in the fluorescence spectrum of probe molecules in this solvent and focus on the solute and perturbation dependence of CΔE(t), the response function of ΔE the difference in the solute-solvent interaction energy between the excited and ground state solutes. The solute dependence is investigated by comparing the solvation dynamics for a given change in the charge distribution of solutes with different Lennard-Jones site diameters and the perturbation dependence by comparing the responses to dipole creation and reversal in each of these solutes. We find that CΔE(t) is strongly dependent on both the solute and the perturbation, especially in the early (< 200 fs) portions of its time-evolution. By examining the time-evolution of the structure around the solutes and the effects of deuteration of the hydroxyl hydrogen, we show that early solvation dynamics is determined by the proximity of the OH groups to the solute and by the ability of the hydroxyl hydrogens in the solute vicinity to rotate freely about the CO bonds. We also demonstrate that changes in the first few solvation shells occur on all the time scales relevant to CΔE(t) relaxation, with the slowest step corresponding to the most sharply-defined structural feature, the solute-solvent hydrogen bond. Comparison of CΔE(t), obtained by nonequilibrium MD simulation of dipole reversal, to the time correlation of equilibrium fluctuations in ΔE confirms our earlier finding, based on the results for dipole creation, that the linear response approximation has a very limited range of validity for solvation dynamics in methanol.