Solvation dynamics of C153 in supercritical fluoroform: a simulation study based on two-site and five-site models of...

Molecular dynamics (MD) simulations of a probe solute (coumarin C153) in supercritical fluoroform are used to study time-dependent solute-solvent interactions. We study the dynamics of solvent reorganization in response to electronic excitation of C153 at a temperature of 1.03 T(c) (the critical temperature) and a series of densities above and below the critical density. Simulations of a two-site and five-site models of fluoroform are presented and compared. The time-dependent solvation response after solute electronic excitation is studied in the two cases, and the five-site results present an earlier onset of exponential decay that is closer to what is expected to be the experimental response. This is confirmed by comparison to experiment. In addition to obtaining the solvation response from nonequilibrium MD trajectories, approximate solvation responses were obtained from equilibrium time correlations of the fluctuations in the solvation energy change in the presence of ground- and excited-state solutes. For the five-site model, the equilibrium excited-state response shows stronger density dependence than the ground-state one. The nonequilibrium response appears to have an intermediate decay rate between the two equilibrium functions. The solute-partial-charge-solvent-induced-dipole interaction was also taken into account by means of a perturbative approach, which improved the agreement with experimental measurements available at densities corresponding to 1.4-1.6 rho(c) (where rho(c) the critical density). From the comparison between the two models, it is possible to conclude that an atomistic description is necessary for correctly representing the portion of solvation dynamics that is related to reorientation. This consideration is supported by providing results for orientational time correlation functions and by comparing the correlation times with the experimental ones.