Ingrosso Francesca, Ladanyi Branka M
Department of Chemistry, Colorado State University, Fort Collins, 80523, USA.
J Phys Chem B. 2006 May 25;110(20):10120-9. doi: 10.1021/jp061170m.
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.
利用超临界氟仿中探针溶质（香豆素C153）的分子动力学（MD）模拟来研究随时间变化的溶质 - 溶剂相互作用。我们研究了在1.03 T(c)（临界温度）以及高于和低于临界密度的一系列密度下，溶剂对C153电子激发的响应动力学。给出并比较了氟仿的两点和五点模型的模拟结果。在这两种情况下研究了溶质电子激发后的随时间变化的溶剂化响应，五点模型的结果呈现出指数衰减的更早起始，更接近预期的实验响应。通过与实验比较得到了证实。除了从非平衡MD轨迹获得溶剂化响应外，还从基态和激发态溶质存在下溶剂化能变化波动的平衡时间相关性中获得了近似溶剂化响应。对于五点模型，平衡激发态响应比基态响应表现出更强的密度依赖性。非平衡响应似乎具有介于两个平衡函数之间的中间衰减率。还通过微扰方法考虑了溶质部分电荷 - 溶剂诱导偶极相互作用，这改善了与在对应于1.4 - 1.6 rho(c)（其中rho(c)为临界密度）的密度下可用的实验测量值的一致性。从两个模型的比较中可以得出结论，对于正确表示与重新定向相关的溶剂化动力学部分，原子描述是必要的。通过提供取向时间相关函数的结果并将相关时间与实验值进行比较，支持了这一考虑。
