Terranova Z L, Corcelli S A
Department of Chemistry and Biochemistry , University of Notre Dame , Notre Dame , Indiana 46556 , United States.
J Phys Chem B. 2018 Jul 5;122(26):6823-6828. doi: 10.1021/acs.jpcb.8b04235. Epub 2018 Jun 22.
Time-dependent Stokes shift (TDSS) measurements provide crucial insights into the dynamics of liquids. The interpretation of TDSS measurements is often aided by molecular dynamics simulations, where solvent response functions are computed either with an equilibrium or nonequilibrium approach. In the nonequilibrium approach, the solvent is at equilibrium with the ground electronic state of the solute and its charge distribution is instantaneously changed to that of the first excited state. The solvation response function is then calculated as a nonequilibrium average of the subsequent evolution of the solvent influence on the electronic energy gap. In the equilibrium approach, the normalized time correlation function of the fluctuations of the solvent-perturbed electronic energy gap is calculated. If the linear response approximation is valid, then the nonequilibrium solvation response function is identical to the equilibrium time correlation function. The nonequilibrium methodology conceptually mimics the experiment, but it is significantly more computationally expensive than the equilibrium approach. In multicomponent systems such as ionic liquids, it is natural to inquire how the various components affect the observed relaxation dynamics. When utilizing the nonequilibrium methodology, the solvation response naturally decomposes into a sum of responses for each component present in the system. However, the equilibrium time correlation function does not decompose unambiguously. Here, we have evaluated a decomposition strategy that is consistent with the linear response approximation for the study of solvation dynamics of coumarin 153 (C153) in the 1-ethyl-3-methyl imidazolium tetrafluoroborate, [emim][BF], ionic liquid. The agreement of the equilibrium and nonequilibrium solvation response functions demonstrates the validity of the linear response approximation for the C153/[emim][BF] system. Moreover, decompositions of the equilibrium time correlation function into contributions of the translational and rovibrational motions of the anions and cations are essentially identical to the same decompositions of the nonlinear solvation response.
时间相关斯托克斯位移（TDSS）测量为深入了解液体动力学提供了关键信息。TDSS测量结果的解释通常借助分子动力学模拟，其中溶剂响应函数可通过平衡或非平衡方法计算得出。在非平衡方法中，溶剂与溶质的基态电子态处于平衡状态，其电荷分布瞬间变为第一激发态的电荷分布。然后，溶剂化响应函数被计算为溶剂对电子能隙后续演化影响的非平衡平均值。在平衡方法中，计算溶剂扰动电子能隙波动的归一化时间相关函数。如果线性响应近似有效，那么非平衡溶剂化响应函数与平衡时间相关函数相同。非平衡方法在概念上模拟了实验，但计算成本比平衡方法高得多。在离子液体等多组分体系中，很自然会探究各种组分如何影响所观察到的弛豫动力学。当使用非平衡方法时，溶剂化响应自然分解为系统中每种组分响应的总和。然而，平衡时间相关函数并不能明确分解。在此，我们评估了一种与线性响应近似一致的分解策略，用于研究香豆素153（C153）在1-乙基-3-甲基咪唑四氟硼酸盐（[emim][BF]）离子液体中的溶剂化动力学。平衡和非平衡溶剂化响应函数的一致性证明了C153/[emim][BF]体系线性响应近似方法的有效性。此外，将平衡时间相关函数分解为阴离子和阳离子平动及转动振动运动的贡献，与非线性溶剂化响应的相同分解基本相同。
