Nonlinear, nonpolar solvation dynamics in water: The roles of electrostriction and solvent translation in the breakdown of linear response
Nonlinear, nonpolar solvation dynamics in water: The roles of electrostriction and solvent translation in the breakdown of linear response
复制标题
DOI：
10.1021/jp000326u
复制
发表时间：
2000-06-08
影响因子：
3.3
通讯作者：
Schwartz, BJ
中科院分区：
文献类型：
--
作者：
Aherne, D;Tran, V;Schwartz, BJ
溶剂分子的运动决定了电子转移和其他化学反应的反应坐标，这一事实引起了人们对溶剂化动力学的极大兴趣，溶剂化动力学研究溶剂如何响应溶质电子状态的变化。在线性响应（LR）的极限中，当由溶质引起的扰动是“小的”时，受激溶质的能隙的弛豫应该表现为与未受扰溶质的弛豫动力学相同，遵循差距远离平衡的自然波动。尽管事实上，除了一个基本单位的电荷到一个小的敬礼结果在溶剂化能量是几十或几百kT，溶剂化动力学的计算机模拟发现，只有少数例外，LR服从溶质电荷的变化。然而，这些工作基本上都不能解释真实的化学反应中溶质的大小、形状以及电荷分布都发生变化的事实。在本文中，我们比较了分子模拟的结果极性和非极性溶剂化动力学的一个简单的Lennard-Jones溶质在灵活的水溶液，探讨LR的有效性。我们发现，当涉及到短程力，LR打破了戏剧性的：惯性和扩散组件的松弛不同于LR预测。对于溶质尺寸的增加，溶质的膨胀驱使第一壳层溶剂分子进入第二壳层。由此产生的非平衡弛豫利用不发生在平衡时的旋光-旋转耦合，导致比LR预测的更快的溶剂化。另一方面，溶质尺寸的减小导致溶剂分子的向内平移运动，其通过使（未占据的）基态的能量不稳定来影响溶质的能隙。非平衡弛豫中涉及的向内运动在平衡时不存在，因为基态的不稳定远大于kT。因为在能量上最重要的溶剂分子，也就是那些离溶质最近的分子，在激发时，离开溶质的可能性和靠近溶质的可能性是一样的。尺寸减小的溶剂化比LR预测的慢得多。在最现实的情况下，当溶质的大小和电荷都改变时，由大小变化引起的溶剂平移运动和由电致伸缩引起的溶剂平移运动，带电溶质和极性溶剂之间的净离子偶极吸引，以相加的方式结合联合收割机。当溶质既获得电荷又膨胀时，由电致伸缩产生的平移运动几乎从向外的溶质膨胀中抵消这些，使得旋转运动主导溶剂响应;剩余的气味净膨胀仅导致LR的轻微击穿。另一方面，当溶质带电且其尺寸减小时，额外的向内溶剂平移超出了电致伸缩所需的平移，这是必要的，导致LR的严重击穿。所有的结果进行了比较与以前的实验和理论研究的溶剂化动力学，溶剂驱动的化学反应的影响进行了讨论。
The fact that the motion of solvent molecules defines the reaction coordinate for electron-transfer and other chemical reactions has generated great interest in solvation dynamics, the study of how the solvent responds to changes in a solute's electronic state. In the limit of linear response (LR), when the perturbation caused by the solute is "small", the relaxation of the excited solute's energy gap should behave identically to the relaxation dynamics of the unperturbed solute following a natural fluctuation of the gap away from equilibrium. Despite the fact that the addition of a fundamental unit of charge to a small salute results in a solvation energy that is tens or hundreds of kT, computer simulations of solvation dynamics have found, with only a few exceptions, that LR is obeyed for changes in solute charge. Essentially none of this work, however, accounts for the fact that the solutes in real chemical reactions undergo changes in size and shape as well as in charge distribution. In this paper, we compare the results of molecular simulations of polar and nonpolar solvation dynamics for a simple Lennard-Jones solute in a flexible-water solution to explore the validity of LR. We find that, when short-range forces are involved, LR breaks down dramatically: both the inertial and diffusive components of the relaxation differ from those predicted by LR. For increases in solute size, expansion of the solute drives the first-shell solvent molecules into the second shell. The resulting nonequilibrium relaxation takes advantage of translation-rotation coupling that does not occur at equilibrium, resulting in faster solvation than that predicted by LR. Decreases in solute size, on the other hand, result in inward translational motions of solvent molecules that affect the solute's energy gap by destabilizing the energy of the (unoccupied) ground state. The inward motions involved in the nonequilibrium relaxation are not present at equilibrium because the destabilization of the ground state is much larger than kT. Because the energetically most important solvent molecules, those closest to the solute, are just as likely to be moving away from the solute as toward it at the time of excitation. solvation for decreases in size is much slower than predicted by LR. In the most realistic cases, when both the size and the charge of the solute change, the solvent translational motions resulting from the size change and those resulting from electrostriction, the net ion-dipole attraction between the charged solute and the polar solvent, combine in an additive fashion. When the solute both gains a charge and expands, the translational motions resulting from electrostriction nearly cancel these from the outward solute expansion so that rotational motions dominate the solvent response; the smell net expansion that remains results in only a minor breakdown of LR. The additional inward solvent translations beyond those required by electrostriction, which are necessary when the solute becomes charged and its size decreases, on the other hand, result in a severe breakdown of LR. All of the results are compared with previous experimental and theoretical studies of solvation dynamics, and the implications for solvent-driven chemical reactions are discussed.