Ghorai Pradip Kr, Matyushov Dmitry V
Department of Chemical Sciences, Indian Institute of Science Education and Research Kolkata, Mohanpur 741246, India.
Department of Physics and School of Molecular Sciences, Arizona State University, PO Box 871504, Tempe, Arizona 85287, United States.
J Phys Chem B. 2020 May 7;124(18):3754-3769. doi: 10.1021/acs.jpcb.0c01773. Epub 2020 Apr 27.
A microscopic theory of solvent response by room-temperature ionic liquids is formulated based on the dynamic longitudinal susceptibility of liquid's charge density. The susceptibility function combines the structural information in terms of reciprocal-space structure factors with the memory function responsible for solvation dynamics. The charge-density structure factors and corresponding intermediate scattering functions are analyzed here by molecular dynamics simulations. They show the existence of two drastically different time scales of charge-density fluctuations. Faster, stretched-exponential dynamics are consistent with dielectric measurements. It contributes to the Stokes-shift dynamics of coumarin-153 optical dye calculated with the new theory and compared to experimental reports. The second, much slower and exponential, relaxation shows the phenomenology of de Gennes narrowing: the relaxation time passes through a strong maximum at the wave vector representing the first peak of the structure factor. This peak, which is particularly sharp for the charge density, contributes significantly to the equilibrium free energy of solvation, thus invalidating dielectric theories of solvation for ionic liquids. Dynamics of charge density fluctuations at the length scale consistent with the sharp peak require long observation times. Electron-transfer reactions occurring on faster time-scales are not affected by these slow dynamics. Nonergodic reorganization energy of electron transfer, accounting for the observation window established by the reaction time, drops sharply when the reaction rate crosses the main peak in the Stokes-shift loss spectrum. The dependence of the reorganization energy on the reaction rate strongly affects the energy-gap law of electron transfer, with a tendency for a shallow or entirely disappearing inverted region.
基于液体电荷密度的动态纵向磁化率，建立了室温离子液体溶剂响应的微观理论。磁化率函数将倒易空间结构因子方面的结构信息与负责溶剂化动力学的记忆函数相结合。本文通过分子动力学模拟分析了电荷密度结构因子和相应的中间散射函数。结果表明存在两种截然不同的电荷密度涨落时间尺度。较快的拉伸指数动力学与介电测量结果一致。它有助于用新理论计算并与实验报告进行比较的香豆素 - 153光学染料的斯托克斯位移动力学。第二种慢得多的指数弛豫表现出德热纳窄化现象：弛豫时间在代表结构因子第一个峰值的波矢处通过一个强烈的最大值。这个峰值对于电荷密度特别尖锐，对溶剂化的平衡自由能有显著贡献，从而使离子液体的溶剂化介电理论失效。与尖锐峰值一致的长度尺度上的电荷密度涨落动力学需要很长的观测时间。在较快时间尺度上发生的电子转移反应不受这些慢动力学的影响。当反应速率穿过斯托克斯位移损耗谱中的主峰时，考虑到反应时间确定的观测窗口，电子转移的非遍历重组能会急剧下降。重组能对反应速率的依赖性强烈影响电子转移的能隙定律，倾向于出现浅的或完全消失的反转区域。
