Heid Esther, Moser Wanda, Schröder Christian
University of Vienna, Faculty of Chemistry, Department of Computational Biological Chemistry, Währingerstraße 19, A-1090 Vienna, Austria.
Phys Chem Chem Phys. 2017 May 3;19(17):10940-10950. doi: 10.1039/c6cp08575j.
The time-dependent fluorescence of a chromophore can be calculated from either nonequilibrium simulations, or, as long as linear response theory holds true, from equilibrium solvent fluctuations in the ground or excited state if the perturbation inflicted by the chromophore is small. The assumption of Gaussian statistics, in contrast, links the nonequilibrium dynamics to solvent fluctuations solely in the excited state, as long as the energy gap distribution is Gaussian throughout the process. The validity of linear response theories on the ground and excited state surface as well as Gaussian statistics is thoroughly tested in this study by calculating the time-dependent Stokes shift of different benzene-like solutes. The effect of the size of change in partial charges of the solute, the multipolar order of charge distribution, the direction of change, as well as the influence of different solvents on the validity of linear response theory is examined by simulating 54 different systems. Calculation of the Gaussian character of the energy distribution in equilibrium, as well as the time-evolution of the peak width in the nonequilibrium simulation sheds light on the validity of Gaussian statistics in a nonstationary regime. We observed that a large intermediate broadening of the width of the energy distribution correlates with a failure of correlation functions to describe the nonequilibrium event. These results are accompanied by analysis of higher order correlation functions, as well as the structure of the solvents water, acetonitrile and methanol around the solute, to yield a comprehensive view, as well as general guidelines, on when and why equilibrium solvent fluctuations can correctly depict solvation dynamics.
发色团的时间相关荧光可以通过非平衡模拟来计算，或者只要线性响应理论成立，在发色团造成的扰动较小的情况下，也可以根据基态或激发态的平衡溶剂涨落来计算。相比之下，高斯统计假设将非平衡动力学仅与激发态的溶剂涨落联系起来，只要整个过程中的能隙分布是高斯分布。在本研究中，通过计算不同苯类溶质的时间相关斯托克斯位移，全面测试了基态和激发态表面线性响应理论以及高斯统计的有效性。通过模拟54个不同的系统，研究了溶质部分电荷变化的大小、电荷分布的多极序、变化方向以及不同溶剂对线性响应理论有效性的影响。计算平衡态下能量分布的高斯特性以及非平衡模拟中峰宽的时间演化，有助于揭示非平稳状态下高斯统计的有效性。我们观察到，能量分布宽度的大幅中间展宽与相关函数无法描述非平衡事件有关。这些结果还伴随着对高阶相关函数以及溶质周围水、乙腈和甲醇等溶剂结构的分析，从而对平衡溶剂涨落何时以及为何能够正确描述溶剂化动力学给出全面的认识以及一般指导原则。
