Fachbereich Physik, Freie Universität Berlin, Arnimallee 14, 14195Berlin, Germany.
J Phys Chem B. 2022 Dec 15;126(49):10295-10304. doi: 10.1021/acs.jpcb.2c05923. Epub 2022 Dec 6.
When described by a one-dimensional reaction coordinate, pair-reaction rates in a solvent depend, in addition to the potential barrier height and the friction coefficient, on the potential shape, the effective mass, and the friction relaxation spectrum, but a rate theory that accurately accounts for all of these effects does not exist. After a review of classical reaction-rate theories, we show how to extract all parameters of the generalized Langevin equation (GLE) and, in particular, the friction memory function from molecular dynamics (MD) simulations of two prototypical pair reactions in water, the dissociation of NaCl and of two methane molecules. The memory exhibits multiple time scales and, for NaCl, pronounced oscillatory components. Simulations of the GLE by Markovian embedding techniques accurately reproduce the pair-reaction kinetics from MD simulations without any fitting parameters, which confirms the accuracy of the approximative form of the GLE and of the parameter extraction techniques. By modification of the GLE parameters, we investigate the relative importance of memory, mass, and potential shape effects. Neglect of memory slows down NaCl and methane dissociation by roughly a factor of 2; neglect of mass accelerates reactions by a similar factor, and the harmonic approximation of the potential shape gives rise to slight acceleration. This partial error cancellation explains why Kramers' theory, which neglects memory effects and treats the potential shape in harmonic approximation, describes reaction rates better than more sophisticated theories. In essence, all three effects, friction memory, inertia, and the potential shape nonharmonicity, are important to quantitatively describe pair-reaction kinetics in water.
当用一维反应坐标来描述时，溶剂中的对反应速率除了依赖于势垒高度和摩擦系数外，还依赖于势形、有效质量和摩擦弛豫谱，但目前并不存在一种能准确考虑所有这些效应的速率理论。在对经典反应速率理论进行回顾后，我们展示了如何从 NaCl 和两个甲烷分子的解离这两个典型对反应在水中的分子动力学（MD）模拟中提取广义朗之万方程（GLE）的所有参数，特别是摩擦记忆函数。该记忆函数表现出多个时间尺度，并且对于 NaCl 具有明显的振荡分量。通过马尔可夫嵌入技术对 GLE 的模拟准确地再现了从 MD 模拟中得到的对反应动力学，而无需任何拟合参数，这证实了 GLE 的近似形式和参数提取技术的准确性。通过修改 GLE 参数，我们研究了记忆、质量和势形效应对反应速率的相对重要性。忽略记忆会使 NaCl 和甲烷的解离速度减慢约 2 倍；忽略质量会使反应速度加快相同的倍数，而势形的谐波近似会导致轻微的加速。这种部分误差抵消解释了为什么 Kramers 理论（忽略记忆效应并以谐波近似处理势形）比更复杂的理论能更好地描述反应速率。本质上，摩擦记忆、惯性和势形非谐性这三种效应对于定量描述水中的对反应动力学都很重要。
