Tay Kafui A, Coudert François-Xavier, Boutin Anne
Laboratoire de Chimie Physique, Université de Paris-Sud, 91405 Orsay Cedex, France.
J Chem Phys. 2008 Aug 7;129(5):054505. doi: 10.1063/1.2964101.
Molecular dynamics simulations are used to study the mechanism and kinetics of hydrated electron diffusion. The electron center of mass is found to exhibit Brownian-type behavior with a diffusion coefficient considerably greater than that of the solvent. As previously postulated by both experimental and theoretical works, the instantaneous response of the electron to the librational motions of surrounding water molecules constitutes the principal mode of motion. The diffusive mechanism can be understood within the traditional framework of transfer diffusion processes, where the diffusive step is akin to the exchange of an extramolecular electron between neighboring water molecules. This is a second-order process with a computed rate constant of 5.0 ps(-1) at 298 K. In agreement with experiment the electron diffusion exhibits Arrhenius behavior over the temperature range of 298-400 K. We compute an activation energy of 8.9 kJ mol(-1). Through analysis of Arrhenius plots and the application of a simple random walk model it is demonstrated that the computed rate constant for exchange of an excess electron is indeed the phenomenological rate constant associated with the diffusive process.
分子动力学模拟用于研究水合电子扩散的机制和动力学。发现电子质心表现出布朗型行为，其扩散系数远大于溶剂的扩散系数。正如之前实验和理论工作所假设的那样，电子对周围水分子旋转运动的瞬时响应构成了主要运动模式。扩散机制可以在传统的转移扩散过程框架内理解，其中扩散步骤类似于相邻水分子之间分子外电子的交换。这是一个二级过程，在298 K时计算得到的速率常数为5.0 ps⁻¹。与实验一致，电子扩散在298 - 400 K的温度范围内表现出阿伦尼乌斯行为。我们计算出活化能为8.9 kJ mol⁻¹。通过对阿伦尼乌斯图的分析和简单随机游走模型的应用，证明了计算得到的多余电子交换速率常数确实是与扩散过程相关的唯象速率常数。
