Hydrogen abstraction from ethanol by atomic hydrogen in aqueous solution is studied using two theoretical approaches: the multipath variational transition state theory (MP-VTST) and a path-integral formalism in combination with free-energy perturbation and umbrella sampling (PI-FEP/UM). The performance of the models is compared to experimental values of H kinetic isotope effects (KIE). Solvation models used in this study ranged from purely implicit, via mixed-microsolvation treated quantum mechanically via the density functional theory (DFT) to fully explicit representation of the solvent, which was incorporated using a combined quantum mechanical-molecular mechanical (QM/MM) potential. The effects of the transition state conformation and the position of microsolvating water molecules interacting with the solute on the KIE are discussed. The KIEs are in good agreement with experiment when MP-VTST is used together with a model that includes microsolvation of the polar part of ethanol by five or six water molecules, emphasizing the importance of explicit solvation in KIE calculations. Both, MP-VTST and PI-FEP/UM enable detailed characterization of nuclear quantum effects accompanying the hydrogen atom transfer reaction in aqueous solution.

中文翻译：

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微溶剂化和量子效应在准确预测动力学同位素效应中的作用：以水溶液中原子氢提取乙醇中的氢原子为例。

使用两种理论方法研究了水溶液中原子氢从乙醇中提取氢的方法：多径变分过渡态理论（MP-VTST）和结合自由能扰动和伞形采样（PI-FEP / UM）的路径积分形式主义）。将模型的性能与H动力学同位素效应（KIE）的实验值进行比较。本研究中使用的溶剂化模型的范围从纯隐式的，通过通过密度泛函理论（DFT）机械地处理过的量子力学的混合微溶剂化到溶剂的完全显式表示，通过组合的量子力学-分子力学（QM / MM）纳入潜在。讨论了过渡态构象和微溶剂化水分子与溶质相互作用的位置对KIE的影响。当MP-VTST与包括五个或六个水分子对乙醇的极性部分进行微溶剂化的模型一起使用时，KIE与实验非常吻合，从而强调了在KIE计算中显式溶剂化的重要性。MP-VTST和PI-FEP / UM均可详细表征伴随水溶液中氢原子转移反应的核量子效应。