Evaluating Simple Ab Initio Models of the Hydrated Electron: The Role of Dynamical Fluctuations
Evaluating Simple Ab Initio Models of the Hydrated Electron: The Role of Dynamical Fluctuations
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评估水合电子的简单从头算模型：动态涨落的作用
DOI：
10.1021/acs.jpcb.0c6356
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发表时间：
2020
期刊：
影响因子：
--
通讯作者：
Schwartz, B. J.
中科院分区：
文献类型：
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作者：
Park, S. J.;Schwartz, B. J.
尽管它在电子转移反应和辐射化学中很重要，但人们对水合电子的基本性质存在分歧，例如它是否存在于空腔中。混合量子/经典模拟的水合电子得到不同的结构，这取决于所采用的赝势，和从头计算模型的计算必要性使用少量的水分子和/或提供不足的统计数据，以比较实验观测。几年前，Kumar等人（J. Phys. Chem. A2015，119，9148）提出了一个水合电子的最小初始模型，其中只有少量明确处理的水分子加上可极化连续模型（PCM）。他们发现，优化的几何形状有四个沃茨围绕中心空腔四面体排列，计算出的垂直脱离能和回转半径与实验吻合得很好，结果在很大程度上与所采用的理论水平无关。然而，该模型是基于一个固定的结构，在0 K，并没有明确纳入熵的贡献或热波动，应该与室温水合electron. Thus，在本文中，我们扩展了Kumar等人的模型。通过运行玻恩奥本海默分子动力学（BOMD）的少量水分子与过量的电子加PCM在室温下。我们发现，当引入热波动，理论选择的水平变得足够关键时，只有四个沃茨被使用的沃茨之一，从集群与某些密度泛函解离。此外，即使具有最佳调谐的距离分离的杂化泛函，在室温下，0 K第一壳层沃茨的四面体取向完全丧失并且中心空腔塌陷，这一过程的驱动因素是显式水分子更倾向于彼此形成氢键，而不是与多余的电子形成氢键。由此产生的平均结构与非腔混合量子/经典模型，使最低限度的4-水BOMD模型遭受类似的问题，noncavity模型，如预测错误的符号的水合电子的摩尔溶剂化体积。我们还使用16个显式水分子加上额外的电子和PCM进行了BOMD。我们发现，包含一个完整的第二溶剂化壳显式水导致的结果变化不大，当只有四个沃茨被使用。事实上，16-水模拟的行为很像水团簇阴离子的行为，其中电子局限于团簇表面，这表明PCM不适合用于最小模型来描述体水合电子的行为。对于4-和16-水模型，我们研究了热运动的引入如何改变预测的吸收光谱，垂直脱离能，和共振拉曼光谱的模拟水合电子。我们还提出了一套结构标准，可以用来数值确定如何腔样（或不）一个特定的水合电子模型。所有的结果都强调，水合电子是一个统计对象，其属性是不充分的捕获，只使用少量的明确的沃茨，并正确处理的热波动是至关重要的理解水合电子的化学和物理行为。
Despite its importance in electron transfer reactions and radiation chemistry, there has been disagreement over the fundamental nature of the hydrated electron, such as whether or not it resides in a cavity. Mixed quantum/classical simulations of the hydrated electron give different structures depending on the pseudopotential employed, andab initiomodels of computational necessity use small numbers of water molecules and/or provide insufficient statistics to compare to experimental observables. A few years ago, Kumar et al. (J. Phys. Chem. A2015,119, 9148) proposed a minimalistab initiomodel of the hydrated electron with only a small number of explicitly treated water molecules plus a polarizable continuum model (PCM). They found that the optimized geometry had four waters arranged tetrahedrally around a central cavity, and that the calculated vertical detachment energy and radius of gyration agreed well with experiment, results that were largely independent of the level of theory employed. The model, however, is based on a fixed structure at 0 K and does not explicitly incorporate entropic contributions or the thermal fluctuations that should be associated with the room-temperature hydrated electron. Thus, in this paper, we extend the model of Kumar et al. by running Born–Oppenheimer molecular dynamics (BOMD) of a small number of water molecules with an excess electron plus PCM at room temperature. We find that when thermal fluctuations are introduced, the level of theory chosen becomes critical enough when only four waters are used that one of the waters dissociates from the cluster with certain density functionals. Moreover, even with an optimally tuned range-separated hybrid functional, at room temperature the tetrahedral orientation of the 0 K first-shell waters is entirely lost and the central cavity collapses, a process driven by the fact that the explicit water molecules prefer to make H-bonds with each other more than with the excess electron. The resulting average structure is quite similar to that produced by a noncavity mixed quantum/classical model, so that the minimalist 4-water BOMD models suffer from problems similar to those of noncavity models, such as predicting the wrong sign of the hydrated electron’s molar solvation volume. We also performed BOMD with 16 explicit water molecules plus an extra electron and PCM. We find that the inclusion of an entire second solvation shell of explicit water leads to little change in the outcome from when only four waters were used. In fact, the 16-water simulations behave much like those of water cluster anions, in which the electron localizes at the cluster surface, showing that PCM is not acceptable for use in minimalist models to describe the behavior of the bulk hydrated electron. For both the 4- and 16-water models, we investigate how the introduction of thermal motions alters the predicted absorption spectrum, vertical detachment energy, and resonance Raman spectrum of the simulated hydrated electron. We also present a set of structural criteria that can be used to numerically determine how cavity-like (or not) a particular hydrated electron model is. All of the results emphasize that the hydrated electron is a statistical object whose properties are inadequately captured using only a small number of explicit waters, and that a proper treatment of thermal fluctuations is critical to understanding the hydrated electron’s chemical and physical behavior.