The Fluxional Nature of the Hydrated Electron: Energy and Entropy Contributions to Aqueous Electron Free Energies
The Fluxional Nature of the Hydrated Electron: Energy and Entropy Contributions to Aqueous Electron Free Energies
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水合电子的通量性质：水电子自由能的能量和熵贡献
DOI：
10.1021/acs.jctc.9b00496
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发表时间：
2020-02-01
影响因子：
5.5
通讯作者：
Schwartz, Benjamin J.
中科院分区：
文献类型：
--
作者：
Glover, William J.;Schwartz, Benjamin J.
关于水合电子的结构，以及它是占据空穴还是含有大量的内部沃茨（非空穴），最近有很多争论。我们在这项工作中解决的问题是，从自由能的角度来看，这些建议的结构有多大的不同？不同的结构是否都沿着一个连续体，或者是否存在显著的差异（即，自由能壁垒（free energy barrier）之间的关系。为了解决这些问题，我们已经进行了一系列的单电子计算，使用伞采样与量子偏置分子动力学沿着一个坐标，直接反映了水分子的数量在水合电子的内部。我们验证了一个标准的腔模型的水合电子的行为基本上是一个硬球：该模型是由排斥在短距离，使水从电子周围的局部体积排出，导致水溶剂化壳像的pseudohalide离子。排斥力远大于室温附近的热能，这解释了为什么这些模型表现出几乎不依赖于温度的特性。另一方面，我们的计算表明，非空腔模型是高度流动的，这意味着热运动导致内部沃茨的数量从有效的零波动（即，空穴型电子）以潜在地高于体积水密度。在非空穴模型中，能量的贡献仍然是排斥的，因为它们有利于空穴的形成，所以结构的波动主要是由熵驱动的：从空间区域驱逐水的熵成本足够大，以至于一些水仍然被驱入电子的内部。随着温度的降低，熵变得不那么重要，非腔电子的结构被预测为变得更像腔，与观察到的水合电子的性质的温度依赖性一致。因此，我们认为，虽然我们研究的具体noncavity模型高估了涉及内部水分子的波动的优势，适当的改进，以正确地捕捉内部沃茨和摩尔溶剂化体积的真实平均数，一个fluxional模型可能是最有意义的理解各种实验性质的水合电子。
There has been a great deal of recent controversy over the structure of the hydrated electron and whether it occupies a cavity or contains a significant number of interior waters (noncavity). The questions we address in this work are, from a free energy perspective, how different are these proposed structures? Do the different structures all lie along a single continuum, or are there significant differences (i.e., free energy barriers) between them? To address these questions, we have performed a series of one-electron calculations using umbrella sampling with quantum biased molecular dynamics along a coordinate that directly reflects the number of water molecules in the hydrated electron's interior. We verify that a standard cavity model of the hydrated electron behaves essentially as a hard sphere: the model is dominated by repulsion at short range such that water is expelled from a local volume around the electron, leading to a water solvation shell like that of a pseudohalide ion. The repulsion is much larger than thermal energies near room temperature, explaining why such models exhibit properties with little temperature dependence. On the other hand, our calculations reveal that a noncavity model is highly fluxional, meaning that thermal motions cause the number of interior waters to fluctuate from effectively zero (i.e., a cavity-type electron) to potentially above the bulk water density. The energetic contributions in the noncavity model are still repulsive in the sense that they favor cavity formation, so the fluctuations in structure are driven largely by entropy: the entropic cost for expelling water from a region of space is large enough that some water is still driven into the electron's interior. As the temperature is lowered and entropy becomes less important, the noncavity electron's structure is predicted to become more cavity-like, consistent with the observed temperature dependence of the hydrated electron's properties. Thus, we argue that although the specific noncavity model we study overestimates the preponderance of fluctuations involving interior water molecules, with appropriate refinements to correctly capture the true average number of interior waters and molar solvation volume, a fluxional model likely makes the most sense for understanding the various experimental properties of the hydrated electron.