Borrelli William R, Guardado Sandoval José L, Mei Kenneth J, Schwartz Benjamin J
Department of Chemistry & Biochemistry, University of California, Los Angeles Los Angeles, California 90095-1569, United States.
J Chem Theory Comput. 2024 Aug 7;20(16):7337-46. doi: 10.1021/acs.jctc.4c00780.
Even though single hydrated electrons ('s) are stable in liquid water, two hydrated electrons can bimolecularly react with water to create H and hydroxide: + + 2HO → H + 2OH. The rate of this reaction has an unusual temperature and isotope dependence as well as no dependence on ionic strength, which suggests that cosolvation of two electrons as a single hydrated dielectron () might be an important intermediate in the mechanism of this reaction. Here, we present an ab initio density functional theory study of this reaction to better understand the potential properties, reactivity, and experimental accessibility of hydrated dielectrons. Our simulations create hydrated dielectrons by first simulating single 's and then injecting a second electron, providing a well-defined time zero for formation and offering insight into a potential experimental route to creating dielectrons and optically inducing the reaction. We find that immediately forms in every member of our ensemble of trajectories, allowing us to study the molecular mechanism of H and OH formation. The subsequent reaction involves separate proton transfer steps with a generally well-defined hydride subintermediate. The time scales for both proton transfer steps are quite broad, with the first proton transfer step spanning times over a few ps, while the second proton transfer step varies over ∼150 fs. We find that the first proton transfer rate is dictated by whether or not the reacting water is part of an H-bond chain that allows the newly created OH to rapidly move by Grotthuss-type proton hopping to minimize electrostatic repulsion with H. The second proton transfer step depends significantly on the degree of solvation of H, leading to a wide range of reactive geometries where the two waters involved can lie either across the dielectron cavity or more adjacent to each other. This also allows the two proton transfer events to take place either effectively concertedly or sequentially, explaining differing views that have been presented in the literature.
尽管单个水合电子（$e^-$）在液态水中是稳定的，但两个水合电子可以与水发生双分子反应生成氢气（$H_2$）和氢氧根离子（$OH^-$）：$e^- + e^- + 2H_2O → H_2 + 2OH^-$。该反应速率具有不寻常的温度和同位素依赖性，且与离子强度无关，这表明两个电子作为单个水合双电子（$e_2^-$）的共溶剂化可能是该反应机制中的一个重要中间体。在此，我们提出了对该反应的从头算密度泛函理论研究，以更好地理解水合双电子的潜在性质、反应活性和实验可及性。我们的模拟通过首先模拟单个$e^-$，然后注入第二个电子来创建水合双电子，为$e_2^-$的形成提供了明确的时间零点，并深入了解了创建双电子和光诱导反应的潜在实验途径。我们发现，在我们的轨迹系综的每个成员中，$e_2^-$都能立即形成，这使我们能够研究$H_2$和$OH^-$形成的分子机制。随后的反应涉及单独的质子转移步骤，通常有一个明确的氢化物亚中间体。两个质子转移步骤的时间尺度都很宽，第一个质子转移步骤跨越几个皮秒的时间，而第二个质子转移步骤在约150飞秒内变化。我们发现，第一个质子转移速率取决于反应的水分子是否是氢键链的一部分，该氢键链允许新生成的$OH^-$通过Grotthuss型质子跳跃快速移动，以最小化与$H_2$的静电排斥。第二个质子转移步骤显著取决于$H_2$的溶剂化程度，导致了广泛的反应几何构型，其中涉及的两个水分子可以位于双电子腔的对面或彼此更相邻。这也使得两个质子转移事件可以有效地协同发生或依次发生，解释了文献中提出的不同观点。
