Narvaez Wilberth A, Park Sanghyun J, Schwartz Benjamin J
Department of Chemistry and Biochemistry, University of California, Los Angeles Los Angeles, California 90095-1569 United States.
J Phys Chem B. 2022 May 26;126(20):3748-3757. doi: 10.1021/acs.jpcb.2c01501. Epub 2022 May 11.
Experimental studies have demonstrated that the hydrated electron's absorption spectrum undergoes a concentration-dependent blue-shift in the presence of electrolytes such as NaCl. The blue-shift increases roughly linearly at low salt concentration but saturates as the solubility limit of the salt is approached. Previous attempts to understand the origin of the concentration-dependent spectral shift using molecular simulation have only examined the interaction between the hydrated electron and a single sodium cation, and these simulations predicted a spectral blue-shift that was an order of magnitude larger than that seen experimentally. Thus, in this paper, we first explore the reasons for the exaggerated spectral blue-shift when a simulated hydrated electron interacts with a single Na. We find that the issue arises from nonpairwise additivity of the Na-e and HO-e pseudopotentials used in the simulation. This effect arises because the solvating water molecules donate charge into the empty orbitals of Na, lowering the effective charge of the cation and thus reducing the excess electron-cation interaction. Careful analysis shows, however, that although this nonpairwise additivity changes the energetics of the electron-Na interaction, the forces between the electron, Na, and water are unaffected, so that mixed quantum/classical (MQC) simulations produce the correct structure and dynamics. With this in hand, we then use MQC simulations to explore the behavior of the hydrated electron as an explicit function of NaCl salt concentration. We find that the simulations correctly reproduce the observed experimental spectral shifting behavior. The reason for the spectral shift is that as the electrolyte concentration increases, the average number of cations simultaneously interacting in contact pairs with the hydrated electron increases from 1.0 at low concentrations to ∼2.5 near the saturation limit. As the number of cations that interact with the electron increases, the cation/electron interactions becomes slightly weaker, so that the corresponding Na-e distance increases with increasing salt concentration. We also find that the dielectric constant of the solution plays little role in the observed spectroscopy, so that the electrolyte-dependent spectral shifts of the hydrated electron are directly related to the concentration-dependent number of closely interacting cations.
实验研究表明，在诸如氯化钠等电解质存在的情况下，水合电子的吸收光谱会发生浓度依赖性蓝移。在低盐浓度下，蓝移大致呈线性增加，但随着盐的溶解度极限临近，蓝移趋于饱和。此前利用分子模拟来理解浓度依赖性光谱位移起源的尝试，仅考察了水合电子与单个钠离子之间的相互作用，而这些模拟预测的光谱蓝移比实验观测到的大一个数量级。因此，在本文中，我们首先探究当模拟的水合电子与单个钠相互作用时，光谱蓝移被夸大的原因。我们发现这个问题源于模拟中使用的钠 - 电子和氢氧根 - 电子赝势的非成对加和性。这种效应的出现是因为溶剂化水分子将电荷注入钠的空轨道，降低了阳离子的有效电荷，从而减少了多余电子与阳离子的相互作用。然而，仔细分析表明，尽管这种非成对加和性改变了电子 - 钠相互作用的能量，但电子、钠和水之间的力并未受到影响，所以混合量子/经典（MQC）模拟能够产生正确的结构和动力学。有了这些认识，我们接着使用MQC模拟来探究水合电子行为随氯化钠盐浓度的具体变化。我们发现模拟结果正确地再现了观测到的实验光谱位移行为。光谱位移的原因是，随着电解质浓度增加，与水合电子以接触对形式同时相互作用的阳离子平均数量从低浓度时的1.0增加到接近饱和极限时的约2.5。随着与电子相互作用的阳离子数量增加，阳离子/电子相互作用会略微变弱，使得相应的钠 - 电子距离随盐浓度增加而增大。我们还发现溶液的介电常数在观测到的光谱学中作用很小，因此水合电子的电解质依赖性光谱位移直接与密切相互作用的阳离子的浓度依赖性数量相关。
