Ab Initio Investigation of the Resonance Raman Spectrum of the Hydrated Electron
Ab Initio Investigation of the Resonance Raman Spectrum of the Hydrated Electron
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DOI：
10.1021/acs.jpcb.9b04895
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发表时间：
2019-09-26
影响因子：
3.3
通讯作者：
Herbert, John M.
中科院分区：
文献类型：
--
作者：
Dasgupta, Saswata;Rana, Bhaskar;Herbert, John M.
根据传统的观点，水或“水合”电子 e(-)(aq) 占据液态水结构中的排除体积（空腔）。然而，某些单电子模型的模拟预测，不成对电子的自旋密度更加离域，没有明显的空腔结构。有人建议，只有后者（非空腔）结构才能解释水合电子的共振拉曼光谱，尽管这一建议是基于使用针对纯液态水而非 e(-)(aq) 开发的经验频率图进行的计算。这里提出的全电子从头计算表明，e(-)(aq) 的空腔和非空腔模型都在 O-H 拉伸区域中提供了显着的红移。这种效应是非特异性的，是由于电子渗透到水分子的前沿轨道而产生的。然而，只有传统的空腔模型才能重现实验观察到的 H-O-D 弯曲分裂（在同位素混合水中），这种分裂是由于电子第一溶剂化壳层中羟基部分的不对称环境而产生的。我们得出的结论是，e(-)(aq) 的空腔模型比离域非空腔模型更符合测量的共振拉曼光谱，尽管之前有相反的建议。此外，混合密度泛函和 Hartree-Fock 理论的计算预测，非空腔液体几何形状只能为额外的电子提供无束缚（连续）态，而实际上该能级应比真空能级低 3 eV 以上。因此，e(-)(aq) 的非腔模型似乎与现有的振动光谱、光电子能谱和量子化学不一致。
According to the conventional picture, the aqueous or "hydrated" electron, e(-)(aq), occupies an excluded volume (cavity) in the structure of liquid water. However, simulations with certain one-electron models predict a more delocalized spin density for the unpaired electron, with no distinct cavity structure. It has been suggested that only the latter (non-cavity) structure can explain the hydrated electron's resonance Raman spectrum, although this suggestion is based on calculations using empirical frequency maps developed for neat liquid water, not for e(-)(aq). All-electron ab initio calculations presented here demonstrate that both cavity and non-cavity models of e(-)(aq) afford significant red-shifts in the O-H stretching region. This effect is nonspecific and arises due to electron penetration into frontier orbitals of the water molecules. Only the conventional cavity model, however, reproduces the splitting of the H-O-D bend (in isotopically mixed water) that is observed experimentally and arises due to the asymmetric environments of the hydroxyl moieties in the electron's first solvation shell. We conclude that the cavity model of e(-)(aq) is more consistent with the measured resonance Raman spectrum than is the delocalized, non-cavity model, despite previous suggestions to the contrary. Furthermore, calculations with hybrid density functionals and with Hartree-Fock theory predict that non-cavity liquid geometries afford only unbound (continuum) states for an extra electron, whereas in reality this energy level should lie more than 3 eV below vacuum level. As such, the non-cavity model of e(-)(aq) appears to be inconsistent with available vibrational spectroscopy, photoelectron spectroscopy, and quantum chemistry.