Lehrstuhl für Theoretische Chemie, Ruhr-Universität Bochum, 44780 Bochum, Germany.
Phys Chem Chem Phys. 2019 Feb 27;21(9):4975-4987. doi: 10.1039/c8cp07489e.
The molecular-level understanding of THz spectra of aqueous solutions under ambient conditions has been greatly advanced in recent years. Here, we go beyond previous analyses by performing ab initio molecular dynamics simulations of glycine in water with artificially frozen solute or solvent molecules, respectively, while computing the total THz response as well as its decomposition into mode-specific resonances based on the "supermolecular solvation complex" technique. Clamping the water molecules and keeping glycine moving breaks the coupling of glycine to the structural dynamics of the solvent, however, the polarization and dielectric solvation effects in the static solvation cage are still at work since the full electronic structure of the quenched solvent is taken into account. The complementary approach of fixing glycine reveals both the dynamical and electronic response of the solvation cage at the level of its THz response. Moreover, to quantitatively account for the electronic contribution solely due to solvent embedding, the solute species is "vertically desolvated", thus preserving the fully coupled solute-solvent motion in terms of the solute's structural dynamics in solution, while its electronic structure is no longer subject to solute-solvent polarization and charge transfer effects. When referenced to the free simulation of Gly(aq), this three-fold approach allows us to decompose the THz spectral contributions due to the correlated solute-solvent dynamics into entirely structural and purely electronic effects. Beyond providing hitherto unknown insights, the observed systematic changes of THz spectra in terms of peak shifts and lineshape modulations due to conformational freezing and frozen solvation cages might be useful to investigate the solvation of molecules in highly viscous H-bonding solvents such as ionic liquids and even in cryogenic ices as relevant to polar stratospheric and dark interstellar clouds.
近年来，人们对环境条件下水溶液的太赫兹光谱的分子水平理解有了很大的提高。在这里，我们通过分别对人工冻结的溶质或溶剂分子进行甘氨酸在水中的从头算分子动力学模拟，超越了以前的分析，同时根据“超分子溶剂络合物”技术计算总太赫兹响应及其分解为特定模式的共振。然而，通过冻结水分子并保持甘氨酸运动，打破了甘氨酸与溶剂结构动力学的耦合，但由于考虑了冻结溶剂的完整电子结构，静态溶剂笼中的极化和介电溶剂化效应仍在起作用。固定甘氨酸的互补方法揭示了溶剂笼在太赫兹响应水平上的动态和电子响应。此外，为了仅定量考虑电子贡献，溶剂嵌入，将溶质物种“垂直去溶剂化”，从而在溶液中保留溶质的结构动力学方面保持完全耦合的溶质-溶剂运动，而其电子结构不再受溶剂-溶质极化和电荷转移效应的影响。当参考甘氨酸的自由模拟时，这种三重方法允许我们将与相关溶质-溶剂动力学相关的太赫兹光谱贡献分解为完全结构和纯电子效应。除了提供迄今为止未知的见解外，由于构象冻结和冻结溶剂笼引起的太赫兹光谱的系统变化，如峰值位移和线形调制，可能有助于研究高度粘性氢键溶剂（如离子液体）中分子的溶剂化作用，甚至在与极地平流层和黑暗星际云相关的低温冰中。
