Tang Fujie, Ohto Tatsuhiko, Hasegawa Taisuke, Xie Wen Jun, Xu Limei, Bonn Mischa, Nagata Yuki
International Center for Quantum Materials, School of Physics, Peking University , 5 Yiheyuan Road, Haidian, Beijing 100871, China.
Max Planck Institute for Polymer Research, Ackermannweg 10, D-55128 Mainz, Germany.
J Chem Theory Comput. 2018 Jan 9;14(1):357-364. doi: 10.1021/acs.jctc.7b00566. Epub 2017 Dec 4.
Free O-H groups of water are often found at the water-hydrophobic medium interface, e.g. for water contact with hydrophobic protein residues, or at the water-air interface. In surface-specific vibrational spectroscopic studies using sum-frequency generation (SFG) spectroscopy, free O-H groups are experimentally well characterized in the O-H stretch region by a sharp 3700 cm peak. Although these free O-H groups are often defined as the O-H groups which are not hydrogen-bonded to other water molecules, a direct correlation between such non-hydrogen-bonded O-H groups and the 3700 cm SFG response has been lacking. Our data show that commonly used hydrogen bond definitions do not adequately capture the free O-H groups contributing to the 3700 cm peak. We thus formulate a new definition for capturing the subensemble of the surface free O-H groups using the intermolecular distance and the angle formed by the water dimer, through the comparison of the ∼3700 cm SFG response and the responses from the selected free O-H groups at the HOD-air interface. Using these optimized free O-H group definitions, we infer the fraction of interfacial water molecules with free O-H groups of 28%, a vibrational lifetime of the free O-H groups of 1.3 ps, and the angle formed by the free O-H groups and the surface normal of 67° at the water-air interface. We expect that this improved free O-H group definition can be helpful in exploring the structure and dynamics of the interfacial water.
水的游离O-H基团通常出现在水与疏水介质的界面处，例如水与疏水蛋白质残基接触时，或者在水-空气界面处。在使用和频产生（SFG）光谱的表面特异性振动光谱研究中，游离O-H基团在O-H伸缩区域通过一个尖锐的3700 cm⁻¹峰在实验上得到了很好的表征。尽管这些游离O-H基团通常被定义为未与其他水分子形成氢键的O-H基团，但这种未形成氢键的O-H基团与3700 cm⁻¹ SFG响应之间一直缺乏直接关联。我们的数据表明，常用的氢键定义并不能充分捕捉到对3700 cm⁻¹峰有贡献的游离O-H基团。因此，我们通过比较约3700 cm⁻¹ SFG响应与HOD-空气界面处选定游离O-H基团的响应，利用分子间距离和水二聚体形成的角度，为捕捉表面游离O-H基团的子集制定了一个新定义。使用这些优化后的游离O-H基团定义，我们推断出具有游离O-H基团的界面水分子比例为28%，游离O-H基团的振动寿命为1.3 ps，并且在水-空气界面处游离O-H基团与表面法线形成的角度为67°。我们期望这个改进后的游离O-H基团定义有助于探索界面水的结构和动力学。
