Theoretical Chemistry Institute and Department of Chemistry, University of Wisconsin, Madison, Wisconsin 53706, USA.
J Chem Phys. 2011 Jul 28;135(4):044701. doi: 10.1063/1.3613623.
We propose a novel interpretation of the water liquid-vapor interface vibrational sum-frequency (VSF) spectrum in terms of hydrogen-bonding classes. Unlike an absorption spectrum, the VSF signal can be considered as a sum of signed contributions from different hydrogen-bonded species in the sample. We show that the recently observed positive feature at low frequency, in the imaginary part of the signal, is a result of cancellation between the positive contributions from four-hydrogen-bonded molecules and negative contributions from those molecules with one or two broken hydrogen bonds. Spectral densities for each of these subgroups span the entire relevant spectral range. Three-body interactions within our newly developed E3B water simulation model prove to be critical in describing the proper balance between different hydrogen-bonded species, as (two-body) SPC/E, TIP4P, and TIP4P/2005 models fail to reproduce the positive feature. The results clarify the molecular origin of the VSF signal, and highlight the importance of many-body interactions for water in heterogeneous situations.
我们提出了一种新的解释，即根据氢键种类来解释水的液-气相振动和频（VSF）谱。与吸收光谱不同，VSF 信号可以被认为是样品中不同氢键化物质的有符号贡献的总和。我们表明，最近在信号的虚部中观察到的低频处的正特征，是四氢键化分子的正贡献与一个或两个氢键断裂的分子的负贡献之间的抵消的结果。这些亚组的谱密度涵盖了整个相关的光谱范围。我们新开发的 E3B 水模拟模型中的三体相互作用被证明在描述不同氢键化物质之间的适当平衡方面至关重要，因为（二体）SPC/E、TIP4P 和 TIP4P/2005 模型无法复制正特征。研究结果阐明了 VSF 信号的分子起源，并强调了多体相互作用对非均相条件下水的重要性。
