光谱学与光谱分析, 2019, 39 (5): 1458, 网络出版: 2019-05-13  

基于新型超低频拉曼光谱的液态水分子高温研究

Temperature Effects on Novel Ultra-Low Frequency Raman Spectroscopy of Liquid Water
作者单位
浙江科技学院理学院应用物理系, 浙江 杭州 310023
摘要
液态水是地球上大多数生化过程的化学支柱, 对生物的新陈代谢是必不可少的。 因此, 它是一个跨科学领域的关键课题。 水的理化特性被认为是氢键衍生结构的结果。 然而, 目前还很难在实验上定量地将水分子的理化特性与氢键结构联系起来形成完整的液态水分子结构理论。 拉曼光谱因其快速、 无损等优点成为表征液态水分子结构及其变化规律的主要手段。 目前, 水分子的拉曼光谱主要研究的是其高频振动模。 然而, 液态水较宽的低频拉曼模是氢键及其局部结构效应的结果, 包含高频峰无法表征的特征信息, 而超低频拉曼特征峰仍能在高温下揭示水分子(超)结构的许多关键细节。 因此, 在实验上实现对水分子的新型高温超低频拉曼光谱(5~200 cm-1频率区域), 探测得到理论预测的全部四种平动特征模, 包括弯曲模(51.7 cm-1)、 扭转模(81.4 cm-1)、 对称(154.0 cm-1)和不对称拉伸模(188.6 cm-1), 并在225.2 cm-1处额外发现了平动-旋转耦合特征模。 所有特征模都被精确指认。 高温超低频拉曼光谱实验发现, 首先在特征峰频率上, 由于高温下氢键断裂导致水分子间的平均结构关联长度(SLG)迅速缩短, 当温度从0 ℃升高到400 ℃, 所有四种超低频特征模的频率都随温度升高而大幅蓝移。 其次在特征峰强度上, 拉伸模的强度在100和200 ℃间出现明显降低。 而弯曲模的强度随着拉伸模频率从高频率到低频率依次升高, 这是理论研究从未涉及的。 最后在斯托克斯/反斯托克斯比值(RS/AS)上, 温度在150~170 ℃时(压强约为2 kbar), RS/AS迅速从1.1增加到1.3; 当温度高于170 ℃时, RS/AS随温度线性变化。 综上所述, 通过对水分子各共振模的频率蓝移、 强度变化, 以及斯托克斯/反斯托克斯比值等特征进行细致研究, 得到温度对水分子结构, 尤其是氢键衍生特性的影响。 该新型高温超低频拉曼光谱方法, 填补了部分理论空白, 为深入全面地理解水分子结构提供了重要的实验依据。
Abstract
Liquid water is the chemical backbone of most biochemical processes on earth and is therefore essential for the metabolism of living beings. As a result it is a key topic across a wide range of scientific disciplines. The physicochemical properties of water are considered to be the result of hydrogen bond derived structures. However, it is still difficult to quantitatively assign the physicochemical properties of water molecules with the hydrogen bond structure in order to form a complete theory of liquid water molecular structure. Raman spectroscopy is one of the main methods to characterize the molecular structure of liquid water because of its fast and non-destructive advantages. At present, Raman spectroscopy of water molecule is mainly concerned with its high frequency vibration modes. Wide low-frequency Raman modes in liquid water are the result of hydrogen bonds and their local structural effects, which contain characteristic information that high-frequency peaks cannot be characterized. The ultra-low-frequency Raman spectroscopy can reveal many key details of water molecules (super-) structure at high temperature. Therefore, this article provides novel precision temperature-dependent ultra-low frequency Raman spectra of water molecules for the first time. All four translational characteristic modes predicted by theory have been experimentally detected, including the bending mode (51.7 cm-1), torsional mode (81.4 cm-1), symmetrical (154.0 cm-1) and asymmetrical stretching mode (188.6 cm-1) , an additional translational-rotational coupled characteristic mode is found at 225.2 cm-1. All feature modules are accurately assigned. From the spectroscopy results, first of all, when the temperature rises from 0 to 400 ℃, due to hydrogen bond breaking and the rapid average structure correlation length (SLG) decreasing, the frequencies of all four ultra-low frequency characteristic modes shift significantly blue with the increase of temperature. Secondly, the strength of the stretching modes drops obviously between 100 and 200 ℃. While The strength of bending mode increases in turn from high frequency to low frequency with the decrease of stretching modes, which has never been involved in theoretical research. Finally, the the Stokes/Anti-Stokes Ratio (RS/AS) increases rapidly from 1.1 to 1.3 at the temperature range 150~170 ℃ (about 2 kbar) and display a linear temperature dependence when the temperature is above 170 ℃. In general, the effects of temperature on the structure of water molecule, especially on hydrogen bond derivation, are obtained by studying the blue shift of resonance modes, the change of intensity and the Stokes/anti-Stokes ratio. This study provides a deep insight into the analysis of hydrogen-bond derived properties and a new experimental spectroscopy method for understanding the structure of water molecule in depth and comprehensively.

沈艳婷, 吴震东, 黄兴涛, 管郡智. 基于新型超低频拉曼光谱的液态水分子高温研究[J]. 光谱学与光谱分析, 2019, 39(5): 1458. SHEN Yan-ting, WU Zhen-dong, HUANG Xing-tao, GUAN Jun-zhi. Temperature Effects on Novel Ultra-Low Frequency Raman Spectroscopy of Liquid Water[J]. Spectroscopy and Spectral Analysis, 2019, 39(5): 1458.

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