液態水中羥基重新定向之同位素效應

利用TIP4P/2005水模型對OH基團在液態水中的重新定向進行古典分子動力學的研究。通過比較四種相同温度和壓力的系統：纯H2O和D2O液體，以及H2O和D2O中稀釋HDO混合物的结果，對水分子的同位素效應進行了檢驗。根據OH基團與另一水分子氧原子間的氫鍵強度，將每個系統中的OH基團分為四個類別，其强度與氧-氧距離相關。在每個系统中，通過模擬計算得到各類别的氫鍵壽命，其中强氫鍵的壽命估計小於10fs。通過定向時間相關函数(OTCF)以及系统中每個分類類别中所有的OH或OD基團，計算旋轉均方位移(RMSD )的平均值，研究了OH或OD基團在短到中間時間尺度上(t<200fs)的重新定向。我們的结果表明，OH或OD基團的RMSD在中間時間尺度上的振盪行為很大程度上取決於系统中水分子的種類：稀HDO在D2O液體中的重新定向受到周圍分子的籠子效應强於HDO在H2O液體中的籠子效應。在二階累積量近似下，提出了四種計算RMSD的方法。我們指出籠内OH或OD基團的擺動角度可以通過中間時間尺度RMSD的平台来估計，由四種方法估算的擺動角基本接近，且與角錐模型的方法相近，但略高於其他文獻由實驗得出的估計結果，而我們對擺動角與氫鍵強度相關性的研究结果與實驗结果一致。

關鍵字

液態水；重新定向；同位素效應；氫鍵

並列摘要

The reorientation of OH-group in liquid water was studied by performing classical molecular dynamics with TIP4P/2005 water model. The isotope effects were examined by comparing the results of four systems at the same temperature and pressure: pure H2O and D2O liquids and the mixtures of dilute HDO in H2O and in D2O. All OH-groups in each system were classified into four categories, according to the strength of the H-bond connecting an OH-group to the O-atom of another molecule, where the strength is related to the oxygen-oxygen distance. For each system , the H-bond lifetime of each category was estimated by simulations and the estimated lifetime of the strongest H-bond was less than 10 fs. The OH- or OD-group reorientation over short to intermediate timescales(t<200fs) was investigated through the orientational time correlation functions (OTCFs) and the ro-tation mean square displacements (RMSDs) averaged for all OH- or OD-groups in a sys-tem and in each classified category. Our results show that the oscillatory behavior in the RMSD of the OH- or OD-group at an intermediate timescale depends on the species of the three water molecules in a system: The reorientation of diluted HDO in D2O liquid experiences stronger cage effects from its surrounding molecules as compared with that of HDO in H2O liquid. Under the second-cumulant approximation, four methods were proposed to calculate the RMSD. We point out that the libration angle of an OH- or OD-group within a cage can be estimated by the plateau in the RMSD at an intermediate time. The libration angles estimated by the four methods were generally close, and com-parable to that by the approach of cone model, but somewhat higher than those reported in other literature. Our results on the correlation of the libration angle with the H-bond strength were consistent with the prediction from experimental measurements.

並列關鍵字

liquid water ； reorientation ； isotope effects ； hydrogen bond

參考文獻

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