Topological Hydrogen-Bond Definition to Characterize the Structure and Dynamics of Liquid Water
Topological Hydrogen-Bond Definition to Characterize the Structure and Dynamics of Liquid Water
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DOI：
10.1021/jp105381s
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发表时间：
2010-12-23
影响因子：
3.3
通讯作者：
Irudayam, Sheeba Jem
中科院分区：
文献类型：
--
作者：
Henchman, Richard H.;Irudayam, Sheeba Jem
将氢键在拓扑上等同于势能面上的局部能井的定义用于研究液态水的结构和动力学。我们证明了这种氢键定义的鲁棒性，而不是使用固定的任意参数，不考虑可变的分子环境，不能有效地解决过渡态的许多其他定义。我们的拓扑定义明确地表明，大多数水分子是双受体，但相当大的比例是单或三受体。几乎所有的氢都参与到氢键中。只有当两个分子之间试图形成两个氢键时，才会形成断裂的氢键。双受体具有四面体的几何结构，较低的势能、熵和密度，以及较慢的动力学。单受体和三受体具有三角和三角双锥体的几何形状，当它们一起考虑时，具有更高的密度、势能和熵、更快的动力学和聚集的趋势。这些计算使用了液态水熵的扩展理论，该理论考虑了氢键数量的变化。氢键转换被证明是明确地依赖于可变数目的氢键接受和间隙水分子的存在。过渡态理论表明，氢键的转换是一个轻度激活的过程，只需要适度的氢键扭曲。根据供体和受体是否已经共用一个氢键，可以观察到三种主要的开关事件。这种转换可以在没有中间物的情况下进行，也可以通过分岔氧或环状二聚体进行，这两种二聚体都有一个断裂的氢键和对称或不对称的形式。开关被发现与整个分子的振动强烈耦合，特别是对于更可移动的单受体和三受体。我们的分析表明，尽管水在氢键数量上是不均匀的，但在不同长度和时间尺度上相邻分子之间的耦合使其性质具有更大的连续性。
A definition that equates a hydrogen bond topologically, with a local energy well in the potential energy surface is used to study the structure and dynamics of liquid water. We demonstrate the robustness of this hydrogen-bond definition versus the many other definitions which use fixed, arbitrary parameters, do not account for variable molecular environments, and cannot effectively resolve transition states. Our topology definition unambiguously shows that most water molecules are double acceptors but sizable proportions are single or triple acceptors. Almost all hydrogens are found to take part in hydrogen bonds. Broken hydrogen bonds only form when two molecules try to form two hydrogen bonds between them. The double acceptors have tetrahedral geometry, lower potential energy, entropy, and density, and slower dynamics. The single and triple acceptors have trigonal and trigonal bipyramidal geometry and when considered together have higher density, potential energy, and entropy, faster dynamics, and a tendency to cluster. These calculations use an extended theory for the entropy of liquid water that takes into account the variable number of hydrogen bonds. Hydrogen-bond switching is shown to depend explicitly on the variable number of hydrogen bonds accepted and the presence of interstitial water molecules. Transition state theory indicates that the switching of hydrogen bonds is a mildly activated process, requiring only a moderate distortion of hydrogen bonds. Three main types of switching events are observed depending on whether the donor and acceptor are already sharing a hydrogen bond. The switch may proceed with no intermediate or via a bifurcated-oxygen or cyclic dimer, both of which have a broken hydrogen bond and symmetric and asymmetric forms. Switching is found to be strongly coupled to whole-molecule vibration, particularly for the more mobile single and triple acceptors. Our analysis suggests that even though water is heterogeneous in terms of the number of hydrogen bonds, the coupling between neighbors on various length and time scales brings about greater continuity in its properties.