Department of Chemistry, University of Rhode Island, Kingston, Rhode Island 02881, United States.
J Phys Chem B. 2011 Apr 28;115(16):4725-44. doi: 10.1021/jp201082p. Epub 2011 Apr 4.
The classical thermodynamic properties of water clusters modeled by the TIP4P potential with and without the inclusion of molecular hydrogen are calculated using Monte Carlo methods. The temperature-dependent heat capacity curves of the dimer and the smaller pure water clusters having ring structures show low-temperature anomalies arising from the onset of a transition from librational motion to free rotational motion. Pure water clusters having cage structures display heat capacity anomalies characteristic of "melting" phase changes. The addition of molecular hydrogen to the water clusters has little impact on the structure of the water core, but low-temperature heat capacity anomalies are observed that arise from the onset of hindered to free rotational and translational motion of the hydrogen molecule with respect to the skeletal moieties of the core. The Gibbs free energy changes associated with the growth of pure water clusters and the addition of molecular hydrogen to the water clusters are determined using a combination of state-integration methods and an application of the Gibbs-Helmholtz equation. For the temperature integration of the Gibbs-Helmholtz equation, a quadrature method is introduced that avoids numerical difficulties arising from singularities in the integrand at low temperatures. For the growth of pure water clusters, the fine structure of the enthaply and the low-temperature Gibbs free energy as a function of cluster size is rationalized using an ansatz of molecular dipole orientations. For the addition of molecular hydrogen to water clusters, the Gibbs free energy change is a virtually flat function of cluster size showing no fine structure.
使用蒙特卡罗方法计算了 TIP4P 势模型化的包含和不包含分子氢的水团簇的经典热力学性质。具有环结构的二聚体和较小的纯水簇的温度依赖热容曲线显示出低温异常，这是由于从旋转运动到自由旋转运动的转变开始引起的。具有笼状结构的纯水簇显示出“熔化”相变的热容异常。向水团簇中添加分子氢对水核的结构几乎没有影响，但观察到低温热容异常，这是由于分子氢相对于核的骨架部分的自由旋转和平移运动的受阻开始引起的。使用状态积分方法的组合和 Gibbs-Helmholtz 方程的应用确定了纯水簇的生长和向水团簇中添加分子氢的 Gibbs 自由能变化。对于 Gibbs-Helmholtz 方程的温度积分，引入了一种求积方法，该方法避免了在低温下积分项中奇点引起的数值困难。对于纯水簇的生长，使用分子偶极取向的假设来合理化焓和低温 Gibbs 自由能作为簇大小的函数的精细结构。对于向水团簇中添加分子氢，吉布斯自由能变化几乎是簇大小的平坦函数，没有精细结构。
