Instituto de Ciencias Físicas, Universidad Autónoma de México, Apdo. Postal 48-3, Cuernavaca, Morelos 62251, Mexico.
Departamento de Física, Centro de Investigación en Ciencias, IICBA, Universidad Autónoma del Estado de Morelos, Cuernavaca, Morelos 62209, Mexico.
J Chem Phys. 2018 Apr 14;148(14):144307. doi: 10.1063/1.5021348.
The hydration features of [Mg(HO)] and [Ca(HO)] clusters with n = 3-6, 8, 18, and 27 were studied by means of Born-Oppenheimer molecular dynamics simulations at the B3LYP/6-31+G** level of theory. For both ions, it is energetically more favorable to have all water molecules in the first hydration shell when n ≤ 6, but stable lower coordination average structures with one water molecule not directly interacting with the ion were found for Mg at room temperature, showing signatures of proton transfer events for the smaller cation but not for the larger one. A more rigid octahedral-type structure for Mg than for Ca was observed in all simulations, with no exchange of water molecules to the second hydration shell. Significant thermal effects on the average structure of clusters were found: while static optimizations lead to compact, spherically symmetric hydration geometries, the effects introduced by finite-temperature dynamics yield more prolate configurations. The calculated vibrational spectra are in agreement with infrared spectroscopy results. Previous studies proposed an increase in the coordination number (CN) from six to eight water molecules for [Ca(HO)] clusters when n ≥ 12; however, in agreement with recent measurements of binding energies, no transition to a larger CN was found when n > 8. Moreover, the excellent agreement found between the calculated extended X-ray absorption fine structure spectroscopy spectra for the larger cluster and the experimental data of the aqueous solution supports a CN of six for Ca.
采用 Born-Oppenheimer 分子动力学模拟方法在 B3LYP/6-31+G**理论水平上研究了[Mg(HO)]和[Ca(HO)]簇的水合特征，n = 3-6、8、18 和 27。对于两种离子，当 n ≤ 6 时，所有水分子都处于第一水合壳中在能量上更有利，但在室温下，对于 Mg 发现了具有一个水分子不直接与离子相互作用的稳定较低配位数的平均结构，显示出较小阳离子质子转移事件的特征，但对于较大阳离子则没有。在所有模拟中，Mg 比 Ca 具有更刚性的八面体结构，没有水分子交换到第二水合壳。发现簇的平均结构对温度有显著影响：虽然静态优化导致紧凑的、球对称的水合几何形状，但有限温度动力学引入的影响导致更扁长的构型。计算得到的振动光谱与红外光谱结果一致。先前的研究提出，当 n≥12 时，[Ca(HO)]簇的配位数（CN）从六增加到八个水分子；然而，与最近的结合能测量结果一致，当 n>8 时，没有发现向更大 CN 的转变。此外，较大簇的计算扩展 X 射线吸收精细结构光谱与水溶液的实验数据之间的极好一致性支持 Ca 的 CN 为六。
