The hydration features of ${\left[\text{Mg}{({\mathrm{H}}_2\mathrm{O})}_n\right]}^{2+}$ and ${\left[\text{Ca}{({\mathrm{H}}_2\mathrm{O})}_n\right]}^{2+}$ 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 Mg²⁺ 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 Mg²⁺ than for Ca²⁺ 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 ${\left[\text{Ca}{({\mathrm{H}}_2\mathrm{O})}_n\right]}^{2+}$ 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 Ca²⁺.

中文翻译：

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Mg（ii）和Ca（ii）的水溶剂化：微水合气相簇的Born-Oppenheimer分子动力学研究

的水合作用 ${\left[\text{镁}{（{\mathrm{H}}_{\mathrm{2个}}\mathrm{Ø}）}_{ñ}\right]}^{\mathrm{2个}+}$ 和 ${\left[\text{钙}{（{\mathrm{H}}_{\mathrm{2个}}\mathrm{Ø}）}_{ñ}\right]}^{\mathrm{2个}+}$通过Born-Oppenheimer分子动力学模拟以B3LYP / 6-31 + G **的理论水平研究了n = 3-6、8、18和27的聚类。对于这两种离子，它是能量更有利为具有所有的水分子在第一水合壳时Ñ发现镁≤6，但稳定的一个水分子不直接与离子相互作用下协调平均结构²⁺在室温下，显示较小阳离子的质子转移事件的特征，而不是较大阳离子的质子转移事件的特征。Mg ^2+的刚性八面体型结构比Ca ^2+的刚性八面体型结构在所有模拟中均观察到，没有水分子交换至第二水合壳。发现了对团簇平均结构的显着热效应：虽然静态优化导致紧凑的球形对称水化几何形状，但有限温度动力学引入的效应却产生了更扁平的结构。计算出的振动光谱与红外光谱结果一致。先前的研究提出，对于${\left[\text{钙}{（{\mathrm{H}}_{\mathrm{2个}}\mathrm{Ø}）}_{ñ}\right]}^{\mathrm{2个}+}$群集时Ñ ≥12; 然而，与最近对结合能的测量结果一致，当n > 8时，未发现转变为较大的CN 。此外，对于较大的团簇，所计算的扩展X射线吸收精细结构光谱光谱与实验数据之间发现了极好的一致性。的水溶液支持Ca ²⁺的CN为6 。