Pappalardo Rafael R, Caralampio Daniel Z, Martínez José M, Sánchez Marcos Enrique
Department of Physical Chemistry, University of Seville, 41012 Seville, Seville, Spain.
Inorg Chem. 2021 Sep 6;60(17):13578-13587. doi: 10.1021/acs.inorgchem.1c01888. Epub 2021 Aug 13.
The physicochemical properties of the three heaviest alkaline-earth cations, Sr, Ba, and Ra in water have been studied by means of classical molecular dynamics (MD) simulations. A specific set of cation-water intermolecular potentials based on ab initio potential energy surfaces has been built on the basis of the hydrated ion concept. The polarizable and flexible model of water MCDHO2 was adopted. The theoretical-experimental comparison of structural, dynamical, energetic, and spectroscopical properties of Sr and Ba aqueous solutions is satisfactory, which supports the methodology developed. This good behavior allows a reasonable reliability for the predicted Ra physicochemical data not experimentally determined yet. Simulated extended X-ray absorption fine-structure (EXAFS) and X-ray absorption near-edge spectroscopy spectra have been computed from the snapshots of the MD simulations and compared with the experimental information available for Sr and Ba. For the Ra case, the Ra L-edge EXAFS spectrum is proposed. Structural and dynamical properties of the aqua ions for the three cations have been obtained and analyzed. Along the [M(HO)] series, the M-O distance for the first-hydration shell is 2.57, 2.81, and 2.93 Å for Sr, Ba, and Ra, respectively. The hydration number also increases when one is going down along the group: 8.1, 9.4, and 9.8 for Sr, Ba, and Ra, respectively. Whereas [Sr(HO)] is a typical aqua ion with a well-defined structure, the Ba and Ra hydration provides a picture exhibiting an average between the ennea- and the deca-hydration. These results show a similar chemical behavior of Ba and Ra aqueous solutions and support experimental studies on the removal of Ra-226 of aquifers by different techniques, where Ra is replaced by Ba. A comparison of the heavy alkaline ions, Rb and Cs, with the heavy alkaline-earth ions is made.
通过经典分子动力学（MD）模拟研究了水中三种最重的碱土金属阳离子Sr、Ba和Ra的物理化学性质。基于水合离子概念，构建了一组基于从头算势能面的特定阳离子-水间分子势能。采用了可极化且灵活的水模型MCDHO2。Sr和Ba水溶液的结构、动力学、能量和光谱性质的理论与实验比较结果令人满意，这支持了所开发的方法。这种良好的表现使得尚未通过实验确定的Ra物理化学数据的预测具有合理的可靠性。从MD模拟的快照中计算出模拟的扩展X射线吸收精细结构（EXAFS）和X射线吸收近边光谱，并与Sr和Ba的现有实验信息进行比较。对于Ra的情况，提出了Ra L边EXAFS光谱。已获得并分析了三种阳离子水合离子的结构和动力学性质。沿着[M(HO)]系列，Sr、Ba和Ra的第一水合层的M-O距离分别为2.57、2.81和2.93 Å。水合数也随着沿着该族向下而增加：Sr、Ba和Ra分别为8.1、9.4和9.8。虽然[Sr(HO)]是具有明确结构的典型水合离子，但Ba和Ra的水合情况呈现出九水合和十水合之间的平均情况。这些结果表明Ba和Ra水溶液具有相似的化学行为，并支持了通过不同技术去除含水层中Ra-226的实验研究，其中用Ba替代Ra。对重碱金属离子Rb和Cs与重碱土金属离子进行了比较。
