Hydration of Heavy Alkaline-Earth Cations Studied by Molecular Dynamics Simulations and X-ray Absorption Spectroscopy

Abstract

The physicochemical properties of the three heaviest alkaline-earth cations, Sr 2+ , Ba 2+ , and Ra 2+ 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 2+ and Ba 2+ aqueous solutions is satisfactory, which supports the methodology developed. This good behavior allows a reasonable reliability for the predicted Ra 2+ 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 2+ and Ba 2+ . For the Ra 2+ case, the Ra L 3 -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(H 2 O) n ] m + series, the M–O distance for the first-hydration shell is 2.57, 2.81, and 2.93 Å for Sr 2+ , Ba 2+ , and Ra 2+ , respectively. The hydration number also increases when one is going down along the group: 8.1, 9.4, and 9.8 for Sr 2+ , Ba 2+ , and Ra 2+ , respectively. Whereas [Sr(H 2 O) 8 ] 2+ is a typical aqua ion with a well-defined structure, the Ba 2+ and Ra 2+ hydration provides a picture exhibiting an average between the ennea- and the deca-hydration. These results show a similar chemical behavior of Ba 2+ and Ra 2+ aqueous solutions and support experimental studies on the removal of Ra-226 of aquifers by different techniques, where Ra 2+ is replaced by Ba 2+ . A comparison of the heavy alkaline ions, Rb + and Cs + , with the heavy alkaline-earth ions is made.

Extended X-ray absorption fine structure spectroscopy measurements and ab initio molecular dynamics simulations reveal the hydration structure of the radium(II) ion

“…To evaluate the dynamical property, the mean residence time (MRT) of water molecules in the first hydration shell was evaluated by the “direct” method ( Hofer et al., 2004 ), which is defined as where , , and are the sampling time (50 ps), average CN, and number of the exchange events that lasted longer ps, respectively. The MRTs for Ra 2+ with and 0.5 ps are shorter than those of Ba 2+ , which is consistent with the previous study ( Pappalardo et al., 2021 ) ( Table 4 ). The MRTs were also calculated for other functionals, suggesting that the dispersion interaction critically affects them (Tabls S1).…”

“… is the average distance between the cation and center of mass of the water molecules in the first hydration shell, defined in a previous study ( Caralampio et al., 2017 ) as follows: where represents the center of mass of the water molecules in the first hydration shell. The obtained values were 0.31 Å (Ra 2+ ) and 0.26 Å (Ba 2+ ), which were compared with the classical MD results ( Pappalardo et al., 2021 ) of 0.24 Å (Ra 2+ ) and 0.22 Å (Ba 2+ ). Consistent result of the value of Ra 2+ being higher than that of Ba 2+ was obtained via the simulation method.…”

“… MCP 2.80–2.95 7–9 – – – Previous study ( Matsuda and Mori, 2014a ) Ra 2+ Sim. MD 2.93 9.8 135 0.034 0.24 Previous study ( Pappalardo et al., 2021 ) Ba 2+ Exp. EXAFS 2.79 ± 0.02 8.0 ± 1.9 – 0.020 – Previous study ( Yamaguchi et al., 2021 ) Ba 2+ Exp.…”

“… AIMD PW91 2.80 8.0 – – – Previous study ( Chaudhari et al., 2015 ) Ba 2+ Sim. MD 2.81 9.4 138 0.034 0.22 Previous study ( Pappalardo et al., 2021 ) Ba 2+ Sim. MD 2.85 8.1 – – – Previous study ( Migliorati, Caruso and D’Angelo, 2019 ) Ba 2+ Sim.…”

“…Classical molecular dynamics, ab initio quantum chemical calculations with the relativistic model core potential (MCP) method, and fragment molecular orbital–molecular dynamics were conducted to study the hydration of Ra 2+ and compare it with other hydrated divalent alkaline earth metal ions ( Matsuda and Mori, 2014a ). Recently, classical molecular dynamics simulation with the force field fitted to the results of quantum mechanical calculations was conducted ( Pappalardo et al., 2021 ). In the hydrated barium ion (Ba 2+ ), an analog element of Ra cases, it has been shown that full- ab initio molecular dynamics (AIMD) simulation is one of the best simulation methods compared with other methods, including QM/MM, a hybrid method of quantum and classical calculations, in the accuracy for reproducing experimental results ( Yamaguchi et al., 2021 ).…”