Hydration of divalent lanthanides, Sm²⁺ and Eu²⁺ : A molecular dynamics study with polarizable AMOEBA force field

The chemistry of divalent lanthanides, Ln²⁺ , is a growing sub-field of heavy element chemistry owing to new synthetic approaches. However, some theoretical aspects of these unusual cations are currently underdeveloped, especially as they relate to their dynamic properties in solution. In this work, we address the hydration of two of the classical Ln²⁺ cations, Sm²⁺ and Eu²⁺ , using atomic multipole optimized energetic for biomolecular applications (AMOEBA) force fields. These cations have not been parameterized to date with AMOEBA, and few studies are available because of their instability with respect to oxidation in aqueous media. Coordination numbers (CN's) of 8.2 and 8.1 respectively for Sm²⁺ and Eu²⁺ , and 8.8 for both Sm³⁺ and Eu³⁺ have been obtained and are in good agreement with the few available AIMD and X-ray absorption fine structures studies. The decreased CN of Ln²⁺ compared with Ln³⁺ arises from progressive water exchange events that indicates the gradual stabilization of 8-coordinate structures with respect to 9-coordinate geometries. Moreover, the effects of the chloride counter anions on the coordination of Ln²⁺ cations have been studied at different chloride concentrations in this work. Lastly, water exchange times of Ln²⁺ cations have been calculated to provide a comprehensive understanding of the behavior of Eu²⁺ and Sm²⁺ in aqueous chloride media.