A general study of actinyl hydration by molecular dynamics simulations using ab initio force fields

A set of new ab initio force fields for aqueous [AnO₂]^2+/+ (An = Np(vi,v), Pu(vi), Am(vi)) has been developed using the Hydrated Ion (HI) model methodology previously used for [UO₂]²⁺. Except for the non-electrostatic contribution of the HI-bulk water interaction, the interaction potentials are individually parameterized. Translational diffusion coefficients, hydration enthalpies, and vibrational normal mode frequencies were calculated from the MD simulations. Physico-chemical properties satisfactorily agree with experiments validating the robustness of the force field strategy. The solvation dynamics and structure for all hexavalent actinoids are extremely similar and resemble our previous analysis of the uranyl cation. This supports the idea of using the uranyl cation as a reference for the study of other minor actinyls. The comparison between the NpO₂ ²⁺ and NpO₂ ⁺ hydration only provides significant differences in first and second shell distances and second-shell mean residence times. We propose a single general view of the [AnO₂]^2+/+ hydration structure: aqueous actinyls are amphiphilic anisotropic solutes which are equatorially conventional spherically symmetric cations capped at the poles by clathrate-like water structures.