Hydration Structure of the Elusive Ac(III) Aqua Ion: Interpretation of X-ray Absorption Spectroscopy (XAS) Spectra on the Basis of Molecular Dynamics (MD) Simulations.

Knowledge of actinoid solution chemistry has been enriched with the recent synthesis and characterization of the elusive Ac(III) aqua ion, the first one of the series, for which extended X-ray absorption fine structure (EXAFS) and X-ray absorption near-edge structure (XANES) spectra has been recorded. Structural analysis combined with Born-Oppenheimer molecular dynamics simulations lead to suggest a 2.63-2.69 Å range for the Ac-O distance, and a coordination number between 9 and 11. A hydration number as high as 11 would imply the appearance of a sharp coordination number contraction at the beginning of the series. In this work, we present a specific Ac(III)-H2O first-principles-based intermolecular potential, which has been developed following the exchangeable Hydrated Ion model. This potential has been used in classical molecular dynamics (MD) simulations of Ac(III) in water. Results show a well-defined Ac(III) ennea-hydrated aqua ion with a mean Ac-O distance of 2.66 ± 0.02 Å, surrounded by a compact second hydration shell formed by ∼20 H2O centered at 4.9 ± 0.1 Å. The results obtained for the first element of the actinoid series confirm the regular contraction of their aqua ions along the series. Simulated EXAFS and XANES spectra have been computed from the structural information provided by the MD simulation. The agreement with the experimental spectra is satisfactory, validating the results from the computer simulation. An observed hump in the experimental XANES spectrum is interpreted and ascribed to the second hydration shell, being an evidence of the consistency of the Ac(III) hydration shells.