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

Abstract

Knowledge of actinoid solution chemistry hasbeen enriched with the recent synthesis and characterizationof the elusive Ac(III) aqua ion, thefirst one of the series, forwhich extended X-ray absorptionfine structure (EXAFS) andX-ray absorption near-edge structure (XANES) spectra hasbeen recorded. Structural analysis combined with Born−Oppenheimer molecular dynamics simulations lead to suggesta2.63−2.69 Å range for the Ac−O distance, and acoordination number between 9 and 11. A hydration numberas high as 11 would imply the appearance of a sharpcoordination number contraction at the beginning of the series. In this work, we present a specific Ac(III)−H2Ofirst-principles-based intermolecular potential, which has been developed following the exchangeable Hydrated Ion model. This potential hasbeen 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 thefirst element of the actinoid series confirm the regularcontraction of their aqua ions along the series. Simulated EXAFS and XANES spectra have been computed from the structuralinformation provided by the MD simulation. The agreement with the experimental spectra is satisfactory, validating the resultsfrom the computer simulation. An observed hump in the experimental XANES spectrum is interpreted and ascribed to thesecond hydration shell, being an evidence of the consistency of the Ac(III) hydration shells.