Theoretical chemical contribution to the simulation of the LIII X-ray absorption edges of uranyl, neptunyl and osmyl hydrates and hydroxides

Abstract

XANES spectroscopy has long been used as a structural and electronic probe of a selected element. Phenomenological application of this technique to actinide cations has proved fruitful to characterize the actinide environment in both solid state and solution compounds. Although powerful XANES simulation codes have been developed, the use of such simulations in order to describe the valence orbitals of the actinide cation is still scarce. The very short life time of the core hole at the L_III edge as well as the low symmetry and large size of the coordination polyhedron are difficulties to be overcome in the analysis of the edge spectra. In this work, three simple molecules have been selected for their similar geometry that is typical of the trans dioxo actinyl compounds: [UO₂(H₂O)₅]²⁺, [NpO₂(H₂O)₅]²⁺, [NpO₂(OH)₄]²⁻. Additional comparison with a transition metal, the osmyl cation [OsO₂(OH)₄]²⁻, is also made. The cation L_III edges have been recorded and compared to edge calculations using FEFF8.2 code. This article is structured in two parts. In the first one, elaboration and optimization of a valid structural model cluster is carried out using molecular dynamics calculations. The influence of the water solvent molecules as well as the hydrogen atoms of the cations’ first coordination sphere are discussed. In the second part, Amsterdam quantum chemical calculations have been carried out on the four clusters and molecular energy levels are qualitatively compared to the data obtained from calculated XANES spectra.