RESUMO
We performed X-ray absorption spectroscopy measurements on the arsenic K-edge of As(III) in solution under acidic conditions. Extended X-ray absorption fine structure (EXAFS) and X-ray near edge structure (XANES) spectra were compared with theoretical calculations which use local atomic structure configurations, either derived from density functional theory (DFT) energy minimization (EM) calculations or based on classical Monte Carlo (MC) simulations, for a As(OH)3 cluster surrounded by water molecules. The nearest arsenic-oxygen distances obtained from the fit of the XAFS spectra are consistent with the distances present in configurations derived from Monte Carlo simulations but not with those obtained from DFT-EM calculations. Calculations of XANES using either DFT-EM or the average configuration obtained from MC simulations do not reproduce the XANES spectra in the vicinity of the absorption edge. However, specific local atomic structural configurations of the As(OH)3 and water molecules, obtained from MC simulations, which show some ordering of water molecules up to 5 Å from the arsenic, reproduce qualitatively the experimental spectra. These results highlight the capability of XANES to yield information about hydration of ions in solution.
RESUMO
Experimental L(III) X-ray absorption near edge structure (XANES) spectra of the distorted octahedral neptunium oxo ions NpO(2)(OH)(4)(2-), NpO(4)(OH)(2)(3-), and NpO(6)(6-) are interpreted using relativistic full multiple scattering calculations of the X-ray absorption process. In this series of compounds, the neptunium cation exhibits two different oxidation states, VI and VII, with coordination spheres from di- to tetra oxo for the first two compounds. The comparison between calculated XANES spectra using the feff code and experimental ones shows that the main features in the spectra are determined by the local coordination around the actinide metal center. Furthermore, the projected density of electronic states (DOS) calculated from the XANES simulations using the feff code are compared to calculations using ADF code. They are both discussed in terms of molecular orbitals and qualitative evolution of bonding within this series of compounds.