Determination of structural parameters of uranyl ions complexed with organic acids using EXAFS

Abstract

Two compounds of known crystal structure, sodium triacetatodioxouranium(VI), Na[UO 2(CH 3COO) 3], and dibenzoatodioxouranium(VI), UO 2[C 6H 5(COO)] 2, were studied by uranium L III-edge extended X-ray absorption fine structure, EXAFS, spectroscopy to differentiate between bidentate and monodentate coordination of carboxylate ions on the basis of the uranium–equatorial oxygen, O eq, bond lengths. Bidentate coordination can be verified by detecting carboxyl carbon atoms and the neighboring distal carbon atom of the organic rest. In contrast, EXAFS spectra for monodentate carboxylate complexes show no evidence of carbon atoms beyond the O eq coordination shell. The mode of coordination was determined by EXAFS analysis for solid uranyl complexes with humic, methoxybenzoic, and salicylic acids. A correlation between the U L III-edge X-ray absorption near-edge structure, XANES, and the U–O eq bond distance according to the relationship Δ E· R(O eq) 2=constant was observed. For the samples studied, the constant was determined to be 197±8 eV Å 2.