Spectrophotometric and DFT characterization of uranyl carboxylate complexes in aqueous solution

This investigation is an advance in the understanding of uranyl chemistry in water solution. Our previous works were addressed to the speciation of uranyl with carboxylic acids by potentiometry and UV-visible absorption spectrophotometry. In this session we propose an enhancement on structural description of complexes by means of a deep analysis of spectrophotometric absorption data and by means of DFT (Density Functional Theory) characterization. The spectrophotometric study of metal - ligand systems can provide information about the structure of complexes, especially if it is supported by theoretical data. Moreover, the recent literature confirms that the theoretical DFT approach provides complementary information and validates the structural and solution chemical information. We report the investigation on coordination compounds of uranyl ion with citric and oxydiacetic (diglycolic) acids in aqueous solution. The different binary systems were previously studied by potentiometric and spectroscopic techniques at t = 25 °C and I = 0.1 mol dm-3. A speciation model was proposed for both metal/ligand systems from potentiometric data. Moreover, the joint elaboration of potentiometric and spectroscopic data obtained on the uranyl - ligand containing solutions allowed us to calculate the individual spectra of the complex species. With the carboxylate ligands we have found that the co-ordination environment produces an increase in the molar absorptivity values and a light bathochromic shift in the position of εmax. The bathochromic shift is higher for citric acid and we observed a remarkable raise of the relative intensity of the vibronic bands which appear at l>440 nm for uranyl- oxydiacetate complexes. Uranyl - carboxylate species assumed from potentiometric speciation, have been checked by means of a computational modelization. Differing coordination geometries have been explored, both in gas phase and in solution (assuming the polarizable continuum model). The different protonation constants of uranyl - citrate complexes have been analyzed and electronic and vibronic spectra computed with time - dependent density functional method. Results are compared with experimental spectra.