Stoichiometry and structure of uranyl (VI) hydroxo dimer and trimer complexes in aqueous solution

The structures of uranyl(VI) hydroxo dimer and trimer in aqueous solution were studied by EXAFS, FTIR, and UV-vis spectroscopy and also by density functional theory (DFT) calculations. DFT calculations show that (UO2)2(OH)22+ have two bridging hydrxo groups with the U-U distance of 3.88 Å which had a good agreement with EXAFS measurement. For hydroxo trimer complex, DFT calculations show that (UO2)3(O)(OH)3+ with oxo bridging in the center is energetically favored above its stoichiometric equivalent (UO2)3(OH)5+. This idea was confirmed by the EXAFS measurements where a shorter U-U distance of 3.83 Å was observed and the presence of oxo bridging in the center was confirmed. Several stable intermediates which lie several tens of kJ/mol above the stable energy minimum, (UO2)3(O)(OH)3+, were identified and their structures, energies, and intramolecular proton transfer reaction are discussed.