Stoichiometry and Structure of Uranyl(VI) Hydroxo Dimer and Trimer Complexes in Aqueous Solution

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Abstract
We studied the structure and stoichiometry of aqueous uranyl(VI) hydroxo dimers and trimers by spectroscopic (EXAFS, FTIR, UV−vis) and quantum chemical (DFT) methods. FTIR and UV−vis spectroscopy were used for the speciation of uranyl complexes in aqueous solution. DFT calculations show that (UO2)2(OH)22+ has two bridging hydroxo groups with a U−U distance of 3.875 Å. This result is in good agreement with EXAFS, where a U−U distance of 3.88 Å was found. For the hydroxo trimer complex, DFT calculations show that the species (UO2)3(O)(OH)3+ with oxo bridging in the center is energetically favored in comparison to its stoichiometric equivalent (UO2)3(OH)5+. This is again in line with the EXAFS result, where a shorter U−U distance of 3.81−3.82 Å and evidence for oxo bridging in the center were found. Several stable intermediates which lie several tens of kJ/mol above that of (UO2)3(O)(OH)3+ were identified, and their structures, energies, and intramolecular proton-transfer reaction are discussed.