Quantum mechanical calculation of aqueuous uranium complexes: carbonate, phosphate, organic and biomolecular species

Agreement between experiment and theory is acceptable in most cases, and the reasons for discrepancies are discussed. Calculated Gibbs free energies are used to constrain which configurations are most likely to be stable under circumneutral pH conditions. Reduction of U(VI) to U(IV) is examined for the U-carbonate and U-catechol complexes.Results on the potential energy differences between U(V)- and U(IV)-carbonate complexes suggest that the cause of slower disproportionation in this system is electrostatic repulsion between UO2 [CO3]35- ions that must approach one another to form U(VI) and U(IV) rather than a change in thermodynamic stability. Calculations on U-catechol species are consistent with the observation that UO22+ can oxidize catechol and form quinone-like species. In addition, outer-sphere complexation is predicted to be the most stable for U-catechol interactions based on calculated energies and comparison to 13C NMR spectra. Outer-sphere complexes (i.e., ion pairs bridged by water molecules) are predicted to be comparable in Gibbs free energy to inner-sphere complexes for a model carboxylic acid. Complexation of uranyl to phosphorus-containing groups in extracellular polymeric substances is predicted to favor phosphonate groups, such as that found in phosphorylated NAG, rather than phosphodiesters, such as those in nucleic acids.The toxicity and radioactivity of U makes it a potentially hazardous element in the environment. In areas of high U concentrations, understanding U chemistry is imperative in order to predict its fate, transport, and risk. Uranium is capable of forming a wide variety of aqueous and surface complexes. Furthermore, redox reactions, mainly between U(VI) and U(IV), are common in subsurface environments (e.g., [1]).Research has focused on the environmental chemistry of U with the goal of managing and remediating U-contaminated sites in the most effective manner ([2-4] and references therein). Recent studies have probed the molecular