Binding modes of oxalate in UO2(oxalate) in aqueous solution studied with first-principles molecular dynamics simulations. Implications for the chelate effect

Car-Parrinello molecular dynamics simulations are reported for aqueous UO2(H2O)(n)(C2O4) (n = 3, 4), calling special attention to the binding modes of oxalate and the thermodynamics of the so-called chelate effect. Based on free energies from thermodynamic integration (BLYP functional), the kappa(1),kappa(1')-binding mode of the oxalate (with one O atom from each carboxylate coordinating) is more stable than kappa(2) (2 O atoms from the same carboxylate) and kappa(1) forms by 23 and 39 kJ mol(-1), respectively. The free energy of binding a fourth water ligand to UO2(H2O)(3)(kappa(1)-C2O4) is computed to be low, 12 kJ mol(-1). Changes of the hydration shell about oxalate during chelate opening are discussed. Composite enthalpies and free energies, obtained from both experiment and quantum-chemical modeling, are proposed for the formation of monodentate UO2(H2O)(4)(kappa(1)-C2O4). These data suggest that the largest entropy change in the overall complex formation occurs at this stage, and that the subsequent chelate closure under water release is essentially enthalpy-driven.