Binding modes of oxalate in UO_2(oxalate) in aqueous solution studied with first-principles molecular dynamics simulations. Implications for the chelate effect

摘要

Car-Parrinello molecular dynamics simulations are reported for aqueous UO_2(H_2O)_n(C_2O_4) (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 κ~1, κ~(1′)-binding mode of the oxalate (with one O atom from each carboxylate coordinating) is more stable than κ~2 (2 O atoms from the same carboxylate) and κ1 forms by 23 and 39 kJ mol-1, respectively. The free energy of binding a fourth water ligand to UO_2(H_2O)_3(κ~1-C_2O_4) 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 UO_2(H_2O)_4(κ~1-C_2O_4). 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.