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

Abstract

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 κ¹,κ^1′-binding mode of the oxalate (with one O atom from each carboxylate coordinating) is more stable than κ² (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 UO₂(H₂O)₃(κ¹-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(κ¹-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.