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 UO(2)(H(2)O)(n)(C(2)O(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 κ(1) forms by 23 and 39 kJ mol(-1), respectively. The free energy of binding a fourth water ligand to UO(2)(H(2)O)(3)(κ(1)-C(2)O(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(2)O)(4)(κ(1)-C(2)O(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.

Acidity constants and redox potentials of uranyl ions in hydrothermal solutions

“…are also lacking at high temperatures. As shown in the previous studies, [38][39][40][41] FPMD based methods can provide reasonable estimates for thermodynamic data, and thus we expect the method presented here can be extended to elevated T-P conditions. Such complexing may change pK a s and U 0 and this can also be addressed by using the FPMD vertical energy gap technique.…”

“…7 Bühl et al successfully applied FPMD to investigate the complexing of UO 2 2+ with both inorganic and organic ligands. [38][39][40][41] The Car-Parrinello MD based metadynamics method was used to investigate the hydration structures and the hydrolysis of uranium, plutonium and curium ions under ambient conditions and favorable agreement was found between the simulation results and the experiment. [42][43][44] The group of Sprik has developed a FPMD based vertical energy gap method for calculating redox potentials and acidity constants.…”

Yttrium speciation in subduction zone fluids from <i>ab initio</i> molecular dynamics simulations

“…Here, we use the constrained molecular dynamics approach and thermodynamic integration in terms of the blue moon sampling (Ciccotti et al, 2005). This method has been used already by different groups to investigate the stability of metal complexes in aqueous solutions, not only at ambient pressure and temperature conditions (Bühl and Golubnychiy, 2007;Bühl and Grenthe, 2011) but also at hydrothermal (Mei et al, 2013(Mei et al, , 2015(Mei et al, , 2016 and deep crustal high density fluid conditions (Mei et al, 2018).…”

Uranium Removal from Seawater by Means of Polyamide 6 Fibers Directly Grafted with Diallyl Oxalate through a Single-Step, Solvent-Free Irradiation Process

“…The structural reported by the aforementioned papers, including the existence of hydrogen-bonded chains, have been supported by XRD and IR studies 14 . Furthermore, according to molecular dynamics simulations, the most stable form of the UO 2 (C 2 O 4 )(H 2 O) 3 complex is a five-coordinate chelate 15 . It is reasonable to expect that the complex UO 2 (C 2 O 4 )(OH) 2 2− , which is probably the dominant form of uranyl under the conditions prevailing near the surface of an oxalate-grafted polymeric surface at pH 8 (see above) may have a similar structure to the five-coordinated UO 2 (C 2 O 4 )(H 2 O) 3 with two of the water molecules replaced by hydroxide groups.…”