Theoretical studies of UO2(OH)(H2O)n+, UO2(OH)2(H2O)n, NpO2(OH)(H2O)n, and PuO2(OH)(H2O)n+ (n≤21) complexes in aqueous solution

Abstract

Extensive ab initio calculations have been carried out to study equilibrium structures, vibrational frequencies, and the nature of chemical bonds of hydrated UO(2)(OH)(+), UO(2)(OH)(2), NpO(2)(OH), and PuO(2)(OH)(+) complexes that contain up to 21 water molecules both in first and second hydration spheres in both aqueous solution and the gas phase. The structures have been further optimized by considering long-range solvent effects through a polarizable continuum dielectric model. The hydrolysis reaction Gibbs free energy of UO(2)(H(2)O)(5) (2+) is computed to be 8.11 kcal/mol at the MP2 level in good agreement with experiments. Our results reveal that it is necessary to include water molecules bound to the complex in the first hydration sphere for proper treatment of the hydrated complex and the dielectric cavity although water molecules in the second hydration sphere do not change the coordination complex. Structural reoptimization of the complex in a dielectric cavity seems inevitable to seek subtle structural variations in the solvent and to correlate with the observed spectra and thermodynamic properties in the aqueous environment. Our computations reveal dramatically different equilibrium structures in the gas phase and solution and also confirm the observed facile exchanges between the complex and bulk solvent. Complete active space multiconfiguration self-consistent field followed by multireference singles+doubles CI (MRSDCI) computations on smaller complexes confirm predominantly single-configurational nature of these species and the validity of B3LYP and MP2 techniques for these complexes in their ground states.

Ab initiomolecular dynamics studies of hydroxide coordination of alkaline earth metals and uranyl

“…The calculated mean U-O yl , U-O w and U-O OH bond lengths and uranium equatorial coordination number are summarised in Table 7. There is an increase of 0.24 Å and 0.11 Å in the U-O yl and U-O w bond lengths, respectively, reflecting the trend reported by both Ingram et al 117 and Cao et al 122 The calculated mean U-O w and U-O OH bond length are 0.08 Å and 0.07 Å longer, respectively, than those reported by Cao et al, 122 and both are 0.07 Å longer than those reported by Ingram et al 117 However, neither of these previous studies included explicit water molecules beyond the first solvation shell, and relied on continuum solvent models to model the long range interactions of the water, which has been shown to impact the accuracy of the first solvation shell in studies with uranyl in water. 91,93,150 In three of these AIMD trajectories, there is migration of a water molecule to the 2nd solvation shell, reducing the equatorial coordination number to 4.…”

“…The gas phase DFT studies of Felmy et al 108 The impact of hydroxide on the first solvation shell of uranyl has been documented in both experimental 80,111,112 and computational literature alike. 80,97,98,[113][114][115][116][117][118][119][120][121][122] The EXAFS and XRD analysis by Clark et al 111 While there is broad consensus in the literature regarding solvation structures of the ions in an aqueous environment, the significant range of reported values for both Ca and Sr hydrates suggests that the systematic study presented here will be a valuable addition to the literature as well as provide an accurate basis for the novel investigation of the hydrated ions in the presence of hydroxide. The impact on the solvation environment of alkaline earth metals due to the presence of hydroxide is investigated here for the first time using AIMD methods, with particular attention paid to identification and characterisation of proton transfer events, along with their timescales and frequency.…”

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

“…had been noted previously. 28 In our CPMD simulations, such conformational differences tend to be blurred by rapid rotation and libration processes. Frequently, H atoms bend away from what looks like an "sp 2 " water coordination (with local C 2v symmetry of the UOH 2 group, cf.…”

Development and application of effective pairwise potentials for UO2n+, NpO2n+, PuO2n+, and AmO2n+ (n = 1, 2) ions with water

“…[49][50][51][52][53][54][55][56][57][58][59] The DFT-B3LYP level of theory and IEF-PCM were used based on the good performance found for similar calculations performed on the hydration of dipositive and monopositive actinyl and hydroxoactinyl ions. 52,[56][57][58][59][60][61][62][63][64][65][66][67] Both prior experiment and present molecular dynamics suggest five H 2 O in the first hydration shell; we use five asymmetric equatorial H 2 O in these hybrid solvation calculations for all cations. For computational expediency in these C 1 calculations, more modest 6-31G(d) and 6-21G basis sets were used for oxygen and hydrogen, respectively.…”