Single-Hydroxide Bridged Dimers of U and Np Actinyls: A Density Functional Study on Their Existence and Structure in Aqueous Solution

Abstract

With quantum chemical calculations at the density functional theory level, we examined the structure and the stability of diactinyl monohydroxo complexes [(AnO2)2(OH)]3+/+ in aqueous solution for An = U(VI), Np(VI), and Np(V). In particular, this study contributes to understanding the hydrolysis of Np(VI) and Np(V), which is less well characterized than for U(VI). [(UO2)2(OH)]3+ is a known hydrolysis complex of U(VI) at low pH. Although not yet found in experiments, [(NpO2)2(OH)]3+ is suggested to exist due to the similarity between Np(VI) and U(VI) complexes, while [(NpO2)2(OH)]+ is a hypothetical species thus far. Our calculations suggest that the An(VI) complexes favor the parallel orientation of actinyls, whereas for the Np(V) complex a perpendicular arrangement is stabilized by hydrogen bonds between aqua ligands and the actinyl oxygen atoms. The Np(VI) complex [(NpO2)2(OH)]3+ features a structure and stability similar to its U(VI) analogue. From calculated formation constants for An(VI) diactinyl monohydroxo complexes, we find qualitative agreement with the experiment for U(VI). Both An(VI) complexes are only slightly less stable than the separate mononuclear constituents, the actinyl aqua and the monohydroxo complex. For the Np(V) species [(NpO2)2(OH)]+, we calculated a considerably lower complexation constant than for its An(VI) analogues, but it is more stable against decay into its constituents. Thus, this complex may exist at about the pH where Np(V) hydrolysis starts at not too low Np(V) concentrations.

Structures of multinuclear U(vi) species on the hydroxylated α-SiO2(001) surface: insights from DFT calculations

“…However, SCMs that consider only mononuclear U(VI) surface complexes can hardly fit the U(VI) adsorption data obtained at high U(VI) concentrations, especially under conditions where multinuclear aqueous U(VI) species dominate. 7,10,30,33,34 In contrast, if multinuclear U(VI) surface species are included, SCMs show better fitting performance to the experimental data. 20,30,35 The formation of multinuclear U(VI) surface species at high U(VI) concentrations and near-neutral pH values is supported by many spectroscopic investigations.…”