X‑ray Magnetic Circular Dichroism Spectra for Uranium Monochalcogenides, UQ (Q = S, Se, and Te) from First Principles

In this study, by applying relativistic full potential density functional theory with the generalized gradient approximation (GGA), GGA + U, and GGA + OP (orbital polarization) methods, we have investigated the magnetic properties of uranium monochalcogenides, UQ (Q = S, Se, and Te). The emphasis here is to calculate X-ray magnetic circular dichroism (XMCD) spectra of U in UQ systems at the U M45, N45, and L23 edges and compare the spectra as well as deduced parameters such as spin, orbital, and total magnetic moments with the available experimental and computational results. The effect of the Hubbard (U) parameter on the 5f electrons of uranium is also scrutinized to probe the electron correlation effects in UQ for their electronic and magnetic properties. The spin and orbital sum rule analyses have been carried out on the computed U M45 and N45 XMCD spectra. The corresponding spin, orbital, and total magnetic moments and the ratio of orbital and spin magnetic moments determined for UQ systems are found to be in good agreement with experiments when we use the GGA + OP method

Disproportionation of the Uranyl(V) Coordination Complexes in Aqueous Solution through Outer-Sphere Electron Transfer

Among the linear actinyl­(VI/V) cations, the uranyl­(V) species are particularly intriguing because they are unstable and exhibit a unique behavior to undergo H+ promoted disproportionation in aqueous solution and form stable uranyl­(VI) and U­(IV) complexes. This study uses density functional theory (DFT) combined with the conductor-like polarizable continuum model approach to investigate [UO2]2+/+ to [UIVO2] reduction free energies (RFEs) and explores the stability of uranyl­(V) complexes in aqueous solution through computing disproportionation free energies (DFEs) for an outer-sphere electron transfer process. In addition to the aqua complex (U1), another three commonly encountered ligands such as chloride (U2), acetate (U3), and carbonate (U4) in aqueous environmental conditions are taken into account. For the U1 complex, the computed 1e– (V/IV) and 2e– (VI/IV) RFEs are in good agreement with experiments. The computed DFEs reveal that the presence of H+ is imperative for the disproportionation to take place. Although the presence of the alkali cations favors the disproportionation to some extent, they cannot fully make the reaction thermodynamically feasible. For the anionic complexes, the high negative charge does not allow for the formation of a cation–cation encounter complex due to Coulombic repulsion. Furthermore, an additional factor is the ligand exchange reaction which is also an energy-demanding step. Therefore, the current study examined the Kern–Orlemann mechanism and our results validate the mechanism based on DFT computed DFEs and propose that for the anionic complexes, an outer-sphere electron transfer is highly probable and our computed protonation free energies further support this claim

Actinyl Adsorption and Reduction on Pyrite Surfaces: Insights from DFT Calculations

Interactions of actinides with pyrite surfaces are highly important in catalyzing their reductive immobilization, thereby controlling the movement of these species in the environment. Here, surface adsorption and subsequent reduction of aqueous actinyl­(VI) on pyrite surfaces were explored using density functional hybrid theory (DFT-B3LYP) combined with a first hydration sphere of water molecules and a dielectric continuum for solvation effects. Adsorption of cationic (AnO2(H2O)5)2+(An = U, Np, Pu) and neutral AnO2(OH)2(H2O)3 actinyl onto a small pyrite cluster (Fe4S8) and the effect of coadsorption on the energetics and electron transfer are evaluated by adding either hydroquinone, H2Q (reduced), or quinone, Q (oxidized). The pyrite surface instantaneously transfers an electron to the adsorbed cationic actinyl. Unpaired electron atomic spin densities confirm the electron transfer from the pyrite surface to An atoms. For the neutral actinyl adsorption, electron transfer is confirmed for neptunyl and plutonyl but not for uranyl. Several factors control the overall adsorption energetics and kinetics, such as the nature of the coadsorbate (H2Q/Q), pyrite surface, actinyl, and charge or protonation state (cationic or neutral). The surface-mediated reduction of adsorbed actinyl occurs by receiving electrons either directly from the sulfide or from the coadsorbed H2Q through the sulfide. In the direct reduction case, an H+ ion is added to the surface-bound cationic actinyl, and the mineral surface acts as an electron donor. In contrast, in the proton-coupled electron transfer (PCET) reduction, the surface mediates the electrons through the surface by synergistically aligning relevant orbitals in line. This results in the less soluble and stable An­(IV). Our results indicate that the pyrite surface promotes a faster PCET reaction for the actinyl reduction under circumneutral (pH 4–7) conditions