Ultrafast Transient Absorption Spectroscopy of UO22+ and [UO2Cl]. J Phys Chem A 2018;122:6970-6977. [PMID: 30095911 DOI: 10.1021/acs.jpca.8b05567]

Quenching Mechanism of Uranyl(VI) by Chloride and Bromide in Aqueous and Non-Aqueous Solutions

A major hindrance in utilizing uranyl(VI) luminescence as a standard analytical tool, for example, in environmental monitoring or nuclear industries, is quenching by other ions such as halide ions, which are present in many relevant matrices of uranyl(VI) speciation. Here, we demonstrate through a combination of time-resolved laser-induced fluorescence spectroscopy, transient absorption spectroscopy, and quantum chemistry that coordinating solvent molecules play a crucial role in U(VI) halide luminescence quenching. We show that our previously suggested quenching mechanism based on an internal redox reaction of the 1:2-uranyl-halide-complex holds also true for bromide-induced quenching of uranyl(VI). By adopting specific organic solvents, we were able to suppress the separation of the oxidized halide ligand X₂^·- and the formed uranyl(V) into fully solvated ions, thereby "reigniting" U(VI) luminescence. Time-dependent density functional theory calculations show that quenching occurs through the outer-sphere complex of U(VI) and halide in water, while the ligand-to-metal charge transfer is strongly reduced in acetonitrile.

Unveiling a Photoinduced Hydrogen Evolution Reaction Mechanism via the Concerted Formation of Uranyl Peroxide

We present a density functional theory study for the photochemical water oxidation reaction promoted by uranyl nitrate upon sunlight radiation. First, we explored the most stable uranyl complex in the absence of light. The reaction in a dark environmen proceeds through the condensation of uranyl monomers to form dimeric hydroxo-bridged species, which is the first step toward a hydrogen evolution reaction (HER). We found a triplet-state-driven mechanism that leads to the formation of uranyl peroxide and hydrogen gas. To describe in detail this reaction path, we characterized the singlet and triplet low-lying states of the dimeric hydroxo-bridged species, including minima, transition states, minimal energy crossing points, and adiabatic energies. Our computational results provide mechanistic insights that are in good agreement with the experimental data available.

Temperature-dependent luminescence spectroscopic and mass spectrometric investigations of U(VI) complexation with aqueous silicates in the acidic pH-range

In this study the complexation of U(VI) with orthosilicic acid (H₄SiO₄) was investigated between pH 3.5 and 5 by combining electrospray ionization mass spectrometry (ESI-MS) and laser-induced luminescence spectroscopy. The ESI-MS experiments performed at a total silicon concentration of 5 · 10^-3M (exceeding the solubility of amorphous silica at both pH-values) revealed the formation of oligomeric sodium-silicates in addition to the UO₂OSi(OH)₃⁺ species. For the luminescence spectroscopic experiments (25 °C), the U(VI) concentration was fixed at 5 · 10^-6M, the silicon concentration was varied between 1.3 · 10^-4-1.3 · 10^-3M (reducing the formation of silicon oligomers) and the ionic strength was kept constant at 0.2 M NaClO₄. The results confirmed the formation of the aqueous UO₂OSi(OH)₃⁺ complex. The conditional complexation constant at 25 °C, log *β = -(0.31 ± 0.24), was extrapolated to infinite dilution using the Davies equation, which led to log *β⁰ = -(0.06 ± 0.24). Further experiments at different temperatures (1-25 °C) allowed the calculation of the molal enthalpy of reaction Δ_rH_m⁰ = 45.8 ± 22.5 kJ·mol^-1 and molal entropy of reaction Δ_rS_m⁰ = 152.5 ± 78.8 J·K^-1·mol^-1 using the integrated van't Hoff equation, corroborating an endothermic and entropy driven complexation process.

Temperature-dependent luminescence spectroscopic investigations of uranyl(vi) complexation with the halides F− and Cl−

Uranyl(vi) complexation with fluoride and chloride was investigated with luminescence spectroscopy, and the strong quenching by chloride was overcome by freezing.