Theoretical Insights into the O _4,5 Edge Spectra in Uranium Compounds Validated by the Experiment

The impact of the chemical environment and oxidation state on the X-ray absorption spectra (XAS) of the uranium O4,5 edge has been investigated using quantum simulations and compared with experimental data. The XAS features of UO2Cl2 are simulated using time-dependent density functional theory and validated against experimental measurements. The results demonstrate that good agreement between theory and experiment is achievable by employing generalized gradient approximation DFT with spin–orbit coupling corrections. Building on this validated computational approach, we examine the impact of the oxidation state by comparing U(VI)O2Cl2 and U(IV)Cl62–, U(VI)O2Cl2 and U6+, and U(IV)Cl62– and U4+. While both U(IV) and U(VI) complexes exhibit similar main edge peaks, distinct differences appear in the pre-edge region. U(VI) exhibits more intense main edge features with greater uranium-ligand orbital mixing while U(IV) shows an enhanced intensity at the main edge and pre-edge. The U(IV) spectrum is red-shifted relative to U(VI), and this was induced by the ligands and change in oxidation states. These results are interpreted by analyzing the orbital plots of hole–electron densities associated with participating electronic transitions. These quantified, system-specific signatures provide practical reference points for distinguishing oxidation state and coordination environment in uranium compounds.