Core-Excited States of Linear and Bent Uranyl Complexes: Insights from High-Energy Resolution X-ray Spectroscopy and...

Advanced X-ray spectroscopic techniques are widely recognized as state-of-the-art tools for probing the electronic structure, bonding, and chemical environments of the heaviest elements in the periodic table. In this study, we employ X-ray absorption near-edge structure measurements in high-energy resolution fluorescence detection (HERFD-XANES) mode to investigate the core states arising from excitations out of the U 3d_3/2 (M₄ edge) levels for molecular complexes in which the uranyl moiety deviates from linearity to varying degrees, and in particular systems containing the UO₂Cl₂ group such as UO₂Cl₂·n(H₂O) and UO₂Cl₂(phen)₂, which in the latter case exhibits a pronounced O-U-O bending angle. These U M₄-edge HERFD-XANES spectra are compared with those of other uranyl complexes reported in the literature. This evaluation is complemented by ab initio relativistic quantum chemistry simulations on the [UO₂(NO₃)₂·n(H₂O)], UO₂Cl₂·n(H₂O), and UO₂Cl₂(phen)₂ systems, using two-component time-dependent density functional theory (TD-DFT) with the CAM-B3LYP functional, employing the Tamm-Dancoff approximation (2c-TDA). Our 2c-TDA simulations show modest deviations from the HERFD-XANES data, with peak splittings differing by less than 1 eV from experimental values. These core-excited states were further characterized by natural transition orbital (NTO) analysis. Overall, our results highlight the influence of equatorial ligands on the spectroscopic signatures, particularly pronounced in UO₂Cl₂(phen)₂, where the U 3d_3/2 → 5f_σu^* satellite transition appears at lower energies compared to the other systems studied.