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

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${_{3/2}}$ (M$_4$ edge) levels for molecular complexes in which the uranyl moiety deviates from linearity to varying degrees, and in particular systems containing the UO$_2$Cl$_2$ group such as UO$_2$Cl$_2$.n(H$_2$O) and UO$_2$Cl$_2$(phen)$_2$, which in the latter case exhibits a pronounced O-U-O bending angle. These U M$_4$ edge HERFD-XANES spectra are compared to those of other linear (Cs$_2$UO$_2$Cl$_4$) or pseudo-linear ([UO$_2$(NO$_3$)$_2$.n(H$_2$O)]) uranyl complexes. This evaluation is complemented by ab initio relativistic quantum chemistry simulations using 2-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 UO$_2$Cl$_2$(phen)$_2$, where the U 3d${_{3/2}} \rightarrow$ $5f$ $\sigma{_u}^{*}$ satellite transition appears at lower energies compared to the other systems studied.