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

Wilken Aldair Misael; Lucia Amidani; Juliane März; Elena F. Bazarkina; Kristina O. Kvashnina; Valérie Vallet; André Severo Pereira Gomes

paper

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

arXiv:2504.05542

Abstract

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 uranyl complexes reported in the literature. This evaluation is complemented by \textit{ab initio} relativistic quantum chemistry simulations on the [UO $_{2}$ (NO $_{3}$ ) $_{2}$ .n(H $_{2}$ O)], UO $_{2}$ Cl $_{2}$ .n(H $_{2}$ O) and UO $_{2}$ Cl $_{2}$ (phen) $_{2}$ systems, 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 $_{3/2}$ $\to$ 5f $σ_{u}^{*}$ satellite transition appears at lower energies compared to the other systems studied.

55 pages, 10 figures, 5 tables