Electronic Structure of Protactinium(V) and Uranyl Complexes from High-Resolution X-ray Spectroscopy and Relativistic Theory
Résumé
High-energy-resolution X-ray spectroscopies such as HERFD-XANES and Resonant Inelastic X-ray Scattering (RIXS) provide powerful probes of the electronic structure and bonding of f-element complexes. Developing predictive theoretical descriptions of these spectra remains challenging, particularly for poorly characterized actinides.
Here we combine HERFD-XANES experiments with relativistic quantum chemical calculations to investigate uranium(VI) and protactinium(V) complexes. Uranyl compounds with varying ligand fields and degrees of O=U=O bending serve as benchmark systems. U M4-edge HERFD-XANES spectra are interpreted using relativistic time-dependent density functional theory (TD-DFT), which captures the main trends in 3d → 5f excitations and ligand-field effects [1,2].
We then examine Pa(V) [3], whose coordination chemistry remains poorly understood. In contrast to the well-established actinyl ions of heavier actinides, Pa(V) may adopt coordination environments with or without a mono-oxo Pa=O bond, and the stability of this bond in solution remains debated. HERFD-XANES spectra recorded at the Pa M4-edge in hydrofluoric and oxalic acid solutions display similar features, motivating a combined experimental–theoretical analysis of Pa(V) speciation.
These spectra are analyzed using TD-DFT together with multi-reference spin–orbit SO-RASSCF/SO-RASPT2 calculations. Predictive Pa M4-edge RIXS maps, computed at the SO-RASSCF/SO-RASPT2 level, reveal fingerprints of metal-centered and charge-transfer excitations and strong sensitivity to the presence or absence of a Pa=O bond. These results provide new insight into the electronic structure of Pa complexes and guide future high-resolution RIXS experiments.
References
1.W. A. Misael; A. S. P. Gomes, Inorg. Chem., 2023, 62, 11589–11601.
2.W. A. Misael et al., Inorg. Chem., 2025, 64, 22487–22502.
3.M. Maloubier et al., Angew. Chem. Int. Ed., 2025, e18403.
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