We applied relativistic multiconfigurational all-electron ab initio calculations including the spin–orbit interaction to calculate the 3d4f resonant inelastic X-ray scattering (RIXS) map (3d_3/2 → 5f_5/2 U M₄ absorption edge and 4f_5/2 → 3d_3/2 U M_β emission) of uranyl (UO₂²⁺). The calculated data are in excellent agreement with experimental results and allow a detailed understanding of the observed features and an unambiguous assignment of all involved intermediate and final states. The energies corresponding to the maxima of the resonant emission and the non-resonant (normal) emission were determined with high accuracy, and the corresponding X-ray absorption near edge structure spectra extracted at these two positions were simulated and agree well with the measured data. With the high quality of our theoretical data, we show that the cause of the splitting of the three main peaks in emission is due to the fine structure splitting of the 4f orbitals induced through the trans di-oxo bonds in uranyl and that we are able to obtain direct information about the energy differences between the 5f and 4f orbitals: Δ5f δ/ϕ – 4f δ/ϕ, Δ5f π* – 4f π, and Δ5f σ* – 4f σ from the 3d4f RIXS map. RIXS maps contain a wealth of information, and ab initio calculations facilitate an understanding of their complex structure in a clear and transparent way. With these calculations, we show that the multiconfigurational protocol, which is nowadays applied as a standard tool to study the X-ray spectra of transition metal complexes, can be extended to the calculation of RIXS maps of systems containing actinides.

Plutonium is used as a major component of new-generation nuclear fuels and of radioisotope batteries for Mars rovers, but it is also an environmental pollutant. Plutonium clearly has high technological and environmental importance, but it has an extremely complex, not well-understood electronic structure. The level of covalency of the Pu 5f valence orbitals and their role in chemical bonding are still an enigma and thus at the frontier of research in actinide science. We performed fully relativistic quantum chemical computations of the electronic structure of the Pu⁴⁺ ion and the PuO₂ compound. Using four different theoretical tools, it is shown that the 5f orbitals have very little covalent character although the 5f(_7/2) a_2u orbital with the highest orbital energy has the greatest extent of covalency in PuO₂. It is illustrated that the Pu M_4,5 edge high-energy resolution X-ray absorption near-edge structure (Pu M_4,5 HR-XANES) spectra cannot be interpreted in terms of dipole selection rules applied between individual 3d and 5f orbitals, but the selection rules must be applied between the total wavefunctions for the initial and excited states. This is because the states cannot be represented by single determinants. They are shown to involve major redistributions on the 5f electrons over the different 5f orbitals. These redistributions could be viewed as shake-up-like excitations in the 5f shell from the lowest orbital energy from J = 5f(_5/2) into higher orbital energy J = 5f(_7/2). We show that the second peak in the Pu M₄ edge and the high-energy shoulder of the Pu M₅ edge HR-XANES spectra probe the 5f(_7/2) a_2u orbital; thus, these spectral features are expected to change upon bond variations. We describe theoretical and spectroscopy tools, which can be applied for all actinide elements in materials with cubic structure.

We show that the structural and electronic properties of UO₂NPs (2–3 nm) are similar to those of bulk UO₂under inert conditions, with U(iv) as the dominating oxidation state, though NPs oxidize with time and under the X-ray beam.

The actinides possess a complex electronic structure, making their chemical and physical properties among the least understood in the periodic table. Advanced spectroscopic tools, able to obtain deep insights into the electronic structure and binding properties of the actinides, are highly desirable. Here, we introduce two sensitive spectroscopic tools: one determines the number of localized 5 f electrons on an actinide atom, and another assesses the covalent character of actinide-ligand bonding. Both tools are based on the multiplet structure present in actinide M ₄ edge core-to-core resonant inelastic X-ray scattering (CC-RIXS) maps. The spectral intensity of different many-body final-state multiplets directly depends on the local many-electron ground-state symmetry including the local 5 f spin configuration. By comparing U M ₄ edge CC-RIXS data for 21 U, Np, Pu and Am compounds, we demonstrate the ability to compare the number of localized 5 f electrons and bond covalency across the actinide series.

The stabilization of uranyl(V) (UO₂ 1 ⁺ ) by Fe(II) in natural systems remains an open question in uranium chemistry. Stabilization of U^VO₂ ¹⁺ by Fe(II) against disproportionation was also demonstrated in molecular complexes. However, the relation between the Fe(II) induced stability and the change of the bonding properties have not been elucidated up to date. We demonstrate that U(V) – oaxial bond covalency decreases upon binding to Fe(II) inducing redirection of electron density from the U(V) – oaxial bond towards the U(V) – equatorial bonds thereby increasing bond covalency. Our results indicate that such increased covalent interaction of U(V) with the equatorial ligands resulting from iron binding lead to higher stability of uranyl(V). For the first time a combination of U M_4,5 high energy resolution X-ray absorption near edge structure (HR-XANES) and valence band resonant inelastic X-ray scattering (VB-RIXS) and ab initio multireference CASSCF and DFT based computations were applied to establish the electronic structure of iron-bound uranyl(V).