Relativistic Multiconfigurational Ab Initio Calculation of X-ray Spectroscopies of Radionuclide Compounds: Tc L 3 -Edge XANES of Tc(IV)-Gluconate Complexes and 3d4f RIXS maps of Uranyl and Neptunyl

The science of nuclear waste disposal includes the detailed understanding of the chemical behaviour of radionuclides in the environment. Many of these radionuclides are transition metals (Tc in our case) or actinide compounds, like uranyl and neptunyl. The thorough understanding of the electronic structure of Technetium (Tc) and actinides (An) and the often puzzling contribution of the 4d and 5f orbitals to chemical bonding are of major interest in chemistry. The electronic structure is best probed by combining X-ray spectroscopy with relativistic multiconfigurational ab initio calculations (see e.g. [1-4]).
99Tc is one of the main fission products of 235U and 239Pu in nuclear reactors. Due to its long half-life and large inventory in spent nuclear fuel, 99Tc is of great relevance in the context of safety assessment of repositories for radioactive waste. 99Tc is also a relevant contaminant associated with sites for plutonium production or nuclear fuel reprocessing, e.g. at the Hanford site. In this context, formation of, e.g., Tc(IV)−gluconate complexes significantly influences the environmental mobility compared to the sparingly soluble Tc(IV)O2 solid.
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We performed ab initio (RASSCF/RASPT2) calculations including spin-orbit couplings (SOC) of Tc K‐/L3‐Edge XANES of the observed Tc(IV)-gluconate species. The experimental XANES spectra [2] differ significantly from the spectrum obtained from Tc(IV)O2. A detailed understanding of this change requires the knowledge of the geometry and electronic structure of the Tc(IV)-gluconate species. Our calculations allow a full understanding of the Tc L3‐Edge XANES of the observed Tc(IV)-gluconate species based on the knowledge of the changes in the electronic structure.
Advanced spectroscopic techniques of An, such as An M4,5 HR-XANES and 3d4f resonant inelastic X-ray scattering (RIXS), are bulk-sensitive techniques, which can be applied in situ. An improved understanding of the An interaction with the constituents of the geo- and ecosphere is highly relevant for the reliable safety assessment of nuclear waste disposals and of An behavior in the environment, and X-ray spectroscopy together with ab initio calculations is an effective tool for this purpose [1,3,4].
We present results of RASSCF/RASPT2 results including SOC of An M4,5 HR-XANES spectra and 3d4f RIXS for uranyl (U(VI)O22+) [1], neptunyl (Np(VI)O22+ and Np(V)O2+) and uranyl halides ([U(VI)O2X4]2-, X=F, Cl, Br) which allow all a detailed interpretation of the experimental X-ray data.

These results allow for a deeper understanding of the experimental M4,5 HR-XANES spectra and 3d4f RIXS map of uranyl with the help of our relativistic multiconfigurational ab initio calculations. The calculations complement the experimental efforts in this field greatly and allows for the interpretation of the experimental data in a clear and transparent way.

[1] R. Polly et al., Inorg. Chem. 2021, 60, 24, 18764–18776.
[2] K. Dardenne et al., Inorg. Chem. 2021, 60, 16, 12285–12298.
[3] T. Vitova et al., Nat. Commun. 2017, 8, 1−9.
[4] .-C. Sergentu et al., J. Phys. Chem. Lett. 2018, 9, 18, 5583–5591.