Combining XAS and UV-vis spectroscopy with ensemble-based quantum calculations: Towards the speciation of radioactive cations in marine environments

• Autores: Gema Raposo Hernandez
• Directores de la Tesis: Rafael Rodríguez Pappalardo (dir. tes.), Enrique Sánchez Marcos (dir. tes.)
• Lectura: En la Universidad de Sevilla ( España ) en 2026
• Idioma: inglés
• Número de páginas: 268
• Enlaces
  • Tesis en acceso abierto en: IdUS
• Resumen

  • The characterisation of f-block element speciation in aqueous solution remains a fundamental challenge in nuclear chemistry, particularly concerning the management of high-level radioactive waste and the optimisation of separation processes. This thesis develops a unified computational strategy to investigate the coordination environment, solvation dynamics, and spectroscopic properties of highly charged metal cations. The approach integrates multireference quantum chemistry, classical and ab initio molecular dynamics, and Wigner sampling to generate theoretical average properties.

    The study of 3d-metal phthalocyanines (FePc, NiPc, CuPc, ZnPc) serves as a starting point for this work. Although distinct from the f-block systems, these complexes provided a useful test case for the structural sensitivity of the EXAFS technique.

    By combining classical and Born–Oppenheimer molecular dynamics, we established the role of interference patterns and the limitations of conventional experimental fitting. Standard fitting procedures often artificially inflate Debye–Waller factors due to the lack of paths considered in the fitting, a finding of general relevance for the EXAFS analysis of complex molecular frameworks.

    New intermolecular potentials are developed for the pentavalent actinyls NpO2+ and PuO2+ at the NEVPT2 level. The multiconfigurational character of these open-shell species is essential for an accurate description of the potential energy surface. DFT methods systematically underestimate the An-Oyl bond lengths, propagating inaccuracies into the EXAFS spectra. These NEVPT2-based force fields resolve the long-standing coordination controversies, confirming through EXAFS and free-energy calculations that both NpO2+ and PuO2+ exist as stable pentahydrates, [AnO2(H2O)5]+, in acidic media.

    The statistical averaging strategy is further extended to UV-Vis electronic spectroscopy. For the trivalent cerium ion Ce3+, combining NEVPT2+SOC calculations with a Gaussian Mixture Models-Nuclear Ensemble Approach (GMM-NEA) yields theoretical spectra in quantitative agreement with experiment. This work, recognised with the VII GEQC-RSEQ Outstanding Publication Award, shows the first direct comparison of the [Ce(H2O)9]3+/Ce(H2O)8]3+ spectra in equilibrium and its temperature dependence from first principles.

    For tetravalent uranium (U4+), the methodology addresses the demanding case of Laporte-forbidden f-f transitions. This study was selected as a Featured Article and cover of Inorganic Chemistry. It demonstrates that the 5f2 manifold requires the extension of the active space to 12 orbitals, 5f+6d, to recover the experimental band positions and intensities, and that the combination of UV-Vis and X-ray absorption can help to assign the ennea-coordination at low pH. The pH-driven speciation of U4+ is then resolved by mixing simulated spectra of aqua and hydroxo complexes in ratios derived from thermodynamic data.

    The final stage of this thesis focuses on the speciation of PuO22+ in the presence of chloride, which is one of the most abundant ions in marine environments. Understanding this interaction can help to predict the evolution of radionuclides in seawater. Using NEVPT2 and Wigner sampling, the systematic red-shift of the main absorption band was reproduced as chloride ligands replace water molecules. This provides specific spectral fingerprints for each individual [PuO2(H2O)mCln](2-n) complex, which are often difficult to distinguish experimentally.

    This research also establishes Wigner sampling as a reliable and efficient alternative to molecular dynamics for systems where force fields are not available, such as hydrolysed species or complexes with chloride. These results show that combining multireference electronic structure with statistical sampling is an effective approach to resolve f-block element speciation in complex aqueous environments.