Defense of thesis Tamara Shaadan

Defense of thesisTamara Shaaban - laboratory PhLAM

Abstract : 

Protactinium (Pa, Z = 91) occupies a unique position between transition metals and the heavier actinides, as both its 5f and 6d orbitals contribute to the ground-state electronic
configuration. In aqueous solution, protactinium exists mainly in the +V oxidation state; however, unlike uranium or neptunium, it does not form a stable di-oxo cation (PaO2+). Instead, it can form a mono-oxo bond (Pa –O) in solution, for which only limited experimental information exists. Because of its scarcity, strong radioactivity, and marked tendency toward hydrolysis, polymerization, and sorption onto solids, experimental investigations of protactinium chemistry remain highly challenging. This thesis therefore combines advanced quantum-chemical modeling and high-resolution X-ray spectroscopy to elucidate the stability, structure, and spectroscopic signatures of Pa(V) mono-oxo complexes.
A theoretical framework was developed to evaluate the thermodynamics of oxo-bond formation and ligand-dependent stabilization in aqueous media using relativistic density
functional theory (DFT) and correlated wave-function methods. The Gibbs free energy of the reaction Pa(OH)4(X) →PaO(OH)2(X)(H2O) was computed for a series of monodentate ligands, hydroxide (OH– ), halides (F– , Cl– , Br– , I – ), and thiocyanate (SCN– ) as well as for bidentate ligands such as nitrate (NO3– ), sulfate (SO42– ), and oxalate (C2O42– ). These calculations showed that Cl– , Br– , I – , SO42– , SCN– , and NO3– effectively stabilize the mono-oxo bond and that the Pa –O interaction exhibits triple bond character. The derived thermodynamic trends provided quantitative guidance for subsequent experimental work.
These predictions were validated by X-ray Absorption Spectroscopy (XANES and EXAFS) measurements performed at the SOLEIL synchrotron within the ANR-CHESS project.
For the first time, a persistent Pa –O bond was observed in concentrated hydrochloric acid, revealing the formation of [PaOCl5(H2O)]2– even in 12M HCl. The experimental Pa –O bond length of 1.83Å agrees remarkably well with theoretical predictions, resolving a discrepancy between theory and experiment. At lower acidities (3m HCl), mixed hydroxo–oxo species such as [PaO(OH)Cl(H2O)4]+ and [PaO(OH)Cl2(H2O)3] were identified, confirming the coexistence of multiple Pa(V) species depending on acidity.
To interpret the HERFD-XANES spectra at the M4-edge of [PaF7]2– and [PaO(C2O4)3]3–, core-excited states were modeled using two-component relativistic TD-DFT (2c-TDACAM B3LYP) and multireference RASPT2 methods. Both approaches gave consistent results, indicating that the main transitions are localized on the Pa and Pa –O fragments
of the respective complexes, with minimal ligand-to-metal charge transfer. Overall, this work provides the first coherent theoretical–experimental description of Pa(V) speciation in aqueous media. It establishes a predictive computational framework for anticipating complex stability and guiding synchrotron experiments. The results demonstrate that the mono-oxo Pa –O bond can persist in highly acidic environments and that theoretical models can now reproduce experimental bond distances and spectral features with good accuracy.

Keywords : actinides, protactinium, x-ray absorption spectroscopy, quantum chemistry