Protactinium (Z = 91) is an enigmatic element for chemists. Positioned between thorium and uranium in the actinide series of the periodic table, it exhibits unique properties that pose challenges for experimental studies and interpretations. For example, the five valence electrons of its free atom can be distributed in numerous ways across the valence 5f, 6d, 7s, and 7p shells, creating a quantum chaos of states even prior to ionization. In solution, protactinium element is expected to be dominated by its stable +V oxidation state (formally free of valence electrons), with the potential occurrence, under specific reducing conditions, of the unstable +IV state (displaying one unpaired 5f electron in the ground state). The limited experimental data on protactinium compounds in aqueous solutions and the gas phase present a challenge to understanding the fundamental chemistry of this element. However, leveraging the available data on Pa +V and Pa +IV, we developed a working hypothesis that guided our research towards obtaining more information on the properties of Pa compounds through quantum chemical calculations. Our recent progress in studying the fundamental chemistry of protactinium has facilitated a better understanding of its enigmatic nature, enabling us to conduct more systematic experimental studies in the future. The synergy between theoretical and experimental studies will likely involve characterizing various Pa +IV compounds under different conditions and analyzing their reactivity, as well as other properties such as chemical composition, stability, and spectroscopic behavior. Through this research, we aim to shed more light on the unique properties of protactinium and contribute to a better understanding of its role in various chemical and physical systems.

This work was supported by the Région des Pays de la Loire (RCT-PaPAn project) and by the French National Research Agency (ANR-21-CE29-0027, LABEX CaPPA/ANR-11-LABX-0005-01 and I-SITE ULNE/ANR-16-IDEX-0004 ULNE)

References
[1]H. Oher et al., Phys. Chem. Chem. Phys 25, 10033 (2023).
[2]C. Le Naour et al., Radiochim. Acta 107, 979 (2019).
[3]M. Mendes et al., Inorg. Chem. 49, 9962 (2010).