Chemistry and speciation of protactinium – a first principles study

National audienceIt is of fundamental interest to understand and predict the chemistry of rare radioelements. In this work, we focus on protactinium (Z = 91), an element that is sandwiched in between thorium and uranium in the periodic table. Protactinium may naturally occur in environment (protactini-um-231 results from the decay of naturally occurring uranium-235) and also appear in thorium-based nuclear fuel cycles. From a chemical point of view, protactinium is a crossing point in the actinide series [1] and its chemistry is hard to predict [2,3]. We hypothesize that relativistic quantum chemistry should allow us to understand the enigmatic chemistry of protactinium and even predict it. For our first study, we have chosen to focus on the coordination sphere of protactinium and on the computation of equilibrium constants for experimentally known systems [3–5]. The occur-rence of 1:1, 1:2 and 1:3 complexes of protactinium(V) with sulfate and oxalate ligands is thus studied by means of quantum mechanical calculations, in particular based on density functional theory. The solvent effects, inherent to solution chemistry, are introduced by means of a polariz-able continuum model [6] and the explicit treatment of water molecules (micro solvation).The coordination sphere of protactinium has been obtained by geometry optimizations per-formed both in the gas phase and in solution. It involves an oxygen atom from the Pa=O mono-oxo bond and also oxygen atoms from the bidentate ligands, and in some cases from additional water molecules. The computation of equilibrium constants and comparison with experimental data is more subtle. First, only apparent constants were experimentally determined. Since the oc-currence of a mono-oxo bond was confirmed by EXAFS [5] even in the case of the 1:3 complex with oxalate ligands (corresponding to the stronger complexation), we hypothesize that this bond is also present in all the studied complexes. Second, number of explicitly treated water molecules should not be randomly chosen, it should ideally (i) lead to saturation of the coordination sphere of protactinium and (ii) be sufficient to stabilize the anionic ligands. We find that adding CN+1 water molecules is enough to satisfy both conditions in all the six studied complexes. By doing so and computing ligand-exchange equilibrium constants, we reproduce well the experimental trends for the exchange of 2 and 3 ligands, while the exchange of only one ligand (1:1 complex-es) is still hard to reproduce from computations.We report recent progress concerning the basic chemistry of protactinium. We have shown that its coordination sphere may include up to 8 oxygen atoms (from the original mono-oxo bond and from ligand and solvent molecule complexation) and find an approximate way of determin-ing trends in equilibrium constants, opening the way for future predictions.[1]Wilson R. et al. (2018) Nat. Commun. 9, 622.[2]Wilson R. (2012) Nat. Chem. 4, 586.[3]Le Naour C. et al. (2019) Radiochim. Acta, 107, 979-991.[4]Le Naour C. et al. (2005) Inorg. Chem. 44, 9542.[5]Mendes M. et al. (2010) Inorg. Chem. 49, 9962-9971.[6]Barone et al. (1997) J. Chem. Phys. 107, 3210-3221.