Stability of the protactinium(V) mono-oxo cation probed by first-principle calculations

This study explores the distinctive behavior of protactinium (Z=91) within the actinide series. In contrast to neighboring elements like uranium or plutonium, protactinium in the pentavalent state diverges by not forming the typical dioxo protactinyl moiety PaO2 + in aqueous phase. Instead, it manifests as a monooxo PaO3+ cation or a Pa5+ . Employing first-principle calculations with implicit and explicit solvation, we investigate two stoichiometrically equivalent neutral complexes: PaO(OH)2 (X)(H2 O) and Pa(OH)4 (X), where X represents various monodentate and bidentate ligands. Calculating the Gibbs free energy for the reaction PaO(OH)2 (X)(H2 O)→Pa(OH)4 (X), we find that the PaO(OH)2 (X)(H2 O) complex is stabilized with Cl- , Br- , I- , NCS- , NO3 - , and SO4 2- ligands, while it is not favored with OH- , F- , and C2 O4 2- ligands. Quantum Theory of Atoms in Molecules (QTAIM) and Natural Bond Orbital (NBO) methods reveal the Pa mono-oxo bond as a triple bond, with significant contributions from the 5f and 6d shells. Covalency of the Pa mono-oxo bond increases with certain ligands, such as Cl- , Br- , I- , NCS- , and NO3 - . These findings elucidate protactinium's unique chemical attributes and provide insights into the conditions supporting the stability of relevant complexes.

On the volatility of protactinium in chlorinating and brominating gas media

A multi-target recoil chamber technique was applied to study online chemical properties of protactinium in chlorinating and brominating gas media using 226Pa (T 1/2 = 1.8 min) decaying by alpha emission (74%) and β+/EC decay (26%). A 58 MeV proton beam passing 15 × 50 μg/cm2 thick 232Th targets enabled production of 226Pa formed in the reaction 232Th(p,7n). Isothermal gas chromatography in quartz columns allowed for the determination of adsorption enthalpies of oxohalides and pure halides of Pa5+ compounds. On the basis of empirical correlations, these adsorption enthalpies (ΔH0 ads) could be converted to sublimation enthalpies (ΔH0 subl). Resulting values for the assumed compounds PaCl5, PaOCl3, PaBr5, and PaOBr3 were 113 ± 15, 329 ± 16, 165 ± 5 and 235 ± 17 kJ/mol, respectively. These values are rather similar to known ΔH0 subl data for group-5 elements Nb, Ta and Db in support of the assumption that Pa is a pseudo-group 5 element.