Hydrolysis Reactions of the High Oxidation State Dimers Th₂O₄, Pa₂O₅, U₂O₆, and Np₂O₆. A Computational Study

Abstract

The energetics of the hydrolysis reactions for high oxidation states of the dimeric actinide species Th2 IVO4, Pa2 VO5, and U2 VIO6 were calculated at the CCSD­(T) level and those for triplet Np2 VIO6 at the B3LYP level. Hydrolysis is initiated by the formation of a Lewis acid/base adduct with H2O (physisorbed product), followed by a proton transfer to form a dihydroxide molecule (chemisorbed product); this process was repeated until the initial actinide oxide is fully hydrolyzed. For Th2O4, hydrolysis (chemisorption) by the initial and subsequent H2O molecules prefers proton transfer to terminal oxo groups before the bridge oxo groups. The overall Th2O4 hydration pathway is exothermic with chemisorbed products preferred over the physisorption products, and the fully hydrolyzed Th2(OH)8 can form exothermically. Hydrolysis of Pa2O5 forms isomers of similar energies with no initial preference for bridge or terminal hydroxy groups. The most exothermic hydrolysis product for Pa is Pa2O­(OH)8 and the most stable species is Pa2O­(OH)8(H2O). Hydrolysis of U2O6 and Np2O6 with strong [OAnO]2+ actinyl groups occurs first at the bridging oxygens rather than at the terminal oxo groups. The U2O6 and Np2O6 pathways predict hydrated products to be more favored than hydrolyzed products, as more H2O molecules are added. The stability of the U and Np clusters is predicted to decrease with increasing number of hydroxyl groups. The most stable species on the hydration reaction coordinate for U and Np is An2O3(OH)6(H2O).