Hydrolysis Reactions of the High Oxidation State Dimers ThO, PaO, UO, and NpO. A Computational Study.

The energetics of the hydrolysis reactions for high oxidation states of the dimeric actinide species ThO, PaO, and UO were calculated at the CCSD(T) level and those for triplet NpO at the B3LYP level. Hydrolysis is initiated by the formation of a Lewis acid/base adduct with HO (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 ThO, hydrolysis (chemisorption) by the initial and subsequent HO molecules prefers proton transfer to terminal oxo groups before the bridge oxo groups. The overall ThO hydration pathway is exothermic with chemisorbed products preferred over the physisorption products, and the fully hydrolyzed Th(OH) can form exothermically. Hydrolysis of PaO forms isomers of similar energies with no initial preference for bridge or terminal hydroxy groups. The most exothermic hydrolysis product for Pa is PaO(OH) and the most stable species is PaO(OH)(HO). Hydrolysis of UO and NpO with strong [O═An═O] actinyl groups occurs first at the bridging oxygens rather than at the terminal oxo groups. The UO and NpO pathways predict hydrated products to be more favored than hydrolyzed products, as more HO 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 AnO(OH)(HO).