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

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).