Hydrolysis Reactions of the High Oxidation State Dimers Th 2 O 4 , Pa 2 O 5 , U 2 O 6 , and Np 2 O 6 . A Computational Study.

The energetics of the hydrolysis reactions for high oxidation states of the dimeric actinide species Th 2 IV O 4 , Pa 2 V O 5 , and U 2 VI O 6 were calculated at the CCSD(T) level and those for triplet Np 2 VI O 6 at the B3LYP level. Hydrolysis is initiated by the formation of a Lewis acid/base adduct with H 2 O (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 Th 2 O 4 , hydrolysis (chemisorption) by the initial and subsequent H 2 O molecules prefers proton transfer to terminal oxo groups before the bridge oxo groups. The overall Th 2 O 4 hydration pathway is exothermic with chemisorbed products preferred over the physisorption products, and the fully hydrolyzed Th 2 (OH) 8 can form exothermically. Hydrolysis of Pa 2 O 5 forms isomers of similar energies with no initial preference for bridge or terminal hydroxy groups. The most exothermic hydrolysis product for Pa is Pa 2 O(OH) 8 and the most stable species is Pa 2 O(OH) 8 (H 2 O). Hydrolysis of U 2 O 6 and Np 2 O 6 with strong [O═An═O] 2+ actinyl groups occurs first at the bridging oxygens rather than at the terminal oxo groups. The U 2 O 6 and Np 2 O 6 pathways predict hydrated products to be more favored than hydrolyzed products, as more H 2 O 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 An 2 O 3 (OH) 6 (H 2 O).