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

Theenergetics of the hydrolysis reactions for high oxidation statesof the dimeric actinide species Th-2 O-IV(4), Pa-2 O-V(5), and U-2 O-VI(6) were calculated at the CCSD(T) level and thosefor triplet Np-2 O-VI(6) at the B3LYPlevel. Hydrolysis is initiated by the formation of a Lewis acid/baseadduct with H2O (physisorbed product), followed by a protontransfer to form a dihydroxide molecule (chemisorbed product); thisprocess was repeated until the initial actinide oxide is fully hydrolyzed.For Th2O4, hydrolysis (chemisorption) by theinitial and subsequent H2O molecules prefers proton transferto terminal oxo groups before the bridge oxo groups. The overallTh(2)O(4) hydration pathway is exothermic with chemisorbedproducts preferred over the physisorption products, and the fullyhydrolyzed Th-2(OH)(8) can form exothermically.Hydrolysis of Pa2O5 forms isomers of similarenergies with no initial preference for bridge or terminal hydroxygroups. 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 ratherthan at the terminal oxo groups. The U2O6 andNp(2)O(6) pathways predict hydrated products tobe more favored than hydrolyzed products, as more H2O moleculesare added. The stability of the U and Np clusters is predicted todecrease with increasing number of hydroxyl groups. The most stablespecies on the hydration reaction coordinate for U and Np is An(2)O(3)(OH)(6)(H2O).