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 [OAnO]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).