Hydrolysis Of Small Oxo/Hydroxo Molecules Containing High Oxidation State Actinides (Th, Pa, U, Np, Pu): A Computational Study

The energetics of hydrolysis reactions for high oxidation states of oxo/hydroxo monomeric actinide species ((ThO2)-O-IV, (PaO2)-O-IV, (UO2)-O-IV, (PaO2)-O-V(OH), (UO2)-O-V(OH), (UO3)-O-VI, (NpO3)-O-VI, (NpO3)-O-VII(OH), and (PuO3)-O-VII(OH)) were calculated at the CCSD(T) level. The first step is the formation of a Lewis acid/base adduct with H2O (hydration), followed by a proton transfer to form a dihydroxide molecule (hydrolysis); this process is repeated until all oxo groups are hydrolyzed. The physisorption (hydration) for each H2O addition was predicted to be exothermic, ca. -20 kcal/mol. The hydrolysis products are preferred energetically over the hydration products for the +IV and +V oxidation states. The compounds with An(VI) are a turning point in terms of favoring hydration over hydrolysis. For An(VII)O(3)(OH), hydration products are preferred, and only two waters can bind; the complete hydrolysis process is now endothermic, and the oxidation state for the An in An(OH)(7) is +VI with two OH groups each having one-half an electron. The natural bond order charges and the reaction energies provide insights into the nature of the hydrolysis/hydration processes. The actinide charges and bond ionicity generally decrease across the period. The ionic character decreases as the oxidation state and coordination number increase so that covalency increases moving to the right in the actinide period.