Gas-Phase Uranyl, Neptunyl, and Plutonyl: Hydration and Oxidation Studied by Experiment and Theory

Abstract

The following monopositive actinyl ions were produced by electrospray ionization of aqueous solutions of An^VIO₂(ClO₄)₂ (An = U, Np, Pu): U^VO₂⁺, Np^VO₂⁺, Pu^VO₂⁺, U^VIO₂(OH)⁺, and Pu^VIO₂(OH)⁺; abundances of the actinyl ions reflect the relative stabilities of the An­(VI) and An­(V) oxidation states. Gas-phase reactions with water in an ion trap revealed that water addition terminates at AnO₂⁺·(H₂O)₄ (An = U, Np, Pu) and AnO₂(OH)⁺·(H₂O)₃ (An = U, Pu), each with four equatorial ligands. These terminal hydrates evidently correspond to the maximum inner-sphere water coordination in the gas phase, as substantiated by density functional theory (DFT) computations of the hydrate structures and energetics. Measured hydration rates for the AnO₂(OH)⁺ were substantially faster than for the AnO₂⁺, reflecting additional vibrational degrees of freedom in the hydroxide ions for stabilization of hot adducts. Dioxygen addition resulted in UO₂⁺(O₂)­(H₂O)_<i>n</i> (<i>n</i> = 2, 3), whereas O₂ addition was not observed for NpO₂⁺ or PuO₂⁺ hydrates. DFT suggests that two-electron three-centered bonds form between UO₂⁺ and O₂, but not between NpO₂⁺ and O₂. As formation of the UO₂⁺–O₂ bonds formally corresponds to the oxidation of U­(V) to U­(VI), the absence of this bonding with NpO₂⁺ can be considered a manifestation of the lower relative stability of Np­(VI)