Gas-Phase Uranyl, Neptunyl,
and Plutonyl: Hydration
and Oxidation Studied by Experiment and Theory
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Abstract
The following monopositive actinyl ions were produced
by electrospray
ionization of aqueous solutions of An<sup>VI</sup>O<sub>2</sub>(ClO<sub>4</sub>)<sub>2</sub> (An = U, Np, Pu): U<sup>V</sup>O<sub>2</sub><sup>+</sup>, Np<sup>V</sup>O<sub>2</sub><sup>+</sup>, Pu<sup>V</sup>O<sub>2</sub><sup>+</sup>, U<sup>VI</sup>O<sub>2</sub>(OH)<sup>+</sup>, and Pu<sup>VI</sup>O<sub>2</sub>(OH)<sup>+</sup>; 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 AnO<sub>2</sub><sup>+</sup>·(H<sub>2</sub>O)<sub>4</sub> (An = U, Np, Pu) and AnO<sub>2</sub>(OH)<sup>+</sup>·(H<sub>2</sub>O)<sub>3</sub> (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 AnO<sub>2</sub>(OH)<sup>+</sup> were substantially faster
than for the AnO<sub>2</sub><sup>+</sup>, reflecting additional vibrational
degrees of freedom in the hydroxide ions for stabilization of hot
adducts. Dioxygen addition resulted in UO<sub>2</sub><sup>+</sup>(O<sub>2</sub>)(H<sub>2</sub>O)<sub><i>n</i></sub> (<i>n</i> = 2, 3), whereas O<sub>2</sub> addition was not observed for NpO<sub>2</sub><sup>+</sup> or PuO<sub>2</sub><sup>+</sup> hydrates. DFT
suggests that two-electron three-centered bonds form between UO<sub>2</sub><sup>+</sup> and O<sub>2</sub>, but not between NpO<sub>2</sub><sup>+</sup> and O<sub>2</sub>. As formation of the UO<sub>2</sub><sup>+</sup>–O<sub>2</sub> bonds formally corresponds to the
oxidation of U(V) to U(VI), the absence of this bonding with NpO<sub>2</sub><sup>+</sup> can be considered a manifestation of the lower
relative stability of Np(VI)