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

The following monopositive actinyl ions were produced by electrospray ionization of aqueous solutions of AnVIO2(ClO4)2 (An = U, Np, Pu): UVO2+, NpVO2+, PuVO2+, UVIO2(OH)+, and PuVIO2(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)n (n = 2, 3), whereas O2 addition was not observed for NpO2+ or PuO2+ hydrates...