Gas-phase hydration of U(IV), U(V), and U(VI) dioxo monocations

The formation of adduct ions consisting of uranium oxycations and water was studied using an ion trap-secondary ion mass spectrometer. The U(IV) and U(V) species [UO(OH)]⁺ and [UO₂]⁺ were produced by bombarding the surface of UO₃ using molecular primary ions, and the U(VI) species [UO₂(OH)]⁺ was generated by O₂ oxidation of [UO(OH)]⁺ in the gas phase. All three ions formed H₂O adducts by termolecular association reactions: [UO(OH)]⁺ (a U(IV) species) added three water molecules, for a total of five ligands; [UO₂]^- (U(V)) added three or four water molecules, for a total of five or six ligands; and [UO₂(OH)]⁺ (U(VI)) added four water molecules for a total of six ligands. Addition of a seventh ligand was not observed in any of the systems. These analyses showed that the optimum extent of ligation increased with increasing oxidation state of the uranium metal. Hard kinetic models were fit to the time-dependent mass spectral data using adaptive simulated annealing (ASA) to estimate reaction rates and rate constants from kinetic data sets. The values determined were validated using stochastic kinetic modeling and resulted in rate data for all forward and reverse reactions for the ensemble of reactive ions present in the ion trap. A comparison of the forward rate constants of the hydration steps showed that in general, formation of the monohydrates was slow, but that hydration efficiency increased upon addition of the second H₂O. Addition of the third H₂O was less efficient (except in the case of [UO₂]⁺), and addition of the fourth H₂O was even more inefficient and did not occur at all in the [UO₂(OH)]⁺ system. Reverse rate constants also decreased with increasing ligation by H₂O, except in the case of [UO(OH)(H₂O)₄]⁺, which prefers to quickly revert to the trihydrate. These findings indicate that stability of the hydrate complexes [UO_yH_z(H₂O)_n]⁺ increases with increasing n, until the optimum number of ligands is achieved. The results enable correlation of uranium hydration behavior with oxidation state.