Gas-Phase Hydration of U(IV), U(V), and U(VI) Dioxo Monocations

Abstract

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 y H 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.

Elucidation of the collision induced dissociation pathways of water and alcohol coordinated complexes containing the uranyl cation

“…Because only the ion at m/z 270 was isolated for storage in the ion trap, the m/z 288 peak observed in Figure 4 is an H 2 O adduct to m/z 270 formed by an association reaction in the gas phase. However, the intensity of m/z 288 (Ͻ5% relative to m/z 270) is vastly lower than the peak at the same m/z that was an apparent product ion generated by the CID of investigation that demonstrated slow hydration rates for the reduced uranyl ion produced by ion induced sputtering of solid UO 3 [42]. We propose instead that the product may be generated via the reaction:…”

“…The formation of abundant H 2 O adducts indicates that several of the uranyl complex ions show a significant tendency to accept ligands via gas-phase association reactions in the ion trap. The reaction kinetics for the addition of one or more H 2 O ligands to UO 2 ϩ and related species generated by ioninduced sputtering of solid UO 3 was recently investigated [42]. While beyond the scope of the present study, the reaction kinetics for the addition of H 2 O ligands to the various CID products generated from the alcohol coordinated complexes containing UO 2 2ϩ were also investigated and will be the subject of a separate report [43].…”

Ions generated from uranyl nitrate solutions by electrospray ionization (ESI) and detected with fourier transform ion-cyclotron resonance (FT-ICR) mass spectrometry

“…In both experiments, the ion-molecule reaction times were the same as for m/z 301 (180 s) but the pressures were slightly different (2.4 ϫ 10 Ϫ8 torr for D 2 O and 4.8 ϫ 10 Ϫ8 torr for CD 3 OD). UO 2 ϩ reacts readily with CD 3 OD by adding three and four CD 3 4 ] ϩ in ion trap studies at higher partial pressures of water [17].…”

“…Many of these studies have focused on the reactions of U ϩ and UO ϩ with alkanes [6,7], alkenes [8], alcohols, thiols and ethers [9], arenes [10], and other small molecules [11][12][13][14][15][16]. Recently, the hydration reactivities of UO ϩ , UO 2 ϩ , and UO 2 2ϩ were examined using secondary ion mass spectrometry [17]. These studies used surface ionization [11,12], pulsed glow discharge [13,14], or laser ablation [6 -11, 13, 16, 17] techniques to create uranium ions for study.…”

Ion-molecule reactions of gas-phase chromium oxyanions: CrxOyHz− + O2

“…O 2 reactions with ionic Cr oxide species were conducted by Bricker and Russell [32], who concluded that O 2 would bind with Cr carbonyl anions in a molecular fashion, but gas-phase studies of Cr oxyanions more typical of the condensed phase have not been conducted. Here, O 2 oxidation of Cr 1,2 O y H z Ϫ species is examined using an ion trap SIMS instrument; this approach has been effective for elucidating reaction pathways and kinetics for ionic Al [33,34], Si [34], and U oxides [35]. The results show that under the ambient conditions of the ion trap (1 ϫ 10 Ϫ4 torr He, 310 K), Cr 1,2 O y H z Ϫ species containing low-coordinate, low oxidation state metal centers underwent oxidation.…”