Gas Phase Reactions of Ions Derived from Anionic Uranyl Formate and Uranyl Acetate Complexes

The speciation and reactivity of uranium are topics of sustained interest because of their importance to the development of nuclear fuel processing methods, and a more complete understanding of the factors that govern the mobility and fate of the element in the environment. Tandem mass spectrometry can be used to examine the intrinsic reactivity (i.e., free from influence of solvent and other condensed phase effects) of a wide range of metal ion complexes in a species-specific fashion. Here, electrospray ionization, collision-induced dissociation, and gas-phase ion-molecule reactions were used to create and characterize ions derived from precursors composed of uranyl cation (U^VIO₂ ²⁺) coordinated by formate or acetate ligands. Anionic complexes containing U^VIO₂ ²⁺ and formate ligands fragment by decarboxylation and elimination of CH₂=O, ultimately to produce an oxo-hydride species [U^VIO₂(O)(H)]^-. Cationic species ultimately dissociate to make [U^VIO₂(OH)]⁺. Anionic complexes containing acetate ligands exhibit an initial loss of acetyloxyl radical, CH₃CO₂?, with associated reduction of uranyl to U^VO₂ ⁺. Subsequent CID steps cause elimination of CO₂ and CH₄, ultimately to produce [U^VO₂(O)]^–. Loss of CH₄ occurs by an intra-complex H⁺ transfer process that leaves U^VO₂ ⁺ coordinated by acetate and acetate enolate ligands. A subsequent dissociation step causes elimination of CH₂=C=O to leave [U^VO₂(O)]^–. Elimination of CH₄ is also observed as a result of hydrolysis caused by ion-molecule reaction with H₂O. The reactions of other anionic species with gas-phase H₂O create hydroxyl products, presumably through the elimination of H₂.

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