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The goals of the present study were (a) to create positively charged organo-uranyl complexes with general formula [UO2 (R)]+ (eg, R═CH3 and CH2 CH3 ) by decarboxylation of [UO2 (O2 C─R)]+ precursors and (b) to identify the pathways by which the complexes, if formed, dissociate by collisional activation or otherwise react when exposed to gas-phase H2 O. Collision-induced dissociation (CID) of both [UO2 (O2 C─CH3 )]+ and [UO2 (O2 C─CH2 CH3 )]+ causes H+ transfer and elimination of a ketene to leave [UO2 (OH)]+ . However, CID of the alkoxides [UO2 (OCH2 CH3 )]+ and [UO2 (OCH2 CH2 CH3 )]+ produced [UO2 (CH3 )]+ and [UO2 (CH2 CH3 )]+ , respectively. Isolation of [UO2 (CH3 )]+ and [UO2 (CH2 CH3 )]+ for reaction with H2 O caused formation of [UO2 (H2 O)]+ by elimination of ·CH3 and ·CH2 CH3 : Hydrolysis was not observed. CID of the acrylate and benzoate versions of the complexes, [UO2 (O2 C─CH═CH2 )]+ and [UO2 (O2 C─C6 H5 )]+ , caused decarboxylation to leave [UO2 (CH═CH2 )]+ and [UO2 (C6 H5 )]+ , respectively. These organometallic species do react with H2 O to produce [UO2 (OH)]+ , and loss of the respective radicals to leave [UO2 (H2 O)]+ was not detected. Density functional theory calculations suggest that formation of [UO2 (OH)]+ , rather than the hydrated UV O2+ , cation is energetically favored regardless of the precursor ion. However, for the [UO2 (CH3 )]+ and [UO2 (CH2 CH3 )]+ precursors, the transition state energy for proton transfer to generate [UO2 (OH)]+ and the associated neutral alkanes is higher than the path involving direct elimination of the organic neutral to form [UO2 (H2 O)]+ . The situation is reversed for the [UO2 (CH═CH2 )]+ and [UO2 (C6 H5 )]+ precursors: The transition state for proton transfer is lower than the energy required for creation of [UO2 (H2 O)]+ by elimination of CH═CH2 or C6 H5 radical.

We recently reported a detailed investigation of the collision-induced dissociation (CID) of [UO2 (NO3 )3 ]- and [UO2 (NO3 )2 (O2 )]- in a linear ion trap mass spectrometer (J. Mass Spectrom. DOI:10.1002/jms.4705). Here, we describe the CID of [UO2 (NO3 )(O2 )]- which is created directly by ESI, or indirectly by simple elimination of O2 from [UO2 (NO3 )(O2 )2 ]- . CID of [UO2 (NO3 )(O2 )]- creates product ions as at m/z 332 and m/z 318. The former may be formed directly by elimination of O2 , while the latter required decomposition of a nitrate ligand and elimination of NO2 . DFT calculations identify a pathway by which both product ions can be generated, which involves initial isomerization of [UO2 (NO3 )(O2 )]- to create [UO2 (O)(NO2 )(O2 )]- , from which elimination of NO2 or O2 will leave [UO2 (O)(O2 )]- or [UO2 (O)(NO2 )]- , respectively. For the latter product ion, the composition assignment of [UO2 (O)(NO2 )]- rather than [UO2 (NO3 )]- is supported by ion-molecule reaction behavior, and in particular, the fact that spontaneous addition of O2 , which is predicted to be the dominant reaction pathway for [UO2 (NO3 )]- is not observed. Instead, the species reacts with H2 O, which is predicted to be the favored pathway for [UO2 (O)(NO2 )]- . This result in particular demonstrates the utility of ion-molecule reactions to assist the determination of ion composition. As in our earlier study, we find that ions such as [UO2 (O)(NO2 )]- and [UO2 (O)(O2 )]- form H2 O adducts, and calculations suggest these species spontaneously rearrange to create dihydroxides.

Electrospray ionization (ESI) can produce a wide range of gas-phase uranyl (UO2 2+ ) complexes for tandem mass spectrometry studies of intrinsic structure and reactivity. We describe here the formation and collision-induced dissociation (CID) of [UO2 (NO3 )3 ]- and [UO2 (NO3 )2 (O2 )]- . Multiple-stage CID experiments reveal that the complexes dissociate in reactions that involve elimination of O2 , NO2 , or NO3 , and subsequent reactions of interesting uranyl-oxo product ions with (neutral) H2 O and/or O2 were investigated. Density functional theory (DFT) calculations reproduce experimental results and show that dissociation of nitrate ligands, with ejection of neutral NO2 , is favored for both [UO2 (NO3 )3 ]- and [UO2 (NO3 )2 (O2 )]- . DFT calculations also suggest that H2 O adducts to products such as [UO2 (O)(NO3 )]- spontaneously rearrange to create dihydroxides and that addition of O2 is favored over addition of H2 O to formally U(V) species.