Developing a comprehensive understanding of the reactivity of uranium-containing species remains an important goal in areas ranging from the development of nuclear fuel processing methods to studies of the migration and fate of the element in the environment. Electrospray ionization (ESI) is an effective way to generate gas-phase complexes containing uranium for subsequent studies of intrinsic structure and reactivity. Recent experiments by our group have demonstrated that the relatively low levels of residual HO in a 2-D, linear ion trap (LIT) make it possible to examine fragmentation pathways and reactions not observed in earlier studies conducted with 3-D ion traps (Van Stipdonk et al. J. Am. Soc. Mass Spectrom. 14, 1205-1214, 2003). In the present study, we revisited the dissociation of complexes composed of uranyl nitrate cation [UO(NO)] coordinated by alcohol ligands (methanol and ethanol) using the 2-D LIT. With relatively low levels of background HO, collision-induced dissociation (CID) of [UO(NO)] primarily creates [UO(O)] by the ejection of NO. However, CID (using He as collision gas) of [UO(NO)] creates [UO(HO)] and UO when the 2-D LIT is used with higher levels of background HO. Based on the results presented here, we propose that product ion spectrum in the previous experiments was the result of a two-step process: initial formation of [UO(O)] followed by rapid exchange of O for HO by ion-molecule reaction. Our experiments illustrate the impact of residual HO in ion trap instruments on the product ions generated by CID and provide a more accurate description of the intrinsic dissociation pathway for [UO(NO)]. Graphical Abstract ᅟ.

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The goals of the present study were (a) to create positively charged organo-uranyl complexes with general formula [UO 2 (R)] + (eg, R═CH 3 and CH 2 CH 3 ) by decarboxylation of [UO 2 (O 2 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 H 2 O. Collision-induced dissociation (CID) of both [UO 2 (O 2 C─CH 3 )] + and [UO 2 (O 2 C─CH 2 CH 3 )] + causes H + transfer and elimination of a ketene to leave [UO 2 (OH)] + . However, CID of the alkoxides [UO 2 (OCH 2 CH 3 )] + and [UO 2 (OCH 2 CH 2 CH 3 )] + produced [UO 2 (CH 3 )] + and [UO 2 (CH 2 CH 3 )] + , respectively. Isolation of [UO 2 (CH 3 )] + and [UO 2 (CH 2 CH 3 )] + for reaction with H 2 O caused formation of [UO 2 (H 2 O)] + by elimination of ·CH 3 and ·CH 2 CH 3 : Hydrolysis was not observed. CID of the acrylate and benzoate versions of the complexes, [UO 2 (O 2 C─CH═CH 2 )] + and [UO 2 (O 2 C─C 6 H 5 )] + , caused decarboxylation to leave [UO 2 (CH═CH 2 )] + and [UO 2 (C 6 H 5 )] + , respectively. These organometallic species do react with H 2 O to produce [UO 2 (OH)] + , and loss of the respective radicals to leave [UO 2 (H 2 O)] + was not detected. Density functional theory calculations suggest that formation of [UO 2 (OH)] + , rather than the hydrated U V O 2 + , cation is energetically favored regardless of the precursor ion. However, for the [UO 2 (CH 3 )] + and [UO 2 (CH 2 CH 3 )] + precursors, the transition state energy for proton transfer to generate [UO 2 (OH)] + and the associated neutral alkanes is higher than the path involving direct elimination of the organic neutral to form [UO 2 (H 2 O)] + . The situation is reversed for the [UO 2 (CH═CH 2 )] + and [UO 2 (C 6 H 5 )] + precursors: The transition state for proton transfer is lower than the energy required for creation of [UO 2 (H 2 O)] + by elimination of CH═CH 2 or C 6 H 5 radical.

With lower levels of background H O, CID experiments reveal that the intrinsic dissociation pathway for [U O (ClO )] leads to [U O (Cl)] , apparently by loss of two O molecules. We propose that the results reported in the earlier CID study reflected a two-step process: initial formation of [U O (Cl)] by CID, followed by a very rapid hydrolysis reaction to leave [U O (OH)] .

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