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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.

Reported here is a comparison of electron transfer dissociation (ETD) and collision-induced dissociation (CID) of solvent-coordinated dipositive uranyl and plutonyl ions generated by electrospray ionization. Fundamental differences between the ETD and CID processes are apparent, as are differences between the intrinsic chemistries of uranyl and plutonyl. Reduction of both charge and oxidation state, which is inherent in ETD activation of [An(VI) O(2) (CH(3) COCH(3) )(4) ](2+) , [An(VI) O(2) (CH(3) CN)(4) ](2) , [U(VI) O(2) (CH(3) COCH(3) )(5) ](2+) and [U(VI) O(2) (CH(3) CN)(5) ](2+) (An = U or Pu), is accompanied by ligand loss. Resulting low-coordinate uranyl(V) complexes add O(2) , whereas plutonyl(V) complexes do not. In contrast, CID of the same complexes generates predominantly doubly-charged products through loss of coordinating ligands. Singly-charged CID products of [U(VI) O(2) (CH(3) COCH(3) )(4,5) ](2+) , [U(VI) O(2) (CH(3) CN)(4,5) ](2+) and [Pu(VI) O(2) (CH(3) CN)(4) ](2+) retain the hexavalent metal oxidation state with the addition of hydroxide or acetone enolate anion ligands. However, CID of [Pu(VI) O(2) (CH(3) COCH(3) )(4) ](2+) generates monopositive plutonyl(V) complexes, reflecting relatively more facile reduction of Pu(VI) to Pu(V).

The presence and position of a single beta-alanine (betaA), gamma-aminobutyric acid (gammaABu) or epsilon-aminocaproic acid (Cap) residue has been shown to have a significant influence on the formation of b(n)+ and y(n)+ product ions from a series of model, protonated peptides. In this study, we examined the effect of the same residues on the formation of analogous [b3 - 1 + cat]+ products from metal (Li+, Na+ and Ag+)-cationized peptides. The larger amino acids suppress formation of b3+ from protonated peptides with general sequence AAXG (where X = beta-alanine, gamma-aminobutyric acid or epsilon-aminocaproic acid), presumably because of the prohibitive effect of larger cyclic intermediates in the 'oxazolone' pathway. However, abundant [b3 - 1 + cat]+ products are generated from metal-cationized versions of AAXG. Using a group of deuterium-labeled and exchanged peptides, we found that formation of [b3 - 1 + cat]+ involves transfer of either amide or alpha-carbon position H atoms, and the tendency to transfer the atom from the alpha-carbon position increases with the size of the amino acid in position X. To account for the transfer of the H atom, a mechanism involving formation of a ketene product as [b3 - 1 + cat]+ is proposed.