Oxygen atom transfer to positive ions: A novel reaction of ozone in the gas phase

In the gas phase, neutral ozone (O₃) transfers an oxygen atom to several positive ions, i.e. the radical cations of pyridines (R−Py^+•; R = H, CH₃, C₂H₅, and Cl), pyrimidine (Pi^+•), and alkyl halides (CH₃X^+•; X = Cl and I), and the halogen cations (X⁺; X = Cl, Br, and I). Reactivity changes drastically within the halogen series (Cl⁺ ≪ Br⁺ ≤ I⁺), whereas no O-transfer occurs to F⁺. The oxide derivatives R−Py⁺−O^•, Pi⁺−O^•, CH₃X⁺−O^•, and XO⁺ are formed, as demonstrated by pentaquadrupole (QqQqQ) double- and triple-stage mass spectrometry. No oxygen atom transfer occurs, however, in “inverse” reactions, i.e., those of ionized ozone (O₃^+•) with the corresponding neutrals; and charge transfer dominates. Ab initio calculations suggest that O-transfer from ozone to ionized pyridine yields ionized pyridine N-oxide via simple nucleophilic addition of ozone as opposed to 1,3-dipolar cycloaddition. Similar nucleophilic addition followed by O₂ loss is also the most likely mechanism for O-transfer from ozone to the ionized alkyl halides and halogen cations. This novel O-transfer reaction to positive ions, which expands our knowledge of the rich chemistry of ozone, introduces a new pathway for the gas-phase oxidation of halogen atoms, pyridines, pyrimidines, alkyl halides, and analogues, and consequently for the gas-phase generation of their chemically interesting but difficult to access ionized oxides.