Differentiation of the pyridine radical cation from its distonic isomers by ion–molecule reactions with dioxygen

In a previous study on the pyridine ion (1) and the pyridine-2-ylid isomer (2), we reported that ions 2 readily react with H2O to produce 2-pyridone ions at m/z 95, by O-atom abstraction. The mechanism for this intriguing reaction, however, was not established. This prompted us to use model chemistry calculations (CBS-QB3) to probe various mechanistic scenarios and to perform complementary experiments with the new, more versatile, ion–molecule reaction chamber of the Mons Autospec 6F mass spectrometer. It appears that H2O is not reacting neutral that produces the 2-pyridone ion of the above reaction, but rather O2 from air co-introduced with the water vapour. Theory and experiment agree that the exothermic reaction of O2 with the pyridine-2-ylid ion leads to loss of 3O from a stable peroxide-type adduct ion at m/z 111. Similarly, pyridine-3-ylid ions (3) generate 3-pyridone ions, but the reaction in this case is thermoneutral. The m/z 111:95 peak intensity ratios in the spectra of the ion–molecule products from ions 2 and 3 may serve to differentiate the isomers.