Azacyclohexatriene-2-ylidene (1), the 2-isomer of pyridine (2), has been generated by one-electron reduction of the corresponding radical cation in neutralization-reionization mass spectrometric experiments. The experimental finding that this molecule is stable on the microsecond time scale is in agreement with results of quantum chemical calculations that indicate both 1 and its radical cation, 1(.+), correspond to minima on the C5H5N and C5H5N.+ potential energy surfaces. The calculations predict that 1 is less stable than pyridine, 2, by 50 and 49 kcal/mol (MP2/6-31G** and CASSCF-MP2/6-31G**, respectively) or 47 kcal/mol (B3LYP/6-31G**), whereas the radical cations 1(.+) and 2(.+) are much closer in energy. The ylid ion 1(.+) is predicted to be 6 and 7 kcal/mol lower in energy than 2(.+) at the MP2 and CASSCF-MP2/6-31G** levels, respectively, and 1 kcal/mol higher according to the hybrid density functional theory. Calculations also suggest that facile isomerization of the ions is prohibited by an energy barrier, amounting to 62 and 57 kcal/mol at MP2/6-31G** and B3LYP/6-31G**, respectively, relative to 1(.+), which is even larger than the 38 kcal/mol obtained at both levels of theory required for the neutral transformation. Despite the substantial impediments, isomerization of excited species is possible since the lowest dissociation channels lie even higher in energy but the experimental observations confirm that neither the ions or neutrals undergo particularly facile isomerization. Using known thermochemical data a value for Delta H-f (1(.+)) = 237 +/- 5 kcal/mol was obtained from the measured appearance energy, 10.14 eV, of the C5H5N.+ ion generated from methyl picolinate, which is completely consistent with the theoretical predictions of 237-242 kcal/mol; derived from the calculated energy differences between the various species and the known heat of formation of 2.