Observation of the Hammick Intermediate: Reduction of the Pyridine-2-ylid Ion in the Gas Phase

Abstract

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,...