Generation of neutral and cationic hydrogen shift isomers of pyridine: a combined experimental and computational investigation11Dedicated to Professor Fulvio Cacace in recognition of his pioneering and stimulating work on gas-phase ion chemistry and physics.

Azacyclohexatriene-3-ylidene (3), an isomer of pyridine (2) and its previously generated Hammick intermediate (1), is accessible in the gas phase by one-electron reduction of the corresponding radical cation in neutralization-reionization mass spectrometric experiments. The experimental finding that this carbene is stable on the microsecond time scale is in agreement with results of quantum chemical calculations, which indicate that all these C5H5N and C5H5N·+ isomers, including also the isomers of azacyclohexatriene-4-ylidene 4 and 4·+, correspond to minima that are separated by significant barriers, thus preventing facile unimolecular isomerization. Although neutral pyridine is much more stable than the ylidic carbenes 1, 3, and 4, for the radical cations the C5H5N·+ isomers are nearly isoenergetic, with 4·+ actually being lowest in energy.