Study of the coupling reaction of pyridine in the gas phase

The coupling reaction of pyridine in the gas phase to form bipyridyl and terpyridyl has been studied by electron ionization using an ion trap mass spectrometer. In contrast to the difficulty in carrying out electrophilic substitutions at carbon atoms in the pyridine ring under highly acidic solvent conditions, reactions in the gas phase overcame the conjugate acidification of pyridine in the solvent phase, thus decreasing the hardness of this electrophilic coupling. Through product ion mass spectra of the ion at m/z 157, we have shown that this ion was protonated bipyridyl rather than the ion/molecule adduct. A computational study of the heat of formation surface also supported the formation of polypyridyls through the electrophilic substitution of pyridine. We have confirmed the reaction through a study of pyridine-d(5) coupling in the gas phase.