GENERATION OF NEUTRAL AND CATIONIC HYDROGEN SHIFT ISOMERS OF PYRIDINE- A COMBINED EXPERIMENTAL AND COMPUTATIONAL INVESTIGATION

Azacyclohexatriene-3-ylidene (3), an isomer of pyridine (2) and its pr eviously 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 exper imental finding that this carbene is stable on the microsecond time sc ale is in agreement with results of quantum chemical calculations, whi ch indicate that all these C5H5N and C5H5N.+ isomers, including also t he isomers of azacyclohexatriene-4-ylidene 4 and 4(.+), correspond to minima that are separated by significant barriers, thus preventing fac ile 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. (C) 1998 Elsevier Science B.V.