Dehalogenation of protonated C-halogeno-1,2,4-triazoles: synthesis of new heterocyclic carbenic and ylid radical cations and contrasting behaviour ofcollision gases

C(3,5)-halogeno-1,2,4-triazoles, protonated under chemical ionization condi tions, are found to undergo easy dehalogenation upon 8 keV collisional acti vation conditions, provided the collision gas is oxygen, not helium. The io ns produced under these reactions are demonstrated to be five-membered cycl ic carbenic ions or ylid ions, isomers of more conventional molecular ions of 1,2,4-triazoles. The same unconventional radical cations can also be pro duced in the low kinetic energy regime (similar to 20-30 eV) if the halogen is bromine, not chlorine. These conclusions were derived from tandem mass spectrometric measurements (collisional activation, neutralization-reioniza tion, and specific ion molecule reactions) performed on a hybrid tandem mas s spectrometer of sector-quadrupole-sector configuration. Quantum chemical calculations using the density functional theory (DFT) at the B3LYP/6-31G(d ,p) + ZPE level were also carried out on the protonated C-halogenated-1,2,4 -triazoles in both singlet and triplet states and their fragmentation produ cts (halogen = Cl and Br). Calculated results suggest that the dehalogenati on, occurring when oxygen gas was employed, is likely to arise from an exci tation of protonated species into their lowest-lying triplet state prior to dissociation. Ionization energies and proton affinities of triazoles were also evaluated. (Int J Mass Spectrom 199 (2000) 221-233) (C) 2000 Elsevier Science B.V.