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

C(3,5)-halogeno-1,2,4-triazoles, protonated under chemical ionization conditions, are found to undergo easy dehalogenation upon 8 keV collisional activation conditions, provided the collision gas is oxygen, not helium. The ions produced under these reactions are demonstrated to be five-membered cyclic carbenic ions or ylid ions, isomers of more conventional molecular ions of 1,2,4-triazoles. The same unconventional radical cations can also be produced 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-reionization, and specific ion molecule reactions) performed on a hybrid tandem mass 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 products (halogen = Cl and Br). Calculated results suggest that the dehalogenation, occurring when oxygen gas was employed, is likely to arise from an excitation 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.