Collisional activation of protonated C-halogenopyrazoles

Collisional activation of protonated 3-halogenopyrazoles (X-Pz, X = Cl, Br and I) in the high or low translational energy regime induced an intense lo ss of X giving C3H4N2.+ radical cations whose structure depends on the natu re of the halogen. Protonated 3-1-Pz generated thus ionized pyrazole a, whe reas protonated 3-Cl-Pz was a precursor of an isomeric species ascribed to a dehydropyrazolium distonic structure b. A mixture Of C3H4N2.+ ions was fo rmed in protonated 3-Br-Pz. B3LYP/6-31++G(d,p) computations confirmed a reg iospecific N-2-protonation, and a low energy content of the distonic ions b or c (50 kJ mol(-1) above a and lying in deep energy wells). Two competiti ve C-H and C-X bond cleavages were invoked to explain the contrasting behav iour of various protonated X-Pz under dehalogenation conditions. (C) 2001 E lsevier Science B.V. All rights reserved.