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 loss of X giving C3H4N2+ radical cations whose structure depends on the nature of the halogen. Protonated 3-I–Pz generated thus ionized pyrazole a, whereas protonated 3-Cl–Pz was a precursor of an isomeric species ascribed to a dehydropyrazolium distonic structure b. A mixture of C3H4N2+ ions was formed in protonated 3-Br–Pz. B3LYP/6-31++G(d,p) computations confirmed a regiospecific N2-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 competitive C–H and C–X bond cleavages were invoked to explain the contrasting behaviour of various protonated X–Pz under dehalogenation conditions.