Protonated 2-Cl and 2-Br-pyridines undergo facile dehalogenation upon high energy (8 keV range) collisional activation provided the target is NO, or even better O2, instead of He or Ar. In the low energy regime (20-30 eV range), debromination occurs more readily than dechlorination, but the peculiar behaviour of O2 in favouring an X-loss over a HX-elimination, is no longer detected (using a MS3 instrument). B3LYP/6-31G(d) calculations on X-pyridines with X=F, Cl and Br on positions 2, 3 and 4, suggest that, if a protonated pyridine is formed in the lower-lying triplet state following strong interaction with O2 or NO having higher spin, the loss of Cl or Br becomes almost spontaneous (F-loss is more difficult). Proton affinity PA(pyridine)=940±15 kJ/mol, is decreased by 5-15 kJ/mol upon halogenation.
