KINETICS AND MECHANISM OF NUCLEOPHILIC DISPLACEMENTS WITH HETEROCYCLES AS LEAVING GROUPS .26. COLLISIONALLY ACTIVATED DISSOCIATION OF N-ALKYLPYRIDINIUM CATIONS TO PYRIDINE AND ALKYL CATIONS IN THE GAS-PHASE

Many VV-alkylpyridinium cations undergo collisionally activated dissociation in the gas phase to form pyridine and the corresponding carbocation. Appearance potentials (A.E.) for this dissociation pathway for a wide variety of pyridiniums Py+-R were estimated quantitatively from the thresholds for fragment ion appearance in the collisionally activated dissociation of these laser-desorbed pyridinium cations. These A.E. were compared with the energy differences [ΔΔHf= ΔHfPy) + ΔHf(R+)-ΔHf(Py+R)] determined by AMI semiempirical MO calculations. These A.E. were frequently equal or close to the calculated energy difference, in particular this was so for all cases when R+ cannot rearrange to a more stable cation. Values considerably below that calculated were found for some cases when cation rearrangement can take place. The results are interpreted as involving Py⃜R+ ion/molecule pair intermediates in which rearrangement of R+ can occur before dissociation. © 1990, American Chemical Society. All rights reserved.