Benzylpyridinium ions, generated via electrospray ionization of dilute solutions of their salts in acetonitrile/water, are probed by collisional activation in an ion-trap mass spectrometer. From the breakdown diagrams obtained, phenomenological appearance energies of the fragment ions are derived. Comparison of the appearance energies with calculated reaction endothermicities shows a reasonably good correlation for this particular class of compounds. In addition, the data indirectly indicate that at threshold the dissociation of almost all of the benzylpyridinium ions under study leads to the corresponding benzylium ions, rather than the tropylium isomers. Substituent effects on the fragmentation for a series of benzylpyridinium ions demonstrate that neither mass effects nor differences in density of states seriously affect the energetics derived from the ion-trap experiments.

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Fragmentation pathways of unsubstituted and substituted benzylpyridinium compounds were investigated using mass-analysed kinetic energy (MIKE) technique in combination with high level of quantum chemical calculations in the gas phase. Fast atom bombardment (FAB) source was used for ionisation of the studied compounds. The formation of both benzylium and tropylium species were investigated. Hybrid Hartree-Fock/Density Functional Theory calculations have been performed to assess the geometries and the energies of the transition states and intermediates. For each cases, different reaction pathways were investigated, and particularly in the case of the formation of tropylium species, the formation of the seven-membered ring before or after the loss of pyridine were studied. The effect of para-methyl and para-methoxy substituents on the activation energy of the rearrangement process to form thermodynamically stable tropylium compounds has been studied. Theoretical calculations showed competition between direct bond cleavage and rearrangement reactions to form benzylium and tropylium compounds, respectively. Experimental results also suggested that the rearrangement process takes place to yield stable tropylium under "soft ionisation techniques", such as FAB.