One question was whether the predominant fragment ion was a benzylium ion as initially assumed, or a tropylium ion, which would entail a rearrangement (Figure ). Pieces of evidence for tropylium are based on kinetic energy release experiments ) and gasphase reactivity of the fragment ion ). The benzylium ion can indeed make adducts with solvent molecules such as acetonitrile ). If this happens, for the survival yield method the adducts peaks must be summed with their respective fragments. Infrared multiple photon dissociation (IRMPD) spectroscopy demonstrates that the formation of benzylium or tropylium depends of the activation regime . In summary, the benzylium ion forms first, and rearrangement into a tropylium ion occurs subsequently, if the reaction time and collision energy are sufficient. Thus, even if a fraction of the fragment ion is in the tropylium form when arriving at the detector, the ratelimiting step to form the fragment ion remains the simple bond cleavage, the relevant transition state is as assumed in the initial concept, and the calculated bond dissociation energies are correct when assuming the simple bond cleavage. Besides, benzylpyridinium ions such as para-ethoxybenzylpyridinium (p-OEt-Bz), para-isopropoxybenzylpyridinium (p-OiPr-Bz) or para-tert-butoxybenzylpyridinium (p-OtBu-Bz) can form other fragments than the expected benzylium ion . As shown in Figure , this unexpected fragment results from an intermolecular elimination formed via a four-membered transition state. ). They compared internal energy calculation performed either by considering only the direct bond cleavage between C and N, or by summing up all fragment ion intensities.