Unimolecular and photoinduced dissociations of aromatic C8H10+center dot molecular ions

Production of C7H7+ in the unimolecular and photoinduced dissociations of e thylbenzene; methylcycloheptatriene; and o-, m-, and p-xylene ions has been investigated using mass-analyzed ion kinetic energy spectrometry. Both the benzylium and tropylium ions were produced near the reaction threshold, wh ereas only the benzylium ion was observed at higher internal energy achieve d by photoabsorption. Experimental data suggest that interconversion betwee n ethylbenzene and methylcycloheptatriene ions occurs more rapidly than the ir dissociations near the reaction threshold, whereas xylene ions undergo r ate-determining isomerization to the methylcycloheptatriene structure, and dissociation occurs from the ethylbenzene/methylcycloheptatriene ion mixtur e. The processes occurring at higher internal energy are quite similar, exc ept that the tropylium channel cannot compete against the benzylium channel because either the ethylbenzene ion-to-methylcycloheptatriene ion conversi on or the dissociation of the methylcycloheptatriene ion to the tropylium i on is slower than the direct dissociation of the ethylbenzene ion. The abov e mechanistic picture is in disagreement with the prediction of a dynamic m odel reported previously. A series of investigations performed for the prod uction of C7H7+ from alkylbenzene ions shows an interesting trend of increa sing benzylium production with the increase in the total number of carbon a toms in the alkyl substituents.