Unimolecular and photoinduced dissociations of aromatic C₈H₁₀^+• molecular ions

Production of C₇H₇⁺ in the unimolecular and photoinduced dissociations of ethylbenzene; 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, whereas only the benzylium ion was observed at higher internal energy achieved by photoabsorption. Experimental data suggest that interconversion between ethylbenzene and methylcycloheptatriene ions occurs more rapidly than their dissociations near the reaction threshold, whereas xylene ions undergo rate-determining isomerization to the methylcycloheptatriene structure, and dissociation occurs from the ethylbenzene/methylcycloheptatriene ion mixture. The processes occurring at higher internal energy are quite similar, except that the tropylium channel cannot compete against the benzylium channel because either the ethylbenzene ion-to-methylcycloheptatriene ion conversion or the dissociation of the methylcycloheptatriene ion to the tropylium ion is slower than the direct dissociation of the ethylbenzene ion. The above mechanistic picture is in disagreement with the prediction of a dynamic model reported previously. A series of investigations performed for the production of C₇H₇⁺ from alkylbenzene ions shows an interesting trend of increasing benzylium production with the increase in the total number of carbon atoms in the alkyl substituents.