Photoionization mass spectrometry of six isomers of C7H8 in the 7–22 eV photon energy range Original Research Article

Photoion mass spectrometry in the 7–22 eV range was used to obtain the parent and fragment photoion yield curves and appearance energies for six isomers of C7H8: toluene, cycloheptatriene, norbornadiene, quadricyclane, spirohepta-4,6-diene and 1,6-heptadiyne. The apparent heats of formation of fragment ions m1+ were determined for various fragmentation pathways and compared with standard thermochemical values of ΔHf(m1+) in order to assign the fragmentation channels. Comparisons between the mass spectra of the six isomers obtained by photoelectron–photoion coincidence measurements at a photon excitation energy of 20 eV, and between the respective apparent ΔHf(m1+) values for these isomers, were used to discuss the possible formation of common isomers during the various dissociative ionization processes. Interconversion of norbornadiene and cycloheptatriene ions is suggested to occur prior to the formation of C7H7+ and other fragment ions in these two isomers. For toluene, isomerization does not appear to be necessary for the formation of low-energy fragment ions except for the tropylium form of C7H7+ and its sequential product C5H5+. The results also suggest that interconversion to an acyclic isomer common to all six species occurs in the formation of the three fragment ions C4H3+, C3H3+ and C2H3+ at high internal energies of the parent ion.