Photoionization mass spectrometry of six isomers of C 7H 8 in the 7–22 eV photon energy range

Abstract

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 C 7H 8: toluene, cycloheptatriene, norbornadiene, quadricyclane, spiro [2,4]hepta-4,6-diene and 1,6-heptadiyne. The apparent heats of formation of fragment ions m 1 + were determined for various fragmentation pathways and compared with standard thermochemical values of ΔH f(m 1 +) 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 ΔH f(m 1 +) 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 C 7H 7 + 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 C 7H 7 + and its sequential product C 5H 5 +. The results also suggest that interconversion to an acyclic isomer common to all six species occurs in the formation of the three fragment ions C 4H 3 +, C 3H 3 + and C 2H 3 + at high internal energies of the parent ion.