Photoionization mass spectrometry of six isomers of C7H8 in the 7-22 eV photon energy range

Photoion mass spectrometry in the 7-22 eV range was used to obtain the pare nt and fragment photoion yield curves and appearance energies for six isome rs of C7H8: toluene, cycloheptatriene, norbornadiene, quadricyclane, spiro[ 2,4]hepta-4,6-diene and 1,6-neptadiyne. The apparent heats of formation of fragment ions m(1)(+) were determined for various fragmentation pathways an d compared with standard thermochemical values Delta H-f(m(1)(+)) in order to assign the fragmentation channels. Comparisons between the mass spectra of the six isomers obtained by photoelectron-photoion coincidence measureme nts at a photon excitation energy of 20 eV, and between the respective appa rent Delta H-f(m(1)(+)) values for these isomers, were used to discuss the possible formation of common isomers during the various dissociative ioniza tion processes. Interconversion of norbornadiene and cycloheptatriene ions is suggested to occur prior to the ro[2,4]hepta-4,6-diene and 1,6-heptadiyn e. The apparent heats formation of C7H7+ and other fragment ions in these t wo isomers. For toluene, isomerization does not appear to pe 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 inter conversion to an acyclic isomer common to all six species occurs in the for mation of the three fragment ions C4H3+, C3H3+ and C2H3+ at high internal e nergies of the parent ion. (C) 2000 Elsevier Science B.V. All rights reserv ed.