THE PHOTOFRAGMENTATION OF NAPHTHALENE AND AZULENE MONOCATIONS IN THE ENERGY-RANGE 7-22 EV

Photoion mass spectrometry was used to study the fragmentation of naphthalene and azulene monocations over the excitation energy range 7-22 eV. Fifteen fragmentation processes in naphthalene and twelve in azulene have been examined in detail. The photoionization mass spectra at 20.58 eV are quasi-identical for the two isomers. This, and the constant value of the difference between the fragment appearance energies (AE) for naphthalene and azulene, equal to the difference in the heats of formations of the neutral parents, suggest that identical products are formed. The unimolecular dissociations fall mainly into (i) a "low energy" group, (AE < 16 eV) and (ii) a "high energy" group (AE > 18 eV). The reactions in (i) have in common the bicyclic precursor C10H8+ ion 18 which decays via rupture of one ring. The group (ii) reactions involve rupture of both rings to give an open chain precursor, the 1,6-bis-ethinyl-hexatriene radical cation 20. Thermodynamic and mechanistic arguments are given to propose specific reaction pathways and product structures. Two general schemes rationalize the low-energy and high-energy ionic decompositions.