FORMATION OF C10H8+ FROM THE BENZENE RADICAL CATION - A CASE FOR THE GROWTH OF POLYCYCLIC AROMATIC HYDROCARBON IONS BY ION MOLECULE REACTIONS IN THE GAS-PHASE

Experimental results are reported which show that diacetylene, but not acetylene, adds rapidly to C6H6+ in the gas phase in helium at 0.35 Torr and 296 +/- 2 K when C6H6+ is produced by the chemical ionisation of benzene with NO+. The experiments were performed with the selected-ion flow tube and Fourier transform ion cyclotron resonance techniques. Chemical reactivity and ion photodissociation methods were employed to investigate the identity of the C10H8+ ion produced by the addition reaction. Results were obtained for the reactions of this cation with deuterium, acetylene, diacetylene, styrene, trimethylamine, nitric oxide, 1,2,4,5-tetramethylbenzene and p-methylaniline, and were compared with the results of the reactions of these same molecules with the C10H8+ radical cation produced directly from naphthalene by chemical ionisation with Si+ and electron ionisation. Laser photodissociation experiments at 355 nm were also carried out on the C10H8+ ion produced from the addition reaction, and by electron ionisation of naphthalene and azulene. The chemical and photophysical behavior of the former two ions was found to be identical which suggests that ionised naphthalene is formed in the addition reaction of ionised benzene and diacetylene. The implications of this result for the growth of polycyclic aromatic hydrocarbons in hydrocarbon flames and in interstellar regions are briefly discussed.