Infrared Spectroscopy of Gas Phase Benzenium Ions: Protonated Benzene and Protonated Toluene, from 750 to 3400 cm^–1

Gas phase C₆H₇⁺ and C₇H₉⁺ ions are studied with infrared photodissociation spectroscopy (IRPD) and the method of rare gas tagging. The ions are produced in a pulsed electric discharge supersonic expansion source from benzene or toluene precursors. We observe exclusively the formation of either the C_2v benzenium ion (protonated benzene) or the para isomer of the toluenium ion (protonated toluene). The infrared spectral signatures associated with each ion are established between 750 and 3400 cm⁻¹. Comparing the gas phase spectrum of the benzenium ion to the spectrum obtained in a superacid matrix [Perkampus, H. H.; Baumgarten, E. Angew. Chem. Int. Ed. 1964, 3, 776], we find that the C_2v structure of the gas phase species is minimally affected by the matrix environment. An intense band near 1610 cm⁻¹ is observed for both ions and is indicative of the allylic π-electron density associated with the six membered ring in these systems. This spectral signature, also observed for alkyl substituted benzenium ions and protonated naphthalene, compares favorably with the interstellar, unidentified infrared emission band near 6.2 µm (1613 cm⁻¹).