Infrared detection of the 1,3-dimethyl cyclopentadienyl cation, an isomer of protonated toluene

Protonated toluene (C7H9+) is a well-known stable carbocation, usually formed by sigma-protonation of the parent molecule. Here, the corresponding C7H9+ ion is formed with a pulsed discharge in a supersonic expansion from a norbornene precursor. The different molecular framework of this precursor makes it possible to access different structural configurations. Infrared photodissociation spectroscopy is employed to characterize the C7H9+ ions produced. Protonated toluene is most abundant, however, new bands in the fingerprint and C-H stretching regions indicate that another isomer is also present. Computational chemistry makes it possible to identify the 1,3-dimethylcyclopentadienyl cation, a less stable isomer not detected previously. (C) 2014 Elsevier B.V. All rights reserved.