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

• Infrared spectroscopy is reported for the C7H9+ cation produced in a discharge of norbornene and hydrogen.
• Computational studies reveal 11 isomers of C7H9+ and their relative energies; p-protonated toluene is the global minimum structure.
• In addition to p-protonated toluene, the infrared spectrum of the 1,3-dimethyl cyclopentadienyl cation is identified for the first time. This structure is significantly less stable than protonated toluene, but is obtained by kinetic trapping in the cold supersonic beam conditions.

Protonated toluene (C7H9+) is a well-known stable carbocation, usually formed by σ-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 CH 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.

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