Mid-Infrared Spectroscopy of C7H7+ Isomers in the Gas Phase: Benzylium and Tropylium.

Both prominent C7H7+ isomers, the benzylium and the tropylium cations, were generated in an electrical discharge/supersonic expansion from toluene and cycloheptatriene precursors. Their infrared spectra were measured in the region of 1000-3500 cm-1 using photodissociation of the respective argon- and nitrogen-tagged complexes with a broadly tunable OPO/OPA laser system. Spectral signatures of both isomers were observed independent of the precursor, albeit in different relative intensities. The spectra were assigned based on scaled harmonic B3LYP-D3/cc-pVTZ frequency computations and comparisons to previous experimental studies. Consistent with its high symmetry, only two bands were observed for the (nitrogen-tagged) tropylium ion at 3036 and 1477 cm-1, corresponding to C-H stretching and C-C-H deformation/C═C stretching vibrations, respectively. Furthermore, the C-H stretching region of the benzylium ion is reported for the first time.