A new method for investigating infrared spectra of protonated benzene (C ₆H ₇) and cyclohexadienyl radical (c-C ₆H ₇) using para-hydrogen

We use protonated benzene (C ₆H ₇) and cyclohexadienyl radical (c-C ₆H ₇) to demonstrate a new method that has some advantages over other methods currently used. C ₆H ₇ and c-C ₆H ₇ were produced on electron bombardment of a mixture of benzene (C ₆H ₆) and para-hydrogen during deposition onto a target at 3.2 K. Infrared (IR) absorption lines of C ₆H ₇ decreased in intensity when the matrix was irradiated at 365 nm or maintained in the dark for an extended period, whereas those of c-C ₆H ₇ increased in intensity. Observed vibrational wavenumbers, relative IR intensities, and deuterium isotopic shifts agree with those predicted theoretically. This method, providing a wide spectral coverage with narrow lines and accurate relative IR intensities, can be applied to larger protonated polyaromatic hydrocarbons and their neutral species which are difficult to study with other methods.