A new method for investigating infrared spectra of protonated benzene (C6H7+) and cyclohexadienyl radical (c-C6H7) using para-hydrogen

Abstract

We use protonated benzene (C6H7+) and cyclohexadienyl radical (c-C6H7) to demonstrate a new method that has some advantages over other methods currently used. C6H7+ and c-C6H7 were produced on electron bombardment of a mixture of benzene (C6H6) and para-hydrogen during deposition onto a target at 3.2 K. Infrared (IR) absorption lines of C6H7+ decreased in intensity when the matrix was irradiated at 365 nm or maintained in the dark for an extended period, whereas those of c-C6H7 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. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.3703502]