Formation of cyclopentadienyl radical from the gas-phase pyrolysis of hydroquinone, catechol, and phenol

The formation of radicals from the gas-phase pyrolysis of hydroquinone, catechol, and phenol over a temperature range of 400-750 degrees C was studied using the technique of low-temperature matrix isolation electron paramagnetic resonance (LTMI EPR). Cooling the reactor effluent from pyrolysis in a nitrogen carrier gas to 77 K produces a cryogenic matrix that exhibits poorly resolved EPR spectra. However, using carbon dioxide as a carrier gas formed a matrix that, upon annealing by slowly raising the matrix temperature followed by rapid recooling to 77 K, yielded more resolved, identifiable spectra. Annealed spectra of all three samples resulted in the generation of EPR spectra above 700 C with 6 lines, hyperfine splitting constant similar to 6.0 G, and peak to peak width similar to 3 G that was readily assignable, based on comparison to the literature and theoretical calculations, as that of cyclopentadienyl radical. Pyrolysis at temperatures below 700 C generated a carbon dioxide matrix isolation spectrum with a high g-value (> 2.0040) that is attributed to oxygen-containing radicals such as semiquinone or phenoxyl. Conclusive identification of anticipated semiquinone, phenoxyl, and hydroxycyclopentadienyl radicals was complicated by the ability of these radicals to exist in carbon-centered and oxygen-centered resonance structures that can give different EPR spectra.