Radicals from the Gas-Phase Pyrolysis of Catechol: 1. o-Semiquinone and ipso-Catechol Radicals

The formation of environmentally persistent free radicals (EPFRs) from the gas-phase pyrolysis of catechol (CT) was studied over a temperature range of 400-750 degrees C using the technique of low-temperature matrix isolation electron paramagnetic resonance (LTMI-EPR) spectroscopy. A split singlet EPR spectrum with a g value of 2.0052 was observed. To aid in the interpretation of this spectrum, a detailed analysis of the potential energy surface of CT decomposition pathways was performed employing CBS-QB3 multilevel and DFT individual methods. The energetically favored channels were the formation of ipso- and alpha-CT isomers of CT as well as o-semiquinone (o-SQ) radicals from ipso- and alpha-CT. ipso-CT as well as open ipso-CT radicals were apparently unstable intermediates that could not be accumulated in sufficiently high concentrations to be detected by EPR. The calculated EPR spectrum of o-SQ radical is consistent with the formation of an oxygen-centered radical with a high g value observed experimentally. The hyperfine splitting of the observed spectrum can be attributed to the formation of hydrogen bonds in radical dimers. The lack of experimental observation of o-benzoquinone, an anticipated pyrolysis product, has been explained based on theoretical calculations.