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

Abstract

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.

Gas Chromatography Mass Spectrometry Identification of Labile Radicals Formed during Pyrolysis of Catechool, Hydroquinone, and Phenol through Neutral Pyrolysis Product Mass Analysis

“…Fluorene detection as the pyrolysis products of CAT is a confirmation that CPD radicals are formed during the pyrolysis/photolysis of CAT as demonstrated in previous studies [39,40,42].…”

“…Therefore OHCPD is formed but was not persistent enough to acquire its EPR spectrum with the various techniques we used in the previous studies [39,40,42].…”

“…Further reaction of phenoxy proceeds by CO elimination to form CPD [39]. In our gasphase phenol thermal degradation study, CPD and phenoxy radicals are, respectively, identified as pyrolysis products and their EPR gas-phase spectra are acquired [39][40][41][42].…”

“…From hydroquinone, a computer-generated hydroxycyclopentadienyl (HO-CPD) radical EPR spectrum, a fiveline spectrum with intensity distribution 1 : 4 : 6 : 4 : 1 was compared with the EPR spectra from the pyrolysis of HQ in all temperature regions (400-1000 ∘ C) [42]. There was no match.…”

Formation of dibenzofuran, dibenzo-p-dioxin and their hydroxylated derivatives from catechol

“…3 Dellinger's group has thoroughly investigated thermal decomposition of CT reporting that both pyrolytic and oxidative decomposition reactions of CT result in the formation of dibenzo-p-dioxin (DD) and dibenzofuran (DF). 2,[5][6][7][8] In one of the contributions of the group, Khachatryan et al, 5 using the technique of low-temperature matrix isolation electron paramagnetic resonance spectroscopy (LTMI-EPR), detected the formation of an o-semiquinone (o-SQ) radical as the most prominent initial intermediate from decomposition of the CT molecule, via fission of one of its phenolic O-H bonds (Scheme 1).…”