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.
Khachatryan
Lavrent
L
Department of Chemistry, Louisiana State University, Baton Rouge, Louisiana 70803, USA.
Adounkpe
Julien
J
Asatryan
Rubik
R
Dellinger
Barry
B
D016227:Benzoquinones / Q000737:chemistry*; D002396:Catechols / Q000737:chemistry*; D003198:Computer Simulation; D004578:Electron Spin Resonance Spectroscopy; D005740:Gases / Q000737:chemistry*; D008872:Microwaves; D015394:Molecular Structure; D011789:Quantum Theory; D013237:Stereoisomerism; D013696:Temperature
D016227:Benzoquinones; D002396:Catechols; D005740:Gases; C025232:semiquinone radicals