UV absorption spectra of methyl-substituted hydroxy-cyclohexadienyl radicals in the gas phase

David Johnson, Séverine Raoult, Robert Lesclaux, Lev N. Krasnoperov

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UV absorption spectra of five methyl-substituted hydroxy-cyclohexadienyl radicals, formed by the addition of the hydroxyl radical (OH) to toluene (methyl benzene), o-, m- and p-xylene (1,2-, 1,3- and 1,4-dimethyl benzene, respectively) and mesitylene (1,3,5-trimethylbenzene), have been determined at 298 K, 1 atm pressure (N2 + O2), and the corresponding absolute absorption cross-sections measured, using laser flash photolysis and time-resolved UV absorption detection. As observed for other cyclohexadienyl-type radicals, a strong absorption band is present in the 260-340 nm spectral region, with maximum cross-sections in the range (0.9-2.2) × 10-17 cm2 molecule-1. The shape of the band varies significantly from one radical to the next for the series of aromatic precursors investigated. The nature and yields of hydroxylated ring-retaining oxidation products, identified in previous studies of the OH-initiated oxidation of aromatic hydrocarbons, and the results of theoretical density functional theory (DFT) calculations indicate that one or more possible isomers of the various OH-adducts may contribute to the observed spectra. Isomers where the OH-group is ortho- (or both ortho- and ipso-) to a substituent methyl-group are likely to be the most abundant but other isomers may also be formed to a significant extent. Nonetheless, the present study provides absorption spectra of the adduct radicals formed from the gas phase addition of OH to the aromatic hydrocarbons considered, near room temperature and 1 atm pressure.

Original languageEnglish (US)
Pages (from-to)98-106
Number of pages9
JournalJournal of Photochemistry and Photobiology A: Chemistry
Issue number1-3 SPEC. ISS.
StatePublished - Dec 14 2005

All Science Journal Classification (ASJC) codes

  • Chemistry(all)
  • Chemical Engineering(all)
  • Physics and Astronomy(all)


  • Hydroxy-cyclohexadienyl radicals
  • UV absorption spectra


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