Thermochemical properties and decomposition pathways of three isomeric semiquinone radicals

Mohammednoor Altarawneh, Bogdan Z. Dlugogorski, Eric M. Kennedy, John C. Mackie

Research output: Contribution to journalArticlepeer-review

33 Citations (Scopus)


Semiquinones are persistent free environmental radicals formed as important initial products from the decomposition of dihydroxylated benzene isomers. This study develops detailed decomposition pathways for the thermal decomposition of the three isomeric semiquinone radicals. Branching ratios based on the calculated high-pressure limit reaction rate constants predict that p-benzoquinone is a major product from the unimolecular decomposition of the p-semiquinone radical, while the formation of o-benzoquinone from the o-semiquinone radical corresponds to a minor channel. This finding is consistent with the absence of o-benzoquinone from the thermal degradation of the 1,2-dihydroxybenzene isomer and the abundance of p-benzoquinone from the thermal decomposition of 1,4-dihydroxybenzene. Ring contraction/CO elimination is shown to be the dominant sink pathway for the o-semiquinone and m-semiquinone radicals. Thermochemical properties, in terms of enthalpies of formation, entropies, and heat capacities for dihydroxylated benzene isomers, semiquinone radicals, and benzoquinones, are evaluated by quantum chemical calculations. Values of the enthalpies of formation calculated by the B3LYP/GTLarge method show good agreement with those obtained at the G3B3 level of theory.

Original languageEnglish
Pages (from-to)1098-1108
Number of pages11
JournalJournal of Physical Chemistry A
Issue number2
Publication statusPublished - Jan 21 2010
Externally publishedYes

ASJC Scopus subject areas

  • Physical and Theoretical Chemistry


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