Absorption spectra of ethynyl, ethenyl, and phenyl peroxyl radicals

M. Krauss, R. Osman

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Abstract

Both the ethenyl and phenyl peroxyl radical absorb in the visible region in aqueous solution. Since the absorption spectra of alkyl peroxyl radicals is invariably in the ultraviolet, these observations were initially surprising. Ab initio determination of the electronic structure of the ground and excited states of the ethynyl, ethenyl, and phenyl peroxyl radicals provides a fundamental understanding of these electronic transitions. The electronic excited states of these radicals are low in energy because the π-type open-shell orbital localized on the oxygen atoms in the ground state couples to a relatively low-energy π or conjugated orbital system in the excited state. In the case of both the ethynyl and phenyl peroxyl radicals, the excitation of the C-C bond π orbital is relatively high in energy, and the in vacuo prediction for the absorption is far to the blue of the transition observed in solution. Large spectral red shifts are predicted, however, because all the radicals are polar in the ground state, and the dipole moment in the relevant excited state is substantially larger. In addition, for the ethenyl peroxyl radical there are two isomeric forms whose ground states are close in energy, and the observed spectrum can be assigned to the convoluted spectra of both isomers.

Original languageEnglish
Pages (from-to)11387-11391
Number of pages5
JournalJournal of Physical Chemistry
Volume99
Issue number29
DOIs
StatePublished - 1995
Externally publishedYes

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