ABSORPTION-SPECTRA OF ETHYNYL, ETHENYL, AND PHENYL PEROXYL RADICALS

Both the ethenyl and phenyl peroxyl radical absorb in the visible regi on in aqueous solution. Since the absorption spectra of alkyl peroxyl radicals is invariably in the ultraviolet, these observations were ini tially 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 electro nic transitions. The electronic excited states of these radicals are l ow in energy because the pi-type open-shell orbital localized on the o xygen atoms in the ground state couples to a relatively low-energy pi 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 bo nd pi orbital is relatively high in energy, and the in vacuo predictio n for the absorption is far to the blue of the transition observed in solution. Large spectral red shifts are predicted, however, because al l the radicals are polar in the ground state, and the dipole moment, i n 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.