Gas phase absorption spectrum and cross sections of vinylperoxy (C2H3O2) radical

The absorption spectrum and cross sections of vinylperoxy (C2H3O2, ethenylperoxy) radical have been determined, for the first time in the gas phase, in the spectral range 220-550 nm. The spectrum exhibits a relatively broad and intense absorption centered at about 232 nm, followed by at least two identifiable and weaker absorptions with maxima at about 340 and 420 nm. The absorption tail persists at longer wavelengths into the visible region, To discern competition between the stabilization of the vinylperoxy isomers and reaction, the effect of pressure on the absorption has been examined, Vinylperoxy radicals in these experiments were produced through photochemical production of vinyl radicals followed by reaction of vinyl radicals with molecular oxygen. Vinyl bromide (C2H3Br) photolyzed at 193 nm was used as the precursor of vinyl radicals, and methyl vinyl ketone (CH3COC2H3) photolyzed at 193 nm was used as a precursor of both methyl and vinyl radicals. In the latter system identical concentrations of methyl and vinyl peroxy radicals were produced, and by employing comparative methods and using the literature values for methylperoxy absorption cross sections, the absolute absorption cross sections for vinylperoxy were determined. Ab initio molecular orbital calculations of CH3O2, C2H3O2. C2H3O, and HCO have been employed to characterize the observed spectrum and to identify the species and assign transitions contributing to the spectrum, These calculations suggest that the observed spectrum can primarily be assigned to two stable isomers (conformations) of the vinylperoxy radical with the O-O bond in a cis or trans position relative to the C-C bond, with a minor contribution to the absorption spectrum from the vinery and formyl radicals. For unsaturated radicals and the weak bonds involved here, accurate geometries are difficult to calculate, but the geometry obtained by gradient optimization using the multiconfiguration self-consistent-field method yields excitation energies that most closely agree with experiment. The relative theoretical oscillator strengths of all relevant vinylperoxy and vinery transitions have been evaluated and assist the analysis of the pressure dependence of the absorption spectrum.