GAS-PHASE ABSORPTION-SPECTRUM AND CROSS-SECTIONS OF VINYLPEROXY (C2H3O2) RADICAL

The absorption spectrum and cross sections of vinylperoxy (C2H3O2, eth enylperoxy) radical have been determined, for the first time in the ga s phase, in the spectral range 220-550 nm. The spectrum exhibits a rel atively broad and intense absorption centered at about 232 nm, followe d by at least two identifiable and weaker absorptions with maxima at a bout 340 and 420 nm. The absorption tail persists at longer wavelength s into the visible region, To discern competition between the stabiliz ation of the vinylperoxy isomers and reaction, the effect of pressure on the absorption has been examined, Vinylperoxy radicals in these exp eriments were produced through photochemical production of vinyl radic als followed by reaction of vinyl radicals with molecular oxygen. Viny l bromide (C2H3Br) photolyzed at 193 nm was used as the precursor of v inyl radicals, and methyl vinyl ketone (CH3COC2H3) photolyzed at 193 n m was used as a precursor of both methyl and vinyl radicals. In the la tter system identical concentrations of methyl and vinyl peroxy radica ls were produced, and by employing comparative methods and using the l iterature values for methylperoxy absorption cross sections, the absol ute absorption cross sections for vinylperoxy were determined. Ab init io molecular orbital calculations of CH3O2, C2H3O2. C2H3O, and HCO hav e 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 assi gned 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 invol ved here, accurate geometries are difficult to calculate, but the geom etry obtained by gradient optimization using the multiconfiguration se lf-consistent-field method yields excitation energies that most closel y agree with experiment. The relative theoretical oscillator strengths of all relevant vinylperoxy and vinery transitions have been evaluate d and assist the analysis of the pressure dependence of the absorption spectrum.