BRANCHING RATIOS IN O(P-3) REACTIONS OF TERMINAL OLEFINS STUDIED BY KINETIC MICROWAVE-ABSORPTION SPECTROSCOPY

O(3P) atom reactions with ethylene, propylene, and 1-butene were studied by use of kinetic microwave absorption spectroscopy where the atomic oxygen was supplied via ArF excimer laser photolysis of SO2. Time evolutions of vinoxy, HCO, and H2CO were pursued. The fraction of vinoxy production in the propylene and 1-butene reactions at 30 mTorr was found to be 0.29 +/- 0.15 and 0.37 +/- 0.15, respectively. The fractions seemed to increase with the decrease in pressure, by comparing the present results with those of several previous researchers. Therefore, at least some part of the vinoxy production fraction is pressure dependent. The reaction scheme explaining the pressure-dependent part is suggested as follows. Initially, a triplet biradical is produced through O(3P) attack to the terminal carbon atom of the C=C double bond, which is then converted to a singlet biradical. Subsequently, a hydrogen atom migrates and then the C-C bond adjacent to the original C=C double bond dissociates to yield the vinoxy and corresponding alkyl radicals. Thus, some part of the vinoxy radical is produced via a quite different mechanism from the case of ethylene reaction, where it is produced via a direct substitution channel on a triplet surface.