Propargyl from the reaction of singlet methylene with acetylene

The technique of infrared kinetic spectroscopy has been used to study the production of propargyl radical from the reaction of singlet methylene with acetylene. The rate constant for this product channel was determined to be (3.5 +/- 0.7) x 10(-10) cm(3) molecule(-1) s(-1) at 295 K, measured relative to the known rate for (CH2)-C-1 with H-2 or CH4. Methylene was produced in the singlet state by excimer laser photolysis of ketene at 308 nm in the presence of acetylene and either H-2 or CH4. Reaction of (CH2)-C-1 with acetylene produces propargyl, and reaction of (CH2)-C-1 with either H-2 or CH4 produces CH3. The intensity of a propargyl infrared absorption line was compared with that of a methyl infrared absorption line, and the rate of formation of propargyl was determined from the ratio of these two intensities and the known rate of reaction of singlet methylene with H-2 (or CH4) to produce CH3. The relative peak infrared absorption cross sections of methyl and propargyl were calibrated under the conditions of the experiment by photolyzing crotyl bromide at 193 nm to produce methyl and propargyl in equal concentrations.