Isomerization and decomposition of chloromethylacetylene

The isomerization and thermal decomposition of chloromethylacetylene (CMA) has been studied with two shock tube techniques. The first experiment (Jerusalem) utilizes single-pulse shock tube methods to measure the isomerization rate of CMA to chloroallene. In addition, equilibrium constants can be estimated at similar to 1200 K. The second experiment (Argonne) monitors Cl-atom formation at temperatures above similar to 1150 K. Absolute yield measurements have been performed over the 1200-1700 K range and indicate that two decomposition channels contribute to CMA destruction, namely, Cl fission and HCl elimination. The results show that the branching fraction between processes is temperature dependent. Therefore, direct Cl-atom fission is accompanied by molecular elimination, undoubtedly giving HCl and one or more isomers of C3H2. MP2 6-31G(d,p) ab initio electronic structure calculations have been used to determine vibration frequencies and moments of inertia for three C3H3Cl isomers. Using these quantities, the experimental equilibrium constants required that Delta H-0(0)(CH2Cl-C=CH reversible arrow CHCl=C=CH2) = -0.24 kcal mole(-1). A potential energy. surface pertinent to the present system has been constructed, and RRKM calculations have been carried out in order to explain the isomerization rates. The isomerization data can be explained with E(0) = 52.3 kcal mole(-1) and [Delta E(down)] = 225 cm(-1). Subsequent semi-empirical Tree and RRKM-Gorin modeling of the Cl atom rate data require E(0) = (67.5 +/- 0.5) kcal mole(-1) with a [Delta E(down)] = (365 +/- 90) cm(-1). This suggests a heat of formation for propargyl radicals of (79.0 +/- 2.5) kcal mole(-1).