Unimolecular dissociation of the propargyl radical intermediate of the $CH+C2H2$ and $C+C2H3$ reactions

This paper examines the unimolecular dissociation of propargyl $(HCCCH2)$ radicals over a range of internal energies to probe the $CH+HCCH$ and $C+C2H3$ bimolecular reactions from the radical intermediate to products. The propargyl radical was produced by $157nm$ photolysis of propargyl chloride in crossed laser-molecular beam scattering experiments. The H-loss and $H2$ elimination channels of the nascent propargyl radicals were observed. Detection of stable propargyl radicals gave an experimental determination of 71.5 $(+5∕−10)kcal∕mol$ as the lowest barrier to dissociation of the radical. This barrier is significantly lower than predictions for the lowest barrier to the radical’s dissociation and also lower than calculated overall reaction enthalpies. Products from both $H2+HCCC$ and $H+C3H2$ channels were detected at energies lower than what has been theoretically predicted. An HCl elimination channel and a minor C–H fission channel were also observed in the photolysis of propargyl chloride.