A crossed molecular beam study on the reaction of methylidyne radicals [CH(X2Π)] with acetylene [C2H2(X1Σg+)]—competing C3H2 + H and C3H + H2 channels

Abstract

We carried out the crossed molecular beam reaction of ground state methylidyne radicals, CH(X²Π), with acetylene, C₂H₂(X¹Σ_g⁺), at a nominal collision energy of 16.8 kJ mol⁻¹. Under single collision conditions, we identified both the atomic and molecular hydrogen loss pathways forming C₃H₂ and C₃H isomers, respectively. A detailed analysis of the experimental data suggested the formation of c-C₃H₂ (31.5 ± 5.0%), HCCCH/H₂CCC (59.5 ± 5.0%), and l-HCCC (9.0 ± 2.0%). The reaction proceeded indirectly via complex formation and involved the unimolecular decomposition of long-lived propargyl radicals to form l-HCCC plus molecular hydrogen and HCCCH/H₂CCC plus atomic hydrogen. The formation of c-C₃H₂ was suggested to be produced via unimolecular decomposition of the cyclopropenyl radical, which in turn could be accessed via addition of the methylidyne radical to both carbon atoms of the acetylene molecule or after an initial addition to only one acetylenic carbon atom via ring closure. This investigation brings us closer to unraveling of the reaction of important combustion radicals—methylidyne—and the connected unimolecular decomposition of chemically activated propargyl radicals. This also links to the formation of C₃H and C₃H₂ in combustion flames and in the interstellar medium.