Crossed beam reactions of methylidyne [CH(X2)] with D2-acetylene [C2D2(X1g+)] and of D1-methylidyne [CD(X2)] with acetylene [C2H2(X1g+)]

The crossed molecular beam reactions of ground state methylidyne, CH(X 2 ), with D2-acetylene, C 2 D 2 (X 1 g + ), and of D1-methylidyne, CD(X 2 ), with acetylene, C 2 H 2 (X 1 g + ), were conducted under single collision conditions at a collision energy of 17 kJ mol 1 . Four competing reaction channels were identified in each system following atomic hydrogen (H/D) and molecular hydrogen (H 2 /D 2 /HD) losses. The reaction dynamics were found to be indirect via complex formation and were initiated by two barrierless-addition pathways of methylidyne/D1-methylidyne to one and to both carbon atoms of the D2-acetylene/acetylene reactant yielding HCCDCD/DCCHCH and c-C 3 D 2 H/c-C 3 H 2 D collision complexes, respectively. The latter decomposed via atomic hydrogen/deuterium ejection to form the thermodynamically most stable cyclopropenylidene species (c-C 3 H 2 , c-C 3 D 2 , c-C 3 DH). On the other hand, the HCCDCD/DCCHCH adducts underwent hydrogen/deuterium shifts to form the propargyl radicals (HDCCCD, D 2 CCCH; HDCCCH, H 2 CCCD) followed by molecular hydrogen losses within the rotational plane of the decomposing complex yielding l-C 3 H/l-C 3 D. Quantitatively, our crossed beam studies suggest a dominating atomic compared to molecular hydrogen loss with fractions of 81 23% vs. 19 10% for the CD/C 2 H 2 and 87 30% vs. 13 4% for the CH/C 2 D 2 systems. The role of these reactions in the formation of interstellar isomers of C 3 H 2 and C 3 H is also discussed. H, D, H 2 , D 2 or HD? The competing reaction pathways.