An ab initio/RRKM study of the reaction mechanism and product branching ratios of the reactions of ethynyl radical with 1,2-butadiene

Ab initio/RRKM calculations have been performed to investigate the mechanism of the C2H + 1,2-butadiene reaction and to compute its product branching ratios under single-collision conditions. The reaction starts with barrierless C2H addition to various sites of H2CCCHCH3 producing different exothermic initial adducts. The chemically activated C6H7 adducts can then isomerize and decompose by splitting H or CH3. With the assumption of equal formation probabilities of all initial adducts, C6H6 (2-ethynyl-1,3-butadiene) + H are predicted as the dominant products (91–84% at collision energies of 0–7 kcal/mol), whereas C5H4 (penta-1,4-diyne) + CH3 (7–12%) and ethynylallene + CH3 (2–3%) are minor products.