An ab initio/RRKM study of the reaction mechanism and product branching ratios of the reactions of ethynyl radical with allene and methylacetylene

Abstract

Ab initio CCSD(T)/cc-pVTZ//B3LYP/6-311G** calculations of the C₅H₅ potential energy surface have been performed to investigate the reaction mechanism of ethynyl radical (C₂H) with C₃H₄ isomers, allene and methylacetylene. They were followed by RRKM calculations of reaction rate constants and product branching ratios under single-collision conditions. The results show that the C₂H + CH₂CCH₂ reaction in a case of statistical behavior is expected to produce 1,4-pentadiyne (56–63%), ethynylallene (22–24%), and pentatetraene (10–15%), with the most favorable pathways including H losses from the initial HCCCH₂CCH₂ adduct leading to either 1,4-pentadiyne or ethynylallene, and a multistep route HCCC(CH₂)₂ → four-member ring → CH₂CCCHCH₂ → CH₂CCCCH₂ + H featuring a formal insertion of C₂H into a double bond of allene followed by H elimination giving rise to pentatetraene. On the contrary, the C₂H + CH₃CCH reaction produces diacetylene + methyl (21–61%) by CH₃ loss from the HCCC(CH)CH₃ initial adduct as well as methyldiacetylene + H (27–56%) and ethynylallene + H (11–22%) by H eliminations from CHCCHCCH₃. The calculated product branching ratios are in general agreement with the available experimental data, although some quantitative deviations from experiment and possible reasons for them are also discussed. The present calculations confirm that the C₂H + C₃H₄ reactions proceed without entrance barriers and lead, via intermediates and transition states residing lower in energy than the initial reactants, to the C₅H₄ + H and C₄H₂ + CH₃ products exothermic by 20–36 kcal mol⁻¹, with strong dependence of the product distribution on the reacting C₃H₄ isomer, making these reactions fast under low-temperature conditions of Titan’s atmosphere where they can serve as a source of more complex unsaturated hydrocarbons.