Department of Chemistry and Biochemistry, Florida International University, Florida 33199, USA.
Phys Chem Chem Phys. 2010 Mar 20;12(11):2606-18. doi: 10.1039/b920977h. Epub 2010 Jan 27.
Ab initio CCSD(T)/cc-pVTZ//B3LYP/6-311G** calculations of the C(5)H(5) potential energy surface have been performed to investigate the reaction mechanism of ethynyl radical (C(2)H) with C(3)H(4) 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 C(2)H + CH(2)CCH(2) 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 HCCCH(2)CCH(2) adduct leading to either 1,4-pentadiyne or ethynylallene, and a multistep route HCCC(CH(2))(2) --> four-member ring --> CH(2)CCCHCH(2) --> CH(2)CCCCH(2) + H featuring a formal insertion of C(2)H into a double bond of allene followed by H elimination giving rise to pentatetraene. On the contrary, the C(2)H + CH(3)CCH reaction produces diacetylene + methyl (21-61%) by CH(3) loss from the HCCC(CH)CH(3) initial adduct as well as methyldiacetylene + H (27-56%) and ethynylallene + H (11-22%) by H eliminations from CHCCHCCH(3). 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 C(2)H + C(3)H(4) reactions proceed without entrance barriers and lead, via intermediates and transition states residing lower in energy than the initial reactants, to the C(5)H(4) + H and C(4)H(2) + CH(3) products exothermic by 20-36 kcal mol(-1), with strong dependence of the product distribution on the reacting C(3)H(4) 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.
已进行了从头算 CCSD(T)/cc-pVTZ//B3LYP/6-311G** 计算，以研究乙炔基自由基 (C(2)H) 与 C(3)H(4) 异构体丙二烯和甲基乙炔的反应机理。然后在单分子碰撞条件下，通过 RRKM 计算了反应速率常数和产物分支比。结果表明，在统计行为的情况下，预计 C(2)H + CH(2)CCH(2) 反应将产生 1,4-戊二炔（56-63%）、乙炔基丙二烯（22-24%）和戊四烯（10-15%），最有利的途径包括初始 HCCCH(2)CCH(2) 加合物中 H 的损失，导致 1,4-戊二炔或乙炔基丙二烯，以及多步途径 HCCC(CH(2))(2) --> 四元环 --> CH(2)CCCHCH(2) --> CH(2)CCCCH(2) + H，其特征是 C(2)H 插入丙二烯的双键，随后消除 H 生成戊四烯。相反，C(2)H + CH(3)CCH 反应通过初始 HCC(CH)CH(3) 加合物中 CH(3) 的损失产生二乙炔+甲基（21-61%），以及通过 CHCCHCCH(3) 中 H 的消除生成甲基二乙炔+H（27-56%）和乙炔基丙二烯+H（11-22%）。计算的产物分支比与现有实验数据基本一致，尽管存在一些与实验的定量偏差及其可能的原因也进行了讨论。本计算证实，C(2)H + C(3)H(4) 反应无需入口势垒，通过中间体和过渡态进行，这些中间体和过渡态的能量低于初始反应物，生成放热 20-36 kcal mol(-1)的 C(5)H(4) + H 和 C(4)H(2) + CH(3)产物，产物分布强烈依赖于反应的 C(3)H(4)异构体，使这些反应在泰坦大气的低温条件下快速进行，在那里它们可以作为更复杂的不饱和烃的来源。
