Mebel Alexander M, Kim Gap-Sue, Kislov Vadim V, Kaiser Ralf I
Department of Chemistry and Biochemistry, Florida International University, Miami, Florida 33199, USA.
J Phys Chem A. 2007 Jul 26;111(29):6704-12. doi: 10.1021/jp0690300. Epub 2007 Mar 29.
Ab initio calculations of the potential energy surface for the C3(1Sigmag+)+C2H2(1Sigmag+) reaction have been performed at the RCCSD(T)/cc-pVQZ//B3LYP/6-311G(d,p) + ZPE[B3LYP/6-311G(d,p)] level with extrapolation to the complete basis set limit for key intermediates and products. These calculations have been followed by statistical calculations of reaction rate constants and product branching ratios. The results show the reaction to begin with the formation of the 3-(didehydrovinylidene)cyclopropene intermediate i1 or five-member ring isomer i7 with the entrance barriers of 7.6 and 13.8 kcal/mol, respectively. i1 rearranges to the other C5H2 isomers, including ethynylpropadienylidene i2, singlet pentadiynylidene i3, pentatetraenylidene i4, ethynylcyclopropenylidene i5, and four- and five-member ring structures i6, i7, and i8 by ring-closure and ring-opening processes and hydrogen migrations. i2, i3, and i4 lose a hydrogen atom to produce the most stable linear isomer of C5H with the overall reaction endothermicity of approximately 24 kcal/mol. H elimination from i5 leads to the formation of the cyclic C5H isomer, HC2C3, +H, 27 kcal/ mol above C3+C2H2. 1,1-H2 loss from i4 results in the linear pentacarbon C5+H2 products endothermic by 4 kcal/mol. The H elimination pathways occur without exit barriers, whereas the H2 loss from i4 proceeds via a tight transition state 26.4 kcal/mol above the reactants. The characteristic energy threshold for the reaction under single collision conditions is predicted be in the range of approximately 24 kcal/mol. Product branching ratios obtained by solving kinetic equations with individual rate constants calculated using RRKM and VTST theories for collision energies between 25 and 35 kcal/mol show that l-C5H+H are the dominant reaction products, whereas HC2C3+H and l-C5+H2 are minor products with branching ratios not exceeding 2.5% and 0.7%, respectively. The ethynylcyclopropenylidene isomer i5 is calculated to be the most stable C5H2 species, more favorable than triplet pentadiynylidene i3t by approximately 2 kcal/mol.
在RCCSD(T)/cc-pVQZ//B3LYP/6-311G(d,p)+ZPE[B3LYP/6-311G(d,p)]水平下，对关键中间体和产物外推至完整基组极限，进行了C3(1Σg+)+C2H2(1Σg+)反应势能面的从头算计算。随后进行了反应速率常数和产物分支比的统计计算。结果表明，反应开始时形成3-(二脱氢亚乙烯基)环丙烯中间体i1或五元环异构体i7，其能垒分别为7.6和13.8 kcal/mol。i1通过闭环、开环过程和氢迁移重排为其他C5H2异构体，包括乙炔基丙二烯基i2、单线态戊二炔基i3、戊四烯基i4、乙炔基环丙烯基i5以及四元和五元环结构i6、i7和i8。i2、i3和i4失去一个氢原子生成最稳定的线性C5H异构体，总反应吸热约24 kcal/mol。i5消除氢导致形成环状C5H异构体HC2C3+H，比C3+C2H2高27 kcal/mol。i4失去1,1-H2生成线性五碳C5+H2产物，吸热4 kcal/mol。氢消除途径没有出口能垒，而i4失去H2通过一个比反应物高26.4 kcal/mol的紧密过渡态进行。预测单碰撞条件下反应的特征能量阈值在约24 kcal/mol范围内。通过使用RRKM和VTST理论计算25至35 kcal/mol碰撞能量下的各个速率常数来求解动力学方程得到的产物分支比表明，l-C5H+H是主要反应产物，而HC2C3+H和l-C5+H2是次要产物，分支比分别不超过2.5%和0.7%。计算得出乙炔基环丙烯基异构体i5是最稳定的C5H2物种，比三线态戊二炔基i3t约有利2 kcal/mol。
