Mebel A M, Kislov V V
Department of Chemistry and Biochemistry, Florida International University, Miami, Florida 33199, USA.
J Phys Chem A. 2009 Sep 10;113(36):9825-33. doi: 10.1021/jp905931j.
Ab initio and density functional calculations using a variety of theoretical methods (CASSCF, B3LYP, CASPT2, CCSD(T), and G3(MP2,CC)) have been carried out to unravel the mechanism of unimolecular isomerization and dissociation of 9,10-dihydrofulvalene C(10)H(10) (S0) formed by barrierless recombination of two cyclopentadienyl radicals. Different reaction pathways on the C(10)H(10) potential energy surface (PES) are found to lead to the production of 9-H-fulvalenyl radical + H, 9-H-naphthyl radical (a naphthalene precursor) + H, and naphthalene + H(2). RRKM calculations of thermal rate constants and product branching ratios at the high pressure limit show that at temperatures relevant to combustion the 9-H-fulvalenyl radical formed by a direct H loss from S0 with endothermicity of 76.3 kcal/mol is expected to be the dominant reaction product. The naphthalene precursor 9,10-dihydronaphthalene (D3) can be produced from the initial S0 adduct by a multistep diradical mechanism involving the formation of a metastable tricyclic diradical intermediate, followed by its three-step opening to a 10-member ring structure, which then undergoes ring contraction producing the naphthalene core structure in D3, with the highest barrier on this pathway being 70.3 kcal/mol. D3 can lose molecular hydrogen producing naphthalene via a barrier of 77.7 kcal/mol relative to the initial adduct. Another possibility is a hydrogen atom elimination in D3 giving rise to the 9-H-naphthyl radical without exit barrier and with overall endothermicity of 59.2 kcal/mol. The pathway to 9-H-naphthyl appears to be preferable as compared to the direct route to 9-H-fulvalenyl at temperatures below 600 K, but the rate constants at these temperatures are too slow for the reaction to be significant. The naphthalene + H(2) channel is not viable at any temperature. The following reaction sequence is suggested for kinetic models to account for the recombination of two cyclopentadienyl radicals: c-C5H5+c-C5H5-->9,10-dihydrofulvalene-->9-H-fulvalenyl+H(C10H10PES), 9-H-fulvalenyl-->naphthalene+H/fulvalene+H(C10H9PES). We conclude that naphthalene can be produced from the recombination of two cyclopentadienyl radicals and is expected to be a favorable product of this reaction sequence at T < 1000 K, but this molecule would be formed through isomerizations and H atom loss on the C(10)H(9) PES (after the initial H elimination from C(10)H(10) S0) and not in conjunction with molecular hydrogen. The alternative product, fulvalene, can potentially contribute to the growth of cyclopentafused polycyclic aromatic hydrocarbons.
已使用多种理论方法（CASSCF、B3LYP、CASPT2、CCSD(T)和G3(MP2,CC)）进行从头算和密度泛函计算，以阐明由两个环戊二烯基自由基无势垒重组形成的9,10 - 二氢富瓦烯C(10)H(10)（S0）的单分子异构化和解离机制。发现C(10)H(10)势能面（PES）上的不同反应途径会导致生成9 - H - 富瓦烯基自由基 + H、9 - H - 萘基自由基（萘的前体）+ H以及萘 + H₂。在高压极限下对热速率常数和产物分支比的RRKM计算表明，在与燃烧相关的温度下，由S0直接失去H形成的9 - H - 富瓦烯基自由基（吸热76.3 kcal/mol）预计将是主要反应产物。萘的前体9,10 - 二氢萘（D3）可通过多步双自由基机制从初始S0加合物生成，该机制涉及形成亚稳的三环双自由基中间体，随后其三步开环形成10元环结构，然后进行环收缩生成D3中的萘核心结构，此途径上的最高势垒为70.3 kcal/mol。D3可通过相对于初始加合物77.7 kcal/mol的势垒失去分子氢生成萘。另一种可能性是D3中氢原子消除生成9 - H - 萘基自由基，无出口势垒，总吸热为59.2 kcal/mol。与在600 K以下温度下生成9 - H - 富瓦烯基的直接途径相比，生成9 - H - 萘基的途径似乎更可取，但这些温度下的速率常数太慢，该反应不太显著。在任何温度下，萘 + H₂通道都不可行。建议动力学模型采用以下反应序列来解释两个环戊二烯基自由基的重组：c - C5H5 + c - C5H5→9,10 - 二氢富瓦烯→9 - H - 富瓦烯基 + H（C₁₀H₁₀ PES），9 - H - 富瓦烯基→萘 + H/富瓦烯 + H（C₁₀H₉ PES）。我们得出结论，萘可由两个环戊二烯基自由基重组产生，预计在T < 1000 K时是该反应序列的有利产物，但该分子将通过C(10)H(9) PES上的异构化和H原子损失形成（在从C(10)H(10) S0初始消除H之后），而不是与分子氢一起形成。另一种产物富瓦烯可能有助于环戊稠合多环芳烃的生长。
