Can the C₅H₅ + C₅H₅ → C₁₀H₁₀ → C₁₀H₉ + H/C₁₀H₈ + H₂ Reaction Produce Naphthalene? An Ab Initio/RRKM Study

Abstract

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 C₁₀H₁₀ (<b>S0</b>) formed by barrierless recombination of two cyclopentadienyl radicals. Different reaction pathways on the C₁₀H₁₀ 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 + H₂. 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 <b>S0</b> with endothermicity of 76.3 kcal/mol is expected to be the dominant reaction product. The naphthalene precursor 9,10-dihydronaphthalene (<b>D3</b>) can be produced from the initial <b>S0</b> 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 <b>D3</b>, with the highest barrier on this pathway being 70.3 kcal/mol. <b>D3</b> 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 <b>D3</b> 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 + H₂ 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:\nc-C5H5+c-C5H5→9,10-dihydrofulvalene→9-H-fulvalenyl+H(C10H10PES)\n9-H-fulvalenyl→naphthalene+H/fulvalene+H(C10H9PES)\nWe 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 <i>T</i> < 1000 K, but this molecule would be formed through isomerizations and H atom loss on the C₁₀H₉ PES (after the initial H elimination from C₁₀H₁₀ <b>S0</b>) and not in conjunction with molecular hydrogen. The alternative product, fulvalene, can potentially contribute to the growth of cyclopentafused polycyclic aromatic hydrocarbons.