Ab Initio MO Study of the Unimolecular Decomposition of the Phenyl Radical

The unimolecular decomposition of the C6H5 radical has been studied by ab initio molecular orbital and statistical-theory calculations. Three low-energy decomposition channels, including the commonly assumed decyclization/fragmentation process yielding n-C4H3 + C2H2, have been identified. With a modified Gaussian-2 method of Mebel et al. (ref 17), the energy barrier for the decyclization of C6H5 was calculated to be 66.5 kcal/mol with the corresponding recyclization energy of 5.6 kcal/mol. The two open-chain 1-dehydrohexa1,3-dien-5-yne radicals (with HCĊH cis and trans structures) may undergo further fragmentation reactions producing n-C4H3 + C2H2 and l-C6H4 (1,5-hexadiyn-3-ene) + H with the predicted barriers of 44.0 and 36.1 kcal/mol, respectively. The dominant decomposition channel of C6H5 was found to take place barrierlessly by C-H breaking, producing o-C6H4 (o-benzyne) + H with the predicted endothermicity of 76.0 kcal/mol. RRKM calculations have been carried out for the production of n-C4H3 + C2H2, l-C6H4 + H, and o-C6H4 + H with the coupled multichannel mechanism, which includes the reversible decyclization/recyclization reactions. The results of the calculations indicate that at T < 1500 K o-C6H4 is the major product of the decomposition reaction. Above 1500 K, the formation of l-C6H4 becomes competitive with its cyclic isomer. However, the formation of the commonly assumed n-C4H3 + C2H2 products was found to be least competitive. Rate constants for all three product channels from C6H5 as well as those from the bimolecular reaction of n-C4H3 with C2H2 producing C6H5, o-C6H4 + H, and l-C6H4 + H have been calculated as functions of temperature and pressure for practical applications.