Decomposition and ring expansion in methylcyclopentadiene: Single-pulse shock tube and modeling study

The thermal reactions of methylcyclopentadiene diluted in argon were studied behind reflected shock waves in a 2 in. i.d. pressurized driver single-pulse shock tube over the temperature range 1070-1270 K and overall densities of∼3×10^-5 mol/cm3. A plethora of products resulting from decompositions and ring expansion were found in the post-shock samples. They were, in order of decreasing abundance, cyclopentadiene, benzene, methane, ethane, naphthalene, acetylene, ethylene, C₄H₄, toluene, C₃H₄, and indene. Very minute yields of some other compounds were also observed. Production of all the products involves free radical reactions. The initiation of the free radical mechanisms in the decomposition of methylcyclopentadiene takes place via ejection of hydrogen atoms from SP³ carbons and dissociation of the methyl group attached to the ring. The H atoms and the methyl radicals initiate free radical reactions by abstraction of hydrogen atoms from SP³ carbons and by dissociative recombination of H atom and removal of a methyl group from the ring. In addition to these dissociations, there are several reactions that involve b-cleavage in the five-membered ring radical intermediates to produce open chain intermediates that then decompose to stable and unstable fragments. The ring expansion process that leads to the production of high yield of benzene takes place mainly from the methylene cyclopentadienyl intermediate. Ring expansion from methylcyclopentadiene itself does not take place. The total decomposition of methylcyclopentadiene in terms of a first-order rate constant is given by k_total = 10^11.31 exp(-46.6×10³/RT) s^-1. A reaction scheme containing 40 species and 105 elementary reactions was composed and computer simulation was performed over the temperature range 1050-1300 K at 25 K intervals. The agreement between the experimental results and the model prediction for most of the products is satisfactory.