Decomposition, Isomerization, and Ring Expansion in 2-Methylindene: Single-pulse Shock Tube and Modeling Study

The thermal reactions of 2-methylindene 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 1050-1300 K and overall densities of ∼3 × 10^-5 mol/cm³. A plethora of products resulting from decompositions, isomerizations, and ring expansion were found in the post shock samples. They were naphthalene as the product of five-membered ring expansion, 1- and 3-methylindene due to isomerizations, and CH₄, C₂H₄ C₂H₆, C₂H ₂, benzene, toluene, ethylbenzene, styrene, phenylacetylene, and indene as the result of fragmentation. Very minute yields of some other compounds were also observed. Except for the isomerizations that take place from the reactant as a starting material, the production of all the other products involve free radical reactions. The initiation of the free radical mechanisms in the decomposition of 2-methylindene 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 a free radical mechanism 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 dissociation reactions there are several breakdown processes that involve cleavage of the five-membered ring to produce both stable and unstable products. The ring expansion process that leads to the production of a high yield of naphthalene takes place only from a methylene indene radical intermediate rather than methylindene itself, whereas isomerizations take place from both the radical intermediates and the molecule. The total decomposition of 2-methylindene in terms of a first-order rate constant is given by: k _total = 10^13.69exp (-59.4 × 10³/RT) s^-1. A reaction scheme containing 34 species and 71 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 species is satisfactory.