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The high temperature decomposition kinetics of toluene and benzyl were investigated by combining a kinetic analysis with the ab initio/master equation study of new reaction channels. It was found that similarly to toluene, which decomposes to benzyl and phenyl losing atomic hydrogen and methyl, also benzyl decomposition proceeds through two channels with similar products. The first leads to the formation of fulvenallene and hydrogen and has already been investigated in detail in recent publications. In this work it is proposed that benzyl can decompose also through a second decomposition channel to form benzyne and methyl. The channel specific kinetic constants of benzyl decomposition were determined by integrating the RRKM/master equation over the C(7)H(7) potential energy surface. The energies of wells and saddle points were determined at the CCSD(T) level on B3LYP/6-31+G(d,p) structures. A kinetic mechanism was then formulated, which comprises the benzyl and toluene decomposition reactions together with a recently proposed fulvenallene decomposition mechanism, the decomposition kinetics of the fulvenallenyl radical, and some reactions describing the secondary chemistry originated by the decomposition products. The kinetic mechanism so obtained was used to simulate the production of H atoms measured in a wide pressure and temperature range using different experimental setups. The calculated and experimental data are in good agreement. Kinetic constants of the new reaction channels here examined are reported as a function of temperature at different pressures. The mechanism here proposed is not compatible with the assumption often used in literature kinetic mechanisms that benzyl decomposition can be effectively described through a lumped reaction whose products are the cyclopentadienyl radical and acetylene.

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The cyclopentadienyl radical (cC(5)H(5)) is among the most stable radical species that can be generated during the combustion and pyrolysis of hydrocarbons and it is generally agreed that its contribution to the gas phase reactivity is significant. In this study the kinetics of one key cC(5)H(5) reaction channel, namely the reaction between cC(5)H(5) and cyclopentadiene (cC(5)H(6)), was investigated using ab initio calculations and RRKM/Master Equation theory. It was found that most of the excited C(5)H(5)_C(5)H(6) adducts formed by the addition of cC(5)H(5) to cC(5)H(6) decompose back to reactants and that the major reaction products are, in order of importance, indene, vinylfulvene (a most probable styrene precursor), phenylbutadiene, and benzene. The preferred reaction pathway of the C(5)H(5)_C(5)H(6) adduct is started by the migration of the tertiary hydrogen of the C(5)H(5) ring to a vicinal carbon and followed by the β-opening of the C(5)H(6) ring, which is the rate determining step. Successive molecular rearrangements lead to decomposition to the four possible products. The kinetic constants for the four reaction channels, calculated at atmospheric pressure and interpolated in cm(3) mol(-1) s(-1) between 900 and 2000 K, are k(indene) = 10(25.197)T(-3.935) exp(-11630/T(K)), k(vinylfulvene) = 10(65.077)T(-14.20) exp(-37567/T(K)), k(benzene) = 10(29.172)T(-4.515) exp(-20570/T(K)), and k(phenylbutadiene) = 10(16.743)T(-1.407) exp(-11804/T(K)). The predictive capability of the reaction set so determined was tested through the simulations of recent cC(5)H(6) pyrolysis and combustion experiments using a detailed kinetic mechanism. A quantitative agreement with experimental data was obtained by assuming that vinylfulvene converts rapidly to stryrene, increasing its reaction channel by a factor of 2, and assuming that phenylbutadiene rapidly decomposes with equal probability to styrene and benzene.