Decomposition of Methylbenzyl Radicals in the Pyrolysis and Oxidation of Xylenes

Alkyl benzyl radicals are important initial products in thermal and combustion reactions of substituted aromatic fuels. The decomposition reactions of the three isomeric methylbenzyl radicals, formed as primary products in xylene combustion, are studied theoretically and are shown to be significantly more complex than previously reported. Thermochemical properties are calculated using the G3X and G3SX model chemistries, with isodesmic and atomization work reactions. G3X atomization calculations reproduce heats of formation for the 14 reference species in the work reactions to a mean unsigned error of 0.23 kcal mol(-1), and maximum error of 0.70 kcal mol(-1), slightly outperforming the G3SX method. For the target molecules the isodesmic and atomization heats of formation agree to within 0.20 kcal mol(-1), on average. We posit that this study approaches the crossover point at which atomization calculations offer improved accuracy over isodesmic ones, for these closed-shell hydrocarbons. Our results suggest that m-xylylene is not the decomposition product of m-methylbenzyl, as was previously reported. Instead, the m-methylbenzyl radical decomposes to p-xylylene (and perhaps some of the less stable o-xylylene) via a ring-contraction/methylene-migration (RCMM) mechanism, with activation energy of around 70 kcal mol(-1). At higher temperatures m-methylbenzyl is predicted to also decompose to 2- and 3-methylfulvenallene + H, with activation energy of around 84 kcal mol(-1). The o-methylbenzyl radical is shown to primarily decompose to o-xylylene + H with bond dissociation energy of 67.3 kcal mol(-1), with fulvenallene + CH3 proposed as a minor product set. Finally, the p-methylbenzyl radical decomposes solely to p-xylylene + H with bond dissociation energy 61.5 kcal mol(-1). Rate expressions are estimated for all reported reactions, based on thermochemical kinetic considerations, with further modeling along with detailed experiments needed to better refine rate constants and branching ratios for methylbenzyl radical decomposition. These calculated reaction mechanisms and rate constants for methylbenzyl radical decomposition are consistent with the experimental data. Our results help explain the ignition behavior of the xylenes, and should lead to improved kinetic models for combustion of these and other alkylated aromatic hydrocarbons.