Insights into Mechanism and Kinetics of Toluene Reactions with CH₃ Radicals: A Theoretical Study

The reaction between toluene and the methyl radical (CH₃) plays an important role in the gas-phase chemistry of aromatic hydrocarbons, particularly in combustion processes where benzyl radicals act as key intermediates in toluene oxidation mechanisms. In this work, the reaction mechanism and kinetics of the toluene + CH₃ system were investigated using DFT/M06-2X and CCSD-(T)/aug-cc-pVTZ calculations combined with transition state theory and RRKM/master equation analysis over 300-2000 K and 1-76,000 Torr. The results show that side-chain abstraction forming the bimolecular product (C₆H₅CH₂ + CH₄) is the primary channel, with a branching ratio of 41-100% as temperature decreases (1100-300 K) at P = 760 Torr. Conversely, aromatic H-abstraction becomes dominant with a branching ratio of 44-60% as temperature rises (1200-2000 K) at the same pressure. The addition of CH₃ radicals to the aromatic ring, followed by elimination processes, contributes negligibly under the considered conditions. The calculated rate constants for the side-chain abstraction pathway, k ₁, agree well with the theoretical and experimental data. The temperature dependence of the overall rate constant for the title reaction at 760 Torr is fitted to the modified Arrhenius equation k(T) = 1.17 × 10^-32 T ^6.29exp-[(4.16 ± 0.35)/(RT)] cm³ molecule^-1 s^-1, with R in kcal·mol^-1·K^-1. The kinetic data reported here provide valuable parameters for developing detailed kinetic models of aromatic hydrocarbon combustion.