Hydrogen abstraction from toluene by OH, H, O, CH3, and HO2 radicals are important reactions in oxidation process of toluene. Geometries and corresponding harmonic frequencies of the reactants, transition states as well as products involved in these reactions are determined at the B3LYP/6-31G(2df,p) level. To achieve highly accurate thermochemical data for these stationary points on the potential energy surfaces, the Gaussian-4(G4) composite method was employed. Torsional motions are treated either as free rotors or hindered rotors in calculating partion functions to determine thermodynamic properties. The obtained standard enthalpies of formation for reactants and some prodcuts are shown to be in excellent agreement with experimental data with the largest error of 0.5 kcal mol(-1). The conventional transition state theory (TST) with tunneling effects was adopted to determine rate constants of these hydrogen abstraction reactions based on results from quantum chemistry calculations. To faciliate its application in kinetic modeling, the obtained rate constants are given in Arrhenius expression: k(T) = AT(n) exp(-EaR/T). The obtained reaction rate constants also agree reasonably well with available expermiental data and previous theoretical values. Branching ratios of these reactions have been determined. The present reaction rates for these reactions have been used in a toluene combustion mechanism, and their effects on some combustion properties are demonstrated.

The AramcoMech 1.3 mechanism, containing 253 species and 1542 reactions for oxidation of hydrocarbon and oxygenated C1-C2 fuels, is reduced with six direct relation graph (DRG)-related methods. The final skeletal mechanism with 81 species and 497 reactions is achieved from the intersection of the resulting skeletal mechanisms obtained with these DRG-related methods. The maximum error for the ignition delay times with this 81-species mechanism does not increase significantly compared with that obtained for the other skeletal mechanisms. This shows that the intersection of skeletal mechanisms from various mechanism reduction methods can effectively remove the redundant species. Ignition delay times of two-component mixtures with the skeletal mechanism also agree very well with those of the detailed mechanism. The skeletal mechanism has also been validated against the detailed mechanism using many other combustion characters of the involved fuels in different reactors and flames. Results from the element flux analysis demonstrate that the reaction paths for these fuels with the detailed mechanism can be reproduced accurately with the 81-species skeletal mechanism. All the important reaction paths are thus retained in the 81-species mechanism. All these results show that the skeletal mechanism is able to provide the combustion properties of C1-C2 fuels that are in good agreement with those of the detailed mechanism. The 81-species skeletal mechanism can be employed as a reaction base for developing mechanisms of other large hydrocarbon or oxygenated fuels.