Theoretical study of an undisclosed reaction class: Direct H-atom abstraction from allylic radicals bymolecular oxygen

The 1-methylallyl (C₄H₇1-3) allylic radical is an important intermediate species in oxidation of linear C4 unsaturated hydrocarbons (1-butene, 2-butene, and 1, 3-butadiene). This study reports the first high-level quantum chemical calculations for an undisclosed reaction class of this radical at intermediate to high temperatures: direct H-atom abstraction from terminal methyl group by molecular oxygen. Moreover, we systematically calculated rate constants for primary, secondary, and tertiary H-atom abstraction from the C₄, C₅, and C₆ allylic radicals, respectively. Our results can be further used as rate rules for kinetic model development of unsaturated hydrocarbon oxidation. All calculations were implemented using two different ab initio solvers: Gaussian and ORCA, three sets of ab initio methods, and two different kinetic solvers: MultiWell and PAPR. Temperature dependent rate constants and thermochemistry were carried out based on transition state theory and statistical thermodynamics, respectively. H-atom abstraction from the primary site of C4 allylic radical is found to be faster than that from secondary and tertiary sites of C5 and C6 allylic radicals, contrary to common understanding. Barrier heights predicted by different ab initio solvers and methods are about 4-5 kcal/mol different, which results in a factor of 4-86 difference in rate constant predictions depending on the temperature. Using the Gaussian solver with Method 2 is found to be the most effective combination of predicting accurate rate constants when compared against experimental data. When comparing two kinetic solvers, both reaction rate coefficients and species thermochemistry show good agreement at a wide range of temperatures, except for the rate coefficients calculated for C5 and C6 reactions (about a factor of 5-17 and 3-4 differences were obtained, respectively). From an application point of view, we incorporated the calculation results into the AramcoMech2.0 model, and found systematic improvements for predicting ignition delay time, laminar flame speed and speciation targets of 2-butene oxidation.