An ab initio kinetic study on H-abstraction reactions of gasoline surrogates by NO2

Nitrogen dioxide (NO2) has an influence on the auto-ignition of hydrocarbon fuels within the engine combustion chamber in the context of exhaust gas recirculation (EGR). This study conducts the first calculation of rate coefficients for the crucial H-abstraction reactions between commonly employed gasoline surrogate components and NO2. A three-component gasoline surrogate fuel, including n-heptane, iso-octane, and toluene, was considered. H-abstraction reactions at all carbon sites were considered in this study, including channels that generate three different nitrous acid (HONO) products. The calculations employed high-precision electronic structure methods, commencing with the lowest-energy conformers determined at the M06-2X/6-311++G(d,p) level of theory. Subsequently, the geometries were re-optimized utilizing the B2PLYP-D3/cc-pVTZ method to obtain the vibrational frequencies and zero-point energies. Single point energies of all involved species were computed using the DLPNO-CCSD(T) method. The rate coefficients were derived employing traditional transition state theory (TST) with the asymmetric Eckart tunneling corrections. Based on the results from this work and literature research for C1-C5 alkane fuels, the barrier heights and rate coefficients for the RH+NO2 reactions were systematically compared. The H-abstraction reaction rates from different carbon sites within the same fuel, the same carbon site from different molecules, and reaction channels forming three different HONO products were investigated. A universal rate rule for H-abstraction reactions of RH+NO2 was proposed based on the results in this work, which provides a reliable reference for analogues for large alkanes.