Oxidation of norbornadiene: Theoretical investigation on H-atom abstraction and related radical decomposition reactions

The chemical kinetics of hydrogen atom (H-atom) abstraction reactions from norbornadiene (NBD) by five radicals (H, O(³P), OH, CH₃, and HO₂), and the unimolecular reactions of three NBD derived radicals, were studied through high-level ab-initio calculations. The geometries optimization and vibrational frequencies calculation for all the reactants, transition states, and products were obtained at the M06-2X/6-311++G(d,p) level of theory. The zero-point energy (ZPE) corrected potential energy surfaces (PESs) were determined at the QCISD(T)/cc-pVDZ, TZ level of theory with basis set corrections from MP2/cc-pVDZ, TZ, QZ methods for single point energy calculations. Conventional transition state theory (TST) was used for the rate constants calculations of H-atom abstraction reactions by five radicals (H, O(³P), OH, CH₃, and HO₂) at temperatures from 298.15 to 2000 K, while the α-site H-atom abstraction reaction rate constant of NBD by OH radical has been obtained through variational transition state theory (VTST). The results show that the H-atom abstraction reactions from the α-carbon atom of NBD are the most critical channels at low temperatures. Total rate constants for H-atom abstraction reactions by OH radical are also the fastest among all of the reaction channels investigated at the temperature range from 298.15 to 2000 K. Rice-Ramsperger-Kassel-Marcus/Master Equation (RRKM/ME) has been used to calculate the pressure- and temperature-dependent rate constants for the unimolecular reactions of three related C₇H₇ product radicals which generated from H-atom abstraction reaction within temperature ranges of 300–2000 K and pressures of 0.01–100 atm. A combination of composite methods has been used to calculate the temperature-dependent thermochemical properties of NBD and related radicals. All the calculated kinetics and thermochemistry data can be utilized in the model development for NBD oxidation.