Theoretical Investigation on the H Atom Abstraction Reaction from C1-C4 Alkanes and Alkenes by ṄH₂ Radicals

The H atom abstraction reactions from alkanes and alkenes by ṄH₂ are decisive in predicting the combustion characteristics of NH₃/CxHy binary fuels. Theoretical investigation is carried out on the energy barriers of H atom abstraction reactions from C1-C4 alkanes/alkenes by ṄH₂ radicals at the QCISD(T)/CBS//M06-2X/6-311++G(d,p) level of theory. Single-point energies of each species are computed using QCISD(T)/cc-pVDZ, TZ level of theories with basis set corrections from MP2/cc-pVDZ, TZ, and QZ methods. One-dimensional hindered rotor potentials are obtained by the M06-2X/cc-pVTZ method with 10° increment. Rate constants of each channel across temperatures of 298.15-2000 K are calculated by solving the RRKM/Master Equation with conventional transition state theory. For alkanes, rate constants order follows k_tertiary> k_secondary> k_primary, while for alkenes the order follows k_allylic> k_primary> k_vinylic. Among the vinylic carbon sites within the same alkene species, the hydrogen atom sharing the same carbon with the allylic carbon on the C-C double bond is the preferred site for the H atom abstraction reaction. The branching ratio results indicate that the abstraction from tertiary or secondary carbon sites on alkanes and allylic carbon sites on alkenes are dominating during the investigated temperature range but become less important as the temperature increases. The data provided in this work are in good agreement with the literature data, but for the ṄH₂+alkenes system, the literature data are scarce and further investigation is needed.