A theoretical kinetic study of 1-butyne, 2-butyne, and 3-methyl-1-butyne combustion

Propargylic radicals are an essential component in molecular weight growth kinetics and can react to form polycyclic aromatic hydrocarbons and soot. To establish rate rules for propargylic radicals, a total of 16 H-atom abstraction reactions of three alkynes, namely 1-butyne (1-C₄H₆), 2-butyne (2-C₄H₆), and 3-methyl-1-butyne (C₅H₈), were calculated using high-level quantum chemistry, in which four active radicals including Ḣ, ȮH, ĊH₃, and HȮ₂ were involved. Considering the completeness of the establishment of rate rules, the H-atom abstraction sites have included the primary and secondary site of 1-C₄H₆, primary site of 2-C₄H₆, and tertiary site of C₅H₈. The M06–2X/6–311++G(d,p) level of density function theory was used for the geometry optimization, vibrational frequency, and dihedral scan calculations. Single point energy calculations were carried out at the CCSD/cc-pVXZ (X = T, Q) level of coupled cluster theory. The rate coefficients for all of the abstraction reactions and the thermochemical quantities of 1-C₄H₆, 2-C₄H₆, and C₅H₈ and their corresponding radical products were calculated. The results indicate that H-atom abstraction by ȮH radicals has the fastest reaction rates for all three species. Moreover, abstraction from the tertiary site on C₅H₈ is the fastest, followed by abstraction of secondary hydrogen atoms on 1-C₄H₆ and primary hydrogen atoms on 2-C₄H₆, while abstraction of primary hydrogen atoms on 1-C₄H₆ are the slowest. NUIGMech1.3 (mechanism of version 1.3 proposed by National University of Ireland, Galway) has been updated based on our calculations, and ignition delay times (IDTs) were predicted using the updated mechanism. The predicted IDTs gave better agreement with the experiment data at pressures of 1, 30 and 50 bar. Sensitivity analyses were performed to understand observed model predicted differences at low to intermediate temperatures (700–1000 K) at 10 bar. The results show that the reactions 1-C₄H₆/2-C₄H₆ + HȮ₂ and HȮ₂ + HȮ_{2 <}=> H₂O₂ + O₂ have a significant effect on promoting and inhibiting reactivity, respectively. Further Flux analyses show that the H-atom abstraction from 1-C₄H₆ and 2-C₄H₆ by ȮH radical is a dominant channel in changing branching ratios.