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 (H) over dot, (O) over dotH, (C) over dotH(3), and H(O) over dot(2) 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 O. H 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 + H(O) over dot(2) and H(O) over dot(2) + H(O) over dot(2) (<)=> H2O2 + O-2 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 (O) over dotH radical is a dominant channel in changing branching ratios.