The formation of resonantly stabilized radicals, such as propargyl (CH2CCH), is a critical step in the formation of polycyclic aromatic hydrocarbons (PAH) and thence soot. These radicals are typically formed either via a CH2 insertion mechanism or via decomposition of an unsaturated hydrocarbon. C4H6 isomers are common intermediates in flames, particularly under fuel rich conditions. 1,3-butadiene + H, for example, is the dominant product in the C2H3 + C2H4 reaction under engine-relevant conditions. Despite the prevalence of C4H6 intermediates, they have received less attention that other unsaturated compounds. This lack of attention is due in part to the complexity of the potential energy surface (PES), which contains multiple dienes, alkynes, 3- and 4-member rings, carbenes, as well as biradicals. Although CH3 + CH2CCH is generally recognized to me the dominant product channels for most C4H6 isomers, there is little agreement on the decomposition mechanism, let alone rate coefficients and branching fractions.
In this talk, we will present high-accuracy calculations on the C4H6 PES, using a compound method with accuracy of 0.3 kcal/mol at the 2-sigma level of uncertainty. Particular attention will be given to multi-reference effects for key transition states, such as those involved homolytic cleavage of either a C-C or C-H bond or are otherwise bi-radical in nature, for which single-reference methods are ill-suited. Variable-Reaction Coordinate Transition State Theory will be used for the barrierless channels, and the entire temperature and pressure dependent C4H6 decomposition kinetics will be computed using RRKM/ME methods. In addition to CH3 + CH2CCH, there are multiple H + C4H5 product channels, in which the C4H5 is a methyl-propargyl isomer. The branching fractions to these pathways are significant, but they have been neglected in all current combustion mechanisms. The results indicate that there is significant scrambling between the C4H6 intermediates prior to decomposition. These new rate coefficients will result in improved mechanisms for PAH growth.