Investigation of product formation in the H + HC = C = CH reaction: a comparison of experimental and theoretical kinetics.

Context: The HCCCH radical plays a crucial role in combustion chemistry, astrophysical processes, and the formation of complex organic molecules, serving as a key intermediate in the synthesis of polycyclic aromatic hydrocarbons and soot precursors. The reactions of HCCCH with small species are significant for understanding the mechanisms of hydrocarbon transformation in combustion, atmospheric chemistry, and interstellar environments. In the present study, the mechanism and kinetics of the H + HCCCH have been thoroughly characterized. The calculated results indicate that the reaction can proceed via H-addition to the HCCCH carbon chain without an energy barrier, forming the adducts (CH). These intermediates can then undergo H-abstraction or carbon-chain cleavage to create various products, in which PR (HCCCH + H) and PR (HCCC + H) are the main products of the reaction system. Furthermore, the triplet potential surface shows the dominant channel forming the product PR (HCCCH + H). In the low-temperature region, PR is dominant, exhibiting a 70% branching ratio at 400 K; at higher temperatures, the PR product prevails, with a 65.7% branching ratio at 2000 K. The bimolecular rate constants of the reaction are positively dependent on temperatures but negatively dependent on pressures. The calculated rate constants in this study agree well with the available literature data. The computational results of the H + HCCCH reaction provide profound insights into the theoretical aspects and offer valuable applications for modeling reaction systems involving the propargyl radicals.

Methods: The B3LYP and CCSD(T) methods, combined with the aug-cc-pVnZ (n = T, Q, 5) basis sets, were employed to optimize structures and calculate single-point energies for all species involved in the reaction. The temperature range (200 - 2000 K) and pressure range (0 - 7600 Torr) were used to calculate the bimolecular rate constants for the dominant reaction pathways. The TST, VRC-TST, and RRKM models, with the small curvature tunneling correction, were employed for the kinetic calculations.