Investigation of product formation in the H + H2C = C = CH reaction: a comparison of experimental and theoretical kinetics

Abstract

Context

The H₂CCCH 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 H₂CCCH 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 + H₂CCCH have been thoroughly characterized. The calculated results indicate that the reaction can proceed via H-addition to the H₂CCCH carbon chain without an energy barrier, forming the adducts (C₃H₄). These intermediates can then undergo H₂-abstraction or carbon-chain cleavage to create various products, in which PR₁ (¹HCCCH + H₂) and PR₄ (H₂CCC + H₂) are the main products of the reaction system. Furthermore, the triplet potential surface shows the dominant channel forming the product PR₁₁ (³HCCCH + H₂). In the low-temperature region, PR₄ is dominant, exhibiting a 70% branching ratio at 400 K; at higher temperatures, the PR₁₁ 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 + H₂CCCH 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.