Reaction Dynamics of Singlet Methylene with Acetylene: A Combined Ab Initio and RRKM Study

The reaction dynamics of singlet methylene (CH₂, ¹A₁) with acetylene (C₂H₂) were investigated using high-level ab initio electronic structure calculations combined with Rice-Ramsperger-Kassel-Marcus (RRKM) theory. The potential energy surface (PES) for the singlet C₃H₄ system was mapped at the CCSD(T)/CBS//B3LYP/6-311G(d,p) level of theory. The results reveal that the reaction proceeds via a barrierless addition of CH₂ to the π-bond of C₂H₂, yielding cyclopropene (<i>c-</i>C₃H₄, i1) as the initial adduct. Subsequent isomerization and dissociation pathways were characterized, and unimolecular rate constants and product branching ratios were determined under single-collision conditions. The dominant product channel involves hydrogen migration followed by dissociation to form the propargyl radical (H₂CCCH) and atomic hydrogen, accounting for 95-89% of the products over collision energies from 0 to 8 kcal mol^-1. Minor channels leading to HCCCH + H₂ (5-10%) and H₂CCC + H₂ (∼1%) were also identified. The present study provides a detailed mechanistic and kinetic framework for the CH₂ (¹A₁) + C₂H₂ reaction with broader implications for hydrocarbon growth processes relevant to combustion chemistry and astrochemical environments.