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 (c-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.