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

The reaction dynamics of singlet methylene (CH, A) with acetylene (CH) 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 CH 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 CH to the π-bond of CH, yielding cyclopropene (CH, 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 (HCCCH) and atomic hydrogen, accounting for 95-89% of the products over collision energies from 0 to 8 kcal mol. Minor channels leading to HCCCH + H (5-10%) and HCCC + H (∼1%) were also identified. The present study provides a detailed mechanistic and kinetic framework for the CH (A) + CH reaction with broader implications for hydrocarbon growth processes relevant to combustion chemistry and astrochemical environments.