Theoretical study on the mechanism of the reaction: HCCCH₂⁺+C₂H₂→c-C ₃H₃⁺+C₂H₂

The gas phase ion-molecule reaction of propargylium (HCCCH₂⁺) with acetylene (C₂H₂) to produce cyclopropenylium (c-C₃H₃⁺) with C₂H₂ has been investigated theoretically at the B3LYP/6-31G(d) and single-point QCISD/6-311G(d,p) levels. The detailed mechanism for the observed isotope exchange between HCCCH₂⁺ and C₂D₂ has also discussed. Three intermediates 1 CH₂CCH₂CCH⁺, 2 H₂C₂·CHCCH₂⁺ and 3 c-C₄H₃-CH₂⁺ are shown to play important roles in the product formation and isotope exchange processes, rather than the low-lying isomers 4 c-C₃H₂-CH₂⁺, 7 c-(CH)₅⁺ and 8 pyramidal C₅H₅⁺. Our calculated results agree well with the available experimental data and may be helpful for understanding the mechanism for combustion processes.