Theoretical Study on the Gas-phase Proton Transfer Reactions of the Formyl and Isoformyl Cations with Acetylene

The gas-phase proton transfer reactions of the formyl and isoformyl cations (HCO⁺ and HOC⁺) with acetylene (C₂H₂) in gas phase have been investigated theoretically at the HF/6-31G(d , p) , B3LYP/6-31G(d , p) and QCISD (T)/6-311G (2df, p)//B3LYP/6-31G (d, p) levels. The possible pathways leading to two dissociation products HC(H)CH⁺+CO (a) and H₂C=CH⁺+CO (b) are probed. It is shown that high-level electron-correlation effect play crucial roles in studying the proton transfer process. The vertical attacks of the protons of HCO⁺ and HOC⁺ towards the π-bond of C₂H₂ involving the respective complexes OC · HC₂H⁺₂ and CO · HC₂H⁺₂ are the most plausible mechanism for proton transfer, which may mainly lead to the π-protonated acetylene, HC (H) CH⁺. The higher exothermicity of the reaction of HOC⁺+C₂H₂ and the lower stability of the involved intermediate suggest that HOC⁺ is more reactive than HCO⁺ for proton transfer reaction. Our calculated results agree well with the available experimental results and may be helpful for understanding the interstellar and combustion chemistry in which HCO⁺ and HOC⁺ are involved.