Abstract: 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/631G(d,p), B3LYP/631G(d,p) and QCISD(T)/6311G(<2df,p)//B3LYP/631G(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·HC₂H₂⁺ and CO·HC₂H₂⁺ 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.

Key words: Formyl and isoformyl cations, Proton transfer, Theoretical study