A crossed-beam study of the dynamics of C2H3+ formation in the reaction of C2H2+ with methane

Abstract

The formation of C₂H₃⁺ and its H/D isotopic variants in the reaction of C₂H₂⁺ with CH₄ and CD₄ was investigated in crossed-beam scattering experiments over the collision energy range 0.8 to 2.0 eV (centre of mass). Analysis of the scattering data shows that the product is formed simultaneously via decomposition of intermediates of two different average lifetimes; the mechanism of the process can be identified with two different pathways on the potential energy hypersurface. Formation of the long-lived intermediate (mean lifetime longer than 4 ps in the entire collision energy region) is interpreted as a reaction path involving intermediates of strongly bound C₃H₆⁺ isomers, and formation of the short-lived intermediate (mean lifetime decreases from about 2 ps to 0.3 ps with collision energy increasing from 0.8 to 2.0 eV) as a reaction path going through H-bonded intermediates (C₂H₂···H···CH₃)⁺. At the collision energy of 2 eV the rate of decomposition of the short-lived intermediate is shown to be in effective competition with the rate of H/D isotope scrambling.