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

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.