A crossed-beam study of the dynamics of C2H3+ formation in the reaction ofC2H2+ with methane

The formation of C2H3+ and its H/D isotopic variants in the reaction of C2H 2+ 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 o f the scattering data shows that the product is formed simultaneously via d ecomposition of intermediates of two different average lifetimes; the mecha nism of the process can be identified with two different pathways on the po tential energy hypersurface. Formation of the long-lived intermediate (mean lifetime longer than 4 ps in the entire collision energy region) is interp reted as a reaction path involving intermediates of strongly bound C3H6+ is omers, and formation of the short-lived intermediate (mean lifetime decreas es 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 th e short-lived intermediate is shown to be in effective competition with the rate of H/D isotope scrambling.