Hydrogen/deuterium exchange in the reactions of methyl cation with methane

Abstract

The reaction of methyl cation with methane to form C 2H 5 + and molecular hydrogen has been studied for several combinations of deuterated reactants. The experiments were carried out at room temperature with a tandem flowing afterglow-selected ion flow tube (FA-SIFT). We observed small deviations from statistical loss of H 2/HD/D 2 and some reactions, notably the 3H/4D and 4H/3D cases, exhibit an asymmetry in product distributions. A matrix method for solving coupled rate equations, originally developed by J. Weiner et al. ( J. Am. Chem. Soc. 95 (1973) 4115), has been extended to include isotope effects in the dissociation of the intermediate complex, and provides very good fits to the experimental data. The data and analysis suggest that even for highly exothermic reactions such as the title reaction, the product distributions can deviate from statistical distributions depending on the relative rates of H/D scrambling within the collision complex and of its dissociation to products. The observed isotope effect is approximately equal to the reciprocal square root of the relative masses of the H 2, HD, and D 2 products.