HYDROGEN DEUTERIUM EXCHANGE IN THE REACTIONS OF METHYL CATION WITH METHANE/

The reaction of methyl cation with methane to form C2H5+ and molecular hydrogen has been studied for several combinations of deuterated reac tants. The experiments were carried out at room temperature with a tan dem flowing afterglow-selected ion Bow tube (FA-SIFT). We observed sma ll 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 dis tributions. A matrix method for solving coupled rate equations, origin ally developed by J. Weiner et al. (J. Am. Chem Sec. 95 (1973) 4115), has been extended to include isotope effects in the dissociation of th e intermediate complex, and provides very good fits to the experimenta l data. The data and analysis suggest that even for highly exothermic reactions such as the title reaction, the product distributions can de viate from statistical distributions depending on the relative rates o f 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. (C) 1997 Elsevier Science B.V.