Isotope Exchange Rates. IV. The Homogeneous Reaction Between CH4 and D2

Abstract

The rate of exchange of deuterium for hydrogen in the homogeneous gas‐phase reaction between D₂ and CH₄ (highly diluted in argon) was measured behind reflected shocks in a single‐pulse shock tube of the ``magic hole'' design. The studies covered the temperature range 1440°—1755°K, at CH₄/D₂ ratios from 0.1 to 3.0. The exchange rate was found to be approximately first order with respect to deuterium and one‐half order with respect to methane and argon. The empirical rate expression and the measured exchange rates cannot be accounted for either by a direct four‐center bimolecular exchange process or by a mechanism involving a dissociation followed by a chain of atom displacement steps. However, these results are readily accounted for by a vibrational excitation mechanism in which the exchange rate is limited by the rate of populating a critical vibrational level for D₂ (approximately the fifth). The activation energy for exchange is 52.0±2.2 kcal/mole.