Homogeneous Exchange Reaction: CD4+CH4→CD3H+CH3D. Single Pulse Shock Tube Studies

The homogeneous exchange reaction between CH₄ and CD₄ was investigated using the single pulse shock tube technique. The temperature range covered was 1340–1745°, and concentrations up to 8% total methane in argon were used. The over‐all rate of exchange is proportional to the $\frac{3}{2}$ power of the methane concentration and is independent of the argon density. The rate of exchange in mixtures containing 4% CH₄ + 1% CD₄ and 4% CD₄ + 1% CH₄ was found to be the same, indicating a negligible isotope effect at the temperature range covered in this study. For mixtures in which [CH₄] = [CD₄] the rate of exchange is given by $\mathrm{d[}{\mathrm{CD}}_3\mathrm{H}{\mathrm{]\hspace{0.17em}/\hspace{0.17em}dt\hspace{0.17em}=\hspace{0.17em}k}}_r[{\mathrm{CH}}_4{]}^{\text{0.75}}[{\mathrm{CD}}_4{]}^{\text{0.75}}$ , where $k_r{\text{\hspace{0.17em}=\hspace{0.17em}4.3\hspace{0.17em}×\hspace{0.17em}10}}^{14}\exp {\text{[−(70\hspace{0.17em}±\hspace{0.17em}2)\hspace{0.17em}×\hspace{0.17em}10}}^3\mathrm{\hspace{0.17em}/\hspace{0.17em}RT]}{\mathrm{cc}}^{\text{0.5}}{\mathrm{mole}}^{\text{−0.5}}·{\sec }^{\text{−1}}$ . It was shown that the exchange proceeds via a chain reaction mechanism carried by CH₃ and CD₃ free radicals. The evaluated rate constants for the chain, $${\mathrm{CH}}_3+{\mathrm{CD}}_4\to {\displaystyle {lim}^{k_3}}{\mathrm{CD}}_3+{\mathrm{CH}}_3\mathrm{D},$$ $${\mathrm{CD}}_3+{\mathrm{CH}}_4\to {\displaystyle {lim}^{k_4}}{\mathrm{CH}}_3+{\mathrm{CD}}_3\mathrm{H},$$ are by a factor of 10 higher than the ones extrapolated from the low temperature studies on the basis of an Arrhenius plot.