Reaction of O(1D) with Methane

The reaction of O(¹D) with CH₄ has been studied by photolysis (2537 Å) of O₃–CH₄ mixtures dissolved in liquid argon at 87°K. The quantum yields of O₃ decomposition and the product yields based on O₃ decomposed were measured. The products found were CH₃OH, CH₂O, and H₂. Added O₂ was found to lower the quantum yields of O₃ decomposition and to increase the product yields. Experiments with added ¹⁸O₂ were carried out to trace the origin of the products. At 6000 Å, which can only give O (³P), no photo‐reaction could be detected. The results indicate that the O(¹D)—CH₄ reaction has essentially zero activation energy and proceeds by three paths: $$\begin{array}{c}\mathrm{O}{(}^1\mathrm{D)+}{\mathrm{CH}}_4+\mathrm{M}\to {\mathrm{CH}}_3\mathrm{OH}+\mathrm{M},\\ \mathrm{O}{(}^1\mathrm{D)+}{\mathrm{CH}}_4\to {\mathrm{CH}}_2\mathrm{O}+{\mathrm{H}}_2,\\ \mathrm{O}{(}^1\mathrm{D)+}{\mathrm{CH}}_4\to {\mathrm{CH}}_3+\mathrm{OH}.\end{array}$$ The effect of added O₂ is attributed to scavenging of methyl radicals which otherwise react with O₃. The reaction of CH₃ with O₃ has zero E_a and occurs at a rate approximately equal to the reaction of CH₃ with O₂. The magnitude of the scavenging effect shows that at least two O₃ molecules are decomposed by each CH₃. By contrast, OH radicals do not react with O₃ in these experiments, indicating a nonzero activation energy for the reaction OH+O₃→HO₂+O₂.