Reactions of translationally excited and thermal fluorine atoms with CH4 and CD4 molecules in solid argon

Abstract

Solid state reactions of fluorine with methane have been studied by ultraviolet laser photolysis of dilute mixtures of the two reagents in solid argon at 13–30 K. Using a combination of EPR and FTIR spectroscopies, three distinct mechanisms of product formation have been identified. At temperatures below 18 K, product formation is dominated by direct photolysis of F2–CH4 heterodimers, resulting in formation of closed-shell complexes HF–CH3F. A small fraction of reaction intermediate, a nonplanar methyl radical trapped in the reaction cage with HF and F, is also formed. This intermediate decays to product on a time scale of 103 s at 13 K after the photolysis period. Above 20 K, photogenerated F atoms undergo thermally activated diffusion in argon, and they react with isolated CH4 molecules to form CH3–HF complexes. The rate constant for this process is ∼10−25 cm3 s−1 at 20 K, and exhibits an activation energy of 1.7 kcal/mol. A third reaction channel involves reaction of F with CH4 to generate isolated planar methyl radicals by escape of the HF product from the reactive site. This channel has been observed for reactions of translationally excited F atoms (below 20 K) and thermally diffusing F atoms (during post-photolysis reactions above 20 K).