Rate theory of methyl recombination at the low temperatures and pressures of planetary atmospheres

The recombination of methyl radicals in low pressures of hydrogen, helium, and nitrogen to form ethane controls the concentrations observed for methyl and the photochemical synthesis of higher hydrocarbons above the Jovian planets. Few measured or theoretical rate constants are available to provide reliable model predictions. RRKM and master equation calculations are reported here, using three levels of transition state detail, to describe existing data and provide consistent and reliable expressions for this rate constant at 65–300 K and any pressure. This gives k∞=3.59×10−10 T−.262 e−37/T cm3/molec/s and k0(H2)=3.32×10−15 T−4.28 e−131/T cm6/molec2/s.