Free Radicals by Mass Spectrometry. IV. The Rate of Combination of Methyl Radicals

Abstract

The rate of combination of methyl radicals produced by the thermal decomposition of mercury dimethyl has been studied with a free radical mass spectrometer over a large range of temperature (161° to 814°C) and at one temperature (735°C) over a range of total pressure from 4.8 mm to 18.5 mm. The combination was found to be complex, consisting of a second-order homogeneous reaction and a first-order, presumably heterogeneous reaction. Only the second-order reaction was studied in any detail. The rate constant or collision efficiency of this reaction was found to increase with decreasing temperature but showed no dependence on the carrier gas pressure. A negative temperature coefficient was found for the combination which, if expressed as an activation energy, was −2.2±0.5 kcal. It seems, however, more reasonable to regard this effect as a change in the effective collision diameter of the methyl radicals with temperature, the change being due to the van der Waals energy of attraction between the radicals.