Pressure dependence of methyl radical combination reaction. Part II. Effect of temperature

The study of the pressure-dependence of the methyl radical combination in the gas phase has been extended. The change of the cross-combination ratio of the methyl and isopropyl radicals with pressure has been followed. The results obtained at 94·5°(photolysis of isopropyl methyl ketone) and at 146·5°(pyrolysis of di-t-butyl peroxide in the presence of isobutyraldehyde) are in agreement with previous results obtained by the same method. A discussion is given of the remarkable differences between our results and those obtained by relating the combination to hydrogen abstraction. The data obtained and the most reliable results at high temperature are tested against the Rice–Ramsperger–Kassel–Marcus theory assuming a nearly loose model for the activated complex. The theory reasonably fits the medium fall-off pressure over a temperature range of 600°.