Elementary reactions in the photochemical oxidation of methyl iodide

Abstract

The photochemical oxidation of gaseous methyl iodide at room temperature was investigated under conditions such that oxygen molecules were competing with iodine molecules and/or iodine atoms for reaction with methyl radicals. The molecular iodine produced in the oxidation was measured by means of its light absorption. A balanced photomultiplier system for the determination of small percentage light absorptions is described. Analysis of the results shows that the contribution of the methyl radical + iodine atom recombination is negligible compared with the methyl radical + iodine molecule reaction for the relative concentrations of iodine atoms and iodine molecules involved. The variation with pressure of the ratio of the rate constant of the methyl radical + iodine molecule reaction to that of the methyl radical + oxygen molecule reaction indicates that the main reaction of methyl radicals with oxygen is a third-order one. This is assumed to be their association to form methylperoxy radicals. Methyl iodide is approximately ten times as efficient as carbon dioxide in acting as a third body for this association. The relative rate of a simultaneous second-order reaction of methyl radicals with oxygen has also been determined. From comparison with other work, the probable orders of magnitude of the rate constants of these elementary reactions of the methyl radical are deduced.