Gas-Phase Photolysis of Methyl Iodide. Reactions of Hot Methyl Radicals with Added Organic Compounds

The photolysis of CH3I and CD3I at 2537 and 3130 Å has been investigated in the presence of CD3CH3, C2D6, CH3CD2CH3, CD3CH2CD3, C4H10–C4D10 (1:1) mixtures, (CH3)3CD, (CH3)3CCD3, C6H6, C6H12, CH3OCH3, CH3COOCD3, CD3OH, C2D5OH, and CD3CH2OH. All mixtures were saturated with iodine in order to prevent the formation of methane by the reaction of thermal methyl radicals. It is shown that, under these conditions, the measured quantum yield of methane formation can be considered to be the probability that a ``hot'' methyl radical with 32 kcal will react with CH3I or the additive. The following observations were made: (1) Intramolecular and intermolecular isotope effects are of minor importance. (2) The probability of abstraction of an H or a D atom from a molecule by a ``hot'' methyl radical increases with the number of available H or D atoms, or with a decrease in the ΔH of the reaction. For instance, it is seen that the relative probabilities per atom for abstraction of a primary, secondary, or tertiary H atom in a hydrocarbon molecule are 1:3.6:10.0. A variation in wavelength, from 2537 Å to 3130 Å, lowers the methane quantum yield by a factor of 10 but does not greatly affect the relative probabilities of abstraction from different sites within a molecule. (3) The quantum yield of methane in the photolysis of CH3I or CH3I—hydrocarbon mixtures is nearly twice that observed for CD3I or the corresponding CD3I—hydrocarbon mixtures at the same wavelength. It is concluded that systematic studies of this type may prove to be useful in determining a priori the probability of reaction with other related organic compounds.