Photolysis of methyl iodide at 77 K at 253.7 nm

Abstract

The 253.7 nm photolysis of films of methyl iodide at 77 K has been studied by detailed product analysis. At least 12 hydrocarbon products are formed along with molecular iodine and trivial amounts of ethyl, propyl and sec-butyl iodides. Material balances are quantitatively consistent with the formation also of C₂H₂I₄ in substantial amount. All products are primary and their relative yields are independent of all reaction variables. The quantum yield of methyl iodide removal is 3.5 × 10^–2 while that of CF₃I is, in contrast, only 6.8 × 10^–4. The results, and comparison with the findings for other alkyl iodides indicate that the quantum yield of methyl radical production is close to that for CF₃I photolysis. The photolysis exhibits the phenomenon of photo-inhibition as do those of all other alkyl iodides so far studied and this is attributed to formation of the complex (CH₃I)₂I₂. The findings are incompatible with a radical mechanism and a mechanism based on the concerted reactions of excited molecular aggregates is suggested.