Photolysis of methyl iodide at 77 K at 253.7 nm

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.