The photochemical decomposition of methyl iodide in presence of nitric oxide I. The reaction of methyl radicals with nitric oxide

The quantum yield of molecular iodine produced in the photolysis of gaseous methyl iodide is markedly increased by the presence of small quantities of nitric oxide. The yield of iodine is determined by the competition between reactions of methyl radicals with iodine molecules to reform methyl iodide and of methyl radicals with nitric oxide to form nitrosomethane. The latter reaction is pressure-dependent and the kinetics may be interpreted in terms of the following steps: CH3 + NO = CH3NO' (ka). CH3NO' = CH3 + NO (kb), CH3NO' + M = CH3NO + M' (kc,M). The apparent third-order constant at low pressure { = (kc,M ka) / kb} is 50 times as great as that measured for the association of methyl radicals with oxygen in the same gas mixture. The minimum lifetime of the energy-rich CH3NO' complex is 10-8 s. The absolute value of ka, the limiting value of the apparent second-order constant for the association, is estimated to be 7 x 1011 ml. mole-1 s-1, with an uncertainty factor of 3.