Direct study on the unimolecular decomposition of methoxy radicals: The role of the tunneling effect

The unimolecular decomposition of methoxy radicals, CH₃O+M → CH₂O+H+M (1a), has been directly studied by laser photolysis-laser-induced fluorescence method in the temperature range 610-740 K. Nearly linear pressure dependence of the first-order decay rate was observed in the investigated pressure range (100-450 Torr), and the second-order rate constants were derived for He and N₂ buffer gases as: k⁰(He) = 2.8 x 10^-9 exp (-84.3 kJ mol^-1/RT) cm³ molecule^-1 s^-1 and k⁰(N₂) = 4.3 x 10^-9 exp(-84.1 kJ mol^-1/RT) cm³ molecule^-1 s^-1, respectively. An RRKM calculation with a semiclassical one-dimensional tunneling treatment showed that the tunneling effect is essentially important in the present experimental conditions. The calculation indicates a non-linear pressure dependence of the rate constant even in the classical low-pressure limiting conditions, which is consistent with the experimental observations. The RRKM model (with threshold energy = 101.7 kJ mol^-1) derived by the best fit to the experimental results is in good agreement with the theoretical investigations and the microcanonical rate constants measured by the stimulated emission pumping method.