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 fluore scence method in the temperature range 610-740 K. Nearly linear pressure de pendence of the first-order decay rate was observed in the investigated pre ssure range (100-450 Torr), and the second-order rate constants were derive d for He and N-2 buffer gases as: k(0)(He) = 2.8 x 10(-9) exp(-84.3 kJ mol( -1)/RT) cm(3) molecule(-1) s(-1) and k(0)(N-2) = 4.3 x 10(-9) exp(-84.1 kJ mol(-1)/RT) cm(3) molecule(-1) s(-1), respectively. An RRKM calculation wit h a semiclassical one-dimensional tunneling treatment showed that the tunne ling effect is essentially important in the present experimental conditions . The calculation indicates a non-linear pressure dependence of the rate co nstant even in the classical low-pressure limiting conditions, which is con sistent with the experimental observations. The RRKM model (with threshold energy = 101.7 kJ mol(-1)) derived by the best fit to the experimental resu lts is in good agreement with the theoretical investigations and the microc anonical rate constants measured by the stimulated emission pumping method.