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

Article abstract of DOI:10.1246/bcsj.73.53

The methoxy radical is one of the important reaction intermediates in combustion and atmospheric photooxidation of hydrocarbons. Since the laser-induced fluorescence (LIF) spectrum of the methoxy radical was found by Inoue et al., several kinetic studies on the bimolecular reactions with O₂, NO₂, and NO, have been reported in the last two decades. The unimolecular decomposition of methoxy radical (CH₃O + M → CH₂O + H + M) generated by the pulsed ArF excimer laser photolysis of methanol at 193 nm was studied by laser photolysis-LIF method at 610-740 K. Nearly linear pressure dependence of the first-order decay rate was seen at 100-450 torr, and the second-order rate constants were derived for He and N₂ buffer gases. The Rice-Ramsperger-Kassel-Marcus calculation indicated a non-linear pressure dependence of the rate constant even in the classical low-pressure limiting conditions, which was consistent with the experimental observations. The RRKM model with threshold energy = 101.7 kJ/mole agreed well with the theoretical investigations and the microcanonical rate constants measured by the stimulated emission pumping method.