A detailed experimental and theoretical study on the decomposition of methoxy radicals

We present a detailed experimental and theoretical study on the pressure and temperature dependence of the rare constant for the thermal unimolecular decomposition of methoxy radicals, according to CH3O + M --> CH2O + H + M. Experimentally, we studied the decomposition of the methoxy radical at temperatures between 680 and 810 K and pressures ranging from 1 to 90 bar helium. The methoxy radicals have been generated by laser flash photolysis of methylbenzoate [C6H5C(O)OCH3] at 193 nm and detected by laser-induced fluorescence. Additionally, we characterized the important features of the potential energy surface by ab initio calculations. The results of these calculations were used to analyze the thermal rate constant applying both the Tree formalism as well as a master equation approach. The following falloff parameters have been extracted: k(1,infinity) = 6.8 x 10(13) exp(-109.5 kJ mol(-1)/RT) s(-1), k(1.0) = [He] 1.9 x 10(-8)(T/1000 K)(-2.)4 exp(-101.7 kJ mol(-1)/RT) cm(3) s(-1) and F-C(He) = 0.715-T/4340 K. Additionally, we reanalyzed the literature data for N-2 as bath gas and we recommend the following falloff parameters for this: k(1.0) = [N-2] 3.1 x 10(-8)(T/1000 K)(-3.0) exp(-101.7 kJ mol(-1)/RT) cm(3) s(-1) and F-C(N-2) = 0.97-T/1950 K. In contradiction to earlier studies we did not find any indications that tunneling markedly contributes to the thermal rate constant under our experimental conditions. We calculated the specific and the high-pressure limiting rate constants using RRKM theory and obtained satisfactory agreement with experimental results. We attribute the strong fluctuations of the specific rate constants to be essentially caused by the properties of the density of states. For the beta C-H scission reactions in alkoxy radicals we suggest for the high-pressure limiting rate constants a common A factor and activation energy of A = 10(13.8+/-0.3) s(-1) and E-a = 94 +/- 6 kJ mol(-1). Consequently, the reverse reactions, i.e. the H-atom additions to the carbon site of the C=O pi bond in aldehydes and ketones, always compete with the direct H-atom abstraction.