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

Abstract

We present a detailed experimental and theoretical study on the pressure and temperature dependence of the rate 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 Troe formalism as well as a master equation approach. The following falloff parameters have been extracted: k_1,∞ = 6.8 × 10¹³ exp(−109.5 kJ mol⁻¹/RT) s⁻¹, k_1,0 = [He] 1.9 × 10⁻⁸(T/1000 K)^−2.4 exp(−101.7 kJ mol⁻¹/RT) cm³ s⁻¹ and F_C(He) = 0.715–T/4340 K. Additionally, we reanalyzed the literature data for N₂ as bath gas and we recommend the following falloff parameters for this: k_1,0 = [N₂] 3.1 × 10⁻⁸(T/1000 K)^−3.0 exp(−101.7 kJ mol⁻¹/RT) cm³ s⁻¹ and F_C(N₂) = 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 β 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 CO π bond in aldehydes and ketones, always compete with the direct H-atom abstraction.