The kinetics of the decay of methyl radicals at high temperatures.

Abstract

Shock tube experiments were conducted to observe the primary reactions of methyl radicals. 0.7%-7% azomethane diluted in argon or nitrogen was used as a precursor for methyl radicals. The rate of disappearance of CH$\sb3$ radicals was measured behind incident shock waves in real time using a high resolution laser schlieren technique. The measurements covered the temperature range 1400-4000 K and the pressure range 0.1-1.1 atm. Selected experimental results were analyzed by an analytic point-by-point approach, which led to a self-consistent determination of the rate coefficient for methyl recombination, supported by the theory of unimolecular reactions. All of the experiments were then analyzed by computer simulation to evaluate the rate coefficients for the primary reactions of CH$\sb3.$ The rate coefficient for $\rm 2CH\sb3\to C\sb2H\sb4+H\sb2,$ which was basically unknown at high temperatures, was obtained by comparing experimental laser-schlieren signals with those from numerical integration of a postulated mechanism of 44 reactions coupled to the conservation equations of mass, energy, and momentum. The important reactions in the mechanism were identified by a sensitivity analysis. Experiments were also conducted at lower temperatures of 560-1300 K. Chemical chaos and oscillation were observed in the pyrolysis of azomethane at 900-1300 K for the first time. The fall-off effect in the decomposition of azomethane was studied theoretically on the basis of the modern theory of unimolecular reactions. The results of our calculation are in agreement with published experimental work. More exact calculations need to be done above 2000 K. Rate constants are calculated for CH$\sb3$ (+ Ar) $\not=$ CH$\sb2$ + H (+ Ar) at the limiting low pressure, the limiting high pressure, as well as the intermediate fall-off ranges. The results show that published experimental rate constants for methyl dissociation correspond to the fall-off region close to the low pressure limit. At the low pressure limit the activation energy is less than the bond dissociation energy, in agreement with experimental results. Forward and backward rate coefficients at the high pressure limit are compared with other theoretical calculations. More theoretical and experimental work is necessary to understand the reverse reaction and its competing reactions, as well as the decomposition channel leading to CH + H$\sb2.$