A SHOCK-TUBE STUDY OF METHYL-METHYL REACTIONS BETWEEN 1200 AND 2400 K

The methyl-methyl reaction was studied in a shock tube using uv narrowline laser absorption to measure time-varying concentration profiles of CH3. Methyl radicals were rapidly formed initially by pyrolysis of various precursors, azomethane, ethane, or methyl iodide, dilute in argon. The contributions of the various product channels, C2H6, C2H5 + H, C2H4 + H-2, and CH2 + CH4, were examined by varying reactant mixtures and temperature. The measured rate coefficients for recombination to C2H6 between 1200 and 1800 K are accurately fit using the unimolecular rate coefficients reported by Wagner and Wardlaw (1988). The rate coefficient for the C2H5 + H channel was found to be 2.4 (+/-0.5) x 10(13) exp(-6480/T) [cm(3)/mol-s] between 1570 and 1780 K, and is in agreement with the value reported by Frank and Braun-Unkhoff (1988). No evidence of a contribution by the C2H4 + H-2 channel was found in ethane/methane/argon mixtures, although methyl profiles in these mixtures should be particularly sensitive to this channel. An upper limit of approximately 10(11) [cm(3)/mol-s] over the range 1700 to 2200 K-was inferred for the rate coefficient of the C2H4 + H-2 channel. Between 1800 and 2200 K, methyl radicals are also rapidly removed by CH3 + H double right arrow (CH2)-C-1 + H-2. In this temperature range, the reverse reaction was found to have a rate coefficient of 1.3 (+/- 0.3) X 10(14) [cm(3)/mol-s], which is 1.8 times the room-temperature value. (C) 1995 John Wiley & Sons, Inc.