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 narrow line laser absorption to measure time-varying concentration profiles o f CH3. Methyl radicals were rapidly formed initially by pyrolysis of v arious precursors, azomethane, ethane, or methyl iodide, dilute in arg on. 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 coe fficient 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 agreem ent with the value reported by Frank and Braun-Unkhoff (1988). No evid ence of a contribution by the C2H4 + H-2 channel was found in ethane/m ethane/argon mixtures, although methyl profiles in these mixtures shou ld be particularly sensitive to this channel. An upper limit of approx imately 10(11) [cm(3)/mol-s] over the range 1700 to 2200 K-was inferre d 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 ri ght arrow (CH2)-C-1 + H-2. In this temperature range, the reverse reac tion was found to have a rate coefficient of 1.3 (+/- 0.3) X 10(14) [c m(3)/mol-s], which is 1.8 times the room-temperature value. (C) 1995 J ohn Wiley & Sons, Inc.