ELECTRON-SPIN-RESONANCE STUDY OF THE REACTION OF HYDROGEN-ATOMS WITH METHANE

The reaction: H + CH4 --> CH3 + H-2 has been investigated in a flow sy stem between 348 and 421 K. Hydrogen atoms were generated in a microwa ve discharge, introduced to the reactor through a movable injector, an d monitored by electron spin resonance. After an initial decay attribu ted to reaction with impurity, the hydrogen atom concentration decayed in a pseudo-first-order manner. Ethane was detected by gas chromatogr aphy, consistent with its formation by the following reaction: 2CH(3) --> C2H6. The amount of ethane formed at 421 K was only 0.015 times th e amount of hydrogen atoms reacting. Most methyl radicals were assumed to have been removed by the process: H + CH3 + M --> CH4 + M. Because of this process, two hydrogen atoms were removed each time the title reaction occurred. Applying this stoichiometric factor, the rate const ant for the elementary reaction was calculated to be 2.5 x 10(3) L mol (-1) s(-1) at 348 K, increasing to 2.0 x 10(4) L mol(-1) s(-1) at 421 K. Most of the previous discrepancy between kinetics and thermochemist ry has bean eliminated; the exothermicity at 0 K was reduced to 0.8 +/ - 0.4 kJ mol(-1), which corresponds to a standard heat of formation of the methyl radical of 145 kT mol(-1). Properties of the activation ba rrier have been inferred from the experimental data with the aid of tr ansition state theory. The fitted barrier height was 63 +/- 1 kJ mol(- 1), the average of five low-frequency vibrational term values was 640 +/- 30 cm(-1), and the characteristic tunnelling temperature was 500 /- 30 K.