Pm. Marquaire et al., ELECTRON-SPIN-RESONANCE STUDY OF THE REACTION OF HYDROGEN-ATOMS WITH METHANE, Canadian journal of chemistry, 72(3), 1994, pp. 600-605
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.