| Summary: | Rate coefficients for the reactions CH3 + H and CH3 + D are presented over the temperature ranges 300 ≤ T/K ≤ 600 and 289 ≤ T/K ≤ 400, respectively. The laser flash photolysis (193 nm) of acetone or acetone/N2O/D2 mixtures was employed to generate CH3 and H or CH3 and D, respectively, and the time dependences of the atom and radical concentrations were monitored by resonance fluorescence and absorption, respectively. Over the experimental pressure range (25-600 Torr of He) the CH3 + H reaction is in the falloff regime, while the rate coefficient for the CH3 + D reaction is pressure independent, because the fragmentation of CH3D* to generate CH2D + H is much faster than that to regenerate CH3 + D under all conditions studied; in consequence the measured rate coefficient corresponds, in effect, to the high-pressure limit, k1∞(D). Fits to the CH3 + H falloff data show that the high-pressure limit, k1∞(H), at 300 K exceeds that predicted from k1∞(D) by at least a factor of 2. This conclusion is confirmed by detailed master equation calculations that incorporate microcanonical dissociation rate coefficients calculated on the basis of a variational RRKM procedure. Parameters are provided that give a satisfactory representation of the CH3 + H rate data, over the experimental pressure and temperature ranges, with a temperature-independent value of k1∞(H) of 4.7 × 10-10 cm3 molecule-1 s-1 with uncertainties of Δ log k1∞(H) ∼ +0.2 to -0.1 at 300 K rising to ∼±0.4 at 600 K. © 1989 American Chemical Society.
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