TEMPERATURE AND ISOTOPE DEPENDENCE OF THE REACTION OF METHYL RADICALSWITH DEUTERIUM ATOMS

The reactions of methyl isotopomers (CH3, CH2D, and CHD2) with excess deuterium atoms have been studied using discharge flow/mass spectromet ry at 298 K and at pressures of similar to 1 Torr (helium), At these l ow pressures the initially formed methane complex is not stabilized. H owever, zero-point energy differences between methyl isotopomers mean that ejection of H from energized methane is favored. In consequence, regeneration of the reactant isotopomer is inefficient and values of k (1a-c) may be extracted from the appropriate methyl radical decay. The experimental values can be used to calculate the high-pressure values for each isotopic reaction: (la) CH3 + D --> CH2D + H, k(1a)(infinity ) = (2.3 +/- 0.6) x 10(-10) cm(3) molecule(-1) s(-1); (Ib) CH2D + D -- > CHD2 + H, k(1b)(infinity) = 2.1 +/- 0.5) x 10(-10) cm(3) molecule(-1 ) s(-1); (Ic) CHD2 + D --> CD3 + H, k(1c)(infinity) = (1.9 +/- 0.5) x 10(-10) cm(3) molecule(-1) s(-1). These, in turn, can be corrected for isotopic substitution and averaged to give a value of (2.9 +/- 0.7) x 10(-10) cm(3) molecule(-1) s(-1) for the limiting high-pressure recom bination rate coefficient of CH3 and H. The errors of similar to 25% a re estimates of both the statistical and systematic errors in the meas urements and calculations. The results are in agreement with an earlie r direct determination of reaction la and recent theoretical calculati ons. The previous direct studies of CH3 + H in the fall off region hav e been reanalyzed using master equation techniques and are now shown t o be in good agreement with current experimental and theoretical calcu lations. Reaction Ic was also studied at 200 K, with k(1c) failing by approximately 35% from its room-temperature value, confirming theoreti cal predictions of a positive temperature dependence for the high-pres sure Limiting rate coefficient for the reaction CH3 + H + M --> CH4 M.