Kinetic study of CH3+HBr and CH3+Br reactions by laser photolysis-transient absorption over 1-100 bar pressure range

Reactions of methyl radicals with hydrogen bromide CH3 + HBr --> CH4 + Br ( 1) and bromine atoms CH3 + Br --> CH3Br (2) were studied using excimer lase r photolysis-transient UV spectroscopy at 297 +/- 3 K over the 1-100 bar bu ffer gas (He) pressure range. Methyl radicals were produced by 193 nm (ArF) laser photolysis of acetone, (CH3)(2)CO, and methyl bromide, CH3Br. Tempor al profiles of methyl radicals were monitored by UV absorption at 216.51 nm (copper hollow cathode lamp with current boosting). The yield of acetyl ra dicals in photolysis of acetone at 193 nm was found to be less than 5% at 1 00 bar He based on the transient absorptions at 222.57 and 224.42 nm. The m easured rate constants for reaction 1 are k(1) = (2.9 +/- 0.7) x 10(-12), ( 3.8 +/- 1.5) x 10(-12), and (3.4 +/- 1.3) x 10(-12) cm(3) molecule(-1) s(-1 ) at the buffer gas (He) pressures of 1.05, 11.2, and 101 bar, respectively . The rate data obtained in this study confirmed high values of the previou s (low pressure) measurements and ruled out the possibility of interference of excited species. The measured rate constant is independent of pressure within the experimental error. The rate constant of reaction of methyl radi cals with bromine atoms (2) was determined relative to the rate constant of methyl radical self-reaction, CH3 + CH3 --> C2H6 (3) in experiments with p hotolysis of CH3Br: k(2)/k(3) = 0.92 +/- 0.32, 1.15 +/- 0.30, and 1.65 +/- 0.26 at 1.05, 11.2, and 101 bar He, respectively. On the basis of the liter ature data for reaction 3, this yields k(2) = (5.8 +/- 2.2) x 10(-11), (7.4 +/- 2.2) x 10(-11), (10.7 +/- 2.3) x 10(-11), and (11.9 +/- 2.5) x 10(-11) cm(3) molecule(-1) s(-1) at 1.05, 11.2, 101 bar (He), and in the high-pres sure limit, respectively.