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

Document Type

Article

Publication Date

10-7-1999

Abstract

Reactions of methyl radicals with hydrogen bromide CH3 + HBr → CH4 + Br (1) and bromine atoms CH3 + Br → CH3Br (2) were studied using excimer laser photolysis-transient UV spectroscopy at 297 ± 3 K over the 1-100 bar buffer gas (He) pressure range. Methyl radicals were produced by 193 nm (ArF) laser photolysis of acetone, (CHa3)2CO, and methyl bromide, CH3Br. Temporal profiles of methyl radicals were monitored by UV absorption at 216.51 nm (copper hollow cathode lamp with current boosting). The yield of acetyl radicals in photolysis of acetone at 193 nm was found to be less than 5% at 100 bar He based on the transient absorptions at 222.57 and 224.42 nm. The measured rate constants for reaction 1 are k1 = (2.9 ± 0.7) × 10-12, (3.8 ± 1.5) × 10-12, and (3.4 ± 1.3) × 10-12 cm3 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 previous (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 radicals with bromine atoms (2) was determined relative to the rate constant of methyl radical self-reaction, CH3 + CH3 → C2H6 (3) in experiments with photolysis of CH3Br: k2/k3 = 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 literature data for reaction 3, this yields k2 = (5.8 ±2.2) × 10-11, (7.4 ± 2.2) × 10-11, (10.7 ± 2.3) × 10-11, and (11.9 ± 2.5) × 10-11 cm3 molecule-1 s-1 at 1.05, 11.2, 101 bar (He), and in the high-pressure limit, respectively. © 1999 American Chemical Society.

Identifier

28944440340 (Scopus)

Publication Title

Journal of Physical Chemistry A

External Full Text Location

https://doi.org/10.1021/jp991183i

ISSN

10895639

First Page

7958

Last Page

7971

Issue

40

Volume

103

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