Kinetic and Mechanistic Study of the Reactions of Atomic Chlorine with CH3CH2Br, CH3CH2CH2Br, and CH2BrCH2Br

Laine, P.L., J.M. Nicovich, and P. Wine (2011), Kinetic and Mechanistic Study of the Reactions of Atomic Chlorine with CH3CH2Br, CH3CH2CH2Br, and CH2BrCH2Br, J. Phys. Chem. A, 115, 1658-1666, doi:10.1021/jp110798q.
Abstract

A laser flash photolysis-resonance fluorescence technique has been employed to investigate the reactions of atomic chlorine with three alkyl bromides (R-Br) that have been identified as short-lived atmospheric constituents with significant ozone depletion potentials (ODPs). Kinetic data are obtained through time-resolved observation of the appearance of atomic bromine that is formed by rapid unimolecular decomposition of radicals generated via abstraction of a β-hydrogen atom. The following Arrhenius expressions are excellent representations of the temperature dependence of rate coefficients measured for the reactions Cl þ CH3CH2Br (eq 1) and Cl þ CH3CH2CH2Br (eq 2) over the temperature range 221-436 K (units are 10-11 cm3 molecule-1 s-1): k1(T) = 3.73 exp(-378/T) and k2(T) = 5.14 exp(þ21/T). The accuracy (2σ) of rate coefficients obtained from the above expressions is estimated to be (15% for k2(T) and þ15/-25% for k1(T) independent of T. For the relatively slow reaction Cl þ CH2BrCH2Br (eq 3), a nonlinear ln k3 vs 1/T dependence is observed and contributions to observed kinetics from impurity reactions cannot be ruled out; the following modified Arrhenius expression represents the temperature dependence (244-569 K) of upper-limit rate coefficients that are consistent with the data: k3(T) e 3.2 × 10-17 T 2 exp(-184/T) cm3 molecule-1 s-1. Comparison of Br fluorescence signal strengths obtained when Cl removal is dominated by reaction with R-Br with those obtained when Cl removal is dominated by reaction with Br2 (unit yield calibration) allows branching ratios for β-hydrogen abstraction (kia/k i, i = 1,2) to be evaluated. The following Arrhenius-type expressions are excellent representations of the observed temperature dependences: k1a/k1 = 0.85 exp(-230/T) and k2a/k2 = 0.40 exp(þ181/T). The accuracy (2σ) of branching ratios obtained from the above expressions is estimated to be (35% for reaction 1 and (25% for reaction 2 independent of T. It appears likely that reactions 1 and 2 play a significant role in limiting the tropospheric lifetime and, therefore, the ODP of CH3CH2Br and CH3CH2CH2Br, respectively.

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Research Program
Upper Atmosphere Research Program (UARP)