Kinetic and Mechanistic Study of the Reactions of Atomic Chlorine with...

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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)