5313
J. Phys. Chem. 1993,97, 5313-5320
Kinetics of the CCl3 Association Reaction with Molecular Oxygen at 298 and 333 K and from 1 to 760 Torr of Total Pressure Frederick F. Fenter' and Phillip D. Lightfoot+ Luboratoire de Photophysique et Photochimie Molbculaire, Universitb de Bordeaux I, 351 Cows de la Libbation 33405 Talence Cedex, France
Jukka T. Niiranent and David Gutman Department of Chemistry, Catholic University of America, Washington, D.C. 20064 Received: December 1, 1992; In Final Form: February 25, 1993
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The reaction CCl3 0 2 M CCl3O2 M (eq 1) was studied at 298 and 333 K as a function of pressure, from 1 to 760 Torr, using two experimental techniques. Low-pressure experiments (1-12 Torr) were carried out with the laser photolysis/photoionizationmass spectrometry technique. The experiments conducted in helium at low pressure yielded rate constants that were about a factor of 2 smaller than in N2, confirming that two previous studies are in good agreement once the third body collisional efficiency of the bath gas is taken into account. High-pressure experiments (2&760 Torr)were carried out with the laser photolysis/UV absorption technique with an apparatus that is described for the first time. The value of kl at 760 Torr and 298 K was found to be (1.65 f 0.12) X 10-12 cm3 molecule-' s-l. All the data taken with N2 as the bath gas were fitted to two different versions of the Troe expression that permits the extrapolation of fall-off data to the high- and low-pressure limiting rate constants; our results are well represented by the following forms of ko and k,: ko( T ) = (6.5 f 0.2) X 10-31(T/300)4.1*0.2cm6 molecule-2 s-1 and k.,,(T) = (2.6 f 0.2) X 10-~2(T/300)-~~~*0~6 cm3 molecule-l.s-'. A study of the secondary chemical reactions allowed us to determine the rate constants for the following two Br atom reactions: Br CC13 M CC13Br M (eq 2) and Br CC1302 BrO CC130 (eq 3). At atmospheric pressure of nitrogen and 298 K, we found that k2 = (6.7 f 1.5) X l W 1cm3 molecule-' s-I and k3 = (6 f 3) X 10-l2 cm3 molecule-' s-l. In addition, we report our spectrum of the CCl3 radical, found to have a banded structure on top of the intense, broad feature that peaks near 215 nm.
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Introduction
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constant is the best determined of this class of reactions:
The reactions of chlorine-containing radicals are receiving renewed scrutiny because of their role in a number of important environmental issues, including statospheric ozone depletion and the incineration and decomposition of chorine-containing polymeric materials.IJ For the chlorinated alkyl radicals (often referred to here as R) involved in these processes, the association reaction with oxygen molecules represents a key elementary step in their oxidation pathway. There is now a growing, but still limited, number of studies on this issue, including measurements of the rate constants for R 02 reactions as a function of temperature and pressure, as well as determinationsof the relevant equilibrium constants. For example, rate constants have been reported for the 0 2 recombination reactions with the fully halogenated species CF3,,v4 CC12F,S and CC13:6-8 most of this kinetic work has been carried out in the low-pressure regime (
