Atmospheric Implications

Bromine-containing compounds have the potential to released Br atoms upon degradation in the atmosphere. Once in the stratosphere, Br atoms are about 45 times more effective than chlorine in destroying stratospheric ozone [22]. An important example of Br-containing compounds is 1-bromopropane, which is currently utilized as a industrial solvent and have been proposed as a replacement for CFCs, controlled under the agreements of fhe Montreal Protocol. 

The stability of n-bromopropane has also been explored by ah initio methods. The majority of the ab initio methods predicted that, between the trans and gauche bromopropane isomers, the trans isomer is the most stable. Only high-level calculations predict that the gauche form is the most stable, in accordance with experimental observation where an abundance of approximately 64% (gauche) and 36% (trans) have been determined of the two isomers. 

Results from this study confirms the experimental findings that the reaction of n-bromopropane with OH radicals should be slower than the reaction with Cl atoms. The present results show that pre- and post-reaction complexes are important in the hydrogen abstraction reactions. A detailed study of these rate constants that incorporates the contribution of pre-reactive complexes, the multi-chaimel nature of these reactions, and temperature dependence is necessary. The results also find that there are subtle reaction preferences for the abstraction of site specific hydrogen on n-bromopropane. While knowledge of the dominant products of the 

OH reaction is experimentally known, the information for the Cl reaction is new. For the OH reaction, the hydrogen atom is preferred while the reaction with Cl atoms both a and p hydrogen are favored. This suggests that the oxidation byproducts of n-bromopropane is going to be different for the OH versus Cl initiation chemistry. The results presented in this computational study should provide useful insight for modeling n-bromopropane global cycle in atmospheric reactivity models. 

The authors greatly thank Dr. Paul C. Redfern, Chemistry Division from Argonne National Laboratory, Argonne, IL 60439 for providing the Br atom basis sets and for assistance in making the G3 calculations possible. 

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The chemistry of the troposphere (the layer of the atmosphere closest to earth s surface) overlaps with low-temperature combustion, as one would expect for an oxidative environment. Consequently, the concerns of atmospheric chemistry overlap with those of combustion chemistry. Monks recently published a tutorial review of radical chemistry in the troposphere. Atkinson and Arey have compiled a thorough database of atmospheric degradation reactions of volatile organic compounds (VOCs), while Atkinson et al. have generated a database of reactions for several reactive species with atmospheric implications. Also, Sandler et al. have contributed to the Jet Propulsion Laboratory s extensive database of chemical kinetic and photochemical data. These reviews address reactions with atmospheric implications in far greater detail than is possible for the scope of this review. For our purposes, we can extend the low-temperature combustion reactions [Equations (4) and (5)], whereby peroxy radicals would have the capacity to react with prevalent atmospheric radicals, such as HO2, NO, NO2, and NO3 (the latter three of which are collectively known as NOy) ... 

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