N-bromopropane

Energetics of the reaction of n-bromopropane with OH radical and Cl atom 237... 

Ab initio molecular orbital theory is utilized to study the hydrogen abstraction reaction of n-bromopropane with hydroxyl radical and chlorine atom. The stability of the trans and gauche isomers of n-bromopropane is explored. The potential energy surface of both reactions is characterized by pre- and post-reactive complexes, as well as transition state structures in both trans and gauche isomeric forms. The importance of these two reactions relies on the ultimate product distribution from both reactions. Differences in the reactivity of 1-bromopropane toward OH and Cl are observed. The reaction of n-bromopropane with OH radical favors the abstraction of hydrogen atoms while the reaction with Cl atoms favors the abstraction of hydrogen atoms at the a and p carbon sites. 

The potential energy surface of hydrogen absfracfions during the reaction of n-bromopropane with OH radicals and Cl atoms is explored in detail. 1-Bromopro-pane contains three different reactive sites from where hydrogen atoms can be abstracted a, /3, and y carbon atoms. The a carbon (primary) atom is defined as the carbon to which the substituent, in this case the bromine atom, is attached. The adjacent carbon atom is called the /3-carbon. The three resulting reaction channels proceed as follows, where X = OH/Cl,... 

In this study, we explored the stability of the n-bromopropane trans and gauche isomers by calculating the isomerization enthalpy at the levels of theory of interest. The results are summarized in Table 11.1. The values vary from —0.12 to... 

FIGURE 11.1 Geometries of n-bromopropane, primary, secondary, and tertiary alkyl radicals. Parameters are calculated at the MP2(full)/6-31G(d) level of theory. 

Figure 11.7B describes the energetics of the abstraction of hydrogen afoms from n-bromopropane by chlorine atoms. The as well as the aH abstraction reaction share the same reactant-like complexes, with binding energy of... 

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. 

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