Reactivity Trends within Alkoxy Radical Reactions Responsible for Chain Branching

Alkoxy radicals are a key component in both the atmospheric and combustion oxidation pathways of traditional and alternative fuels. An accurate description of their chemistry as it alters with reaction conditions is essential to understanding atmospheric and combustion processes involving hydrocarbons. Experimental and theoretical data on alkoxy radicals and their reactions are scarce, especially for larger chain systems and high temperatures. The present work investigates all unimolecular reactions of the methoxy through heptoxy radicals using the CBS-Q, G2, and G4 composite computational methods. After analysis of the resulting thermodynamic and kinetic parameters, discussions about the relative importance of each reaction group and their effects on chain branching in the oxidation reaction pathways of hydrocarbons are presented. These results are then compared to similar processes in alkyl and alkylperoxy radicals. Where discrepancies are found among these three radical systems, discussions about possible causes are presented. Of particular interest is the observation that 1,6 H-migration reactions are not the dominant pathway in alkoxy radicals, as they are in both alkyl and alkylperoxy radicals, at low temperatures. However, these H-migrations are expected to play a larger role in reaction mechanisms than previously believed, particularly at atmospherically relevant temperatures. This will lead to greater diversity in the intermediate and end product species, which will in turn add complexity to other atmospheric processes, such as aerosol formation and tropospheric ozone production. The current work significantly extends the range of alkoxy radicals that are relevant to models for new fuel systems. Based on the results of this study, recommendations regarding the selection of model systems for future studies are presented.