Department of Chemistry, Purdue University, West Lafayette, Indiana 47907-1393, United States.
J Am Chem Soc. 2011 Nov 16;133(45):18208-19. doi: 10.1021/ja204806b. Epub 2011 Oct 25.
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.
烷氧基自由基是传统和替代燃料在大气和燃烧氧化途径中的关键组成部分。准确描述它们随反应条件而变化的化学性质对于理解涉及碳氢化合物的大气和燃烧过程至关重要。烷氧基自由基及其反应的实验和理论数据都很稀缺，特别是对于较大链系统和高温条件。本工作使用 CBS-Q、G2 和 G4 复合计算方法研究了甲氧基到庚氧基自由基的所有单分子反应。在分析得到的热力学和动力学参数后，讨论了每个反应基团的相对重要性及其对碳氢化合物氧化反应途径中链分支的影响。然后将这些结果与烷基和过烷基自由基中的类似过程进行了比较。在这三个自由基体系中，如果发现有差异，就会提出可能的原因。特别值得注意的是，在低温下，1,6 H-迁移反应并不是烷氧基自由基中的主要途径，而在烷基和过烷基自由基中则是主要途径。然而，这些 H-迁移反应预计将在反应机制中发挥比以前认为的更大的作用，特别是在与大气相关的温度下。这将导致中间产物和最终产物的种类更加多样化，从而增加其他大气过程的复杂性，如气溶胶形成和对流层臭氧生成。目前的工作大大扩展了与新型燃料系统模型相关的烷氧基自由基的范围。基于这项研究的结果，提出了关于未来研究模型系统选择的建议。
