Mechanisms of Reactions of the RO Radicals

Abstract

The atmospheric chemistry of alkoxy radicals determines the first-generation oxidation products of organic compounds in the atmosphere. There are three competing fates for alkoxy radicals: reaction with molecular oxygen (O2), isomerization, and decomposition (Atkinson and Arey, 2003b; Devolder, 2003; Orlando et al., 2003b; Calvert et al., 2008). Reaction with O2 preserves the carbon chain of the parent alkane and results in the production of a carbonyl compound and HO2. Unimolecular decomposition usually results in the formation of an alkyl radical and a carbonyl compound with a shortening of the carbon chain. Unimolecular isomerization usually leads to multifunctional oxidation products (e.g., 1,4-hydroxycarbonyls and 1,4-hydroxynitrates) and a preservation of the carbon chain. These potentially competing pathways are illustrated in Figure VI-A-1 for the 2-pentoxy radical: Absolute rate coefficients for these processes have been obtained for only a few of the smaller alkoxy radicals. For example, rate coefficients have been firmly established only over a range of temperatures for reaction of a subset of the C1–C6 alkoxy radicals with O2; dissociation rate coefficients have only been directly measured for ethoxy, 2-propoxy, 2-butoxy, and tert-butoxy radicals (Balla et al., 1985; Blitz et al., 1999; Caralp et al., 1999; Devolder et al., 1999; Fittschen et al., 1999, 2000; Falgayrac et al., 2004); and no direct measurement of isomerization rates have been reported to date. A large portion of the database describing the atmospheric behavior of alkoxy radicals has been built up primarily from two sources: (1) environmental chamber experiments, where end-product distributions observed under atmospheric conditions have been used to infer relative rates of competing alkoxy radical reactions (e.g., Carter et al., 1976; Cox et al., 1981; Niki et al., 1981a; Eberhard et al., 1995; Aschmann et al., 1997; Orlando et al., 2000a; Cassanelli et al., 2006); and (2) from theoretical methodologies that lend themselves well to the study of unimolecular processes (e.g., Somnitz and Zellner, 2000a, 2000b, 2000c; Mereau et al., 2000a, 2000b; Fittschen et al., 2000; Lin and Ho, 2002; Mereau et al., 2003; Davis and Francisco, 2011). An overview of these three classes of competing alkoxy radical reactions (reaction with O2, unimolecular decomposition, and isomerization) is given in this section.