Alkoxy Radicals (RO)
Alkoxy radicals (RO) are central intermediates in the atmospheric oxidation of hydrocarbons. Numerous reaction pathways are available, shown in Figure 5: isomerization, dissociation, and hydrogen-atom abstraction by oxygen. None of the three pathways are radical-termination steps; isomerization and dissociation form alkyl radicals, whereas reaction with O2 propagates the cycle, forming HO2. In the case of β-hydroxy alkoxy radicals, formed in alkene oxidation, decomposition leads to the formation of α-hydroxy alkyl radicals, which react with O2 to form HO2; this reaction is far faster than RO + O2 reactions. This branching is complicated further by the fact that RO is formed chemically activated, so there is additional competition between collisional stabilization and chemical reaction, dependent on the exothermicity of the parent reaction (RO2 + NO versus RO2 + RO2). Therefore the branching ratio of the RO reactions, which play a major role in the rate of HOX cycling, may exhibit complex dependence on pressure, temperature, and structure of the parent hydrocarbon.
Reaction pathways of alkoxy radicals; shown here are reactions of the β-hydroxy-1-butoxy radical, formed in the atmospheric oxidation of 1-butene. Not shown is the additional complexity arising from chemical activation of the radical.
Most studies on alkoxy radical branching are indirect, relying on molecular product yield measurements [Atkinson, 1997b], or more recently, on the rate of OH regeneration [Reitz et al., 2002]. Direct measurement of alkoxy radicals and their reaction products, however, is the ideal approach to the measurement of these branching ratios.
In initial studies, RO radicals will be generated by photolysis of a precursor (RONO) in a sidearm, with SF6 for rapid thermalization. Concentration of RO radicals will be monitored as a function of time using CRDS detection axes. Studies will be carried out at low temperatures (150 K-200 K) so that reactions occur over measurable timescales. Measurements as a function of temperature and oxygen concentration will yield insight into the branching between the different pathways as well as the barriers for each. These may be compared against results from electronic structure/RRKM calculations. The simultaneous measurement of reaction products, namely HO2 and alkyl (or hydroxyalkyl) radicals, would also be of great benefit, as the individual pathways could all be monitored in isolation.
With this understanding of the reaction rates of thermal alkoxy radicals, the more complex behavior of RO radicals formed in hydrocarbon oxidation may be observed. As shown in a set of recent experiments [e.g., Orlando et al., 1998], formation of chemically activated RO leads to branching that may be dependent on a complex combination of pressure, temperature, O2 concentration, and R group identity. By observing RO formed in the oxidation of VOCs (not only alkanes and alkenes but also aldehydes, ketones, etc.) over a wide range of these conditions, these processes will be better understood.