Peroxy radical self-reactions

The overall reaction sequence leading to CO2 formation, through the HCHO and CO intermediate "stable products, is shown in Figure 5.2. When NO, levels are sufficiently high that reaction of the peroxy radicals HO2 and CH3O2 with NO predominates over peroxy radical self-reactions, the methane oxidation chain depicted in Figure 5.2 can be written as... 

Table 6 collects the available HCFC-based peroxy radical self-reaction rate constants. There are two important points to keep in mind regarding this table. The first is that the peroxy radical self-reaction proceeds by two major channels ... 

TABLE 6 HFC- and HCFC-Based Peroxy Radical Self-Reactions... 

The data in Table 6 suggest a few general trends. The room-temperature HCFC-based peroxy radical self-reaction rate constants all lie in the rather... 

The results of LACTOZ have provided an extended kinetic data base for the following classes of reactions reactions of OH with VOCs, reactions of NO3 with VOCs and peroxy radicals, reactions of O3 with alkenes, reactions of peroxy radicals (self reactions, reaction with HO2, other RO2, NO, NO2), reactions of alkoxy radicals (reactions with O2, decomposition, isomerisation), thermal decomposition of peroxynitrates. Photolysis parameters (absorption cross-section, quantum yields) have been refined or obtained for the first time for species which photolyse in the troposphere. Significantly new mechanistic information has also been obtained for the oxidation of aromatic compounds and biogenic compounds (especially isoprene). These different data allow the rates of the processes involved to be modelled, especially the ozone production from the oxidation of hydrocarbons. The data from LACTOZ are summarised in the tables given in this report and have been used in evaluations of chemical data for atmospheric chemistry conducted by international evaluation groups of NASA and lUPAC. 

We have confirmed that the rate constant of the f-butyl peroxy radical self-reaction is very slow, with a fairly high activation energy. However, as above, the effect of functional groups on the rate constant should be investigated. 

It is possible to rationalise the first trend in terms of steric effects associated with the increasing alkyl chain length and branching. The same trend is observed in the peroxy radical self-reaction kinetics. The second trend is more difficult to explain but may reflect a decrease in the RO-O bond strength caused by the electron withdrawing halogen atom. Ab initio theoretical studies are needed to shed further light on the reactivity trends. 

However in low NOx conditions peroxy radicals primarily react through self and cross peroxy-peroxy reactions to form methyl hydrogen peroxide (CH3OOH) and hydrogen peroxide (H202). H02 is also recycled back to OH through the reaction with 03 (Reaction 9). 

Determination of Rate Constants for the Self-Reactions of Peroxy Radicals by Electron Spin Resonance Spectroscopy... 

Rate constants for the self-reactions of a number of tertiary and secondary peroxy radicals have been determined by electron spin resonance spectroscopy. The pre-exponential factors for these reactions are in the normal range for bi-molecular radical-radical reactions (109 to 1011 M"1 sec 1). Differences in the rate constants for different peroxy radicals arise primarily from differences in the activation energies of their self reactions. These activation energies can be large for some tertiary peroxy radicals (—10 kcal. per mole). The significance of these results as they relate to the mechanism of the self reactions of tertiary and secondary peroxy radicals is discussed. Rate constants for chain termination in oxidizing hydrocarbons are summarized. 

There is excellent agreement between the decay constants obtained by ceric ion oxidation of secondary hydroperoxides and the rate constants for chain termination in hydrocarbon autoxidation determined by the rotating sector. The agreement suggests that secondary peroxy radicals do not undergo many nonterminating interactions, so that most self-reactions of secondary peroxy radicals must be chain terminating. 

A number of critical questions require additional study before the details of the self-reactions of peroxy radicals can be specified with confidence. More precise values of absolute rate constants and their temperature coefficients for a variety of radicals under various experimental conditions are required. 

Figure 6.31 shows plots of measured peroxy radical concentrations for low-NOx conditions at Cape Grim, Tasmania (Penkett et al., 1997). Overlaid are plots of 7(0 D) and /(O D) 05. The plot of /(O D) 05 provides a better match. The slower decay in the peroxy radical concentration at dusk is due to the slow decay due to self-reactions, with some contribution from the CH302 + O, reaction (k 1 X 10"17 cnr1 molecule 1 s 1 Tyndall et al., 1998) and perhaps a small contribution from deposition (Monks et al., 1996). 

Mention has already been made of the relatively small reactivity of allyl peroxy radicals compared with other alkyl peroxy radicals. Jost (88, 96) has reasoned that paraffins react by a small number of long chains, whereas olefins oxidize by a large number of short chains. Olefins are thus attacked more readily than paraffins but form less reactive allyl radicals. In addition, during oxidation chain transfer occurs in which alkyl radicals are replaced by allyl radicals. Shorter chains would then be expected. Comparison of the precombustion products of iso-octane and diisobutylene (154) indicates that marked self-inhibition of reaction chains was occurring in the latter case. 

The mechanisms behind lipid oxidation of foods has been the subject of many research projects. One reaction in particular, autoxida-tion, is consistently believed to be the major source of lipid oxidation in foods (Fennema, 1993). Autoxidation involves self-catalytic reactions with molecular oxygen in which free radicals are formed from unsaturated fatty acids (initiation), followed by reaction with oxygen to form peroxy radicals (propagation), and terminated by reactions with other unsaturated molecules to form hydroperoxides (termination O Connor and O Brien, 1994). Additionally, enzymes inherent in the food system can contribute to lipid oxidization. 

Han P, Bartels DM (1994) Encounters of H and D atoms with 02 in water relative diffusion and reaction rates. In Gauduel Y, Rossky P (eds) AIP conference proceedings 298. "Ultrafast reaction dynamics and solvent effects." AIP Press, New York, 72 pp Hasegawa K, Patterson LK (1978) Pulse radiolysis studies in model lipid systems formation and behavior of peroxy radicals in fatty acids. Photochem Photobiol 28 817-823 Herdener M, Heigold S, Saran M, Bauer G (2000) Target cell-derived superoxide anions cause efficiency and selectivity of intercellular induction of apoptosis. Free Rad Biol Med 29 1260-1271 Hildenbrand K, Schulte-Frohlinde D (1997) Time-resolved EPR studies on the reaction rates of peroxyl radicals of polyfacrylic acid) and of calf thymus DNA with glutathione. Re-examination of a rate constant for DNA. Int J Radiat Biol 71 377-385 Howard JA (1978) Self-reactions of alkylperoxy radicals in solution (1). In Pryor WA(ed) Organic free radicals. ACS Symp Ser 69 413-432... 

Lee S-H, Mendenhall GD (1988) Relative yields of excited ketones from self-reactions of alkoxyl and alkylperoxyl radical pairs. J Am Chem Soc 110 4318-4323 Leitzke A, Reisz E, Flyunt R, von Sonntag C (2001) The reaction of ozone with cinnamic acids - formation and decay of 2-hydroperoxy-2-hydroxy-acetic acid. J Chem Soc Perkin Trans 2 793-797 Lodhi ZH, Walker RW (1991) Oxidation of allyl radicals kinetic parameters for the reactions of allyl radicals with H02 and 02 between 400 and 480 °C. J Chem Soc Faraday Trans 87 2361-2365 Martini M, Termini J (1997) Peroxy radical oxidation of thymidine. Chem Res Toxicol 10 234-241... 

Although this mechanism appears to be a very attractive explanation for the observed low activation energy and high value of the rate constant of self-reaction of secondary peroxy radicals, it is not out of the question that ionic or at least very polar reaction steps specific to a condensed system are also involved. 

There exists another pathway of self-reaction of two secondary or primary alkyl peroxy radicals which is even more favorable from the viewpoint of exothermicity than it is Russell s mechanism. It is the reaction in which two molecules of ketone and hydrogen peroxide (or hydrogen and oxygen) are formed as follows ... 

The reactivity of peroxy radicals towards other peroxy radicals varies over many orders of magnitude depending on the nature of the R group. Consequently, it is not possible to provide a simple accounting of the importance of peroxy self and cross reactions compared to the other possible loss mech-... 

Laboratory studies of the autooxidation of hydrocarbons (Ingold, 1969 Howard, 1971) have revealed that the self-reactions of alkylperoxy radicals among each other are slow with rate coefficients generally smaller than that for the CH302+CH302 reaction. These reactions need not be considered in the atmosphere. A possible exception may be the reaction of ROO radicals with CH302, which is the most abundant peroxy radical in the atmosphere. 

Under some experimental conditions, including low hydrocarbon concentration and high rate of radical formation, the interaction of methyl-peroxy and cumylperoxy radicals rather than self-reaction of cumyl-peroxy radicals accounts for most of the terminating interactions. The competing reaction for methylperoxy is abstraction from cumene, viz. 

Self-Reaction Kinetics. Of all peroxy radical reactions, the self-reaction between two identical peroxy radicals is perhaps the most studied. The measurement of peroxy radical UV absorption cross sections, discussed above, often occurs under the assumption that all the chlorine or fluorine atoms produced by photolysis are converted quantitatively into peroxy radicals however, this assumption must be corrected for by the loss of peroxy radicals from self-reaction. Furthermore, studies of RO2 -b NO or RO2 -f HO2 reactions usually take place at sufficiently high RO2 concentrations to require knowledge of the self-reaction rate constant, in order to interpret the results of the kinetics measurements. Both concerns make laboratory studies of peroxy self-reaction kinetics an important issue. In contrast, the steady-state atmospheric concentrations of HCFC-based peroxy radicals are probably too small for their self-reactions to be relevant to atmospheric chemistry. In this context, the most important peroxy-peroxy radical reactions would be between the HCFC-based peroxy radicals and CH3O2, but such reactions have not been studied to date. 

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