Self-reactions of peroxy radicals

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

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. 

Self-reactions of peroxy radicals may occur in the atmosphere under low NO concentrations, particularly since the reactions are fast, with rate constants which... 

Self-reactions of peroxy radicals may occur in the atmosphere under low NO concentrations, particularly for the fastest cases, where rate constants may reach values as high as 10" cm molecule" s". In addition, self-reactions must be well characterised before studying other reactions of peroxy radicals, particularly those with HO2 and CH3O2. The general mechanism for RO2 self-reactions is the following ... 

Shallcross, D.E., M.T. Raventos-Duran, M.W. Bardwell, A. Bacak, Z. Solman, and C.J. Percival (2005), A semi-empirical correlation for the rate coefficients for cross- and self-reactions of peroxy radicals in the gas-phase, Atmos. Environ., 39, 763-771. 

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... 

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. 

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. 

Percent product distribution acetone 24.5 5.1, 2-methyl-2-propanol 18.8 4.0, 2-methyl-2-hydroperoxypropane 36.7 7.5, 2-methyl-propanal 14.0 3.9, 2-methyl-propanol 4.4 1.3, tertiary butylperoxide < 1.7. The peroxy radicals involved are primary 2-methyl-1-propylperoxy, primary methylperoxy and tertiary 2-methyl-2-propylperoxy. The relatively large yield of tertiary butanol is due to the interaction between CH3OO and tertiary butylperoxy radicals. Computer simulations based on the known rate coefficients for the self-reactions of these radicals [2] gave = 3 x 10" cm molecule s for the cross combination reaction. To simulate the observed ratio of primary alcohol and aldehyde requires a rate coefficient p 3 x 10" cm molecule s for the interaction between 2-methyl-1-propylperoxy and tertiary 2-methyl-2-propyl-peroxy radicals. The oxidation mechanism is quantitatively well understood. 

Atmospheric chemistry of peroxy radicals. Part 1. Thermal decomposition of peroxynitrates. Part 2. Self-reaction of aUcylperoxy radicals - a product study, in P.M. Borrell, P. Borrell, T. Cvita , W. Seiler (eds), Proc. EUROTRAC Symp. 92, SPB Academic PubL, The Hague 1993, pp. 394-398. 

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. 

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-... 

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. 

Only few data are available for self-reactions of secondary alkylperoxy radicals. One of the principal contributions of the project has been the study of the cyclo-alkylperoxy radicals C-C5H9O2 and c-CeUi 02 radicals. The corresponding rate constants are 40 times smaller than the only linear secondary peroxy radical studied so far, /-C3H7O2, and exhibit a slight positive temperature dependence. More data would be necessary for a better description of such reactions but, due to their relatively low rate constants (< 5 x lO" " cm molecule s ), their contribution to atmospheric chemistry is negligible. 

The work performed within the duration of the LACTOZ subproject has allowed laboratory data to be obtained on the UV absorption spectra, and kinetics and products of the self-reactions and reactions with HO2 for a range of peroxy radicals. Some additional kinetic and mechanistic information related to other aspects of VOC degradation has also been obtained, particularly concerning the behaviour of some of the "oxy" radicals (RO) formed from reactions of the peroxy radicals. Much of this work has appeared in the open literature [1-12]. 

Self-reaction rate coefficients (kj) and branching ratios (a) obtained in this laboratory for a series of peroxy radicals, are presented in Table 2, where a is defined as the fraction of the reaction proceeding via the propagating channel, i.e. ksJks ... 

Kinetic studies of peroxy radicals The self-reaction CH3O2 + CH3O2... 

Percent product distribution propanal 50.6 1.2, 2-hydroxy-butanal 7.0 0.3, l-hydroxy-butan-2-one 24.2 0.7, 1,2 dihydroxybutane 18.2 0.7. Computer assisted analysis of the product distribution showed that addition of the OH radical occurs to 26 % at the inner and to 74% at the outer position of the double bond. These reactions produced the corresponding primary and secondary hydroxy-alkylperoxy radicals. The branching ratio for the radical propagating channel of the self-reaction of the secondary peroxy radicals was determined to be issa/ iss = 0.75 0.02 28 % of the hydroxy-alkoxyl radical thus formed reacted with oxygen to produce hydroxyketone. If it is assumed that the rate coefficient for the reaction of the hydroxy-alkoxyl radical with oxygen is 8 x 10 cm molecule s the rate coefficient for the decomposition of this radical to produce propanal is 1 x 10 s V... 

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