Kinetics and mechanism of reactions between aromatic olefins and hydroxyl radicals

The rates of the reactions of hydroxyl radicals (OH) with styrene, α-methylstyrene, and β-methylstyrene have been measured by irradiating mixtures of these aromatic olefins and NO in an environmental chamber at 298 K. Experimental conditions were used whereby the competition of ozone with OH in oxidizing the hydrocarbons could be considered negligible. The rate constant values, obtained by a relative method using isooctane as reference hydrocarbon, are: styrene (5.3 ± 0.5) × 10^?11 cm³/molec·s, α-methylstyrene (5.3 ± 0.6) × 10^?11 cm³/molec·s, and β-methylstyrene (6.0 ± 0.6) × 10^?11 cm³/molec·s. A simplified kinetic treatment of the experimental data shows that styrene and β-methylstyrene are stoichiometrically converted to benzaldehyde, suggesting that OH attack occurs only on the aliphatic moiety of the aromatic olefins. Benzaldehyde was observed to undergo consecutive oxidation by OH, and its maximum formation yield was about 60%. A reaction mechanism is proposed where the primary rate-determining OH attack leads to the formation of 1-hydroxy-2-phenyl-2-ethenyl radicals, from which benzaldehyde is formed through fast intermediate reactions.     

The multidipole effect in reactions of polyfunctional peroxy radicals

Conclusions The rate constant of the reaction of peroxide radicals of acrylic and methacrylic esters of polyols with nonpolar cumene is not dependent on the number of ester groups, and with polar 2,4,6-tri-tert-butylphenol decreases with an increase in the number of ester groups in the peroxide radical. Both findings are in agreement with the model of a dipole-dipole interaction in reactions of polyfunctional compounds.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 4, pp. 778–781, April, 1985.     

Kinetics and thermodynamics of the exchange reactions of peroxy radicals with hydroperoxides

The enthalpies and equilibrium constants of the exchange reactions of peroxy radicals with hydroperoxides of various structures are calculated. The experimental data on the reactions of hydrogen atom abstraction by the peroxy radicals from the hydroperoxides are analyzed, and the kinetic parameters characterizing these reactions are calculated using the intersecting parabolas method. The activation energies and rate constants for nine reactions of H atom abstraction by a peroxy radical from the OOH group of a peroxide are calculated using the above parameters. The geometric parameters of the transition states for the reactions are calculated. The low triplet repulsion plays an important role in the fast occurrence of the reactions. The polar interaction in the transition state is manifested in the reactions of the peroxy radicals with hydroperoxides containing a polar group.     

Fluorocarbon oxy and peroxy radicals

This review deals with the most fundamental fluorocarbon oxy (x=1) and peroxy (x=2) radicals CF₃O_x, FC(O)O_x, CF₃C(O)O_x and CF₃OC(O)O_x. Their role in atmospheric and synthetic chemistry is described as well as their formation and characterisation in the gas phase and isolated in low temperature matrices. Reservoirs and hence thermal sources for oxy and peroxy radicals are peroxides (ROOR) and peroxynitrates (ROONO₂), while catenated trioxides (ROOOR) are precursors for both oxy and peroxy radicals simultaneously. The synthesis and some properties of these precursors are also described.     

The structure and spectra of organic peroxy radicals

Recent spectroscopic and computational work on organic peroxy radicals, RO(2), is reviewed and extended with an emphasis on radicals where R is an alkyl group. Detailed experimental spectral, structural relationships are developed and show the dependence of spectral properties on the number of carbon atoms in the radical, and its isomeric and conformeric structure. These empirical relationships are explored and rationalized with the help of a series of quantum chemistry calculations, which are in turn benchmarked by the experimental data. The application of the spectra as a diagnostic for sensitive and selective measurement of radical concentrations for different RO(2) species in an isomer- and conformer-specific manner is described. Future areas of research including investigation of additional peroxy species and high resolution spectra of cold radicals are discussed.     

Reactivity of alkylperoxyl radicals in reactions with aromatic nitrons

Interaction of tetradecylperoxyl radicals with two aromatic nitrons has been studied.

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Generation and decay of peroxy radicals in deformed polyethylene

The rate of peroxy radical accumulation as a function of strain at various temperatures in AC1220 high molecular weight polyethylene has been determined by EPR spectroscopy. The results of isothermal radical decay experiments are used, where appropriate, to correct the apparent accumulation rate to the actual rate. An exponential dependence of radical concentration [R], on true strain is observed at all temperatures investigated in the range from 160 to 294°K. For constant effective strain, measured from the approximate strain at which radical accumulation initiates, it is found that d[R]/de exhibits two sharp transitions as a function of temperature. One of these, at low temperature, is believed to be associated with the glass transition of the amorphous phase of the material; the other, at higher temperature, is believed to occur as a result of a change in the rate-controlling mechanism of deformation.     

Kinetics study of the aromatic bicyclic peroxy radical + NO reaction: overall rate constant and nitrate product yield measurements

The measurements of the overall bicyclic peroxy radical + NO rate constant for the 1,3,5-trimethylbenzene (1,3,5-TMB) system and of the nitrate product yields for the benzene, toluene, p-xylene, and 1,3,5-TMB systems were performed via the turbulent flow chemical ionization mass spectrometry technique. While the overall rate constant was found to be consistent with the value used in the most detailed aromatic oxidation kinetic model (Master Chemical Mechanism, MCM), the nitrate product yields were found to be generally lower than predicted by the MCM and to have a different aromatic species-specific dependence than the MCM predicts.     

Solvent effect on reversible self-termination reactions of aromatic free radicals

Rates and thermodynamic data have been obtained for the reversible self-termination reaction: Involving aromatic 2-(4′dimethylaminophenyl)indandione-1,3-yl (I), 2-(4′diphenylaminophenyl)indandione-1,3-yl (II), and 2,6 di-tert-butyl-4-(β-phthalylvinyl)-phenoxyl (III) radicals in different solvents. The type of solvent does not tangibly affect the 2k₁ of Radical(I), obviously due to a compensation effect. The log(2k₁) versus solvent parameter E_T(30) curves for the recombination of radicals (II) and (III) have been found to be V shaped, the minimum corresponding to chloroform. The intensive solvation of Radical (II) by chloroform converts the initially diffusion-controlled recombination of the radical into an activated reaction. The log (2k_?1) of the dimer of Radical (I) has been found to be a linear function of the Kirkwood parameter (ε - 1)/(2ε + 1), the dissociation rate increasing with the dielectic constant of the solvent. The investigation revealed an isokinetic relationship for the decay of the dimer of Radical (I), an isokinetic temperature β = 408 K and isoequilibrium relationship for the reversible recombination of Radical (I) with β° = 651 K. For Radical (I) dimer decay In(2k_?1) = const + 0.8 In K, where K is the equilibrium constant of this reversible reaction. The transition state of Radical (I) dimer dissociation reaction looks more like a pair of radicals than the initial dimer. The role of specific solvation in radical self-termination reactions is discussed.     

The mechanism of spin polarization in aromatic free radicals

 The spin-polarization mechanism in aromatic systems is analyzed with reference to the prototypical phenoxyl, cyclohexadienyl and benzyl radicals. In particular, a decomposition into “first-order” and “second-order” contributions is proposed, which helps to rationalize the different nature of the spin density for atoms in α or in β positions with respect to the radical center. The different weights of the two contributions are discussed on the basis of Hartree–Fock and density functional computations. Received: 17 September 1999 / Accepted: 3 February 2000 / Published online: 29 June 2000     

Ambiguous reactivity of hydroxyperoxide radicals in reactions with aromatic amines

Conclusions It was experimentally shown that hydroxyperoxide radicals reduce diphenyl nitrogenous radicals to diphenylamlne. An explanation of the phenomenon of multiple chain terminations by aromatic amine molecules in the alcohols which are being oxidized was given.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 12, pp. 2764–2766, December, 1967.     

Reactions of alkoxy and peroxy radicals with carbon monoxide

Experimental rate constants of the reactions HO^· + CO → H^· + CO₂, RO^· + CO → R^· + CO₂, HO ₂ ^· + CO → HO^· + CO₂, and RO ₂ ^· + CO → RO^· + CO₂ are analyzed in the framework of the intersecting-parabolas model. The transition states of the additions of the methoxy and methylperoxy radicals to carbon monoxide were calculated by quantum-chemical methods. The reactions occur in two consecutive steps: first the HO^· (RO^·, RO ₂ ^· ) radical adds to CO and then the resulting unstable intermediate radical decomposes to evolve CO₂. The kinetic parameters of these reactions are calculated by two methods (using the intersecting-parabolas model and the quantum-chemical method). The activation energies and rate constants of a series of R _i O^· + CO and R _i O ₂ ^· + CO reactions are calculated. A comparison of the kinetic parameters suggests close similarity between the transition states in the additions of the O-centered radicals to CO and olefins.     

Recombination of polyatomic ester peroxy radicals in solution

Termination rate constants of peroxy radicals of seven polyatomic esters in benzene and perflouorooctane media have been measured by pulse photolysis. Recombination of peroxy radicals for all esters examined proves to be diffusion controlled. Reactivity of peroxy radicals in the termination reaction grows with increasing number of ester groups.

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The kinetics of the interaction of peroxy radicals. II. Primary and secondary alkyl peroxy

A chain mechanism is proposed to account for the very rapid termination reactions observed between alkyl peroxy radicals containing α-C—H bonds which are from 10⁴ to 10⁶ faster than the termination of tertiary alkyl peroxy radicals. The new mechanism is with termination by . \documentclass{article}\pagestyle{empty}\begin{document}$ {\rm R}\overline {{\rm CHOO}} $\end{document} is the zwitterion originally postulated by Criegee to account for the chemistry of O₃-olefin addition. Heats of formation are estimated for \documentclass{article}\pagestyle{empty}\begin{document}$ \overline {{\rm CH}_2 {\rm OO,}} {\rm }\overline {{\rm RCHOO}} $\end{document}, and \documentclass{article}\pagestyle{empty}\begin{document}$ ({\rm C}\overline {{\rm H}_3 )_2 {\rm COO}} $\end{document} and it is shown that all steps in the mechanism are exothermic. The second step can account for (¹Δ)O₂ which has been observed. k₁ is estimated to be 10^9–2/θ liter/M sec where θ = 2.303RT in kcal/mole. The second and third steps constitute a chain termination process where chain length is estimated at from 2 to 10. This mechanism for the first time accounts for minor products such as acid and ROOH found in termination reactions. Trioxide (step 3) is shown to be important below 30°C or in very short time observations (<10 s at 30°C). Solvent effects are also shown to be compatible with the new mechanism.     

Hydroxy peroxy nitrites and nitrates from OH initiated reactions of isoprene

The reaction of hydroxy peroxy radicals (RO(2)) with NO represents one of the most crucial tropospheric processes, leading to terrestrial ozone formation or NO(x)() removal and chain termination. We investigate the formation of hydroxy peroxy nitrites (ROONO) and nitrates (RONO(2)) from the OH-isoprene reactions using DFT and ab initio theories and variational RRKM/master equation (vRRKM/ME) formalism. The binding energies of ROONO from NO addition to RO(2) are determined to be in the range of 20-22 kcal mol(-)(1), and the bond dissociation energies of ROONO to form an alkoxy radical (RO) and NO(2) range from 6 to 9 kcal mol(-)(1). Isomerization of ROONO to RONO(2) is exothermic by 22-28 kcal mol(-)(1). The entrance and exit channels of the RO(2)-NO reaction are found to be barrierless, and the rate constants to form ROONO are calculated to be 3 x 10(-)(12) to 2 x 10(-)(11) cm(3) molecule(-)(1) s(-)(1) using the canonical variational transition state theory. The vRRKM/ME analysis reveals negligible stabilization of excited ROONO and provides an assessment of ROONO isomerization to RONO(2).