Branching ratios in the hydroxyl reaction with propene

The branching ratios for the reactions of attachment of hydroxyl radical to propene and hydrogen-atom abstraction were measured at 298 K over the buffer gas pressure range 60-400 Torr (N(2)) using a subatmospheric pressure turbulent flow reactor coupled with a chemical ionization quadrupole mass spectrometer. Isotopically enriched water H(2)(18)O was used to produce (18)O-labeled hydroxyl radicals in reaction with fluorine atoms. The β-hydroxypropyl radicals formed in the attachment reactions 1a and 1b , OH + C(3)H(6) → CH(2)(OH)C(?)HCH(3) (eq 1a ) and OH + C(3)H(6) → C(?)H(2)CH(OH)CH(3) (eq 1b ), were converted to formaldehyde and acetaldehyde in a sequence of secondary reactions in O(2)- and NO-containing environment. The (18)O-labeling propagates to the final products, allowing determination of the branching ratio for the attachment channels of reaction 1. The measured branching ratio for attachment is β(1b) = k(1b)/(k(1a) + k(1b)) = 0.51 ± 0.03, independent of pressure over the 60-400 Torr pressure range. An upper limit on the hydrogen-abstraction channel, OH + C(3)H(6) → H(2)O + C(3)H(5) (eq 1c ), was determined by measuring the water yield in reactions of OH and OD radicals (produced via H(D) + NO(2) → OH(OD) + NO reactions) with C(3)H(6) as k(1c)/(k(1a) + k(1b) + k(1c)) < 0.05 (at 298 K, 200 Torr N(2)).     

Reactivity of peroxy radicals in the reaction with nickel bis(diisopropyldithiophosphate)

The rate constants of the reaction of peroxy radicals C₆H₅CH(00^.)R with nickel bis(diisopropyldithiophosphate) change as function of the nature of the substituent R in the order (75°C) H > CH₃ > iso-C₃H₇ > n-C₄H₉ > tert-C₄H₉; the order conforms with the decreasing order of Charton's steric constants of the substituents R.Translated from Teoreticheskaya i Eksperimental'naya Khimiya, Vol. 28, No. 3, pp. 236–238, May–June, 1992.     

Temperature dependence of pentyl nitrate formation from the reaction of pentyl peroxy radicals with NO

Alkyl nitrate yields from the reaction of 1-pentyl, 2-pentyl and 2-methyl-2-butyl peroxy radicals with NO have been determined over the temperature range (261-305 K) and at 1 bar pressure from the photo-oxidation of the iodoalkane precursors in air-NO mixtures. Yields were observed to increase with decreasing temperature and, contrary to previous observations, along the series primary < secondary congruent with tertiary. Our results suggests a significant temperature dependence for the formation of nitrates from the reaction of pentyl peroxy radicals with NO and represent an extension in the temperature range over which this reaction has been studied experimentally in the past.     

Studies on the role of peroxy and hydroperoxy groups in polymer photodegradation

Interactions of carbonyl singlet and triplet excited states with peroxides and hydroperoxides were examined in order to ascertain the reasons why the latter two compounds act as nearly diffusion controlled quenchers of ketone-containing polymer photodegradation. From singlet state fluorescence quenching and laser flash photolysis measurements of triplet decays, direct interaction with the carbonyl excited states was ruled out. These processes occurred with rate constants at least one to two orders of magnitude below the diffusion controlled limit. Trapping of the 1,4-biradical precursor to chain scission, thereby preventing its disproportionation was also tentatively eliminated. Results obtained using benzoyl peroxide suggest that quencher photodissociation into alkoxy radicals might represent the key step in the quenching of ketone-containing polymer photodegradation.     

Enthalpies of the formation of aliphatic carbonyl-containing radicals

A collection of data on enthalpies of the formation(Δ_f H ^o) of aliphatic carbonyl-containing radicals is analyzed and expanded. The Δ_f H ^o values for 29 carbonyl-containing radicals are determined for the first time, and are strongly revised for 17 carbonyl-containing radicals using the literature data on the dissociation energies of the bonds in molecules. The data is analyzed on the basis of the structureproperty (enthalpy of formation) relation within the additive-group approach, with the determination and specification of the parameters. It is concluded that the Δ_f H ^o values of carbonyl-containing radicals calculated from the obtained parameters (a total of 96 compounds was considered) agree well with the experimental data.     

Formation of alkyl nitrates from the reaction of branched and cyclic alkyl peroxy radicals with NO

The yields of C₅ and C₆ alkyl nitrates from neopentane, 2-methylbutane, 2-methylpentane, 3-methylpentane, and cyclohexane have been measured in irradiated CH₃ONONO-alkane-air mixtures at 298 ± 2 K and 735-torr total pressure. Additionally, OH radical rate constants for neopentyl nitrate, 3-nitro-2-methylbutane, 2-nitro-2-methylpentane, 2-nitro-3-methylpentane, and cyclohexyl nitrate, relative to that for n-butane, have been determined at 298 ± 2 K. Using a rate constant for the reaction of OH radicals with n-butane of 2.58 × 10^?12 cm³ molecule^?1 s^?1, these OH radical rate constants are (in units of 10^?12 cm³ molecule^?1 s^?1): neopentyl nitrate, 0.87 ± 0.21; cyclohexyl nitrate, 3.35 ± 0.36; 3-nitro-2-methylbutane, 1.75 ± 0.06; 2-nitro-2-methylpentane, 1.75 ± 0.22; and 2-nitro-3-methylpentane, 3.07 ± 0.08. After accounting for consumption of the alkyl nitrates by OH radical reaction and for the yields of the individual alkyl peroxy radicals formed in the reaction of OH radicals with the alkanes studied, the alkyl nitrate yields (which reflect the fraction of the individual RO₂ radicals reacting with NO to form RONO₂) determined were: neopentyl nitrate, 0.0513 ± 0.0053; cyclohexyl nitrate, 0.160 ± 0.015; 3-nitro-2-methylbutane, 0.109 ± 0.003; 2-nitro-2methylbutane, 0.0533 ± 0.0022; 2-nitro-2-methylpentane, 0.0350 ± 0.0096; 3- + 4-nitro-2-methylpentane, 0.165 ± 0.016; and 2-nitro-3-methylpentane, 0.140 ± 0.014. These results are discussed and compared with previous literature values for the alkyl nitrates formed from primary and secondary alkyl peroxy radicals generated from a series of n-alkanes.     

Mechanistic pathways for the reaction of quercetin with hydroperoxy radical

The extensive theoretical study of the interaction of one of the most abundant and reactive flavonols, quercetin, with hydroperoxy radical (HOO·), using the M052X/6-31 + Gd, p level of theory, was performed. Results indicating that quercetin is not a planar molecule are in accord with the X-ray analysis. The applied method successfully reproduces the bond dissociation enthalpy, and reveals that the reaction of quercetin with the hydroperoxy radical is governed by a hydrogen atom transfer mechanism. It is confirmed that the 3′OH and 4′OH are the most reactive sites, and that the reaction in the 3′OH position is faster than that in the 4′OH position.     

An ESR study of the decay reaction of isotactic polypropylene peroxy radicals in air

The decay of peroxy radicals trapped in irradiated isotactic polypropylene has been studied by ESR in air at various temperatures between 284 and 309 K. All the ESR spectra obtained at the various reaction stages are shown to be composed of two components arising from a mobile fraction and an immobile fraction. Only the mobile peroxy radicals decay; those belonging to the immobile fraction are stable. Various reaction mechanisms are examined in order to explain the experimental results; it is concluded that the decay reaction is controlled by diffusion of peroxy radicals and that the immobile peroxy radicals play no role in the decay reaction. Intermolecular hydrogen abstraction of the peroxy radicals, rather than intramolecular abstraction, is suggested as the rate-determining reaction.     

Tertiary aminomethylphenols and methylene bisphenols with isobornyl substituents in the reaction with diphenylpicrylhydrazyl and peroxy radicals in ethylbenzene

The effect of the structure of aminomethylphenols and methylene bisphenols with isobornyl substituents on their reactivity in interactions with peroxy radicals in ethylbenzene and with 1,1-diphenyl-2-picrylhydrazyl (DPPH) is studied. Isobornylphenols with o-aminomethyl substituents, as opposed to p-aminomethyl derivatives, were found to possess rather low activity in the initiated oxidation of ethylbenzene, due to the formation of intramolecular hydrogen bonds between the hydrogen atom of the OH group and the nitrogen atom of the aminomethyl substituent. An increase in activity of o-aminomethyl-substituted phenols with increasing polarity of the medium is observed in the reaction with DPPH. The reaction rate constants for the interaction between two isomeric 2,2′- and 4,4′-methylene-bisphenols having isobornyl moieties and ethylbenzene peroxy radicals are measured. The ratio between activities of the first and second OH groups in 2,2-methylene-bisphenol is shown to be close to 50.     

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.     

Determination of the rate constants of the reaction of pentaerythritol tetravalerate peroxy radicals with polyfunctional phosphites at high temperature

The effectiveness of a number of aromatic phosphites as inhibitors of the high-temperature autooxidation of pentaerythritol tetravalerate was studied, and the rate constants of the reaction of its peroxide radicals with aromatic phosphites at 200°C were determinedInstitute of Chemistry, Bashkir Science Center, Urals Branch of the Russian Academy of Sciences, 450054 Ufa. Institute of Organic Chemistry, Academy of Sciences of the Uk-raine, 252000 Kiev. Translated from Izvestiya Akademii Nauk, Seriya Khimicheskaya, No. 2, pp. 289–291, February, 1992.     

Intramolecular hydrogen bond in the hydroxycyclohexadienyl peroxy radicals

The hydroxycyclohexadienyl peroxy radicals (HO? C₆H₆? O₂) produced from the reaction of OH‐benzene adduct with O₂ were studied with density functional theory (DFT) calculations to determine their characteristics. The optimized geometries, vibrational frequencies, and total energies of 2‐hydroxycyclohexadienyl peroxy radical II_s and 4‐hydroxycyclohexadienyl peroxy radical III_s were calculated at the following theoretical levels, B3LYP/6‐31G(d), B3LYP/6‐311G(d,p), and B3LYP/6‐311+G(d,p). Both were shown to contain a red‐shifted intramolecular hydrogen bond (O? H … O? H bond). According to atoms‐in‐molecules (AIM) analysis, the intramolecular hydrogen bond in the 2‐hydroxycyclohexadienyl peroxy radical II_s is stronger than that one in 4‐hydroxycyclohexadienyl peroxy radical III_s, and the former is the most stable conformation among its isomers. Generally speaking, hydrogen bonding in these radicals plays an important role to make them more stable. Based on natural bond orbital (NBO) analysis, the stabilization energy between orbitals is the main factor to produce red‐shifted intramolecular hydrogen bond within these peroxy radicals. The hyperconjugative interactions can promote the transfer of some electron density to the O? H antibonding orbital, while the increased electron density in the O? H antibonding orbital leads to the elongation of the O? H bond and the red shift of the O? H stretching frequency. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007     

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.     

Hydrogen bonding affects the persistency of alkyl peroxy radicals

The effect of 1,1,1,3,3,3-hexafluoro-2-phenylpropan-2-ol (HFPP) on the persistency of sec-alkyl peroxy radicals has been investigated. The formation of a hydrogen-bonding complex between HFPP and the radical resulted in a large increase in the lifetime of the radical. This variation was accompanied by a decrease in the g-value. An enthalpy change of -3.4 kcal mol-1 was estimated for the formation of the hydrogen-bonded complex.     

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.     

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.