Radical Decomposition Research Articles

Formation of a Criegee intermediate in the low-temperature oxidation of dimethyl sulfoxide

Dimethyl sulfoxide (DMSO) is the major sulfur-containing constituent of the Marine Boundary Layer. It is a significant source of H2SO4 aerosol/particles and methane sulfonic acid via atmospheric oxidation processes, where the mechanism is not established. In this study, several new, low-temperature pathways are revealed in the oxidation of DMSO using CBS-QB3 and G3MP2 multilevel and B3LYP hybrid density functional quantum chemical methods. Unlike analogous hydrocarbon peroxy radicals the chemically activated DMSO peroxy radical, [CH3S(=O)CH2OO*]*, predominantly undergoes simple dissociation to a methylsulfinyl radical CH3S*(=O) and a Criegee intermediate, CH2OO, with the barrier to dissociation 11.3 kcal mol(-1) below the energy of the CH3S(=O)CH2* + O2 reactants. The well depth for addition of O2 to the CH3S(=O)CH2 precursor radical is 29.6 kcal mol(-1) at the CBS-QB3 level of theory. We believe that this reaction may serve an important role in atmospheric photochemical and irradiated biological (oxygen-rich) media where formation of initial radicals is facilitated even at lower temperatures. The Criegee intermediate (carbonyl oxide, peroxymethylene) and sulfinyl radical can further decompose, resulting in additional chain branching. A second reaction channel important for oxidation processes includes formation (via intramolecular H atom transfer) and further decomposition of hydroperoxide methylsulfoxide radical, *CH2S(=O)CH2OOH over a low barrier of activation. The initial H-transfer reaction is similar and common in analogous hydrocarbon radical + O2 reactions; but the subsequent very low (3-6 kcal mol(-1)) barrier (14 kcal mol(-1) below the initial reagents) to beta-scission products is not common in HC systems. The low energy reaction of the hydroperoxide radical is a beta-scission elimination of *CH2S(=O)CH2OOH into the CH2=S=O + CH2O + *OH product set. This beta-scission barrier is low, because of the delocalization of the *CH2 radical center through the -S(=O) group, to the -CH2OOH fragment in the transition state structure. The hydroperoxide methylsulfoxide radical can also decompose via a second reaction channel of intramolecular OH migration, yielding formaldehyde and a sulfur-centered hydroxymethylsulfinyl radical HOCH2S*(=O). The barrier of activation relative to initial reagents is 4.2 kcal mol(-1). Heats of formation for DMSO, DMSO carbon-centered radical and Criegee intermediate are evaluated at 298 K as -35.97 +/- 0.05, 13.0 +/- 0.2 and 25.3 +/- 0.7 kcal mol(-1) respectively using isodesmic reaction analysis. The [CH3S*(=O) + CH2OO] product set is shown to form a van der Waals complex that results in O-atom transfer reaction and the formation of new products CH3SO2* radical and CH2O. Proper orientation of the Criegee intermediate and methylsulfinyl radical, as a pre-stabilized pre-reaction complex, assist the process. The DMSO radical reaction is also compared to that of acetonyl radical.

Thermochemistry and kinetics of acetonylperoxy radical isomerisation and decomposition: a quantum chemistry and CVT/SCT approach

CBS-QB3 calculations have been used to determine thermochemical and kinetic parameters of the isomerisation and decomposition reactions of the acetonylperoxy radical, CH3C(O)CH2OO* , which has been formed via the reaction of acetonyl radical with O2 leading to the formation of an energised peroxy adduct with a calculated well depth of near 111 kJ mol(-1). This species can undergo subsequent 1,5 and 1,3 H-shifts to give the primary and secondary radicals: C*H2C(O)CH2OOH and CH3C(O)C*HOOH, respectively, or rearrange to give a 3-methyl-1,2-dioxetan-3-yloxy radical. Rate constants for isomerisation and subsequent decomposition have been estimated using canonical variational transition state theory with small curvature tunneling cvt/sct. The variational effect for the isomerisation channels is only moderate but the tunneling correction is significant at temperatures up to 1000 K; the formation of a primary radical by a 1,5-shift is the main reaction channel and the competition with the secondary one starts only at around 1500 K. The fate of the primary acetonylhydroperoxy radical is predominantly to form oxetan-3-one while the ketene and 1-oxy-3-hydroxyacetonyl radical channels only compete with the formation of oxetan-3-one at temperatures >1200 K. In addition, consistent and reliable enthalpies of formation have been computed for the molecules acetonylhydroperoxide, 1,3-dihydroxyacetone, methylglyoxal and cyclobutanone, and for some related radicals.

Secondary decomposition of C3H5 radicals formed by the photodissociation of 2-bromopropene

The photodissociation of 2-bromopropene at 193 nm produces C(3)H(5) radicals with a distribution of internal energies that spans the threshold for the secondary decomposition of the 2-propenyl radicals into C(3)H(4)+H. Just above this threshold, the decomposition rate is on the nanosecond time scale, and in the present study, time-resolved velocity-map ion imaging is used to gain insight into this process. The results provide information on the energy dependence of the secondary dissociation process. In addition, comparison of the results with theoretical predictions of the energy dependence of the dissociation rate provides information on the branching between fragment rotational and vibrational energies in the primary photodissociation process.

Modification of Multi-Walled Carbon Nanotubes by Diels-Alder and Sandmeyer Reactions

Random (L) and aligned (A) multi-walled carbon nanotubes (MWNTs) were modified by Diels-Alder (DA) [4+2] cycloaddition, Sandmeyer (SM) reaction and by catalytic oxidation (OX). The properties of modified carbon nanotubes were studied by dispersability tests, elemental analysis, thermogravimetry/mass spectrometry, X-ray photoelectron spectroscopy, and NMR spectroscopy. The cycloaddition reaction could only be successfully performed with the L-MWNTs in molten and in solution state by using an aluminum chloride homogeneous catalyst. The efficiency and thermal stability of the solution phase cycloaddition were much higher than in the case of modification in the molten phase. The functionalization of both types of MWNTs by Sandmeyer reaction was carried out by copper(I) and iron(ll) ions that helped in the radical decomposition of diazonium salts. Successful functionalization of nanotubes is achieved by a long decomposition time of the thermally activated diazonium salts. To the contrary, in the case of radical decomposition of diazonium salts, the time is not a decisive parameter. The dispersability tests have proved the changes in the physical features of modified carbon nanotubes depending on the hydrophobic and hydrophilic character of the solvents. The presence of the modifying groups and their fragments from the functionalized MWNTs has been demonstrated by thermogravimetry/mass spectrometry (TG/MS). Relatively high concentration of sulfur atoms was detected by X-ray photoelectron spectroscopy in nanotubes modified by sulfur substituent groups. In the case of catalytic oxidation, the X-ray photoelectron spectroscopic signal of oxygen bound to nanotubes showed considerable change as compared to pristine nanotubes. Due to the high thermal stability of modified multi-walled carbon nanotubes, the functionalized derivatives are applicable in several industrial fields.

Ethylene Polymerization under Mild Conditions Using Simple Aluminum Alkyls and Chloro Activators

Methylaluminum sesquichloride (Al2Cl3Me3, MAS), in the presence of ethyl trichloroacetate (ETAC), is found to be an effective catalyst for the production of polyethylene. Optimal catalyst productivities are obtained for a MAS:ETAC ratio of 1:1 at a reaction temperature of 75 °C. The polymerization is first order in both ethylene and MAS/ETAC concentration. NMR studies indicate that a 1:1 adduct is formed between binuclear MAS and ETAC, which is postulated to be a direct precursor to the active species. The productivity at elevated temperatures is limited by a radical decomposition process. Screening of various additives revealed that many halogenated organic reagents can act as activators, including carbon tetrachloride and p-toluene sulfonyl chloride. Improved productivities are obtained using sulfonyl chloride activators due to circumvention of radical decomposition pathways.

Isomer-Specific Fuel Destruction Pathways in Rich Flames of Methyl Acetate and Ethyl Formate and Consequences for the Combustion Chemistry of Esters

The influences of fuel-specific destruction pathways on flame chemistry are determined for two isomeric ester fuels, methyl acetate, CH3(CO)OCH3, and ethyl formate, H(CO)OC2H5, used as model representatives for biodiesel compounds, and their potential for forming air pollutants is addressed. Measurements are presented of major and intermediate species mole fractions in premixed, laminar flat flames using molecular-beam sampling and isomer-selective VUV-photoionization mass spectrometry. The observed intermediate species concentrations depend crucially on decomposition of the different radicals formed initially from the fuels. The methyl acetate structure leads to preferential formation of formaldehyde, while the ethyl formate isomer favors the production of acetaldehyde. Ethyl formate also yields higher concentrations of the C2 species (C2H2 and C2H4) and C4 species (C4H2 and C4H4). Benzene concentrations, while larger for ethyl formate, are at least an order of magnitude smaller for both flames than seen for simple hydrocarbon fuels (ethylene, ethane, propene, and propane).

Kinetics of the Reaction of Methyl Radical with Hydroxyl Radical and Methanol Decomposition

The CH3 + OH bimolecular reaction and the dissociation of methanol are studied theoretically at conditions relevant to combustion chemistry. Kinetics for the CH3 + OH barrierless association reaction and for the H + CH2OH and H + CH3O product channels are determined in the high-pressure limit using variable reaction coordinate transition state theory and multireference electronic structure calculations to evaluate the fragment interaction energies. The CH3 + OH --> 3CH2 + H2O abstraction reaction and the H2 + HCOH and H2 + H2CO product channels feature localized dynamical bottlenecks and are treated using variational transition state theory and QCISD(T) energies extrapolated to the complete basis set limit. The 1CH2 + H2O product channel has two dynamical regimes, featuring both an inner saddle point and an outer barrierless region, and it is shown that a microcanonical two-state model is necessary to properly describe the association rate for this reaction over a broad temperature range. Experimental channel energies for the methanol system are reevaluated using the Active Thermochemical Tables (ATcT) approach. Pressure dependent, phenomenological rate coefficients for the CH3 + OH bimolecular reaction and for methanol decomposition are determined via master equation simulations. The predicted results agree well with experimental results, including those from a companion high-temperature shock tube determination for the decomposition of methanol.

Ab initio and DFT study of the molecular conformations and the thermochemistry of the CH 2[dbnd]CHC(O)OONO 2 (APAN) atmospheric molecule and of the CH 2[dbnd]CHC(O)OO and CH 2[dbnd]CHC(O)O radicals

The molecular structures and conformational mobilities of the atmospheric peroxyacryloyl nitrate, CH 2 CHC(O)OONO 2 (APAN) molecule and its radical decomposition products CH 2 CHC(O)OO and CH 2 CHC(O)O were studied by ab initio and density functional methods. The potential energy curves for the internal rotations were calculated using the B3LYP hybrid functional with the 6-311++G(d,p) basis set. The equilibrium conformation of APAN, determined at B3LYP/6-311++G(3df,3pd) level, is characterized by a structure in which groups of atoms adjacent to the peroxide bond lie in almost perpendicular planes, τ(COON) = 86.4°. A quantum statistical analysis showed that only a 7% of the internal rotors of the above molecules can freely rotate at room temperature. From isodesmic energies calculated at the G3MP2//B3LYP/6-311++G(3df,3pd) level of theory, standard enthalpies of formation at 298 K for APAN, CH 2 CHC(O)OO and CH 2 CHC(O)O are predicted to be −34.8, −8.9, and −16.0 kcal mol −1. The resulting O N and O O bond dissociation enthalpies for the channels CH 2 CHC(O)OONO 2 → CH 2 CHC(O)OO + NO 2 and CH 2 CHC(O)OONO 2 → CH 2 CHC(O)O + NO 3 of 34.1 and 36.4 kcal mol −1 are significant larger than those reported for similar processes of the atmospherically relevant PAN species.

Combustion Modeling and Kinetic Rate Calculations for a Stoichiometric Cyclohexane Flame. 1. Major Reaction Pathways

The Utah Surrogate Mechanism was extended in order to model a stoichiometric premixed cyclohexane flame (P = 30 Torr). Generic rates were assigned to reaction classes of hydrogen abstraction, beta scission, and isomerization, and the resulting mechanism was found to be adequate in describing the combustion chemistry of cyclohexane. Satisfactory results were obtained in comparison with the experimental data of oxygen, major products and important intermediates, which include major soot precursors of C2-C5 unsaturated species. Measured concentrations of immediate products of fuel decomposition were also successfully reproduced. For example, the maximum concentrations of benzene and 1,3-butadiene, two major fuel decomposition products via competing pathways, were predicted within 10% of the measured values. Ring-opening reactions compete with those of cascading dehydrogenation for the decomposition of the conjugate cyclohexyl radical. The major ring-opening pathways produce 1-buten-4-yl radical, molecular ethylene, and 1,3-butadiene. The butadiene species is formed via beta scission after a 1-4 internal hydrogen migration of 1-hexen-6-yl radical. Cascading dehydrogenation also makes an important contribution to the fuel decomposition and provides the exclusive formation pathway of benzene. Benzene formation routes via combination of C2-C4 hydrocarbon fragments were found to be insignificant under current flame conditions, inferred by the later concentration peak of fulvene, in comparison with benzene, because the analogous species series for benzene formation via dehydrogenation was found to be precursors with regard to parent species of fulvene.

Photodegradation of glyphosate in the ferrioxalate system

The photoinduced degradation of glyphosate (GLP) in the ferrioxalate system was investigated under irradiation with a 250 W metal halide lamp ( λ ≥ 365 nm). The efficiency of orthophosphates release, representing the photodegradation efficiency of GLP, increased with decreasing the initial concentrations of GLP and Fe(III)/oxalate ratios. At acidic pH value in the range of 3.5–5.0, higher efficiency of orthophosphates release up to 60.6% was achieved, while the efficiency dropped to 42.1% at pH 6.0. The photochemical process mainly involved the predominant species of iron(III), namely Fe(C 2O 4) 2 − and Fe(C 2O 4) 3 3−, which lead to the formation of hydroxyl radicals in the presence of dissolved oxygen under UV–vis irradiation. Also, the complexation of GLP with Fe(III) obviously increased the light absorption of GLP and facilitated its degradation by direct photolysis. The ninhydrin test for primary amines showed that the GLP was attacked by hydroxyl radicals with C N cleavage to yield aminomethylphosphonic acid (AMPA) and C P cleavage to yield sarcosine. The photodegradation may be enhanced by the decomposition of reactive radicals produced through ligand-to-metal charge transfer (LMCT) of ferric–GLP complexes.

High-Temperature Shock Tube Measurements of Methyl Radical Decomposition

We have studied the two-channel thermal decomposition of methyl radicals in argon, involving the reactions CH3 + Ar --> CH + H2 + Ar (1a) and CH3 + Ar --> CH2 + H + Ar (1b), in shock tube experiments over the 2253-3527 K temperature range, at pressures between 0.7 and 4.2 atm. CH was monitored by continuous-wave, narrow-line-width laser absorption at 431.1311 nm. The collision-broadening coefficient for CH in argon, 2gamma(CH-Ar), was measured via repeated single-frequency experiments in the ethane pyrolysis system behind reflected shock waves. The measured 2gamma(CH-Ar) value and updated spectroscopic and molecular parameters were used to calculate the CH absorption coefficient at 431.1311 nm (23194.80 cm(-1)), which was then used to convert raw traces of fractional transmission to quantitative CH concentration time histories in the methyl decomposition experiments. The rate coefficient of reaction 1a was measured by monitoring CH radicals generated upon shock-heating highly dilute mixtures of ethane, C2H6, or methyl iodide, CH3I, in an argon bath. A detailed chemical kinetic mechanism was used to model the measured CH time histories. Within experimental uncertainty and scatter, no pressure dependence could be discerned in the rate coefficient of reaction 1a in the 0.7-4.2 atm pressure range. A least-squares, two-parameter fit of the current measurements, applicable between 2706 and 3527 K, gives k(1a) (cm(3) mol(-1) s(-1)) = 3.09 x 1015 exp[-40700/T (K)]. The rate coefficient of reaction 1b was determined by shock-heating dilute mixtures of C2H6 or CH3I and excess O2 in argon. During the course of reaction, OH radicals were monitored using the well-characterized R(1)(5) line of the OH A-X (0,0) band at 306.6871 nm (32606.52 cm(-1)). H atoms generated via reaction 1b rapidly react with O2, which is present in excess, forming OH. The OH traces are primarily sensitive to reaction 1b, reaction 9 (H + O2 --> OH + O) and reaction 10 (CH3 + O2 --> products), where the rate coefficients of reactions 9 and 10 are relatively well-established. No pressure dependence could be discerned for reaction 1b between 1.1 and 3.9 atm. A two-parameter, least-squares fit of the current data, valid over the 2253-2975 K temperature range, yields the rate expression k(1b) (cm(3) mol(-1) s(-1)) = 2.24 x 10(15) exp[-41600/T (K)]. Theoretical calculations carried out using a master equation/RRKM analysis fit the measurements reasonably well.

Thermal decomposition of flame retarded polycarbonates

The effect of tetrabromobisphenol A (TBBA), ammonium polyphosphate (APP) and tris(2,4-di-ter-dibuthylphenyl-phosphite) (TBP) flame retardants were examined on the thermal decomposition of polycarbonate by pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) and thermogravimetry/mass spectrometry (TG/MS) techniques. The maximal rate of polycarbonate decomposition shifted to a slightly lower temperature in the presence of TBBA. More than 20 bromine containing products were identified among the pyrolysis products of the polycarbonate sample containing TBBA flame retardant at 550 and 700 °C temperature. Significant decrease of the total evaluated volatile compounds was found at 550 °C from the APP blended polycarbonate and at 700 °C pyrolysis temperature in the presence of both phosphor-containing flame retardants investigated. The presence of the examined phosphorous flame retardants has no effect on the temperature regarding the radical decomposition of polycarbonate, while partial hydrolysis takes place at a lower temperature due to APP. The polycarbonate chain keenly decomposes under gradual heating in the temperature range of ammonia evolution from APP. The product analysis indicates that APP accelerates the formation of phenol and isopropylene phenol from the bisphenol A segments of polycarbonate.

Ab initio study on the decomposition of first excited state HOOO radicals

The HOOO radical plays a crucial role in atmospheric processes involving the OH radical and O(2) molecule. We present an ab initio molecular orbital theory study on the decomposition reaction of the first excited state HOOO((2)A') with respect to OH and O(2). The geometries and harmonic vibrational frequencies of all stationary points are calculated at the CASSCF and MRCI levels of theory in conjunction with the 6-31+G(d,p) basis set. The potential energy profile of the decomposition reaction is studied at the CASSCF/6-31+G(d,p) level of theory, in which the complete valence orbitals and electrons are included in the active space. The energies of the potential energy profile are further refined at the CASPT2 and MRCI levels of the theory. Additionally, we have determined the interesting reaction process: the HOOO((2)A') radical with C(s) symmetry does not dissociate to OH((2)Pi) and O(2)((3)Sigma(-)(g)) directly as this is forbidden by orbital symmetry, but dissociates to OH((2)Pi) and O(2)((3)Sigma(-)(g)) via the change in symmetry from C(s) to C(infinity v) symmetry with a low barrier.

Theory, measurements, and modeling of OH and HO2 formation in the reaction of cyclohexyl radicals with O2

The production of OH and HO(2) in Cl-initiated oxidation of cyclohexane has been measured using pulsed-laser photolytic initiation and continuous-laser absorption detection. The experimental data are modeled by master equation calculations that employ new G2(MP2)-like ab initio characterizations of important stationary points on the cyclo-C(6)H(11)O(2) surface. These ab initio calculations are a substantial expansion on previously published characterizations, including explicit consideration of conformational changes (chair-boat, axial-equatorial) and torsional potentials. The rate constants for the decomposition and ring-opening of cyclohexyl radical are also computed with ab initio based transition state theory calculations. Comparison of kinetic simulations based on the master equation results with the present experimental data and with literature determinations of branching fractions suggests adjustment of several transition state energies below their ab initio values. Simulations with the adjusted values agree well with the body of experimental data. The results once again emphasize the importance of both direct and indirect components of the kinetics for the production of both HO(2) and OH in radical + O(2) reactions.

Functions of Trace Metals in Fibers on Dyeing of Wool Fiber with Oxidation Dyes

To clarify what components of CMC contribute to the oxidation reaction, the effects of pretreatment of wool fibers using ethylenediaminetetraacetic acid (EDTA) solution and premordanting with the solution of metal salts on the dye uptake of the dinuclear oxidation dyes into wool fibers were investigated. The dye uptake of the wool samples pretreated with EDTA solutions decreased apparently compared to that of the intact wool sample. On the contrary, the wool samples premordanted with copper, iron and nickel salts allowed the dye uptake of oxidation dye inside the wool fiber to increase. These findings indicate that the metal ions presented primarily within the intercellular materials play an important role in developing the colored oxidation dye on⁄into the fiber. The presence of transition metal ions in the intercuticular material causes hydrogen peroxide to undergo radical decomposition, leading to oxidizing species by means of Haber-Weiss mechanism. They take part as initiator of oxidation reaction, where the dye precursor is electrochemically oxidized to its corresponding Wurster salt.

Photodegradation of atrazine and ametryn with visible light using water soluble porphyrins as sensitizers

The photodegradation of the herbicides atrazine and ametryn with visible light in aerated neutral aqueous solutions and 5, 10, 15, 20-tetrakis (2,6-dichloro-3-sulfophenyl) porphyrin or 5, 10, 15, 20-tetrakis (4-sulfophenyl) porphyrin as sensitizers are reported for the first time. Our findings show that the degradation percentage reached 30% for atrazine and 63% for ametryn. The final photoproducts were characterized as dealkylated s-triazines. Photolysis of the pesticides in the presence of a singlet oxygen quencher showed only a minor contribution of this type of mechanism, while a bimolecular quenching reaction between the triplet state of the sensitizer and the pesticides is excluded by flash photolysis studies. It is proposed that the mechanism may involve the formation of a superoxide radical anion from the triplet state of the sensitizer and molecular oxygen, followed by a radical decomposition pathway.

Graphitic Structures by Design

It is shown that self-supporting graphitic structures of specific shape can be grown in a variety of forms, from nanoscale to macroscale, on metal templates, in a fuel-rich mixture of ethylene and oxygen at temperatures between 750 and 900 K. The evidence presented suggests graphite can be grown in any shape created from catalytic metals (e.g., Ni) under the proper conditions of temperature and gas composition. Structures produced include macroscale bodies, centimeters in dimension, composed of micrometer-scale graphite elements such as graphite "foam" and regular graphite "lattices". Nanoscale hollow graphite spheres were also produced. The production rate in the apparatus employed was roughly shown to be 1 layer/s and was steady with time over several hours. The process of producing self-supporting bodies generally produces hollow graphite structures, as the underlying metal template must be removed by acid following the completion of graphite growth. The process is believed to be possible only in an environment, such as combustion, in which a high concentration of particular radical species is present in the vicinity of the template surface. The following process is postulated: (i) a single layer of graphite is formed from gas-phase radicals by the catalytic action of the metal template, (ii) additional graphite growth is "autocatalytic" and occurs via the decomposition of radicals on the surface and the incorporation of "free" carbon atoms, or other radical fragments, into "edge sites" on the graphite surface.

Interaction between hydrogen peroxide and cobalt(II) acetylacetonate in the system of reverse micelles of triton X-100 nonionic surfactant in cyclohexane

The yield of free radicals upon the decomposition of hydrogen peroxide catalyzed by cobalt acetylacetonate (Co(acac)2) in the systems of reverse micelles of TX-100/n-hexanol and AOT in cyclohexane at 37°C was studied with the inhibitor method using a stable nitroxyl radical as a spin trap. It is shown that, in micellar AOT solutions in cyclohexane as well as in n-decane, H2O2 and Co(acac)2 in practice do not react, because H2O2 is localized in a micelle water pool and Co(acac)2, in the organic phase. Therefore, the generation of radicals is not observed in AOT solutions in cyclohexane, whereas, in aqueous solution, Co(acac)2 catalyzes the radical decomposition of H2O2. In the system of mixed reverse micelles of TX-100 and n-hexanol in cyclohexane, at equal overall concentrations of H2O2 and Co(acac)2, the rate of radical formation is much higher than in aqueous solution; i.e., the micellar catalysis of the radical decomposition of H2O2 takes place. It follows from measurements of UV and ESR spectra and the kinetics of changes in the content of peroxides in the reaction mixture that TX-100 and n-hexanol react with free radicals formed upon H2O2 decomposition and with atmospheric oxygen.

Reactivity of quinones as alkyl radical acceptors

The enthalpies of the addition of 11 alkyl radicals to ortho-and para-benzoquinones and substituted para-benzoquinones and the enthalpies of formation of various alkoxyphenoxyl radicals have been calculated. Experimental data for the addition of alkyl radicals to quinones are analyzed in terms of the intersection of two parabolic potential curves, and parameters characterizing this class of reactions are calculated. The classical potential barrier of the thermally neutral reaction of alkyl radical addition to benzoquinone is E e,0 = 82.1 kJ/mol. This class of reactions is compared to other classes of free-radical addition reactions. The interaction between the electrons of the reaction center and the π electrons of the aromatic ring is a significant factor in the activation energy. Activation energies, rate constants, and the geometric parameters of the transition state have been calculated for 40 reactions of alkyl radical addition to quinones. Strong polar interaction has been revealed in the addition of polar macroradicals to quinones, and its contribution to the activation energy has been estimated. Kinetic parameters, activation energies, and rate constants have been calculated for the reverse reactions of alkoxyphenoxyl radical decomposition to quinone and alkyl. The competition between chain termination and propagation reactions in alkoxyphenol-inhibited hydrocarbon oxidation is discussed.

Unimolecular decomposition of aromatic bicyclic alkoxy radicals and their acyclic radicals

The unimolecular decomposition of bicyclic alkoxy radicals and their subsequent ring-cleavage acyclic radicals from the OH-initiated reactions of toluene is investigated by means of theoretical quantum chemistry methods. The reaction and activation energies of ring-cleavage of the bicyclic alkoxy radicals are in the ranges of −45.9 to −54.5 kcal mol −1 and 5.5–11.3 kcal mol −1, respectively. The activation energies of C–C bond cleavage of the acyclic radicals are in the range of 2.5–27.0 kcal mol −1. Kinetic calculations employing RRKM/ME formalism have been performed to assess the fate of chemically excited acyclic radicals for prompt decomposition or stabilization. The results indicate that the favorable decomposition pathways of the bicyclic alkoxy radicals correspond to the formation of dicarbonyl compounds along with glyoxal and methylglyoxal.