Uncovering the fundamental chemistry of alkyl + O2 reactions via measurements of product formation

The reactions of alkyl radicals (R) with molecular oxygen (O(2)) are critical components in chemical models of tropospheric chemistry, hydrocarbon flames, and autoignition phenomena. The fundamental kinetics of the R + O(2) reactions is governed by a rich interplay of elementary physical chemistry processes. At low temperatures and moderate pressures, the reactions form stabilized alkylperoxy radicals (RO(2)), which are key chain carriers in the atmospheric oxidation of hydrocarbons. At higher temperatures, thermal dissociation of the alkylperoxy radicals becomes more rapid and the formation of hydroperoxyl radicals (HO(2)) and the conjugate alkenes begins to dominate the reaction. Internal isomerization of the RO(2) radicals to produce hydroperoxyalkyl radicals, often denoted by QOOH, leads to the production of OH and cyclic ether products. More crucially for combustion chemistry, reactions of the ephemeral QOOH species are also thought to be the key to chain branching in autoignition chemistry. Over the past decade, the understanding of these important reactions has changed greatly. A recognition, arising from classical kinetics experiments but firmly established by recent high-level theoretical studies, that HO(2) elimination occurs directly from an alkylperoxy radical without intervening isomerization has helped resolve tenacious controversies regarding HO(2) formation in these reactions. Second, the importance of including formally direct chemical activation pathways, especially for the formation of products but also for the formation of the QOOH species, in kinetic modeling of R + O(2) chemistry has been demonstrated. In addition, it appears that the crucial rate coefficient for the isomerization of RO(2) radicals to QOOH may be significantly larger than previously thought. These reinterpretations of this class of reactions have been supported by comparison of detailed theoretical calculations to new experimental results that monitor the formation of products of hydrocarbon radical oxidation following a pulsed-photolytic initiation. In this article, these recent experiments are discussed and their contributions to improving general models of alkyl + O(2) reactions are highlighted. Finally, several prospects are discussed for extending the experimental investigations to the pivotal questions of QOOH radical chemistry.     

Stabilization mechanisms of hindered amines

As a result of studying the interaction of hindered amine stabilizers (2, 2, 6, 6-tetramethylpiperidines) with simple hydroperoxides, peroxy radicals, and acylperoxy radicals, the last two in AIBN-initiated oxidation experiments in chlorobenzene, the following conclusions have been reached:

• 1 Hindered amines have multiple mechanisms of functioning as photostabilizers of polymers.
• 2 Reactions between tetramethylpiperidines and simple hydroperoxides are too slow at moderate temperatures to make a significant contribution to polymer stabilization.
• 3 Reactions between tetramethylpiperidines and alkylperoxy radicals at moderate temperatures occur at varying rates with varying effectiveness for stabilization. With favorable alignment among reaction rates for oxidation propagation and termination, reactions between tetramethylpiperidines and alkylperoxy radicals can play a significant role in oxidation inhibition.
• 4 Hydrocarbon polymer photooxidation proceeds by two major paths - the usually accepted alkyl radical/alkylperoxy radical/hydroperoxide route and the usually neglected aldehyde/acyl radical/acylperoxy radical/peracid route.
• 5 Hindered amine stabilizers are able to participate in inhibiting both photooxidation reactions - they trap acylperoxy radicals, converting them to carboxylic acids and are converted to nitroxyl radicals in the process; the nitroxyl radicals trap alkyl radicals and the hindered amines trap alkylperoxy radicals to inhibit the other oxidation pathway.
• 6 Nitroxyls are regenerated from N-alkyloxy hindered amines in a fast, efficient reactions with acylperoxy radicals and in slow reactions with alkylperoxy radicals. We postulate neither reaction yields peroxides: carboxylic acids and oxidized alkyloxy substituents are obtained from the first reaction; alcohols and oxidized alkyloxy substituents are obtained from the second reaction.

     

Detection of radical intermediates in the photo-oxidation of alkanols by ceric ammonium nitrate-CAN. an EPR study

The photolysis of acetonitrile solutions of alkanols in the presence of ceric ammonium nitrate-CAN, at different temperatures, leads to the formation of alkyl, peroxyl and nitroxyl radicals identified by EPR spectroscopy. The involvement of free alkoxy and alkylperoxy radicals as intermediates in the reaction path will be discussed.     

Thermochemistry and reaction paths in the oxidation reaction of benzoyl radical: C6H5C•(═O)

Alkyl substituted aromatics are present in fuels and in the environment because they are major intermediates in the oxidation or combustion of gasoline, jet, and other engine fuels. The major reaction pathways for oxidation of this class of molecules is through loss of a benzyl hydrogen atom on the alkyl group via abstraction reactions. One of the major intermediates in the combustion and atmospheric oxidation of the benzyl radicals is benzaldehyde, which rapidly loses the weakly bound aldehydic hydrogen to form a resonance stabilized benzoyl radical (C6H5C(?)═O). A detailed study of the thermochemistry of intermediates and the oxidation reaction paths of the benzoyl radical with dioxygen is presented in this study. Structures and enthalpies of formation for important stable species, intermediate radicals, and transition state structures resulting from the benzoyl radical +O2 association reaction are reported along with reaction paths and barriers. Enthalpies, ΔfH298(0), are calculated using ab initio (G3MP2B3) and density functional (DFT at B3LYP/6-311G(d,p)) calculations, group additivity (GA), and literature data. Bond energies on the benzoyl and benzoyl-peroxy systems are also reported and compared to hydrocarbon systems. The reaction of benzoyl with O2 has a number of low energy reaction channels that are not currently considered in either atmospheric chemistry or combustion models. The reaction paths include exothermic, chain branching reactions to a number of unsaturated oxygenated hydrocarbon intermediates along with formation of CO2. The initial reaction of the C6H5C(?)═O radical with O2 forms a chemically activated benzoyl peroxy radical with 37 kcal mol(-1) internal energy; this is significantly more energy than the 21 kcal mol(-1) involved in the benzyl or allyl + O2 systems. This deeper well results in a number of chemical activation reaction paths, leading to highly exothermic reactions to phenoxy radical + CO2 products.     

Free radical induced oxidation of alkoxyphenols: Some insights into the processes of photoyellowing of papers

The photoyellowing of lignin-rich papers has been demonstrated to depend on the formation of phenoxyl radical intermediates and their ultimate conversion into various products, including quinones. Molecular oxygen has also been observed as a necessary adjunct to this process, although the mechanism is not understood. This work demonstrates the requirement for the reaction of O² with active radical intermediates (in processes analogous to autoxidation reactions) in order for the photoyellowing of the phenolic moieties to occur. Photo-oxidations of a variety of alkoxyphenols and their reactions with model alkyl, alkoxy and alkylperoxy radicals are studied by CIDEP.     

Systematic computational study on the unimolecular reactions of alkylperoxy (RO2), hydroperoxyalkyl (QOOH), and hydroperoxyalkylperoxy (O2QOOH) radicals

Unimolecular isomerization and decomposition reactions of alkylperoxy (RO(2)), hydroperoxyalkyl (QOOH), and hydroperoxyalkylperoxy (O(2)QOOH) radicals play important roles in the low-temperature oxidation of hydrocarbons. In this study, these reactions have been investigated by the CBS-QB3 quantum chemical method, and the variation of the rate parameters by the structural change of alkyl groups has been studied systematically for the rule-based construction of the low-temperature oxidation mechanisms of arbitrary noncyclic alkanes. The results can be well-interpreted in terms of the group additivity and the ring-strain effect of the cyclic transition states. To extract the important processes needed for the chemical kinetic modeling, the competing reaction channels were compared in detail by steady-state analysis with the high-pressure limiting rate constants. The importance of some reactions of O(2)QOOH radicals, which have not been considered in the previous modeling studies, such as the hydrogen exchange reactions between -OOH and -OO(?) groups and hydrogen shift reactions from non-OOH sites, is suggested.     

Ab initio study of chain branching reactions involving second generation products in hydrocarbon combustion mechanisms

sec-Alkyl radicals are key reactive intermediates in the hydrocarbon combustion and atmospheric decomposition mechanisms that are formed by the abstraction of hydrogen from an alkane, or as a second generation product of n-alkyl H-migrations, C-C bond scissions in branched alkyl radicals, or the bimolecular reaction between olefins and n-alkyl radicals. Since alkanes and branched alkanes, which the sec-alkyl radicals are derived from, make up roughly 40-50% of traditional fuels an understanding of their chemistry is essential to improving combustion systems. The present work investigates all H-migration reactions initiated from an sec-alkyl radical that involve the movement of a secondary hydrogen, for the 2-butyl through 4-octyl radicals, using the CBS-Q, G2, and G4 composite methods. The resulting thermodynamic and kinetic parameters are compared to similar reactions in n-alkyl radicals in order to determine underlying trends. Particular attention is paid to the effect of cis/trans and 1,3-diaxial interactions on activation energies and rate coefficients. When combined with our previous work on n-alkyl radical H-migrations, a complete picture of H-migrations in unbranched alkyl radicals is obtained. This full data set suggests that the directionality of the remaining branched chains has a minimal effect on the rate coefficients for all but the largest viable transition states, which is in stark contrast to the differences predicted by the structurally similar dimethylcycloalkanes. In fact the initial location of the secondary radical site has a greater effect on the rate than does the directionality of the remaining alkyl chains. The activation energies for secondary to secondary reactions are much closer to those of the secondary to primary H-migrations. However, the rate coefficients are found to be closer to the corresponding primary to primary reaction values. A significant ramification of these results is that there will be multiple viable reaction pathways for these reactions instead of only one dominant pathway as previously believed.     

Theoretical studies of atmospheric reaction mechanisms in the troposphere

The chemistry of the atmosphere encompasses a vast number of reactions acting on a plethora of intermediates. These reactions, occurring sequentially and in parallel, give rise to intertwined and irreducible mechanisms describing the complex chemical transformations of organic and inorganic compounds in the atmosphere. The complexity of this system is that it requires combined experimental, theoretical, and modeling approaches to elucidate the characteristics of the individual reactions, and their mutual interaction. In this review, we describe recent results from quantum chemical and theoretical kinetic studies of relevance to atmospheric chemistry. The review first summarizes the most commonly used theoretical methodologies. It then examines the VOC oxidation initiation channels by OH, O(3), NO(3) and Cl, followed by the reactions of the alkyl, alkoxy, alkylperoxy and Criegee intermediates active in the subsequent oxidation steps. Specific systems such as the oxidation of aromatics and the current state of knowledge on OH-regeneration in VOC oxidation are also discussed, as well as some inorganic reactions.     

Laboratory studies of organic peroxy radical chemistry: an overview with emphasis on recent issues of atmospheric significance

Organic peroxy radicals (often abbreviated RO(2)) play a central role in the chemistry of the Earth's lower atmosphere. Formed in the atmospheric oxidation of essentially every organic species emitted, their chemistry is part of the radical cycles that control the oxidative capacity of the atmosphere and lead to the formation of ozone, organic nitrates, organic acids, particulate matter and other so-called secondary pollutants. In this review, laboratory studies of this peroxy radical chemistry are detailed, as they pertain to the chemistry of the atmosphere. First, a brief discussion of methods used to detect the peroxy radicals in the laboratory is presented. Then, the basic reaction pathways - involving RO(2) unimolecular reactions and bimolecular reactions with atmospheric constituents such as NO, NO(2), NO(3), O(3), halogen oxides, HO(2), and other RO(2) species - are discussed. For each of these reaction pathways, basic reaction rates are presented, along with trends in reactivity with radical structure. Focus is placed on recent advances in detection methods and on recent advances in our understanding of radical cycling processes, particularly pertaining to the complex chemistry associated with the atmospheric oxidation of biogenic hydrocarbons.     

A theoretical study of the OH-initiated gas-phase oxidation mechanism of β-pinene (C10H16): first generation products

An extensive mechanism for the OH-initiated oxidation of β-pinene up to the first-generation products was derived based on quantum chemical calculations, theoretical kinetics, and structure-activity relationships. The resulting mechanism deviates from earlier explicit mechanisms in several key areas, leading to a different product yield prediction. Under oxidative conditions, the inclusion of ring closure reactions of unsaturated alkoxy radicals brings the predicted nopinone and acetone yields to an agreement with the experimental data. Routes to the formation of other observed products, either speciated or observed as peaks in mass spectrometric studies, are also discussed. In pristine conditions, we predict significant acetone formation following ring closure reactions in alkylperoxy radicals; in addition, we predict some direct OH recycling in subsequent H-migration reactions in alkylperoxy radicals. The uncertainties on the key reactions are discussed. Overall, the OH-initiated oxidation of β-pinene is characterized by the formation of a few main products, and a very large number of products in minor to very small yields.     

Atmospheric reactions of alkoxy and β-hydroxyalkoxy radicals

Alkoxy and β-hydroxyalkoxy radicals are key intermediates formed in the atmospheric degradations of alkanes and alkenes, respectively. In the troposphere, these alkoxy radicals can decompose, isomerize, and react with O₂. The literature data concerning the rates of these reactions are evaluated, and predictive schemes allowing the calculation of rate constants for these alkoxy radical reactions for atmospheric purposes are proposed. Good agreement between calculated reaction rates and experimental data concerning the absolute and relative importance of these reaction pathways is obtained, and alkoxy and β-hydroxyalkoxy radical reaction rates for radicals for which experimental data are not presently available can now be calculated for use in atmospheric modeling. © 1997 John Wiley & Sons, Inc.     

Toluene combustion: reaction paths, thermochemical properties, and kinetic analysis for the methylphenyl radical + O2 reaction

Aromatic compounds such as toluene and xylene are major components of many fuels. Accurate kinetic mechanisms for the combustion of toluene are, however, incomplete, as they do not accurately model experimental results such as strain rates and ignition times and consistently underpredict conversion. Current kinetic mechanisms for toluene combustion neglect the reactions of the methylphenyl radicals, and we believe that this is responsible, in part, for the shortcomings of these models. We also demonstrate how methylphenyl radical formation is important in the combustion and pyrolysis of other alkyl-substituted aromatic compounds such as xylene and trimethylbenzene. We have studied the oxidation reactions of the methylphenyl radicals with O2 using computational ab initio and density functional theory methods. A detailed reaction submechanism is presented for the 2-methylphenyl radical + O2 system, with 16 intermediates and products. For each species, enthalpies of formation are calculated using the computational methods G3 and G3B3, with isodesmic work reactions used to minimize computational errors. Transition states are calculated at the G3B3 level, yielding high-pressure limit elementary rate constants as a function of temperature. For the barrierless methylphenyl + O2 and methylphenoxy + O association reactions, rate constants are determined from variational transition state theory. Multichannel, multifrequency quantum Rice-Ramsperger-Kassel (qRRK) theory, with master equation analysis for falloff, provides rate constants as a function of temperature and pressure from 800 to 2400 K and 1 x 10(-4) to 1 x 10(3) atm. Analysis of our results shows that the dominant pathways for reaction of the three isomeric methylphenyl radicals is formation of methyloxepinoxy radicals and subsequent ring opening to methyl-dioxo-hexadienyl radicals. The next most important reaction pathway involves formation of methylphenoxy radicals + O in a chain branching process. At lower temperatures, the formation of stabilized methylphenylperoxy radicals becomes significant. A further important reaction channel is available only to the 2-methylphenyl isomer, where 6-methylene-2,4-cyclohexadiene-1-one (ortho-quinone methide, o-QM) is produced via an intramolecular hydrogen transfer from the methyl group to the peroxy radical in 2-methylphenylperoxy, with subsequent loss of OH. The decomposition of o-QM to benzene + CO reveals a potentially important new pathway for the conversion of toluene to benzene during combustion. A number of the important products of toluene combustion proposed in this study are known to be precursors of polyaromatic hydrocarbons that are involved in soot formation. Reactions leading to the important unsaturated oxygenated intermediates identified in this study, and the further reactions of these intermediates, are not included in current aromatic oxidation mechanisms.     

Evidence for alkoxy radical isomerization in photooxidations of C4C6 alkanes under simulated atmospheric conditions

We have calculated approximate room temperature rate constants for intramolecular hydrogen shift isomerizations of alkyl, alkoxy and alkylperoxy radical intermediates in photochemical smog and found that alkoxy radicals with δ hydrogens appear to undergo 1,5 hydrogen shifts at ambient conditions. Product distributions observed during irradiations of alkane (n-butane, n-pentane and n-hexane)NO_xair systems in two quite-different ≈6000-liter environmental chambers were found to be consistent with this prediction. For example, in the n-pentaneNO _xair system the observed large yields of 3-pentanone relative to 2-pentanone were consistent with room temperature rate constants which we have estimated for the corresponding hydrogen shift isomerizations. While such isomerizations have been well recognized in studies of free radicals and high temperature hydrocarbon oxidations, they are not taken into account in current kinetic mechanisms for alkane oxidation under ambient atmospheric conditions. This has implications for the construction of computer models of photo-chemical smog formation.     

Real-time quantitative analysis of volatile products generated during solid-state polypropylene thermal oxidation

Analysis of volatile organic compounds (VOCs) during PP thermal oxidation under three oxygen partial pressures (0%, 21% and 100% of atmospheric pressure) at 140 °C was performed by proton transfer reaction coupled with Fourier transform ion cyclotron resonance mass spectrometry. Six main VOCs were identified: acetone, acetic acid, 2,4-pentanedione, acetaldehyde, formaldehyde and methyl acrolein. Their formation was shown to obey two main reaction pathways, both involving methyne units as driving oxidation sites: (i) the widely accepted chain scission mechanism of tertiary alkoxy radicals, which generates primary radicals undergoing secondary reactions leading to the oxidation of methylene units; (ii) the chain scission mechanism occurring on tertiary alkyl radical, which is proposed here as a realistic path leading to methyl acrolein. The relative proportions of the six main VOCs depend on the oxygen partial pressure, which mostly impacts the oxidation of methylene units rather than the competition between the two previous paths.     

Mechanisms of antioxidant action: Auto-synergistic behaviour of nitrones

The antioxidant activities of a number of nitrones have been studied in model systems initiated by both alkoxy radical generators (hydroperoxides) and an alkylperoxy radical generator (azo-bis-isobutyronitrile in the presence of oxygen). Simple nitrones are ineffective in the presence of the latter but are highly effective in the presence of the former. An auto-synergistic mechanism has been shown to be involved due to the ability of nitrones to destroy hydroperoxides in a stoichiometric reaction and at the same time to scavenge alkoxy radicals formed by homolysis of hydroperoxides. Nitrones containing additional conventional phenolic antioxidant functions are effective in an alkylperoxy radical initiated system but do not show the usual structure-activity relationships.     

Ab initio study of hydrogen migration in 1-alkylperoxy radicals

Alkylperoxy and hydroperoxyalkyl radicals are key reactive intermediates in hydrocarbon oxidation mechanisms. An understanding of the interconversion of these two species via a hydrogen migration reaction is of fundamental importance to the prediction of chain branching reactions and end product composition. An extensive ab initio investigation of the hydrogen migration reaction in 1-ethyl, 1-propyl, 1-butyl, 1-pentyl, and 1-hexylperoxy radicals is conducted to assess the validity of using cycloalkanes to model the ring strain of their transition states as well as the effect of both location of the migrating hydrogen and directionality of the remaining alkyl chain in the transition state of the reaction involving a secondary hydrogen. The G2 and CBS-Q composite methods are used to determine the activation energy and enthalpy of reaction relative to the alkylperoxy radical. Both methods show good agreement with five experimentally determined reaction enthalpies, having root mean squared deviations of 0.7 and 1.3 kcal mol(-1) for the CBS-Q and G2 methods, respectively. The effect of hydrogen abstraction site and transition state geometry, particularly axial and equatorial geometries of the remaining alkyl chain, on the activation energy, Arrhenius A-factor, tunneling, and rate coefficient are discussed. Differences between terminal adjacent and nonterminal adjacent secondary sites result in small but consistent differences in barrier height. Failure of key assumptions within the cycloalkane based estimation method leads to the break down in the accuracy for both small and large transition states. For large transition states, the breakdown of these assumptions also results in the failure of the current cycloalkane method as a conceptual model. Of great interest is the observed alteration in the preferred H-migration from the 1,5 to the 1,6 H-migration within the temperature region where these reactions are particularly important to the combustion mechanism.     

Rate constant estimation for C1 to C4 alkyl and alkoxyl radical decomposition

Rate coefficients for alkyl and alkoxy radical decomposition are important in combustion, biological, and atmospheric processes. In this paper, rate constant expressions for C₁? C₄ alkyl and alkoxy radicals decomposition via β‐scission are recommended based on the reverse, exothermic reaction, the addition of a hydrogen atom or an alkyl radical to an olefin or carbonyl species with the decomposition reaction calculated using microscopic reversibility. The rate expressions have been estimated based on a wide‐range study of available experimental data. Rate coefficients for hydrogen atom and alkyl radical addition to an olefin show a strong temperature curvature. In addition, it is found that there is a correlation between the activation energy for addition and (i) the type of atom undergoing addition and (ii) whether this radical adds to the internal or terminal carbon atom of the olefin. Rate coefficients for alkoxy radical decomposition show a strong correlation to the ionization potential of the alkyl radical leaving group and on the enthalpy of reaction. It is shown that the activation energy for alkyl radical addition to a carbonyl species can be estimated as a function of the alkyl radical ionization potential and enthalpy of reaction. © 2006 Wiley Periodicals, Inc. Int J Chem Kinet 38: 250–275, 2006     

High-pressure rate rules for alkyl + O2 reactions. 2. The isomerization, cyclic ether formation, and β-scission reactions of hydroperoxy alkyl radicals

The unimolecular reactions of hydroperoxy alkyl radicals (QOOH) play a central role in the low-temperature oxidation of hydrocarbons as they compete with the addition of a second O(2) molecule, which is known to provide chain-branching. In this work we present high-pressure rate estimation rules for the most important unimolecular reactions of the β-, γ-, and δ-QOOH radicals: isomerization to RO(2), cyclic ether formation, and selected β-scission reactions. These rate rules are derived from high-pressure rate constants for a series of reactions of a given reaction class. The individual rate expressions are determined from CBS-QB3 electronic structure calculations combined with canonical transition state theory calculations. Next we use the rate rules, along with previously published rate estimation rules for the reactions of alkyl peroxy radicals (RO(2)), to investigate the potential impact of falloff effects in combustion/ignition kinetic modeling. Pressure effects are examined for the reaction of n-butyl radical with O(2) by comparison of concentration versus time profiles that were obtained using two mechanisms at 10 atm: one that contains pressure-dependent rate constants that are obtained from a QRRK/MSC analysis and another that only contains high-pressure rate expressions. These simulations reveal that under most conditions relevant to combustion/ignition problems, the high-pressure rate rules can be used directly to describe the reactions of RO(2) and QOOH. For the same conditions, we also address whether the various isomers equilibrate during reaction. These results indicate that equilibrium is established between the alkyl, RO(2), and γ- and δ-QOOH radicals.     

Reactions of o-benzyne with propargyl and benzyl radicals: potential sources of polycyclic aromatic hydrocarbons in combustion

The kinetics and mechanisms of the reactions of o-benzyne with propargyl and benzyl radicals have been investigated computationally. The possible reaction pathways have been explored by quantum chemical calculations at the M06-2X/6-311+G(3df,2p)//B3LYP/6-311G(d,p) level and the mechanisms have been investigated by the Rice-Ramsperger-Kassel-Marcus theory/master-equation calculations. It was found that the o-benzyne associates with the propargyl and benzyl radicals without pronounced barriers and the activated adducts easily isomerize to five-membered ring species. Indenyl radical and fluorene + H were predicted to be dominantly produced by the reactions of o-benzyne with propargyl and benzyl radicals, respectively, with the rate constants close to the high-pressure limits at temperatures below 2000 K. The related reactions on the two potential energy surfaces, namely, the reaction between fulvenallenyl radical and acetylene and the decomposition reactions of indenyl and α-phenylbenzyl radicals were also investigated. The high reactivity of o-benzyne toward the resonance stabilized radicals suggested a potential role of o-benzyne as a precursor of polycyclic aromatic hydrocarbons in combustion.