Reactions Of Alkylperoxy Radicals Research Articles

Theoretical Study on Reactions of Alkylperoxy Radicals.

We carried out a theoretical study on geometries, relative energies of stationary points, and reaction rate constants for ethyl + O2, propyl + O2, and butyl + O2 reactions, which are important reactions in the low-temperature oxidation of corresponding alkanes. Geometries with CCSD(T)/aug-cc-pVTZ for the ethyl + O2 system are adopted as the benchmark to choose a proper exchange-correlation functional for geometry optimization. Our results show that B3LYP with 6-311+G(d,p) can provide reliable structures for this system, and structures of the other two systems are determined with this functional. The performances of the explicitly correlated CCSD(T)-F12a and the locally correlated DLPNO-CCSD(T) methods on barrier heights and reaction energies are evaluated by comparing their results with those of CCSD(T)/aug-cc-pVQZ for the ethyl + O2 system. Our results indicate that reliable energy differences for this system are achieved with CCSD(T)-F12a using the cc-pVDZ-F12 basis set, and this method is employed in calculating single-point energies for the other two systems. The single-reference equation-of-motion spin-flip coupled-cluster method is adopted to obtain the potential energy surface of the barrierless reaction C2H5· + O2 → CH3CH2OO·, and the results are compared with those using broken-symmetry density functional theory and the Morse potential. Differences between energies with these methods are <1.6 kcal/mol, but the difference in the rate constants could be sizable at temperatures <500 K, and rate constants obtained in this work are reliable only for temperatures >500 K. Pressure-dependent rate constants for these reactions are determined using the Rice-Ramsperger-Kassel-Marcus/Master equation method. The obtained reaction energies, barrier heights, and rate constants could be valuable for reactions between the large alkane radical and O2, which are important in the low-temperature combustion of fuels such as kerosene and gasoline.

New insight into the spatiotemporal variability and source apportionments of C&amp;lt;sub&amp;gt;1&amp;lt;/sub&amp;gt;–C&amp;lt;sub&amp;gt;4&amp;lt;/sub&amp;gt; alkyl nitrates in Hong Kong

Abstract. C1–C4 alkyl nitrates (RONO2) were measured concurrently at a mountain site, Tai Mo Shan (TMS), and an urban site, Tsuen Wan (TW), at the base of the same mountain in Hong Kong from September to November 2010. Although the levels of parent hydrocarbons were much lower at TMS (p &lt; 0.05), similar alkyl nitrate levels were found at both sites regardless of the elevation difference, suggesting various source contributions of alkyl nitrates at the two sites. Prior to using a positive matrix factorization (PMF) model, the data at TW were divided into "meso" and "non-meso" scenarios for the investigation of source apportionments with the influence of mesoscale circulation and regional transport, respectively. Secondary formation was the prominent contributor of alkyl nitrates in the meso scenario (60 ± 2 %, 60.2 ± 1.2 pptv), followed by biomass burning and oceanic emissions, while biomass burning and secondary formation made comparable contributions to alkyl nitrates in the non-meso scenario, highlighting the strong emissions of biomass burning in the inland Pearl River delta (PRD) region. In contrast to TW, the alkyl nitrate levels measured at TMS mainly resulted from the photooxidation of the parent hydrocarbons at TW during mesoscale circulation, i.e., valley breezes, corresponding to 52–86 % of the alkyl nitrate levels at TMS. Furthermore, regional transport from the inland PRD region made significant contributions to the levels of alkyl nitrates (∼ 58–82 %) at TMS in the non-meso scenario, resulting in similar levels of alkyl nitrates observed at the two sites. The simulation of secondary formation pathways using a photochemical box model found that the reaction of alkyl peroxy radicals (RO2) with nitric oxide (NO) dominated the formation of RONO2 at both sites, and the formation of alkyl nitrates contributed negatively to O3 production, with average reduction rates of 4.1 and 4.7 pptv pptv−1 at TMS and TW, respectively.

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.

High-Pressure Rate Rules for Alkyl + O2 Reactions. 1. The Dissociation, Concerted Elimination, and Isomerization Channels of the Alkyl Peroxy Radical

The reactions of alkyl peroxy radicals (RO(2)) play a central role in the low-temperature oxidation of hydrocarbons. In this work, we present high-pressure rate estimation rules for the dissociation, concerted elimination, and isomerization reactions of RO(2). These rate rules are derived from a systematic investigation of sets of reactions within a given reaction class using electronic structure calculations performed at the CBS-QB3 level of theory. The rate constants for the dissociation reactions are obtained from calculated equilibrium constants and a literature review of experimental rate constants for the reverse association reactions. For the concerted elimination and isomerization channels, rate constants are calculated using canonical transition state theory. To determine if the high-pressure rate expressions from this work can directly be used in ignition models, we use the QRRK/MSC method to calculate apparent pressure and temperature dependent rate constants for representative reactions of small, medium, and large alkyl radicals with O(2). A comparison of concentration versus time profiles obtained using either the pressure dependent rate constants or the corresponding high-pressure values reveals 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).

Products of the OH radical-initiated reactions of 2-propyl nitrate, 3-methyl-2-butyl nitrate and 3-methyl-2-pentyl nitrate

In the atmosphere, alkyl nitrates formed from the reactions of alkyl peroxy radicals with NO are chemically removed by photolysis and by reaction with OH radicals. Products of the gas-phase reactions of OH radicals with 2-propyl nitrate, 3-methyl-2-butyl nitrate and 3-methyl-2-pentyl nitrate at room temperature have been investigated. The products observed and quantified were: from 2-propyl nitrate, acetone (58 ± 18%); from 3-methyl-2-butyl nitrate, acetaldehyde (113 ± 39% from gas chromatographic analyses and 70 ± 25% from FT-IR analyses), acetone (55 ± 8%), and 3-methyl-2-butanone (17 ± 2%); and from 3-methyl-2-pentyl nitrate, acetaldehyde (120 ± 26% from gas chromatographic analyses and 80 ± 21% from FT-IR analyses), propanal (≤1.1%), 2-butanone (33 ± 3%), 2-methylbutanal (≤1.1%), and 3-methyl-2-pentanone (9 ± 1%), where the percentage molar yields are given in parentheses. Using these measured product yields together with predicted reaction schemes indicates that these products account for 58 ± 18%, 86 ± 15% and 63 ± 7% of the overall reaction pathways of the 2-propyl nitrate, 3-methyl-2-butyl nitrate and 3-methyl-2-pentyl nitrate reactions, respectively. The NO2 present in the −ONO2 group in these nitrates will be released in the reaction pathways leading to the observed products.

Modeling the Organic Nitrate Yields in the Reaction of Alkyl Peroxy Radicals with Nitric Oxide. 1. Electronic Structure Calculations and Thermochemistry

Alkyl nitrates (RONO2) are minor products formed in the atmospheric reactions of alkyl peroxy radicals (RO2•) with nitric oxide. The major products are alkoxy radicals (RO•) and NO2. The alkyl nitrate channel is important in the troposphere because RONO2 formation results in removal of NOx and trapping of free radicals; both effects reduce the rate of ozone production. We have used electronic structure calculations at the G3 and B3LYP/6-311++G** levels to calculate the geometries, energies, and vibrational frequencies for major stationary points on the potential energy surfaces for R = H, CH3, C2H5, n-C3H7, i-C3H7, and 2-C5H11. Selected calculations have been made at the G2 and QCISD(T)/cc-pVTZ levels. Reaction energies are found to be rather insensitive to the size of the alkyl group. Corrections to the reaction energies are estimated, and a generic set of reaction energies are suggested. The B3LYP/6-311++G** barriers for the isomerization of ROONO to RONO2 are found to be much too high to account for ob...

Modeling the Organic Nitrate Yields in the Reaction of Alkyl Peroxy Radicals with Nitric Oxide. 2. Reaction Simulations

Master equation calculations are used to model gas-phase literature experimental data for alkyl nitrate formation via the following reaction system of reversible reactions: (1) RO2 + NO ↔ ROONO, (2) ROONO ↔ RO + NO2, (3) ROONO ↔ RONO2, and (4) RONO2 ↔ RO + NO2 for R = CH3, i-C3H7, and 2-C5H11. The structures and thermochemistry of the stable species are based on electronic structure calculations described in the preceding companion paper in this issue (Lohr et al. J. Phys. Chem. A 2003, 107, xxx−xxx). Literature data for recombination rate constants are used to constrain the model calculations. Several transition state models and a range of energy transfer parameters are investigated. The results for R = CH3 show that a wide variety of plausible transition state models for k-4 gives good agreement with experiment for reaction (−4), because changes in assumed energy transfer parameters can compensate for differences between the transition state models. It is concluded that recombination reactions are good sources of absolute energy transfer parameters only when transition state properties are known with great accuracy. Although satisfactory models are obtained for the individual systems, the parameters cannot be transferred reliably from one system to another. Master equation models can be made to reproduce the experimental 2-pentyl nitrate yields from the title reaction as long as 〈ΔE〉down, the average energy transferred in deactivating collisions, is assumed to be surprisingly, and perhaps unphysically, small (∼25 cm-1), regardless of assumptions about the barrier to isomerization reaction 3. Several critical assumptions in the master equation models are examined, but none of them accounts for the small value of 〈ΔE〉down. It is concluded that new experiments should be carried out to verify or possibly revise the pressure-dependent alkyl nitrate yield data currently available in the literature.

Hindered Amine Mechanisms: Part III—Investigations using isotopic labelling

The literature ‘peroxide’ mechanism and our proposed mechanism for the regeneration of hindered amine nitroxyls have been tested with isotopically labelled reactants. Both mechanisms deal with the reaction of alkylperoxy radicals with N-alkyloxy hindered amines, which are products of the reaction of nitroxyl radicals with alkyl radicals. The ‘peroxide’ mechanism predicts the regeneration of nitroxyl and formation of dialkyl peroxide. Our mechanism predicts the regeneration of nitroxyl and formation of ketones, alcohols and carboxylic acids, depending on the oxidizing substrate. The results of this work confirm the new proposed regeneration mechanism but not the previous ‘peroxide’ mechanism. The features of the confirmed mechanism are ▪ This basic mechanism is also proposed for acylperoxy radicals ▪

Alkyl nitrate formation from the atmospheric photoxidation of alkanes; a revised estimation method

The available experimental data concerning the yields of alkyl nitrates in the reactions of alkyl peroxy radicals with NO have been used to derive a revised expression for the estimation of alkyl nitrate yields in the atmospheric photooxidation of alkanes as a function of temperature and pressure. This revised expression gives more reasonable predictions of alkyl nitrate yields under high altitude tropospheric conditions than that which has been previously published.

ChemInform Abstract: The UV Absorption Spectra and Kinetics of the Self Reactions of CH2ClO2 and CH2FO2 Radicals in the Gas Phase

The ultraviolet absorption spectra of chloromethylperoxy and fluoromethylperoxy radicals, CH2ClO2 and CH2FO2, and the kinetics of their respective self reactions have been studied in the gas phase using a flash photolysis technique. The absorption spectra for both radicals were quantified over the wavelength range 210 and 290 nm. The measured absorption cross-sections were used to derive the observed self-reaction rate constants (for reactions 1 and 2) over the temperature range 228–380 K, defined as –d[CH2XO2]/dt = 2k[CH2XO2]2, where X represents Cl or F. The rate constants at 298 K were found to be independent of pressure over the range 25–400 torr N2 with values of k1(298 K) = (3.78 ± 0.45) × 10−12 and k2(298 K) = (3.07 ± 0.65) × 10−12 in units of cm3 molecule−1 s−1. The kinetic data over the complete temperature range are represented by the Arrhenius expressions: where the error limits represent 2σ from linear least squares analysis. These results are discussed with respect to previous measurements of the absorption spectra and reactions of alkylperoxy radicals.

The UV absorption spectra and kinetics of the self reactions of CH2ClO2 and CH2FO2 radicals in the gas phase

AbstractThe ultraviolet absorption spectra of chloromethylperoxy and fluoromethylperoxy radicals, CH2ClO2 and CH2FO2, and the kinetics of their respective self reactions have been studied in the gas phase using a flash photolysis technique. The absorption spectra for both radicals were quantified over the wavelength range 210 and 290 nm. The measured absorption cross‐sections were used to derive the observed self‐reaction rate constants (for reactions 1 and 2) over the temperature range 228–380 K, defined as –d[CH2XO2]/dt = 2k[CH2XO2]2, where X represents Cl or F. magnified image The rate constants at 298 K were found to be independent of pressure over the range 25–400 torr N2 with values of k1(298 K) = (3.78 ± 0.45) × 10−12 and k2(298 K) = (3.07 ± 0.65) × 10−12 in units of cm3 molecule−1 s−1. The kinetic data over the complete temperature range are represented by the Arrhenius expressions: where the error limits represent 2σ from linear least squares analysis. These results are discussed with respect to previous measurements of the absorption spectra and reactions of alkylperoxy radicals.

Singlet oxygen production from the reactions of alkylperoxy radicals. Evidence from 1268-nm chemiluminescence

Singlet oxygen production from the reactions of alkylperoxy radicals. Evidence from 1268-nm chemiluminescence

Metal complexes as antioxidants. V. Inhibition of hydrocarbon autoxidation by cupric complexes of dialkyldithiophosphoric and dialkyldithiocarbamic acids

Liquid-phase reactions of alkylperoxy radicals with cupric complexes of dialkyldithiophosphoric and dialkyldithiocarbamic acids have been examined. These complexes have been shown to be extremely efficient peroxy radical scavenging antioxidants for hydrocarbon autoxidation. Initial rates of oxygen absorption by typical hydrocarbons were extremely slow implying inhibition rate constants &gt; 106 M−1 s−1. The initial rate of disappearance of the dithiocarbamate was equal to the rate of chain initiation (Ri) whereas the rate of disappearance of the dithiophosphate was twice as fast as Ri. In both cases the rate of complex disappearance slowed down as the complex was consumed. Autoxidation of styrene commenced as soon as the complex disappeared while cumene did not absorb oxygen for a considerable length of time after complex destruction. Cumylperoxy radicals were converted to α-cumyl alcohol, α-methylstyrene, and acetophenone by reaction with these complexes and the copper ions were eventually precipitated as copper sulphate. In the case of the dithiocarbamate three intermediate cupric complexes were detectee by epr spectroscopy.

The gas-phase reactions of alkylperoxy radicals generated by a photochemical technique

The gas-phase reactions of alkylperoxy radicals generated by a photochemical technique

The extension to long-chain alkanes and to high temperatures of the hydroperoxide chain mechanism of autoxidation

The liquid-phase oxidation of 2-methylhexadecane has been studied to determine how adequately the hydroperoxide chain mechanism describes the oxidation of high molecular weight alkanes and to elucidate the changes in this mechanism caused by increased temperature. The reaction between 2-methylhexadecane and molecular oxygen has been studied at temperatures of 145 to 230 °C and the oxidation products analysed by gas-chromatographic and chemical methods. Over 160 products including hydroperoxides, alkanes, alcohols, carbonyl compounds and acids have been identified and the dependence of their yields on time, temperature and oxygen concentration measured. The attack on 2-methylhexadecane at temperatures throughout this range is selective, indicating that chain propagation occurs predominantly by the reaction of alkylperoxy radicals with alkane molecules followed by the addition of oxygen to the alkyl radicals so formed. At low temperatures the former reaction is rate-determining. However, an increase in temperature increases the rate of this reaction and reduces the concentration of oxygen dissolved in the alkane; the combined effect of these two factors causes the addition of oxygen to alkyl radicals to become rate-determining at temperatures above ca . 210 °C. As a result of this change, the concentration of alkyl radicals relative to that of alkylperoxy radicals increases with temperature. Consequently, both the yield of alkanes, and the fraction of secondary alkyl radicals that react with 2-methylhexadecane molecules to form more stable tertiary radicals, increase. These results help to predict the necessary properties of a high temperature antioxidant; compounds that react specifically with alkyl radicals rather than with alkylperoxy radicals should function thus.