Kinetics of the reactions of OH radicals with some oxygenated volatile organic compounds under simulated atmospheric conditions

Some relative rate experiments have been carried out at room temperature and at atmospheric pressure. This concerns the OH-oxidation of some oxygenated volatile organic compounds including methanol (k₁), ethanol (k₂), MTBE (k₃), ethyl acetate (k₄), n-propyl acetate (k₅), isopropyl acetate (k₆), n-butyl acetate (k₇), isobutyl acetate (k₈), and t-butyl acetate (k₉). The experiments were performed in a Teflon-film bag smog chamber. The rate constants obtained are (in cm³ molecule⁻¹ s⁻¹): k₁=(0.90±0.08)×10⁻¹²; k₂=(3.88±0.11)×10⁻¹²; k₃=(2.98±0.06)×10⁻¹²; k₄=(1.73±0.20)×10⁻¹²; k₅=(3.56±0.15)×10⁻¹²; k₆=(3.97±0.18)×10⁻¹²; k₇=(5.78±0.15)×10⁻¹²; k₈=(6.77±0.30)×10⁻¹²; and k₉=(0.56±0.11)×10⁻¹². The agreement between the obtained rate constants and some previously published data has allowed for most of the studied compounds to point out a coherent group of values and to suggest recommended values. Atmospheric implications are also discussed. © 1998 John Wiley & Sons, Inc. Int J Chem Kinet 30: 839–847, 1998     

Relative rate constants for the reactions of hydroxyl radicals and chlorine atoms with a series of aliphatic alcohols

Rate constants for the gas‐phase reactions of hydroxyl radicals and chlorine atoms with a series of alcohols have been determined by using the relative method. The experiments were performed at 295 ± 2 K and at 1 atmospheric pressure. The obtained values of the rate constants in units of 10^?12 cm³ molecule^?1 s^?1 are as follows:

Relative rate constant measurements for the gas-phase reactions of hydroxyl radicals with 4-methyl-2-pentanone,trans-4-octene,and trans-2-heptene

The relative OH reaction rates from the simulated atmospheric oxidation of 4-methyl-2-pentanone, trans-4-octene, and trans-2-heptene have been measured. Reactions were carried out at 297 ± 2 K in 100-liter FEP Teflon®-film bags. The OH radicals were produced from the photolysis of methyl nitrite. The measured rate constants (×10¹¹ cm³ molecule^?1 s^?1) were as follows: 6.77 ± 0.50 for trans-4-octene, 1.40 ± 0.07 for 4-methyl-2-pentanone, and 6.70 ± 0.23 for trans-2-heptene using an absolute rate constant of 2.63 × 10¹¹ cm³ molecule^?1 s^?1 for the reaction of OH with propene; the principal reference organic. © John Wiley & Sons, Inc.     

Kinetics of the reactions of OH radicals with selected acetates and other esters under simulated atmospheric conditions

The relative hydroxyl radical reaction rate constants from the simulated atmospheric oxidation of selected acetates and other esters have been measured. Reactions were carried out at 297 ± 2 K in 100-liter FEP Teflon®-film bags. The OH radicals were generated from the photolysis of methyl nitrite in pure air. Using a rate constant of 2.63 × 10^?11 cm³ molecule^?1 s^?1 for the reaction of OH radicals with propene, the principal reference organic compound, the rate constants (×10¹² cm³ molecule^?1 s^?1) obtained for the acetates and esters used in this study are:

Measurement and estimation of rate constants for the reactions of hydroxyl radical with several alkanes and cycloalkanes

Relative rate experiments were used to measure ratios of chemical kinetics rate constants as a function of temperature for the reactions of OH with isobutane, isopentane, 2-methylpentane, 3-methylpentane, 2,3-dimethylbutane, 2,3-dimethylpentane, 2,4-dimethylpentane, 2,3,4-trimethylpentane, n-heptane, n-octane, cyclopentane, cyclohexane, and cycloheptane. The results have been used to calibrate a structure-reactivity rate constant estimation method for k(298 K) which, when combined with previously determined relationships between k(298 K) and the Arrhenius parameters, is capable of determining the temperature dependence accurately. The estimation method reproduces most of the observed rate data within experimental accuracy but appears to fail for 2,3-dimethylbutane, which has an anomalously high rate constant. Curvature in the Arrhenius plots at low temperatures is not present for compounds with a single type of C-H bond and, for compounds with different C-H bonds, is shown to be consistent with effects due to different group sites on the molecule.     

Gas phase reactions of hydroxyl radicals arenes

At elevated temperatures. OH is found to abstract H from benzene derivatives; in air, phenols are then generated via reversible addition of ArOO· to arene.     

Relative rate constants of reactions of alkyl radicals of methacrylates and acrylates with oxygen and stable nitroxyl radicals

Conclusions In methacrylates and acrylates undergoing oxidation, stable nitroxyl radicals react with the alkyl radicals of the monomers. The relative rate constants of the reactions of and O₂ with alkyl radicals for the different esters range from 0.36 to 5.20.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 4, pp. 753–756, April, 1977.We thank V. A. Golubev for providing the stable nitroxyl radicals.     

Formation of hydroxyl radicals by collapsing ozone microbubbles under strongly acidic conditions

Hydroxyl radicals are strong oxidants in aqueous solution, reacting rapidly with a wide range of dissolved compounds. In contrast, ozone is a highly selective oxidant. Understanding the process by which ozone is transformed into hydroxyl radicals is important in the treatment of wastewater and drinking water. We use electron spin-resonance spectroscopy to demonstrate that when microbubbles of ozone in strongly acidic aqueous solution collapse, the ozone progressively decomposes and large quantities of hydroxyl radicals are generated. Moreover, the degradation of polyvinyl alcohol, which is ozone resistant, was also observed during the collapse of the microbubbles. These findings indicate that ozone microbubbles are potentially useful in future water-treatment applications.     

Gas-phase reaction of n-butyl acetate with the hydroxyl radical under simulated tropospheric conditions: Relative rate constant and product study

The gas-phase reaction of n-butyl acetate with hydroxyl radicals has been studied in an environmental smog chamber at 298 K atmospheric pressure, and simulated tropospheric concentrations. The rate constant for this reaction has been determined by a relative method and the experimental result, relative to n-octane used as reference compound, is This value appears to be about 25% higher than absolute rate constants found in the literature, but agrees very well with the other relative determination. Two reaction products have been identified and their production yield has been estimated, each accounting for about (15 ± 5)% of the overall OH reaction processes. The two observed products are \centerline{ 2--oxobutil acetate ($\rm CH_3$--C0--0--$\rm CH_2$--CO--$\rm CH_2$--$\rm CH_3$)} and \centerline{ 2--oxobutil acetate ($\rm CH_3$--C0--0--$\rm CH_2$--$\rm CH_2$--CO--$\rm CH_3$)} The accuracy of the relative rate constant obtained is examined considering the evolution of the reactivity of the alkoxy end of the esters. Formation mechanisms for the two observed products are proposed and the likely other degradation channels are discussed. © 1996 John Wiley & Sons, Inc.     

A kinetic and mechanistic study of the reactions of OH radicals and Cl atoms with 3,3,3-trifluoropropanol under atmospheric conditions

Product distribution studies of the OH radical and Cl atom initiated oxidation of CF3CH2CH2OH in air at 1 atm and 298 +/- 5 K have been carried out in laboratory and outdoor atmospheric simulation chambers in the presence and absence of NOx. The results show that CF3CH2CHO is the only primary product and that the aldehyde is fairly rapidly removed from the system. In the absence of NOx the major degradation product of CF3CH2CHO is CF3CHO, and the combined yields of the two aldehydes formed from CF3CH2CH2OH are close to unity (0.95 +/- 0.05). In the presence of NOx small amounts of CF3CH2C(O)O2NO2 were also observed (<15%). At longer reaction times CF3CHO is removed from the system to give mainly CF2O. The laser photolysis-laser induced fluorescence technique was used to determine values of k(OH + CF3CH2CH2OH) = (0.89 +/- 0.03) x 10(-12) and k(OH + CF3CH2CHO) = (2.96 +/- 0.04) x 10(-12) cm3 molecule(-1) s(-1). A relative rate method has been employed to measure the rate coefficients k(OH + CF3CH2CH2OH) = (1.08 +/- 0.05) x 10(-12), k(OH + C6F13CH2CH2OH) = (0.79 +/- 0.08) x 10(-12), k(Cl + CF3CH2CH2OH) = (22.4 +/- 0.4) x 10(-12), and k(Cl + CF3CH2CHO) = (25.7 +/- 0.4) x 10(-12) cm3 molecule(-1) s(-1). The results from this investigation are discussed in terms of the possible importance of emissions of fluorinated alcohols as a source of fluorinated carboxylic acids in the environment.     

Temperature dependence of the relative rate constants of the reactions of alkanes with oh radicals in aqueous solution

A study was carried out on the rate constant ratio (k _RH/k _EtH) in reactions of alkanes C₃H₈, n-C₄H₁₀, n-C₅H₁₂, n-C₆H₁₄, i-C₄H₁₀, c-C₅H₁₀, and c-C₆H₁₂ with OH radicals in water at 5-55°C and the relative activation parameters A _RH/A _EtH and E _EtH – E _RH. The values of E _EtH – E _RH in water and the gas phase have opposite signs. The values of k _RH/k _EtH decrease with increasing temperature in the gas phase but increase in water. The behavior of these reactions in water may be attributed to a solvent cage effect.     

A reinvestigation of the rate constants for the reactions of ozone with cyclopentene and cyclohexene under atmospheric conditions

The rates of decay of ozone in a large excess of the cycloalkene have been re-measured by an improved experimental procedure, and the following second-order rate constants (cm³ molecule^?1 s^?1) have been determined in synthetic air at atmospheric pressure: The results are discussed in relation to existing literature data on the kinetics of ozonecycloalkene reactions.     

Absolute rate constants for some intermolecular reactions of alpha-aminoalkylperoxyl radicals. Comparison with alkylperoxyls

Seven alpha-aminoalkylperoxyl radicals have been generated by 355 nm laser flash photolysis (LFP) of oxygen-saturated di-tert-butyl peroxide containing mono-, di-, and trialkylamines and a dialkylarylamine. All these peroxyls possess absorptions in the near-UV (strongest for the trialkylamine-derived peroxyls) which permits direct monitoring of the kinetics of their reactions with many substrates. The measured rate constants for hydrogen atom abstraction from some phenols and oxygen atom transfer to triphenylphosphine demonstrated that all seven alpha-aminoalkylperoxyls have similar reactivities toward each specific substrate. More importantly, a comparison with literature data for alkylperoxyls shows that alpha-aminoalkylperoxyls and these alkylperoxyls have essentially the same reactivities. The combination of LFP and alkylamines provides a quick, reliable method for determining absolute rate constants for alkylperoxyl radical reactions, an otherwise laborious task.     

Pulse radiolysis studies on reactions of hydroxyl radicals with selenocystine derivatives

Reactions of hydroxyl radicals (*OH) with selenocystine (SeCys) and two of its analogues, diselenodipropionic acid (SeP) and selenocystamine (SeA), have been studied in aqueous solutions at pHs of 1, 7, and 10 using the pulse radiolysis technique coupled with absorption detection. All of these diselenides react with *OH radicals with rate constants of approximately 10(10) M(-1) s(-1), producing diselenide radical cations ( approximately 1-5 micros after the pulse), with an absorption maximum at 560 nm, by elimination of H(2)O or OH(-) from hydroxyl radical adducts. Assignment of the 560 nm band to the diselenide radical cation was made by comparing the transient spectra with those produced upon reaction of diselenides with specific one-electron oxidants, Cl(2)(*-) (pH 1) and Br(2)(*-) radicals (pHs of 7 and 10). SeP having a carboxylic acid functionality showed quantitative conversion of hydroxyl radical adducts to radical cations. The compounds SeCys and SeA, having an amino functional group, in addition to the radical cations, produced a new transient with lambda(max) at 460 nm, at later time scales ( approximately 20-40 micros after the pulse). The rate and yield of formation of the 460 nm band increased with increasing concentrations of either SeCys or SeA. In analogy with similar studies reported for analogous disulfides, the 460 nm transient absorption band has been assigned to a triselenide radical adduct. The one-electron reduction potentials of the compounds were estimated to be 0.96, 1.3, and 1.6 V versus NHE, respectively, for SeP, SeCys, and SeA at pH 7. From these studies, it has been concluded that the electron-donating carboxylic acid group decreases the reduction potential and facilitates quantitative conversion of hydroxyl radical adducts to radical cations, while the electron-withdrawing NH(3)(+) group not only increases the reduction potential but also leads to fragmentation of the hydroxyl radical adduct to selenyl radicals, which are converted to triselenide radical adducts.     

Polar transition states in reactions of aromatic hydrocarbons with hydroxyl radicals

The mechanism of formation of radical adducts in the radical hydroxylation of aromatic hydrocarbons has been investigated. The analysis of correlations between the logarithms of the rate constants of the radical hydroxylation of substituted benzenes, naphthalenes, and condensed hydrocarbons in aqueous solutions and the ⁺ constants of the substituents, the ionization potentials, the localization energies of the arenes, and also the quantum-chemical calculations of the surfaces of potential energies of the hydroxylation of benzene and naphthalene leads to the conclusion that an ion pair in the excited state, formed by the cation radical of the hydrocarbon and the hydroxyl anion, can serve as the model for the transition states of the above-mentioned reactions. A decrease in the ionization potential of the arene leads to an increase in the coulombic interaction in this pair, to its stabilization, and to an increase in the reaction rate.Deceased.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 22, No. 1, pp. 96–99, January–February, 1986.     

Fast kinetics of the reactions of hydroxyl radicals with nitrone spin traps

The technique of spin trapping with nitrone spin traps nas gained wide acceptance as a method for estimating·OH yields in ESR studies. In our study, fast optical kinetic techniques applied to a series of these traps (PBN, 2-PyBN, 3-PyBN, 4-PyBN, 3-PyOBN and 4-PyOBN) reveal relaxation spectra that indicate two absorption maxima with different time constants, with all except 4-PyOBN showing second order behavior. These two spectral regions show different kinetics. Thus, two reaction sites are indicated, only one of which is necessarily a measure of initial · OH when ESR methods are used. One other trap (DMPO) after · OH reaction decays in one mode suggesting that its final product might be useful as a measure of initial · OH. Also, our ESR evidence shows that OH detection can be improved significantly by spin trapping -hydroxyalkyl radicals formed by · OH attack on alcohols.     

Studies on some radical transfer reactions by entrapping the radicals as polymer end groups. I. Reaction of hydroxyl radicals with amines

The reaction of OH radicals with a number of amines has been studied by entrapping the resultant radicals as polymer end groups which have been detected and estimated by the sensitive dye partition technique. Expressions have been developed relating the average amounts of end groups per polymer molecule to the rate constant of the radical transfer reaction, the rate constants determined for reaction with n-butyl, n-hexyl, and n-octyl amine being 1.00 × 10¹⁰, 1.31 × 10¹⁰, and 1.46 × 10¹⁰ mol^?1 L s^?1, respectively, at 25°C. The order of reactivity for amines of different classes has been found to be as primary < secondary > tertiary, the rate constants for reaction with n-butyl, dibutyl, and tributyl amine being 1.00 × 10¹⁰, 1.81 × 10¹⁰, and 1.67 × 10¹⁰ mol^?1 L s^?1, respectively, at 25°C. The change in the reactivity of the amine with chain length and amine class has been explained by activation and deactivation of the CH₂ group from which H abstraction by OH radicals occurs, respectively, by the alkyl group and by the protonated amino nitrogen under the acidic condition of the medium. Between pH 1.00 and 2.17, the rate of the reaction with n-butyl amine remains practically unchanged, but from pH 2.20 to 2.72 the rate constant increases with increasing pH, indicating that deprotonation of the positively charged nitrogen starts at about pH 2.20. The method is simple and accurate and can be applied to detect and estimate very reactive radicals.     

Theoretical studies on the reactions of hydroxyl radicals with trimethylsilane and tetramethylsilane

The multiple-channel reactions OH + SiH(CH₃)₃ → products (R1) and the single-channel reaction OH + Si(CH₃)₄ → Si(CH₃)₃CH₂ + H₂O (R2) have been studied by means of the direct dynamics method at the BMC-CCSD//MP2/6-311+G(2d,2p) level. The optimized geometries, frequencies and minimum energy path are all obtained at the MP2/6-311+G(2d,2p) levels, and energy information is further refined by the BMC-CCSD (single-point) level. The rate constants for every reaction channels are calculated by canonical variational transition states theory (CVT) with small-curvature tunneling (SCT) contributions over the temperature range 200–2,000 K. The theoretical total rate constants are in good agreement with the available experimental data, and the three-parameter expression k ₁ = 2.53×10⁻²¹ T ^3.14 exp(1, 352.86/T), k ₂ = 6.00 × 10⁻¹⁹ T ^2.54 exp(−106.11/T) (in unit of cm³ molecule⁻¹ s⁻¹) over the temperature range 200–2,000 K are given. Our calculations indicate that at the low temperature range, for reaction R1, H-abstraction is favored for the SiH group, while the abstraction from the CH₃ group is a minor channel. Electronic Supplementary Material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

An FT-IR study of the isomerization of 1-butoxy radicals under atmospheric conditions

Relative rate-studies of the reactions of 1-butoxy radicals have been carried out using a 47 L static reactor with detection of end products by FT-IR spectroscopy. Experiments were performed at 700 torr total pressure and over the temperature range 253–295 K. The chemistry of 1-butoxy is characterized by a competition between reaction with oxygen CH₃CH₂CH₂CH₂O + O₂ → n-C₃H₇CHO + HO₂ (R2), which yields butanal and isomerization CH₃CH₂CH₂CH₂O → CH₂CH₂CH₂CH₂OH (R3), to form a hydroxylated carbonyl-product. A reference spectrum attributed to the product of 1-butoxy isomerization was obtained and used to determine the competition between 1-butoxy isomerization versus reaction with oxygen. The results indicate that isomerization is the dominant fate of 1-butoxy radicals at ambient temperature and pressure and that its importance decreases with decreasing temperature. The rate-coefficient ratio k₃/k₂ (molecule cm⁻³) = 5.5 × 10²³ exp[(−25.1 ± 0.9 kJmol⁻¹)/RT] was obtained. This agrees with other estimates based on methods without monitoring of the isomerization product.