Reaction Of OH Radicals Research Articles

Photochemical properties of trans-1-chloro-3,3,3-trifluoropropene (trans-CHCl═CHCF3): OH reaction rate constant, UV and IR absorption spectra, global warming potential, and ozone depletion potential.

Measurements of the rate constant for the gas-phase reactions of OH radicals with trans-1-chloro-3,3,3-trifluoropropene (trans-CHCl═CHCF3) were performed using a flash photolysis resonance-fluorescence technique over the temperature range 220-370 K. The reaction rate constant exhibits a noticeable curvature of the temperature dependence in the Arrhenius plot, which can be represented by the following expression: kt-CFP (220-370 K) = 1.025 × 10(-13) × (T/298)(2.29) exp(+384/T) cm(3)molecule(-1) s(-1). The room-temperature rate constant was determined to be kt-CFP (298 K) = (3.29 ± 0.10) × 10(-13) cm(3) molecule(-1) s(-1), where the uncertainty includes both two standard errors (statistical) and the estimated systematic error. For atmospheric modeling purposes, the rate constant below room temperature can be represented by the following expression: kt-CFP (220-298 K) = (7.20 ± 0.46) × 10(-13) exp[-(237 ± 16)/T] cm(3) molecule(-1) s(-1). There was no difference observed between the rate constants determined at 4 kPa (30 Torr) and 40 kPa (300 Torr) at both 298 and 370 K. The UV and IR absorption cross sections of this compound were measured at room temperature. The atmospheric lifetime, global warming potential, and ozone depletion potential of trans-CHCl═CHCF3 were estimated.

The experimental and mechanism study of novel heterogeneous Fenton-like reactions using Fe3−xTixO4 catalysts for Hg0 absorption

Novel heterogeneous Fenton-like Fe3−xTixO4 catalysts prepared by co-precipitation method were used for Hg0 absorption in a self-designed bubbling reactor system. Characterization by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) showed that Ti successfully entered the structure of Fe3−xTixO4 catalysts and was enriched on the surface of Fe3−xTixO4. Important factors affecting Fe3−xTixO4 catalysts activity toward Hg0 absorption, such as H2O2 concentration and Ti content in Fe3−xTixO4, were considered. Results showed that Fe3−xTixO4 can effectively decompose H2O2 to generate highly reactive OH radicals, and oxidize Hg0 to Hg2+ in Fenton-like solutions. The mechanism model of Hg0 absorption by heterogeneous Fenton-like reaction was preliminarily established. Theoretically calculated results using the model well agreed with the experimental results, indicating that Hg0 absorption is simultaneously controlled by solid–liquid surface oxidation and gas–liquid film oxidation. These findings can help investigations on the mechanism of Hg0 absorption by the heterogeneous Fenton-like reaction.

FTIR gas-phase kinetic study on the reactions of some acrylate esters with OH radicals and Cl atoms.

Acrylate esters are α,β-unsaturated esters that contain vinyl groups directly attached to the carbonyl carbon. These compounds are widely used in the production of plastics and resins. Atmospheric degradation processes of these compounds are currently not well understood. The kinetics of the gas phase reactions of OH radicals with methyl 3-methylacrylate and methyl 3,3-dimethylacrylate were determined using the relative rate technique in a 50 L Pyrex photoreactor using in situ FTIR spectroscopy at room temperature (298 ± 2 K) and atmospheric pressure (708 ± 8 Torr) with air as the bath gas. Rate coefficients obtained were (in units cm(3) molecule(-1) s(-1)): (3.27 ± 0.33) × 10(-11) and (4.43 ± 0.42) × 10(-11), for CH3CH═CHC(O)OCH3 and (CH3)2CH═CHC(O)OCH3, respectively. The same technique was used to study the gas phase reactions of hexyl acrylate and ethyl hexyl acrylate with OH radicals and Cl atoms. In the experiments with Cl, N2 and air were used as the bath gases. The following rate coefficients were obtained (in cm(3) molecule(-1) s(-1)): k3 (CH2═CHC(O)O(CH2)5CH3 + Cl) = (3.31 ± 0.31) × 10(-10), k4(CH2═CHC(O)OCH2CH(CH2CH3)(CH2)3CH3 + Cl) = (3.46 ± 0.31) × 10(-10), k5(CH2═CHC(O)O(CH2)5CH3 + OH) = (2.28 ± 0.23) × 10(-11), and k6(CH2═CHC(O)OCH2CH(CH2CH3)(CH2)3CH3 + OH) = (2.74 ± 0.26) × 10(-11). The reactivity increased with the number of methyl substituents on the double bond and with the chain length of the alkyl group in -C(O)OR. Estimations of the atmospheric lifetimes clearly indicate that the dominant atmospheric loss process for these compounds is their daytime reaction with the hydroxyl radical. In coastal areas and in some polluted environments, Cl atom-initiated degradation of these compounds can be significant, if not dominant. Maximum Incremental Reactivity (MIR) index and global warming potential (GWP) were also calculated, and it was concluded that these compounds have significant MIR values, but they do not influence global warming.

Gas-phase kinetics of OH radicals reaction with a series of fluorinated acrylates and methacrylates at atmospheric pressure and 298 K

Rate coefficients for the gas phase reactions of OH radicals with 2,2,2-trifluoroethylmethacrylate (k1) 1,1,1,3,3,3-hexafluoroisopropylacrylate (k2), 1,1,1,3,3,3-hexafluoroisopropylmethacrylate (k3), and 2,2,2-trifluoroethylacrylate (k4) were determinated to be k1 = (2.54 ± 0.12) × 10−11, k2 = (1.41 ± 0.11) × 10−11, k3 = (1.65 ± 0.14) × 10−11 and k4 = (1.25 ± 0.13) × 10−11 cm3 molecule−1 s−1, respectively. Kinetic experiments were performed at room temperature and atmospheric pressure using the relative-rate technique with GC-FID analysis. This study is the first kinetic for these reactions of OH radicals under atmospheric pressure. Additionally, the rate constants obtained are compared with other halogenated and non halogenated acrylates and methacrylates to develop structure/reactivity relationships in terms of the presence of fluorine and ester groups and the hydrocarbon chain length. The atmospheric implications of the reactions were assessed by the estimation of the tropospheric lifetimes of the title reactions.

Atmospheric reactions of methylcyclohexanes with Cl atoms and OH radicals: determination of rate coefficients and degradation products.

As the result of biogenic and anthropogenic activities, large quantities of chemical compounds are emitted into the troposphere. Alkanes, in general, and cycloalkanes are an important chemical class of hydrocarbons found in diesel, jet and gasoline, vehicle exhaust emissions, and ambient air in urban areas. In general, the primary atmospheric fate of organic compounds in the gas phase is the reaction with hydroxyl radicals (OH). The oxidation by Cl atoms has gained importance in the study of atmospheric reactions because they may exert some influence in the boundary layer, particularly in marine and coastal environments, and in the Arctic troposphere. The aim of this paper is to study of the atmospheric reactivity of methylcylohexanes with Cl atoms and OH radicals under atmospheric conditions (in air at room temperature and pressure). Relative kinetic techniques have been used to determine the rate coefficients for the reaction of Cl atoms and OH radicals with methylcyclohexane, cis-1,4-dimethylcyclohexane, trans-1,4-dimethylcyclohexane, and 1,3,5-trimethylcyclohexane at 298 ± 2K and 720 ± 5Torr of air by Fourier transform infrared) spectroscopy and gas chromatography-mass spectrometry (GC-MS) in two atmospheric simulation chambers. The products formed in the reaction under atmospheric conditions were investigated using a 200-L Teflon bag and employing the technique of solid-phase microextraction coupled to a GC-MS. The rate coefficients obtained for the reaction of Cl atoms with the studied compounds are the following ones (in units of 10(-10)cm(3)molecule(-1)s(-1)): (3.11 ± 0.16), (2.89 ± 0.16), (2.89 ± 0.26), and (2.61 ± 0.42), respectively. For the reactions with OH radicals the determined rate coefficients are (in units of 10(-11)cm(3)molecule(-1)s(-1)): (1.18 ± 0.12), (1.49 ± 0.16), (1.41 ± 0.15), and (1.77 ± 0.23), respectively. The reported error is twice the standard deviation. A detailed mechanism for ring-retaining product channels is proposed to justify the observed reaction products. The global tropospheric lifetimes estimated from the reported OH- and Cl-rate coefficients show that the main removal path for the investigated methylcyclohexanes is the reaction with OH radicals. But in marine environments, after sunrise, Cl reactions become more important in the tropospheric degradation. Thus, the estimated lifetimes range from 16 to 24h for the reactions of the OH radical (calculated with [OH] = 10(6)atomscm(-3)) and around 7-8h in the reactions with Cl atoms in marine environments (calculated with [Cl] = 1.3 × 10(5)atomscm(-3)). The reaction of Cl atoms and OH radicals and methylcylohexanes can proceed by H abstraction from the different positions.

The mechanism and dynamic studies for the reactions of OH radical with CH3−nFnOCF2CHFCl (n = 0, 1, and 2)

The mechanism and dynamic properties for the multi-channel reactions of CH3OCF2CHFCl+OH (R1), CH2FOCF2CHFCl+OH (R2), and CHF2OCF2CHFCl+OH (R3) were carried out theoretically. The geometric parameters were optimized at the BMK/6-311+G(d,p) level. Subsequently, the energies were refined at the BMC-CCSD level. Based on the information of partial potential energy surface, the rate constants were evaluated by using the canonical variational transition state theory (CVT) with a small-curvature tunneling correction (SCT) method. For every reaction, there are two possible H-abstraction positions, i.e., CH3−nFn (n=0, 1, and 2) group and CHFCl group. The major reaction channel and the effect of fluorine substitution on the reactivity are explored. Both questions are elucidated by analyzing the reaction energy, barrier height, bond dissociation energy, and rate constants.

Methyl-Perfluoroheptene-Ethers (CH3OC7F13): Measured OH Radical Reaction Rate Coefficients for Several Isomers and Enantiomers and Their Atmospheric Lifetimes and Global Warming Potentials

Mixtures of methyl-perfluoroheptene-ethers (CH3OC7F13, MPHEs) are currently in use as replacements for perfluorinated alkanes (PFCs) and poly-ether heat transfer fluids, which are persistent greenhouse gases with lifetimes >1000 years. At present, the atmospheric processing and environmental impact from the use of MPHEs is unknown. In this work, rate coefficients at 296 K for the gas-phase reaction of the OH radical with six key isomers (including stereoisomers and enantiomers) of MPHEs used commercially were measured using a relative rate method. Rate coefficients for the six MPHE isomers ranged from ∼ 0.1 to 2.9 × 10(-12) cm(3) molecule(-1) s(-1) with a strong stereoisomer and -OCH3 group position dependence; the (E)-stereoisomers with the -OCH3 group in an α- position relative to the double bond had the greatest reactivity. Rate coefficients measured for the d3-MPHE isomer analogues showed decreased reactivity consistent with a minor contribution of H atom abstraction from the -OCH3 group to the overall reactivity. Estimated atmospheric lifetimes for the MPHE isomers range from days to months. Atmospheric lifetimes, radiative efficiencies, and global warming potentials for these short-lived MPHE isomers were estimated based on the measured OH rate coefficients along with measured and theoretically calculated MPHE infrared absorption spectra. Our results highlight the importance of quantifying the atmospheric impact of individual components in an isomeric mixture.

Reactions of Benzene and 3-Methylpyrrole with the•OH and•OOH Radicals: An Assessment of Contemporary Density Functional Theory Methods

A high-level quantum chemistry investigation has been carried out for the addition and abstraction reactions by the radicals (•)OH and (•)OOH to and from the model alkenes 3-methylpyrrole and benzene. These models were chosen to reflect the functionalities contained in the side chain of the amino acid tryptophan. The W1BD procedure was used to calculate benchmark barriers and reaction energies for the smaller model system of (•)OOH addition to ethylene. It was found that the CBS-QB3 methodology compares best with the W1BD benchmark, demonstrating a mean absolute deviation (MAD) from W1BD of 3.9 kJ mol(-1). For the reactions involving the (•)OH radical and benzene or 3-methylpyrrole, addition is favored over abstraction in all cases. In particular the CBS-QB3 calculations suggest a barrierless addition reaction of the (•)OH radical to position two of 3-methylpyrrole. For the analogous addition and abstraction reactions involving the (•)OOH radical, the same order of reactivity was found, albeit with higher barriers. A number of other processes involving the addition of the (•)OOH radical were also investigated. The main findings of these studies determined that the initial (•)OOH barrier of stepwise addition to 3-methylpyrrole (+18.8 kJ mol(-1)) is significantly smaller than the concerted addition barrier (+71.5 kJ mol(-1)). This conclusion contrasts starkly with the situation for ethylene in which it is well established that the concerted process has the smaller barrier. A considerable variety of contemporary density functional theory procedures have been tested to examine their accuracy in predicting the CBS-QB3 results. It was found that the best overall performing method was UBMK with an MAD of 7.3 kJ mol(-1). A number of other functionals additionally performed well. They included UM06, RM06, UXYG3 and RXYG3, all of which have MADs of less than 8 kJ mol(-1).

H2SO4 formation from the gas-phase reaction of stabilized Criegee Intermediates with SO2: Influence of water vapour content and temperature

The importance of gas-phase products from alkene ozonolysis other than OH radicals, most likely stabilized Criegee Intermediates (sCI), for the process of atmospheric SO2 oxidation to H2SO4 has been recently discovered. Subjects of this work are investigations on H2SO4 formation as a function of water vapour content (RH = 2–65%) and temperature (278–343 K) starting from the ozonolysis of trans-2-butene and 2,3-dimethyl-2-butene (TME). H2SO4 production other than via the OH radical reaction was attributed to the reaction of SO2 with sCI, i.e. acetaldehyde oxide arising from trans-2-butene ozonolysis and acetone oxide from TME. Measurements have been conducted in an atmospheric pressure flow tube using NO3−–CI-APi-TOF mass spectrometry for H2SO4 detection. The sCI yields derived from H2SO4 measurements at 293 K were 0.49 ± 0.22 for acetaldehyde oxide and 0.45 ± 0.20 for acetone oxide. Our findings indicate a H2SO4 yield from sCI + SO2 of unity or close to unity. The deduced rate coefficient ratio for the reaction of sCI with H2O and SO2, k(sCI + H2O)/k(sCI + SO2), was found to be strongly dependent on the structure of the Criegee Intermediate, for acetaldehyde oxide at 293 K: (8.8 ± 0.4)·10−5 (syn- and anti-conformer in total) and for acetone oxide: <4·10−6. H2SO4 formation from sCI was pushed back with rising temperature in both reaction systems most probably due to an enhancement of sCI decomposition. The ratio k(dec)/k(sCI + SO2) increased by a factor of 34 (acetone oxide) increasing the temperature from 278 to 343 K. In the case of acetaldehyde oxide the temperature effect is less pronounced. The relevance of atmospheric H2SO4 formation via sCI + SO2 is discussed in view of its dependence on the structure of the Criegee Intermediate.

Gas-Phase Reaction of Methyl Isothiocyanate and Methyl Isocyanate with Hydroxyl Radicals under Static Relative Rate Conditions

Gaseous methyl isothiocyanate (MITC), the principal breakdown product of the soil fumigant metam sodium (sodium N-methyldithiocarbamate), is an inhalation exposure concern to persons living near treated areas. Inhalation exposure also involves gaseous methyl isocyanate (MIC), a highly reactive and toxic transformation product of MITC. In this work, gas-phase hydroxyl (OH) radical reaction rate constants of MITC and MIC have been determined using a static relative rate technique under controlled laboratory conditions. The rate constants obtained are 15.36 × 10(-12) cm(3) molecule(-1) s(-1) for MITC and 3.62 × 10(-12) cm(3) molecule(-1) s(-1) for MIC. The average half-lives of MITC and MIC in the atmosphere are estimated to be 15.7 and 66.5 h, respectively. The molar conversion of MITC to MIC for OH radical reactions is 67% ± 8%, which indicates that MIC is the primary product of the MITC-OH reaction in the gas phase.

Products and Mechanism of the Reactions of OH Radicals and Cl Atoms with Methyl Methacrylate (CH2═C(CH3)C(O)OCH3) in the Presence of NOx

The OH radical and Cl atom initiated photodegradation of methyl methacrylate has been investigated in a 1080 L quartz-glass environmental chamber at 298 ± 2 K and atmospheric pressure of synthetic air using in situ FTIR spectroscopy to monitor the reactants and products. The major products observed in the OH reaction were methyl pyruvate (92 ± 16%) together with formaldehyde (87 ± 12%) as a coproduct from the C1-C2 bond cleavage channel of the intermediate 1,2-hydroxyalkoxy radical, formed by the addition of OH to the terminal carbon of the double bond which is designated C1. For the Cl atom reaction, the products identified were chloroacetone (41 ± 6%) together with its coproduct formaldehyde (35 ± 5%) and methyl pyruvate (24 ± 4%) together with its coproduct formylchloride (25 ± 4%). The results show that the fate of the intermediate 1,2-chloroalkoxy radical involves not only cleavage of the C1-C2 bond but also quite substantial cleavage of the C2-C3 bond. The present results are compared with previous studies of acrylates, showing different branching ratios for the OH and Cl addition reactions in the presence of NOx. Atmospheric implications are discussed.

Products of the OH Radical-Initiated Reactions of Furan, 2- and 3-Methylfuran, and 2,3- and 2,5-Dimethylfuran in the Presence of NO

Products of the gas-phase reactions of OH radicals with furan, furan-d4, 2- and 3-methylfuran, and 2,3- and 2,5-dimethylfuran have been investigated in the presence of NO using direct air sampling atmospheric pressure ionization tandem mass spectrometry (API-MS and API-MS/MS), and gas chromatography with flame ionization and mass spectrometric detectors (GC-FID and GC-MS) to analyze samples collected onto annular denuders coated with XAD solid adsorbent and further coated with O-(2,3,4,5,6-pentafluorobenzyl)hydroxylamine for derivatization of carbonyl-containing compounds to their oximes. The products observed were unsaturated 1,4-dicarbonyls, unsaturated carbonyl-acids and/or hydroxy-furanones, and from 2,5-dimethylfuran, an unsaturated carbonyl-ester. Quantification of the unsaturated 1,4-dicarbonyls was carried out by GC-FID using 2,5-hexanedione as an internal standard, and the measured molar formation yields were: HC(O)CH═CHCHO (dominantly the E-isomer) from OH + furan, 75 ± 5%; CH3C(O)CH═CHCHO (dominantly the E-isomer) from OH + 2-methylfuran, 31 ± 5%; HC(O)C(CH3)═CHCHO (a E-/Z-mixture) from OH + 3-methylfuran, 38 ± 2%; and CH3C(O)C(CH3)═CHCHO from OH + 2,3-dimethylfuran, 8 ± 2%. In addition, a formation yield of 3-hexene-2,5-dione from OH + 2,5-dimethylfuran of 27% was obtained from a single experiment, in good agreement with a previous value of 24 ± 3% from GC-FID analyses of samples collected onto Tenax solid adsorbent without derivatization.

Mechanism, kinetics and atmospheric fate of CF3CH[dbnd]CH2, CF3CF[dbnd]CH2, and CF3CF[dbnd]CF2 by its reaction with OH-radicals: CVT/SCT/ISPE and hybrid meta-DFT methods

The dual level direct dynamic study is carried out for the reactions of CF3CHCH2, CF3CFCH2 and CF3CFCF2 with hydroxyl radicals. The dynamic calculations are performed using the variational transition state theory (VTST) with interpolated single-point energies (ISPE) method at M06-2X/MG3S//M06-2X/6-31+G(d,p) level of theory. All the possible reactions such as abstraction and addition–elimination pathways are explored for the title reactions. The temperature dependent rate coefficients using canonical variational transition state theory with small curvature tunneling for the reaction of OH radicals with test molecules over the temperature range of 200–3000K are computed. The predicted rate coefficients (in 10−12cm3molecule−1s−1) using CVT/SCT/ISPE methodology for the reaction of CF3CHCH2, CF3CFCH2 and CF3CFCF2 with OH radicals are 1.48, 1.02 and 1.77, respectively, are in good agreement with reported ones at 298K. The atmospheric lifetimes for the test molecules CF3CHCH2, CF3CFCH2 and CF3CFCF2 are calculated at 277K to be 8, 11 and 6 days, respectively. Global warming potentials are also reported for the different time horizon of 20, 100 and 500 years.

Relative rate coefficient measurements of OH radical reactions with (Z)-2-hexen-1-ol and (E)-3-hexen-1-ol under simulated atmospheric conditions

The relative rate technique was used to determine the rate coefficients of the reactions of OH radicals with (Z)-2-hexen-1-ol (k1), and (E)-3-hexen-1-ol (k2), at (296 ± 2) K and (750 ± 10) Torr of N2 or pure air. The reactions were investigated using a 200 L Teflon reaction chamber and a gas chromatograph coupled with flame-ionization detection. The following rate coefficients were derived, in units of cm3 mol−1 s−1: k1 = (1.1 ± 0.4) × 10−10 and k2 = (0.8 ± 0.1) × 10−10. This is the first experimental determination of k1 and k2. A comparison between the experimental rate coefficients (kexp) and the calculated rate coefficients using the structure–activity relationship (SAR) method (kSAR), for the reaction of different unsaturated alcohols with OH radicals is presented. The atmospheric lifetimes of the studied alcohols were estimated considering the rate coefficients of their reactions with OH and NO3 radicals. The radiative efficiencies (REs) were obtained from the infrared spectra of the two hexenols and the global warming potentials (GWPs) were then estimated. Atmospheric implications of the alcohols emission are briefly discussed.

Theoretical study of a reaction mechanism of tropospheric interest: CH3CH2F + OH

Dual-level electronic structure calculation has been performed to investigate the mechanism and all possible channels of OH radical reaction with CH3CH2F. Geometries and frequencies are computed at the B3LYP/6-311G(d,p) level of theory for all stationary points and complexes and transition states are located. Potential energy surfaces are constructed at the PMP2/cc-pVTZ//B3LYP/6-311G(d,p) level + ZPE correction. Four types of reaction channels are identified: hydrogen abstraction, fluorine abstraction and attack on carbon atom along or perpendicular to the C–C bond axis. Hydrogen abstraction channels have lower barriers and are more exothermic, while out-of-plane β–H abstraction with the lowest barrier is competitive with a–H abstraction. Due to the high energy barrier, contributions of non-H abstraction channels are excluded. The influence of hydrogen bonding interaction is clearly observed in the barrier heights.

Fe(iii)-photocatalytic partial oxidation of benzyl alcohol to benzaldehyde under UV-solar simulated radiation

A great deal of interest is recorded among researchers in the identification of new catalytic systems that make possible the selective oxidation of organic species in the presence of non-toxic solvents, primarily water, through the use of inexpensive catalysts. The possibility to selectively oxidize benzyl alcohol to benzaldehyde is studied in the present work by using ferric ions as homogeneous catalysts and oxygen as an oxidant under UV-solar simulated radiation. Due to the possibility that Fe(III) aquo-complex photolysis could generate undesired reactive OH radicals with the consequent occurrence of side reactions, most of the runs are carried out at pH = 0.5 at which these events have a reduced incidence. The results indicate that benzyl alcohol can be partially converted into benzaldehyde with yield and selectivity values higher than 40% and 80% respectively for the conditions adopted, with a minor occurrence of benzoic acid formation. Reaction schemes to account for the experimental observations are provided.

Experimental and Theoretical Study of the Kinetics and Mechanism of the Reaction of OH Radicals with Dimethyl Ether

The reaction of OH with dimethyl ether (CH3OCH3) has been studied from 195 to 850 K using laser flash photolysis coupled to laser induced fluorescence detection of OH radicals. The rate coefficient from this work can be parametrized by the modified Arrhenius expression k = (1.23 ± 0.46) × 10(-12) (T/298)(2.05±0.23) exp((257 ± 107)/T) cm(3) molecule(-1) s(-1). Including other recent literature data (923-1423 K) gives a modified Arrhenius expression of k1 = (1.54 ± 0.48) × 10(-12) (T/298 K)(1.89±0.16) exp((184 ± 112)/T) cm(3) molecule(-1) s(-1) over the range 195-1423 K. Various isotopomeric combinations of the reaction have also been investigated with deuteration of dimethyl ether leading to a normal isotope effect. Deuteration of the hydroxyl group leads to a small inverse isotope effect. To gain insight into the reaction mechanisms and to support the experimental work, theoretical studies have also been undertaken calculating the energies and structures of the transition states and complexes using high level ab initio methods. The calculations also identify pre- and post-reaction complexes. The calculations show that the pre-reaction complex has a binding energy of ~22 kJ mol(-1). Stabilization into the complex could influence the kinetics of the reaction, especially at low temperatures (<300 K), but there is no direct evidence of this occurring under the experimental conditions of this study. The experimental data have been modeled using the recently developed MESMER (master equation solver for multi energy well reactions) code; the calculated rate coefficients lie within 16% of the experimental values over the temperature range 200-1400 K with a model based on a single transition state. This model also qualitatively reproduces the observed isotope effects, agreeing closely above ~600 K but overestimating them at low temperatures. The low temperature differences may derive from an inadequate treatment of tunnelling and/or from an enhanced role of an outer transition state leading to the pre-reaction complex.

Atmospheric Chemistry of Methyl and Ethyl N,N,N′,N′-Tetramethylphosphorodiamidate and O,S-Dimethyl Methylphosphonothioate

Rate constants for the reactions of OH radicals with methyl N,N,N',N'-tetramethylphosphorodiamidate [CH3OP(O)[N(CH3)2]2; MTMPDA], ethyl N,N,N',N'-tetramethylphosphorodiamidate [C2H5OP(O)[N(CH3)2]2; ETMPDA], and O,S-dimethyl methylphosphonothioate [CH3OP(O)(CH3)SCH3; OSDMMP] have been measured over the temperature range 281-349 K at atmospheric pressure of air using a relative rate method. The rate expressions obtained were 4.96 × 10(-12) e((1058±71)/T) cm(3) molecule(-1) s(-1) (1.73 × 10(-10) cm(3) molecule(-1) s(-1) at 298 K) for OH + MTMPDA, 4.46 × 10(-12) e((1144±95)/T) cm(3) molecule(-1) s(-1) (2.07 × 10(-10) cm(3) molecule(-1) s(-1) at 298 K) for OH + ETMPDA, and 1.31 × 10(-13) e((1370±229)/T) cm(3) molecule(-1) s(-1) (1.30 × 10(-11) cm(3) molecule(-1) s(-1) at 298 K) for OH + OSDMMP. The rate constant for OH + OSDMMP was independent of O2 content over the range 2.1-71% O2 at 296 ± 2 K. In addition, rate constants for the reactions of NO3 radicals and O3 with MTMPDA, of (1.4 ± 0.1) × 10(-12) cm(3) molecule(-1) s(-1) and <3.5 × 10(-19) cm(3) molecule(-1) s(-1), respectively, were measured at 297 ± 2 K. Products of the OH radical- and, for MTMPDA, NO3 radical-initiated reactions were investigated using gas chromatography and in situ atmospheric pressure ionization mass spectrometry. A product of molecular weight 180 was observed from the OH and NO3 radical-initiated reactions of MTMPDA, and this is attributed to CH3OP(O)[N(CH3)2]N(CH3)CHO. Similarly, a product of molecular weight 194 was observed from the OH + ETMPDA reaction and attributed to C2H5OP(O)[N(CH3)2]N(CH3)CHO. Possible reaction mechanisms are discussed.

Rate coefficients for the gas-phase reaction of OH radical with α-pinene: an experimental and computational study

The rate coefficient for the gas-phase reaction of OH radical with α-pinene was measured at 298 K using relative rate methods, with propylene as a reference compound. The ratio of the rate coefficient for the reaction of OH radicals with α-pinene to that of with OH radicals with propylene was measured to be 1.77 ± 0.21. Considering the absolute value of the rate coefficient of the reaction of OH radicals with propylene as (3.01 ± 0.42)×10−11 cm3 molecule−1 s−1, the rate coefficient for the reaction of OH radicals with α-pinene was determined to be (5.33 ± 0.79)×10−11 cm3 molecule−1 s−1. To gain a deeper insight into the reaction mechanism, theoretical calculations were also carried out on this reaction. The rate coefficient of OH radical with α-pinene was calculated using canonical variational transition state theory with small-curvature tunnelling. The kinetics data obtained over the temperature range of 200–400 K were used to derive the Arrhenius expression: k(T) = 3.8×10−28 T5.2 exp[2897/T] cm3 molecule−1 s−1. The OH-driven atmospheric lifetime (τ) and ozone formation potential of α-pinene were calculated and reported in this work.

Free radical destruction of haloacetamides in aqueous solution

Haloacetamides are disinfection byproducts (DBPs) formed through chlorine/chloramine disinfection processes in drinking waters and have been recently highlighted in the US national Reconnaissance Survey. These species occur at low concentrations, but have been determined to have high cytotoxicity and mutagenicity and therefore may represent a human health hazard. Advanced oxidation/reduction processes (AO/RPs) which utilize free radical reactions are new alternatives to degrade these species. This study reports the absolute bimolecular reaction rate constants for 12 haloacetamides with •OH radical and hydrated electron. The •OH radical reaction rates varied from 5.18 × 107 to 1.14 × 1010 M−1 s−1, and hydrated electron reaction rates varied from 5.00 × 109 to 3.64 × 1010 M−1 s−1. Results obtained using an ion chromatograph indicated that ARPs are suitable for dehaloliazation of haloacetamides. These data are required for both evaluating AO/RPs for the destruction of these compounds and for studies of their fate and transport in surface waters where radical chemistry may be important in assessing their lifetime.