Gas-phase oxidation of methyl crotonate and ethyl crotonate. kinetic study of their reactions toward OH radicals and Cl atoms

Rate coefficients for the reactions of hydroxyl radicals and chlorine atoms with methyl crotonate and ethyl crotonate have been determined at 298 K and atmospheric pressure. The decay of the organics was monitored using gas chromatography with flame ionization detection (GC-FID), and the rate constants were determined using the relative rate method with different reference compounds. Room temperature rate coeficcients were found to be (in cm(3) molecule(-1) s(-1)): k(1)(OH + CH(3)CH═CHC(O)OCH(3)) = (4.65 ± 0.65) × 10(-11), k(2)(Cl + CH(3)CH═CHC(O)OCH(3)) = (2.20 ± 0.55) × 10(-10), k(3)(OH + CH(3)CH═CHC(O)OCH(2)CH(3)) = (4.96 ± 0.61) × 10(-11), and k(4)(Cl + CH(3)CH═CHC(O)OCH(2)CH(3)) = (2.52 ± 0.62) × 10(-10) with uncertainties representing ±2σ. This is the first determination of k(1), k(3), and k(4) under atmospheric pressure. The rate coefficients are compared with previous determinations for other unsaturated and oxygenated VOCs and reactivity trends are presented. In addition, a comparison between the experimentally determined k(OH) with k(OH) predicted from k vs E(HOMO) relationships is presented. On the other hand, product identification under atmospheric conditions has been performed for the first time for these unsaturated esters by the GC-MS technique in NO(x)-free conditions. 2-Hydroxypropanal, acetaldehyde, formaldehyde, and formic acid were positively observed as degradation products in agreement with the addition of OH to C2 and C3 of the double bond, followed by decomposition of the 2,3- or 3,2-hydroxyalkoxy radicals formed. Atmospheric lifetimes, based on of the homogeneous sinks of the unsaturated esters studied, are estimated from the kinetic data obtained in the present work.     

High-accuracy measurements of OH(•) reaction rate constants and IR and UV absorption spectra: ethanol and partially fluorinated ethyl alcohols

Rate constants for the gas phase reactions of OH(?) radicals with ethanol and three fluorinated ethyl alcohols, CH(3)CH(2)OH (k(0)), CH(2)FCH(2)OH (k(1)), CHF(2)CH(2)OH (k(2)), and CF(3)CH(2)OH (k(3)) were measured using a flash photolysis resonance-fluorescence technique over the temperature range 220 to 370 K. The Arrhenius plots were found to exhibit noticeable curvature for all four reactions. The temperature dependences of the rate constants can be represented by the following expressions over the indicated temperature intervals: k(0)(220-370 K) = 5.98 × 10(-13)(T/298)(1.99) exp(+515/T) cm(3) molecule(-1) s(-1), k(0)(220-298 K) = (3.35 ± 0.06) × 10(-12) cm(3) molecule(-1) s(-1) [for atmospheric modeling purposes, k(0)(T) is essentially temperature-independent below room temperature, k(0)(220-298 K) = (3.35 ± 0.06) × 10(-12) cm(3) molecule(-1) s(-1)], k(1)(230-370 K) = 3.47 × 10(-14)(T/298)(4.49) exp(+977/T) cm(3) molecule(-1) s(-1), k(2)(220-370 K) = 3.87 × 10(-14)(T/298)(4.25) exp(+578/T) cm(3) molecule(-1) s(-1), and k(3)(220-370 K) = 2.48 × 10(-14)(T/298)(4.03) exp(+418/T) cm(3) molecule(-1) s(-1). The atmospheric lifetimes due to reactions with tropospheric OH(?) were estimated to be 4, 16, 62, and 171 days, respectively, under the assumption of a well-mixed atmosphere. UV absorption cross sections of all four ethanols were measured between 160 and 215 nm. The IR absorption cross sections of the three fluorinated ethanols were measured between 400 and 1900 cm(-1), and their global warming potentials were estimated.     

Atmospheric chemistry of i-butanol

Smog chamber/FTIR techniques were used to determine rate constants of k(Cl + i-butanol) = (2.06 ± 0.40) × 10(-10), k(Cl + i-butyraldehyde) = (1.37 ± 0.08) × 10(-10), and k(OH + i-butanol) = (1.14 ± 0.17) × 10(-11) cm(3) molecule(-1) s(-1) in 700 Torr of N(2)/O(2) diluent at 296 ± 2K. The UV irradiation of i-butanol/Cl(2)/N(2) mixtures gave i-butyraldehyde in a molar yield of 53 ± 3%. The chlorine atom initiated oxidation of i-butanol in the absence of NO gave i-butyraldehyde in a molar yield of 48 ± 3%. The chlorine atom initiated oxidation of i-butanol in the presence of NO gave (molar yields): i-butyraldehyde (46 ± 3%), acetone (35 ± 3%), and formaldehyde (49 ± 3%). The OH radical initiated oxidation of i-butanol in the presence of NO gave acetone in a yield of 61 ± 4%. The reaction of chlorine atoms with i-butanol proceeds 51 ± 5% via attack on the α-position to give an α-hydroxy alkyl radical that reacts with O(2) to give i-butyraldehyde. The atmospheric fate of (CH(3))(2)C(O)CH(2)OH alkoxy radicals is decomposition to acetone and CH(2)OH radicals. The atmospheric fate of OCH(2)(CH(3))CHCH(2)OH alkoxy radicals is decomposition to formaldehyde and CH(3)CHCH(2)OH radicals. The results are consistent with, and serve to validate, the mechanism that has been assumed in the estimation of the photochemical ozone creation potential of i-butanol.     

Studies of the gas phase reactions of linalool, 6-methyl-5-hepten-2-ol and 3-methyl-1-penten-3-ol with O3 and OH radicals

The reactions of three unsaturated alcohols (linalool, 6-methyl-5-hepten-2-ol, and 3-methyl-1-penten-3-ol) with ozone and OH radicals have been studied using simulation chambers at T ～ 296 K and P ～ 760 Torr. The rate coefficient values (in cm(3) molecule(-1) s(-1)) determined for the three compounds are linalool, k(O3) = (4.1 ± 1.0) × 10(-16) and k(OH) = (1.7 ± 0.3) × 10(-10); 6-methyl-5-hepten-2-ol, k(O3) = (3.8 ± 1.2) × 10(-16) and k(OH) = (1.0 ± 0.3) × 10(-10); and 3-methyl-1-penten-3-ol, k(O3) = (5.2 ± 0.6) × 10(-18) and k(OH) = (6.2 ± 1.8) × 10(-11). From the kinetic data it is estimated that, for the reaction of O(3) with linalool, attack at the R-CH═C(CH(3))(2) group represents around (93 ± 52)% (k(6-methyl-5-hepten-2-ol)/k(linalool)) of the overall reaction, with reaction at the R-CH═CH(2) group accounting for about (1.3 ± 0.5)% (k(3-methyl-1-penten-3-ol)/k(linalool)). In a similar manner it has been calculated that for the reaction of OH radicals with linalool, attack of the OH radical at the R-CH═C(CH(3))(2) group represents around (59 ± 18)% (k(6-methyl-5-hepten-2-ol)/k(linalool)) of the total reaction, while addition of OH to the R-CH═CH(2) group is estimated to be around (36 ± 6)% (k(3-methyl-1-penten-3-ol)/k(linalool)). Analysis of the products from the reaction of O(3) with linalool confirmed that addition to the R-CH═C(CH(3))(2) group is the predominant reaction pathway. The presence of formaldehyde and hydroxyacetone in the reaction products together with compelling evidence for the generation of OH radicals in the system indicates that the hydroperoxide channel is important in the loss of the biradical [(CH(3))(2)COO]* formed in the reaction of O(3) with linalool. Studies on the reactions of O(3) with the unsaturated alcohols showed that the yields of secondary organic aerosols (SOAs) are higher in the absence of OH scavengers compared to the yields in their presence. However, even under low-NO(X) concentrations, the reactions of OH radicals with 3-methyl-1-penten-3-ol and 6-methyl-5-hepten-2-ol will make only a minor contribution to SOA formation under atmospheric conditions. Relatively high yields of SOAs were observed in the reactions of OH with linalool, although the initial concentrations of reactants were quite high. The importance of linalool in the formation of SOAs in the atmosphere requires further investigation. The impact following releases of these unsaturated alcohols into the atmosphere are discussed.     

Atmospheric chemistry of isoflurane, desflurane, and sevoflurane: kinetics and mechanisms of reactions with chlorine atoms and OH radicals and global warming potentials

The smog chamber/Fourier-transform infrared spectroscopy (FTIR) technique was used to measure the rate coefficients k(Cl + CF(3)CHClOCHF(2), isoflurane) = (4.5 ± 0.8) × 10(-15), k(Cl + CF(3)CHFOCHF(2), desflurane) = (1.0 ± 0.3) × 10(-15), k(Cl + (CF(3))(2)CHOCH(2)F, sevoflurane) = (1.1 ± 0.1) × 10(-13), and k(OH + (CF(3))(2)CHOCH(2)F) = (3.5 ± 0.7) × 10(-14) cm(3) molecule(-1) in 700 Torr of N(2)/air diluent at 295 ± 2 K. An upper limit of 6 × 10(-17) cm(3) molecule(-1) was established for k(Cl + (CF(3))(2)CHOC(O)F). The laser photolysis/laser-induced fluorescence (LP/LIF) technique was employed to determine hydroxyl radical rate coefficients as a function of temperature (241-298 K): k(OH + CF(3)CHFOCHF(2)) = (7.05 ± 1.80) × 10(-13) exp[-(1551 ± 72)/T] cm(3) molecule(-1); k(296 ± 1 K) = (3.73 ± 0.08) × 10(-15) cm(3) molecule(-1), and k(OH + (CF(3))(2)CHOCH(2)F) = (9.98 ± 3.24) × 10(-13) exp[-(969 ± 82)/T] cm(3) molecule(-1); k(298 ± 1 K) = (3.94 ± 0.30) × 10(-14) cm(3) molecule(-1). The rate coefficient of k(OH + CF(3)CHClOCHF(2), 296 ± 1 K) = (1.45 ± 0.16) × 10(-14) cm(3) molecule(-1) was also determined. Chlorine atoms react with CF(3)CHFOCHF(2) via H-abstraction to give CF(3)CFOCHF(2) and CF(3)CHFOCF(2) radicals in yields of approximately 83% and 17%. The major atmospheric fate of the CF(3)C(O)FOCHF(2) alkoxy radical is decomposition via elimination of CF(3) to give FC(O)OCHF(2) and is unaffected by the method used to generate the CF(3)C(O)FOCHF(2) radicals. CF(3)CHFOCF(2) radicals add O(2) and are converted by subsequent reactions into CF(3)CHFOCF(2)O alkoxy radicals, which decompose to give COF(2) and CF(3)CHFO radicals. In 700 Torr of air 82% of CF(3)CHFO radicals undergo C-C scission to yield HC(O)F and CF(3) radicals with the remaining 18% reacting with O(2) to give CF(3)C(O)F. Atmospheric oxidation of (CF(3))(2)CHOCH(2)F gives (CF(3))(2)CHOC(O)F in a molar yield of 93 ± 6% with CF(3)C(O)CF(3) and HCOF as minor products. The IR spectra of (CF(3))(2)CHOC(O)F and FC(O)OCHF(2) are reported for the first time. The atmospheric lifetimes of CF(3)CHClOCHF(2), CF(3)CHFOCHF(2), and (CF(3))(2)CHOCH(2)F (sevoflurane) are estimated at 3.2, 14, and 1.1 years, respectively. The 100 year time horizon global warming potentials of isoflurane, desflurane, and sevoflurane are 510, 2540, and 130, respectively. The atmospheric degradation products of these anesthetics are not of environmental concern.     

Atmospheric chemistry of dichlorvos

Dichlorvos [2,2-dichlorovinyl dimethyl phosphate, (CH(3)O)(2)P(O)OCH═CCl(2)] is a relatively volatile in-use insecticide. Rate constants for its reaction with OH radicals have been measured over the temperature range 296-348 K and atmospheric pressure of air using a relative rate method. The rate expression obtained was 3.53 × 10(-13) e((1367±239)/T) cm(3) molecule(-1) s(-1), with a 298 K rate constant of (3.5 ± 0.7) × 10(-11) cm(3) molecule(-1) s(-1), where the error in the 298 K rate constant is the estimated overall uncertainty. In addition, rate constants for the reactions of NO(3) radicals and O(3) with dichlorvos, of (2.5 ± 0.5) × 10(-13) cm(3) molecule(-1) s(-1) and (1.7 ± 1.0) × 10(-19) cm(3) molecule(-1) s(-1), respectively, were measured at 296 ± 2 K. Products of the OH and NO(3) radical-initiated reactions were investigated using in situ atmospheric pressure ionization mass spectrometry (API-MS) and (OH radical reaction only) in situ Fourier transform infrared (FT-IR) spectroscopy. For the OH radical reaction, the major initial products were CO, phosgene [C(O)Cl(2)] and dimethyl phosphate [(CH(3)O)(2)P(O)OH], with equal (to within ±10%) formation yields of CO and C(O)Cl(2). The API-MS analyses were consistent with formation of (CH(3)O)(2)P(O)OH from both the OH and NO(3) radical-initiated reactions. In the atmosphere, the dominant chemical loss processes for dichlorvos will be daytime reaction with OH radicals and nighttime reaction with NO(3) radicals, with an estimated lifetime of a few hours.     

Kinetic study of the daytime atmospheric fate of (z)-3-hexenal

Rate coefficients for three daytime atmospheric reactions of (Z)-3-hexenal (3HA)-photolysis (J(1)), reaction with OH radicals (k(2)), and reaction with ozone (k(3))-were measured at 760 Torr and 298 K using a 6 m(3) photochemical reaction chamber. The UV absorption cross sections (σ(3HA)(λ)) were obtained in the wavelength range 240-350 nm. The photodissociation rate of 3HA relative to that of NO(2) was measured by a solar simulator at 760 Torr and was determined to be J(1)/J(NO2) = (4.7 ± 0.4) × 10(-3). Using the obtained σ(3HA)(λ) and J(1)/J(NO2), the effective photodissociation quantum yield was calculated to be Φ(3HA) = 0.25 ± 0.06. The rate coefficient for the reaction with OH radicals was measured by the relative rate method with three reference compounds and was determined to be k(2) = (6.9 ± 0.9) × 10(-11) cm(3) molecule(-1) s(-1). The rate coefficient for the reaction with ozone was measured by an absolute method and was determined to be k(3) = (3.5 ± 0.2) × 10(-17) cm(3) molecule(-1) s(-1). Using the obtained rate coefficients, the daytime atmospheric lifetime of 3HA was estimated.     

Reaction OH + OH studied over the 298-834 K temperature and 1-100 bar pressure ranges

Self-reaction of hydroxyl radicals, OH + OH → H(2)O + O (1a) and OH + OH → H(2)O(2) (1b), was studied using pulsed laser photolysis coupled to transient UV-vis absorption spectroscopy over the 298-834 K temperature and 1-100 bar pressure ranges (bath gas He). A heatable high-pressure flow reactor was employed. Hydroxyl radicals were prepared using reaction of electronically excited oxygen atoms, O((1)D), produced in photolysis of N(2)O at 193 nm, with H(2)O. The temporal behavior of OH radicals was monitored via transient absorption of light from a dc discharge in H(2)O/Ar low-pressure resonance lamp at ca. 308 nm. The absolute intensity of the photolysis light was determined by accurate in situ actinometry based on the ozone formation in the presence of molecular oxygen. The results of this study combined with the literature data indicate that the rate constant of reaction 1a, associated with the pressure independent component, decreases with temperature within the temperature range 298-414 K and increases above 555 K. The pressure dependent rate constant for (1b) was parametrized using the Troe expression as k(1b,inf) = (2.4 ± 0.6) × 10(-11)(T/300)(-0.5) cm(3) molecule(-1) s(-1), k(1b,0) = [He] (9.0 ± 2.2) × 10(-31)(T/300)(-3.5±0.5) cm(3) molecule(-1) s(-1), F(c) = 0.37.     

Rates of water exchange on the [Fe4(OH)2(hpdta)2(H2O)4]0 molecule and its implications for geochemistry

The ammonium salt of [Fe(4)O(OH)(hpdta)(2)(H(2)O)(4)](-) is soluble and makes a monospecific solution of [Fe(4)(OH)(2)(hpdta)(2)(H(2)O)(4)](0)(aq) in acidic solutions (hpdta = 2-hydroxypropane-1,3-diamino-N,N,N',N'-tetraacetate). This tetramer is a diprotic acid with pK(a)(1) estimated at 5.7 ± 0.2 and pK(a)(2) = 8.8(5) ± 0.2. In the pH region below pK(a)(1), the molecule is stable in solution and (17)O NMR line widths can be interpreted using the Swift-Connick equations to acquire rates of ligand substitution at the four isolated bound water sites. Averaging five measurements at pH < 5, where contribution from the less-reactive conjugate base are minimal, we estimate: k(ex)(298) = 8.1 (±2.6) × 10(5) s(-1), ΔH(++) = 46 (±4.6) kJ mol(-1), ΔS(++) = 22 (±18) J mol(-1) K(-1), and ΔV(++) = +1.85 (±0.2) cm(3) mol(-1) for waters bound to the fully protonated, neutral molecule. Regressing the experimental rate coefficients versus 1/[H(+)] to account for the small pH variation in rate yields a similar value of k(ex)(298) = 8.3 (±0.8) × 10(5) s(-1). These rates are ～10(4) times faster than those of the [Fe(OH(2))(6)](3+) ion (k(ex)(298) = 1.6 × 10(2) s(-1)) but are about an order of magnitude slower than other studied aminocarboxylate complexes, although these complexes have seven-coordinated Fe(III), not six as in the [Fe(4)(OH)(2)(hpdta)(2)(H(2)O)(4)](0)(aq) molecule. As pH approaches pK(a1), the rates decrease and a compensatory relation is evident between the experimental ΔH(++) and ΔS(++) values. Such variation cannot be caused by enthalpy from the deprotonation reaction and is not well understood. A correlation between bond lengths and the logarithm of k(ex)(298) is geochemically important because it could be used to estimate rate coefficients for geochemical materials for which only DFT calculations are possible. This molecule is the only neutral, oxo-bridged Fe(III) multimer for which rate data are available.     

High accuracy measurements of OH reaction rate constants and IR absorption spectra: substituted 2-propanols

Rate constants for the gas phase reactions of OH radicals with 2-propanol and three fluorine substituted 2-propanols, (CH(3))(2)CHOH (k(0)), (CF(3))(2)CHOH (k(1)), (CF(3))(2)C(OH)CH(3) (k(2)), and (CF(3))(3)COH (k(3)), were measured using a flash photolysis resonance-fluorescence technique over the temperature range 220-370 K. The Arrhenius plots were found to exhibit noticeable curvature for all four reactions. The temperature dependences of the rate constants can be represented by the following expressions: k(0)(T) = 1.46 × 10(-11) exp{-883/T} + 1.30 × 10(-12) exp{+371/T} cm(3) molecule(-1) s(-1); k(1)(T) = 1.19 × 10(-12) exp{-1207/T} + 7.85 × 10(-16) exp{+502/T } cm(3) molecule(-1) s(-1); k(2)(T) = 1.68 × 10(-12) exp{-1718/T} + 7.32 × 10(-16) exp{+371/T} cm(3) molecule(-1) s(-1); k(3)(T) = 3.0 × 10(-20) × (T/298)(11.3) exp{+3060/T} cm(3) molecule(-1) s(-1). The atmospheric lifetimes due to reactions with tropospheric OH were estimated to be 2.4 days and 1.9, 6.3, and 46 years, respectively. UV absorption cross sections were measured between 160 and 200 nm. The IR absorption cross sections of the three fluorinated compounds were measured between 450 and 1900 cm(-1), and their global warming potentials were estimated.     

Rate constants for the reactions between OH and perfluorinated alkenes

The rate constants for the reactions of OH radicals with fully fluorinated alkenes containing different numbers of -CF(3) groups next to olefinic carbon, CF(2)═CF(2), CF(2)═CFCF(3), CF(3)CF═CFCF(3), and (CF(3))(2)C═CFC(2)F(5), were measured between 230 and 480 K using the flash photolysis resonance fluorescence technique to give the following expressions: k(C(2)F(4))(250-480 K) = 1.32 × 10(-12) × (T/298 K)(0.9) × exp(+600 K/T) cm(3) molecule(-1) s(-1), k(C(3)F(6))(230-480 K) = 9.75 ×10(-14) × (T/298 K)(1.94) × exp(+922 K/T) cm(3) molecule(-1) s(-1), k(trans-C(4)F(8))(230-370 K) = 7.50 × 10(-14) × (T/298 K)(1.68) × exp(+612 K/T) cm(3) molecule(-1) s(-1), k(cis-C(4)F(8))(230-370 K) = 2.99 × 10(-14) × (T/298 K)(2.61) × exp(+760 K/T) cm(3) molecule(-1) s(-1), and k(C(6)F(12))(250-480 K) = 2.17 × 10(-15) × (T/298 K)(3.90) × exp(+1044 K/T) cm(3) molecule(-1) s(-1). The kinetics of the OH reaction in an industrial sample of octofluoro-2-propene (a mixture of the cis- and trans-isomers of CF(3)CF═CFCF(3)) was studied to determine the "effective" reaction rate constant for the typically industrial mixture: k()(230-480 K) = 7.89 × 10(-14) × (T/298 K)(1.71) × exp(+557 K/T) cm(3) molecule(-1) s(-1). On the basis of these results, the atmospheric lifetimes were estimated to be 1.2, 5.3, 21, 34, and 182 days for CF(2)═CF(2), CF(3)CF═CF(2), trans-CF(3)CF═CFCF(3), cis-CF(3)CF═CFCF(3), and (CF(3))(2)C═CFC(2)F(5), respectively. The general pattern of halolalkene reactivity toward OH is discussed.     

Kinetics of elementary steps in the reactions of atomic bromine with isoprene and 1,3-butadiene under atmospheric conditions

Laser flash photolysis of CF(2)Br(2) has been coupled with time-resolved detection of atomic bromine by resonance fluorescence spectroscopy to investigate the gas-phase kinetics of early elementary steps in the Br-initiated oxidations of isoprene (2-methyl-1,3-butadiene, Iso) and 1,3-butadiene (Bu) under atmospheric conditions. At T ≥ 526 K, measured rate coefficients for Br + isoprene are independent of pressure, suggesting that hydrogen transfer (1a) is the dominant reaction pathway. The following Arrhenius expression adequately describes all kinetic data at 526 K ≤ T ≤ 673 K: k(1a)(T) = (1.22 ± 0.57) × 10(-11) exp[(-2100 ± 280)/T] cm(3) molecule(-1) s(-1) (uncertainties are 2σ and represent precision of the Arrhenius parameters). At 271 K ≤ T ≤ 357 K, kinetic evidence for the reversible addition reactions Br + Iso ? Br-Iso (k(1b), k(-1b)) and Br + Bu ? Br-Bu (k(3b), k(-3b)) is observed. Analysis of the approach to equilibrium data allows the temperature- and pressure-dependent rate coefficients k(1b), k(-1b), k(3b), and k(-3b) to be evaluated. At atmospheric pressure, addition of Br to each conjugated diene occurs with a near-gas-kinetic rate coefficient. Equilibrium constants for the addition/dissociation reactions are obtained from k(1b)/k(-1b) and k(3b)/k(-3b), respectively. Combining the experimental equilibrium data with electronic structure calculations allows both second- and third-law analyses of thermochemistry to be carried out. The following thermochemical parameters for the addition reactions 1b and 3b at 0 and 298 K are obtained (units are kJ mol(-1) for Δ(r)H and J mol(-1) K(-1) for Δ(r)S; uncertainties are accuracy estimates at the 95% confidence level): Δ(r)H(0)(1b) = -66.6 ± 7.1, Δ(r)H(298)(1b) = -67.5 ± 6.6, and Δ(r)S(298)(3b) = -93 ± 16; Δ(r)H(0)(3b) = -62.4 ± 9.0, Δ(r)H(298)(3b) = -64.5 ± 8.5, and Δ(r)S(298)(3b) = -94 ± 20. Examination of the effect of added O(2) on Br kinetics under conditions where reversible adduct formation is observed allows the rate coefficients for the Br-Iso + O(2) (k(2)) and Br-Bu + O(2) (k(4)) reactions to be determined. At 298 K, we find that k(2) = (3.2 ± 1.0) × 10(-13) cm(3) molecule(-1) s(-1) independent of pressure (uncertainty is 2σ, precision only; pressure range is 25-700 Torr) whereas k(4) increases from 3.2 to 4.7 × 10(-13) cm(3) molecule(-1) s(-1) as the pressure increases from 25 to 700 Torr. Our results suggest that under atmospheric conditions, Br-Iso and Br-Bu react with O(2) to produce peroxy radicals considerably more rapidly than they undergo unimolecular decomposition. Hence, the very fast addition reactions appear to control the rates of Br-initiated formation of Br-Iso-OO and Br-Bu-OO radicals under atmospheric conditions. The peroxy radicals are relatively weakly bound, so conjugated diene regeneration via unimolecular decomposition reactions, though unimportant on the time scale of the reported experiments (milliseconds), is likely to compete effectively with bimolecular reactions of peroxy radicals under relatively warm atmospheric conditions as well as in 298 K competitive kinetics experiments carried out in large chambers.     

Rate constants for the gas-phase reactions of OH and O3 with β-ocimene, β-myrcene, and α- and β-farnesene as a function of temperature

The rate constants for the gas-phase reactions of hydroxyl radicals and ozone with the biogenic hydrocarbons β-ocimene, β-myrcene, and α- and β-farnesene were measured using the relative rate technique over the temperature ranges 313-423 (for OH) and 298-318 K (for O?) at about 1 atm total pressure. The OH radicals were generated by photolysis of H?O?, and O? was produced from the electrolysis of O?. Helium was used as the diluent gas. The reactants were detected by online mass spectrometry, which resulted in high time resolution, allowing large amounts of data to be collected and used in the determination of the Arrhenius parameters. The following Arrhenius expressions have been determined for these reactions (in units of cm3 molecules?1 s?1): for β-ocimene + OH, k = (4.35(-0.66)(+0.78)) × 10?11 exp[(579 ± 59)/T]; for β-ocimene + O?, k = (3.15(-0.95)(+1.36)) × 10?1? exp[-(626 ± 110)/T]; for β-myrcene + O?, k = (2.21(-0.66)(+0.94)) × 10?1? exp[-(520 ± 109)/T]; for α-farnesene + OH, k(OH) = (2.19 ± 0.11) × 10?1? for 23-413 K; for α-farnesene + O?, k = (3.52(-2.54)(+9.09)) × 10?12 exp[-(2589 ± 393)/T]; for β-farnesene + OH, k(OH) = (2.88 ± 0.15) × 10?1? for 323-423 K; for β-farnesene + O?, k = (1.81(-1.19)(+3.46)) × 10?12 exp[-(2347 ± 329)/T]. The Arrhenius parameters here are the first to be reported. The reactions of α- and β-farnesene with OH showed no significant temperature dependence. Atmospheric residence times due to reactions with OH and O? were also presented.     

13C, 18O, and D fractionation effects in the reactions of CH3OH isotopologues with Cl and OH radicals

A relative rate experiment is carried out for six isotopologues of methanol and their reactions with OH and Cl radicals. The reaction rates of CH2DOH, CHD2OH, CD3OH, (13)CH3OH, and CH3(18)OH with Cl and OH radicals are measured by long-path FTIR spectroscopy relative to CH3OH at 298 +/- 2 K and 1013 +/- 10 mbar. The OH source in the reaction chamber is photolysis of ozone to produce O((1)D) in the presence of a large excess of molecular hydrogen: O((1)D) + H2 --> OH + H. Cl is produced by the photolysis of Cl2. The FTIR spectra are fitted using a nonlinear least-squares spectral fitting method with measured high-resolution infrared spectra as references. The relative reaction rates defined as alpha = k(light)/k(heavy) are determined to be: k(OH + CH3OH)/k(OH + (13)CH3OH) = 1.031 +/- 0.020, k(OH + CH3OH)/k(OH + CH3(18)OH) = 1.017 +/- 0.012, k(OH + CH3OH)/k(OH + CH2DOH) = 1.119 +/- 0.045, k(OH + CH3OH)/k(OH + CHD2OH) = 1.326 +/- 0.021 and k(OH + CH3OH)/k(OH + CD3OH) = 2.566 +/- 0.042, k(Cl + CH3OH)/k(Cl + (13)CH3OH) = 1.055 +/- 0.016, k(Cl + CH3OH)/k(Cl + CH3(18)OH) = 1.025 +/- 0.022, k(Cl + CH3OH)/k(Cl + CH2DOH) = 1.162 +/- 0.022 and k(Cl + CH3OH)/k(Cl + CHD2OH) = 1.536 +/- 0.060, and k(Cl + CH3OH)/k(Cl + CD3OH) = 3.011 +/- 0.059. The errors represent 2sigma from the statistical analyses and do not include possible systematic errors. Ground-state potential energy hypersurfaces of the reactions were investigated in quantum chemistry calculations at the CCSD(T) level of theory with an extrapolated basis set. The (2)H, (13)C, and (18)O kinetic isotope effects of the OH and Cl reactions with CH3OH were further investigated using canonical variational transition state theory with small curvature tunneling and compared to experimental measurements as well as to those observed in CH4 and several other substituted methane species.     

A kinetic study of Mg+ and Mg-containing ions reacting with O3, O2, N2, CO2, N2O and H2O: implications for magnesium ion chemistry in the upper atmosphere

Reactions between Mg(+) and O(3), O(2), N(2), CO(2) and N(2)O were studied using the pulsed laser photo-dissociation at 193 nm of Mg(C(5)H(7)O(2))(2) vapour, followed by time-resolved laser-induced fluorescence of Mg(+) at 279.6 nm (Mg(+)(3(2)P(3/2)-3(2)S(1/2))). The rate coefficient for the reaction Mg(+) + O(3) is at the Langevin capture rate coefficient and independent of temperature, k(190-340 K) = (1.17 ± 0.19) × 10(-9) cm(3) molecule(-1) s(-1) (1σ error). The reaction MgO(+) + O(3) is also fast, k(295 K) = (8.5 ± 1.5) × 10(-10) cm(3) molecule(-1) s(-1), and produces Mg(+) + 2O(2) with a branching ratio of (0.35 ± 0.21), the major channel forming MgO(2)(+) + O(2). Rate data for Mg(+) recombination reactions yielded the following low-pressure limiting rate coefficients: k(Mg(+) + N(2)) = 2.7 × 10(-31) (T/300 K)(-1.88); k(Mg(+) + O(2)) = 4.1 × 10(-31) (T/300 K)(-1.65); k(Mg(+) + CO(2)) = 7.3 × 10(-30) (T/300 K)(-1.59); k(Mg(+) + N(2)O) = 1.9 × 10(-30) (T/300 K)(-2.51) cm(6) molecule(-2) s(-1), with 1σ errors of ±15%. Reactions involving molecular Mg-containing ions were then studied at 295 K by the pulsed laser ablation of a magnesite target in a fast flow tube, with mass spectrometric detection. Rate coefficients for the following ligand-switching reactions were measured: k(Mg(+)·CO(2) + H(2)O → Mg(+)·H(2)O + CO(2)) = (5.1 ± 0.9) × 10(-11); k(MgO(2)(+) + H(2)O → Mg(+)·H(2)O + O(2)) = (1.9 ± 0.6) × 10(-11); k(Mg(+)·N(2) + O(2)→ Mg(+)·O(2) + N(2)) = (3.5 ± 1.5) × 10(-12) cm(3) molecule(-1) s(-1). Low-pressure limiting rate coefficients were obtained for the following recombination reactions in He: k(MgO(2)(+) + O(2)) = 9.0 × 10(-30) (T/300 K)(-3.80); k(Mg(+)·CO(2) + CO(2)) = 2.3 × 10(-29) (T/300 K)(-5.08); k(Mg(+)·H(2)O + H(2)O) = 3.0 × 10(-28) (T/300 K)(-3.96); k(MgO(2)(+) + N(2)) = 4.7 × 10(-30) (T/300 K)(-3.75); k(MgO(2)(+) + CO(2)) = 6.6 × 10(-29) (T/300 K)(-4.18); k(Mg(+)·H(2)O + O(2)) = 1.2 × 10(-27) (T/300 K)(-4.13) cm(6) molecule(-2) s(-1). The implications of these results for magnesium ion chemistry in the atmosphere are discussed.     

Atmospheric chemistry of t-CF3CH=CHCl: products and mechanisms of the gas-phase reactions with chlorine atoms and hydroxyl radicals

FTIR-smog chamber techniques were used to study the products and mechanisms of the Cl atom and OH radical initiated oxidation of trans-3,3,3-trifluoro-1-chloro-propene, t-CF(3)CH=CHCl, in 700 Torr of air or N(2)/O(2) diluent at 296 ± 2 K. The reactions of Cl atoms and OH radicals with t-CF(3)CH=CHCl occur via addition to the >C=C< double bond; chlorine atoms add 15 ± 5% at the terminal carbon and 85 ± 5% at the central carbon, OH radicals add approximately 40% at the terminal carbon and 60% at the central carbon. The major products in the Cl atom initiated oxidation of t-CF(3)CH=CHCl were CF(3)CHClCHO and CF(3)C(O)CHCl(2), minor products were CF(3)CHO, HCOCl and CF(3)COCl. The yields of CF(3)C(O)CHCl(2), CF(3)CHClCOCl and CF(3)COCl increased at the expense of CF(3)CHO, HCOCl and CF(3)CHClCHO as the O(2) partial pressure was increased over the range 10-700 Torr. Chemical activation plays a significant role in the fate of CF(3)CH(O)CHCl(2) and CF(3)CClHCHClO radicals. In addition to reaction with O(2) to yield CF(3)COCl and HO(2) the major competing fate of CF(3)CHClO is Cl elimination to give CF(3)CHO (not C-C bond scission as previously thought). As part of this study k(Cl + CF(3)C(O)CHCl(2)) = (2.3 ± 0.3) × 10(-14) and k(Cl + CF(3)CHClCHO) = (7.5 ± 2.0) × 10(-12) cm(3) molecule(-1) s(-1) were determined using relative rate techniques. Reaction with OH radicals is the major atmospheric sink for t-CF(3)CH=CHCl. Chlorine atom elimination giving the enol CF(3)CH=CHOH appears to be the sole atmospheric fate of the CF(3)CHCHClOH radicals. The yield of CF(3)COOH in the atmospheric oxidation of t-CF(3)CH=CHCl will be negligible (<2%). The results are discussed with respect to the atmospheric chemistry and environmental impact of t-CF(3)CH=CHCl.     

Atmospheric chemistry of (Z)-CF3CH═CHCF3: OH radical reaction rate coefficient and global warming potential

Rate coefficients, k, for the gas-phase reaction of the OH radical with (Z)-CF(3)CH═CHCF(3) (cis-1,1,1,4,4,4-hexafluoro-2-butene) were measured under pseudo-first-order conditions in OH using pulsed laser photolysis (PLP) to produce OH and laser-induced fluorescence (LIF) to detect it. Rate coefficients were measured over a range of temperatures (212-374 K) and bath gas pressures (20-200 Torr; He, N(2)) and found to be independent of pressure over this range of conditions. The rate coefficient has a non-Arrhenius behavior that is well-described by the expression k(1)(T) = (5.73 ± 0.60) × 10(-19) × T(2) × exp[(678 ± 10)/T] cm(3) molecule(-1) s(-1) where k(1)(296 K) was measured to be (4.91 ± 0.50) × 10(-13) cm(3) molecule(-1) s(-1) and the uncertainties are at the 2σ level and include estimated systematic errors. Rate coefficients for the analogous OD radical reaction were determined over a range of temperatures (262-374 K) at 100 Torr (He) to be k(2)(T) = (4.81 ± 0.20) × 10(-19) × T(2) × exp[(776 ± 15)/T], with k(2)(296 K) = (5.73 ± 0.50) × 10(-13) cm(3) molecule(-1) s(-1). OH radical rate coefficients were also measured at 296, 345, and 375 K using a relative rate technique and found to be in good agreement with the PLP-LIF results. A room-temperature rate coefficient for the O(3) + (Z)-CF(3)CH═CHCF(3) reaction was measured using an absolute method with O(3) in excess to be <6 × 10(-21) cm(3) molecule(-1) s(-1). The atmospheric lifetime of (Z)-CF(3)CH═CHCF(3) due to loss by OH reaction was estimated to be ~20 days. Infrared absorption spectra of (Z)-CF(3)CH═CHCF(3) measured in this work were used to determine a (Z)-CF(3)CH═CHCF(3) global warming potential (GWP) of ~9 for the 100 year time horizon. A comparison of the OH reactivity of (Z)-CF(3)CH═CHCF(3) with other unsaturated fluorinated compounds is presented.     

AsS2Cl-an Arsenic(V) compound? Formation, stability and structure of gaseous AsSCl and AsS2Cl--a combined experimental and theoretical study

By reaction of solid As(4)S(4) with gaseous Cl(2) at a temperature of 410 K gaseous AsSCl and AsS(2)Cl are formed. Unexpectedly in AsS(2)Cl the arsenic is not of formal oxidation state +V but +III: the molecular structure of AsS(2)Cl is arranged as a 1-chloro-dithia-arsirane and comprises an hitherto unknown AsS(2) three-membered ring. Thermodynamic data on AsSCl and AsS(2)Cl are obtained by mass spectrometry (MS). The experimental data are extended and confirmed by ab initio quantum chemical calculations (QC). The following values are given: Δ(f)H(0)(298)(AsSCl) = -5.2 kJ mol(-1) (MS), Δ(f)H(0)(298)(AsSCl) = 1.7 kJ mol(-1) (QC), S(0)(298)(AsSCl) = 296.9 J K(-1) mol(-1) (QC) and c(p)(0)(T)(AsSCl) = 55.77 + 3.97 × 10(-3)T- 4.38 × 10(5)T(-2)- 1.83 × 10(-6)T(2) and Δ(f)H(0)(298)(AsS(2)Cl) = -39.0 kJ mol(-1) (MS), Δ(f)H(0)(298)(AsS(2)Cl) = -20.2 kJ mol(-1) (QC), S(0)(298)(AsS(2)Cl) = 321.3 J K(-1) mol(-1) (QC) and c(p)(0)(T)(AsS(2)Cl) = 80.05 + 5.09 × 10(-3)T- 7.61 × 10(5)T(-2)- 2.35 × 10(-6)T(2) (298.15 K < T < 1000 K) (QC). The ionization energies are determined (IP(AsSCl) = 10.5, IP(AsS(2)Cl) = 10.2 eV). The IR spectrum of AsSCl is detected by means of matrix isolation spectroscopy. The estimated force constant f(As=S) = 4.47 mdyn·?(-1) gives rise to an As=S double bond.     

Laboratory studies of CHF2CF2CH2OH and CF3CF2CH2OH: UV and IR absorption cross sections and OH rate coefficients between 263 and 358 K

Fluorinated alcohols, such as 2,2,3,3-tetrafluoropropanol (TFPO, CHF(2)CF(2)CH(2)OH) and 2,2,3,3,3-pentafluoropropanol (PFPO, CF(3)CF(2)CH(2)OH), can be potential replacements of hydrofluorocarbons with large global warming potentials, GWPs. IR absorption cross sections for TFPO and PFPO were determined between 4000 and 500 cm(-1) at 298 K. Integrated absorption cross sections (S(int), base e) in the 4000-600 cm(-1) range are (1.92 ± 0.34) × 10(-16) cm(2) molecule(-1) cm(-1) and (2.05 ± 0.50) × 10(-16) cm(2) molecule(-1) cm(-1) for TFPO and PFPO, respectively. Uncertainties are at a 95% confidence level. Ultraviolet absorption spectra were also recorded between 195 and 360 nm at 298 K. In the actinic region (λ > 290 nm), an upper limit of 10(-23) cm(2) molecule(-1) for the absorption cross sections (σ(λ)) was reported. Photolysis in the troposphere is therefore expected to be a negligible loss for these fluoropropanols. In addition, absolute rate coefficients for the reaction of OH radicals with CHF(2)CF(2)CH(2)OH (k(1)) and CF(3)CF(2)CH(2)OH (k(2)) were determined as a function of temperature (T = 263-358 K) by the pulsed laser photolysis/laser induced fluorescence (PLP-LIF) technique. At room temperature, the average values obtained were k(1) = (1.85 ± 0.07) × 10(-13) cm(3) molecule(-1) s(-1) and k(2) = (1.19 ± 0.03) × 10(-13) cm(3) molecule(-1) s(-1). The observed temperature dependence of k(1)(T) and k(2)(T) is described by the following expressions: (1.35 ± 0.23) × 10(-12) exp{-(605 ± 54)/T} and (1.36 ± 0.19) × 10(-12) exp{-(730 ± 43)/T} cm(3) molecule(-1) s(-1), respectively. Since photolysis of TFPO and PFPO in the actinic region is negligible, the tropospheric lifetime (τ) of these species can be approximated by the lifetime due to the homogeneous reaction with OH radicals. Global values of τ(OH) were estimated to be of 3 and 4 months for TFPO and PFPO, respectively. GWPs relative to CO(2) at a time horizon of 500 years were calculated to be 8 and 12 for TFPO and PFPO, respectively. Despite the higher GWP relative to CO(2), these species are not expected to significantly contribute to the greenhouse effect in the next decades since they are short-lived species and will not accumulate in the troposphere even as their emissions grow up.     

Atmospheric chemistry of two biodiesel model compounds: methyl propionate and ethyl acetate

The atmospheric chemistry of two C(4)H(8)O(2) isomers (methyl propionate and ethyl acetate) was investigated. With relative rate techniques in 980 mbar of air at 293 K the following rate constants were determined: k(C(2)H(5)C(O)OCH(3) + Cl) = (1.57 ± 0.23) × 10(-11), k(C(2)H(5)C(O)OCH(3) + OH) = (9.25 ± 1.27) × 10(-13), k(CH(3)C(O)OC(2)H(5) + Cl) = (1.76 ± 0.22) × 10(-11), and k(CH(3)C(O)OC(2)H(5) + OH) = (1.54 ± 0.22) × 10(-12) cm(3) molecule(-1) s(-1). The chlorine atom initiated oxidation of methyl propionate in 930 mbar of N(2)/O(2) diluent (with, and without, NO(x)) gave methyl pyruvate, propionic acid, acetaldehyde, formic acid, and formaldehyde as products. In experiments conducted in N(2) diluent the formation of CH(3)CHClC(O)OCH(3) and CH(3)CCl(2)C(O)OCH(3) was observed. From the observed product yields we conclude that the branching ratios for reaction of chlorine atoms with the CH(3)-, -CH(2)-, and -OCH(3) groups are <49 ± 9%, 42 ± 7%, and >9 ± 2%, respectively. The chlorine atom initiated oxidation of ethyl acetate in N(2)/O(2) diluent gave acetic acid, acetic acid anhydride, acetic formic anhydride, formaldehyde, and, in the presence of NO(x), PAN. From the yield of these products we conclude that at least 41 ± 6% of the reaction of chlorine atoms with ethyl acetate occurs at the -CH(2)- group. The rate constants and branching ratios for reactions of OH radicals with methyl propionate and ethyl acetate were investigated theoretically using transition state theory. The stationary points along the oxidation pathways were optimized at the CCSD(T)/cc-pVTZ//BHandHLYP/aug-cc-pVTZ level of theory. The reaction of OH radicals with ethyl acetate was computed to occur essentially exclusively (～99%) at the -CH(2)- group. In contrast, both methyl groups and the -CH(2)- group contribute appreciably in the reaction of OH with methyl propionate. Decomposition via the α-ester rearrangement (to give C(2)H(5)C(O)OH and a HCO radical) and reaction with O(2) (to give CH(3)CH(2)C(O)OC(O)H) are competing atmospheric fates of the alkoxy radical CH(3)CH(2)C(O)OCH(2)O. Chemical activation of CH(3)CH(2)C(O)OCH(2)O radicals formed in the reaction of the corresponding peroxy radical with NO favors the α-ester rearrangement.