Rate constants for the gas-phase reactions of OH radicals with dimethyl phosphonate over the temperature range of 278-351 K and for a series of other organophosphorus compounds at approximately 280 K

Rate constants for the gas-phase reactions of OH radicals with dimethyl phosphonate [DMHP; (CH3O)2P(O)H] were measured over the temperature range of 278-351 K at atmospheric pressure of air using a relative rate method with 4-methyl-2-pentanone as the reference compound. The Arrhenius expression obtained was 1.01 x 10(-12) e((474 +/- 159)/T) cm(3) molecule(-1) s(-1), where the indicated error is two least-squares standard deviations and does not include uncertainties in the rate constants for the reference compound. Rate constants for the gas-phase reactions of OH radicals with dimethyl methylphosphonate [DMMP, (CH3O)2P(O)CH3], dimethyl ethylphosphonate [DMEP, (CH3O)2P(O)C2H5], diethyl methylphosphonate [DEMP, (C2H5O)2P(O)CH3], diethyl ethylphosphonate [DEEP, (C2H5O)2P(O)C2H5], and triethyl phosphate [TEP, (C2H5O)3PO] were also measured at 278 and/or 283 K for comparison with a previous study (Aschmann, S. M.; Long, W. D.; Atkinson, R. J. Phys. Chem. A, 2006, 110, 7393). With the experimental procedures employed, experiments conducted at temperatures below the dew point where a water film was present on the outside of the Teflon reaction chamber resulted in measured rate constants which were significantly higher than those expected from the extrapolation of rate data obtained at temperatures (283-348 K) above the dew point. Using rate constants measured at > or = 283 K, the resulting Arrhenius expressions (in cm(3) molecule(-1) s(-1) units) are 6.25 x 10(-14) e((1538 +/- 112)/T) for DMMP (283-348 K), 9.03 x 10(-14) e((1539 +/- 27)/T) for DMEP (283-348 K), 4.35 x 10(-13) e((1444 +/- 148)/T) for DEMP (283-348 K), 4.08 x 10(-13) e((1485 +/- 328)/T) for DEEP (283-348 K), and 4.07 x 10(-13) e((1448 +/- 145)/T) for TEP (283-347 K), where the indicated errors are as above. Aerosol formation at 296 +/- 2 K from the reactions of OH radicals with these organophosphorus compounds was relatively minor, with aerosol yields of < or = 8% in all cases.     

Temperature-dependent rate constants for the gas-phase reactions of OH radicals with 1,3,5-trimethylbenzene, triethyl phosphate, and a series of alkylphosphonates

Rate constants for the reactions of OH radicals with dimethyl methylphosphonate [DMMP, (CH3O)2P(O)CH3], dimethyl ethylphosphonate [DMEP, (CH3O)2P(O)C2H5], diethyl methylphosphonate [DEMP, (C2H5O)2P(O)CH3], diethyl ethylphosphonate [DEEP, (C2H5O)2P(O)C2H5], triethyl phosphate [TEP, (C2H5O)3PO] and 1,3,5-trimethylbenzene have been measured over the temperature range 278-348 K at atmospheric pressure of air using a relative rate method. alpha-Pinene (for DEMP, DEEP, TEP and 1,3,5-trimethylbenzene) and di-n-butyl ether (for DMMP and DMEP) were used as the reference compounds, and rate constants for the reaction of OH radicals with di-n-butyl ether were also measured over the same temperature range using alpha-pinene and n-decane as the reference compounds. The Arrhenius expressions obtained for these OH radical reactions (in cm3 molecule(-1) s(-1) units) are 8.00 x 10(-14)e(1470+/-132)/T for DMMP (296-348 K), 9.76 x 10(-14)e(1520+/-14)/T for DMEP (296-348 K), 4.20 x 10(-13)e(1456+/-227)/T for DEMP (296-348 K), 6.46 x 10(-13)e(1339+/-376)/T for DEEP (296-348 K), 4.29 x 10(-13)e(1428+/-219)/T for TEP (296-347 K), and 4.40 x 10(-12)e(738+/-176)/T for 1,3,5-trimethylbenzene (278-347 K), where the indicated errors are two least-squares standard deviations and do not include the uncertainties in the rate constants for the reference compounds. The measured rate constants for di-n-butyl ether are in good agreement with literature data over the temperature range studied (278-348 K).     

Atmospheric chemistry of diethyl methylphosphonate, diethyl ethylphosphonate, and triethyl phosphate

Rate constants for the reactions of OH radicals and NO(3) radicals with diethyl methylphosphonate [DEMP, (C(2)H(5)O)(2)P(O)CH(3)], diethyl ethylphosphonate [DEEP, (C(2)H(5)O)(2)P(O)C(2)H(5)], and triethyl phosphate [TEP, (C(2)H(5)O)(3)PO] have been measured at 296 +/- 2 K and atmospheric pressure of air using relative rate methods. The rate constants obtained for the OH radical reactions (in units of 10(-11) cm(3) molecule(-1) s(-1)) were as follows: DEMP, 5.78 +/- 0.24; DEEP, 6.45 +/- 0.27; and TEP, 5.44 +/- 0.20. The rate constants obtained for the NO(3) radical reactions (in units of 10(-16) cm(3) molecule(-1) s(-1)) were the following: DEMP, 3.7 +/- 1.1; DEEP, 3.4 +/- 1.4; and TEP, 2.4 +/- 1.4. For the reactions of O(3) with DEMP, DEEP, and TEP, an upper limit to the rate constant of <6 x 10(-20) cm(3) molecule(-1) s(-1) was determined for each compound. Products of the reactions of OH radicals with DEMP, DEEP, and TEP were investigated using in situ atmospheric pressure ionization mass spectrometry (API-MS) and, for the TEP reaction, gas chromatography with flame ionization detection (GC-FID) and in situ Fourier transform infrared (FT-IR) spectroscopy. The API-MS analyses show that the reactions are analogous, with formation of one major product from each reaction: C(2)H(5)OP(O)(OH)CH(3) from DEMP, C(2)H(5)OP(O)(OH)C(2)H(5) from DEEP, and (C(2)H(5)O)(2)P(O)OH from TEP. The FT-IR and GC-FID analyses showed that the major products (and their molar yields) from the TEP reaction are (C(2)H(5)O)(2)P(O)OH (65-82%, initial), CO(2) (80 +/- 10%), and HCHO (55 +/- 5%), together with lesser yields of CH(3)CHO (11 +/- 2%), CO (11 +/- 3%), CH(3)C(O)OONO(2) (8%), organic nitrates (7%), and acetates (4%). The probable reaction mechanisms are discussed.     

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.     

Theoretical methodology for prediction of tropospheric oxidation of dimethyl phosphonate and dimethyl methylphosphonate

Rate constants for the reactions of OH radicals with dimethyl phosphonate [DMHP, (CH(3)O)(2)P(O)H] and dimethyl methylphosphonate [DMMP, (CH(3)O)(2)P(O)CH(3)] have been calculated by ab initio structural methods and semiclassical dynamics modeling and compared with experimental measurements over the temperature range 250-350 K. The structure and energetics of reactants and transition structures are determined for all hydrogen atom abstraction pathways that initiate the atmospheric oxidation mechanism. Structures are obtained at the CCSD/6-31++G** level of chemical theory, and the height of the activation barrier is determined by a variant of the G2MP2 method. A Transfer Hamiltonian is used to compute the minimum energy path in the neighborhood of the transition state (TS). This calculation provides information about the curvature of the potential energy surface in the neighborhood of the TS, as well as the internal forces that are needed by the semiclassical flux-flux autocorrelation function (SCFFAF) dynamics model used to compute the temperature-dependent reaction rate constants for the various possible abstraction pathways. The computed temperature-dependent rate curves frequently lie within the experimental error bars.     

Kinetics and products of the gas-phase reactions of divinyl sulfoxide with OH and NO3 radicals and O3

Using relative rate methods, rate constants for the gas-phase reactions of divinyl sulfoxide [CH 2CHS(O)CHCH 2; DVSO] with NO 3 radicals and O 3 have been measured at 296 +/- 2 K, and rate constants for the reaction with OH radicals have been measured over the temperature range of 277-349 K. Rate constants obtained for the NO 3 radical and O 3 reactions at 296 +/- 2 K were (6.1 +/- 1.4) x 10 (-16) and (4.3 +/- 1.0) x 10 (-19) cm (3) molecule (-1) s (-1), respectively. For the OH radical reaction, the temperature-dependent rate expression obtained was k = 4.17 x 10 (-12)e ((858 +/- 141)/ T ) cm (3) molecule (-1) s (-1) with a 298 K rate constant of (7.43 +/- 0.71) x 10 (-11) cm (3) molecule (-1) s (-1), where, in all cases, the errors are two standard deviations and do not include the uncertainties in the rate constants for the reference compounds. Divinyl sulfone was observed as a minor product of both the OH radical and NO 3 radical reactions at 296 +/- 2 K. Using in situ Fourier transform infrared spectroscopy, CO, CO 2, SO 2, HCHO, and divinyl sulfone were observed as products of the OH radical reaction, with molar formation yields of 35 +/- 11, 2.2 +/- 0.8, 33 +/- 4, 54 +/- 6, and 5.4 +/- 0.8%, respectively, in air. For the experimental conditions employed, aerosol formation from the OH radical-initiated reaction of DVSO in the presence of NO was minor, being approximately 1.5%. The data obtained here for DVSO are compared with literature data for the corresponding reactions of dimethyl sulfoxide.     

Products of the gas-phase reactions of OH radicals with (C2H5O)2P(S)CH3 and (C2H5O)3PS

Products of the gas-phase reactions of OH radicals with O,O-diethyl methylphosphonothioate [(C2H5O)2P(S)CH3, DEMPT] and O,O,O-triethyl phosphorothioate [(C2H5O)3PS, TEPT] have been investigated at room temperature and atmospheric pressure of air using in situ atmospheric pressure ionization mass spectrometry (API-MS) and, for the TEPT reaction, gas chromatography and in situ Fourier transform infrared (FT-IR) spectroscopy. Combined with products quantified previously by gas chromatography, the products observed were: from the DEMPT reaction, (C2H5O)2P(O)CH3 (21+/-4% yield) and C2H5OP(S)(CH3)OH or C2H5OP(O)(CH3)SH (presumed to be C2H5OP(O)(CH3)SH by analogy with the TEPT reaction); and from the TEPT reaction, (C2H5O)3PO (54-62% yield), SO2 (67+/-10% yield), CH3CHO (22-40% yield) and, tentatively, (C2H5O)2P(O)SH. The FT-IR analyses showed that the formation yields of HCHO, CO, CO2, peroxyacetyl nitrate [CH3C(O)OONO2], organic nitrates, and acetates from the TEPT reaction were <5%, 3+/-1%, <7%, <2%, 5+/-3%, and 3+/-2%, respectively. Possible reaction mechanisms are discussed.     

High-temperature rate constants for CH3OH + Kr --> products, OH + CH3OH --> products, OH + (CH3)(2)CO --> CH2COCH3 + H2O, and OH + CH3 --> CH) + H2O

The reflected shock tube technique with multipass absorption spectrometric detection of OH radicals at 308 nm (corresponding to a total path length of approximately 4.9 m) has been used to study the dissociation of methanol between 1591 and 2865 K. Rate constants for two product channels [CH3OH + Kr --> CH3 + OH + Kr (1) and CH3OH + Kr --> 1CH2 + H2O + Kr (2)] were determined. During the course of the study, it was necessary to determine several other rate constants that contributed to the profile fits. These include OH + CH3OH --> products, OH + (CH3)2CO --> CH2COCH3 + H2O, and OH + CH3 --> 1,3CH2 + H2O. The derived expressions, in units of cm(3) molecule(-1) s(-1), are k(1) = 9.33 x 10(-9) exp(-30857 K/T) for 1591-2287 K, k(2) = 3.27 x 10(-10) exp(-25946 K/T) for 1734-2287 K, kOH+CH3OH = 2.96 x 10-16T1.4434 exp(-57 K/T) for 210-1710 K, k(OH+(CH3)(2)CO) = (7.3 +/- 0.7) x 10(-12) for 1178-1299 K and k(OH+CH3) = (1.3 +/- 0.2) x 10(-11) for 1000-1200 K. With these values along with other well-established rate constants, a mechanism was used to obtain profile fits that agreed with experiment to within <+/-10%. The values obtained for reactions 1 and 2 are compared with earlier determinations and also with new theoretical calculations that are presented in the preceding article in this issue. These new calculations are in good agreement with the present data for both (1) and (2) and also for OH + CH3 --> products.     

Kinetic and product study of the gas-phase reactions of OH radicals, NO(3) radicals, and O(3) with (C(2)H(5)O)(2)P(S)CH(3) and (C(2)H(5)O)(3)PS

Rate constants for the reactions of OH radicals and NO3 radicals with O,O-diethyl methylphosphonothioate [(C(2)H(5)O)(2)P(S)CH(3); DEMPT] and O,O,O-triethyl phosphorothioate [(C(2)H(5)O)(3)PS; TEPT] have been measured using relative rate methods at atmospheric pressure of air over the temperature range 296-348 K for the OH radical reactions and at 296 +/- 2 K for the NO(3) radical reactions. At 296 +/- 2 K, the rate constants obtained for the OH radical reactions (in units of 10(-11) cm(3) molecule(-1) s(-1)) were 20.4 +/- 0.8 and 7.92 +/- 0.27 for DEMPT and TEPT, respectively, and those for the NO(3) radical reactions (in units of 10(-15) cm(3) molecule(-1) s(-1)) were 2.01 +/- 0.20 and 1.03 +/- 0.10, respectively. Upper limits to the rate constants for the reactions of O(3) with DEMPT and TEPT of <6 x 10(-20) cm(3) molecule(-1) s(-1) were determined in each case. Rate constants for the OH radical reactions, measured relative to k(OH + alpha-pinene) = 1.21 x 10(-11) e(436/T) cm(3) molecule(-1) s(-1), resulted in the Arrhenius expressions k(OH + DEMPT) = 1.08 x 10(-11) e(871+/-25)/T cm(3) molecule(-1) s(-1) and k(OH + TEPT) = 8.21 x 10(-13) e(1353+/-49)/T cm(3) molecule(-1) s(-1) over the temperature range 296-348 K, where the indicated errors are two least-squares standard deviations and do not include the uncertainties in the reference rate constant. Diethyl methylphosphonate was identified and quantified from the OH radical and NO(3) radical reactions with DEMPT, with formation yields of 21 +/- 4%, independent of temperature, from the OH radical reaction and 62 +/- 11% from the NO(3) radical reaction at 296 +/- 2 K. Similarly, triethyl phosphate was identified and quantified from the OH radical and NO(3) radical reactions with TEPT, with formation yields of 56 +/- 9%, independent of temperature, from the OH radical reaction and 78 +/- 15% from the NO(3) radical reaction at 296 +/- 2 K.     

Atmospheric chemistry of CF3CH2CH2OH: kinetics, mechanisms and products of Cl atom and OH radical initiated oxidation in the presence and absence of NOX

Relative rate techniques were used to study the kinetics of the reactions of Cl atoms and OH radicals with CF(3)CH(2)C(O)H and CF(3)CH(2)CH(2)OH in 700 Torr of N(2) or air diluent at 296 +/- 2 K. The rate constants determined were k(Cl+CF(3)CH(2)C(O)H) = (1.81 +/- 0.27) x 10(-11), k(OH+CF(3)CH(2)C(O)H) = (2.57 +/- 0.44) x 10(-12), k(Cl+CF(3)CH(2)CH(2)OH) = (1.59 +/- 0.20) x 10(-11), and k(OH+CF(3)CH(2)CH(2)OH) = (6.91 +/- 0.91) x 10(-13) cm(3) molecule(-1) s(-1). Product studies of the chlorine initiated oxidation of CF(3)CH(2)CH(2)OH in the absence of NO show the sole primary product to be CF(3)CH(2)C(O)H. Product studies of the chlorine initiated oxidation of CF(3)CH(2)CH(2)OH in the presence of NO show the primary products to be CF(3)CH(2)C(O)H (81%), HC(O)OH (10%), and CF(3)C(O)H. Product studies of the chlorine initiated oxidation of CF(3)CH(2)C(O)H in the absence of NO show the primary products to be CF(3)C(O)H (76%), CF(3)CH(2)C(O)OH (14%), and CF(3)CH(2)C(O)OOH (< or =10%). As part of this work, an upper limit of k(O(3)+CF(3)CH(2)CH(2)OH) < 2 x 10(-21) cm(3) molecule(-1) s(-1) was established. Results are discussed with respect to the atmospheric chemistry of fluorinated alcohols.     

Ketalization of phosphonium ions by 1,4-dioxane: selective detection of the chemical warfare agent simulant DMMP in mixtures using ion/molecule reactions

Phosphonium ions CH(3)P(O)OCH(3)(+) (93 Th) and CH(3)OP(O)OCH(3)(+) (109 Th) react with 1,4-dioxane to form unique cyclic ketalization products, 1,3,2-dioxaphospholanium ions. By contrast, a variety of other types of ions having multiple bonds, including the acylium ions CH(3)CO(+) (43 Th), CH(3)OCO(+) (59 Th), (CH(3))(2)NCO(+) (72 Th), and PhCO(+) (105 Th), the iminium ion H(2)C[double bond]NHC(2)H(5)(+) (58 Th) and the carbosulfonium ion H(2)C[double bond]SC(2)H(5)(+) (75 Th) do not react with 1,4-dioxane under the same conditions. The characteristic ketalization reaction can also be observed when CH(3)P(OH)(OCH(3))(2)(+), viz. protonated dimethyl methylphosphonate (DMMP), collides with 1,4-dioxane, as a result of fragmentation to yield the reactive phosphonium ion CH(3)P(O)OCH(3)(+) (93 Th). This novel ion/molecule reaction is highly selective to phosphonium ions and can be applied to identify DMMP selectively in the presence of ketone, ester, and amide compounds using a neutral gain MS/MS scan. This method of DMMP analysis can be applied to aqueous solutions using electrospray ionization; it shows a detection limit in the low ppb range and a linear response over the range 10 to 500 ppb.     

Reflected shock tube studies of high-temperature rate constants for OH + CH4 --> CH3 + H2O and CH3 + NO2 --> CH3O + NO

The reflected shock tube technique with multipass absorption spectrometric detection of OH radicals at 308 nm has been used to study the reactions OH + CH(4) --> CH(3) + H(2)O and CH(3) + NO(2) --> CH(3)O + NO. Over the temperature range 840-2025 K, the rate constants for the first reaction can be represented by the Arrhenius expression k = (9.52 +/- 1.62) x 10(-11) exp[(-4134 +/- 222 K)/T] cm(3) molecule(-1) s(-1). Since this reaction is important in both combustion and atmospheric chemistry, there have been many prior investigations with a variety of techniques. The present results extend the temperature range by 500 K and have been combined with the most accurate earlier studies to derive an evaluation over the extended temperature range 195-2025 K. A three-parameter expression describes the rate behavior over this temperature range, k = (1.66 x 10(-18))T(2.182) exp[(-1231 K)/T] cm(3) molecule(-1) s(-1). Previous theoretical studies are discussed, and the present evaluation is compared to earlier theoretical estimates. Since CH(3) radicals are a product of the reaction and could cause secondary perturbations in rate constant determinations, the second reaction was studied by OH radical production from the fast reactions CH(3)O --> CH(2)O + H and H + NO(2) --> OH + NO. The measured rate constant is 2.26 x 10(-11) cm(3) molecule(-1) s(-1) and is not dependent on temperature from 233 to 1700 K within experimental error.     

Experimental study of dimethyl methylphosphonate decomposition over anatase TiO2

Removal from air and decomposition of dimethyl methylphosphonate (DMMP) over high surface area anatase TiO(2) at ambient temperature have been quantitatively studied by employing Fourier transform infrared (FTIR) technique under static conditions. In the first scenario of air purification, DMMP underwent reactive adsorption that upon completion was followed by photocatalytic oxidation. DMMP was captured over the TiO(2) surface at the speed of external diffusion. Hydrolysis of adsorbed DMMP led to methanol and methyl methylphosphonate (MMP). At low DMMP coverage quantity, it hydrolyzed completely with the formation of completely surface-bound methanol at 1% relative humidity (RH) and mostly gaseous methanol at 50% RH. Photocatalytic oxidation generated CO(2) as the only carbonaceous gaseous product and bidentate formates as the intermediate surface product. At high DMMP coverage quantity, it was captured incompletely and hydrolyzed partially with CH(3)OH in the gas phase only, 50% RH enhancing both processes. Photocatalytic oxidation generated gaseous HCOOH, CO, and CO(2) and was incomplete due to catalyst deactivation by nonvolatile products. In the second scenario of air purification, DMMP underwent adsorption, hydrolysis, and photooxidation at the same time. It resulted in the quickest removal of DMMP from the gas phase and completion of oxidation in 30 min, suggesting this process for practical air decontamination. At least 0.8 nm(2) of TiO(2) surface per each DMMP molecule should be available for complete purification of air.     

Atmospheric chemistry of propionaldehyde: kinetics and mechanisms of reactions with OH radicals and Cl atoms, UV spectrum, and self-reaction kinetics of CH3CH2C(O)O2 radicals at 298 K

The kinetics and mechanism of the reactions of Cl atoms and OH radicals with CH3CH2CHO were investigated at room temperature using two complementary techniques: flash photolysis/UV absorption and continuous photolysis/FTIR smog chamber. Reaction with Cl atoms proceeds predominantly by abstraction of the aldehydic hydrogen atom to form acyl radicals. FTIR measurements indicated that the acyl forming channel accounts for (88 +/- 5)%, while UV measurements indicated that the acyl forming channel accounts for (88 +/- 3)%. Relative rate methods were used to measure: k(Cl + CH3CH2CHO) = (1.20 +/- 0.23) x 10(-10); k(OH + CH3CH2CHO) = (1.82 +/- 0.23) x 10(-11); and k(Cl + CH3CH2C(O)Cl) = (1.64 +/- 0.22) x 10(-12) cm3 molecule(-1) s(-1). The UV spectrum of CH3CH2C(O)O2, rate constant for self-reaction, and rate constant for cross-reaction with CH3CH2O2 were determined: sigma(207 nm) = (6.71 +/- 0.19) x 10(-18) cm2 molecule(-1), k(CH3CH2C(O)O2 + CH3CH2C(O)O2) = (1.68 +/- 0.08) x 10(-11), and k(CH3CH2C(O)O2 + CH3CH2O2) = (1.20 +/- 0.06) x 10(-11) cm3 molecule(-1) s(-1), where quoted uncertainties only represent 2sigma statistical errors. The infrared spectrum of C2H5C(O)O2NO2 was recorded, and products of the Cl-initiated oxidation of CH3CH2CHO in the presence of O2 with, and without, NO(x) were identified. Results are discussed with respect to the atmospheric chemistry of propionaldehyde.     

Visible absorption spectrum of the CH3CO radical

The visible absorption spectrum of the acetyl radical, CH(3)CO, was measured between 490 and 660 nm at 298 K using cavity ring-down spectroscopy. Gas-phase CH(3)CO radicals were produced using several methods including: (1) 248 nm pulsed laser photolysis of acetone (CH(3)C(O)CH(3)), methyl ethyl ketone (MEK, CH(3)C(O)CH(2)CH(3)), and biacetyl (CH(3)C(O)C(O)CH(3)), (2) Cl + CH(3)C(O)H --> CH(3)C(O) + HCl with Cl atoms produced via pulsed laser photolysis or in a discharge flow tube, and (3) OH + CH(3)C(O)H --> CH(3)CO + H(2)O with two different pulsed laser photolysis sources of OH radicals. The CH(3)CO absorption spectrum was assigned on the basis of the consistency of the spectra obtained from the different CH(3)CO sources and agreement of the measured rate coefficients for the reaction of the absorbing species with O(2) and O(3) with literature values for the CH(3)CO + O(2) + M and CH(3)CO + O(3) reactions. The CH(3)CO absorption spectrum between 490 and 660 nm has a broad peak centered near 535 nm and shows no discernible structure. The absorption cross section of CH(3)CO at 532 nm was measured to be (1.1 +/- 0.2) x 10(-19) cm(2) molecule(-1) (base e).     

Kinetics and mechanisms of CF3CHFOCH3, CF3CHFOC(O)H, and FC(O)OCH3 reactions with OH radicals

The kinetics and mechanism of oxidation of CF3CHFOCH3 was studied using an 11.5-dm3 environmental reaction chamber. OH radicals were produced by UV photolysis of an O3-H2O-He mixture at an initial pressure of 200 Torr in the chamber. The rate constant of the reaction of CF3CHFOCH3 with OH radicals (k1) was determined to be (1.77 +/- 0.69) x 10(-12) exp[(-720 +/- 110)/T] cm3 molecule(-1)(s-1) by means of a relative rate method at 253-328 K. The mechanism of the reaction was investigated by FT-IR spectroscopy at 298 K. CF3CHFOC(O)H, FC(O)OCH3, and COF2 were determined to be the major products. The branching ratio (k1a/k1b) for the reactions CF3CHFOCH3 + OH --> CF3CHFOCH2* + H2O (k1a) and CF3CHFOCH3 + OH --> CF3CF*OCH3 + H2O (k1b) was estimated to be 4.2:1 at 298 K from the yields of CF3CHFOC(O)H, FC(O)OCH3, and COF2. The rate constants of the reactions of CF3CHFOC(O)H (k2) and FC(O)OCH3 (k3) with OH radicals were determined to be (9.14 +/- 2.78) x 10(-13) exp[(-1190 +/- 90)/T] and (2.10 +/- 0.65) x 10(-13) exp[(-630 +/- 90)/T] cm3 molecule(-1)(s-1), respectively, by means of a relative rate method at 253-328 K. The rate constants at 298 K were as follows: k1 = (1.56 +/- 0.06) x 10-13, k2 = (1.67 +/- 0.05) x 10-14, and k3 = (2.53 +/- 0.07) x 10-14 cm3 molecule(-1)(s-1). The tropospheric lifetimes of CF3CHFOCH3, CF3CHFOC(O)H, and FC(O)OCH3 with respect to reaction with OH radicals were estimated to be 0.29, 3.2, and 1.8 years, respectively.     

Interaction of formic and acetic acid with ice surfaces between 187 and 227 K. Investigation of single species- and competitive adsorption

The physical adsorption of formic (HC(O)OH) and acetic (CH(3)C(O)OH) acid on ice was measured as a function of concentration and temperature. At low concentrations, the gas-ice interaction could be analysed by applying Langmuir adsorption isotherms to determine temperature dependent partition constants, K(Lang). Using temperature independent saturation coverages (N(max)) of (2.2 +/- 0.5) x 10(14) molecule cm(-2) and (2.4 +/- 0.6) x 10(14) molecule cm(-2) for HC(O)OH and CH(3)C(O)OH, respectively, we derive K(Lang)(HC(O)OH) = 1.54 x 10(-24) exp (6150/T) and K(Lang)(CH(3)C(O)OH) = 6.55 x 10(-25) exp (6610/T) cm(3) molecule(-1). Via a van't Hoff analysis, adsorption enthalpies were obtained for HC(O)OH and CH(3)C(O)OH. Experiments in which both acids or HC(O)OH and methanol interacted with the ice surface simultaneously were adequately described by competitive adsorption kinetics. The results are compared to previous measurements and used to calculate the equilibrium partitioning of these trace gases to ice surfaces under conditions relevant to the atmosphere.     

Structures of [(CO2)n(CH3OH)m](-) (n = 1-4, m = 1, 2) cluster anions

The infrared photodissociation spectra of [(CO 2) n (CH 3OH) m ] (-) ( n = 1-4, m = 1, 2) are measured in the 2700-3700 cm (-1) range. The observed spectra consist of an intense broad band characteristic of hydrogen-bonded OH stretching vibrations at approximately 3300 cm (-1) and congested vibrational bands around 2900 cm (-1). No photofragment signal is observed for [(CO 2) 1,2(CH 3OH) 1] (-) in the spectral range studied. Ab initio calculations are performed at the MP2/6-311++G** level to obtain structural information such as optimized structures, stabilization energies, and vibrational frequencies of [(CO 2) n (CH 3OH) m ] (-). Comparison between the experimental and the theoretical results reveals the structural properties of [(CO 2) n (CH 3OH) m ] (-): (1) the incorporated CH 3OH interacts directly with either CO 2 (-) or C 2O 4 (-) core by forming an O-HO linkage; (2) the introduction of CH 3OH promotes charge localization in the clusters via the hydrogen-bond formation, resulting in the predominance of CO 2 (-).(CH 3OH) m (CO 2) n-1 isomeric forms over C 2O 4 (-).(CH 3OH) m (CO 2) n-2 ; (3) the hydroxyl group of CH 3OH provides an additional solvation cite for neutral CO 2 molecules.     

Rate coefficients for the gas-phase reaction of OH radicals with dimethyl sulfide: temperature and O2 partial pressure dependence

Rate coefficients for the gas-phase reaction of hydroxyl (OH) radicals with dimethyl sulfide (CH(3)SCH(3), DMS) have been determined using a relative rate technique. The experiments were performed under different conditions of temperature (250-299 K) and O(2) partial pressure (approximately 0 Torr O(2)-380 Torr O(2)), at a total pressure of 760 Torr bath gas (N(2) + O(2)), in a 336 l reaction chamber, using long path in situ Fourier transform (FTIR) absorption spectroscopy to monitor the disappearance rates of DMS and the reference compounds (ethene, propene and 2-methylpropene). OH was produced by the photolysis of H(2)O(2). The following Arrhenius expressions adequately describe the rate coefficients as a function of temperature (units are cm(3) molecule(-1) s(-1)): k = (1.56 +/- 0.20) x 10(-12) exp[(369 +/- 27)/T], for approximately 0 Torr O(2); (1.31 +/- 0.08) x 10(-14) exp[(1910 +/- 69)/T], for 155 Torr O(2); (5.18 +/- 0.71) x 10(-14) exp[(1587 +/- 24)/T], for 380 Torr O(2). The results are compared with previous investigations.     

Kinetics of the reactions of Cl*(2P(1/2)) and Cl(2P(3/2)) atoms with CH3OH, C2H5OH, n-C3H7OH, and i-C3H7OH at 295 K

The title reactions were studied using laser flash photolysis/laser-induced-fluorescence (FP-LIF) techniques. The two spin-orbit states, Cl*(2P(1/2)) and Cl(2P(3/2)), were detected using LIF at 135.2 and 134.7 nm, respectively. Measured reaction rate constants were as follows (units of cm3 molecule(-1) s(-1)): k(Cl(2P(3/2))+CH3OH) = (5.35 +/- 0.24) x 10(-11), k(Cl(2P(3/2))+C2H5OH) = (9.50 +/- 0.85) x 10(-11), k(Cl(2P(3/2))+n-C3H7OH) = (1.71 +/- 0.11) x 10(-10), and k(Cl(2P(3/2))+i-C3H7OH) = (9.11 +/- 0.60) x 10(-11). Measured rate constants for total removal of Cl*(2P(1/2)) in collisions with CH3OH, C2H5OH, n-C3H7OH, and i-C3H7OH were (1.95 +/- 0.13) x 10(-10), (2.48 +/- 0.18) x 10(-10), (3.13 +/- 0.18) x 10(-10), and (2.84 +/- 0.16) x 10(-10), respectively; quoted errors are two-standard deviations. Although spin-orbit excited Cl*(2P(1/2)) atoms have 2.52 kcal/mol more energy than Cl(2P(3/2)), the rates of chemical reaction of Cl*(2P(1/2)) with CH3OH, C2H5OH, n-C3H7OH, and i-C3H7OH are only 60-90% of the corresponding Cl(2P(3/2)) atom reactions. Under ambient conditions spin-orbit excited Cl* atoms are responsible for 0.5%, 0.5%, 0.4%, and 0.7% of the observed reactivity of thermalized Cl atoms toward CH3OH, C2H5OH, n-C3H7OH, and i-C3H7OH, respectively.