Rate Constants For Reactions Of OH Radicals Research Articles

Probing the OH Oxidation of Pinonic Acid at the Air-Water Interface Using Field-Induced Droplet Ionization Mass Spectrometry (FIDI-MS).

Gas and aqueous phases are essential media for atmospheric chemistry and aerosol formation. Numerous studies have focused on aqueous-phase reactions as well as coupled gas/aqueous-phase mass transport and reaction. Few studies have directly addressed processes occurring at the air-water interface, especially involving surface-active compounds. We report here the application of field-induced droplet ionization mass spectrometry (FIDI-MS) to chemical reactions occurring at the atmospheric air-water interface. We determine the air-water interfacial OH radical reaction rate constants for sodium dodecyl sulfate (SDS), a common surfactant, and pinonic acid (PA), a surface-active species and proxy for biogenic atmospheric oxidation products, as 2.87 × 10-8 and 9.38 × 10-8 cm2 molec-1 s-1, respectively. In support of the experimental data, a comprehensive gas-surface-aqueous multiphase transport and reaction model of general applicability to atmospheric interfacial processes is developed. Through application of the model, PA is shown to be oxidized exclusively at the air-water interface of droplets with a diameter of 5 μm under typical ambient OH levels. In the absence of interfacial reaction, aqueous- rather than gas-phase oxidation is the major PA sink. We demonstrate the critical importance of air-water interfacial chemistry in determining the fate of surface-active species.

Application of an Artificial Neural Network to the Prediction of OH Radical Reaction Rate Constants for Evaluating Global Warming Potential.

Rate constants for reactions of chemical compounds with hydroxyl radical are a key quantity used in evaluating the global warming potential of a substance. Experimental determination of these rate constants is essential, but it can also be difficult and time-consuming to produce. High-level quantum chemistry predictions of the rate constant can suffer from the same issues. Therefore, it is valuable to devise estimation schemes that can give reasonable results on a variety of chemical compounds. In this article, the construction and training of an artificial neural network (ANN) for the prediction of rate constants at 298 K for reactions of hydroxyl radical with a diverse set of molecules is described. Input to the ANN consists of counts of the chemical bonds and bends present in the target molecule. The ANN is trained using 792 (•)OH reaction rate constants taken from the NIST Chemical Kinetics Database. The mean unsigned percent error (MUPE) for the training set is 12%, and the MUPE of the testing set is 51%. It is shown that the present methodology yields rate constants of reasonable accuracy for a diverse set of inputs. The results are compared to high-quality literature values and to another estimation scheme. This ANN methodology is expected to be of use in a wide range of applications for which (•)OH reaction rate constants are required. The model uses only information that can be gathered from a 2D representation of the molecule, making the present approach particularly appealing, especially for screening applications.

Kinetics of the Gas-Phase Reactions of OH and NO3 Radicals and O3 with Allyl Alcohol and Allyl Isocyanate

Rate constants for the gas-phase reactions of OH radical, NO(3) radical, and ozone with allyl alcohol (AAL) and allyl isocyanate (AIC) have been measured using relative rate methods at atmospheric pressure in purified air. The experimental Arrhenius expression obtained for the reaction of the OH radical with AAL is (1.68 +/- 0.89) x 10(-12) x exp(1100/T) cm(3) molecule(-1) s(-1), for T = 282-315 K; the Arrhenius expression for the reaction of OH radical with AIC is (1.94 +/- 1.04) x 10(-14) x exp(2207/T) cm(3) molecule(-1) s(-1), for T = 282-317 K, where the indicated errors are one least-squares standard deviation. All OH radical reaction rate constants have been measured relative to k(OH + alpha-pinene) and k(OH + 1,3,5-trimethylbenzene). The rate constant for the gas-phase reaction of OH radical with allyl alcohol-d(6) isotopomer (AAL-d(6)) has been measured at T = 298 K, and the value is 5.10 x 10(-11) cm(3) molecule(-1) s(-1). The kinetic isotope effect is small, with k(AAL)/k(AAL-d(6)) = 1.32. Rate constants for the gas-phase reactions of NO(3) radical with AAL [relative to k(NO(3) +methacrolein)] and O(3) [relative to k(O(3) + beta-pinene)] have been measured, and the values are 7.7 x 10(-15) cm(3) molecule(-1) s(-1) at T = 298 K and 1.6 x 10(-17) cm(3) molecule(-1) s(-1) at T = 296 K, respectively. Rate constants for the gas-phase reactions of NO(3) radical and O(3) with AIC have been measured, and the values are 9.4 x 10(-16) cm(3) molecule(-1) s(-1) at T = 299 K and 5.54 x 10(-18) cm(3) molecule(-1) s(-1) at T = 299 K, respectively. Multireference ab initio calculations at the MRMP2/6-311G(d,p) level have been carried out for reactions of OH radical with AAL and AIC. Results indicate that prereactive hydrogen bonded complexes form in the entrance channels for these reactions.

New technique for generating high concentrations of gaseous OH radicals in relative rate measurements

AbstractWe have developed a technique for generating high concentrations of gaseous OH radicals in a reaction chamber. The technique, which involves the UV photolysis of O3 in the presence of water vapor, was used in combination with the relative rate method to obtain rate constants for reactions of OH radicals with selected species. A key improvement of the technique is that an O3/O2 (3%) gas mixture is continuously introduced into the reaction chamber, during the UV irradiation period. An important feature is that a high concentration of OH radicals [(0.53–1.2) × 1011 radicals cm−3] can be produced during the irradiation in continuous, steady‐state experiment. Using the new technique in conjunction with the relative rate method, we obtained the rate constant for the reaction of CHF3 (HFC‐23) with OH radicals, k1. We obtained k1(298 K) = (3.32 ± 0.20) × 10−16 and determined the temperature dependence of k1 to be (0.48 ± 0.13) × 10−12 exp[−(2180 ± 100)/T] cm3 molecule−1 s−1 at 253–328 K using CHF2CF3 (HFC‐125) and CHF2Cl (HCFC‐22) as reference compounds in CHF3–reference–H2O gas mixtures. The value of k1 obtained in this study is in agreement with previous measurements of k1. This result confirms that our technique for generating OH radicals is suitable for obtaining OH radical reaction rate constants of ∼10−16 cm3 molecule−1 s−1, provided the rate constants do not depend on pressure. In addition, it also needed to examine whether the reactions of sample and reference compound with O3 interfere the measurement when selecting this technique. © 2003 Wiley Periodicals, Inc. Int J Chem Kinet 35: 317–325, 2003

Atmospheric Gas-Phase Reactions of Selected Phosphorus-Containing Compounds

The kinetics of the gas-phase reactions of phosphorus oxychloride [P(O)Cl3], methylphosphonic dichloride [CH3P(O)Cl2], dimethyl phosphonate [(CH3O)2P(O)H], and trimethyl phosphite [(CH3O)3P] with the OH radical were measured at 298 ± 2 K and 1 atm (∼740 Torr) of dry air by a relative rate method which employed the photolysis of CH3ONO as the source of OH radicals and analysis of the reaction mixtures by in-situ Fourier transform infrared (FT-IR) spectroscopy. The measured values of the OH radical reaction rate constants (cm3 molecule-1 s-1) are the following: P(O)Cl3, < 4 × 10-14; CH3P(O)Cl2, (6.4 ± 2.1) × 10-14; (CH3O)2P(O)H, (5.08 ± 0.53) × 10-12; and (CH3O)3P, (7.1 ± 0.9) × 10-10, corresponding to estimated tropospheric lifetimes of >290 days, 180 days, 2.3 days, and 0.016 day, respectively, for the reactions of these compounds with OH radicals. In support of the OH radical rate constant measurement for (CH3O)3P, the absolute rate constant for the NO2 + (CH3O)3P reaction was measured and determined to be (3.8 ± 0.4) × 10-18 cm3 molecule-1 s-1 at 298 ± 2 K. The products of the reaction of dimethyl phosphonate with the OH radical and those of trimethyl phosphite with the OH radical, NO2, and O3 were investigated by FT-IR spectroscopy and atmospheric pressure ionization mass spectrometry (API-MS).

Atmospheric Degradation of Volatile Methyl-Silicon Compounds

The current widespread use of poly(dimethylsiloxane)s (PDMS) in a broad range of applications leads to their release into soil environments where they degrade to monomeric products, primarily dimethylsilanediol, most of which enter the atmosphere by volatilization. The major degradation pathway of volatile organosilicon compounds in the atmosphere is expected to be a reaction with hydroxyl (OH) radicals. In this work, the kinetics of the gas-phase reactions of dimethylsilanediol, trimethylsilanol, and tetramethylsilane with the OH radical were measured using a relative rate method which employed the N2H4 + O3 reaction as a nonphotolytic source of OH radicals, with analysis by Fourier transform infrared (FT-IR) spectroscopy in a 5870 L chamber. The measured values of the OH radical reaction rate constants (cm3 molecule-1 s-1) at 298 ± 2 K are as follows: dimethylsilanediol, (8.1 ± 1.0) × 10-13; trimethylsilanol, (7.2 ± 0.8) × 10-13; and tetramethylsilane, (8.5 ± 0.9) × 10-13. These values lead to an estimate of tropospheric lifetimes with respect to reaction with the OH radical of ca. 15 days for these organosilicon compounds. FT-IR spectroscopy and atmospheric pressure ionization mass spectrometry (API-MS) were employed to analyze the products of OH radical- and Cl atom-initiated photooxidations of dimethylsilanediol and trimethylsilanol. Infrared signatures of the probable formate ester intermediate products from both silanols were detected. API-MS analyses indicated the formation of methylsilanetriol from dimethylsilanediol, of both dimethylsilanediol and methylsilanetriol from trimethylsilanol, and of the corresponding intermediate formate esters. Possible reaction mechanisms are discussed.

Kinetic study of the gas-phase photolysis and OH radical reaction of E,Z- and E,E-2,4-Hexadienedial

Unsaturated 1,6-dicarbonyls like 2,4-hexadienedial are ring opening products in the OH initiated photo-oxidation of aromatic hydrocarbons. In the present study, the photolysis of E,Z- and E,E-2,4-hexadienedial has been investigated under natural sunlight conditions in a large volume outdoor reaction chamber. In the case of the E,Z-isomer, an extremely rapid isomerization into the E,E-form was observed. The photoisomerization frequency, relative to that of NO2, was found to be J(E,Z-2,4-hexadienedial)/J(NO2) = (0.148 ± 0.012). A more complex photolysis behavior was observed for E,E-2,4-hexadienedial. Here, a fast equilibrium preceded a comparably slow photolysis. For the equilibrium reaction, relative frequencies of J(E,E-2,4-hexadienedial → EQUI)/J(NO2) = (0.113 ± 0.009) and J(EQUI → E,E-2,4-hexadienedial)/J(NO2) = (0.192 ± 0.016) were obtained, giving an equilibrium constant of K = (0.59 ± 0.07). For the photolysis frequencies, ratios of J(E,E-2,4-hexadienedial → products)/J(NO2) = J(EQUI → products)/J(NO2) = (1.22 ± 0.45)·10−2 were determined. Qualitative aerosol measurements during the experiments showed that the photolysis of 2,4-hexadienedials is a source of secondary organic aerosol. In addition to the photolysis study, OH radical reaction rate constants were determined, values of (7.4 ± 1.9)·10−11 and (7.6 ± 0.8)·10−11 cm3 s−1 were obtained for E,Z- and E,E-2,4-hexadienedial, respectively. The results indicate that the dominant fate of E,Z-2,4-hexadienedial in the atmosphere will be photoisomerization, while for E,E-2,4-hexadienedial, both photolysis and OH radical reaction will be important sinks. © 1999 John Wiley & Sons, Inc. Int J Chem Kinet 31: 689–697, 1999

Kinetics of the Gas-Phase Reactions of OH and NO3 Radicals and O3 with the Monoterpene Reaction Products Pinonaldehyde, Caronaldehyde, and Sabinaketone

Using a relative rate method, rate constants have been measured for the gas-phase reactions of OH and NO3 radicals with pinonaldehyde, caronaldehyde and sabinaketone at 296 ± 2 K. The OH radical reaction rate constants obtained are (in units of 10−12 cm3 molecule−1 s−1): pinonaldehyde, 48 ± 8; caronaldehyde, 48 ± 8; and sabinaketone, 5.1 ± 1.4, and the NO3 radical reaction rate constants are (in units of 10−14 cm3 molecule−1 s−1): pinonaldehyde, 2.0 ± 0.9; caronaldehyde, 2.5 ± 1.1; and sabinaketone, 0.036 ± 0.023, where the error limits include the estimated overall uncertainties in the rate constants for the reference compounds. Upper limits to the O3 reaction rate constants were also obtained, of <2 × 10−20 cm3 molecule−1 s−1 for pinonaldehyde and caronaldehyde, and <5 × 10−20 cm3 molecule−1 s−1 for sabinaketone. These reaction rate constants are combined with estimated ambient tropospheric concentrations of OH radicals, NO3 radicals and O3 to calculate tropospheric lifetimes and dominant transformation process(es) of these and other monoterpene reaction products.

Rate Constants for the Gas-Phase Reactions of the OH Radical with Selected Carbamates and Lactates

Using a relative rate method, rate constants have been measured for the gas-phase reactions of the volatile carbamates CH3NHC(O)OCH3, CH3NHC(O)OCH2CH3, CH3CH2NHC(O)OCH3 and CH3CH2NHC(O)OCH2CH3 and the lactates CH3CH(OH)C(O)OCH3, and CH3CH(OH)C(O)OCH2CH3 with the OH radical at 296 ± 2 K of (in units of 10-12 cm3 molecule-1 s-1) 4.3 ± 1.2, 8.3 ± 2.2, 10.5 ± 2.9, 14.4 ± 3.7, 2.8 ± 0.8, and 3.9 ± 1.1, respectively, where the error limits include the estimated overall uncertainties in the rate constants for the reference compounds. Rate constants were also measured for the reactions of the carbamates with NO3 radicals and O3, with respective NO3 radical and O3 reaction rate constants (in cm3 molecule-1 s-1 units) of CH3NHC(O)OCH3, (8.5 ± 3.2) × 10-16 and <4 × 10-20; CH3NHC(O)OCH2CH3, (9.4 ± 3.5) × 10-16 and <4 × 10-20; CH3CH2NHC(O)OCH3, (1.8 ± 0.8) × 10-15 and <6 × 10-20; and CH3CH2NHC(O)OCH2CH3, (2.6 ± 1.3) × 10-15 and <3 × 10-20. The OH radical reaction rate constants for the carbamates have been used to extend a previously developed estimation method to carbamates of the structure R1NHC(O)OR2 (where R1 and R2 are alkyl groups only), allowing OH radical reaction rate constants and hence tropospheric lifetimes to be calculated for carbamates used in agriculture for which experimental data are not available.

Rate Constants for the Gas-Phase Reactions of the OH Radical with Dichlorobiphenyls, 1-Chlorodibenzo-p-dioxin, 1,2-Dimethoxybenzene, and Diphenyl Ether: Estimation of OH Radical Reaction Rate Constants for PCBs, PCDDs, and PCDFs

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Estimation of hydroxyl radical reaction rate constants for gas-phase organic compounds using a structure-reactivity relationship: An update

The structure-reactivity approach proposed by Atkinson (1986, Chem. Rev. 86, 69-201) and extended by Atkinson (1987, Int. J. Chem. Kinet. 19, 799-828) for the calculation of rate constants for the gas-phase reactions of the OH radical with organic compounds has been re-investigated using the presently available database. Substituent group factors for several new groups are derived, including those for fluorinated eithers. Using a large fraction of the available database to derive the parameters needed to calculate the OH radical reaction rate constants, the 298 K rate constants of ∼90% of approximately 485 organic compounds are predicted to within a factor of 2 of the experimental values. Disagreements between calculated and experimental rate constants most commonly occur for halogen-containing compounds, and in particular for haloalkanes, haloalkenes and halogenated ethers. Disagreements also arise for ethers, especially for polyethers and cycloethers. The present estimation technique is reasonably reliable when used within the database used in its derivation, but extrapolation to organic compounds outside of this database results in a lack of assurance of its reliability, and its use for organic compounds which belong to classes other than those used in its development is discouraged.

OH radical reaction rate constants for polycyclic alkanes: Effects of ring strain and consequences for estimation methods

AbstractUsing a relative rate method, rate constants for the gas‐phase reactions of the OH radical with trans‐pinane [(1R, 2R)‐2, 6, 6‐trimethylbicyclo[3.1.1]heptane], tricyclene (1, 7, 7‐trimethyltricyclo[2.2.1.02, 6]heptane), and quadricyclane (quadricyclo[2.2.1.02, 6.03, 5]heptane) of (1.34 ± 0.29) × 10−11 cm3 molecule−1 s−1, (2.86 ± 0.62) × 10−12 cm3 molecule−1 s−1 and (1.83 ± 0.41) × 10−12 cm3 molecule−1 s−1, respectively, have been determined at 296 ± 2 K. These rate constants are compared with values calculated from an empirical estimation method and used to refine this estimation technique for the calculation of OH radical reaction rate constants for polycyclic systems. © John Wiley &amp; Sons, Inc.

Rate constants for the gas‐phase reactions of Cl atoms with a series of hydrofluorocarbons and hydrochlorofluorocarbons at 298 ± 2 K

AbstractA relative rate method has been used to determine rate constants for the gas‐phase reactions of a series of hydrofluorocarbons (HFCs) and hydrochlorofluorocarbons (HCFCs) at 298 ± 2 K and atmospheric pressure of air. Based on a rate constant for the reaction of the Cl atom with CH4 of (1.0 ± 0.2) − 10−13 cm3 molecule−1 s−1 at 298 K, the following Cl atom reaction rate constants (in units of 10−15 cm3 molecule−1 s−1) were obtained: CH3F, 340 ± 70; CH3CHF2, 240 ± 50; CH2FCl, 110 ± 25; CHFCl2, 21 ± 4; CHCl2CF3, 14 ± 3; CHFClCF3, 2.7 ± 0.6; CH3CFCl2, 2.4 ± 0.5; CHF2Cl, 2.0 ± 0.4; CH2FCF3, 1.6 ± 0.3; CH3CF2Cl, 0.37 ± 0.08; and CHF2CF3, 0.24 ± 0.05. These Cl atom reaction rate constants are compared with literature data and with the corresponding OH radical reaction rate constants. © John Wiley &amp; Sons, Inc.

Atmospheric reactions of chloroethenes with the OH radical

AbstractThe kinetics and products of the homogeneous gas‐phase reactions of the OH radical with the chloroethenes were investigated at 298 ± 2 K and atmospheric pressure. Using a relative rate technique and ethane as a scavenger for the chlorine atoms produced in these OH radical reactions, rate constants (in units of 10−12 cm3 molecule−1s−1) of 8.11 ± 0.24, 2.38 ± 0.14, and 1.80 ± 0.03 were obtained for 1,1‐dichloroethene, cis‐1, 2‐dichloroethene and trans‐1,2‐dichloroethene, respectively. Under these conditions, the major products observed by long pathlength FT‐IR absorption spectroscopy were HCHO and HC(O)Cl from vinyl chloride; HC(O)Cl from cis‐ and trans‐1,2‐dichloroethene; HCHO and COCl2 from 1,1‐dichloroethene; HC(O)Cl and COCl2 from trichloroethene; and COCl2 from tetrachloroethene. In the absence of a Cl atom scavenger, significant yields of the chloroacetyl chlorides, CHxCl3−xC(O)Cl, were observed from 1,1‐dichloro‐, trichloro‐ and tetrachloroethene, indicating that these products resulted from reactions involving chlorine atoms. The yields of all of these products are reported and the mechanisms of these gas‐phase reactions discussed. In addition, OH radical reaction rate constants were redetermined, in the presence of a Cl atom scavenger, for cis‐ and trans‐1,3‐dichloropropene and 3‐chloro‐2‐chloromethyl‐1‐propene, being (in units of 10−12 cm3 molecule−1 s−1) 8.45 ± 0.41, 14.4 ± 0.8, and 33.5 ± 3.0, respectively.

ChemInform Abstract: A Structure‐Activity Relationship for the Estimation of Rate Constants for the Gas‐Phase Reactions of OH Radicals with Organic Compounds.

AbstractThe structure‐activity relationship published by the author (1986) is further updated and extended to allow OH radical reaction rate constants for organic compounds containing ‐NH2, =NH, and =N‐ groups and S atoms to be calculated at room temp. and atmospheric pressure of air.

A structure‐activity relationship for the estimation of rate constants for the gas‐phase reactions of OH radicals with organic compounds

AbstractA previous technique for the calculation of rate constants for the gas‐phase reactions of the OH radical with organic compounds has been updated and extended to include sulfur‐ and nitrogen‐containing compounds. The overall OH radical reaction rate constants are separated into individual processes involving (a) H‐atom abstraction from CH and OH bonds in saturated organics, (b) OH radical addition to &gt;CC&lt; and CC unsaturated bonds, (c) OH radical addition to aromatic rings, and (d) OH radical interaction with NH2, &gt;NH, &gt;N, SH, and S groups. During its development, this estimation technique has been tested against the available database, and only for 18 out of a total of ca. 300 organic compounds do the calculated and experimental room temperature rate constants disagree by more than a factor of 2. This suggests that this technique has utility in estimating OH radical reaction rate constants at room temperature and atmospheric pressure of air, and hence atmospheric lifetimes due to OH radical reaction, for organic compounds for which experimental data are not available. In addition, OH radical reaction rate constants can be estimated over the temperature range ca. 250–1000 K for those organic compounds which react via H‐atom abstraction from CH and OH bonds, and over the temperature range ca. 250–500 K for compounds containing &gt;CC&lt; bond systems.

Atmospheric loss processes of 1,2-dibromo-3-chloropropane and trimethyl phosphate

The reactions of 1,2-dibromo-3-chloropropane (DBCP) and trimethyl phosphate with O/sub 3/ and the OH radical have been investigated at 296 +/- 2 K and atmospheric pressure of air by long-path Fourier transform infrared spectroscopic and gas chromatography-mass spectrometry techniques. For reactions with O/sub 3/, upper limits to the rate constants of < 3 x 10/sup -20/ cm/sup 3/ molecule/sup -1/ s/sup -1/ for 1,2-dibromo-3-chloropropane and < 6 x 10/sup -20/ cm/sup 3/ molecule/sup -1/ s/sup -1/ for trimethyl phosphate were determined, showing that reaction with O/sub 3/ is not an important atmospheric loss process for these compounds. With the use of relative rate techniques, OH radical reaction rate constants of 4.4 x 10/sup -13/ cm/sup 3/ molecule/sup -1/ s/sup -1/ for 1,2-dibromo-3-chloropropane and 7.4 x 10/sup -12/ cm/sup 3/ molecule/sup -1/ s/sup -1/ for trimethyl phosphate were obtained, leading to calculated tropospheric lifetimes of approx.55 and approx.3 days, respectively, for the reactions of these compounds with OH radicals. CH/sub 2/BrCOCH/sub 2/Cl has been tentatively identified as a product of the reaction of OH radicals with 1,2-dibromo-3-chloropropane. 14 references, 3 figures.