Rate Constants For Reactions Of OH Research Articles

Photochemical behaviour and toxicity evolution of phenylbenzoate liquid crystal monomers in water

Liquid crystal monomers (LCMs) are a group of emerging pollutants that pose potential environmental risks because of their ubiquitous occurrence and toxicity. Understanding their environmental transformation is essential for assessing the ecological risk. In this study, we investigated the photochemical transformation kinetics, mechanism, and photo-induced toxicity of three phenylbenzoate LCMs in water. Their apparent photolytic rate constants were within (0.023 – 0.058) min-1, and the half-lives were < 30.0 min, showing lower persistence in water. Dissolved organic matter significantly inhibited their photolysis because of light-shielding effect and quenching of excited triplet states of LCMs. Their photolysis mainly occurred through excited triplet states, and the reactive oxygen species (i.e., ⋅OH, 1O2 and ⋅O2-) contributed to their degradation. The main photolysis pathways were ester bond cleavage, ⋅OH substitution/addition, and defluorination. Experiments and computational simulation revealed that some ·OH addition/substitution products have similar toxicity with LCMs. Additionally, the ∙OH reaction rate constants (kOH) of LCMs were determined to be > 1 × 109 M-1 s-1, evidence for their high reactivity toward ⋅OH. We have further developed reliable methods to estimate kOH of other phenylbenzoate-like LCMs with quantum chemical calculations. These results are useful for understanding the transformation and fate of LCMs in aquatic environments.

Atmospheric photo-oxidation of myrcene: OH reaction rate constant, gas-phase oxidation products and radical budgets

Abstract. The photo-oxidation of myrcene, a monoterpene species emitted by plants, was investigated at atmospheric conditions in the outdoor simulation chamber SAPHIR (Simulation of Atmospheric PHotochemistry In a Large Reaction Chamber). The chemical structure of myrcene consists of one moiety that is a conjugated π system (similar to isoprene) and another moiety that is a triple-substituted olefinic unit (similar to 2-methyl-2-butene). Hydrogen shift reactions of organic peroxy radicals (RO2) formed in the reaction of isoprene with atmospheric OH radicals are known to be of importance for the regeneration of OH. Structure–activity relationships (SARs) suggest that similar hydrogen shift reactions like in isoprene may apply to the isoprenyl part of RO2 radicals formed during the OH oxidation of myrcene. In addition, SAR predicts further isomerization reactions that would be competitive with bimolecular RO2 reactions for chemical conditions that are typical for forested environments with low concentrations of nitric oxide. Assuming that OH peroxy radicals can rapidly interconvert by addition and elimination of O2 like in isoprene, bulk isomerization rate constants of 0.21 and 0.097 s−1 (T=298 K) for the three isomers resulting from the 3′-OH and 1-OH addition, respectively, can be derived from SAR. Measurements of radicals and trace gases in the experiments allowed us to calculate radical production and destruction rates, which are expected to be balanced. The largest discrepancies between production and destruction rates were found for RO2. Additional loss of organic peroxy radicals due to isomerization reactions could explain the observed discrepancies. The uncertainty of the total radical (ROx=OH+HO2+RO2) production rates was high due to the uncertainty in the yield of radicals from myrcene ozonolysis. However, results indicate that radical production can only be balanced if the reaction rate constant of the reaction between hydroperoxy (HO2) and RO2 radicals derived from myrcene is lower (0.9 to 1.6×10-11 cm3 s−1) than predicted by SAR. Another explanation of the discrepancies would be that a significant fraction of products (yield: 0.3 to 0.6) from these reactions include OH and HO2 radicals instead of radical-terminating organic peroxides. Experiments also allowed us to determine the yields of organic oxidation products acetone (yield: 0.45±0.08) and formaldehyde (yield: 0.35±0.08). Acetone and formaldehyde are produced from different oxidation pathways, so that yields of these compounds reflect the branching ratios of the initial OH addition to myrcene. Yields determined in the experiments are consistent with branching ratios expected from SAR. The yield of organic nitrate was determined from the gas-phase budget analysis of reactive oxidized nitrogen in the chamber, giving a value of 0.13±0.03. In addition, the reaction rate constant for myrcene + OH was determined from the measured myrcene concentration, yielding a value of (2.3±0.3)×10-10 cm3 s−1.

Atmospheric chemistry of ketones: Reaction of OH radicals with 2-methyl-3-pentanone, 3-methyl-2-pentanone and 4-methyl-2-pentanone.

This work reports new kinetic and mechanistic information on the atmospheric chemistry of ketones. Both absolute and relative rate methods were used to determine the rate constants for OH reactions with 2-methyl-3-pentanone (2M3P), 3-methyl-2-pentanone (3M2P) and 4-methyl-2-pentanone (4M2P), three widely used compounds in the industry. This work constitutes the first temperature dependence study of the reactions of OH with 2M3P and 3M2P. The following rate constants values are recommended at 298 K (in 10-12 cm3 molecule-1 s-1): kOH+2M3P = 3.49 ± 0.5; kOH+3M2P = 6.02 ± 0.14 and kOH+4M2P = 11.02 ± 0.42. The following Arrhenius expressions (in units of cm3 molecule-1 s-1) adequately describe the measured rate constants for OH reactions with 2M3P and 3M2P in the temperature range 263-373 K: k2M3P = (2.33 ± 0.06) × 10-12 exp((127.4 ± 18.6)/T) and k3M2P = (1.05 ± 0.14) × 10-12 exp((537 ± 41)/T). Products studies from the reactions of OH with the investigated ketones were conducted in a 7.3 m3 simulation chamber using PTR-ToF-MS, UHPLC-MS and GC-MS. A series of short chain carbonyl compounds including formaldehyde, acetone, acetaldehyde, 2-butanone and 2-methypropanal were observed as products. Combining the yields of carbonyls measured with those estimated from the SAR method, we propose various mechanistic degradation schemes of the investigated ketones initiated by reaction with OH radicals.

Aqueous-Phase Oxidation of Terpene-Derived Acids by Atmospherically Relevant Radicals.

Terpene-derived acids formed through the atmospheric gas-phase oxidation of terpenes are able to efficiently undergo a phase transfer into the aqueous phase. The subsequent aqueous-phase oxidation of such compounds has not been intensely studied. Accordingly, the aqueous-phase second-order rate constants of the oxidation reactions of cis-pinonic acid (CPA) and (+)-camphoric acid (+CA) with hydroxyl radicals (•OH), nitrate radicals (NO3•), and sulfate radicals (SO4•-) were investigated as a function of temperature and pH in the present study. For CPA and +CA the following •OH reaction rate constants at T = 298 K are determined: ksecond(CPA, pH<2) = (2.8 ± 0.1) × 109 L mol-1 s-1, ksecond(CPA, pH>8) = (2.7 ± 0.3) × 109 L mol-1 s-1, ksecond(+CA, pH<2) = (2.1 ± 0.1) × 109 L mol-1 s-1, ksecond(+CA, pH=5.3) = (2.7 ± 0.3) × 109 L mol-1 s-1, ksecond(+CA, pH>8) = (2.7 ± 0.1) × 109 L mol-1 s-1. In order to assess the atmospheric impact of the aqueous-phase oxidation of such compounds, atmospheric aqueous-phase lifetimes were calculated for two model scenarios based on CAPRAM 3.0i. The aqueous-phase oxidation under remote conditions emerges to be the most favored pathway with lifetimes of 5 ± 1 h.

Importance of sulfate radical anion formation and chemistry in heterogeneous OH oxidation of sodium methyl sulfate, the smallest organosulfate

Abstract. Organosulfates are important organosulfur compounds present in atmospheric particles. While the abundance, composition, and formation mechanisms of organosulfates have been extensively investigated, it remains unclear how they transform and evolve throughout their atmospheric lifetime. To acquire a fundamental understanding of how organosulfates chemically transform in the atmosphere, this work investigates the heterogeneous OH radical-initiated oxidation of sodium methyl sulfate (CH3SO4Na) droplets, the smallest organosulfate detected in atmospheric particles, using an aerosol flow tube reactor at a high relative humidity (RH) of 85 %. Aerosol mass spectra measured by a soft atmospheric pressure ionization source (direct analysis in real time, DART) coupled with a high-resolution mass spectrometer showed that neither functionalization nor fragmentation products are detected. Instead, the ion signal intensity of the bisulfate ion (HSO4−) has been found to increase significantly after OH oxidation. We postulate that sodium methyl sulfate tends to fragment into a formaldehyde (CH2O) and a sulfate radical anion (SO4 ⋅ −) upon OH oxidation. The formaldehyde is likely partitioned back to the gas phase due to its high volatility. The sulfate radical anion, similar to OH radical, can abstract a hydrogen atom from neighboring sodium methyl sulfate to form the bisulfate ion, contributing to the secondary chemistry. Kinetic measurements show that the heterogeneous OH reaction rate constant, k, is (3.79 ± 0.19) × 10−13 cm3 molecule−1 s−1 with an effective OH uptake coefficient, γeff, of 0.17 ± 0.03. While about 40 % of sodium methyl sulfate is being oxidized at the maximum OH exposure (1.27 × 1012 molecule cm−3 s), only a 3 % decrease in particle diameter is observed. This can be attributed to a small fraction of particle mass lost via the formation and volatilization of formaldehyde. Overall, we firstly demonstrate that the heterogeneous OH oxidation of an organosulfate can lead to the formation of sulfate radical anion and produce inorganic sulfate. Fragmentation processes and sulfate radical anion chemistry play a key role in determining the compositional evolution of sodium methyl sulfate during heterogeneous OH oxidation.

Aqueous OH Radical Reaction Rate Constants for Organophosphorus Flame Retardants and Plasticizers: Experimental and Modeling Studies.

Aqueous ·OH reaction rate constants ( kOH) for organophosphate esters (OPEs) are essential for assessing their environmental fate and removal potential in advanced oxidation processes (AOPs). Herein experimental and in silico approaches were adopted to obtain kOH values for a variety of OPEs. The determined kOH for 18 OPEs varies from 4.0 × 108 M-1 s-1 to 1.6 × 1010 M-1 s-1. Based on the experimental kOH values, a quantitative structure-activity relationship model that involves molecular structural information on the number of heavy atoms, content index, and the most negative charge of C atoms was developed for predicting kOH of other OPEs. Furthermore, appropriate density functional theory (DFT) and solvation models were selected, which together with transition state theory were employed to predict kOH of three representative OPEs. The deviation between the DFT calculated and the experimental kOH values ( kcal/ kexp) is within 2. Half-lives of the OPEs were estimated to be 0.5-22791.3 days in natural waters and 0.044-19.7 s in AOPs, indicating the OPEs are potentially persistent in natural waters and can be quickly eliminated by AOPs. The determined kOH values and the in silico methods offer a scientific base for assessing OPEs fate in aquatic environments.

Modeling the pH and temperature dependence of aqueousphase hydroxyl radical reaction rate constants of organic micropollutants using QSPR approach.

Designing of advanced oxidation process (AOP) requires knowledge of the aqueous phase hydroxyl radical (●OH) reactions rate constants (k OH), which are strictly dependent upon the pH and temperature of the medium. In this study, pH- and temperature-dependent quantitative structure-property relationship (QSPR) models based on the decision tree boost (DTB) approach were developed for the prediction of k OH of diverse organic contaminants following the OECD guidelines. Experimental datasets (n=958) pertaining to the k OH values of aqueous phase reactions at different pH (n=470; 1.4×106 to 3.8×1010M-1s-1) and temperature (n=171; 1.0×107 to 2.6×1010M-1s-1) were considered and molecular descriptors of the compounds were derived. The Sanderson scale electronegativity, topological polar surface area, number of double bonds, and halogen atoms in the molecule, in addition to the pH and temperature, were found to be the relevant predictors. The models were validated and their external predictivity was evaluated in terms of most stringent criteria parameters derived on the test data. High values of the coefficient of determination (R 2) and small root mean squared error (RMSE) in respective training (>0.972, ≤0.12) and test (≥0.936, ≤0.16) sets indicated high generalization and predictivity of the developed QSPR model. Other statistical parameters derived from the training and test data also supported the robustness of the models and their suitability for screening new chemicals within the defined chemical space. The developed QSPR models provide a valuable tool for predicting the ●OH reaction rate constants of emerging new water contaminants for their susceptibility to AOPs.

Quantitative structure‐reactivity relationships of hydroxyl radical rate constants for linear and cyclic volatile methylsiloxanes

An accurate understanding of the fate of volatile methylsiloxanes (VMS) in air is crucial for determining their persistence and concentrations in the environment. Although oxidation by atmospheric hydroxyl radicals (•OH) is considered as a major degradation mechanism for airborne VMS, the existing bimolecular rate constants with •OH measured and modeled for any given VMS compound varied greatly, depending on the approaches used to generate the data. The objectives of the present study were to measure •OH reaction rate constants for 4 cyclic and 4 linear VMS based on a relative rate method using a newly designed atmospheric chamber and to establish structure-reactivity relationships for the kinetics. In the past, the reaction rate constants for VMS were generally recognized to increase with the number of the methyl groups per molecule, the only differential factor in the existing models. However, the new measurements indicated that molecular structure should also be considered in the prediction of the reaction rates. Better empirical models were developed by simple and multiple linear regressions of the measured values from the present study and the literature. A high correlation existed for the reaction rates with the number of the methyl group attached at 2 distinct siloxane structures (i.e., linear and cyclic VMS). Even better correlations were obtained with one or 2 molecular descriptors that are directly related to the size of VMS, which, in turn, not only depend on the number of methyl groups, but the linear/cyclic structures as well for permethylsiloxanes. Environ Toxicol Chem 2017;36:3240-3245. © 2017 SETAC.

Modeling the reactivities of hydroxyl radical and ozone towards atmospheric organic chemicals using quantitative structure-reactivity relationship approaches.

The persistence and the removal of organic chemicals from the atmosphere are largely determined by their reactions with the OH radical and O3. Experimental determinations of the kinetic rate constants of OH and O3 with a large number of chemicals are tedious and resource intensive and development of computational approaches has widely been advocated. Recently, ensemble machine learning (EML) methods have emerged as unbiased tools to establish relationship between independent and dependent variables having a nonlinear dependence. In this study, EML-based, temperature-dependent quantitative structure-reactivity relationship (QSRR) models have been developed for predicting the kinetic rate constants for OH (kOH) and O3 (kO3) reactions with diverse chemicals. Structural diversity of chemicals was evaluated using a Tanimoto similarity index. The generalization and prediction abilities of the constructed models were established through rigorous internal and external validation performed employing statistical checks. In test data, the EML QSRR models yielded correlation (R (2)) of ≥0.91 between the measured and the predicted reactivities. The applicability domains of the constructed models were determined using methods based on descriptors range, Euclidean distance, leverage, and standardization approaches. The prediction accuracies for the higher reactivity compounds were relatively better than those of the low reactivity compounds. Proposed EML QSRR models performed well and outperformed the previous reports. The proposed QSRR models can make predictions of rate constants at different temperatures. The proposed models can be useful tools in predicting the reactivities of chemicals towards OH radical and O3 in the atmosphere.

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.

The gas-phase degradation of chlorpyrifos and chlorpyrifos-oxon towards OH radical under atmospheric conditions

The OH initiated oxidation of chlorpyrifos (a widely used insecticide) and its photooxidation product chlorpyrifos-oxon were investigated at the large outdoor European Photoreactor (EUPHORE). The rate constants for reaction of chlorpyrifos and chlorpyrifos oxon with OH radicals were measured using a conventional relative rate method. The value of the OH reaction rate constants with chlorpyrifos and chlorpyrifos-oxon were determined to be k=(9.1±2.1)×10−11cm3molecule−1s−1 and (1.7±0.9)×10−11cm3molecule−1s−1 at 303±5K and atmospheric pressure. They gave an atmospheric lifetime in relation to the reaction with OH of approximately 2h and 11h for chlorpyrifos and chlorpyrifos-oxon, respectively. Photolysis was found to be unimportant relative to reaction with OH. The main products detected in the gas phase from the reaction of OH with chlorpyrifos were SO2, chlorpyrifos-oxon, 3,5,6-trichloro-2-pyridinol and diethylphosphate with molar yields of 17±5%, ∼10%, 8±4% and 30±9%, respectively.

On Rates and Mechanisms of OH and O3 Reactions with Isoprene-Derived Hydroxy Nitrates

Eight distinct hydroxy nitrates are stable products of the first step in the atmospheric oxidation of isoprene by OH. The subsequent chemical fate of these molecules affects global and regional production of ozone and aerosol as well as the location of nitrogen deposition. We synthesized and purified 3 of the 8 isoprene hydroxy nitrate isomers: (E/Z)-2-methyl-4-nitrooxybut-2-ene-1-ol and 3-methyl-2-nitrooxybut-3-ene-1-ol. Oxidation of these molecules by OH and ozone was studied using both chemical ionization mass spectrometry and thermo-dissociation laser induced fluorescence. The OH reaction rate constants at 300 K measured relative to propene at 745 Torr are (1.1 ± 0.2) × 10(-10) cm(3) molecule(-1) s(-1) for both the E and Z isomers and (4.2 ± 0.7) × 10(-11) cm(3) molecule(-1) s(-1) for the third isomer. The ozone reaction rate constants for (E/Z)-2-methyl-4-nitrooxybut-2-ene-1-ol are (2.7 ± 0.5) × 10(-17) and (2.9 ± 0.5) × 10(-17) cm(3) molecule(-1) s(-1), respectively. 3-Methyl-2-nitrooxybut-3-ene-1-ol reacts with ozone very slowly, within the range of (2.5-5) × 10(-19) cm(3) molecule(-1) s(-1). Reaction pathways, product yields, and implications for atmospheric chemistry are discussed. A condensed mechanism suitable for use in atmospheric chemistry models is presented.

Rate Constants and Products of the OH Reaction with Isoprene-Derived Epoxides

Recent laboratory and field work has shown that isoprene-derived epoxides (IEPOX) are crucial intermediates that can explain the existence of a variety of compounds found in ambient secondary organic aerosol (SOA). However, IEPOX species are also able to undergo gas phase oxidation, which competes with the aerosol phase processing of IEPOX. In order to better quantify the atmospheric fate of IEPOX, the gas phase OH reaction rate constants and product formation mechanisms have been determined using a flow tube chemical ionization mass spectrometry technique. The new OH rate constants are generally larger than previous estimations and some features of the product mechanism are well predicted by the Master Chemical Mechanism Version 3.2 (MCM v3.2), while other features are at odds with MCM v3.2. Using a previously proposed kinetic model for the quantitative prediction of the atmospheric fate of IEPOX, it is found that gas phase OH reaction is an even more dominant fate for chemical processing of IEPOX than previously suggested. The present results suggest that aerosol phase processing of IEPOX will be competitive with gas phase OH oxidation only under SOA conditions of high liquid water content and low pH.

Heterogeneous oxidation of folpet and dimethomorph by OH radicals: A kinetic and mechanistic study

This study investigates the heterogeneous OH oxidation of folpet and dimethomorph, two fungicides identified in the atmosphere of the Champagne-Ardenne region. Kinetic experiments were conducted in the relative mode, using terbuthylazine as a reference compound. The experimental method employed makes use of a simulation chamber coupled to a GC/MS analytical system. Meanwhile, the identification of (4-chlorophenyl)(3,4-dimethoxyphenyl)methanone (CPMPM), a degradation product of the OH oxidation of dimethomorph, is achieved using SPME/GC/MS. Moreover, a degradation mechanism of dimethomorph is proposed, and the heterogeneous OH reactivity of CPMPM is evaluated. The obtained OH reaction rate constants (cm3 molecule−1 s−1) are: kZ-dimethomorph = (2.0 ± 1.2) 10−14, kE-dimethomorph = (1.7 ± 1.2) 10−14, kFolpet = (1.6 ± 0.9) 10−13 and kCPMPM = (1.9 ± 1.0) 10−12. The implicated tropospheric life-times are up to 2 months, which shows that the investigated pesticides are relatively persistent towards oxidation removal processes. CPMPM, the identified product of OH oxidation of dimethomorph, is less persistent with a life time of only 6 days relative to heterogeneous oxidation by OH radicals.

Correction to “OH reaction rate constant, IR absorption spectrum, ozone depletion potentials and global warming potentials of 2‐bromo‐3,3,3‐trifluoropropene”

Correction to “OH reaction rate constant, IR absorption spectrum, ozone depletion potentials and global warming potentials of 2‐bromo‐3,3,3‐trifluoropropene”

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.

Photolysis, OH reactivity and ozone reactivity of a proxy for isoprene-derived hydroperoxyenals (HPALDs)

The C(5)-hydroperoxyenals (C(5)-HPALDs) are a newly-recognized class of multi-functional hydrocarbons produced during the hydroxyl radical (OH)-initiated oxidation of isoprene. Recent theoretical calculations suggest that fast photolysis of these compounds may be an important OH source in high-isoprene, low-NO regions. We report experimental constraints for key parameters of photolysis, OH reaction and ozone reaction of these compounds as derived from a closely-related, custom-synthesized C(6)-HPALD. The photolysis quantum yield is 1.0 ± 0.4 over the range 300-400 nm, assuming an absorption cross section equal to the average of those measured for several analogous enals. The yield of OH from photolysis was determined as 1.0 ± 0.8. The OH reaction rate constant is (5.1 ± 1.8) × 10(-11) cm(3) molecule(-1) s(-1) at 296 K. The ozone reaction rate constant is (1.2 ± 0.2) × 10(-18) cm(3) molecule(-1) s(-1) at 296 K. These results are consistent with previous first-principles estimates, though the nature and fate of secondary oxidation products remains uncertain. Incorporation of C(5)-HPALD chemistry with the above parameters in a 0-D box model, along with experimentally-constrained rates for C(5)-HPALD production from isomerization of first-generation isoprene hydroxyperoxy radicals, is found to enhance modeled OH concentrations by 5-16% relative to the traditional isoprene oxidation mechanism for the chemical regimes of recent observational studies in rural and remote regions. This enhancement in OH will increase if C(5)-HPALD photo-oxidation products also photolyze to yield additional OH or if the C(5)-HPALD production rate is faster than has been observed.

Direct rate constant for the reaction of OH radicals with the biofuel molecule ethyl levulinate

Second generation (2G) biofuels are produced from non-edible lignocellulosic biomass providing sustainable alternatives for fossil fuels. The OH reaction rate constant has been determined for the 2G biofuel ethyl levulinate (CH3C(O)CH2CH2C(O)OCH2CH3, ELA) (1) for the first time. The direct reaction kinetic technique of pulsed laser photolysis (PLP) coupled with resonance fluorescence (RF) detection of OH radicals has been used to determine the rate constant value of k 1 = (3.43 ± 0.36) × 10−12 cm3 molecule−1 s−1 (298 K), where the error given designates 2σ accuracy. Implications for atmospheric chemistry have been discussed.

Predicting rate constants of hydroxyl radical reactions with organic pollutants: Algorithm, validation, applicability domain, and mechanistic interpretation

The reaction with hydroxyl radical ( OH) is the most important removal process in the daytime for organic pollutants in the atmosphere, thus the OH reaction rate constants ( k OH) are important to assessing the fate of organic pollutants in the troposphere. In this study, experimental data for log k OH of 722 organic chemicals were employed to develop quantitative structure–activity relationships (QSARs) for k OH, applying 22 molecular structural descriptors and partial least squares (PLS) regression. The QSAR development followed the OECD guidelines, with special attention to validation, applicability domain and mechanistic interpretation. For the established model, the leave-many-out cross-validated Q CUM 2 = 0.865, R 2 = 0.878, and RMSE = 0.391 log units, indicating good robustness and predictivity. The predictive capability was also evaluated by external validation with Q EXT 2 = 0.872. The applicability domain of the model is composed of compounds containing C, H, N, O, S, F, Cl, Br, I, and Si atoms in various functional groups and analyzed by Williams plot. The main molecular structural factors governing k OH are the compactness of the molecule, the molecular ability of donating electrons and the number of halogen atoms in a molecule.

Indirect Photolysis of Organic Compounds: Prediction of OH Reaction Rate Constants through Molecular Orbital Calculations

The MOOH approach is a perturbational molecular orbital method to predict rate constants of indirect photolysis of organic compounds through reaction with OH radicals. It employs the semiempirical AM1 scheme as the underlying quantum chemical model. The original method introduced by Klamt has been reparametrized using an up-to-date set of 675 compounds with experimental rate constants and outperforms the prominent Atkinson increment scheme for this training set as well as for an extended set of 805 compounds, yielding an overall root-mean-square error of 0.32 log units. The discussion includes detailed comparative analyses of the model performances for individual compound classes. The present model calibration refers mainly to monofunctional compounds but performs already reasonably well for multifunctional compounds. For predictive applications, both the Atkinson scheme and the alternative, independent AM1-MOOH model can be used as components of a consensus modeling approach, arriving at increased confidence in cases where the different models agree.