Rate Constants For Reactions Of Radicals Research Articles

Temperature- and pH- Dependent OH Radical Reaction Kinetics of Tartaric and Mucic Acids in the Aqueous Phase.

Tartaric acid and mucic acid are dicarboxylic acids (DCAs), a substance class often found in atmospheric aerosols and cloud droplets. The hydroxyl radical (•OH)-induced oxidation in the aqueous phase is known to be an important loss process of organic compounds such as DCAs. However, the study of •OH kinetics of DCAs in the aqueous phase is still incomplete. In the present study, the rate constants of the •OH reactions of tartaric acid and mucic acid in the aqueous phase were determined by the thiocyanate competition kinetics method as a function of temperature and pH. The following T-dependent Arrhenius expressions (in units of L mol-1 s-1) were first derived for the •OH reactions with tartaric acid─k(T, H2A) = (3.3 ± 0.1) × 1010 exp[(-1350 ± 110 K)/T], k(T, HA-) = (3.6 ± 0.1) × 1010 exp[(-580 ± 110 K)/T], and k(T, A2-) = (3.3 ± 0.1) × 1010 exp[(-1190 ± 170 K)/T]─as well as mucic acid─k(T, H2A) = (2.2 ± 0.1) × 1010 exp[(-1140 ± 150 K)/T], k(T, HA-) = (4.8 ± 0.1) × 1010 exp[(-1280 ± 170 K)/T], and k(T, A2-) = (2.1 ± 0.1) × 1010 exp[(-970 ± 70 K)/T]. A general trend of the •OH rate constant is found as > > . The pH- and temperature-dependent rate constants of the OH radical reactions allow an accurate description of the source and sink processes in the tropospheric aqueous phase.

Iron (III) hydroxocomplex-methyl viologen dication system as a prospective tool for determination of hydroxyl radical reaction rate constants with environmental pollutants.

Reactivity of oxidative species with target pollutants is one of the crucial parameters for application of any system based on advanced oxidation processes (AOPs). This work presents new useful approach how to determine the hydroxyl radical reaction rate constants (kOH) using UVA laser flash photolysis technique. Fe (III) hydroxocomplex at pH 3 was applied as a standard source of hydroxyl radicals and methyl viologen dication (MV2+) was used as selective probe for •OH radical. Application of MV2+ allows to determine kOH values even for compounds which do not generate themselves optically detectable transient species in reaction with hydroxyl radicals. Validity of this approach was tested on a wide range of different persistent pesticides and its main advantages and drawbacks in comparison with existing steady-state and time-resolved techniques were discussed.

Predicting Rate Constants of Hydroxyl Radical Reactions with Alkanes Using Machine Learning.

The hydrogen abstraction reactions of the hydroxyl radical with alkanes play an important role in combustion chemistry and atmospheric chemistry. However, site-specific reaction constants are difficult to obtain experimentally and theoretically. Recently, machine learning has proved its ability to predict chemical properties. In this work, a machine learning approach is developed to predict the temperature-dependent site-specific rate constants of the title reactions. Multilayered neural network (NN) models are developed by training the site-specific rate constants of 11 reactions, and several schemes are designed to improve the prediction accuracy. The results show that the proposed NN models are robust in predicting the site-specific and overall rate constants.

Computational studies of fluorinated propenes: The fluorine "cis effect," barrier heights of the internal rotation of CX3 (X = H or F) group, and π-bond strengths

The (E)- and (Z)-geometrical isomers of fluorinated propenes (CWZ = CY−CX3, W, X, Y, and Z = H or F) are computationally investigated. Our calculations show that geometrical isomers with two fluorine atoms located at the cis positions of the CC bond tend to undergo additional energy lowering compared to the trans isomers. Natural bond orbital analysis suggests that the origin of this lowering of energy is the same as that of the typical fluorine cis effects observed for 1,2-dihaloethenes. The barrier height magnitude of the internal rotation of the methyl or trifluoromethyl group is shown to be sensitive to the substitution patterns in the fluorinated propenes. The change in the rotational barrier heights is closely related to the differences in the magnitude of intramolecular repulsive non-bonded steric interactions. Furthermore, the π-bonds in fluorinated propenes are shown to be weaker for the derivatives possessing two fluorine atoms at both ends of the CC bond. The OH radical reactions of the fluorinated propenes are initiated by the attack of the OH radicals on the π-bond. Nevertheless, no obvious correlation is found between the rate constants for the OH radical reactions and the π-bond strengths of the fluorinated propenes.

Efficacy of ozone for removal of pesticides, metals and indicator virus from reverse osmosis concentrates generated during potable reuse of municipal wastewaters

This study evaluated ozone treatment to address concerns regarding the discharge to marine waters of chemical contaminants and pathogens in reverse osmosis (RO) concentrates generated during the potable reuse of municipal wastewaters. Previous studies indicated that contaminants can be sorted into five groups based on their reaction rate constants with ozone and hydroxyl radical to predict degradation of chemical contaminants during ozonation of municipal effluents. Spiking representatives of each group into five RO concentrate samples, this study demonstrated that the same contaminant grouping scheme could be used to predict contaminant degradation during ozonation of RO concentrates, despite the higher concentrations of ozone and hydroxyl radical scavengers. The predictive capability of the contaminant grouping scheme was further validated for four contaminants of concern in RO concentrates, including the pesticides fipronil and imidacloprid, and the metal chelates Ni-EDTA and Cu-EDTA. After measuring their ozone and hydroxyl radical reaction rate constants, these compounds were assigned to contaminant groups, and their degradation during ozonation matched predictions. Addition of 300 mg/L CaO at pH 11 achieved partial removal of the native nickel and copper by precipitation. Ozone pretreatment further enhanced precipitation of nickel, but not copper. Ozonation achieved 5-log inactivation of MS2 in all five concentrate samples at 1.18 mg O3/mg DOC. Ozonation at 0.9 mg O3/mg DOC formed 139–451 μg/L bromate. Pretreatment of RO concentrates with chlorine and ammonia reduced bromate formation by a maximum of 48% but increased total halogenated DBP concentrations from 20 μg/L to 36 μg/L. Regardless, neither bromate nor trihalomethane concentrations exceeded threshold concentrations of concern for discharge to marine waters.

Rate Constants for the Reactions of OH Radicals with the ( E)/( Z) Isomers of CFCl=CFCl and ( E)-CHF=CHF.

The rate constants for the OH radical reactions with halogenated ethenes were investigated experimentally and computationally. The rate constants for the reactions of OH radicals with ( E)-CFCl=CFCl ( k1), ( Z)-CFCl=CFCl ( k2), and ( E)-CHF=CHF ( k3) were measured using flash and laser photolysis methods. The temporal profile of the OH radical was monitored by a laser-induced fluorescence technique. Kinetic measurements were carried out over the temperature range of 250-430 K. Arrhenius rate constants were determined to be k1 = (1.67 ± 0.06) × 10-12·exp[(140 ± 10) K/ T], k2 = (1.75 ± 0.04) × 10-12·exp[(140 ± 10) K/ T], and k3 = (3.99 ± 0.15) × 10-12·exp[(260 ± 10) K/ T] cm3 molecule-1 s-1. The quoted uncertainties are 95% confidence levels and do not include systematic errors. Infrared absorption spectra were measured at room temperature. The atmospheric lifetimes and the global warming potentials of ( E)-CFCl=CFCl, ( Z)-CFCl=CFCl, and ( E)-CHF=CHF were estimated to be 4.3, 4.2, and 1.2 days and 0.035, 0.036, and 0.0056, respectively. The ozone depletion potentials of ( E)-CFCl=CFCl and ( Z)-CFCl=CFCl were determined to be 0.00011 and 0.00010, respectively. The photochemical ozone creation potentials of the halogenated ethenes were less than 1/4 that of ethene. In addition, the ( E)/( Z) differences in the energy and IR spectra of the CFCl=CFCl and CHF=CHF molecules were computationally examined. The reactivities of these halogenated ethenes toward OH radicals were investigated through the combination of DFT and ab initio computations. The rate constants calculated for the OH radical reactions of these halogenated ethenes showed reasonable agreement with the experimentally determined values. Our computational results for the CFCl=CFCl and CHF=CHF ( E)/( Z) isomeric pairs indicated that the rate constants toward OH radicals are larger for the higher-energy geometrical isomers than for the lower-energy counterparts.

Response of heat release to equivalence ratio variations in high Karlovitz premixed H2/air flames at 20 atm

This paper presents three-dimensional direct numerical simulations of lean premixed H2/air flames with equivalence ratios 0.4, 0.5 and 0.6, respectively. The initial Karlovitz number is around 2335 and the pressure is 20 atm, which is relevant to gas turbine conditions. The heat release in reaction zones under different equivalence ratios is examined statistically with the aim to extend our understanding of lean combustion under high-pressure conditions. With increasing equivalence ratio, the relative thickness of reaction zone (δf/δL) is increasing for both laminar and turbulent flames, but the extent of increase is reduced under high equivalence ratio. By examining the local structures of flame fronts, it is found that trenches and plateaus of local equivalence ratio are located on separate sides of the reaction zone edge. Due to the decreased Lewis number under high equivalence ratio, the trench ‘depth’ and plateau ‘height’ are reduced. For the flame under ultra-lean conditions, there are some spots with temperatures above adiabatic temperature. This is attributed to the high-fraction of radicals in these regions, which will promote heat release. Furthermore, the heat release rates of elementary reactions are investigated with the analysis of radical fractions and rate constants. When the mixture equivalence ratio varies, the local heat release is changed in different temperature windows due to the combined effects of radical fractions and reaction rate constants.

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.

Rate Constants for the Reactions of OH Radical with the ( E)/( Z) Isomers of CF3CF═CHCl and CHF2CF═CHCl.

The rate constants for the reactions of OH radical with ( E)- and ( Z)-isomers of CF3CF═CHCl and CHF2CF═CHCl have been measured over the temperature range of 250-430 K. Kinetic measurements have been performed using flash and laser photolysis methods combined with laser-induced fluorescence. Arrhenius rate constants have been determined as k(( E)-CF3CF═CHCl) = (1.09 ± 0.03) × 10-12 · exp[(50 ± 10)K/ T], k(( Z)-CF3CF═CHCl) = (8.02 ± 0.19) × 10-13 · exp[-(100 ± 10)K/ T], k(( E)-CHF2CF═CHCl) = (1.50 ± 0.03) × 10-12 · exp[(160 ± 10)K/ T], and k(( Z)-CHF2CF═CHCl) = (1.36 ± 0.03) × 10-12 · exp[(360 ± 10)K/ T] cm3 molecule-1 s-1. Infrared absorption spectra have also been measured at room temperature. The atmospheric lifetimes of ( E)-CF3CF═CHCl, ( Z)-CF3CF═CHCl, ( E)-CHF2CF═CHCl, and ( Z)-CHF2CF═CHCl have been estimated as 8.9, 20, 4.6, and 2.6 days, respectively, and their global warming potentials and ozone depletion potentials were determined as 0.23, 0.88, 0.060, and 0.016 and 0.00010, 0.00023, 0.000057, and 0.000030, respectively. Additionally, the rate constants for OH radical addition and IR spectra of these compounds were determined computationally. Consistent with experiment, our calculations indicate that the reactivity toward OH radical addition is reduced as ( Z)-CHF2CF═CHCl > ( E)-CHF2CF═CHCl > ( E)-CF3CF═CHCl > ( Z)-CF3CF═CHCl, where the ( E)/( Z) reactivity is reversed for CF3CF═CHCl and CHF2CF═CHCl. The calculations reproduced the observed temperature dependencies of the rate constants for the OH radical reactions, which is slightly positive for ( Z)-CF3CF═CHCl but negative for the other compounds.

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.

Temperature dependence of hydroxyl radical reactions with chloramine species in aqueous solution

The absolute temperature-dependent kinetics for the reaction between hydroxyl radicals and the chloramine water disinfectant species monochloramine (NH2Cl), as well as dichloramine (NHCl2) and trichloramine (NCl3), have been determined using electron pulse radiolysis and transient absorption spectroscopy. These radical reaction rate constants were fast, with values of 6.06 × 108, 2.57 × 108, and 1.67 × 108 M−1 s−1 at 25 °C for NH2Cl, NHCl2, and NCl3, respectively. The corresponding temperature dependence of these reaction rate constants, measured over the range 10–40 °C, is well-described by the transformed Arrhenius equations:ln k (OH & NH2Cl) = (23.68 ± 0.23) – {(8570 ± 580)/(8.314*T/K)}ln k (OH & NHCl2) = (21.96 ± 0.56) – {(6110 ± 400)/(8.314*T/K)}ln k (OH & NCl3) = (21.26 ± 0.29) – {(5770 ± 720)/(8.314*T/K)}giving activation energies of 8.57 ± 0.58, 6.11 ± 0.40, and 5.77 ± 0.72 kJ mol−1 for these three chloramines, respectively. These data will aid water utilities in predicting hydroxyl radical partitioning and chemical contaminant removal efficiencies under real-world advanced oxidation process treatment conditions.

Modeling the aqueous phase reactivity of hydroxyl radical towards diverse organic micropollutants: An aid to water decontamination processes

The rate constants of the hydroxyl radical reactions (kOH) with organic micropollutants (OMPs) in aqueous medium are important in designing the advanced oxidation processes (AOPs) for their removal. In this study, a quantitative structure-property relationship (QSPR) model for the prediction of kOH of diverse and emerging OMPs was developed in accordance with the OECD guidelines. A large experimental data set (n = 995) comprised of compounds with kOH values ranging from 7.9 × 105 to 6.8 × 1010 M−1 s−1 was considered and several molecular descriptors were calculated. As a result, five descriptors were found to be important in predicting the kOH values which related to the electronegativity, topological polar surface area, double bonds, average molecular weight, and halogen atoms in the molecule. The optimal model was validated internally and externally and several statistical stringent parameters were derived. High values of the coefficient of determination (R2) and small root mean squared error (RMSE) in the training (0.954; 0.17) and validation (0.925; 0.14) sets indicated high generalization and predictivity of the developed model. Other statistical parameters derived from the training and validation data also supported the robustness of the model. The proposed model outperformed the earlier QSARs reported for kOH prediction. Overall, the developed QSPR model provides a valuable tool for an initial assessment of the susceptibility of organic micropollutants to AOPs.

Hydroxyl radical generation with a high power ultraviolet light emitting diode (UV-LED) and application for determination of hydroxyl radical reaction rate constants

We propose a simple, efficient and selective hydroxyl radical generation system based on the photolysis of submicromolar concentrations of nitrite using a high-power ultraviolet light emitting diode (UV-LED). Hydroxyl radical formation by the 6.75-W UV-LED was at least 10 times greater than that by a 300-W Xe lamp. In the UV-LED system, the hydroxyl radical formation rate from nitrite was about four orders of magnitude larger than that from nitrate and two orders of magnitude larger than that from hydrogen peroxide. Such efficient and selective hydroxyl radical formation can be attributed to the overlap of the emission spectrum of the UV-LED and absorption of nitrite. The system was used to determine the reaction rate constants between hydroxyl radicals and chemicals based on the competition method with terephthalate as the hydroxyl radical probe. The reaction rate constants between hydroxyl radicals and some low-molecular-weight organic compounds and inorganic halide salts with various reaction rate constants were determined. The values obtained ranged from 105 to 1010 M−1 s−1, and agreed well with previously reported values. The potential of the developed method to determine the reaction rate constants of hydroxyl radicals is discussed.

Room-temperature and temperature-dependent QSRR modelling for predicting the nitrate radical reaction rate constants of organic chemicals using ensemble learning methods

Experimental determinations of the rate constants of the reaction of NO3 with a large number of organic chemicals are tedious, and time and resource intensive; and the development of computational methods has widely been advocated. In this study, we have developed room-temperature (298 K) and temperature-dependent quantitative structure–reactivity relationship (QSRR) models based on the ensemble learning approaches (decision tree forest (DTF) and decision treeboost (DTB)) for predicting the rate constant of the reaction of NO3 radicals with diverse organic chemicals, under OECD guidelines. Predictive powers of the developed models were established in terms of statistical coefficients. In the test phase, the QSRR models yielded a correlation (r2) of >0.94 between experimental and predicted rate constants. The applicability domains of the constructed models were determined. An attempt has been made to provide the mechanistic interpretation of the selected features for QSRR development. The proposed QSRR models outperformed the previous reports, and the temperature-dependent models offered a much wider applicability domain. This is the first report presenting a temperature-dependent QSRR model for predicting the nitrate radical reaction rate constant at different temperatures. The proposed models can be useful tools in predicting the reactivities of chemicals towards NO3 radicals in the atmosphere, hence, their persistence and exposure risk assessment.

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.

Magnetic field effects on bistability and bifurcation phenomena in lipid peroxidation.

A simple model based on physically evident assumptions was constructed that may predict the strong effect of low intensity magnetic fields (MFs) on living systems. As a model system, we studied MF effect on lipid peroxidation reaction under steady state conditions. It was assumed that external MF affects rate constants of radical reactions taking place in elementary stages of lipid peroxidation reaction. It was shown that under certain conditions, low intensity MF may provoke a bifurcation transition in the system followed by a significant change in its characteristics. This means that, in principle, an external MF may play a critical role in lipid peroxidation reaction and a cell life.

Predicting rate constants of hydroxyl radical reactions with alkenes and aromatics

The radicals obtained from the additions of alkenes with the hydroxyl radical (•OH) and from the H-atom abstraction of aromatics initiated by the OH radical were, respectively, used to calculate quantum chemical descriptors for quantitative structure-activity relationship (QSAR) models for the rate constants (kOH) of the reactions of alkenes and aromatics with the OH radical in the troposphere. Quantum chemical descriptors used as the inputs for the support vector machine (SVM) models were calculated with the unrestricted hybrid density functional theory (DFT), at the UB3LYP level of theory with 6-31G(d) basis set. The average root-mean-square errors (RMSE) for 70 alkenes and 80 aromatics are 0.101 and 0.236 log units, respectively, which show the SVM models in this paper are accuracy and acceptable. To develop QSAR models for kOH, calculating quantum chemical descriptors from the reaction radicals of alkenes and from aromatic radicals formed by the H-atom abstraction is feasible.

Development of linear free energy relationships for aqueous phase radical-involved chemical reactions.

Aqueous phase advanced oxidation processes (AOPs) produce hydroxyl radicals (HO•) which can completely oxidize electron rich organic compounds. The proper design and operation of AOPs require that we predict the formation and fate of the byproducts and their associated toxicity. Accordingly, there is a need to develop a first-principles kinetic model that can predict the dominant reaction pathways that potentially produce toxic byproducts. We have published some of our efforts on predicting the elementary reaction pathways and the HO• rate constants. Here we develop linear free energy relationships (LFERs) that predict the rate constants for aqueous phase radical reactions. The LFERs relate experimentally obtained kinetic rate constants to quantum mechanically calculated aqueous phase free energies of activation. The LFERs have been applied to 101 reactions, including (1) HO• addition to 15 aromatic compounds; (2) addition of molecular oxygen to 65 carbon-centered aliphatic and cyclohexadienyl radicals; (3) disproportionation of 10 peroxyl radicals, and (4) unimolecular decay of nine peroxyl radicals. The LFERs correlations predict the rate constants within a factor of 2 from the experimental values for HO• reactions and molecular oxygen addition, and a factor of 5 for peroxyl radical reactions. The LFERs and the elementary reaction pathways will enable us to predict the formation and initial fate of the byproducts in AOPs. Furthermore, our methodology can be applied to other environmental processes in which aqueous phase radical-involved reactions occur.

Kinetic measurements of the reactivity of hydrogen peroxide and ozone towards small atmospherically relevant aldehydes, ketones and organic acids in aqueous solutions

Abstract. Free radical reactions are an important degradation process for organic compounds within the aqueous atmospheric environment. Nevertheless, non-radical oxidants such as hydrogen peroxide and ozone also contribute to the degradation and conversion of these substances (Tilgner and Herrmann, 2010). In this work, kinetic investigations of non-radical reactions were conducted using UV / Vis spectroscopy (dual-beam spectrophotometer and stopped flow technique) and a capillary electrophoresis system applying pseudo-first order kinetics to reactions of glyoxal, methylglyoxal, glycolaldehyde, glyoxylic, pyruvic and glycolic acid as well as methacrolein (MACR) and methyl vinyl ketone (MVK) with H2O2 and ozone at 298 K. The measurements indicate rather small rate constants at room temperature of k2nd < 3 M−1 s−1 (except for the unsaturated compounds exposed to ozone). Compared to radical reaction rate constants the values are about 10 orders of magnitude smaller (kOH• ~109 M−1 s−1). However, when considering the much larger non-radical oxidant concentrations compared to radical concentrations in urban cloud droplets, calculated first-order conversion rate constants change the picture towards H2O2 reactions becoming more important, especially when compared to the nitrate radical. For some reactions mechanistic suggestions are also given.

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

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