Reaction Of OH Radicals Research Articles

Theoretical study on the reactions of a series of polybromobenzenes with OH radicals: mechanism, kinetics, and QSAR

Polybromobenzenes are a kind of monocyclic aromatic flame retardants that are used as a substitute for polybrominated diphenyl ethers and hexabromocyclododecane. In this paper, the reaction mechanism and rate constants for the reaction of OH radicals with a series of polybromobenzenes such as hexabromobenzene (HBB), 1,2,4,5-tetrabromobenzenes (1,2,4,5-TeBB), pentabromobenzene (PEBB), pentabromoethylbenzene (PBEB), pentabromotoluene (PBT), and 2,4,5-tribromotoluene (2,4,5-TrBT) have been investigated by quantum chemical method. The reaction mechanism was obtained at the MPWB1K/6-311+g(3df,2p)//MPWB1K/6-31+g(d,p) level of theory and the rate constants were deduced over the temperature range of 200–370 K using canonical variational transition state (CVT) theory with the small curvature tunneling (SCT) method. The rate constants of OH radicals with HBB, 1,2,4,5-TeBB, PEBB, PBEB, PBT, and 2,4,5-TrBT are determined to be 5.72 × 10−13, 1.23 × 10−12, 8.78 × 10−13, 9.23 × 10−13, 6.46 × 10−13, and 1.69 × 10−12, respectively, at 298 K and 1 atm. The estimated atmospheric lifetimes of HBB (20.08 days), 1,2,4,5-TeBB (9.65 days), PEBB (13.5 days), PBEB (12.9 days), PBT (18.4 days), and 2,4,5-TrBT (7.0 days) determined by OH radicals indicate that polybromobenzenes have the potential for long-range transport. The genetic function approximation is used to study the quantitative structure–activity relationship. The coefficients indicate that the ELUMO has the highest correlation to logkOH.

Rate Constants for the Reactions of OH Radicals with Fluorinated Ethenes: Kinetic Measurements and Correlation between Structure and Reactivity.

The rate constants for the reaction of OH radicals with four fluorinated ethenes (CF2═CHF, ( E)-CHF═CHF, CF2═CH2, and CHF═CH2) have been measured over the temperature range of 250-430 K. Kinetic measurements have been carried out using flash photolysis and laser photolysis methods combined with a laser-induced fluorescence technique. The Arrhenius expressions for the rate constant have been determined as k(CF2═CHF) = (3.12 ± 0.11) × 10-12 exp[(270 ± 10)/ T], k(( E)-CHF═CHF) = (3.75 ± 0.08) × 10-12 exp[(230 ± 10)/ T], k(CF2═CH2) = (1.15 ± 0.07) × 10-12 exp[(230 ± 20)/ T], and k(CHF═CH2) = (1.16 ± 0.09) × 10-12 exp[(390 ± 20)/ T] cm3 molecule-1 s-1. Infrared absorption spectra of the fluorinated ethenes have been measured at room temperature. The atmospheric lifetimes and global warming potentials of the fluorinated ethenes have been estimated. The correlation between the reactivity and the structure of the halogenated ethenes has been investigated by considering the structure containing the atoms attached to the carbons on both sides of the double bond. The calculated rate constants of 14 halogenated ethenes showed agreement with the measured rate constants within a factor of 2, except for that of one compound.

Infrared Chemiluminescence Study of the Reaction of Hydroxyl Radical with Formamide and the Secondary Unimolecular Reaction of Chemically Activated Carbamic Acid.

Reactions of OH and OD radicals with NH2CHO and ND2CHO were studied by Fourier transform infrared emission spectroscopy of the product molecules from a fast-flow reactor at 298 K. Vibrational distributions of the HOD and H2O molecules from the primary reactions with the C-H bond were obtained by computer simulation of the emission spectra. The vibrational distributions resemble those for other direct H atom abstraction reactions, such as with acetaldehyde. The highest observed level gives an estimate of the C-H bond dissociation energy in formamide of 90.5 ± 1.3 kcal mol-1. Observation of CO2, ammonia, and secondary water chemiluminescence gave evidence that recombination of OH and NH2CO forms carbamic acid (NH2COOH) with excitation energy of 103 kcal mol-1, which decomposes through two pathways forming either NH3 + CO2 or H2O + HNCO. The branching fraction for ammonia formation was estimated to be 2-3 times larger than formation of water. This observation was confirmed by RRKM calculation of the decomposition rate constants. A new simulation method was developed to analyze infrared emission from NH3, NH2D, ND2H, and ND3. Dynamical aspects of the primary and secondary reactions are discussed based on the vibrational distributions of CO2 and those of H/D isotopes of water and ammonia.

Chemical interaction of dual-fuel mixtures in low-temperature oxidation, comparing n-pentane/dimethyl ether and n-pentane/ethanol

With the aim to study potential cooperative effects in the low-temperature oxidation of dual-fuel combinations, we have investigated prototypical hydrocarbon (C5H12) / oxygenated (C2H6O) fuel mixtures by doping n-pentane with either dimethyl ether (DME) or ethanol (EtOH). Species measurements were performed in a flow reactor at an equivalence ratio of ϕ = 0.7, at a pressure of p = 970 mbar, and in the temperature range of 450–930 K using electron ionization molecular-beam mass spectrometry (EI-MBMS). Series of different blending ratios were studied including the three pure fuels and mixtures of n-pentane containing 25% and 50% of C2H6O. Mole fractions and signals of a significant number of species with elemental composition CnH2n+xOy (n = 1–5, x = 0–(n + 2), y = 0–3) were analyzed to characterize the behavior of the mixtures in comparison to that of the individual components. Not unexpectedly, the overall reactivity of n-pentane is decreased when doping with ethanol, while it is promoted by the addition of DME. Interestingly, the present experiments reveal synergistic interactions between n-pentane and DME, showing a stronger effect on the negative temperature coefficient (NTC) for the mixture than for each of the individual components. Reasons for this behavior were investigated and show several oxygenated intermediates to be involved in enhanced OH radical production. Conversely, ethanol is activated by the addition of n-pentane, again involving key OH radical reactions. Although the main focus here is on the experimental results, we have attempted, in a first approximation, to complement the experimental observations by simulations with recent kinetic models. Interesting differences were observed in this comparison for both, fuel consumption and intermediate species production. The inhibition effect of ethanol is not predicted fully, and the synergistic effect of DME is not captured satisfactorily. The exploratory analysis of the experimental results with current models suggests that deeper knowledge of the reaction chemistry in the low-temperature regime would be useful and might contribute to improved prediction of the low-temperature oxidation behavior for such fuel mixtures.

Rate constants for the reactions of OH radicals with CF3CX=CY2 (X = H, F, CF3, Y = H, F, Cl).

The rate constants of OH radicals with CF3CF=CCl2, CF3CH=CF2, CF3CF=CH2, CF3CH=CH2, and (CF3)2C=CH2 have been measured over the temperature range 250-430K. Kinetic measurements have been carried out using flash photolysis and laser photolysis methods combined respectively with laser-induced fluorescence technique. The Arrhenius rate parameters have been determined as k(CF3CF=CCl2) = (6.50 ± 0.22) × 10-13∙exp[(200 ± 10)/T], k(CF3CH=CF2) = (4.85 ± 0.14) × 10-13∙exp[(120 ± 10)/T], k(CF3CF=CH2) = (1.54 ± 0.03) × 10-12∙exp[- (100 ± 10)/T], k(CF3CH=CH2) = (1.06 ± 0.02) × 10-12∙exp[(80 ± 10)/T], and k((CF3)2C=CH2) = (8.75 ± 0.23) × 10-13∙exp[- (20 ± 10)/T]cm3molecule-1s-1. Infrared absorption spectra of the halogenated alkenes have been measured at room temperature. The atmospheric lifetime, global warming potential, ozone depleting potential, and photochemical ozone creation potential have been estimated. The change in the reactivity of halogenated alkenes by the substitution has been examined by considering the structure containing the atoms or atomic groups attached to the carbons on both sides of the double bond.

An experimental and modeling study on the low temperature oxidation of surrogate for JP-8 part II: Comparison between neat 1,3,5-trimethylbenzene and its mixture with n-decane

The low temperature oxidation of neat 1,3,5-trimethylbenzene (T135MB) and n-decane/T135MB mixture as a surrogate for JP-8 has been investigated in a jet-stirred reactor over the temperature range of 500–1100 K at atmospheric pressure under fuel-rich condition with residence time from 2.33 to 1.06 s. Mole fraction profiles of 29 intermediates including light hydrocarbons, oxygenated and aromatic compounds were identified by gas chromatographic techniques. In general, the concentrations of intermediates tend to increase progressively with temperature from 925 K in neat T135MB oxidation, while these species exhibit bimodal distributions from 550 K in the oxidation of n-decane/T135MB mixture (surrogate). By considering the calculated rate constants of T135MB and analogous coupling reactions between T135MB and n-decane, a detailed kinetic mechanism involving 910 species and 5329 reactions was established with a reasonable agreement with the experimental results. The low temperature chemistry of T135MB and surrogate was analyzed including the NTC behavior below 800 K. The oxidation process of T135MB is occurring mainly by H-abstraction with OH radical and subsequent reactions. The difference is that in the NTC region H-abstraction by OH radicals is the major consumption pathway for T135MB in the surrogate. But for neat T135MB, the dominant channels change to the H-abstractions by H-atoms, OH and CH3 radicals. Addition of n-decane can promote the oxidation of T135MB by providing OH radicals in the low temperature oxidation of surrogate fuel. Moreover, the model was also validated against the experimental data on n-decane and JP-8 combustion, including species profiles in low temperature jet-stirred reactor oxidation and high temperature flow reactor pyrolysis as well as ignition delay times. These extended results yielded overall satisfactory agreement, and will benefit for further application of practical JP-8 fuels, particularly for their combustion properties at wide temperature range.

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.

Reactions of OH radicals with 2-methyl-1-butyl, neopentyl and 1-hexyl nitrates. Structure-activity relationship for gas-phase reactions of OH with alkyl nitrates: An update

The kinetics of the reactions 2-methyl-1-butyl (2M1BNT), neopentyl (NPTNT) and 1-hexyl nitrates (1HXNT) with OH radicals has been studied using a low pressure flow tube reactor combined with a quadrupole mass spectrometer. The rate constants of the title reactions were determined under pseudo-first order conditions from kinetics of OH consumption in excess of nitrates. The overall rate coefficients, k2M1BNT = 1.54 × 10−14 (T/298)4.85 exp (1463/T) (T = 278–538 K), kNPTNT = 1.39 × 10−14 (T/298)4.89 exp (1189/T) (T = 278–500 K) and k1HXNT = 2.23 × 10−13 (T/298)2.83 exp (853/T) cm3molecule−1s−1 (T = 306–538 K) (with conservative 15% uncertainty), were determined at a total pressure of 1 Torr of helium. The yield of trimethylacetaldehyde ((CH3)3CCHO), resulting from the abstraction by OH of an α-hydrogen atom in neopentyl nitrate, followed by α-substituted alkyl radical decomposition, was determined as 0.31 ± 0.06 at T = 298 K. The calculated tropospheric lifetimes of 2M1BNT, NPTNT and 1HXNT indicate that reaction of these nitrates with OH represents an important sink of these compounds in the atmosphere. Based on the available kinetic data, we have updated the structure-activity relationship (SAR) for reactions of alkyl nitrates with OH at T = 298 K. Good agreement (within 20%) is obtained between experimentally measured rate constants (total and that for H-atom abstraction from α carbon) and those calculated from SAR using new substituents factors for almost all the experimental data available.

A computational investigation of the sulphuric acid‐catalysed 1,4‐hydrogen transfer in higher Criegee intermediates

AbstractCriegee intermediates (CIs) are formed during the ozonolysis of unsaturated hydrocarbons in the troposphere. The fate of CIs is of critical importance to tropospheric oxidation chemistry, particularly in the context of radical and secondary organic aerosol formation. Using the high‐level ab initio G4(MP2) method, we investigate the 1,4 hydrogen shift reaction in CIs formed from ozonolysis of two common biogenic hydrocarbons: isoprene and α‐pinene. We consider the uncatalysed reaction, as well as the reaction catalysed by a water molecule and by sulphuric acid. We show that sulphuric acid is a very effective catalyst, leading to a barrierless tautomerization relative to the free reactants and to very low reaction barrier heights relative to the reactant complexes. In particular, we obtain reaction barrier heights of Δ = 24.5 (isoprene CI) and 8.4 (α‐pinene CI) kJ mol−1 relative to the reactant complexes. Given the reaction of OH radicals with SO2 in the troposphere can ultimately yield sulphuric acid, these findings may have significant consequences for current atmospheric chemical models for regions of high sulphur concentrations.

The experimental observation, mechanism and kinetic studies on the reaction of hexachloro-1,3-butadiene initiated by typical atmospheric oxidants

Hexachloro-1,3-butadiene (HCBD) is a persistent organic pollutant in the environment. When its samples were collected and observed, the levels of HCBD in its source and high mountains are higher than in urban cities, oil factories and countryside. The density functional theory is applied to the degradation mechanism of HCBD with Cl, NO3, HO2, OH and O3. Those reactions are optimized and calculated at two carbon sites of double bonds, and then the subsequent reactions of the OH-initiated intermediates with O2 and NO are taken as examples. Ozonization reactions of HCBD including the formation of primary and secondary ozonides are investigated. The Criegee intermediates created in the ozonization reactions can react with O2, SO2, NO2 and H2O. Reaction rate constants of the Cl, NO3, HO2, OH and O3 initiated reactions with HCBD are calculated within 200 to 400 K with the transition state theory method, and the rate constants of the Cl, NO3, HO2, OH and O3 at 298.15 K are 4.51 × 10−13, 1.32 × 10−20, 4.33 × 10−29, 6.33 × 10−16, 5.80 × 10−27 cm3 molecule−1 s−1, respectively. The reactions of OH and Cl radicals with HCBD are more important than those of NO3, HO2 and O3 according to the reaction rate branching ratio. Both the temperature and reaction rate could change with the height.

Kinetics and Products of the Reaction of OH Radicals with ClNO from 220 to 940 K.

The kinetics and products of the reaction of OH radicals with ClNO have been studied in a flow reactor coupled with an electron impact ionization mass spectrometer at nearly 2 Torr total pressure of helium and over a wide temperature range, T = 220-940 K. The rate constant of the reaction OH + ClNO → products was determined under pseudo-first order conditions, monitoring the kinetics of OH consumption in excess of ClNO: k1 = 1.48 × 10-18 × T2.12 exp(146/T) cm3 molecule-1 s-1 (uncertainty of 15%). HOCl, Cl, and HONO were observed as the reaction products. As a result of quantitative detection of HOCl and Cl, the partial rate constants of the HOCl + NO and Cl + HONO forming reaction pathways were determined in the temperature range 220-940 K: k1a = 3.64 × 10-18 × T1.99 exp(-114/T) and k1b = 4.71 × 10-18 × T1.74 exp(246/T) cm3 molecule-1 s-1 (uncertainty of 20%). The dynamics of the title reaction and, in particular, non-Arrhenius behavior observed for both k1a and k1b in a wide temperature range, seems to be an interesting topic for theoretical research.

Absolute and relative-rate measurement of the rate coefficient for reaction of perfluoro ethyl vinyl ether (C2F5OCFCF2) with OH

The rate coefficient (k1) for the reaction of OH radicals with perfluoro ethyl vinyl ether (PEVE, C2F5OCF[double bond, length as m-dash]CF2) has been measured as a function of temperature (T = 207-300 K) using the technique of pulsed laser photolysis with detection of OH by laser-induced fluorescence (PLP-LIF) at pressures of 50 or 100 Torr N2 bath gas. In addition, the rate coefficient was measured at 298 K and in one atmosphere of air by the relative-rate technique with loss of PEVE and reference reactant monitored in situ by IR absorption spectroscopy. The rate coefficient has a negative temperature dependence which can be parameterized as: k1(T) = 6.0 × 10-13 exp[(480 ± 38/T)] cm3 molecule-1 s-1 and a room temperature value of k1 (298 K) = (3.0 ± 0.3) × 10-12 cm3 molecule-1 s-1. Highly accurate rate coefficients from the PLP-LIF experiments were achieved by optical on-line measurements of PEVE and by performing the measurements at two different apparatuses. The large rate coefficient and the temperature dependence indicate that the reaction proceeds via OH addition to the C[double bond, length as m-dash]C double bond, the high pressure limit already being reached at 50 Torr N2. Based on the rate coefficient and average OH levels, the atmospheric lifetime of PEVE was estimated to be a few days.

Competition kinetics of OH radical reactions with oxygenated organic compounds in aqueous solution: rate constants and internal optical absorption effects.

Oxygenated organic compounds are omnipresent in the troposphere, due to their strong emissions from either biogenic or anthropogenic sources. Additionally, the degradation and oxidation processes of volatile organic compounds (VOCs) result in the production of oxygenated organic compounds in the troposphere. The degradation and conversion of these compounds are often initiated by radical reactions and occur in the gas phase as well as in the aqueous phase, including cloud droplets, fog, haze, rain or hygroscopic particles containing 'aerosol liquid water (ALW)'. In the present study, the temperature-dependent OH radical reactions with oxygenated organic compounds in the aqueous phase have been investigated by laser flash photolysis. To determine the rate constants, the OH radical - thiocyanate anion competition kinetics method has been used. Once the organic reactant has an absorption at the excitation wavelength of the photolysis laser, the initial OH concentration decreases. This internal absorption effect leads to an overestimated rate constant of the investigated compound. The present study considers this contribution in order to clarify the internal absorption effect of the investigated organic compounds. The following rate constants for OH radical oxidation reactions of the oxygenated organic compounds have been obtained: acetone (2-propanone) k298K = (7.6 ± 1.0) × 107 L mol-1 s-1, 1-hydroxypropan-2-one k298K = (1.1 ± 0.1) × 109 L mol-1 s-1, 1,3-dihydroxypropan-2-one k298K = (1.5 ± 0.1) × 109 L mol-1 s-1, 2,3-dihydroxypropanal k298K = (1.3 ± 0.1) × 109 L mol-1 s-1, butane-1,3-diol k298K = (2.5 ± 0.1) × 109 L mol-1 s-1, butane-2,3-diol k298K = (2.0 ± 0.1) × 109 L mol-1 s-1 and hexane-1,2-diol k298K = (4.6 ± 0.4) × 109 L mol-1 s-1. With the rate constants obtained and their T-dependencies, the source and sink processes of oxygenated organic compounds in the tropospheric aqueous phase are arrived at precisely. These findings might enhance the predictive capabilities of models such as the chemical aqueous-phase radical mechanism (CAPRAM).

Atmospheric chemistry of hexa- and penta-fluorobenzene: UV photolysis and kinetics and mechanisms of the reactions of Cl atoms and OH radicals.

Photochemical reactors were used to study the kinetics and mechanisms of reactions of Cl atoms and OH radicals with hexa- and penta-fluorobenzene (C6F6, C6F5H) in 700 Torr total pressure of N2, air, or O2 diluent at 296 ± 2 K. C6F6 and C6F5H undergo ring-opening following 254 nm UV irradiation, but with small quantum yields (φ < 0.03). Reaction of Cl atoms with C6F6 proceeds via adduct formation, while the reaction of Cl atoms with C6F5H proceeds via hydrogen abstraction and adduct formation. C6F6-Cl and C6F5H-Cl adducts decompose rapidly (k ∼ 105-106 s-1) reforming the reactants, and react with Cl atoms to form products. The fraction of adduct reacting with Cl atoms increases with steady state Cl atom concentration, resulting in an increasing apparent effective Cl atom rate constant. The rate constant for the H-abstraction channel for Cl + C6F5H is estimated at (7.3 ± 5.7) × 10-16 cm3 molecule-1 s-1. Establishment of the equilibrium between the adducts and the aromatic reactant + Cl occurs rapidly with equilibrium constants of K([adduct]/[aromatic][Cl]) = (1.96 ± 0.11) × 10-16 and (9.28 ± 0.11) × 10-17 cm3 molecule-1 for C6F6 and C6F5H, respectively. Reaction of the adducts with O2 occurs slowly with estimated rate constants of <7 and <4 × 10-18 cm3 molecule-1 s-1 for C6F6-Cl and C6F5H-Cl, respectively. The rate constants for reaction of OH radicals with C6F6 and C6F5H were determined to be (2.27 ± 0.49) × 10-13 and (2.56 ± 0.62) × 10-13 cm3 molecule-1 s-1, respectively. UV and IR spectra of C6F6 and C6F5H at 296 ± 1 K were collected and calibrated. Results are discussed in the context of available literature data for reactions of Cl atoms and OH radicals with halogenated aromatic compounds.

An unprecedented route of [rad]OH radical reactivity evidenced by an electrocatalytical process: Ipso-substitution with perhalogenocarbon compounds

Hydroxyl radical (OH) is ubiquitous in the environment and in metabolism. It is one of the most powerful oxidants and can react instantaneously with surrounding chemicals. Currently, three attack modes of OH have been identified: hydrogen atom abstraction, addition to unsaturated bond and electron transfer. Perhalogenocarbon compounds such as CCl4 are therefore supposed to be recalcitrant to OH as suggested by numerous authors due to the absence of both hydrogen atom(s) and unsaturated bond(s). Here, we report for the first time a fourth attack mode of OH through ipso-substitution of the halogen atom. This breakthrough offers new scientific insight for understanding the mechanisms of OH oxidation in the related research areas of research. It is especially a great progress in organic contaminants removal from water. In this study, CCl4 is successfully degraded and mineralized in aqueous media using a green and efficient electrocatalytical production of homogeneous and heterogeneous OH. Maximum degradation rate of 0.298 min−1 and mineralization yield of 82% were reached. This opens up new possibilities of emerging water pollutants elimination such as fluorosurfactants.

Study of the Characteristics of Elementary Processes in a Chain Hydrogen Burning Reaction in Oxygen

The characteristics of possible chain explosive hydrogen burning reactions in an oxidizing medium are calculated on the potential energy surface. Specifically, reactions H2 + O2 → H2O + O, H2 + O2 → HO2 + H, and H2 + O2 → OH + OH are considered. Special attention is devoted to the production of a pair of fast highly reactive OH radicals. Because of the high activation threshold, this reaction is often excluded from the known kinetic scheme of hydrogen burning. However, a spread in estimates of kinetic characteristics and a disagreement between theoretical predictions with experimental results suggest that the kinetic scheme should be refined.

Benchmarking of DFT functionals for the kinetics and mechanisms of atmospheric addition reactions of OH radicals with phenyl and substituted phenyl‐based organic pollutants

Abstract•OH addition reactions play a pivotal role in the atmospheric transformation of a number of phenyl and substituted phenyl‐based persistent and toxic organic pollutants. Here, we screened appropriate DFT functionals to predict reaction mechanisms and rate constants (kOH) of the •OH additions by taking benzene and substituted benzenes (C6H5F, C6H5Cl, C6H5Br, C6H5CH3, C6H5OH) as model compounds. By comparing the kOH values calculated with DFT methods to experimental values, we found that the ωB97 functional is the best among the 18 functionals considered (using the basis sets 6‐31 + G(d,p) for optimizations and 6‐311++G(3df,2pd) for single point energy calculations) in the temperature range of 230‐330 K. In addition, we found that some other functionals performed well in specific conditions, e.g., BMKD3 is good for benzene, halogenated benzenes and C6H5CH3, and CAM‐B3LYP is good for the reaction of C6H5OH at room temperature. Based on the diversity of the electronic structures of the selected model compounds and the frequent occurrence of certain substituents (CH3, OH, F, Cl, and Br) in the target compounds, the functionals recommended here can be used for future study of the reaction mechanisms and kOH values for •OH addition to phenyl and substituted phenyl‐based persistent and toxic organic pollutants.

Metal Sulfates Enhanced Plasma Oxidization of Diesel Particulate Matter

The activities for nonthermal plasma-induced oxidation of diesel particulate matter (DPM) with various metal sulfates were investigated. It was found that the metal sulfates, MgSO4, K2SO4, and CaSO4 $\cdot$ 2H2O, enhanced the energy yield of DPM oxidation while other sulfates, Na2SO4, Fe2(SO4)3, and Al2(SO4) $\cdot$ 8H2O, did not affect the energy yield. The addition of water vapor caused a promotion of DPM oxidation regardless of a metal sulfate was presented. The effect of temperature and sulfate adding amount was also investigated. The metal sulfates enhanced DPM oxidation is possibly due to the adsorption of O atoms onto the sulfates. The positive effects of H2O might be thought of as due to the formation of reactive OH radicals and the promotion of the hydrolysis of the oxygen-containing compounds on DPM surface.

Characterizing chemical transformation of organophosphorus compounds by 13C and 2H stable isotope analysis

Continuous and excessive use of organophosphorus compounds (OPs) has led to environmental contaminations which raise public concerns. This study investigates the isotope fractionation patterns of OPs in the aquatic environment dependence upon hydrolysis, photolysis and radical oxidation processes. The hydrolysis of parathion (EP) and methyl parathion (MP) resulted in significant carbon fractionation at lower pH (pH2–7, εC=−6.9~−6.0‰ for EP, −10.5~−9.9‰ for MP) but no detectable carbon fractionation at higher pH (pH12). Hydrogen fractionation was not observed during any of the hydrolysis experiments. These results indicate that compound specific isotope analysis (CSIA) allows distinction of two different pH-dependent pathways of hydrolysis. Carbon and hydrogen isotope fractionation were determined during UV/H2O2 photolysis of EP and tris(2-chloroethyl) phosphate (TCEP). The constant δ2H values determined during the OH radical reaction of EP suggested that the rate-limiting step proceeded through oxidative attack by OH radical on the PS bond. The significant H isotope enrichment suggested that OH radical oxidation of TCEP was caused by an H-abstraction during the UV/H2O2 processes (εH=−56±3‰). Fenton reaction was conducted to validate the H isotope enrichment of TCEP associated with radical oxidation, which yielded εH of −34±5‰. Transformation products of OPs during photodegradation were identified using Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (FT-ICR MS). This study highlights that the carbon and hydrogen fractionation patterns have the potential to elucidate the transformation of OPs in the environment.

Enhancement of photocatalytic decarboxylation on TiO2 by water-induced change in adsorption-mode

Decarboxylation is an important process in the photocatalytic degradation of organic pollutants. In this study, the adsorption and photocatalytic decarboxylation of acetic acid and trichloroacetic acid were investigated by in-situ diffuse reflectance Fourier transform infrared spectroscopy (DRIFTS), which can distinguish carboxylate groups with different adsorption modes. We found that water molecules promote photocatalytic decarboxylation reaction and the promotional effect of water is attributed to the changed adsorption mode of the carboxylate group by the co-adsorbed water molecules. The IR study shows that degradation of the monodentate-coordinated acetic acids is much faster than that of the bidentate-coordinated one. The kinetic isotope effect studies and analysis of the intermediate products indicates that the degradation of acetic acids originates from the direct oxidation by holes even in the presence of water, rather than from the reaction of OH radicals as generally believed. DFT-based molecular dynamics calculations revealed that the formation of the monodentate-coordinated carboxylate group in the presence of water is attributed to competition between the O atom of the carboxylate group and water for the surface Ti sites. Because of its higher electron density, the monodentate carboxylate group is easier to be directly oxidized by holes than the bidentate group.