Reaction Of OH Radicals Research Articles

Highly efficient peroxymonosulfate activation of single-atom Fe catalysts via integration with Fe ultrafine atomic clusters for the degradation of organic contaminants

Recently, Fe single-atom catalysts activating peroxymonosulfate (PMS) have been of great potential to degrade recalcitrant organic pollutants. However, the mechanism of the unfavorable metal agglomerates produced throughout high-temperature pyrolysis on the Fenton-like reaction still remains confused. Herein, an efficient and stable catalyst integrating Fe single atoms with ultrafine atomic clusters (FeUAC@FeSA-NC) was successfully synthesized. The FeUAC@FeSA-NC/PMS system exhibited nearly 100% sulfamethoxazole (SMX) decomposition performance over a broad range of pH 3.0–9.0. Considering the difference in SMX degradation rate with or without Fe ultrafine atomic clusters, the catalytic performance of leached Fe ions, and the permissible Fe leaching concentration, it was not necessary to acid leach the FeUAC@FeSA-NC catalyst before use. Density functional theory (DFT) calculations manifested that the isolated Fe single-atom site neighbored by ultrafine atomic clusters acted as the optimal active site for PMS activation, and simultaneously the pyrrolic N served as the SMX adsorption site. The reactive species, including 1O2, O2−, SO4− and OH radicals and activated metal-peroxo species (Fe-PMS*), played the key role in FeUAC@FeSA-NC/PMS system. The satisfactory SMX removal efficiency and mineralization rate in actual wastewater imply that this efficient heterogeneous catalytic PMS system is a promising approach for sustainable wastewater reclamation applications.

OH Radical and Chlorine Atom Kinetics of Substituted Aromatic Compounds: 4-chlorobenzotrifluoride (p-ClC6H4CF3).

The mechanisms for the OH radical and Cl atom gas-phase reaction kinetics of substituted aromatic compounds remain a topic of atmospheric and combustion chemistry research. 4-Chlorobenzotrifluoride (p-chlorobenzotrifluoride, p-ClC6H4CF3, PCBTF) is a commonly used substituted aromatic volatile organic compound (VOC) in solvent-based coatings. As such, PCBTF is classified as a volatile chemical product (VCP) whose release into the atmosphere potentially impacts air quality. In this study, rate coefficients, k1, for the OH + PCBTF reaction were measured over the temperature ranges 275-340 and 385-940 K using low-pressure discharge flow-tube reactors coupled with a mass spectrometer detector in the ICARE/CNRS (Orléans, France) laboratory. k1(298-353 K) was also measured using a relative rate method in the thermally regulated atmospheric simulation chamber (THALAMOS; Douai, France). k1(T) displayed a non-Arrhenius temperature dependence with a negative temperature dependence between 275 and 385 K given by k1(275-385 K) = (1.50 ± 0.15) × 10-14 exp((705 ± 30)/T) cm3 molecule-1 s-1, where k1(298 K) = (1.63 ± 0.03) × 10-13 cm3 molecule-1 s-1 and a positive temperature dependence at elevated temperatures given by k1(470-950 K) = (5.42 ± 0.40) × 10-12 exp(-(2507 ± 45) /T) cm3 molecule-1 s-1. The present k1(298 K) results are in reasonable agreement with two previous 296 K (760 Torr, syn. air) relative rate measurements. The rate coefficient for the Cl-atom + PCBTF reaction, k2, was also measured in THALAMOS using a relative rate technique that yielded k2(298 K) = (7.8 ± 2) × 10-16 cm3 molecule-1 s-1. As part of this work, the UV and infrared absorption spectra of PCBTF were measured (NOAA; Boulder, CO, USA). On the basis of the UV absorption spectrum, the atmospheric instantaneous UV photolysis lifetime of PCBTF (ground level, midlatitude, Summer) was estimated to be 3-4 days, assuming a unit photolysis quantum yield. The non-Arrhenius behavior of the OH + PCBTF reaction over the temperature range 275 to 950 K is interpreted using a mechanism for the formation of an OH-PCBTF adduct and its thermochemical stability. The results from this study are included in a discussion of the OH radical and Cl atom kinetics of halogen substituted aromatic compounds for which only limited temperature-dependent kinetic data are available.

Study on the Mechanism and Kinetics of O3/H2O2 Advanced Oxidation Reaction

With the advent of the era of Industry 4.0, the composition of industrial wastewater is complex and toxic, and at the same time, the country and people have higher and higher requirements for the environment, making water pollution prevention and control a lot of attention. The article systematically expounds the reaction mechanism of O3 single oxidation and O3/H2O2 advanced oxidation process, analyzes the superiority of O3/H2O2 process compared with O3 single oxidation, and experimentally explores the effect of O3/H2O2 process on DDNP production wastewater. The orthogonal experiment results show that the primary and secondary relationship of the influencing factors of O3/H2O2 process is pH>reaction time>O3 concentration>H2O2 dosage, the optimal process is pH=9, reaction time 30min, O3 concentration 8.64mg/L, H2O2 dosage 2ml/L, the effluent after treatment reaches the industrial discharge standard. By increasing the acidity and inhibiting the reaction of OH radicals under the condition of pH 3, the reaction order of the characteristic pollutants (nitrophenols) in the direct oxidation of O3 was determined to be first-order, and the rate constant was 0.148 L/moL·min.

Thermochemistry and Kinetics of the Atmospheric Oxidation Reactions of Propanesulfinyl Chloride Initiated by OH Radicals: A Computational Approach.

The thermochemistry and kinetics of the atmospheric oxidation mechanism for propanesulfinyl chloride (CH3-CH2-CH2-S(═O)Cl; PSICl) initiated by the hydroxyl (OH) radical were investigated with high level quantum chemistry calculations and the Master equation solver for multi-energy well reaction (Mesmer) kinetic code. The mechanism for the oxidation of PSICl in the presence of OH radical can proceed via H-abstraction and substitution pathways. The CCSD(T)/aug-cc-pV(T+d)Z//MP2/aug-cc-pV(T+d)Z level calculated energies revealed addition of the OH radical to the S-atom of the sulfinyl (-S(═O)) moiety, followed by cleavage of the Cl-S(═O) single bond, leading to formation of propanesulfinic acid (PSIA) and the Cl radical to be the major pathway when compared to all other possible channels. The transition state barrier height for this reaction was found to be -3.0 kcal mol-1 relative to the energy of the starting PSICl + OH radical reactants. The rate coefficients were calculated for all possible paths in the atmospherically relevant temperature range of 200-320 K and at 1 atm. The rate coefficient for the formation of the PSIA + Cl radical from the PSICl + OH radical reaction was found to be 8.2 × 10-12 cm3 molecule-1 s-1 at 298 K and a pressure of 1 atm. From branching ratio calculations, it was revealed that the reaction resulting in the formation of the PSIA + Cl radical contributed ∼52% to the total reaction. The overall rate coefficient for the PSICl + OH reaction was also calculated and found to be 1.6 × 10-11 cm3 molecule-1 s-1 at 298 K and a pressure of 1 atm. In the aggregate, the results indicate the atmospheric lifetime of PSICl to be ∼12-20 h in the temperature range between 200 and 320 K, which suggests that its contribution to global warming is negligible. However, the degradation products revealed to be formed in its interactions with the OH radical, which include that SO2, Cl radical, HO2 radical, and propylene have significant effects on the formation of acid rain, secondary organic aerosols, the ozone layer, and global warming.

Kinetic insights into the reaction of hydroxyl radicals with 1,4-pentadiene: A combined experimental and theoretical study

Olefinic hydrocarbons are one of the main intermediates of the oxidation of large hydrocarbons, and they are also found in distillate transportation fuels. Combustion characteristics of olefinic species, particularly non-conjugated ones, are not well understood. This work reports chemical insights into the reaction of OH radicals with 1,4-pentadiene (14PTDN), an important non-conjugated diolefin. High-temperature rate coefficients of OH + 14PTDN were measured in a shock tube over T = 881 – 1314 K and p ∼ 1150 Torr. The progress of the reaction was monitored by detecting OH radicals near 307 nm using a UV laser-absorption technique. OH radicals were generated by the fast thermal decomposition of tert‑butyl hydroperoxide (TBHP). Stochastic RRKM-based Master Equation (RRKM-ME) simulations were carried out on the potential energy surface computed at M06–2X/aug-cc-pVTZ level of theory to gain mechanistic insights. The theoretical model captured high-temperature experimental data from this work and previously reported data at low temperatures (294 - 468 K). This combined experimental and theoretical study reveals: (i) no discernible pressure dependence of the total rate constants, (ii) mechanism shift occurs with temperature, (iii) similar to conjugated dienes + OH reactions, the addition of OH radicals to 14PTDN dominates compared to abstraction pathways for T < 500 K, (iv) among the bimolecular product channels originating from the OH-adducts, only vinyl alcohol + allyl radical is important, contributing ∼28% of the reactant consumption at 0.1 Torr and 400 K, (iv) beyond 1000 K, the addition channel contributes negligibly small for all pressures. To our knowledge, this is the first detailed experimental and theoretical kinetic study of the reaction of 1,4-pentadiene with OH radicals. The expression for the theoretical total rate coefficients (in units of cm3 molecule−1s−1) is provided as k(T)=0.172×T−3.82exp(−125.6KT)+1.49×10−20T3.09exp(−53.7KT) over T = 200 – 2000 K and P = 760 Torr.

Formation of persistent free radicals and reactive chlorine species during photochemical processes of Polychlorophenols: Effect of temperature, humidity and particles

Persistent free radicals (PFRs) generated by polychlorophenol pollutions (CPs) have attracted widespread attention, while the hazards of reactive chlorine species (RCS) generated during the oxidation process of PFRs to environment and human health have been largely ignored. In this study, the PFRs and RCS generation pathways in gas and on Si3O10H8 cluster from 2,4,6-trichlorophenol (2,4,6-TCPs) and pentachlorophenol (PCPs) initiated by OH radical are investigated by quantum chemistry method. In the gas phase, 2,4,6-TCPs mainly undergoes H-abstraction reaction to generate PFRs, while on Si3O10H8 cluster, the OH-addition reaction at the phenolic hydroxyl site becomes dominant and the addition product is dehydrated into PFRs under the synergisticeffectof H2O and Si3O10H8 cluster. Conversely, PCPs mainly undergoes H-abstraction reaction both in the gas phase and on Si3O10H8 clusters to form PFRs. This is because the PCPs form strong hydrogen bonds with Si3O10H8 clusters and the (highest occupied molecular orbital) HOMO of PCPs moves to higher energy levels, which reduces the electron binding ability and promotes the electrophilic reaction of OH radical. Introducing a carbonyl group ortho to the carbonyl group of phenolate-type PFRs leads to spontaneous ring-opening and subsequent decomposition into a series of RCS that can react with Cl, NO2 and O2 to form phosgene, chloroformyl peroxynitrate and peroxychloroformyl radical, respectively. These generated chlorine-containing compounds are highly irritating to human eyes and skin and have potential environmental toxicity.

Water Oxidation and Hydrogen Evolution with Organic Photooxidants: A Theoretical Perspective.

In this Perspective, we discuss a novel water-splitting scenario, namely the direct oxidation of water molecules by organic photooxidants in hydrogen-bonded chromophore-water complexes. In comparison with the established scenario of semiconductor-based water splitting, the distance of electron transfer processes is thereby reduced from mesoscopic scales to the Ångström scale, and the time scale is reduced from milliseconds to femtoseconds, which suppresses competing loss processes. The concept is illustrated by computational studies for the heptazine-H2O complex. The excited-state landscape of this complex has been characterized with ab initio electronic-structure methods and the proton-coupled electron-transfer dynamics has been explored with nonadiabatic dynamics simulations. A unique feature of the heptazine chromophore is the existence of a low-lying and exceptionally long-lived 1ππ* state in which a substantial part of the photon energy can be stored for hundreds of nanoseconds and is available for the oxidation of water molecules. The calculations reveal that the absorption spectra and the photochemical functionalities of heptazine chromophores can be systematically tailored by chemical substitution. The options of harvesting hydrogen and the problems posed by the high reactivity of OH radicals are discussed.

Low-lying electronic states of ethanol investigated by theoretical and synchrotron radiation methods

We investigate the ethanol absorption spectrum in the range 5.64–10.78 eV (220–115 nm), by combining ab initio theoretical and experimental methods in order to provide the most accurate and up-to-date information about the electronic state spectroscopy of ethanol. In particular, absolute cross-section values are reported from high-resolution vacuum ultraviolet (VUV) photoabsorption measurements. The present VUV spectrum reveals several new features not previously reported in the literature, with particular reference to the Rydberg (ndσ, ndσ′ ← (3a′′/13a)), n ≥ 3 members of the Rydberg (ndπ′(a′) ← (3a′′/13a)) and n = 3 members of the Rydberg (npσ′ ← (10a′/12a)) transitions. The experimental absolute photoabsorption cross sections have subsequently been used to calculate the photolysis lifetime of ethanol in the Earth's atmosphere (0–50 km), showing that solar photolysis is expected to be a weak sink at altitudes lower than 40 km to ●OH radical reactions. Potential energy curves for the lowest-lying excited electronic states, as a function of the O–H and the C–OH coordinates, are also obtained employing the equation of motion coupled cluster single and doubles (EOM-CCSD) and time dependant density functional theory (TD-DFT) methods. These show clear dissociation character at inter-nuclear distances greater than 2.0 Å for the RO–H and 2.2 Å for the RC–OH bond lengths, indicating the rather complex multidimensional character of the potential energy surfaces involved.

Theoretical evidence for the formation of perfluorocarboxylic acids form atmospheric oxidation degradation of fluorotelomer acrylates.

The atmospheric oxidation degradation of fluorotelomer acrylates (FTAcs) has been proposed as a potential source of perfluorocarboxylic acids (PFCAs) in remote locations. In this paper, detailed reactions of the main oxidant OH radicals with 4:2 FTAc in the atmosphere have been investigated by using density functional theory (DFT) calculation. All possible pathways involved in the oxidation process were presented and discussed. Based on the mechanism, transition state theory (TST) was used to predict the rate constants of the key elementary steps including the initial reactions of OH radical with n:2 FTAcs and the subsequent reactions of the main intermediates. Studies show that the reaction processes of OH radical addition to C = C bond are dominant and the fluorotelomer glyoxylate and formaldehyde are the major products. At 296K, the calculated overall rate constant of 4:2 FTAc with OH radical is 1.19 × 10-11 cm3 molecule-1s-1 with an atmospheric lifetime of 23.3h. In the atmosphere, fluorotelomer glyoxylate will continue to be oxidized, which will lead to the formation of PFCAs ultimately. In addition, atmospheric reactions of more carbons FTAc (CnF2n+1CH2CH2OC(O)CH = CH2, n = 6, 8, 10) are also discussed in the presence of O2/NOx.

Reactions of OH and OD radicals with ethanethiol and diethylsulfide: Branching ratio and vibrational energy disposal for the product water molecules

Reactions of OH and OD radicals with C 2 H 5 SH and (C 2 H 5 ) 2 S were studied by Fourier-transform infrared emission spectroscopy of the water product molecules from a fast-flow reactor. Vibrational excitation of the H 2 O, HOD and D 2 O product molecules was determined by computer modeling of the chemiluminescence spectra. Spectra of the abstraction of H-atom from the ethyl group (C-H) and sulfur atom (S-D) were separated using deuterated reactant, C 2 H 5 SD. The branching ratio was found to be 0.24 ± 0.05 (C-H):0.76 (S-D) both for OH and OD reactants. For the OH + C 2 H 5 SH reaction the branching fraction for the (C-H) channel of 0.25 ± 0.06 was obtained. Vibrational energy disposal for S-H abstraction is similar to that for the reaction with CH 3 SH, whereas abstraction from the ethyl group demonstrates higher bending excitation than abstraction from the methyl group of CH 3 SH. A similar trend was found for abstraction from the ethyl and methyl groups of (C 2 H 5 ) 2 S and CH 3 SCH 3 .

Evaluation of OH Radical Reaction Positions in 3-Ring PAHs Using Transition State Energy and Atomic Charge Calculations

In this study, transition state energy and atomic charge were calculated using the Gaussian 09 program with focus on three-ring PAHs, such as acenaphthylene and anthracene, which are most likely found in contaminated sites. The calculation results were then compared with the radical reaction positions reported in the existing literature. Because the energy difference between the reactant and the transition state according to the reaction position was very small, no distinct correlation was obtained when results were compared with those of the OH radical test findings reported in the literature. It was also found that the charge calculation makes it possible to accurately predict the radical reaction position of the target material. In addition, MK and HLY charges were found to be more accurate than CHelpG charges in predicting the radical reaction positions. The charge calculation can also be applied in predicting radical reaction positions for hazardous materials with different molecular structures.

Evaluating and elucidating the reactivity of OH radicals with atmospheric organic pollutants: Reaction kinetics and mechanisms by machine learning

The rate constants of the reactions of OH radicals with atmospheric organic pollutants (AOPs) are crucial physicochemical parameters to guide in the elucidation of the kinetics and mechanisms of the reactive landscape. The experimental and theoretical difficulties in revealing the reactivity of these degradation processes motivated us to develop a protocol based on machine learning combining molecular fingerprints to estimate their rate constants. The present workflow is based on Organization for Economic Cooperation and Development (OECD) guidelines and state-of-the-art techniques involving (i) data collection including 903 AOPs cataloged in the literature, (ii) pre-processing and structuring of data, (iii) development of models based on three machine learning algorithms, (iv) the standard reference of validation, and (v) mechanistic interpretation. The results show that the built model has a high predictive capacity – Rcv2 > 0.959 and RMSEcv < 0.090 for the training set and Rext2 and Qext2 > 0.889 and RMSEext < 0.084 for the test set. Additionally, through the SHapley Additive exPlanations (SHAP) method, it was possible to establish insight into the contribution of chemical classes to reaction kinetics and mechanism and to discuss it consistently with current experimental and theoretical observations. The availability of the evaluated reaction rate constants permitted to elucidate the role of AOPs in the photochemical ozone balance. Finally, to disseminate use of our results, we have presented them in a user-friendly web application that permits compilation of kinetic parameters, and that permits future implementations to account for the temperature dependence in the environmental relevant range, and the consideration of a wider class of chemicals and processes in the mechanistic networks of atmospheric reactivity.

Ozonation/adsorption hybrid treatment system for improved removal of natural organic matter and organic micropollutants from water – A mini review and future perspectives

Elevated concentrations of natural organic matter (NOM) and organic micropollutants (OMPs) can contaminate the quality of drinking water, and current water treatment technologies are not always successful in removing all their constituents. Ozonation and adsorption are two advanced processes with different removal mechanisms used to treat NOM and OMPs. Their treatment efficiency depends on the strength and kinetics of adsorption and ozonation (ozone molecule and OH radical (OH•) reaction) of the individual NOM constituents and OMPs. They are individually able to remove many of the NOM fractions and OMPs but not satisfactory in removing the vast array of their components which differ in their physico-chemical characteristics, for example molecular weight, charge, functional groups, aromaticity, and hydrophobicity/hydrophilicity. Significant progress has been made by integrating these processes (ozonation followed by activated carbon (AC) adsorption) but they need further improvement to efficiently target all NOM fractions and the various OMPs. Ozonation transforms the larger NOM molecules into smaller molecular sizes with lower aromaticity and hydrophobicity, subsequently resulting in reduced adsorption. The reduced adsorption of these molecules diminishes their competition against OMP adsorption resulting in increased OMP removal. Adsorption can remove unoxidized pollutants as well as the by-products of ozonation, and some of them are suspected to be human carcinogens. Of the commonly used adsorbents, anion exchange resin and AC, the former has higher affinity towards negatively charged humic fraction and OMPs. Conversely, the latter has higher affinity towards the hydrophobic constituents and smaller sized constituents which diffuse into AC pores and get adsorbed. Biofilm formed by long-term use of AC also contributes to enhanced removal of NOM and OMPs. This paper briefly reviews the currently available literature on removing NOM and OMPs by the ozonation/adsorption integrated process. It also suggests a new method for further increasing the efficiency of this process.

Insight into enhanced Fenton-like degradation of antibiotics over CuFeO2 based nanocomposite: To improve the utilization efficiency of [rad]OH/O2[rad]- via minimizing its migration distance

In Fenton or Fenton-like processes, the key step is to catalyze H2O2 and produce highly reactive OH radicals. More efforts are then focus on designing efficient heterogeneous Fenton catalysts by activating H2O2 to generate OH at the highest possible steady state concentration. In this study, using the antibiotic ofloxacin as target organic pollutant, we firstly demonstrate a point of view for improving OH utilization efficiency by regulating surface chemical reactions to minimizing its migration distance to the target pollutant. C doped g-C3N4 incorporated CuFeO2 (CCN/CuFeO2) exhibited almost ten times higher ofloxacin degradation rate constant than our previously reported CuFeO2 {012} catalyst (0.1634 vs 0.0179 min−1). Since similar amount of OH was generated, the different inhibition effect of tert-butyl alcohol and nitrobenzene on the ofloxacin degradation confirmed that the much-enhanced ofloxacin degradation was attributed to the surface Fenton reaction process. According to XPS and EXAFS characterization, the C–O–Cu bond between g-C3N4 and CuFeO2 established a closed-circuit surface Fenton reaction mechanism. H2O2 was adsorbed and decomposed into OH/O2- over ≡Cu + site in CuFeO2. The successful construction of CCN/CuFeO2 creates a negative surface potential and benefits the enrichment of target antibiotics from water, which greatly reduces the migration distance of OH/O2•- to adjacent pollutant and then increases the OH/O2- utilization efficiency by avoiding the unwanted quenching. Hence, CCN/CuFeO2 possesses superior Fenton catalytic activity and long-term stability.

Automatic Approach to Explore the Multireaction Mechanism for Medium-Sized Bimolecular Reactions via Collision Dynamics Simulations and Transition State Searches

We develop a broadly applicable computational method for the automatic exploration of the bimolecular multireaction mechanism. The current methodology mainly involves the high-energy Born-Oppenheimer molecular dynamics (BOMD) simulation and the successive reaction pathway construction. Several computational tricks are introduced, which include the selection of the reactive regions based on the electronic structure calculations and the employment of the virtual collision dynamics simulations with the monitoring of the atomic distance before the BOMD simulation. These prescreening steps largely reduce the number of trajectories in the BOMD simulations and significantly save the computational cost. The hidden Markov model combined with the modified atomic connectivity matrix is used for the detection of reaction events in each BOMD trajectory. Starting from several geometries close to the reaction events, the further intermediate optimization and transition state searches are conducted. The proposed method allows us to build the complicated multireaction mechanism of medium-sized bimolecular systems automatically. Here, we examine the feasibility and efficiency of the current method by its performance in searching the mechanisms of two prototype reactions in environmental science, which are the penicillin G anion + H2O and penicillin G anion + OH radical reactions. The result indicates that the proposed theoretical method is a powerful protocol for the automatic search of the bimolecular reaction mechanisms for medium-sized compounds.

Gas-Phase Reactivity of OH Radicals With Ammonia (NH3) and Methylamine (CH3NH2) at Around 22 K

Interstellar molecules containing N atoms, such as ammonia (NH3) and methylamine (CH3NH2), could be potential precursors of amino acids like the simplest one, glycine (NH2CH2COOH). The gas-phase reactivity of these N-bearing species with OH radicals, ubiquitous in the interstellar medium, is not known at temperatures of cold dark molecular clouds. In this work, we present the first kinetic study of these OH-reactions at around 22 K and different gas densities [(3.4–16.7) × 1016 cm−3] in helium. The obtained rate coefficients, with ± 2σ uncertainties, can be included in pure gas-phase or gas-grain astrochemical models to interpret the observed abundances of NH3and CH3NH2. We observed an increase ofk1andk2with respect to those previously measured by others at the lowest temperatures for which rate coefficients are presently available: 230 and 299 K, respectively. This increase is about 380 times for NH3and 20 times for CH3NH2. Although the OH + NH3reaction is included in astrochemical kinetic databases, the recommended temperature dependence fork1is based on kinetic studies at temperatures above 200 K. However, the OH + CH3NH2reaction is not included in astrochemical networks. The observed increase ink1at ca. 22 K does not significantly change the abundance of NH3in a typical cold dark interstellar cloud. However, the inclusion ofk2at ca. 22 K, not considered in astrochemical networks, indicates that the contribution of this destruction route for CH3NH2is not negligible, accounting for 1/3 of the assumed main depletion route (reaction with HCO+) in this IS environment.k1(OH+NH3)=(2.7±0.1)×10−11cm3s-1k2(OH+CH3NH2)=(3.9±0.1)×10−10cm3s-1

Reaction of OH radicals with CH3NH2 in the gas phase: experimental (11.7-177.5 K) and computed rate coefficients (10-1000 K).

Nitrogen-bearing molecules, like methylamine (CH3NH2), can be the building blocks of amino acids in the interstellar medium (ISM). At the ultralow temperatures of the ISM, it is important to know its gas-phase reactivity towards interstellar radicals and the products formed. In this work, the kinetics of the OH + CH3NH2 reaction was experimentally and theoretically investigated at low- and high-pressure limits (LPL and HPL) between 10 and 1000 K. Moreover, the CH2NH2 and CH3NH yields were computed in the same temperature range for both pressure regimes. A pulsed CRESU (French acronym for Reaction Kinetics in a Uniform Supersonic Flow) apparatus was employed to determine the rate coefficient, k(T), in the 11.7-177.5 K range. A drastic increase of k(T) when the temperature is lowered was observed in agreement with theoretical calculations, evaluated by the competitive canonical unified statistical (CCUS) theory, below 300 K in the LPL regime. The same trend was observed in the HPL regime below 350 K, but the theoretical k(T) values were higher than the experimental ones. Above 200 K, the calculated rate coefficients are improved with respect to previous computational studies and are in excellent agreement with the experimental literature data. In the LPL, the formation of CH3NH becomes largely dominant below ca. 100 K. Conversely, in the HPL regime, CH2NH2 is the only product below 100 K, whereas CH3NH becomes dominant at 298 K with a branching ratio similar to the one found in the LPL regime (≈70%). At T > 300 K, both reaction channels are competitive independently of the pressure regime.

The effect of hydrogen bonding on the reactivity of OH radicals with prenol and isoprenol: a shock tube and multi-structural torsional variational transition state theory study.

The presence of two functional groups (OH and double bond) in C5 methyl-substituted enols (i.e., isopentenols), such as 3-methyl-2-buten-1-ol (prenol) and 3-methyl-3-buten-1-ol (isoprenol), makes them excellent biofuel candidates as fuel additives. As OH radicals are abundant in both combustion and atmospheric environments, OH-initiated oxidation of these isopentenols over wide ranges of temperatures and pressures needs to be investigated. In alkenes, OH addition to the double bond is prominent at low temperatures (i.e., below ∼700 K), and H-atom abstraction dominates at higher temperatures. However, we find that the OH-initiated oxidation of prenol and isoprenol displays a larger role for OH addition at higher temperatures. In this work, the reaction kinetics of prenol and isoprenol with OH radicals was investigated over the temperature range of 900-1290 K and pressure of 1-5 atm by utilizing a shock tube and OH laser diagnostic. To rationalize these chemical systems, variational transition state theory calculations with multi-structural torsional anharmonicity and small curvature tunneling corrections were run using a potential energy surface characterized at the UCCSD(T)/jun-cc-pVQZ//M06-2X/6-311++G(2df,2pd) level of theory. A good agreement was observed between the experiment and theory, with both predicting a non-Arrhenius behavior and negligible pressure effects. OH additions to the double bond of prenol and isoprenol were found to be important, with at least 50% contribution to the total rate constants even at temperatures as high as 700 and 2000 K, respectively. This behavior was attributed to the stabilizing effect induced by hydrogen bonding between the reacting OH radical and the OH functional group of isopentenols at the saddle points. These stabilizing intermolecular interactions help mitigate the entropic effects that hinder association reactions as temperature increases, thus extending the prominent role of addition pathways to high temperatures. The site-specific rate constants were also found to be slower than their analogous reactions of OH + n-alkenes.

T- and pH-dependent OH radical reaction kinetics with glycine, alanine, serine, and threonine in the aqueous phase.

Glycine, alanine, serine, and threonine are essential amino acids originating from biological activities. These substances can be emitted into the atmosphere directly. In the present study, the aqueous phase reaction kinetics of hydroxyl radicals (˙OH) with the four amino acids is investigated using the competition kinetics method under controlled temperature and pH conditions. The following T-dependent Arrhenius expressions are derived for the ˙OH reactions with glycine, k(T, H2A+) = (9.1 ± 0.3) × 109 × exp[(-2360 ± 230 K)/T], k(T, HA±) = (1.3 ± 0.1) × 1010 × exp[(-2040 ± 240 K)/T]; alanine, k(T, H2A+) = (1.4 ± 0.1) × 109 × exp[(-1120 ± 320 K)/T], k(T, HA±) = (5.5 ± 0.2) × 109 × exp[(-1300 ± 200 K)/T]; serine, k(T, H2A+) = (1.1 ± 0.1) × 109 × exp[(-470 ± 150 K)/T], k(T, HA±) = (3.9 ± 0.1) × 109 × exp[(-720 ± 130 K)/T]; and threonine, k(T, H2A+) = (5.0 ± 0.1) × 1010 × exp[(-1500 ± 100 K)/T], k(T, HA±) = (3.3 ± 0.1) × 1010 × exp[(-1320 ± 90 K)/T] (in units of L mol-1 s-1). The energy barriers of the ˙OH-induced H atom abstractions were simulated by the density functional theory (DFT) calculation performed with GAUSSIAN using the method of M06-2X and the basis set of 6-311++G(3df,2p). According to the calculation results, the -COOH and -NH3+ groups with strong negative inductive effects increase the energy barriers and thus decrease the ˙OH reaction rate constants. In contrast, the presence of a -OH or -CH3 group with weak negative or positive inductive effects can reduce energy barriers and hence increase the ˙OH reaction rate constants. To improve the previous structure-activity relationship, the contribution factors of -NH3+ at Cα-atom and Cβ-atom are determined as 0.07 and 0.15, respectively. Aqueous phase ˙OH oxidation acts as an important sink of the amino acids in the atmosphere, and can be accurately described by the obtained Arrhenius expressions under atmospheric conditions.

Emerging investigator series: ozone uptake by urban road dust and first evidence for chlorine activation during ozone uptake by agro-based anti-icer: implications for wintertime air quality in high-latitude urban environments.

High-latitude urban regions provide a unique and complex range of environmental surfaces for uptake of trace pollutant gases, including winter road maintenance materials (e.g., gravel, rock salts, and anti-icer, a saline solution applied to roads during winter). In an effort to reduce the negative environmental and economic impacts of road salts, many municipalities have turned to agro-based anti-icing materials that are rich in organic material. To date, the reactivity of both anti-icer and saline road dust with pollutant gases remain unexplored, which limits our ability to assess the potential impacts of these materials on air quality in high-latitude regions. Here, we used a coated-wall flow tube to investigate the uptake of ozone, an important air pollutant, by road dust collected in Edmonton, Canada. At 25% relative humidity (RH) and 50 ppb ozone, γBET for ozone uptake by this sample is (8.0 ± 0.7) × 10-8 under dark conditions and (2.1 ± 0.1) × 10-7 under illuminated conditions. These values are 2-4× higher than those previously obtained by our group for natural mineral dusts, but are not large enough for suspended road dust to influence local ozone mixing ratios. In a separate set of experiments, we also investigated the uptake of ozone by calcium chloride (i.e., road salt) and commercial anti-icer solution. Although ozone uptake by pure calcium chloride was negligible, ozone uptake by anti-icer was significant, which implies that the reactivity of anti-icer is conferred by its organic content. Importantly, ozone uptake by anti-icer-and, to a lesser extent, road dust doped with anti-icer-leads to the release of inorganic chlorine gas, which we collected using inline reductive trapping and quantified using ion chromatography. To explain these results, we propose a novel pathway for chlorine activation: here, ozone oxidation of the anti-icer organic fraction (in this case, molasses) yields reactive OH radicals that can oxidize chloride. In summary, this study demonstrates the ability of road dust and anti-icer to influence atmospheric oxidant mixing ratios in cold-climate urban areas, and highlights previously unidentified air quality impacts of winter road maintenance decisions.