Formation Of Peroxy Radicals Research Articles

Photoredox-Catalyzed 1,4-Peroxidation-Sulfonylation of Enynones: A Three-Component Radical Coupling Approach for the Synthesis of Highly Functionalized Allenes.

An Eosin Y-catalyzed visible light-promoted 1,4-peroxidation-sulfonylation of enynones was achieved to give tetrasubstituted allenes. The photoredox catalysis of Eosin Y allowed the concomitant formation of peroxy and sulfonyl radicals, where the preferential peroxy radical addition to the alkene moiety of enynones resulted in the subsequent α-keto radical-sulfonyl radical cross couplings. The developed photoredox catalysis of Eosin Y demonstrates a regioselective 1,4-diradical addition strategy, opening up a new possibility of diradical functionalization of conjugate systems.

Theoretical investigation into activation of hydroperoxides by excited quinones under ultraviolet irradiation

In this work, quantum chemical calculations have been applied to explore the reaction of hydroperoxides (hydrogen peroxide, CH3OOH, peroxomonosulfate and peracetic acid) with the first triplet states of quinones (p-benzoquinone, tetrachloro-p-benzoquinone and 2-methoxy-p-benzoquinone) (3Q*), which were excited from the ground states of quinones by the UV irradiation. With the much larger forward energy barriers than the reverse ones, the attack of the ionized hydroperoxides on the CO and CH(Cl) bonds of 3Q* was kinetically unfeasible, which was independent of the substitutes in the structures. Surprisingly, based on the bond lengths, the charge distributions and the molecular orbitals, the single electron transfer from the ionized hydroperoxides to 3Q* was tentatively determined for the activation of ionized hydroperoxides by 3Q*, with the formation of peroxy radicals and semiquinone radicals. The peroxy radicals may be able to transform to other highly oxidative radicals (e.g., sulfate radical). Moreover, not only the electron-withdrawing groups but also the electron-donating ones in the structures might not inhibit the single electron transfer from the ionized hydroperoxides to 3Q*. With quinones as the photosensitizers, the UV irradiation enriched the types of highly reactive species during the activation of hydroperoxides, which may make the combination of quinones and hydroperoxides suitable for the degradation of different pollutants. These theoretical findings may help to verify the influence of UV irradiation on the activation of hydroperoxides by quinones for the decontamination and promote the understanding of the reaction of chromophoric dissolved organic matter with hydroperoxides in natural waters.

Probing the low temperature oxidation chemistry of RP-3 kerosene: Experimental and kinetic modeling investigation

Elucidating the formation of intermediates, especially active species like peroxy radicals and hydroperoxides, is crucial to explore the low temperature reaction network and validate the kinetic model. While low temperature oxidation kinetics of single hydrocarbon has been widely studied, the comprehensive speciation study on the transportation fuels is still lacking and challenging due to the complex compositions of real fuels. This work aims to investigate the speciation pool and develop predictable kinetic model for the low temperature oxidation of RP-3 kerosene. An atmospheric jet stirred reactor combined with synchrotron vacuum ultraviolet photoionization mass spectrometry was used to investigate the RP-3 kerosene oxidation in the temperature range of 480–770 K and equivalence ratios of 0.5, 1.0 and 2.0. Major products and key intermediates were quantified and categorized into several groups, including peroxy radicals, hydroperoxides, alkenes, alcohols, aldehydes, acids and aromatics. Furthermore, a three-component surrogate fuel consisting of 60.0% n-dodecane, 31.5% ethylcyclohexane, and 8.5% n-butylbenzene (by mole) was formulated to represent the RP-3 kerosene. A kinetic model for the three-component surrogate fuel was developed based on the comprehensively validated kinetic model of single components. This model was validated against the low temperature oxidation speciation data, ignition delay times and laminar flame speeds of RP-3 over wide range of conditions. Rate of production and sensitivity analysis were performed to explore the low temperature oxidation reaction network. The results indicate that the negative temperature coefficient behavior of RP-3 is dominated by the reactions of n-alkane, which is easier to proceed low temperature chain-propagation reactions leading to the production active OH radicals. The reaction network of each fuel component is effectively coupled from the view of OH regeneration and consumption, and finally affects the distribution of speciation pool during RP-3 low temperature oxidation. Regarding the characteristic speciation pool, it is found that the formation and consumption of peroxy radicals, hydroperoxides, alkene, aldehyde, and alcohol intermediates are controlled by the reaction network of alkane. While the formation and depletion of aromatic intermediates are affected by the interaction kinetics of fuel components with different chemical functionalities. These results improve the current understanding of the speciation pool and interaction kinetics during RP-3 low temperature oxidation. The kinetic model of RP-3 kerosene developed and validated in this work could be applied for numerical simulation of aero-engine.

Probing O2-dependence of tetrahydrofuranyl reactions via isomer-resolved speciation

Low-temperature oxidation of tetrahydrofuran involves competing reactions that depend on temperature, pressure, and oxygen concentration, including ring-opening and subsequent oxidation of initial radicals (R˙), HOO-elimination yielding dihydrofuran isomers, and the formation of peroxy radicals (ROO˙). The latter species, upon isomerization, lead to hydroperoxy-substituted radicals (Q˙OOH) that undergo reaction either via unimolecular decomposition or second-O2-addition. Quantitative measurements of partially oxidized intermediates formed from each type of reaction provide critical constraints that are required for accurate modeling of combustion. To examine the influence of temperature and oxygen concentration on intermediates from tetrahydrofuran, isomer-resolved speciation measurements were conducted at 810 Torr in a jet-stirred reactor (JSR) from 500 K to 1000 K. Resulting from negative-temperature coefficient behavior, species concentrations peaked at two temperatures, 600 K and 800 K, which were then selected for separate experiments to quantify O2-dependence using concentrations of 0.37 · 1018 – 7.40 · 1018 molecules cm–3.Utilizing vacuum-ultraviolet absorption spectroscopy and electron-impact mass spectrometry, several species related to R˙ + O2 reactions were quantified for the first time, including explicit resolution and quantification of constitutional isomers tetrahydrofuran-3-one, butanedial, and allyl formate; the latter two resulting from ring-opening reactions of Q˙OOH radicals. For the majority of species, including dihydrofuran isomers, tetrahydrofuran-3-one, oxirane, and methyloxirane, clear dependence on O2 exists that is not captured quantitatively by chemical kinetics mechanisms. The latter two species arise from reactions of ring-opened α-tetrahydrofuranyl radicals while the others are derived initially from R˙ + O2 reactions. The experiments herein provide new targets for refinement of chemical kinetics mechanisms of tetrahydrofuran. The discrepancies existing between the measured and model-predicted species profiles indicate that sub-mechanisms for important intermediates may require additional elementary reactions, such as for dihydrofuran isomers. In addition, rates for O2-addition to tetrahydrofuranyl radicals in chemical kinetics mechanisms, which employ rate rules from alkyl radicals, merit scrutiny.

EXPLORATION OF COPPER-BASED COORDINATION POLYMER AS CATALYST: HIGH YIELD OXIDATIVE COUPLING OF BENZYLAMINE TO IMINE

Benzylamine to imine conversion through oxidative coupling reaction has been achieved with almost 91% yield under very mild conditions by applying a one-dimensional coordination polymer as a catalyst and tert-butyl hydroperoxide (TBHP) as an oxidizing agent. The 1D-coordination polymer, [{Cu2(2,3-pydc)2(2,2ʹ-bpy) (H2O)2}.4H2O]n(A) has been re-synthesized by different synthetic process using the ligands 2,3- pyridinedicarboxylic acid (2,3-pydc) and 2,2Ꞌ-bipyridine (2,2Ꞌ-bpy) and it has been characterized by utilizing different physicochemical techniques such as IR-spectroscopy, UV-Vis spectroscopy, elemental analysis and TGDTA analysis including reestablishment of X-ray single crystal structure. The proposed mechanism of the reaction indicates that the catalytic reaction is facilitated by a flexible and coordinatively unsaturated Cu(II) center and through the formation of tert-butyl peroxy radical.

The reaction of organic peroxy radicals with unsaturated compounds controlled by a non-epoxide pathway under atmospheric conditions.

Today, the reactions of gas-phase organic peroxy radicals (RO2) with unsaturated Volatile Organic Compounds (VOC) are expected to be negligible at room temperature and ignored in atmospheric chemistry. This assumption is based on combustion studies (T ≥ 360 K), which were the only experimental data available for these reactions until recently. These studies also reported epoxide formation as the only reaction channel. In this work, the products of the reactions of 1-pentylperoxy (C5H11O2) and methylperoxy (CH3O2) with 2,3-dimethyl-2-butene ("2,3DM2B") and isoprene were investigated at T = 300 ± 5 K with Proton Transfer Reaction Time-of-Flight Mass Spectrometry (PTR-ToF-MS) and Gas Chromatography/Electron Impact Mass Spectrometry. Unlike what was expected, the experiments showed no measurable formation of epoxide. However, RO2 + alkene was found to produce compounds retaining the alkene structure, such as 3-hydroxy-3-methyl-2-butanone (C5H10O2) with 2,3DM2B and 2-hydroxy-2-methyl-3-butenal (C5H8O2) and methyl vinyl ketone with isoprene, suggesting that these reactions proceed through another reaction pathway under atmospheric conditions. We propose that, instead of forming an epoxide, the alkyl radical produced by the addtion of RO2 onto the alkene reacts with oxygen, producing a peroxy radical. The corresponding mechanisms are consistent with the products observed in the experiments. This alternative pathway implies that, under atmospheric conditions, RO2 + alkene reactions are kinetically limited by the initial addition step and not by the epoxide formation proposed until now for combustion systems. Extrapolating the combustion data to room temperature thus underestimates the rate coefficients, which is consistent with those recently reported for these reactions at room temperature. While slow for many classes of RO2, these reactions could be non-negligible at room temperature for some functionalized RO2. They might thus need to be considered in laboratory studies using large alkene concentrations and in biogenically-dominated regions of the atmosphere.

Characterization of the Autoxidation of Terpenes at Elevated Temperature Using High-Resolution Mass Spectrometry: Formation of Ketohydroperoxides and Highly Oxidized Products from Limonene.

Low-temperature experiments on the oxidation of limonene-O2-N2 mixtures were conducted in a jet-stirred reactor (JSR) over a range of temperatures (520-800 K) under fuel-lean conditions (equivalence ratio φ = 0.5) with a short residence time (1.5 s) and a pressure of 1 bar. Collected samples of the reaction mixtures were analyzed by (i) online Fourier transform infrared spectroscopy (FTIR) and (ii) Orbitrap Q-Exactive high-resolution mass spectrometry after direct injection or chromatographic separation using reversed-phase ultra-high-performance liquid chromatography (RP-UHPLC) and soft ionization (with positive or negative heated electrospray ionization and atmospheric-pressure chemical ionization). H/D exchange using deuterated water (D2O) and a reaction with 2,4-dinitrophenylhydrazine (2,4-DNPH) were performed to probe the presence of OH, OOH, and C═O groups in the oxidized products. A broad range of oxidation products ranging from water to highly oxygenated products containing five and more O atoms were detected (C7H10O4,5, C8H12O2,4, C8H14O2,4, C9H12O, C9H14O1,3-5, C10H12O2, C10H14O1-9, C10H16O2-5, and C10H18O6). Mass spectrometry analyses were only qualitative, and quantification was performed with FTIR. The results are discussed in terms of reaction routes involving the initial formation of peroxy radicals, H atom transfer, and O2 addition sequences producing a large set of chemical products, including ketohydroperoxides and more oxygenated products. Carbonyl compounds derived from the Waddington oxidation mechanism on exo- and endo-double bonds (C═C) were observed in addition to their products of further oxidation. Products of the Korcek mechanism (carboxylic acids and carbonyls) were also detected.

Peroxy Radical and Product Formation in the Gas-Phase Ozonolysis of α-Pinene under Near-Atmospheric Conditions: Occurrence of an Additional Series of Peroxy Radicals O,O-C10H15O(O2)yO2 with y = 1-3.

Ozonolysis of α-pinene, C10H16, and other monoterpenes is considered to be one of the important chemical process in the atmosphere leading to condensable vapors, which are relevant to aerosol formation and, finally, for Earth's radiation budget. The formation of peroxy (RO2) radicals, O,O-C10H15(O2)xO2 with x = 0-3, and closed-shell products has been probed from the ozonolysis of α-pinene for close to atmospheric reaction conditions. (The "O,O" in the chemical formulas indicates the two carbonyl groups formed in the ozonolysis.) An additional series of RO2 radicals, O,O-C10H15O(O2)yO2 with y = 1-3, emerged in the presence of NO additions of (1.7-50) × 109 molecules cm-3, whose formation can be explained via different processes starting from alkoxy (RO) radicals, such as the RO-driven autoxidation. The main closed-shell product is a substance with the composition C10H16O3, probably pinonic acid, obtained with a molar yield (lower limit) of 0.26 independent of NO. Total molar product yields accounted for up to 0.71 indicating reasonable detection sensitivity of the analytical technique applied. For the isomeric O,O-C10H15O2 radicals, an average rate coefficient k(RO2 + NO) = (1.5 ± 0.3) × 10-11 cm3 molecule-1 s-1 at 295 ± 2 K was determined. Product analysis showed a lowering in the formation of highly oxygenated organic molecules (HOMs) by a factor of ∼2.2 when adding 5 × 1010 molecules cm-3 of NO. The comparison with former results revealed that total HOM suppression by NO in the α-pinene ozonolysis is slightly stronger than in the OH + α-pinene reaction.

EVALUATION OF IN VITRO ANTIOXIDANT AND IN VIVO HEPATOPROTECTIVE ACTIVITY OF BAUHINIA VARIEGATA ROOT ETHANOLIC EXTRACT AGAINST CCL4 INDUCED LIVER INJURY IN RATS

Plant derived herbal formulations and remedies provide an effective means for the treatment of various types of disease that are dogmatic and incurable in other types of systems of medicines, but it is essential to explore and establish the scientific basis for therapeutic action of herbal plant medicines. Bauhinia variegata root ethanolic extract was studied for the hepatoprotective activity against CCl4 induced liver injury in rats. For estimation of hepatoprotective role of B. variegata, total bilirubin count, serum enzymes level and finally histopathological study of liver were performed. This extract’s DPPH radical scavenging potential was also studied. The ethanolic extract of B. variegata root administered orally to animal showed significant depletion in CCL4 induced increased level of SGPT, SGOT, ALP and total bilirubin concentration. Significantly (p<0.05), hepatotoxicity is reversed by treatment with Liv 52 syrup also. For initiation of biochemical analysis, the histopathological studies are provided supportive evidence. This extract also showed better activity in quenching DPPH radical. The ethanolic extract of B. variegata root shows antioxidant property by preventing the formation of trichloromethyl peroxy radicals, and thus reduce tissue damage, which is examined and confirmed by the histopathological studies. Therefore, the hepatoprotective activity of ethanolic extract of B. variegata root may be due to its antioxidant potential. Previously studies have reported that plants containing flavonoids possess antioxidant properties. The antioxidant and hepatoprotective properties of the test plant may be attributed to the presence flavonoids. B. variegata root ethanolic extract is shown to have hepatoprotective and antioxidant action.

Impact of pyruvic acid photolysis on acetaldehyde and peroxy radical formation in the boreal forest: theoretical calculations and model results

Abstract. Based on the first measurements of gas-phase pyruvic acid (CH3C(O)C(O)OH) in the boreal forest, we derive effective emission rates of pyruvic acid and compare them with monoterpene emission rates over the diel cycle. Using a data-constrained box model, we determine the impact of pyruvic acid photolysis on the formation of acetaldehyde (CH3CHO) and the peroxy radicals CH3C(O)O2 and HO2 during an autumn campaign in the boreal forest. The results are dependent on the quantum yield (φ) and mechanism of the photodissociation of pyruvic acid and the fate of a likely major product, methylhydroxy carbene (CH3COH). With the box model, we investigate two different scenarios in which we follow the present IUPAC (IUPAC Task Group on Atmospheric Chemical Kinetic Data Evaluation, 2021) recommendations with φ = 0.2 (at 1 bar of air), and the main photolysis products (60 %) are acetaldehyde + CO2 with 35 % C–C bond fission to form HOCO and CH3CO (scenario A). In the second scenario (B), the formation of vibrationally hot CH3COH (and CO2) represents the main dissociation pathway at longer wavelengths (∼ 75 %) with a ∼ 25 % contribution from C–C bond fission to form HOCO and CH3CO (at shorter wavelengths). In scenario 2 we vary φ between 0.2 and 1 and, based on the results of our theoretical calculations, allow the thermalized CH3COH to react with O2 (forming peroxy radicals) and to undergo acid-catalysed isomerization to CH3CHO. When constraining the pyruvic acid to measured mixing ratios and independent of the model scenario, we find that the photolysis of pyruvic acid is the dominant source of CH3CHO with a contribution between ∼ 70 % and 90 % to the total production rate. We find that the photolysis of pyruvic acid is also a major source of the acetylperoxy radical, with contributions varying between ∼ 20 % and 60 % dependent on the choice of φ and the products formed. HO2 production rates are also enhanced, mainly via the formation of CH3O2. The elevated production rates of CH3C(O)O2 and HO2 and concentration of CH3CHO result in significant increases in the modelled mixing ratios of CH3C(O)OOH, CH3OOH, HCHO, and H2O2.

Highly oxygenated organic molecule (HOM) formation in the isoprene oxidation by NO<sub>3</sub> radical

Abstract. Highly oxygenated organic molecules (HOM) are found to play an important role in the formation and growth of secondary organic aerosol (SOA). SOA is an important type of aerosol with significant impact on air quality and climate. Compared with the oxidation of volatile organic compounds by ozone (O3) and hydroxyl radical (OH), HOM formation in the oxidation by nitrate radical (NO3), an important oxidant at nighttime and dawn, has received less attention. In this study, HOM formation in the reaction of isoprene with NO3 was investigated in the SAPHIR chamber (Simulation of Atmospheric PHotochemistry In a large Reaction chamber). A large number of HOM, including monomers (C5), dimers (C10), and trimers (C15), both closed-shell compounds and open-shell peroxy radicals (RO2), were identified and were classified into various series according to their formula. Their formation pathways were proposed based on the peroxy radicals observed and known mechanisms in the literature, which were further constrained by the time profiles of HOM after sequential isoprene addition to differentiate first- and second-generation products. HOM monomers containing one to three N atoms (1–3N-monomers) were formed, starting with NO3 addition to carbon double bond, forming peroxy radicals, followed by autoxidation. 1N-monomers were formed by both the direct reaction of NO3 with isoprene and of NO3 with first-generation products. 2N-monomers (e.g., C5H8N2On(n=7–13), C5H10N2On(n=8–14)) were likely the termination products of C5H9N2On⚫, which was formed by the addition of NO3 to C5-hydroxynitrate (C5H9NO4), a first-generation product containing one carbon double bond. 2N-monomers, which were second-generation products, dominated in monomers and accounted for ∼34 % of all HOM, indicating the important role of second-generation oxidation in HOM formation in the isoprene + NO3 reaction under our experimental conditions. H shift of alkoxy radicals to form peroxy radicals and subsequent autoxidation (“alkoxy–peroxy” pathway) was found to be an important pathway of HOM formation. HOM dimers were mostly formed by the accretion reaction of various HOM monomer RO2 and via the termination reactions of dimer RO2 formed by further reaction of closed-shell dimers with NO3 and possibly by the reaction of C5–RO2 with isoprene. HOM trimers were likely formed by the accretion reaction of dimer RO2 with monomer RO2. The concentrations of different HOM showed distinct time profiles during the reaction, which was linked to their formation pathway. HOM concentrations either showed a typical time profile of first-generation products, second-generation products, or a combination of both, indicating multiple formation pathways and/or multiple isomers. Total HOM molar yield was estimated to be 1.2 %-0.7%+1.3%, which corresponded to a SOA yield of ∼3.6 % assuming the molecular weight of C5H9NO6 as the lower limit. This yield suggests that HOM may contribute a significant fraction to SOA yield in the reaction of isoprene with NO3.

On the similarities and differences between the products of oxidation of hydrocarbons under simulated atmospheric conditions and cool flames

Abstract. Atmospheric oxidation chemistry and, more specifically, photooxidation show that the long-term oxidation of organic aerosol (OA) progressively erases the initial signature of the chemical compounds and can lead to a relatively uniform character of oxygenated organic aerosol (OOA). This uniformity character observed after a long reaction time seems to contrast with the great diversity of reaction mechanisms observed in the early stages of oxidation. The numerous studies carried out on the oxidation of terpenes, and more particularly on limonene for its diversity of reaction sites (endo- and oxocyclic), allow this evolution to be studied. We have selected, for their diversity of experimental conditions, nine studies of limonene oxidation at room temperature over long reaction times to be compared to the present data set obtained at elevated temperature and short reaction time in order to investigate the similarities in terms of reaction mechanisms and chemical species formed. Here, the oxidation of limonene–oxygen–nitrogen mixtures was studied using a jet-stirred reactor at elevated temperature and atmospheric pressure. Samples of the reacting mixtures were collected and analyzed by high-resolution mass spectrometry (Orbitrap) after direct injection or after separation by reverse-phase ultra-high-pressure liquid chromatography and soft ionization, i.e., (+/-) HESI and (+/-) APCI. Unexpectedly, because of the diversity of experimental conditions in terms of continuous-flow tank reactor, concentration of reactants, temperature, reaction time, mass spectrometry techniques, and analysis conditions, the results indicate that among the 1138 presently detected molecular formulae, many oxygenates found in earlier studies of limonene oxidation by OH and/or ozone are also produced under the present conditions. Among these molecular formulae, highly oxygenated molecules and oligomers were detected in the present work. The results are discussed in terms of reaction pathways involving the initial formation of peroxy radicals (RO2), isomerization reactions yielding keto-hydroperoxides, and other oxygenated intermediates and products up to C25H32O17, products which could derive from RO2 autoxidation via sequential H shift and O2 addition (C10H14O3,5,7,9,11) and products deriving from the oxidation of alkoxy radicals (produced by RO2 self-reaction or reaction with HO2) through multiple H shifts and O2 additions (C10H14O2,4,6,8,10). The oxidation of RO2, with possible occurrence of the Waddington mechanism and of the Korcek mechanism, involving H shifts is also discussed. The present work demonstrates similitude between the oxidation products and oxidation pathways of limonene under simulated atmospheric conditions and in those encountered during the self-ignition of hydrocarbons at elevated temperatures. These results complement those recently reported by Vereecken and Nozière and confirm for limonene the existence of an oxidative chemistry of the alkylperoxy radical beyond 450 K based on the H shift (Nozière and Vereecken, 2019; Vereecken and Nozière, 2020).

Double High-Level Ozone and PM2.5 Co-Pollution Episodes in Shanghai, China: Pollution Characteristics and Significant Role of Daytime HONO

In recent years, high fine particulate (PM2.5) pollution episodes with high ozone (O3) levels have been observed in Shanghai from time to time. However, their occurrence and characteristics remain poorly understood. Meanwhile, as a major precursor of tropospheric hydroxyl radical (OH) that initiates the formation of hydroperoxyl and organic peroxy radicals, HONO would inevitably affect the formation of O3, but its role in the formation of O3 during the double high-level PM2.5 and O3 pollution episodes remains unclear. In this study, the characteristics of the double high pollution episodes and the role of HONO in O3 formation in these episodes were investigated based on field observation in urban Shanghai from 2014 to 2016. Results showed that high PM2.5 pollution and high O3 pollution could occur simultaneously. The cases with data of double high O3 and PM2.5 concentrations accounted for about 1.0% of the whole sampling period. During the double high pollution episodes, there still existed active photochemical processes, while the active photochemical processes at high PM2.5 concentration were conductive to the production and accumulation of O3 under a VOC-limited regime and a calm atmospheric condition including high temperature, moderately high relative humidity, and low wind speed, which in turn enhanced the conversions of SO2 and NO2 and the formation and accumulation of secondary sulfate and nitrate aerosols and further promoted the increase of PM2.5 concentration and the deterioration of air pollution. Further analysis indicated that the daytime HONO concentration could be strongly negatively correlated with O3 concentration in most of the double high pollution episodes, revealing the dominant role of HONO in O3 formation during these pollution episodes. This study provides important field measurement-based evidence for understanding the significant contribution of daytime HONO to O3 formation, and helps to clarify the formation and coexistence mechanisms of the double high-level O3 and PM2.5 pollution episodes.

Chemiluminescence studies on the photooxidation of isotactic polypropylene

Photooxidation of polypropylene (PP) was investigated by chemiluminescence (CL) after UV irradiation of main wavelengths of 254 nm and 365 nm in air at an ambient temperature. From the kinetic analysis of CL intensity change just after irradiation, CL intensity at the time of the cessation of UV irradiation, (I(t=0)), was obtained. The irradiation time dependence of I(t=0) gave a time-course, where the CL intensity increased with time in the first stage and reached a steady state, and after maintaining the same level for a while, it largely increased in the later stage. The steady state intensity, which is considered the rate of peroxy radical formation, was found to be dependent on the crystallinity of PP. The lower the crystallinity is, the higher the steady state intensity is, indicating the preferential photoinitiation reaction to occur in the amorphous phase. The isothermal CL measurement at above the melting point of the samples, which were stored at 25 °C for various periods after UV irradiation, suggests that there are two types of CL emitting sources of different lifetimes; one is quenched in about one day and the other survives for more than 10 days. The temperature rising measurement also indicates the existence of two types of CL emitting sources, too. Further, thermal oxidation before UV irradiation was found to clearly promote the photooxidation. In contrast, thermal oxidation in oxygen was increasingly accelerated by the pre-photooxidation.

Atmospheric Chemistry of Allylic Radicals from Isoprene: A Successive Cyclization-Driven Autoxidation Mechanism.

The atmospheric chemistry of isoprene has broad implications for regional air quality and the global climate. Allylic radicals, taking 13-17% yield in the isoprene oxidation by •Cl, can contribute as much as 3.6-4.9% to all possible formed intermediates in local regions at daytime. Considering the large quantity of isoprene emission, the chemistry of the allylic radicals is therefore highly desirable. Here, we investigated the atmospheric oxidation mechanism of the allylic radicals using quantum chemical calculations and kinetics modeling. The results indicate that the allylic radicals can barrierlessly combine with O2 to form peroxy radicals (RO2•). Under ≤100 ppt NO and ≤50 ppt HO2• conditions, the formed RO2• mainly undergo two times "successive cyclization and O2 addition" to finally form the product fragments 2-alkoxy-acetaldehyde (C2H3O2•) and 3-hydroperoxy-2-oxopropanal (C3H4O4). The presented reaction illustrates a novel successive cyclization-driven autoxidation mechanism. The formed 3-hydroperoxy-2-oxopropanal product is a new isomer of the atmospheric C3H4O4 family and a potential aqueous-phase secondary organic aerosol precursor. Under >100 ppt NO condition, NO can mediate the cyclization-driven autoxidation process to form C5H7NO3, C5H7NO7, and alkoxy radical-related products. The proposed novel autoxidation mechanism advances our current understanding of the atmospheric chemistry of both isoprene and RO2•.

Experimental Evidence of Dioxole Unimolecular Decay Pathway for Isoprene-Derived Criegee Intermediates.

Ozonolysis of isoprene, one of the most abundant volatile organic compounds emitted into the Earth's atmosphere, generates two four-carbon unsaturated Criegee intermediates, methyl vinyl ketone oxide (MVK-oxide) and methacrolein oxide (MACR-oxide). The extended conjugation between the vinyl substituent and carbonyl oxide groups of these Criegee intermediates facilitates rapid electrocyclic ring closures that form five-membered cyclic peroxides, known as dioxoles. This study reports the first experimental evidence of this novel decay pathway, which is predicted to be the dominant atmospheric sink for specific conformational forms of MVK-oxide (anti) and MACR-oxide (syn) with the vinyl substituent adjacent to the terminal O atom. The resulting dioxoles are predicted to undergo rapid unimolecular decay to oxygenated hydrocarbon radical products, including acetyl, vinoxy, formyl, and 2-methylvinoxy radicals. In the presence of O2, these radicals rapidly react to form peroxy radicals (ROO), which quickly decay via carbon-centered radical intermediates (QOOH) to stable carbonyl products that were identified in this work. The carbonyl products were detected under thermal conditions (298 K, 10 Torr He) using multiplexed photoionization mass spectrometry (MPIMS). The main products (and associated relative abundances) originating from unimolecular decay of anti-MVK-oxide and subsequent reaction with O2 are formaldehyde (88 ± 5%), ketene (9 ± 1%), and glyoxal (3 ± 1%). Those identified from the unimolecular decay of syn-MACR-oxide and subsequent reaction with O2 are acetaldehyde (37 ± 7%), vinyl alcohol (9 ± 1%), methylketene (2 ± 1%), and acrolein (52 ± 5%). In addition to the stable carbonyl products, the secondary peroxy chemistry also generates OH or HO2 radical coproducts.

Atmospheric oxidation of HFE-7300 [n-C2F5CF(OCH3)CF(CF3)2] initiated by •OH/Cl oxidants and subsequent degradation of its product radical: a DFT approach.

To understand the atmospheric chemistry of hydrofluoroethers, we have studied the oxidation of a highly fluorinated compound n-C2F5CF(OCH3)CF(CF3)2 (HFE-7300) by OH/Cl oxidants. Here, we have employed M06-2X functional along with a 6-31 + G(d,p) basis set to obtain the optimized structures, various forms of energies, and different modes of frequencies for all species. We have characterized energies of all species on the potential energy surface, and it indicates that H-abstraction from n-C2F5CF(OCH3)CF(CF3)2 by Cl atom is kinetically more dominant than the H-abstraction reaction initiated by OH radical. In contrast, the calculated energy change (ΔrH°298 and ΔrG°298) results govern that OH-initiated H-abstraction reaction is highly exothermic and spontaneous compared to the Cl-initiated H-abstraction reaction. Rate constants are estimated using transition state theory as well as canonical variation transition state theory at the temperature range 200-1000 K and 1 atm pressure. The calculated rate constants of the H-abstraction channels are found to be in good agreement with the reported experimental rate constant at 298 K. Moreover, we have estimated the atmospheric lifetimes of HFE-7300 for the reaction with OH radical and Cl atom and are found to be 1.75 and 153.93 years, respectively. Additionally, the global warming potentials for HFE-7300 molecule are also estimated for 20-, 100-, and 500-year time horizons. Further, subsequent aerial oxidation of product radical (n-C2F5CF(OCH2)CF(CF3)2) in the presence of NO radical is performed, and it produced alkoxy radical via formation of peroxy radical. This alkoxy radical undergoes unimolecular decompositions via two different ways and formed n-C2F5CF(OCHO)CF(CF3)2 and n-C2F5CF(OH) CF(CF3)2 products.

An improved model to calculate equilibrium constants for formation of peroxy radical–water complexes

Recent experimental results show that the kinetics of some radical–radical reactions important for atmospheric pollution formation are faster when a radical–molecule complex forms as one step in the reaction mechanism. Calculated radical–molecule equilibria are needed to accurately describe the concentrations of complexes formed in these experiments as well as in the atmosphere. Here we report calculation of the equilibrium constant for complexation of hydroperoxy (HO2·) and 2-hydroxyethylperoxy radicals (HOCH2CH2O2·) with one water molecule by directly calculating the canonical partition function of the reactant and product species. We demonstrate the accuracy of the calculation using formation of the water dimer as a test case. Ab initio calculations provide the binding energy, rotational constants, and vibrational frequencies of both monomers and complexes. We demonstrate the failure of the harmonic approximation in the partition function for describing the low-frequency vibrational modes of the complexes. Instead, we model one dissociative hydrogen bond mode using a Lennard-Jones 6–3 potential and the other low-frequency vibrational modes using one- and twofold hindered rotors. The contributions of weakly bound states of the long-range dipole–dipole potential (Lennard-Jones 6–3) and of vibration–rotation coupling are not as important as the contribution of twofold hindered rotors. We also discuss methods for including multiple hydrogen-bonding configurations (local minima) when calculating equilibrium constants for formation of complexes.

Reaction mechanism and kinetics of the degradation of terbacil initiated by OH radical – A theoretical study

The reaction of terbacil with OH radical is studied by using electronic structure calculations. The reaction of terbacil with OH radical is found to proceed by H-atom abstraction, Cl-atom abstraction and OH addition reactions. The initially formed alkyl radical will undergo atmospheric transformation in the presence of molecular oxygen leading to the formation of peroxy radical. The reaction of peroxy radical with other atmospheric oxidants, such as HO2 and NO radicals is studied. The rate constant is calculated for the H-atom abstraction reactions over the temperature range of 200–1000 K. The results obtained from electronic structure calculations and kinetic study show that the H-atom abstraction reaction is more favorable. The calculated lifetime of terbacil is 24 h in normal atmospheric OH concentration. The rate constant calculated for H-atom abstraction reactions is 6 × 10−12, 4.4 × 10−12 and 3.2 × 10−12 cm3molecule−1s−1, respectively which is in agreement with the previous literature value of 1.9 × 10−12 cm3molecule−1s−1.

A theoretical investigation on the mechanism and kinetics of the gas-phase reaction of naphthalene with OH radical

The oxidation of naphthalene initiated by OH radical is studied by using density functional theory methods, M06-2X, ωB97XD and MPWB1K with 6-311G(d,p) basis set. The relative energy of the reactive species is also calculated at CCSD(T)/6-311G(d,p), CCSD(T)/cc-pVDZ, M06-2X/aug-cc-pVTZ and ωB97XD/aug-cc-pVTZ level of theories with the geometry optimized at M06-2X/6-311G(d,p) level of theory. The reactions were found to proceed via electrophilic addition of OH radical to the aromatic ring of naphthalene and H-atom abstraction from the aromatic ring of naphthalene by OH radical. The OH addition and H-atom abstraction reactions are occurring favorably at C1 position of naphthalene. The H-atom abstraction is dominant than OH addition reaction at high temperature (400–873 K). The reactions subsequent to the initial OH radical addition and H-atom abstraction reactions are studied in detail. The reaction of the initially formed intermediates with O2 results in the formation of peroxy radical and 1-naphthol. The isomerization of peroxy radical and reaction of peroxy radical with HO2, NO and NO2 leads to the formation of epoxynaphthalene, hydroperoxide adducts and nitro naphthalene. We have calculated the rate constant for favorable initial and secondary reactions. This study provides thermochemical and kinetic data for the atmospheric oxidation of naphthalene by OH radical and demonstrates the secondary reactions of naphthalene derivatives.