Acetylperoxyl Radicals Research Articles

Unusually improved peracetic acid activation for ultrafast organic compound removal through redox-inert Mg incorporation into active Co3O4

Peracetic acid (PAA) is increasingly used in advanced oxidation processes (AOPs) for water purification, yet there remains a critical need for highly-efficient and low-cost activators. Here, we constructed abundant oxygen vacancies (OVs) into redox-active Co3O4 by incorporating redox-inert Mg species (MgCoOx), achieving ultrafast degradation of sulfamethoxazole (SMX) via PAA activation. The MgCoOx/PAA system successfully degraded SMX within 3 min, with a removal rate 154.8 times higher than the Co3O4/PAA system, surpassing even previously reported single-atom catalysts. The primary active species identified was the acetylperoxyl radical (CH3C(O)OO•), with CoIV-oxo acting as a secondary active species that produced 18O-labeled sulfone compounds. We proposed a novel mechanism involving redox-inert Mg species that simultaneously strengthened PAA adsorption and activation. The enriched surface hydroxyl groups after Mg incorporation elevated the affinity for PAA binding. Meanwhile, the reduced Co average valence state and enhanced electron transfer capability facilitated PAA activation. This study offers an in-depth knowledge of redox-inert alkaline earth metals in PAA-AOPs.

Revisiting iodide species transformation in peracetic acid oxidation: unexpected role of radicals in micropollutants decontamination and iodate formation

Peracetic acid (PAA) is an alternative disinfectant for saline wastewaters, and hypohalous acids are typically regarded as the reactive species for oxidation and disinfection. However, new results herein strongly suggest that reactive radicals instead of HOI primarily contributed to decontamination during PAA treatment of iodine-containing wastewater. The presence of I- could greatly accelerate the micropollutants (e.g., sulfamethoxazole (SMX)) transformation by PAA. Chemical probes experiments and electron paramagnetic resonance analysis demonstrate acetylperoxyl radical rather than reactive iodine species primarily responsible for SMX degradation. The kinetic model was developed to further distinguish and quantify the contribution of radicals and iodine species, as well as to elucidate the transformation pathways of iodine species. Density functional theory calculations indicated that the nucleophilic attack of I- on the peroxide bond of PAA could form unstable O−I bond, with the transition state energy barrier for radical generation lower than that for HOI formation. The transformation of iodine species was regulated by acetylperoxyl radical to generate nontoxic IO3-, greatly alleviating the iodinated DBPs formation in saline wastewaters. This work provides mechanistic insights in radical-regulated iodine species transformation during PAA oxidation, paving the way for the development of viable and eco-friendly technology for iodide containing water treatment.

Unveiling the environmental significance of acetylperoxyl radical: Reactivity quantification and kinetic modeling.

Acetylperoxyl radical (CH3C(O)OO•) is among highly reactive organic radicals which are known to play crucial roles in atmospheric chemistry, aqueous chemistry and, most recently, peracetic acid (PAA)-based advanced oxidation processes. However, fundamental knowledge for its reactivity is scarce and severely hampers the understanding of relevant environmental processes. Herein, three independent experimental approaches were exploited for revelation and quantification of the reaction rates of acetylperoxyl radical. First, we developed and verified laser flash photolysis of biacetyl, ultraviolet (UV) photolysis of biacetyl, and pulse radiolysis of acetaldehyde, each as a clean source of CH3C(O)OO•. Then, using competition kinetics and selection of suitable probe and competitor compounds, the rate constants between CH3C(O)OO• and compounds of diverse structures were determined. The three experimental approaches complemented in reaction time scale and ease of operation, and provided cross-validation of the rate constants. Moreover, the formation of CH3C(O)OO• was verified by spin-trapped electron paramagnetic resonance, and potential influence of other reactive species in the systems was assessed. Overall, CH3C(O)OO• displays distinctively high reactivity and selectivity, reacting especially favorably with naphthyl and diene compounds (k ∼ 107-108 M-1 s-1) but sluggishly with N- and S-containing groups. Significantly, we demonstrated that incorporating acetylperoxyl radical-oxidation reactions significantly improved the accuracy in modeling the degradation of environmental micropollutants by UV/PAA treatment. This study is among the most comprehensive investigation for peroxyl radical reactivity to date, and establishes a robust methodology for investigating organic radical chemistry. The determined rate constants strengthen kinetic databases and improve modeling accuracy for natural and engineered systems.

Non-metallic Lewis acid sites enhance the activation of peracetic acid by cobalt-iron spinel for micropollutant decontamination: The critical role of Lewis acid sites as electron shuttles

Spinel is a common activator for peroxides, e.g., peracetic acid (PAA), in wastewater treatment, but suffers from insufficient electron transfer capacity at the active site and thus the limited activation capacity. Lewis acids are recognized for their capacity to interact with electrons and activate active sites, playing a crucial role in enhancing oxidant adsorption, shortening catalytic reaction distances, and increasing reaction kinetics. Herein, tellurium (Te), a typical Lewis acid, was introduced into cobalt-iron spinel (TCF) as an electron shuttler to improve the electron transfer capacity and thus boost the PAA activation. Results show that the degradation rate of sulfamethoxazole (SMX) with PAA activated via TCF is 4.0 times higher compared with cobalt-iron spinel (CF). Electron spin resonance (ESR) analysis and quenching experiments indicated acetylperoxyl radicals (CH3C(O)OO·) as the dominant reactive species in SMX degradation. Electrochemical tests revealed that the introduction of Lewis acid improved the charge transfer ability and charge utilization efficiency of the catalysts, exhibiting less electronic resistance and superior redox properties. Density functional theory (DFT) calculation suggests that the introduction of Lewis acid optimizes the electron distribution of the catalyst, and reduces the crucial reaction energy barriers for PAA activation. Additionally, the distinctive structural environment of Lewis acid renders TCF to exhibit excellent stability and resistance to complicated matrices in PAA activation. This work provides valuable insights into the Lewis acid regulating bimetallic catalyst performance and understanding the intrinsic interaction mechanisms, thereby establishing a critical groundwork for addressing prevailing wastewater treatment.

Efficient activation of peracetic acid by cobalt modified nitrogen-doped carbon nanotubes for drugs degradation: Performance and mechanism insight

Peracetic acid (PAA) has garnered significant attention as a novel disinfectant owing to its remarkable oxidative capacity and minimal potential to generate byproducts. In this study, we prepared a novel catalyst, denoted as cobalt modified nitrogen-doped carbon nanotubes (Co@N-CNTs), and evaluated it for PAA activation. Modification with cobalt nanoparticles (∼4.8 nm) changed the morphology and structure of the carbon nanotubes, and greatly improved their ability to activate PAA. Co@N-CNTs/PAA catalytic system shows outstanding catalytic degradation ability of antiviral drugs. Under neutral conditions, with a dosage of 0.05 g/L Co@N-CNT-9.8 and 0.25 mM PAA, the removal efficiency of acyclovir (ACV) reached 98.3% within a mere 10 min. The primary reactive species responsible for effective pollutant degradation were identified as acetylperoxyl radicals (CH3C(O)OO•) and acetyloxyl radicals (CH3C(O)O•). In addition, density functional theory (DFT) proved that Co nanoparticles, as the main catalytic sites, were more likely to adsorb PAA and transfer more electrons than N-doped graphene. This study explored the feasibility of PAA degradation of antiviral drugs in sewage, and provided new insights for the application of heterogeneous catalytic PAA in environmental remediation.

Transformation of 2,4,6-tribromophenol in the acetylperoxyl radical based oxidation process and formation of brominated byproducts

Acetylperoxyl radical (CH3C(O)OO•), as a peracetic acid (PAA) derived specific radical, has a promising future in environmental remediation. However, the research about the transformation mechanism of halogenated organics with CH3C(O)OO• and the subsequent formation of disinfection byproducts (DBPs) is still in the gap stage. 2,4,6-tribromophenol (2,4,6-TBP) was chosen as a target brominated compound to study its transformation and the formation mechanism of brominated byproduct in the neutral Co(II)/PAA system. The results showed that 94.7 % of 2,4,6-TBP with 50 µM was transformed and its corresponding pseudo-first-order kinetic constant is about 0.540 min−1. Interestingly, CH3C(O)OO• and reactive bromine species were found to dominate 2,4,6-TBP transformation. 2,4,6-TBP had the best transformation under neutral conditions· H2O2, HCO3–, and natural organic matter (NOM) significantly inhibited 2,4,6-TBP transformation, while Cl− had little effect on 2,4,6-TBP transformation. In addition, the main transformation pathways of 2,4,6-TBP, including debromination, C–C coupling, self-coupling, and CH3C(O)OO• cross-coupling were proposed based on the analysis of transformation products by LC-MS/MS and DFT theoretical calculations. In identified intermediates, there are some DBPs, including bromate, bromoform, and dibromoacetic acid. These DBPs increased gradually in approximately 24 h, moreover, Suwannee River NOM improved DBPs production. Therefore, the formation mechanism of DBPs during the treatment of brominated organics with CH3C(O)OO• was proposed for the first time, which extended the theory of the formation mechanism of DBPs in CH3C(O)OO•-based water treatment.

Chemiluminescence-assisted study on a peracetic acid-based advanced oxidation process activated with cobalt phosphide.

In recent years, the elimination of organic pollutants using advanced oxidation processes (AOPs) based on peracetic acid (PAA) has drawn increasing attention due to the high oxidative potential and low byproducts. However, to explore more efficient and stable PAA-based AOPs, there is still great room for study on the activation of PAA and degradation mechanism in the reaction process. In this study, a new PAA-based AOP activated by metal-organic framework-derived cobalt phosphide (CoP) and accompanied by chemiluminescence (CL) behaviour was explored. The CoP/PAA system could efficiently degrade 99.98% of RhB (20 mg L-1 ) within 5min at pH7 compared with the conventional Co3 O4 /PAA system (merely 17.29%), and the degradation process was matched well with the pseudo-first-order kinetic, and the kinetic constants was ~23.7 times higher than that of Co3 O4 (0.546 min-1 for CoP vs. 0.023 min-1 for Co3 O4 ). In the CoP/PAA/RhB process, the CL intensity was related to the concentration of 1 O2 , O2 •- and acetyl peroxyl radicals [CH3 C(O)OO• and CH3 C(O)O•]. Therefore, CL analysis, combined with quenching tests and electron paramagnetic resonance analysis, was used to study the degradation mechanism in detail, and 1 O2 was confirmed as the dominant contributor for the dye degradation.

The dual pathway mechanisms of peroxyacetic acid activation by CoMn2O4 spinel for efficient levofloxacin degradation

In this work, CoMn2O4 spinel catalyst was used for the first time to activate peroxyacetic acid (PAA) for the degradation of fluoroquinolone antibiotic levofloxacin (LVF). The results showed that the degradation efficiency of CoMn2O4/PAA system reached 93% within 15 min at initial LVF concentration of 20 mg/L. In addition, the bimetallic oxide CoMn2O4 with spinel structure showed significantly higher activation performance than the single metal oxide (Co3O4 and MnO2). The excellent PAA activation performance by CoMn2O4 was attributed to the redox cycles between Co3+/Co2+, Mn3+/Mn2+, and Mn4+/Mn3+, as well as the synergistic interaction between Co and Mn species. Radical quenching and electron paramagnetic resonance (EPR) spectra results indicated that acetylperoxyl radical (CH3C(O)OO·) was the dominant radical species, and this work highlighted that 1O2 played an important role in non-radical process in PAA-based AOPs for the first time. The production of 1O2 may be attributed to the interaction between reactive oxygen (O*) and PAA. In addition, the CoMn2O4/PAA system has been proved to have great stability and reusability, and exhibit selective degradation of organic pollutants which contained electron-rich groups. Overall, this work presents perspective for the application of antibiotic remediation by CoMn2O4/PAA process.

Co–Mn spinel oxides trigger peracetic acid activation for ultrafast degradation of sulfonamide antibiotics: Unveiling critical role of Mn species in boosting Co activity

Activation of peracetic acid (PAA) to generate powerful oxidizing species has become a promising advanced oxidation processes (AOPs) in wastewater treatment, yet the development of low-cost and high-performance activators is still a primary challenge. Herein, a range of Co–Mn spinel oxides (Co3-xMnxO4) with varying levels of Co and Mn were successfully elaborated, in which Co1.1Mn1.9O4 exhibited remarkable performance in PAA activation, outperforming most reported heterogeneous catalysts. Extensive quenching experiments and electron spin resonance (ESR) analysis indicated that acetylperoxyl radical (CH3C(O)OO●) was the predominated oxidizing species responsible for sulfamethoxazole (SMX) degradation. Density functional theory (DFT) calculations revealed that doping with Mn not only promoted the electron transfer and accelerated reduction of Co(III) to Co(II), but also lowered the energy barrier for PAA activation. Moreover, the prominent chemisorption and activation of PAA with Co1.1Mn1.9O4 was also benefitted from the significant role of Mn in optimizing the distribution of bonding and antibonding states on Co 3d orbitals. Unexpectedly, high levels of Cl−greatly facilitated SMX degradation due to the mass production of HOCl from the chain reactions of various radicals with Cl−. This work provides new insights into bimetallic activation of PAA, and the knowledge obtained will further advance the application of PAA-based AOPs.

Activation of Peracetic Acid with CuFe2O4 for Rhodamine B Degradation: Activation by Cu and the Contribution of Acetylperoxyl Radicals.

Advanced oxidation processes (AOPs) demonstrate great micropollutant degradation efficiency. In this study, CuFe2O4 was successfully used to activate peracetic acid (PAA) to remove Rhodamine B. Acetyl(per)oxyl radicals were the dominant species in this novel system. The addition of 2,4-hexadiene (2,4-HD) and Methanol (MeOH) significantly inhibited the degradation efficiency of Rhodamine B. The ≡Cu2+/≡Cu+ redox cycle dominated PAA activation, thereby producing organic radicals (R-O˙) including CH3C(O)O˙ and CH3C(O)OO˙, which accounted for the degradation of Rhodamine B. Increasing either the concentration of CuFe2O4 (0–100 mg/L) or PAA (10–100 mg/L) promoted the removal efficiency of this potent system. In addition, weakly acid to weakly alkali pH conditions (6–8) were suitable for pollutant removal. The addition of Humid acid (HA), HCO3−, and a small amount of Cl− (10–100 mmol·L−1) slightly inhibited the degradation of Rhodamine B. However, degradation was accelerated by the inclusion of high concentrations (200 mmol·L−1) of Cl−. After four iterations of catalyst recycling, the degradation efficiency remained stable and no additional functional group characteristic peaks were observed. Taking into consideration the reaction conditions, interfering substances, system stability, and pollutant-removal efficiency, the CuFe2O4/PAA system demonstrated great potential for the degradation of Rhodamine B.

Insights into a novel CuS/percarbonate/tetraacetylethylenediamine process for sulfamethazine degradation in alkaline medium

This work presents a novel CuS/percarbonate/tetraacetylethylenediamine (CuS/SPC/TAED) process for the degradation of sulfamethazine (SMT). Results indicated that the CuS/SPC/TAED process enabled the efficient generation of peracetic acid (PAA), which can be efficiently activated by CuS in alkaline reaction media, and 93.6% of SMT was degraded in 30 min. Mechanism study revealed that the available reactive oxygen species (ROS) including hydroxyl radical (•OH), carbonate radical (CO3•−), superoxide radical (O2•−), singlet oxygen (1O2), and organic radicals (R−O•). Among them, R−O• (acetyloxyl radical (CH3CO2•) and acetylperoxyl radical (CH3CO3•)) were confirmed to be the primary species that contributed to SMT degradation. Simultaneously, the role of sulfur species and carbonate ions were explored. It was found that the reductive O2•− and sulfur species rendered the efficient redox of Cu species. Besides, the effects of key influencing factors including SPC/TAED mole ratio, CuS dosage, initial pH, temperature, and nontarget matrix constituents on SMT degradation were examined. Finally, the degradation intermediates of SMT was identified, and the toxicity of these products was estimated by quantitative structure-activity relationship (QSAR) analysis. Overall, this work offers a new and simple strategy for antibiotic-polluted water remediation.

Hydrogen atom abstraction mechanism for organic compound oxidation by acetylperoxyl radical in Co(II)/peracetic acid activation system

Peracetic acid (PAA) has been widely used as an alternative disinfectant in wastewater treatment, and PAA-based advanced oxidation processes (AOPs) have drawn increasing attention recently. Among the generated reactive species after PAA activation, acetylperoxyl radical (CH3CO3•) plays an important role in organic compounds degradation. However, little is known about the reaction mechanism on CH3CO3• attack due to the challenging of experimental analysis. In this study, a homogeneous PAA activation system was built up using Co(II) as an activator at neutral pH to generate CH3CO3• for phenol degradation. More importantly, reaction mechanism on CH3CO3•-driven oxidation of phenol is elucidated at the molecular level. CH3CO3• with lower electrophilicity index but much larger Waals molecular volume holds different phenol oxidation route compared with the conventional •OH. Direct evidences on CH3CO3• formation and attack mechanism are provided through integrated experimental and theoretical results, indicating that hydrogen atom abstraction (HAA) is the most favorable route in the initial step of CH3CO3•-driven phenol oxidation. HAA reaction step is found to produce phenoxy radicals with a low energy barrier of 4.78 kcal mol−1 and free energy change of -12.21 kcal mol−1. The generated phenoxy radicals will undergo further dimerization to form 4-phenoxyphenol and corresponding hydroxylated products, or react with CH3CO3• to generate catechol and hydroquinone. These results significantly promote the understanding of CH3CO3•-driven organic pollutant degradation and are useful for further development of PAA-based AOPs in environmental applications.

Ferrate(VI)-peracetic acid oxidation process: Rapid degradation of pharmaceuticals in water

This paper presents, for the first time, an oxidation process of ferrate(VI) (Fe(VI), FeO42-)-peracetic acid (CH3C(O)OOH, PAA) to rapidly degrade micropollutants in water. The effect of concentrations of Fe(VI) and PAA showed that complete removal (degradation below detection limit) of a pharmaceutical, carbamazepine (CBZ, 10.0 µM), could be achieved in 60 s at a molar ratio of Fe(VI) to CBZ of 20.0 and 100.0 µM PAA under mild alkaline condition (pH 9.0). The results of the experiments performed in the presence of quenching agents, i.e., tert-butyl alcohol and 2,4-hexadiene, suggested that the iron-based reactive species (Fe(V)/Fe(IV)) and/or acetylperoxyl radicals (CH3C(O)OO●/CH3C(O)O●) were most likely responsible for the rapid oxidation of CBZ, rather than the hydroxyl radical (●OH). The major role of Fe(V)/Fe(IV) was supported by the high mass balance between loss of PMSO and formation of PMSO2 (>90%) during the oxidation of PMSO by Fe(VI)-PAA, which was not observed in the absence of iron species, i.e., PAA-Co(II)-PMSO system having acetylperoxyl radicals as reactive species. Significantly, the Fe(VI)-PAA oxidation process could degrade several other pharmaceuticals (i.e., sulfamethoxazole, sulfadimethoxine, trimethoprim, atenolol, propranolol, and caffeine) within 60 s, indicating its great potential in eliminating pollutants of emerging concern from water.

Activation of peracetic acid with cobalt anchored on 2D sandwich-like MXenes (Co@MXenes) for organic contaminant degradation: High efficiency and contribution of acetylperoxyl radicals

A novel Co@MXenes catalyst with unique structure was prepared through randomly anchoring cobalt on 2D sandwich-like MXenes to activate peracetic acid (PAA) for water decontamination. Compared to the conventional Co3O4 nanomaterial, the Co@MXenes catalyst exhibits superior performance for PAA activation under neutral condition. In-depth investigation revealed that the Ti3C2Tx MXene substrate triggered the ≡ Co(III)/≡ Co(II) cycle, which greatly enhanced the activation of PAA. The XPS and ICP techniques further demonstrated that Co@MXenes was highly stable with extremely low cobalt ion leaching and hence a superior reusability was obtained. Moreover, acetylperoxyl radical (CH3CO3) was identified as the primary radical species responsible for organic micropollutants degradation, exhibiting high selectivity towards contaminants containing electron-rich groups. The novel technology exhibited a high tolerance of Cl− and good performance in real wastewater treatment. This work provides a promising effective catalyst for PAA activation and will facilitate PAA application in wastewater decontamination.

Cobalt/Peracetic Acid: Advanced Oxidation of Aromatic Organic Compounds by Acetylperoxyl Radicals.

Peracetic acid (PAA) is increasingly used as an alternative disinfectant and its advanced oxidation processes (AOPs) could be useful for pollutant degradation. Co(II) or Co(III) can activate PAA to produce acetyloxyl (CH3C(O)O•) and acetylperoxyl (CH3C(O)OO•) radicals with little •OH radical formation, and Co(II)/Co(III) is cycled. For the first time, this study determined the reaction rates of PAA with Co(II) (kPAA,Co(II) = 1.70 × 101 to 6.67 × 102 M-1·s-1) and Co(III) (kPAA,Co(III) = 3.91 × 100 to 4.57 × 102 M-1·s-1) ions over the initial pH 3.0-8.2 and evaluated 30 different aromatic organic compounds for degradation by Co/PAA. In-depth investigation confirmed that CH3C(O)OO• is the key reactive species under Co/PAA for compound degradation. Assessing the structure-activity relationship between compounds' molecular descriptors and pseudo-first-order degradation rate constants (k'PAA• in s-1) by Co/PAA showed the number of ring atoms, EHOMO, softness, and ionization potential to be the most influential, strongly suggesting the electron transfer mechanism from aromatic compounds to the acetylperoxyl radical. The radical production and compound degradation in Co/PAA are most efficient in the intermediate pH range and can be influenced by water matrix constituents of bicarbonate, phosphate, and humic acids. These results significantly improve the knowledge regarding the acetylperoxyl radical from PAA and will be useful for further development and applications of PAA-based AOPs.

Peroxyl radical reactions with carotenoids in microemulsions: Influence of microemulsion composition and the nature of peroxyl radical precursor

The reactions of acetylperoxyl radicals with different carotenoids (7,7′-dihydro-β-carotene and ζ-carotene) in SDS and CTAC microemulsions of different compositions were investigated using laser flash photolysis (LFP) coupled with kinetic absorption spectroscopy. The primary objective of this study was to explore the influence of microemulsion composition and the type of surfactant used on the yields and kinetics of various transients formed from the reaction of acetylperoxyl radicals with carotenoids. Also, the influence of the site (hydrocarbon phases or aqueous phase) of generation of the peroxyl radical precursor was examined by using 4-acetyl-4-phenylpiperidine hydrochloride (APPHCl) and 1,1-diphenylacetone (11DPA) as water-soluble and lipid-soluble peroxyl radical precursors, respectively. LFP of peroxyl radical precursors with 7,7′-dihydro-β-carotene (77DH) in different microemulsions gives rise to the formation of three distinct transients namely addition radical (λmax=460nm), near infrared transient1 (NIR, λmax=700nm) and 7,7′-dihydro-β-carotene radical cation (77DH•+, λmax=770nm). In addition, for ζ-carotene (ZETA) two transients (near infrared transient1 (NIR1, λmax=660nm) and ζ-carotene radical cation (ZETA•+, λmax=730–740nm)) are generated following LFP of peroxyl radical precursors in the presence of ζ-carotene (ZETA) in different microemulsions. The results show that the composition of the microemulsion strongly influences the observed yield and kinetics of the transients formed from the reactions of peroxyl radicals (acetylperoxyl radicals) with carotenoids (77DH and ZETA). Also, the type of surfactant used in the microemulsions influences the yield of the transients formed. The dependence of the transient yields and kinetics on microemulsion composition (or the type of surfactant used in the microemulsion) can be attributed to the change of the polarity of the microenvironment of the carotenoid. Furthermore, the nature of the peroxyl radical precursor used (water-soluble or lipid-soluble peroxyl radical precursors) has little influence on the yields and kinetics of the transients formed from the reaction of peroxyl radicals with carotenoids. In the context of the interest in carotenoids as radical scavenging antioxidants, the fates of the addition radicals (formed from the reaction of carotenoid with peroxyl radicals) and carotenoid radical cations are discussed.

A practical implementation of semi-classical transition state theory for polyatomics

An algorithm is described for computing rate constants with semi-classical transition state theory (SCTST) formulated by Miller and coworkers. SCTST incorporates non-separable coupling among all degrees of freedom and multi-dimensional quantum mechanical tunneling along the curved reaction path. The algorithm, which is practical for reactants containing dozens of atoms, predicts both microcanonical and canonical rate constants. In addition, the quantum chemistry code CFOUR has been extended to efficiently compute fully-coupled vibrational anharmonicities for transition states at the CCSD(T) level of theory. The new methods are demonstrated for the H 2 + OH reaction and the ‘tail-biting’ isomerization of acetylperoxyl radical.

Laser flash photolysis of new water-soluble peroxyl radical precursor

Laser flash photolysis (LFP) of commercially available water-soluble 4-acetyl-4-phenylpiperidine hydrochloride (APP) in the presence of oxygen gives rise to acetylperoxyl radical (ACP). The formation of ACP during LFP of APP has been established by determining their rate constants with ABTS 2− in methanolic and aqueous solutions (0.15 ± 0.02 and 2.0 ± 0.2 × 10 9 M −1 s −1, respectively). These rate constants are in agreement with those reported in the literature. The reactivity of ACP was investigated with crocin in aqueous and methanolic solutions. Moreover, the rate constant for the reaction of ACP with nitrite anion (NO 2 −) in aqueous solution was determined to be 4.45 ± 0.4 × 10 6 M −1 s −1.Overall, APP can be used as ACP precursor in aqueous and organic solutions using the more accessible LFP technique than the normally used (especially in aqueous solutions) pulse radiolysis technique.

Master equation simulations of competing unimolecular and bimolecular reactions: application to OH production in the reaction of acetyl radical with O2

Master equation calculations were carried out to simulate the production of hydroxyl free radicals initiated by the reaction of acetyl free radicals (CH3(C=O).) with molecular oxygen. In particular, the competition between the unimolecular reactions and bimolecular reactions of vibrationally excited intermediates was modeled by using a single master equation. The vibrationally excited intermediates (isomers of acetylperoxyl radicals) result from the initial reaction of acetyl free radical with O2. The bimolecular reactions were modeled using a novel pseudo-first-order microcanonical rate constant approach. Stationary points on the multi-well, multi-channel potential energy surface (PES) were calculated at the DFT(B3LYP)/6-311G(2df,p) level of theory. Some additional calculations were carried out at the CASPT2(7,5)/6-31G(d) level of theory to investigate barrierless reactions and other features of the PES. The master equation simulations are in excellent agreement with the experimental OH yields measured in N2 or He buffer gas near 300 K, but they do not explain a recent report that the OH yields are independent of pressure in nearly pure O2 buffer gas.

Laser spectrometry and kinetics of selected elementary reactions of the acetonyl radical

The laser induced fluorescence excitation spectrum (LIF) and the ultraviolet absorption spectrum (TA) of the acetonyl radical (CH3C(O)CH2) were remeasured by using the time-resolved fast discharge flow (DF) and laser flash photolysis (LP) experimental techniques (T = 298 K). The absorption cross section of σ(acetonyl, 207 nm) = (3.16 ± 0.61) × 10−18 cm2 molecule−1 was determined calibrated against the acetyl-peroxyl radical (CH3C(O)O2) in LP/TA measurements. The kinetics of the reactions of CH3C(O)CH2 with the open shell reaction partners O2 (1), NO (2), NO2 (3) and H (4) were studied by using the DF method with LIF detection of the acetonyl radical at 298 ± 1 K and 2.85 ± 0.05 mbar He pressure. The rate constants for the overall reactions were determined in units of cm3 molecule−1 s−1 to be k1 = (3.49 ± 0.51) × 10−13, k2 = (1.04 ± 0.19) × 10−11, k3 = (3.25 ± 0.65) × 10−11 and k4 ≥ 3 × 10−10 with 2σ accuracy given. The acetonyl radical was found to react similarly to alkyl radicals by comparison with literature results. A reduced reactivity was observed toward O2 and NO that might be attributed to the resonance stabilisation of the acetonyl radical. No such effect was observed for the NO2 and H atom reactions.