Mechanism Of Photochemical Transformation Research Articles

Photochemical transformation of anthracene (ANT) in surface soil: Chlorination and hydroxylation

To reveal the fate of anthracene (ANT) in soil, the photodegradation behavior of ANT was systematically studied using SiO2 to simulate a soil environment. Under xenon lamp irradiation, more than 90% of ANT loaded on SiO2 could be removed after 240 min. Moreover, the effects of water content, chloride ions (Cl-) and humic acid (HA) were examined. It was found that the presence of water and HA can significantly inhibit the photolysis of ANT on SiO2, while the addition of chloride alone has no obvious effect. However, when water is present, the inhibition effect of chloride became more obvious. According to radical quenching experiments and electron paramagnetic resonance (EPR) spectra, hydroxyl radicals (•OH) and chlorine radicals (Cl•) were formed in the system. Possible reaction pathways were speculated based on products identified by mass spectrometry. ANT was attacked by •OH to form hydroxylated products, which can be further hydroxylated and oxidized with the final formation of ring-opening products. ANT directly excited by light may also react with Cl• to produce chlorinated polycyclic aromatic hydrocarbons (Cl-PAHs). Finally, the experimental results were verified on real soil. This study provides important information for understanding the photochemical transformation mechanism of ANT at the soil/air interface.

Photochemical transformation mechanisms of dissolved organic matters (DOM) derived from different bio-stabilization sludge

Bio-stabilization sludge contains numerous dissolved organic matter (DOM) that could enter aquatic environments by soil leaching after sludge land use, but a clear understanding of their photochemical behavior is still lacking. In this study, we systematically investigated the photoactivity and photochemical transformation of aerobic composting sludge-derived DOM (DOMACS) and anaerobic digestion sludge-derived DOM (DOMADS) by using multispectral analysis coupled with Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). The results indicated that DOMACS and DOMADS have a higher proportion of highly unsaturated and phenolic compounds (HuPh)with high DBEwa, but the different polyphenols (Polyph) abundance of them, causing the different photoactivity between them. DOMACS had much higher apparent quantum yields (AQY) for triplet states of dissolved natural organic matter (3DOM*) and hydroxyl radical (•OH) but slightly lower AQY for singlet oxygen (1O2) than DOMADS under simulated sunlight conditions. As the irradiation time increased, HuPh and Polyph (associated with humic-like substances) contained in DOMACS (DOMADS) decreased by 12.0% (14.1%) and 3.0% (0.2%), respectively, with concurrent decrease in average molecular weight and aromaticity moieties, resulting in more generation of aliphatic compounds. Furthermore, based on 27 types of photochemical transformation reactions, DOMACS containing higher fractions of O10-15 and N1-3Oy class preferred dealkyl group and carboxylic acid reactions, whereas DOMADS composed of more N4Oy and S2Oy fragments preferred oxygen addition and anmine reactions. Consequently, photochemical transformations reduced the Cd (II) ion activity in the presence of DOMACS (DOMADS). This study is believed to unveil the photochemical transformation of bio-stabilization sludge-derived DOM and its impact on pollutants' fate in the aquatic environment.

Elucidating photolysis mechanisms of ketamine by quantum chemical calculations

Ketamine (KET) is a common psychoactive substance, but its photochemical transformation mechanism in aqueous environment has not been elucidated. Density functional theory and time-dependent density functional theory were used to investigate the photodegradation process of KET in water. The calculation results show that during direct photodegradation: The photodegradation reaction of KET in the S1 state is the cleavage of the nitrogen atom connected to the cyclohexanone group and the methyl bond; KET in the T1 state can be photodechlorinated. During indirect photodegradation: Electron rearrangement occurs after the reaction of KET with fluorenone (FL), thereby sensitizing and degrading KET. Cleavage of nitrogen and methyl bonds; photodechlorination and FL degradation of KET are the main reactions during photodegradation.

Photochemical transformation of hexachlorobenzene (HCB) in solid-water system: Kinetics, mechanism and toxicity evaluation

As one of the first batch of persistent organic pollutants (POPs) included in Stockholm Convention, hexachlorobenzene (HCB) has attracted great attention because of its wide occurrence and great environmental risks. Considering the easy adsorption of HCB on solids and the complexity of natural particles, we systematically investigated the photodegradation of HCB on the surface of silica gel (SG) in aqueous solution in this work to reveal its fate in natural waters. Under mercury lamp irradiation, more than 90% of HCB loaded on SG could be removed after 240 min. Moreover, the effects of solution pH and water constituents were examined, and results showed that the presence of NO2−, NO3−, Fe3+ and humic acid (HA) significantly inhibited the reaction due to the scavenging of ROS and/or competitive absorption of light. According to radical quenching experiments and electron paramagnetic resonance (EPR) spectra, hydroxyl radicals and singlet oxygen generated on the surface of SG could participate in the transformation of HCB, but •OH played a dominant role. Based on products identified by high performance liquid chromatography-mass spectrometry (HPLC-MS) and gas chromatography-mass spectrometry (GC-MS), two main pathways were proposed for the removal of HCB, including dechlorination and hydroxylation which represent direct and indirect photodegradation, respectively, and the occurrence of these two reactions was further supported by density functional theory (DFT) calculations. From the quantitative analysis of penta-chlorobenzene, it was estimated that dechlorination and hydroxylation contributed to approximately 44.4% and 55.6% of initial HCB degradation, respectively. Furthermore, toxicity predictions by the ecological structure–activity relationship model (ECOSAR) suggested that the toxicity of HCB was decreased in the photodegradation process. This study would provide important information for understanding the photochemical transformation mechanism of HCB at the solid/water interface.

Mechanisms, kinetics and eco-toxicity assessment of singlet oxygen, sulfate and hydroxyl radicals-initiated degradation of fenpiclonil in aquatic environments

Fenpiclonil is an agricultural phenylpyrrole fungicide, which raise the concern about its ecotoxicological effects. In this paper, we investigate the indirect photochemical transformation mechanisms, environmental persistence and eco-toxicity of fenpiclonil initiated by various active oxidants (1O2, •OH and SO4•‾) in aquatic environments. The results shown that 1O2 can react with pyrrole ring by cycloaddition pathways to form the endo-peroxides. In addition, •OH and SO4•‾ initial mechanisms are calculated, suggesting that •OH-initiated mechanisms play a dominant role in the degradation process of fenpiclonil at high rate constants (2.26 ×109 M−1 s−1, at 298 K). The kinetic calculation results indicate that high temperature is more favorable for the degradation of fenpiclonil. To better understand the adverse effects of the transformation products formed during the subsequent reaction of •OH-adduct IM10, the computational toxicology has been used for the toxicity estimation. The results show that aquatic toxicity of these products decrease with degradation process, especially the decomposition products (TP3 and TP4). However, TP1 and TP2 are still toxic and developmental toxicant. The study provides guidance for further experimental research and industrial application of fungicide degradation from the perspective of theoretical calculation.

Mechanistic insights into toluene degradation under VUV irradiation coupled with photocatalytic oxidation

Volatile organic compounds (VOCs) exists ubiquitously in chemical industries and were regarded as major contributors to air pollution, which should be strictly regulated. Vacuum ultraviolet irradiation coupled with photocatalytic oxidation (VUV-PCO) has been considered as an efficient approach to VOCs removal due to high-energy photons which could break down VOCs directly and be absorbed by photocatalysts to generate free radicals for further oxidation. However, the photochemical transformation mechanisms of VOCs have not been fully revealed. Herein, we systematically analyzed the intermediates using proton-transfer-reaction mass spectrometer (PTR-MS) to explore the transformation mechanisms of toluene degradation in VUV and VUV-PCO processes. VUV-PCO process displayed superior toluene degradation efficiency (50 %) and mineralization efficiency (65 %) compared with single VUV photolysis (35 %) and UV photocatalysis (5 %). TiO2 was deeply involved into CO2 generalization by amplifying the advantages of VUV system and further mineralizing the intermediates. In VUV and VUV-PCO processes, O2 participation changed the intermediates distribution by increasing multiple oxygenated products, while the introduction of water contributed to the formation and degradation of most intermediates. A possible degradation mechanism of toluene under VUV irradiation combined with TiO2 was proposed. This study provides a deep mechanistic insight into VOCs degradation by VUV-PCO process.

Transformation of amino acid tyrosine in chromophoric organic matter solutions: Generation of peroxide and change of bioavailability.

Studying the photochemical transformation mechanism of dissolved free amino acids (DFAA) in chromophoric dissolved organic matter (CDOM) solution facilitates the understanding of DFAA's environmental fate and bioavailability change upon solar irradiation in natural surface waters. Tyrosine oxidation product (Tyr-OH) was synthesized to quantify the primary transformation product (tyrosine peroxide, Tyr-OOH) in CDOM solution. Both reactions between superoxide radical anion (O2-) and tyrosine radical (Tyr) and between singlet oxygen (1O2) and tyrosine (Tyr) yield Tyr-OOH, which is subsequently transformed into Tyr-OH. The reaction between O2- and Tyr not only generated Tyr-OOH but also caused the regeneration of Tyr. O2- and 1O2 contributed 30-44% to Tyr's transformation in CDOM solutions at pH 8.0, in which 1O2 oxidation accounted for 6-11%. The contribution of O2- to Tyr's phototransformation process was the difference between the total contribution of O2- and 1O2 and the individual contribution of 1O2. Compared with the fast assimilation of Tyr, Tyr-OH was stable in natural water under dark incubation, indicating that phototransformation decreased the bioavailability of Tyr.

Photoinduced inhibition of DNA repair enzymes and the possible mechanism of photochemical transformations of the ruthenium nitrosyl complex [RuNO(β-Pic)2(NO2)2OH

In this work we have demonstrated that the ruthenium nitrosyl complex [RuNO(β-Pic)2(NO2)2OH] is suitable for investigation of the inactivation of DNA repair enzymes in vitro. Photoinduced inhibition of DNA glycosylases such as E. coli Endo III, plant NtROS1, mammalian mNEIL1 and hNEIL2 occurs to an extent of ≥90% after irradiation with the ruthenium complex. The photophysical and photochemical processes of [RuNO(β-Pic)2(NO2)2OH] were investigated using stationary and time-resolved spectroscopy, and mass spectrometry. A possible mechanism of the photo-processes was proposed from the combined spectroscopic study and DTF calculations, which reveal that the photolysis is multistage. The primary and secondary photolysis stages are the photo-induced cleavage of the Ru-NO bond with the formation of a free nitric oxide and RuIII complex followed by ligand exchange with solvent. For E. coli Endo III, covalent interaction with the photolysis product was confirmed by UV-vis and mass spectrometric methods.

Photochemistry of Dithiocarbamate Cu(S2CNEt2)2 Complex in CHCl3. Transient Species and TD-DFT Calculations.

Nanosecond laser flash photolysis was used to study the mechanism of photochemical transformations of the diethyldithiocarbamate Cu(II) complex (Cu(dtc)2, where dtc- ≡ -S2CNEt2 anion) in chloroform solutions. The electron transfer from the excited Cu(dtc)2 complex to a solvent molecule leads to the appearance of the primary intermediate, the [ClCu(dtc)(dtcCHCl2)] complex, where a dtcCHCl2 molecule is coordinated with a copper ion via one sulfur atom. In the fast reaction (k = 2.1 × 109 M-1 s-1) with Cu(dtc)2, this complex forms a long-lived dimer [ClCu(dtc)(dtcCHCl2)Cu(dtc)2]. This intermediate decays during several seconds (k = 5.6 × 10-2 s-1) into the final product, i.e., a diamagnetic dimer [ClCu(dtc)Cu(dtc)2]. To determine the structure of intermediate complexes the quantum chemical calculations were carried out using DFT, TD-DFT, and PCM (Polarizable Continuum Model) methods.

Bioaccumulation and ecotoxicity increase during indirect photochemical transformation of polycyclic musk tonalide: A modeling study

Polycyclic musks (PCMs) have recently caused a worldwide environmental concern due to their bioaccumulation potential and ecotoxicological effects. Herein, the OH-initiated indirect photochemical transformation mechanism, environmental fate and ecotoxicity of PCMs (by taking tonalide as an example) were theoretically studied. Results show that tonalide can be degraded readily through OH-addition and H-abstraction pathways, with total rate constants of 6.03 × 109–15.8 × 109 M−1 s−1. The OH-addition pathways were dominant at low temperature (<∼287 K), whereas H-abstraction was the dominant pathway at high temperature. Further, the bioconcentration factors (BCF) and aquatic toxicities to fish of all transformation products from H-abstraction pathways were smaller than tonalide. In contrast, these values of most intermediates from OH-addition pathways were up to 8 times higher than tonalide. Particularly, the resultant phenolic product PC1 had a BCF of 5590 L/kg wet-wt, which exceeds the cutoff criterion set for the typically persistent organic pollutants as critically bioaccumulative. Notably, PC1 would mainly be produced under anaerobic aquatic conditions at low temperatures. Therefore, particular attention should be paid to the indirect photochemical products and parental PCMs, particularly the intermediates from OH-addition pathway.

Indirect photochemical transformations of acyclovir and penciclovir in aquatic environments increase ecological risk.

Acyclovir and penciclovir, 2 antiviral drugs, are increasingly detected in aquatic environments. The present study explores the natural photochemical transformation mechanisms and fate of these drugs, examining direct and indirect photochemical transformation under simulated sunlight irradiation. The 2 antiviral drugs are photostable under certain conditions but significantly degrade in the presence of chromophoric dissolved organic matter (DOM). The degradation rate associated with the drugs' indirect photochemical transformation scaled with chromophoric DOM concentration. Quenchers and sensitizers were used to identify indirect photochemical transformation mechanism. Results suggested that both pharmaceuticals could be transformed by reacting with (1)O2, (•)OH, and excited chromophoric DOM. The (1)O2 played an important role in indirect photochemical transformation. Furthermore, the reaction kinetics between their substructural molecules, guanine, isocytosine, and imidazole, with different reactive oxygen species were evaluated to determine which substrate functionalities were most susceptible to singlet oxygenation. Imidazole was identified as the reaction site for (1)O2, and preliminary (1)O2 oxidation mechanisms were further evaluated based on liquid chromatographic-tandem mass spectrometric results. Finally, aquatic ecotoxicity assessment of phototransformed solutions revealed that the degradation of acyclovir and penciclovir may not ultimately diminish environmental risk because of either formation of more toxic intermediates than parent pharmaceuticals or some synergistic effects existing between the intermediates.

Photostimulated changes in WO3 nanosized films

During the irradiation of WO3 films d = 7–160 nm thick by light at λ = 320 nm (I = (1.5–7) × 1015 quantum cm−2 s−1), absorption band at λ = 850 nm appeared along with absorption band edge shift to shorter waves. The subsequent irradiation of samples at λ = 850 nm caused the disappearance of the longwave absorption band. The intrinsic absorption edge of WO3 films was determined (λ = 320 nm). The degree of transformations of WO3 films increased under atmospheric conditions as the intensity of incident light and the time of irradiation (1–140 min) grew and as film thickness decreased. A mechanism of photochemical transformations of WO3 films was suggested. This mechanism included the generation of electron-hole pairs, the recombination of part of nonequilibrium charge carriers, the formation of [eV a 2+ e] centers, and the isolation of photolysis products.

Mechanism and efficiency of photochemical transformation of ≡SiNNO• radicals grafted to activated Aerosil surface: Calculation of potential energy surface elements for the lower excited state of the radical

Assignment of the bands in the optical absorption spectrum of ≡SiNNO• radicals grafted to activated Aerosil surface was performed. A photochemical transformation mechanism was proposed, and the quantum yields of the reaction of ≡SiNNO• radicals relevant to the long-wavelength absorption band at 77 K were determined in the absence and presence of adsorbed CO2 and SF6 on the surface. For the model radical F3SiNNO•, the parameters characterizing the equilibrium geometry of excited states and the geometry of transition states involved in the reaction were determined by quantum-chemical calculations.

Correlation between Macro‐ and Microstructural Changes in Ge:SiO2 and SiO2 Glasses under Intense Ultraviolet Irradiation

The photochemical transformation mechanism of defects in germanosilicate and silica glasses under ultraviolet (UV) laser irradiation has been investigated based on the changes in Raman spectra before and after irradiation. Two types of silica glasses, fused silica (type I) and dry synthetic silica (type IV), and germanosilicate optical fiber preforms were irradiated by intense UV photons from excimer lasers. Spectral changes in optical absorption and Raman spectra were examined to clarify a correlation between the microscopic defect formation and the macroscopic structural changes causing a photorefractive effect. Successive generation of E′ centers through divalent centers is closely correlated with changes in Raman spectra, indicating that large structural changes in the glass network involved in this process would be the origin of photon‐induced densification of the glasses. In addition, it has been proposed that the successive generation of E′ centers is mediated by transient divalent centers converted from relaxed cation homobondings.

Reaction mechanism and structure of products in photochemical transformations of copper(ii) heptafluorobutyrate in methanol

The mechanism of photochemical transformations of copper(II) heptafluorobutyrate in methanol is studied. The structure of products and intermediate compounds is investigated. Photolysis occurs as a two- step process. The first step involves changes in the coordination sphere of the Cu(II) ion. The second step is photoreduction of Cu(II) forming metallic copper and Cu(II) fluoride as a complex with methanol. In the presence of oxygen, the first step is reversible. A scheme of these processes is suggested.

One-electron photooxidation of carbazole in the presence of carbon tetrachloride. Part II. Carbon tetrachloride as a reaction medium. Use of ammonia after irradiation and during irradiation

In part I of this series of papers we proposed the mechanism of electron transfer as the primary photochemical reaction in the carbazole – carbon tetrachloride system along with a secondary photochemical reaction initiated by transformations of the radical cation of carbazole in the solvent cage resulting in intermediates:[Formula: see text]In this paper we discuss the influence of ammonia, used after and during irradiation, on the mechanism of secondary transformation and the formation of thermodynamically stable products in the system studied. Such compounds as N-cyanocarbazole, 1-cyanocarbazole, and 3-cyanocarbazole have been formed as the main products during neutralization of the photolyte solution by ammonia gas. The mechanism of formation of these compounds has been explained by the chemical reaction of ammonia with cations α and γi. If ammonia is present in the solution of carbazole in CCl4 during irradiation, such products as N,N′-dicarbazyl and N-cyanocarbazole are mainly formed along with 3-(N-carbazyl)carbazole, 3,9-di-(N-carbazyl)carbazole, and N-cyano-3-(N-carbazyl)carbazole. In such a case, reactions of radicals β are the determining factors in the secondary photochemical transformations. Radicals β are formed by the reaction involving ammonia with radical cations of carbazole. All the results in this paper have been discussed taking under consideration the influence of the reaction media on the mechanism of photochemical transformation of carbazole.

Fluorescence Excitation and Photodecomposition of the First Excited Singlet Cyclobutanone (1A2): A Study of Predissociation of and Collisional Energy Transfer from the Vibronically Selected Species

Fluorescence emission quantum yields (ΦF) from the first excited singlet state of cyclobutanone (CB) in the gas phase have been measured in the exciting wavelength (λex) range of 3050–3350 Å at ∼0.5 Å resolution. The value of ΦF at 5 torr is ∼0.002 above λex=3190 Å, but it suffers a sharp decline below this wavelength because of the onset of molecular predissociation, e.g., ΦF&amp;lt;4×10−6 for λex&amp;lt;3100 Å. Fluorescence stabilization phenomena are studied by addition of C3H8 up to 2200 torr. A simple kinetic treatment of the data provides (i) the estimated singlet lifetime value (τs) of 10−9−10−12 sec below λex=3140 Å; and (ii) the collision efficiency value of ∼0.5 for the upward transition as well as for the downward transition (of vibrational energy transfer). The primary photochemical transformation mechanism for the CB(S1*) is discussed in light of the new estimated lifetime information.

Tracer Study of Photochemically Excited Cyclobutanone-2-t and Cyclobutanone. II. Detailed Mechanism, Energetics, Unimolecular Decomposition Rates, and Intermolecular Vibrational Energy Transfer

A detailed photochemical transformation mechanism for the electronically excited cyclobutanones is proposed. It is inferred that the quantum yield of the radiationless, internal conversion process (S ↝ S0) is significantly large and varies from 0.7 at 3130-Å to 0.5 at 2480-Å excitation, while the remaining quantum yield is being attributed to the competing intersystem crossing process (S1 ↝ T1). Energetic and mechanistic differences of two nondegenerate photochemical decomposition pathways are discussed and compared to the singular thermal decomposition pathway. The rates of internal conversion, intersystem crossing (S1 ↝ T1), and the decomposition of CB*(T1) could be greater than 1 × 1011 sec−1 since no observable perturbations of these rate processes by various gases appear at a gas kinetic collision frequency of 1 × 1010 sec−1. The highly vibrationally excited CB**(S0) produced by an internal conversion process following the monochromatic photoactivation of the ground state CB(S0) decomposes unimolecularly. The observed specific rate constant k(E) of CB**(S0) for its decomposition varies from 0.7 × 1010 sec−1 at E = 93 kcal/mole to 5 × 1010 sec−1 at E = 114 kcal/mole, within the limitations of the strong collision assumption, and these experimental values of k(E)'s are in good agreement with the values of k(E) obtained with an approximate RRKM calculation. Efficiencies per collision of deactivating CB**(S0) with E = 93 kcal/mole have been determined for monatomic gases (He, Ne, Ar, Kr, and Xe), diatomic gases (H2, D2, and O2), and organic molecules (C3H6 and C3H8), and they vary from 0.06 for He to 1.00 for C3H6.

The Photochemical Reaction Products of Indole in Sunlight

From the transformation products of indole by the action of sunlight, a crystalline material, m.p. 245° (dec.) was isolated and identified as 2, 2-diindyl-ϕ-indoxyl (II), not being the ether of indoxyl proposed by B. Oddo.The identification of anthranilic acid (III) which occurred simultaneously in the products was performed by separation using counter-current process, followed by the paper chromatographic and spectrophotometric methods.These results indicate a possible mechanism of photochemical transformation in which both oxidative degradation and oxidative addition took place.