Quantitative structure-property relationships on photodegradation of polybrominated diphenyl ethers

Photodegradation of 1,3,5-Tris-(2,3-dibromopropyl)-1,3,5-triazine-2,4,6-trione and decabromodiphenyl ethane flame retardants: Kinetics, Main products, and environmental implications

Photodegradation has been demonstrated as one of the important environmental factors affecting the fate of contaminants such as brominated flame retardants (BFRs). However, a number of emerging BFRs, particularly those with high bromine substitution, have rarely been investigated for their photodegradation kinetics. Our study evaluated photodegradation of two highly brominated FRs, 1,3,5-tris-(2,3-dibromopropyl)-1,3,5-triazine-2,4,6-trione (TDBP-TAZTO) and decabromodiphenyl ethane (DBDPE), under various conditions. The results indicated that the degradation kinetics was affected by UV irradiation wavelength, intensity, solvent type, as well as the structural characteristics. TDBP-TAZTO exhibited degradation half-lives (t_1/2) of 23.5-6931 min under various UV irradiation conditions and 91.2 days under natural sunlight. Its degradation was much slower than that of DBDPE which exhibited t_1/2 of 0.8-101.9 min under UV and 41.3 min under natural sunlight. A variety of degradation products were detected as a result of different breakdown pathways. This indicated that photodegradation could substantially influence the fate of these highly brominated FRs, resulting in a cocktail of degradation products as environmentally occurring contaminants. This could also complicate the evaluation of the ecological risks of these target flame retardants, given that degradation products generally possess physicochemical properties and biological effects different from their parent chemicals.

Photodegradation of polychlorinated diphenyl sulfides (PCDPSs) under simulated solar light irradiation: Kinetics, mechanism, and density functional theory calculations

The direct photolysis of 25 individual polychlorinated diphenyl sulfides (PCDPSs) substituted with 1-7 chlorine atoms was investigated using a 500-W Xe lamp. Photolysis of PCDPSs followed pseudo-first-order kinetics, with the higher chlorinated diphenyl sulfides generally degrading faster than the lower chlorinated congeners. A quantitative structure-activity relationship model to predict the photolysis rates of PCDPSs was developed using 16 fundamental quantum chemical descriptors. We found that the substitution pattern for chlorine atoms, the dipole moment, and E_LUMO - E_HOMO were major factors in the photolysis of PCPDSs. The reaction kinetics, products, and photodegradation pathways of 2,2',3',4,5-pentachlorodiphenyl sulfide (PeCDPS) suggest hydroxylation, direct photooxidation, the C-S bond cleavage reaction, and hydroxyl substitution were mainly involved in the photodegradation process, leading to the formation of 13 intermediates, detected by an electrospray time-of-flight mass spectrometer. The initial reaction sites of PCDPSs under photolysis were rationalized by density functional theory calculations. Anions (Cl^-, SO₄^2-, NO₃^-, and HCO₃^-) and Co²⁺ had no influence on the removal of PeCDPS, while Fe³⁺, Cu²⁺, and HA decreased the photolysis efficiency of PeCDPS. This report is the first to develop a logk quantitative structure-property relationships (QSPR) model of 25 PCDPSs and to describe mechanistic pathways for the photolysis of PeCDPS.

Peculiar and rapid photocatalytic degradation of tetrabromodiphenyl ethers over Ag/TiO2 induced by interaction between silver nanoparticles and bromine atoms in the target

As a typical moderately-brominated diphenylethers, 2,2',4,4'-tetrabromodiphenyl ether (BDE47) is hardly debrominated by a conventional TiO2-mediated photocatalysis. However, its reductive debromination was rapid achieved over silver nanoparticle-loaded TiO2 (Ag/TiO2) in UV-irradiated anoxic acetonitrile-water within 13 min. An "Ag-promoted electron transfer and C-Br cleavage" concept was proposed based on experimental results and density functional theory calculations. Ag(0) exerted affinity interaction with bromine atoms, and the storing of electrons on Ag(0) increased the binding interaction, which elongated the C-Br bond of BDE47 and facilitated its cleavage. The initiating of the BDE47 debromination on Ag(0) required an induction period to enrich a critical amount of electrons, leading to a stronger driving force for both injecting electron to BDE47 and stretching the C-Br bond. Stronger photo-excitation, higher polar solvent, and a moderate Ag(0) load strengthened the interfacial electron transfer over Ag/TiO2, and thereby shortening the induction time and accelerating the BDE47 degradation.

Modification of PBDEs (BDE-15, BDE-47, BDE-85 and BDE-126) biological toxicity, bio-concentration, persistence and atmospheric long-range transport potential based on the pharmacophore modeling assistant with the full factor experimental design

In this study, the properties of AhR binding affinity, bio-concentration factor, half-life and vapor pressure were selected as the typical indicators of biological toxicity, bio-concentration, persistence and atmospheric long-range transport potential for polybrominated diphenyl ethers (PBDEs), respectively. A three-dimensional pharmacophore modeling assistant with a full factor experimental design for each property was used to reveal the significant pharmacophore features and the substituent effects to obtain reasonable modified schemes for the selected target PBDEs. Finally, the performances of the persistent organic pollutant (POP) properties, the synthesis feasibility and the fire resistance of the modified compounds were evaluated. The most influential pharmacophore feature for all POP properties was the hydrophobic group, especially the vinyl and propyl groups. Modified compounds with two additional hydrophobic groups exhibited a better regulatory performance. The average reduction in the proportions of the four POP properties for the modified compounds (except for 3-phenyl-BDE-15) was 70.60%, 52.44%, 47.04% and 70.88%. In addition, the energy and the C-Br bond dissociation enthalpy of the four typical PBDEs were higher than those of the modified compounds (except for 3-phenyl-BDE-15), indicating the synthesis feasibility and the lower energy barrier of the modified compounds to release Br free radicals to provide fire resistance.

Chemical and photochemical degradation of polybrominated diphenyl ethers in liquid systems - A review

Polybrominated diphenyl ethers (PBDEs) are brominated flame retardants which have received a great deal of attention due to their persistence, potential to bioaccumulate and possible toxic effects. PBDEs have been globally detected in humans, wildlife and environment, highlighting the urgency of looking for effective removal technologies to mitigate their spread and accumulation in the environment. Among all environmental compartments, the water has raised particular attention. This paper aims to provide information about the suitability of the main degradation processes investigated to date (photolysis, zerovalent iron and TiO2 photocatalysis) for the degradation of PBDEs in water matrices. The most relevant criteria behind the design of a system for such purpose are discussed in detail for each individual process. The comparative analysis suggests that the oxidative degradation by TiO2 is the most appropriated technology to treat waters contaminated with PBDEs because higher debromination and mineralization degrees are achieved, preventing the formation/accumulation of lower brominated PBDE congeners and promoting the cracking of aromatic cores.

Dehalogenation of persistent halogenated organic compounds: A review of computational studies and quantitative structure-property relationships

Dehalogenation is one of the highly important degradation reactions for halogenated organic compounds (HOCs) in the environment, which is also being developed as a potential type of the remediation technologies. In combination with the experimental results, intensive efforts have recently been devoted to the development of efficient theoretical methodologies (e.g. multi-scale simulation) to investigate the mechanisms for dehalogenation of HOCs. This review summarizes the structural characteristics of neutral molecules, anionic species and excited states of HOCs as well as their adsorption behavior on the surface of graphene and the Fe cluster. It discusses the key physiochemical properties (e.g. frontier orbital energies and thermodynamic properties) calculated at various levels of theory (e.g. semiempirical, ab initio, density functional theory (DFT) and the periodic DFT) as well as their connections to the reactivity and reaction pathway for the dehalogenation. This paper also reviews the advances in the linear and nonlinear quantitative structure-property relationship models for the dehalogenation kinetics of HOCs and in the mathematical modeling of the dehalogenation processes. Furthermore, prospects of further expansion and exploration of the current research fields are described in this article.

Prediction of the Fate of Organic Compounds in the Environment From Their Molecular Properties: A Review

A comprehensive review of quantitative structure-activity relationships (QSAR) allowing the prediction of the fate of organic compounds in the environment from their molecular properties was done. The considered processes were water dissolution, dissociation, volatilization, retention on soils and sediments (mainly adsorption and desorption), degradation (biotic and abiotic), and absorption by plants. A total of 790 equations involving 686 structural molecular descriptors are reported to estimate 90 environmental parameters related to these processes. A significant number of equations was found for dissociation process (pKa), water dissolution or hydrophobic behavior (especially through the KOW parameter), adsorption to soils and biodegradation. A lack of QSAR was observed to estimate desorption or potential of transfer to water. Among the 686 molecular descriptors, five were found to be dominant in the 790 collected equations and the most generic ones: four quantum-chemical descriptors, the energy of the highest occupied molecular orbital (EHOMO) and the energy of the lowest unoccupied molecular orbital (ELUMO), polarizability (α) and dipole moment (μ), and one constitutional descriptor, the molecular weight. Keeping in mind that the combination of descriptors belonging to different categories (constitutional, topological, quantum-chemical) led to improve QSAR performances, these descriptors should be considered for the development of new QSAR, for further predictions of environmental parameters. This review also allows finding of the relevant QSAR equations to predict the fate of a wide diversity of compounds in the environment.

Excited States and photodebromination of selected polybrominated diphenyl ethers: computational and quantitative structure--property relationship studies

This paper presents a density functional theory (DFT)/time-dependent DFT (TD-DFT) study on the lowest lying singlet and triplet excited states of 20 selected polybrominateddiphenyl ether (PBDE) congeners, with the solvation effect included in the calculations using the polarized continuum model (PCM). The results obtained showed that for most of the brominated diphenyl ether (BDE) congeners, the lowest singlet excited state was initiated by the electron transfer from HOMO to LUMO, involving a π–σ* excitation. In triplet excited states, structure of the BDE congeners differed notably from that of the BDE ground states with one of the specific C–Br bonds bending off the aromatic plane. In addition, the partial least squares regression (PLSR), principal component analysis-multiple linear regression analysis (PCA-MLR), and back propagation artificial neural network (BP-ANN) approaches were employed for a quantitative structure-property relationship (QSPR) study. Based on the previously reported kinetic data for the debromination by ultraviolet (UV) and sunlight, obtained QSPR models exhibited a reasonable evaluation of the photodebromination reactivity even when the BDE congeners had same degree of bromination, albeit different patterns of bromination.

Estimating stepwise debromination pathways of polybrominated diphenyl ethers with an analogue Markov Chain Monte Carlo algorithm

A stochastic process was developed to simulate the stepwise debromination pathways for polybrominated diphenyl ethers (PBDEs). The stochastic process uses an analogue Markov Chain Monte Carlo (AMCMC) algorithm to generate PBDE debromination profiles. The acceptance or rejection of the randomly drawn stepwise debromination reactions was determined by a maximum likelihood function. The experimental observations at certain time points were used as target profiles; therefore, the stochastic processes are capable of presenting the effects of reaction conditions on the selection of debromination pathways. The application of the model is illustrated by adopting the experimental results of decabromodiphenyl ether (BDE209) in hexane exposed to sunlight. Inferences that were not obvious from experimental data were suggested by model simulations. For example, BDE206 has much higher accumulation at the first 30 min of sunlight exposure. By contrast, model simulation suggests that, BDE206 and BDE207 had comparable yields from BDE209. The reason for the higher BDE206 level is that BDE207 has the highest depletion in producing octa products. Compared to a previous version of the stochastic model based on stochastic reaction sequences (SRS), the AMCMC approach was determined to be more efficient and robust. Due to the feature of only requiring experimental observations as input, the AMCMC model is expected to be applicable to a wide range of PBDE debromination processes, e.g. microbial, photolytic, or joint effects in natural environments.

Photolytic debromination pathway of polybrominated diphenyl ethers in hexane by sunlight

The objective of this work is to identify the photolytic debromination pathways of polybrominated diphenyl ethers (PBDEs). Thirteen PBDEs (BDEs 209, 208, 207, 206, 196, 183, 154, 153, 100, 99, 85, 47 and 28) in hexane were individually exposed to sunlight for up to 64 h. A total of 180 PBDEs were screened and 74 BDE debromination products were detected. The disappearance rate constant increased exponentially with increasing number of bromines. While no evident difference in debromination preference among ortho, meta and para bromines was found for heavier congeners, the vulnerability rank order was meta ≥ ortho > para for the lighter congeners (≤8 Br). The total molar mass of PBDEs continuously decreased during sunlight exposure, indicating PBDEs were transformed to non-PBDE compounds. A stochastic least squares debromination pathway model was developed to simulate the reactions and determine the yields to extend the results beyond the experimental observations.

Prediction of nitrobenzene toxicity to the algae (Scenedesmus obliguus) by quantitative structure-toxicity relationship (QSTR) models with quantum chemical descriptors

In this study, Quantitative structure-toxicity relationship (QSTR) models were developed to predict the toxicity of nitrobenzene to the algae (Scenedesmus obliguus). Quantum chemical descriptors computed by PM3 Hamiltonian were used as predictor variables. The cross-validated Q²(cum) value for the optimal QSTR models is 0.867, indicating good predictive capability. The toxicity of nitrobenzenes (pC) was found to be affected by the molecular structure, the heat of formation (ΔH(f)) and dipole moment (μ(z)). Contrary to the μ(z) values of nitrobenzenes, the ΔH(f) values increase with increase in pC values and the energy of the highest occupied molecular orbital. Increasing the largest positive atomic charge on a nitrogen atom and the most positive net atomic charge on a hydrogen atom of the nitrobenzene leads to decrease in pC values. Nitrobenzenes with larger absolute hardness tend to be more stable and less toxic to the algae.

QSAR models for predicting toxicity of polychlorinated dibenzo-p-dioxins and dibenzofurans using quantum chemical descriptors

By partial least square regression, simple quantitative structure-activity relationship (QSAR) models were developed for the toxicity of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs). Quantum chemical descriptors computed by semi-empirical PM3 method were used as predictor variables. Three optimal QSAR models are developed for 25 PCDDs, 35 PCDFs, 25 PCDDs and 35 PCDFs together, respectively. The cross-validated Q (cum) (2) values for the three QSAR models of 25 PCDDs, 35 PCDFs, 25 PCDDs and 35 PCDFs together are 0.816, 0.629 and 0.603, respectively, indicating good predictive capabilities for the biological toxicity of these PCDD/Fs. The present study suggests that quantum chemical descriptors of POPs indeed govern the binding affinity of these chemicals for aryl hydrocarbon receptors. Moreover, different models contain different molecular descriptors to define respective equation, which suggests that the relationship between molecular structure and the binding affinity of these chemicals for aryl hydrocarbon receptors is complex.

Quantitative structure - photodegradation relationships of polybrominated diphenyl ethers, phenoxyphenols and selected organochlorines

Among other developments, the technological revolution has lead to introduction of new chemicals to better serve in instruments and materials. The consequences of the extensive increase in use of new chemicals can be detected in the environment world wide, i.e. in wildlife and humans. To ensure this problem to be minimised in the future, new chemicals need to be subjected to predictive assessments before commercialised. To facilitate screening, qualitative structure-activity relationships, quantitative structure-activity relationships may be applied to describe reactivity of chemicals. Physico-chemical properties of chemicals such as partition coefficients and half-lives for the various environmental compartments are essential input data in multimedia environmental fate models. In this study we examine how structural characteristics can quantitatively describe laboratory determined photolytic half-lives of halogenated compounds of different classes, such as polybrominated diphenyl ethers (PBDEs), hydroxylated brominated diphenyl ethers (OH-PBDEs), and other organohalogens. A total of 30 chemicals with experimentally measured half-lives are used. Results reveal that the most important descriptors for describing the half-lives of the brominated compounds are the energy gap (GAP-1) between HOMO-1 and LUMO, the lowest partial charge on a halogen atom (Qhal-), topological polar surface area (TPSA), the atom with highest radical superdelocalizability (Rad-super+) and LUMO density (LUMO+).

Quantitative structure-property relationship studies for direct photolysis rate constants and quantum yields of polybrominated diphenyl ethers in hexane and methanol

The direct photolysis of 18 individual polybrominated diphenyl ethers (PBDEs) substituted with 1-7 bromine atoms was investigated in hexane and methanol under UV irradiation. Based on the determined photolysis rate constants (k(p)) and calculated quantum yields (Phi), four quantitative structure-property relationship (QSPR) models were developed by partial least squares (PLS) method and 20 fundamental molecular structural descriptors. The fitting results showed that all of four QSPR models had good predictability and correlations between observed and predicted photolysis data were significant. The predominant molecular descriptors governing photolysis rate constants of PBDEs in hexane were Mw; Sm; qBr; and qBr+], while in methanol the significant variables were alpha, TE, M(w) and S(m). In terms of the QSPR models for quantum yields of PBDEs in hexane and methanol, the governing molecular descriptors were almost the same. Molecular weight (M(w)) and three atomic charge descriptors (qBr, qH, qBr) were all presented in the four QSPR models, implying that photolysis rate constants and quantum yields were affected by the bromination degree and substitution pattern of PBDE molecules.

UVA/B-induced formation of free radicals from decabromodiphenyl ether

Polybrominated diphenyl ether (PBDE) flame retardants are ubiquitous in the environment and in humans. A deca-bromodiphenyl ether mixture (deca-BDE) is the dominating commercial PBDE product today. Deca-BDE is degraded by UV to PBDEs with fewer bromines. We hypothesized that photodegradation of deca-BDE results in the formation of free radicals. We employed electron paramagnetic resonance (EPR) with spin trap agents to examine the free radicals formed from UV irradiation of a deca-BDE mixture (DE-83R). The activating wavelength for deca-BDE photochemistry was in the UVA to UVB range. The yields of radicals from irradiated deca-BDE in tetrahydrofuran, dimethylformamide, and toluene were about 9-, 4-, and 7-fold higher, respectively, than from irradiated solvent alone. Radical formation increased with deca-BDE concentration and irradiation time. The quantum yield of radical formation of the deca-BDE mixture was higher than with an octa-BDE mixture (DE-79; approximately 2-fold), decabromobiphenyl (PBB 209; approximately 2-fold), decachlorobiphenyl (PCB 209; approximately 3-fold), and diphenyl ether (DE; approximately 6-fold), indicating the positive effects of bromine and an ether bond on radical formation. Analysis of hyperfine splittings of the spin adducts suggests that radical formation is initiated or significantly enhanced by debromination paired with hydrogen abstraction from the solvents. To our knowledge this is the first study that uses EPR to demonstrate the formation of free radicals during the photolytic degradation of PBDEs. Our findings strongly suggestthe potential of negative consequences due to radical formation during UV exposure of PBDEs in biological systems.

Development and validation of a congener-specific photodegradation model for polybrominated diphenyl ethers

With the phaseout of the manufacture of some polybrominated diphenyl ether (PBDE) formulations, namely penta-brominated diphenyl ether (BDE) and octa-BDE, and the continued use of the deca-BDE formulation, it is important to be able to predict the photodegradation of the more highly brominated congeners. A model was developed and validated to predict the products and their relative concentrations from the photodegradation of PBDEs. The enthalpies of formation of the 209 PBDE congeners were calculated, and the relative reaction rate constants were obtained. The predicted reaction rate constants for PBDEs show linear correlation with previous experimental results. Because of their large volume use, their presence in the environment, and/or importance in the photodegradation of the deca-BDE formulation, BDE-209, BDE-184, BDE-100, and BDE-99 were chosen for further ultraviolet photodegradation experiments in isooctane. The photodegradation model successfully predicted the products of the photochemical reactions of PBDEs in experimental studies. A gas chromatography retention time model for PBDEs was developed using a multiple linear regression analysis and, together with the photodegradation model and additional PBDE standards, provided a way to identify unknown products from PBDE photodegradation experiments. Based on the results of the photodegradation experiments, as well as the model predictions, it appears that the photodegradation of PBDEs is a first-order reaction and, further, that the rate-determining step is the stepwise loss of bromine. Our results suggest that, based on photodegradation, over time, BDE-99 will remain the most abundant penta-BDE, while BDE-49 and BDE-66 will increase greatly and will be comparable in abundance to BDE-47.

Quantitative structure-property relationships for octanol-water partition coefficients of polybrominated diphenyl ethers

Theoretical molecular descriptors were tested against logK(OW) values for polybrominated diphenyl ethers (PBDEs) using the Partial Least-Squares Regression method which can be used to analyze data with many variables and few observations. A quantitative structure-property relationship (QSPR) model was successfully developed with a high cross-validated value (Q(cum)(2)) of 0.961, indicating a good predictive ability and stability of the model. The predictive power of the QSPR model was further cross-validated. The values of logK(OW) for PBDEs are mainly governed by molecular surface area, energy of the lowest unoccupied molecular orbital and the net atomic charges on the oxygen atom. All these descriptors have been discussed to interpret the partitioning mechanism of PBDE chemicals. The bulk property of the molecules represented by molecular surface area is the leading factor, and K(OW) values increase with the increase of molecular surface area. Higher energy of the lowest unoccupied molecular orbital and higher net atomic charge on the oxygen atom of PBDEs result in smaller K(OW). The energy of the lowest unoccupied molecular orbital and the net atomic charge on PBDEs oxygen also play important roles in affecting the partition of PBDEs between octanol and water by influencing the interactions between PBDEs and solvent molecules.

Prediction of biodegradation rate constants of hydroxylated polychlorinated biphenyls by fungal laccases from Trametes versicolor and Pleurotus ostreatus

Quantitative structure-activity relationship (QSAR) models for fungal laccase-catalyzed degradation of different hydroxylated polychlorinated biphenyls (OH-PCBs) were developed using some fundamental quantum chemical descriptors. The cross-validated Q(2)(cum )values for the two optimal QSAR models are as high as 0.958 and 0.961 for laccases from Trametes versicolor and Pleurotus ostreatus, respectively, indicating good predictive abilities for laccase-catalyzed degradation of OH-PCBs. Results from this study show that increasing heat of formation (DeltaH(f)) and frontier molecular orbital energy (i.e. E(LUMO) + E(HOMO)) values or decreasing frontier molecular orbital energy (i.e. E(HOMO-1)) and core-core repulsion energy (CCR) values leads to the increase of OH-PCB degradation rates by laccases.

Different reaction mechanisms of diphenylether and 4-bromodiphenylether with nitrous acid in the 355 nm laser flash photolysis of mixed aqueous solution

The microscopic reaction mechanisms of diphenylether (DPE) and 4-bromodiphenylether (4-BrDPE) with nitrous acid (HNO(2)) in the absence of O(2) have been explored by the 355nm laser flash photolysis. It was proposed that OH radical, from the photolysis of HNO(2), added to DPE forms the C(12)H(10)O-OH adduct while added to 4-BrDPE forms the 4-BrDPE-OH and 4-BrOH-DPE adducts. The first-order decay rate constants of the C(12)H(10)O-OH adduct, 4-BrDPE-OH adduct and 4-BrOH-DPE adduct were measured to be (1.86+/-0.14)x10(5)s(-1), (2.19+/-0.04)x10(5)s(-1) and (1.56+/-0.03)x10(5)s(-1), respectively. The final photolysis products of DPE and HNO(2) identified by GC/MS analysis were phenol, o-hydroxydiphenylether, p-hydroxydiphenylether and p-nitrodiphenylether, while the final photolysis product of 4-BrDPE and HNO(2) identified by LC/MS analysis was mainly the dimer.

Photochemical degradation of six polybrominated diphenyl ether congeners under ultraviolet irradiation in hexane

The photodegradation of six individual PBDE congeners (BDE-28, 47, 99, 100, 153, 183) in hexane was investigated under UV light in the sunlight region, employing a mercury lamp filtered with Pyrex glass. All photodegradation reactions followed the pseudo-first-order kinetics, with the half-lives ranging from 0.26h for BDE-183 to 6.46h for BDE-100. The photochemical reaction rates of PBDEs decreased with decreasing number of bromine substituents in the molecule, also in some cases were influenced by the PBDE substitution pattern. Principal photoproducts detected were less brominated PBDEs, and no PBDE-solvent adducts were found. Consecutive reductive debromination was confirmed as the main mechanism for the photodegradation of PBDEs in hexane. In general, debromination firstly occurred on the more substituted rings, when the numbers of bromine atoms on the two phenyl rings were unequal. For less brominated PBDEs, the photoreactivity of bromines at various positions of phenyl rings decreased in the order: ortho>para; while for higher brominated PBDEs, the difference became not significant.

Behavior and prediction of photochemical degradation of chlorinated polycyclic aromatic hydrocarbons in cyclohexane

The photochemical degradation of 11 chlorinated polycyclic aromatic hydrocarbons (ClPAHs) and the corresponding 5 parent PAHs was examined to simulate the compound's fate on aerosol surfaces. All the ClPAHs and PAHs decayed according to the first-order reaction rate kinetics. The photolysis rates of ClPAHs varied greatly according to the skeleton of PAHs; the rates of chlorophenanthrenes (ClPhes) and 1-chloropyrene were higher than those of corresponding parent PAHs, whereas chlorofluoranthenes, 7-chlorobenz[a]anthracene and 6-chlorobenzo[a]pyrene were more stable under irradiation compared to respective parent PAH. Considering the photoproducts of ClPhes detected, the oxidation could occur immediately at positions of the highest frontier electron density. Finally, the quantitative structure-property relationship models were developed for direct photolysis half-lives and average quantum yields of the ClPAHs and parent PAHs, in which the significant factors affecting photolysis were E(LUMO+1), total energy and surface area, and E(LUMO), E(LUMO)-E(HOMO) and total energy, respectively.

Photolysis of mono- through deca-chlorinated biphenyls by ultraviolet irradiation in n-hexane and quantitative structure-property relationship analysis

The photolysis of 16 polychlorinated biphenyls (PCBs) (including mono- through deca-chlorinated) in n-hexane was investigated under ultraviolet irradiation using a 500-W high-pressure mercury lamp. Photolysis of PCBs follows pseudo-first-order reaction kinetics, with photolysis rate constants ranging between 0.0011 s(-1) for PCB-52 and 0.0574 s(-1) for PCB-118. The degradation rates of PCBs by high-pressure mercury lamp irradiation were remarkably independent with respect to the degree of chlorination. Furthermore, partial least squares (PLS) models were developed to provide insight into which aspect of the molecular structure influenced PCB photolysis rate constants. It was found that the photolysis rates of PCBs increased with an increase in the net charge on the carbon atom (qc), (E(LUMO)-E(HOMO))2, and the Y-axis dipole moment (mu(y)) values, or the decrease in the energy of the second highest occupied molecular orbital (E(HOMO-1)), energy of the lowest unoccupied molecular orbital (E(LUMO)), E(LUMO) + E(HOMO), E(LUMO)--E(HOMO), most positive atomic charge (q+), and the twist angle of the chlorine atom (TA) values.

Quantitative structure-activity relationships for prediction of the toxicity of hydroxylated and quinoid PCB metabolites

Quantitative structure-activity relationship (QSAR) models were developed for the in vitro potencies to downregulate gap junctional intercellular communication (GJIC) of hydroxylated polychlorinated biphenyls (OH-PCBs) and PCB quinines using partial least squares (PLS) regression. Quantum chemical descriptors computed by the semiempirical AM1, PM3 and MNDO methods were used as predictor variables. The cross-validated Q2cum values for the three optimal QSAR models are 0.784, 0.789 and 0.755, respectively, indicating good predictive capabilities for the acute inhibition of GJIC (IC(50)) of oxygenated PCB derivatives. The slightly higher Q2cum value of the model using computed molecular descriptors from the PM3 Hamiltonian suggested a slightly better predictive power than the models developed using AM1 or MNDO. However, given these dispersion parameters in these three optimal models, there would not be a significant difference between the Q2cum values. Results from this study showed that the logarithmic scale of IC(50) is affected by different molecular structural descriptors.