Mechanism for the growth of polycyclic aromatic hydrocarbons from the reactions of naphthalene with cyclopentadienyl and indenyl

Unraveling the atmospheric oxidation mechanism and kinetics of naphthalene: Insights from theoretical exploration

Naphthalene, the most abundant polycyclic aromatic hydrocarbon in the atmosphere, significantly influences OH consumption and secondary organic aerosol (SOA) formation. Naphthoquinone (NQ) is a significant contributor to ring-retaining SOA from naphthalene degradation, impacting the redox properties and toxicity of ambient particles. However, inconsistencies persist regarding concentrations of its isomers, 1,2-NQ and 1,4-NQ. In present work, our theoretical investigation into naphthalene's reaction with OH and subsequent oxygenation unveils their role in SOA formation. The reaction kinetics of initial OH and subsequent O2 oxidation was extensively studied using high-level quantum chemical methods (DLPNO-CCSD(T)/aug-ccpVQZ//M052x-D3/6-311++G(d,p)) combined with RRKM/master equation simulations. The reactions mainly proceed through electrophilic addition and abstraction from the aromatic ring. The total rate coefficient of naphthalene + OH at 300 K and 1 atm from our calculation (7.2 × 10-12 cm3 molecule-1 s-1) agrees well with previous measurements (∼1 × 10-11 cm3 molecule-1 s-1). The computed branching ratios facilitate accurate product yield determination. The largest yield of 1-hydroxynaphthalen-1-yl radical (add1) producing the major precursor of RO2 is computed to be 93.8 % in the ambient environment. Our calculated total rate coefficient (5.2 × 10-16 cm3 molecule-1 s-1) for add1 + O2 closely matches that of limited experimental data (8.0 × 10-16 cm3 molecule-1 s-1). Peroxy radicals (RO2) generated from add1 + O2 include 4-cis/trans-(1-hydroxynaphthalen-1-yl)-peroxy radical (add1-4OOadd-cis/trans, 66.0 %/17.5 %), 2-cis/trans-(1-hydroxynaphthalen-1-yl)-peroxy radical (add1-2OOadd-cis/trans, 10.3 %/6.3 %). Regarding the debated predominance of 1,4-NQ (corresponding to the parent RO2, i.e., add1-4OOadd-cis/trans) and 1,2-NQ (corresponding to the parent RO2, i.e., add1-2OOadd-cis/trans) in the atmosphere, our findings substantiate the dominance of 1,4-NQ. This study also indicates potential weakening of 1,4-NQ's dominance due to competition from decomposition reactions of add1-4OOadd-cis/trans and add1-2OOadd-cis/trans. Precise reaction kinetics data are essential for characterizing SOA transformation derived from naphthalene and assessing their climatic impacts within modeling frameworks.

The mechanism of photodegradation reaction of different dissociation forms of tetrabromobisphenol S in water with free radicals and the ecotoxicity evaluation of related products

Tetrabromobisphenol S (TBBPS) is a widely used brominated flame retardant that has attracted environmental concern due to its abundant presence in water. The objective of this study is to systematically analyze the direct photolysis and degradation mechanisms of TBBPS in two different dissociation forms in water, as well as to evaluate their toxicological effects induced by •OH, 1O2, and •NO2 radicals. The degradation mechanism of TBBPS is investigated with density functional theory (DFT) and time-dependent density functional theory (TDDFT) methods, and the toxicity of the degradation products is assessed through toxicological studies. The results of the study indicate that the OH-addition and H-abstraction reactions are favorable pathways for •OH-induced TBBPS degradation. The H-abstraction reaction of TBBPS0 with •OH was more favorable than the •OH addition reaction. However, in the degradation of TBBPS-, the •OH addition reaction was favored over the H-abstraction reaction. Additionally, the indirect photolysis of TBBPS by 1O2 and •NO2 in water was found to be easier for TBBPS- compared to TBBPS0, with degradation mechanisms involving Br-substitution and NO2-addition reactions. The higher Ea values calculated indicate that the degradation of TBBPS by 1O2 and •NO2 in water has been a secondary reaction. The direct photolysis reaction pathway of TBBPS in water has involved the cleavage of the S1-C7 and S1-C16 bonds. For TBBPS0 in the S1/T1 states, the primary reaction pathway is the cleavage of the S1-C16 bond, while for TBBPS-, the primary reaction pathway is the cleavage of the S1-C7 bond. Furthermore, the computational toxicology results indicate a slight increase in the toxicity levels of most products, highlighting the significance of investigating the degradation byproducts of TBBPS in greater detail.

Computational Study of Photodegradation Process and Conversion Products of the Antidepressant Citalopram in Water

Citalopram (CIT) is a commonly prescribed medication for depression. However, the photodegradation mechanism of CIT has not yet been fully analyzed. Therefore, the photodegradation process of CIT in water is studied by density functional theory and time-dependent density functional theory. The calculated results show that during the indirect photodegradation process, the indirect photodegradation of CIT with ·OH occurs via OH-addition and F-substitution. The minimum activation energy of C10 site was 0.4 kcal/mol. All OH-addition and F-substitution reactions are exothermic. The reaction of ¹O₂ with CIT includes the substitution of ¹O₂ for F and an addition reaction at the C14 site. The E_a value of this process is 1.7 kcal/mol, which is the lowest activation energy required for the reaction of ¹O₂ with CIT. C-C/C-N/C-F cleavage is involved in the direct photodegradation process. In the direct photodegradation of CIT, the activation energy of the C7-C16 cleavage reaction was the lowest, which was 12.5 kcal/mol. Analysis of the E_a values found that OH-addition and F-substitution, the substitution of ¹O₂ for F and addition at the C14 site, as well as the cleavage reactions of C6-F/C7-C16/C17-C18/C18-N/C19-N/C20-N are the main pathways of photodegradation of CIT.

Photodegradation fate of different dissociation species of antidepressant paroxetine and the effects of metal ion Mg2+: Theoretical basis for direct and indirect photolysis

Paroxetine (abbreviated as PXT) has been widely used as one of the standard antidepressants for the treatment of depression. PXT has been detected in the aqueous environment. However, the photodegradation mechanism of PXT remains unclear. The present study aimed to use density functional theory and time-dependent density functional theory to study the photodegradation process of two dissociated forms of PXT in water. The main mechanisms include direct and indirect photodegradation via reaction with ·OH and ¹O₂ and photodegradation mediated by the metal ion Mg²⁺. Based on the calculations, PXT and PXT-Mg²⁺ complexes in water are photodegraded mainly indirectly and directly. It was found that PXT and PXT-Mg²⁺ complexes were photodegraded by H-abstraction, OH-addition and F-substitution. The main reaction of PXT indirect photolysis is OH-addition reaction, while the main reaction of PXT⁰-Mg²⁺ complex is H-abstraction. All the reaction pathways of H-abstraction, OH-addition and F-substitution are exothermic. PXT⁰ reacts more readily with ·OH or ¹O₂ in water than PXT⁺. However, the higher activation energy of PXT with ¹O₂ indicates that the ¹O₂ reaction plays a minor role in the photodegradation pathway. The direct photolysis process of PXT includes ether bond cleavage, defluorination, and dioxolane ring-opening reaction. In the PXT-Mg²⁺ complex, the direct photolysis process occurs via a dioxolane ring opening. Additionally, Mg²⁺ in water has a dual effect on the direct and indirect photolysis of PXT. In other words, Mg²⁺ can inhibit or promote their photolytic reactions. Overall, PXT in natural water mainly undergo direct and indirect photolysis reactions with ·OH. The main products include direct photodegradation products, hydroxyl addition products and F-substitution products. These findings provide critical information for predicting the environmental behavior and transformation of antidepressants.

Effects of steam and CO2 on gasification tar composition and evolution of aromatic compounds

The removal of tar is conducive to improving the energy efficiency of downstream equipment and reducing the damage caused to it. In this study, a two-stage continuous feeding apparatus was developed to explore the yield and characteristics of tar produced from the co-gasification of microcrystalline cellulose (MCC) and polyethylene (PE) under separate and mixed atmospheres of steam and CO₂. The tar yield can effectively reduce to 2.27 % when the steam and feedstock mass ratio (S/F) is 0.8. CO₂ can partially substitute the steam in the gasification process, which can effectively promote a decrease in benzofuran. Furthermore, Gaussian software was employed to analyze the evolution mechanism of aromatic compounds. When the temperature is more than 800 °C, hydrogen consumption in the benzene cracking process is reduced, which is instrumental in improving the quality of syngas. Naphthalene is prone to form through the recombination of two cyclopentadienyls. Controlling the cyclization of cyclopentadienyls is a critical step in reducing the formation of polycyclic aromatic hydrocarbons. H and OH radicals are critical in phenol and benzofuran cracking, respectively. Although radicals act differently on specific aromatic compounds, the gasification effect of CO₂ is less than that of steam because steam can provide both H and OH radicals, whereas CO₂ needs to consume H radicals to generate OH radicals. The results provide beneficial guidance for controlling tar formation.

Emission characteristics and influencing mechanisms of PAHs and EC from the combustion of three components (cellulose, hemicellulose, lignin) of biomasses

Biomass burning is an important source of polycyclic aromatic hydrocarbons (PAHs) and elemental carbon (EC), but the formation mechanisms are still unclear. Cellulose, hemicellulose, and lignin are the three major components of biomass. In this study, the three-components extracted from three typical biomass raw materials were used for laboratory combustion experiments, to investigate the differences in the emission factors and chemical compositions of PAHs and EC. The average emission factors of the 16 kinds of PAHs were showing as lignin (135 ± 180 mg/kg) > cellulose (97.8 ± 124 mg/kg) > hemicellulose (48.9 ± 65.2 mg/kg), and the average emission factors of EC presented in the descending order of cellulose (1.65 ± 3.02 g/kg), lignin (1.30 ± 1.04 g/kg), and hemicellulose (0.450 ± 0.480 g/kg), respectively. The proportion of naphthalene emitted from cellulose and hemicellulose combustion is higher, while fluoranthene and pyrene accounted significantly higher proportion for lignin. Moreover, the influence of ignition temperature and oxygen content on the emission characteristics of PAHs and EC were also discussed. The influence of ignition temperature on the emission of EC and PAHs is more significant compared to oxygen content, because it obviously promoted the PAHs and EC formations through resonance-stabilized hydrocarbon-radical chain reaction (RSR) pathway. However, correlation analysis combined with cluster analysis showed that the RSR-pathway probably had different effects on PAH growth for the three-components, as the indene-involved RSR-pathway were mainly related to 4-6 ring PAHs for cellulose and lignin (except fluoranthene and pyrene), but 2-4 ring PAHs for hemicellulose. We also found that the fitted results according to the proportion of three-components were significantly higher than the measured values of raw materials for indene, medium-molecular-weight PAHs, and soot-EC. These results presented the different formation pathways for medium-molecular-weight PAHs and the two EC components emitted by biomass combustion, which are worthy of further studies in exploring the generation mechanisms of PAHs and EC.

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

The atmospheric oxidation degradation of fluorotelomer acrylates (FTAcs) has been proposed as a potential source of perfluorocarboxylic acids (PFCAs) in remote locations. In this paper, detailed reactions of the main oxidant OH radicals with 4:2 FTAc in the atmosphere have been investigated by using density functional theory (DFT) calculation. All possible pathways involved in the oxidation process were presented and discussed. Based on the mechanism, transition state theory (TST) was used to predict the rate constants of the key elementary steps including the initial reactions of OH radical with n:2 FTAcs and the subsequent reactions of the main intermediates. Studies show that the reaction processes of OH radical addition to C = C bond are dominant and the fluorotelomer glyoxylate and formaldehyde are the major products. At 296 K, the calculated overall rate constant of 4:2 FTAc with OH radical is 1.19 × 10^-11 cm³ molecule^-1 s^-1 with an atmospheric lifetime of 23.3 h. In the atmosphere, fluorotelomer glyoxylate will continue to be oxidized, which will lead to the formation of PFCAs ultimately. In addition, atmospheric reactions of more carbons FTAc (C_nF_2n+1CH₂CH₂OC(O)CH = CH₂, n = 6, 8, 10) are also discussed in the presence of O₂/NO_x.

Formation and growth mechanisms of polycyclic aromatic hydrocarbons: A mini-review

Polycyclic aromatic hydrocarbons (PAHs) are mostly formed during the incomplete combustion of organic materials, but their importance and presence in materials science, and astrochemistry has also been proven. These carcinogenic persistent organic pollutants are essential in the formation of combustion generated particles as well. Due to their significant impact on the environment and human health, to understand the formation and growth of PAHs is essential. Therefore, the most important growth mechanisms are reviewed, and presented here from the past four decades (1981-2021) to initiate discussions from a new perspective. Although, the collected and analyzed observations are derived from both experimental, and computational studies, it is neither a systematic nor a comprehensive review. Nevertheless, the mechanisms were divided into three main categories, acetylene additions (e.g. HACA), vinylacetylene additions (HAVA), and radical reactions, and discussed accordingly.

Thermal Decomposition Kinetics of the Indenyl Radical: A Theoretical Study

Quantum chemistry and statistical reaction rate theory calculations have been performed to investigate the products and kinetics of indenyl radical decomposition. Three competitive product sets are identified, including formation of a cyclopentadienyl radical (c-C₅H₅) and diacetylene (C₄H₂), which has not been included in prior theoretical kinetics investigations. Rate coefficients for indenyl decomposition are determined from master equation simulations at 1800-2400 K and 0.01-100 atm, and temperature- and pressure-dependent rate coefficient expressions are incorporated into a detailed chemical kinetic model for indene pyrolysis. Indenyl is found to predominantly decompose to o-benzyne (o-C₆H₄) + propargyl (C₃H₃), with lesser amounts of fulvenallenyl (C₇H₅) + C₂H₂ and c-C₅H₅ + C₄H₂.

Atmospheric oxidation of indene initiated by OH radical in the presence of O2 and NO: A mechanistic and kinetic study

The atmospheric degradation of polycyclic aromatic hydrocarbons (PAHs) can generate organic pollutants that contribute to the formation of secondary organic aerosols (SOAs) and exacerbate their carcinogenicity. Indene is an example of styrene-like bicyclic hydrocarbons that are not fully aromatic. The OH-initiated atmospheric oxidation of indene in the presence of O₂ and NO was investigated using quantum chemical methods at M06-2X/6-311++G(3df,2p)//M06-2X/6-311+G(d,p) level. The oxidation products are oxygenated polycyclic aromatic hydrocarbons (OPAHs) containing hydroxyindene, indenone, dialdehydes and 2-(formylmethyl)benzaldehyde. Calculation results showed that 7-indene radical, which is the precursor of various PAHs, has a high production ratio that is 35.29% in the initial reaction, indicating that the OH-initiated oxidation increase the environmental risks of indene in the atmosphere. The rate constants for the crucial elementary reactions were calculated based on Rice-Ramsperger-Kassel-Marcus (RRKM) theory. The overall rate constant of the initial reaction is calculated to be 1.04 × 10^-10 cm³ molecule^-1 s^-1 and the atmospheric lifetime of indene is determined as 2.74 h. This work provides a comprehensive understanding on the oxidation mechanisms of indene and the findings could help to clarify the fate of indene in the atmosphere.

The Gas-Phase Formation Mechanism of Dibenzofuran (DBF), Dibenzothiophene (DBT), and Carbazole (CA) from Benzofuran (BF), Benzothiophene (BT), and Indole (IN) with Cyclopentadienyl Radical

Benzofuran (BF), benzothiophene (BT), indole (IN), dibenzofuran (DBF), dibenzothiophene (DBT), and carbazole (CA) are typical heterocyclic aromatic compounds (NSO-HETs), which can coexist with polycyclic aromatic hydrocarbons (PAHs) in combustion and pyrolysis conditions. In this work, quantum chemical calculations were carried out to investigate the formation of DBF, DBT, and CA from the reactions of BF, BT, and IN with a cyclopentadienyl radical (CPDyl) by using the hybrid density functional theory (DFT) at the MPWB1K/6-311+G(3df,2p)//MPWB1K/6-31+G(d,p) level. The rate constants of crucial elementary steps were deduced over 600-1200 K, using canonical variational transition state theory with a small-curvature tunneling contribution (CVT/SCT). This paper showed that the production of DBF, DBT, and CA from the reactions of BF, BT, and IN with CPDyl involved six elementary steps: the addition reaction, ring closure, the first H shift, C-C cleavage, the second H shift, and elimination of CH3 or H. The cleavage of the C-C bond was regarded as the rate-determining step for each pathway due to the extremely high barrier. The 1-methyl substituted products were more easily formed than the 4-methyl substituted products. The main products were DBF and 1-methyl-DBF, DBT and 1-methyl-DBT, and CA and 1-methyl-CA for reactions of BF, BT, and IN with CPDyl, respectively. The ranking of DBF, DBT, and CA formation potential was as follows: DBT and methyl-DBT formation > DBF and methyl-DBF formation > CA, and methyl-CA formation. Comparison with the reaction of naphthalene with CPDyl indicated that the reactions of CPDyl attacking a benzene ring and a furan/thiophene/pyrrole ring could be inferred to be comparable under high temperature conditions.

Theoretical investigation on atmospheric oxidation of fluorene initiated by OH radical

Atmospheric oxidation of fluorene and its derivatives initiated by OH radicals was investigated theoretically with quantum chemical calculation methods [M06-2X/6-311++G(3df,2p)//M06-2X/6-311+G(d,p)]. It revealed that the OH addition pathways form hydroxyfluorene and ring-opening product dialdehyde while the H abstraction pathways lead to the formation of 9-fluorenone. Subsequent oxidation of 9-fluorenone has considerable potential to form dibenzo-p-dioxin and nitrofluorenone according to the present calculation results. The atmospheric lifetime of fluorene relative to the reactions with OH radicals is deduced to be 12.51 h based on the calculated overall rate constant (2.29 × 10^-11 cm³ molecule^-1 s^-1) at 298 K and 1 atm. The oxidation products of fluorene in the atmosphere are generally more toxic and persistent. This work provides a comprehensive explanation for atmospheric oxidation processes of fluorene and facilitates clarifying its environmental risks.

A DFT/TDDFT study on the mechanisms of direct and indirect photodegradation of tetrabromobisphenol A in water

Tetrabromobisphenol A (TBBPA) is the most widely used commercial brominated flame retardant. However, the mechanisms underlying the photodegradation of TBBPA remain unclear. Here we use density functional theory and time-dependent density functional theory to examine the photodegradation of the two species of TBBPA in water: TBBPA (neutral form) and TBBPA^- (anionic form). The study includes direct photodegradation and indirect photodegradation of TBBPA with ·OH and ¹O₂. The results of the calculations indicate that indirect photodegradation of TBBPA and TBBPA^- with ·OH occurs via OH-addition and Br-substitution. All of the OH-addition and Br-substitution pathways are exothermic. Indirect photodegradation of TBBPA and TBBPA^- by ¹O₂ proceeds via H abstraction by ¹O_2.E_a was higher for H abstraction of TBBPA than H abstraction of TBBPA^-. The mechanisms for the direct photodegradation of TBBPA and TBBPA^- include debromination, C1C7/C7C13 cleavage, and cyclization. CBr cleavage was observed in the optimized geometries of TBBPA and TBBPA^- at the lowest excited triplet state. However, high E_a values and an endothermic nature indicated that C1C7/C7C13 cleavage and cyclization reactions were not the main pathways. OH-adducts, Br-substitution products, H-abstraction (by ¹O₂) products, and debromination products were the main products of photodegradation of TBBPA. These findings provide useful information for risk assessment and pollution control of brominated flame retardants.

Formation Mechanism of Benzo(a)pyrene: One of the Most Carcinogenic Polycyclic Aromatic Hydrocarbons (PAH)

The formation of polycyclic aromatic hydrocarbons (PAHs) is a strong global concern due to their harmful effects. To help the reduction of their emissions, a crucial understanding of their formation and a deep exploration of their growth mechanism is required. In the present work, the formation of benzo(a)pyrene was investigated computationally employing chrysene and benz(a)anthracene as starting materials. It was assumed a type of methyl addition/cyclization (MAC) was the valid growth mechanism in this case. Consequently, the reactions implied addition reactions, ring closures, hydrogen abstractions and intramolecular hydrogen shifts. These steps of the mechanism were computed to explore benzo(a)pyene formation. The corresponding energies of the chemical species were determined via hybrid density funcional theory (DFT), B3LYP/6-31+G(d,p) and M06-2X/6-311++G(d,p). Results showed that the two reaction routes had very similar trends energetically, the difference between the energy levels of the corresponding molecules was just 6.13 kJ/mol on average. The most stable structure was obtained in the benzo(a)anthracene pathway.

Carcinogenic Metabolic Activation Process of Naphthalene by the Cytochrome P450 Enzyme 1B1: A Computational Study

The metabolic activation and transformation of naphthalene by the cytochrome P450 enzyme (CYP 1B1) plays an important role in its potential carcinogenicity. The process has been explored by a quantum mechanics/molecular mechanics (QM/MM) computational method. Molecular dynamic simulations were performed to explore the interaction between naphthalene and CYP 1B1. Naphthalene involves α- and β-carbon, the electrophilic addition of which would result in different reaction pathways. Our computational results show that both additions on α- and β-carbon can generate naphthalene 1,2-oxide. The activation barrier for the addition on β-carbon is higher than that for the α-carbon by 2.6 kcal·mol^-1, which is possibly caused by the proximity between β-carbon and the iron-oxo group of Cpd I in the system. We also found that naphthalene 1,2-oxide is unstable and the O-C bond cleavage easily occurs via cellular hydronium ion, hydroxyl radical/anion; then it will convert to the potential ultimate carcinogen 1,2-naphthoquinone. The results demonstrate and inform a detailed process of generating naphthalene 1,2-oxide and new predictions for its conversion.

Theoretical Insights into the Reaction Mechanism between 2,3,7,8-Tetrachlorodibenzofuran and Hydrogen Peroxide: A DFT Study

A detailed knowledge of the reactivity of 2,3,7,8-tetrachlorodibenzofuran (TCDF) at the molecular level is important to better understand the transformation of dioxins analogous to TCDF in the environment. To clarify the reactivity of the organic hydroperoxides toward TCDF, the reaction of the TCDF with hydrogen peroxide (H2O2) and its anion has been investigated theoretically. For the reaction of the neutral H2O2, a molecular complex can be formed between TCDF and H2O2 first. Then, the nucleophilic aromatic substitution of TCDF by H2O2 occurs in the presence of the water molecules to form an intermediate containing an O-O bond. Finally, the O-O bond cleavages homolytically for the above intermediate. On the other hand, as for the reaction of the anion of H2O2 (HO2 -), the nucleophilic addition of HO2 - to TCDF can also occur besides the nucleophilic aromatic substitution reaction mentioned above, resulting in the dissociation of the C-O bond of TCDF. Unlike the reaction involving neutral H2O2, no water molecules are required. In addition, the selected substitution effects, such as F-, Br-, and CH3-substituents, on the reactivity of the above reaction have also been explored. Hopefully, the present results can enable us to gain insights into the reactivity of the organic hydroperoxides with TCDF-like environmental pollutants.

Theoretical Investigations on the Reactivity of Hydrogen Peroxide toward 2,3,7,8-Tetrachlorodibenzo-p-dioxin

Acquiring full knowledge of the reactivity of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is crucial for the better understanding of the transformation and degradation of TCDD-like dioxins in the environment. To clarify the reactivity of the organic hydroperoxides toward TCDD, in this study, the reactions between the neutral/anion of the hydrogen peroxide (H₂O₂) and TCDD have been systematically investigated theoretically. It was found that the neutral H₂O₂ is relatively difficult to react with TCDD compared with its anion, exhibiting the pH dependence of the title reaction. As for the anion of H₂O₂, it reacts with TCDD through two reaction mechanisms, i.e., nucleophilic substitution and nucleophilic addition. For the former, the terminal O atom of HO₂- nucleophilically attacks the C atom of the C-Cl bond in TCDD to form an intermediate containing an O-O bond, accompanying the dissociation of the chlorine atom. For the latter, the terminal O atom of HO₂- can be easily attached to the C atom of the C-O bond in TCDD, resulting in the decomposition of C-O bond and the formation of an intermediate containing an O-O bond. For these formed intermediates in both reaction mechanisms, their O-O bonds can be homolytically cleaved to produce different radicals. In addition, the selected substitution effects including F-, Br-, and CH₃- substituents on the above reactions have also been studied. Hopefully, the present results can provide new insights into the reactivity of the organic hydroperoxides toward TCDD-like environmental pollutants.

Computational evidence for the degradation mechanism of haloalkane dehalogenase LinB and mutants of Leu248 to 1-chlorobutane

The catalytic degradation ability of the haloalkane dehalogenase LinB toward 1-chlorobutane (1-CB) was studied using a combined quantum mechanics/molecular mechanics (QM/MM) approach. Two major processes are involved in the LinB-catalyzed removal of halogens: dechlorination and hydrolyzation. The present study confirmed the experimentally proposed reaction path at the molecular level. Moreover, based on nucleophilic substitution mechanism (SN2 reaction), dechlorination was found to be the rate-determining step of the entire reaction process. In this study, the Boltzmann-weighted average barrier for dechlorination was determined to be 17.0 kcal mol-1, which is fairly close to the experimental value (17.4 kcal mol-1). The state of His107 and the influence of Leu248 on the dechlorination process were also explored. In addition, an intriguing phenomenon was discovered: the potential energy barrier decreased by 7.5 kcal mol-1 when the Leu248 residue was mutated into Phe248. This discovery might be of great help to design new mutant enzymes or novel biocatalysts.

Theoretical Study of the Reactions of H Atoms with CH3I and CH2I2

High level ab initio methods have been used to provide reliable kinetic data for the H + CH₃I and H + CH₂I₂ gas-phase reactions. The (H, I)-abstraction and I-substitution reaction pathways were identified. The structures were determined on the potential energy surface at the MP2/aug-cc-pVTZ level of theory. The energetics was then refined using the coupled cluster theory. For the iodinated species, the spin-orbit coupling was calculated using the MRCI approach. The core valence and the scalar relativistic corrections were considered. Thermal rate constants were reported using the canonical transition-state theory (TST) and compared to computed values with the canonical variational transition-state theory (CVT) using the zero curvature tunneling (ZCT) and the small curvature tunneling (SCT) corrections over a wide temperature range (250-2500 K) to show the importance of quantum tunneling effects at low temperatures. They are given by the following expressions for the overall reactions using the CVT/SCT method: k_H+CH3I( T) = 1.07 × 10^-17 × T^2.13 exp(2.68 (kJ mol^-1)/ RT) and k_H+CH2I2( T) = 5.73 × 10^-21 × T^2.97 exp(3.15 (kJ mol^-1)/ RT). The I-abstraction is predicted to be the major pathway for both H + CH₃I and H + CH₂I₂ reactions. The obtained kinetic parameters for the H + CH₃I reaction are in excellent agreement with their experimental counterparts over the temperature range 300-750 K. On the basis of our calculated reaction enthalpies, a new evaluation of the standard enthalpy of formation at 298 K of CH₂I and CHI₂ has been provided. Obtained values are Δ_f H°_298K (CH₂I) = 219.5 kJ mol^-1 and Δ_f H°_298K(CHI₂) = 296.3 kJ mol^-1.

The growth mechanism of polycyclic aromatic hydrocarbons from the reactions of anthracene and phenanthrene with cyclopentadienyl and indenyl

Polycyclic aromatic hydrocarbons (PAHs) are highly toxic, mutagenic and/or carcinogenic to humans. To reduce the emission of PAHs, it's significant and indispensable to explore the PAH formation mechanism. In the present work, the growth mechanism of PAHs from the reactions of anthracene and phenanthrene with cyclopentadienyl and indenyl radicals was investigated with the aid of high-accuracy quantum chemistry calculation. The rate constants of key elementary steps were calculated by meaning of the canonical variation transition-state (CVT) theory with the small curvature tunneling (SCT) correction over the temperature range of 400-1400 K. The mechanism of the PAH formation involves in six elementary steps, addition reaction, ring closure, intramolecular H-shift, cleavage of CC bond, intramolecular H-shift and unimolecular elimination of CH₃ or H. The cleavage of CC bond is the rate-determining step due to the high barrier. The formation of PAHs from the reactions of anthracene with cyclopentadienyl and indenyl radicals is easier than that from the reactions of phenanthrene.

Nanofiller Presence Enhances Polycyclic Aromatic Hydrocarbon (PAH) Profile on Nanoparticles Released during Thermal Decomposition of Nano-enabled Thermoplastics: Potential Environmental Health Implications

Nano-enabled products are ultimately destined to reach end-of-life with an important fraction undergoing thermal degradation through waste incineration or accidental fires. Although previous studies have investigated the physicochemical properties of released lifecycle particulate matter (called LCPM) from thermal decomposition of nano-enabled thermoplastics, critical questions about the effect of nanofiller on the chemical composition of LCPM still persist. Here, we investigate the potential nanofiller effects on the profiles of 16 Environmental Protection Agency (EPA)-priority polycyclic aromatic hydrocarbons (PAHs) adsorbed on LCPM from thermal decomposition of nano-enabled thermoplastics. We found that nanofiller presence in thermoplastics significantly enhances not only the total PAH concentration in LCPM but most importantly also the high molecular weight (HMW, 4-6 ring) PAHs that are considerably more toxic than the low molecular weight (LMW, 2-3 ring) PAHs. This nano-specific effect was also confirmed during in vitro cellular toxicological evaluation of LCPM for the case of polyurethane thermoplastic enabled with carbon nanotubes (PU-CNT). LCPM from PU-CNT shows significantly higher cytotoxicity compared to PU which could be attributed to its higher HMW PAH concentration. These findings are crucial and make the case that nanofiller presence in thermoplastics can significantly affect the physicochemical and toxicological properties of LCPM released during thermal decomposition.