Wenling Feng

Insights into the one-electron reduction behavior of tetrachloro-o-benzoquinone: a DFT and molecular dynamics study

In this study, the one-electron reduction behavior of tetrachloro-o-benzoquinone (o-TCBQ) has been systematically investigated at the B3LYP/6-311++G** level of theory in combination with the ab initio molecular dynamics. It was found that the explicit water molecules have slight effects on the geometry of o-TCBQ. On the contrary, the introduction of an electron can make the geometry change significantly. Moreover, the C2v symmetry of neutral and anionic o-TCBQ in the gas phase has been changed to be C2 symmetry in solution. All the electron affinity and vertical detachment energies are positive in the gas phase and in solution, increasing with the increasing of the dielectric constant of the bulk solvent. Therefore, explicit water molecules and bulk solvents can efficiently enhance the electron-accepting ability of the o-TCBQ, reflecting the intrinsic nature of o-TCBQ as a good electron acceptor in different media.

Theoretical studies on the spin trapping of the 2-chloro-5-hydroxy-1,4-benzoquinone radical by 5,5-dimethyl-1-pyrroline N-oxide (DMPO): the identification of the C–O bonding spin adduct

The detection and identification of related radicals is crucial for the elucidation of the reaction mechanisms for metal-independent decomposition of hydroperoxides by halogenated quinones. In this study, the spin trapping of the 2-chloro-5-hydroxy-1,4-benzoquinone radical (CBQ) produced in the reaction of 2,5-dichloro-1,4-benzoquinone and t-butylhydroperoxide by 5,5-dimethyl-1-pyrroline N-oxide (DMPO) and its subsequent reaction processes have been systematically investigated at the B3LYP/6-311++G(d,p) level of theory in combination with the atoms in molecules (AIM) theory, natural bond orbital (NBO) theory, and ab initio molecular dynamics. It was found that DMPO and CBQ can not only form the C–C bonding spin adduct observed experimentally, but also can form the C–O bonding spin adduct. This point has been further tested by the spin trapping of the other halogenated CBQ radicals. After that, the keto–enol tautomerization occurs for the formed C–C bonding spin adduct, where the explicit water molecule plays an important catalytic role in assisting the proton transfer process. Subsequently, spontaneous proton transfer has been observed from the hydroxyl group of the CBQ fragment to the adjacent O atom of the DMPO fragment in the formation process of the oxidation state of the spin adduct. These results not only help deepen our understanding of the spin trapping mechanism of CBQ-type radicals by DMPO, but also can provide important clues to the clarification of the reaction mechanism between halogenated quinone and organic hydroperoxides.

An identification of the C–C bonding spin adduct in the spin trapping of N-methyl benzohydroxamic acid radical by 5,5-dimethyl-1-pyrroline N-oxide

A detailed knowledge of the spin trapping of the radicals by spin traps is crucial for the elucidation of the reaction mechanisms involving radicals and the rational design of the novel efficient spin traps experimentally. In this study, the spin trapping of N-methyl benzohydroxamic acid radical (·N-MeBHA) produced in the reaction of 2,5-dichloro-1,4-benzoquinone (DCBQ) with N-methyl benzohydroxamic acid has been systematically investigated employing 5,5-dimethyl-1-pyrroline N-oxide (DMPO) as a spin trap theoretically. The tautomerization behavior of ·N-MeBHA radical has been firstly investigated, and four tautomers including C- and N-centered forms have been located. After that, the nature of the formed spin adducts has been explored as well as the thermodynamic and kinetic parameters associated with the spin-trapping process. Moreover, the reaction of the ·N-MeBHA radical with the C-centered quinone ketoxy radical has been studied. Besides the available C–N bonding product identified experimentally, more stable C–C bonding products have also been observed. Additionally, significant catalytic role of explicit water molecules should be highlighted in the tautomerization reaction of the ·N-MeBHA radical and the keto–enol tautomerization reaction of the final products. This study demonstrates for the first time the possibility of the existences of the C-centered ·N-MeBHA radical and the C–C bonding product in the reaction of DCBQ and N-MeBHA, providing new insights into the reaction mechanisms between polyhalogenated quinones and hydroxamic acids.

Insights Into the Reaction Mechanism of Criegee Intermediate CH2OO with Methane and Implications for the Formation of Methanol

Criegee intermediates (CIs) play a key role in controlling the atmospheric budget of hydroxyl radical, organic acids, and secondary organic aerosols. In this study, the detailed reaction mechanisms of the simplest Criegee intermediate CH2OO and its derivatives with methane (CH4) have been systematically investigated theoretically. Two pathways A and B have been identified for the title reaction. In pathway A, CIs can act as an oxygen donor by inserting its terminal oxygen atom into the C-H bond of alkanes, resulting in the formation of alcohol species. The corresponding energy barriers ranging from 6.5 to 24.1 kcal/mol are associated with the O-O bond strength of CIs. Meanwhile, this pathway is more favorable thermodynamically, where the free energy changes (enthalpy changes) range from -81.1 (-78.3) to -110.9 (-109.0) kcal/mol, respectively. In pathway B, an addition reaction to produce the hydroperoxides occurs, accompanying the hydrogen transfer from the alkanes to the terminal oxygen atom of CIs. The corresponding energy barriers ranging from 17.3 to 30.9 kcal/mol are higher than those in pathway A. Further calculations of the rate constants suggest that pathway A is the most favorable reaction channel and the rate constant exhibits a positive temperature dependence. In addition, the conformation-dependent reactivity for the title reaction has been observed. The present findings can enable us to better understand the potential reactivity of CIs in the presence of the alkane species.

A DFT study on the reaction mechanism between tetrachloro-o-benzoquinone and H2O2 and an alternative reaction approach to produce the hydroxyl radical

The formation of hydroxyl and alkoxyl radicals in the reaction of halogenated quinones and organic hydroperoxides can be used to elucidate the potential carcinogenicity of polyhalogenated aromatic environmental pollutants. To further enrich the understanding of the reactivity of the halogenated quinones with organic hydroperoxides, in this study, the reaction mechanism of tetrachloro-o-benzoquinone (o-TCBQ) with H2O2 has been systematically investigated at the B3LYP/6-311++G** level. It was found that a molecular complex was formed as the first step of the title reaction. After that, the nucleophilic attack of H2O2 on o-TCBQ occurs to produce an unstable intermediate containing an O–O bond. Subsequently, the unstable intermediate decomposes homolytically via the cleavage of the O–O bond, resulting in the formation of the OH radical. Note that explicit water molecules play an important positive role in the nucleophilic attack process. The nucleophilic attack process is the rate-determining step in the whole reaction. Moreover, selected substitution effects on the title reaction have also been studied. In addition, as an alternative reaction approach, it was found that the formed unstable intermediate containing an O–O bond mentioned above can be produced directly from the nucleophilic attack of the anionic form of H2O2 on o-TCBQ in the absence of explicit water molecules.

Theoretical insights into the reaction mechanism between tetrachloro-o-benzoquinone and N-methyl benzohydroxamic acid

Acquiring the detailed reaction mechanism between halogenated quinones and hydroxamic acids is crucial for better understanding of the potential applications of benzohydroxamic acids in the detoxification of the carcinogenic polyhalogenated quinoid metabolites of pentachlorophenol. In this study, the reaction mechanism between tetrachloro-o-benzoquinone (o-TCBQ) and N-methyl benzohydroxamic acid (N-MeBHA) has been systematically investigated at the B3LYP/6-311++G(d,p) level. It was found that o-TCBQ can react with the anion of N-MeBHA (N-MeBHA−) under mild conditions. As the first step of reaction, a molecular complex is formed between o-TCBQ and N-MeBHA− followed by the nucleophilic attack of the O atom of N-MeBHA− at the C atom attached to the Cl atom of o-TCBQ, resulting in the formation of an unstable intermediate containing an N–O bond. Subsequently, the unstable intermediate decomposes via the homolytic cleavage of the N–O bond to produce N-centered and O-centered radicals. For the O-centered radical, it can isomerize to a C-centered form upon structural relaxation. Finally, these radicals react with each other to form the major C–N bonding products and minor C–O bonding products. In addition, it was found that the reactivity of o-TCBQ with N-MeBHA is higher than that of tetrachloro-p-benzoquinone (p-TCBQ).

Theoretical insights into the reaction mechanisms between 2,3,7,8-tetrachlorodibenzofuran and the methylidyne radical

To explore the potential role of the methylidyne radical (CH) in the transformation of 2,3,7,8-tetrachlorodibenzofuran (TCDF), in this study, the detailed reaction mechanisms between TCDF and CH radical have been systematically investigated employing the B3LYP method of density functional theory (DFT) in combination with the atoms in molecules (AIM) theory and ab initio molecular dynamics. It was found that the title reaction is a multi-channel reaction, i.e., the CH radical can attack the C–X (X = C, Cl, H, O) bonds of TCDF via the insertion modes, resulting in the formation of 13 products. Thermodynamically, the whole reaction processes are exothermic and spontaneous since all the enthalpy and Gibbs free energy changes are negative values in the formation processes. Moreover, the thermodynamic stability of the products is controlled by the distribution of the single unpaired electron. Kinetically, the most favorable reaction channel is the insertion of the CH radical into the C–C bond except for the C atoms attached to the chlorine atom. Moreover, the dominant products have been further confirmed by the molecular dynamics. Meanwhile, the IR spectra and hyperfine coupling constants of the dominant products have been investigated to provide helpful information for their identification experimentally. In addition, the reactivity of the CH radical toward the F- and Br-substituted TCDFs has also been investigated. Expectedly, the present findings can enable us to better understand the reactivity of the CH radical toward organic pollutants analogous to TCDF in the atmosphere.

Theoretical Insights into the Electron Capture Behavior of H2SO4···N2O Complex: A DFT and Molecular Dynamics Study

Both sulfuric acid (H2SO4) and nitrous oxide (N2O) play a central role in the atmospheric chemistry in regulating the global environment and climate changes. In this study, the interaction behavior between H2SO4 and N2O before and after electron capture has been explored using the density functional theory (DFT) method as well as molecular dynamics simulation. The intermolecular interactions have been characterized by atoms in molecules (AIM), natural bond orbital (NBO), and reduced density gradient (RDG) analyses, respectively. It was found that H2SO4 and N2O can form two transient molecular complexes via intermolecular H-bonds within a certain timescale. However, two molecular complexes can be transformed into OH radical, N2, and HSO4− species upon electron capture, providing an alternative formation source of OH radical in the atmosphere. Expectedly, the present findings not only can provide new insights into the transformation behavior of H2SO4 and N2O, but also can enable us to better understand the potential role of the free electron in driving the proceeding of the relevant reactions in the atmosphere.

Theoretical Insights into the Interaction Mechanisms between Nitric Acid and Nitrous Oxide Initiated by an Excess Electron

Nitric acid (HNO3) and nitrous oxide (N2O) play an important role in the atmospheric chemistry in regulating the global environment and climate changes. In this study, the interaction mechanisms between them have been systematically investigated before and after the electron capture employing the density functional theory in combination with the AIM, NBO, and ab initio molecular dynamics calculations. It was found that HNO3 and N2O can form transient complexes through intermolecular H-bonds. HNNO, OH, and NO2 free radicals can be produced after the electron capture of the formed complexes, providing an alternative source of these radicals in the atmosphere. The present results not only can provide new insights into the transformation of the HNO3 and N2O atmospheric species but also can enable us to better understand the potential role of the free electron in the atmosphere.

Theoretical Investigations on the Reactivity of Methylidyne Radical toward 2,3,7,8-Tetrachlorodibenzo-p-Dioxin: A DFT and Molecular Dynamics Study

To explore the potential reactivity of the methylidyne radical (CH) toward 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), the reaction mechanism between them has been systematically investigated employing the density functional theory (DFT) and ab initio molecular dynamics simulations. The relevant thermodynamic and kinetic parameters in the possible reaction pathways have been discussed as well as the IR spectra and hyperfine coupling constants (hfcc’s) of the major products. Different from the reaction of the CH radical with 2,3,7,8-tetrachlorodibenzofuran, CH radical can attack all the C-C bonds of TCDD to form an initial intermediate barrierlessly via the cycloaddition mechanism. After then, the introduced C-H bond can be further inserted into the C-C bond of TCDD, resulting in the formation of a seven-membered ring structure. The whole reactions are favorable thermodynamically and kinetically. Moreover, the major products have been verified by ab initio molecular dynamics simulations. The distinct IR spectra and hyperfine coupling constants of the major products can provide some help for their experimental detection and identification. In addition, the reactivity of the CH radical toward the F- and Br-substituted TCDDs has also been investigated. Hopefully, the present findings can provide new insights into the reactivity of the CH radical in the transformation of TCDD-like dioxins.