Theoretical Insights into the Reaction Mechanism between 2,3,7,8

Publication Date (Web): January 7, 2019 ... A detailed knowledge of the reactivity of 2,3,7,8-tetrachlorodibenzofuran (TCDF) at the molecular level is...
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Article Cite This: ACS Omega 2019, 4, 358−367

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Theoretical Insights into the Reaction Mechanism between 2,3,7,8Tetrachlorodibenzofuran and Hydrogen Peroxide: A DFT Study Nana Bai, Weihua Wang,* Yun Zhao, Wenling Feng, and Ping Li* Key Laboratory of Life-Organic Analysis, School of Chemistry and Chemical Engineering, Qufu Normal University, No. 57 Jingxuan West Road, Qufu 273165, P. R. China

ACS Omega 2019.4:358-367. Downloaded from pubs.acs.org by 94.231.219.237 on 01/12/19. For personal use only.

S Supporting Information *

ABSTRACT: 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.

1. INTRODUCTION

addition, it was also found that water molecules are necessary for the production of free radicals.26,27 From the structural viewpoint, PCDFs have similar geometrical features as those of the chlorinated benzoquinones. For instance, all of them have planar ring structures and C−Cl bonds. Meanwhile, they are also good electron acceptors because they have positive electron affinities.28−31 In this case, it is expected that PCDFs and chlorinated benzoquinones may have some similarities in reactivity. Inspired by the reactivity of organic hydroperoxides with chlorinated benzoquinones,21−27 we wonder if the organic hydroperoxides can also react with the PCDFs so as to transform the PCDFs. If possible, what are the detailed reaction mechanisms? Clearly, the answers of the above questions can give us a full understanding of the reactivity of organic hydroperoxides as well as the transformation of PCDFs environmental pollutants. Unfortunately, no relevant studies have been reported so far to the best of our knowledge. Probably, the high toxicity of the PCDFs limits the performance of the relevant experiments. Therefore, taking 2,3,7,8-tetrachlorodibenzofuran (TCDF) as a model compound of PCDFs, its reactions with H2O2 and its anion have been explored using the density functional theory

It is well known that persistent organic pollutants (POPs) are one of the most difficult-to-treat pollutants because they are resistant to environmental degradation via biological, chemical, and photolytic processes under natural conditions. They have potential adverse impacts on human health and the environment because of their high toxicity. As one of the typical POPs, polychlorinated dibenzofurans (PCDFs) belonging to the dioxins have attracted extensive attentions. There are many anthropogenic and natural sources to produce PCDFs in the presence of the chlorine source. To this day, more and more studies have been performed to understand the formation1−8 and degradation9−20 of PCDFs. Moreover, many methods have been developed for the thermal and chemical decomposition of PCDFs. However, only the active hydroxyl, NO3, and H radicals can react with PCDFs due to their unique structures. Therefore, seeking novel species to react with PCDFs is important for the transformation of PCDFs. Recently, it has been reported that the organic hydroperoxides (e.g., H2O2) can readily react with the chlorinated benzoquinones (e.g., tetrachloro-p-benzoquinone and 2,5-dichlorobenzoquinone), where OH, organic alkoxyl radicals, and other highly reactive free radicals have been produced experimentally.21−24 Moreover, these produced radicals can lead to the DNA oxidation, which can be used to account for the potential carcinogenicity of the halogenated aromatic compounds.25 In © 2019 American Chemical Society

Received: April 14, 2018 Accepted: July 19, 2018 Published: January 7, 2019 358

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Scheme 1. Proposed Reaction Mechanism between TCDF and HO2−

Figure 1. Optimized transition states for the direct nucleophilic attack of H2O2 on TCDF. The selected distances are given in angstrom, which is true for the following figures.

(DFT). The detailed reaction mechanisms have been discussed for the whole reaction as well as the substitution effects of F-, Br-, and CH3-groups on the reactivity of the title reaction. Expectedly, the present findings not only can further enrich the understanding of the potential reactivity of organic hydroperoxides but can also provide useful clues to the performance of the related experimental studies on the transformation of the PCDFs environmental pollutants.

Table 1. Energy Barriers for the Direct Nucleophilic Attack on TCDF by H2O2 in Terms of Different Attack Modesa attack modes

ΔE1

ΔE2

ΔE2 − ΔE1

mode a mode b mode c mode d

51.06/50.13 51.28/50.85 53.82/53.88 52.08/52.08

51.30 49.42 51.11 52.15

0.24 −1.86 −2.71 0.08

a

All of the units are in kcal/mol. The data behind the slash refer to the results at the M06-2X/6-311++G(d,p) level of theory. ΔE1 and ΔE2 refer to the results in the gas phase and in aqueous solution calculated with the SMD solvation model, respectively.

2. RESULTS AND DISCUSSION As displayed in Scheme 1, the reaction mechanisms of the title reaction have been proposed. Similar to the reaction between tetrachloro-p-benzoquinone and H2O2,26,27 the whole reaction is initiated by the formation of a molecular complex (MC). Then, the nucleophilic attack of H2O2 to TCDF takes place to form an intermediate (IM) containing an O−O bond. Finally, the O−O bond homolytically decomposes, leading to the production of radicals. The above reaction mechanisms were testified as follows. 2.1. Reaction of TCDF with the Neutral H2O2. First of all, the direct reaction between TCDF and H2O2 has been investigated. In view of the C2v symmetry of TCDF and the possible orientations of the H atom in H2O2, four nucleophilic attack modes of H2O2 to TCDF, i.e., modes a, b, c, and d, have been considered. As shown in Figure 1, the corresponding transition states TSn (n = a−d) for each mode have been located. As presented in Table 1, the calculated energy barriers for the four nucleophilic attack modes are 51.06, 51.28, 53.82, and 52.08 kcal/mol, respectively. Obviously, such high-energy barriers suggest that it is very difficult for the title reaction to take place under normal conditions. Can solvent effects promote the nucleophilic attack processes? To answer this question, the solvent effects in aqueous solution have been considered by

employing the solvent model density (SMD) model. As a result, as presented in Table 1, the energy barriers of the four reaction processes have been changed ranging from −2.71 to 0.24 kcal/ mol upon going from the gas phase to solution, suggesting slight solvent effects on the reaction involving the neutral H2O2. Therefore, the nucleophilic attack processes are still difficult to proceed even in the solution. Moreover, given the fact that the above nucleophilic attack process involves the proton transfer (PT) process from H2O2 to the dissociated chlorine atom of TCDF and water molecule is a good catalyst in assisting the PT, the reaction between TCDF and H2O2 has been explored with the help of the different numbers of water molecules ranging from 1 to 3 based on the attack mode b. For simplicity, the symbols MC(nw), IM(nw), and TS(nw) have been used to denote the located MCs, intermediates, and transition states, where nw stands for the number of the water molecules introduced. Here, four reaction pathways have been named as A, B, C, and D in terms of the numbers of the introduced water molecules in the reaction, corresponding to the presence of 0, 1, 2, and 3 water molecules, respectively. 359

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Figure 2. Optimized molecular complexes (MCs), intermediates (IMs), and transition states (TSs) for the available reaction pathways.

2.1.1. Formation of the MCs. First, the MCs formed during the reaction have been investigated based on the intrinsic reaction coordinate (IRC) calculations of the located transition states TS1(nw)n=0−3 in the nucleophilic attack processes. As shown in Figure 2, different MCs MC(nw)n=0−3 have been obtained. In view of the high toxicity of TCDF and the instability

of MCs, it is necessary to characterize them theoretically. Therefore, the atoms-in-molecule (AIM) analyses of these MCs have been performed. Correspondingly, the molecular graphs and the topological analyses of these MCs have been given in Figure S1 and Table S1 of the Supporting Information (SI), respectively. 360

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increasing the numbers of water molecules, which is crucial for the subsequent reaction steps probably. 2.1.2. Nucleophilic Aromatic Substitution Process. As displayed in Figure 2, all of the transition states TS1(nw)n=0−3 in the nucleophilic attack process have been located, where all of them have been verified by the IRC calculations. As displayed in Figure 4, the microscopic details during the nucleophilic attack process can be observed. For instance, for the process without involving water molecules, one of the O atoms (O22) of H2O2 directly attacks the C atom (C11) of TCDF. Meanwhile, the H atom (H23) of H2O2 and the Cl atom (Cl19) of TCDF begin to dissociate simultaneously. As a result, the intermediate IM(0w) can be obtained accompanied by the dissociation of the H23 and Cl19 atoms. Note that the O22−H23 bond of H2O2 begins to increase significantly until the O22 atom approaches the C11 atom of TCDF within a certain distance, reflecting the asynchrony of the nucleophilic aromatic substitution reaction. As for the process involving one water molecule, as displayed in Figure 4, the introduced H2O molecule accepts the proton of H2O2 and gives its own proton to the chlorine atom of TCDF simultaneously, reflecting the catalytic role of H2O in the PT. Moreover, no zwitterionic species have been observed during the PT process. Therefore, the above PT processes should proceed in one step. Similarly, the same phenomena have also been observed in the PT processes assisted with 2 and 3 water molecules. To further clarify the catalytic role of water molecules, the selected distances associated with the nucleophilic attack processes have been analyzed. As shown in Figure 2, it was found that the distances between the dissociated chlorine atom and its adjacent C atom (RC···Cl) in the TSs decreased upon introducing the water molecules. Moreover, the RC···Cl decreases upon increasing the number of water molecules. For instance, the RC···Cl is 2.293, 2.201, 2.158, and 2.135 Å in the transition states TS1(nw) involving 0, 1, 2, and 3 water molecules, respectively. However, the opposite is true for the distance between the attacking O atom of H2O2 and the attacked C atom of TCDF. In other words, H2O2 and TCDF have no obvious geometrical deformation upon introducing the water molecules relative to the direct reaction, implying the decrease in the energy barriers in the presence of water molecules. Expectedly, as shown in Figure 3 and Table 2, the original energy barrier has been reduced significantly with the help of explicit water molecules. For instance, the energy barrier is

As displayed in Figure 2, TCDF, H2O2, and H2O interact with each other via the intermolecular H-bonds in the MCs. As shown in Figure S1 of the SI, this point can be further verified by the location of the corresponding bond critical points (BCPs). As for the introduced H2O molecules, they interact with both the TCDF and H2O2 via intermolecular H-bonds. As presented in Table S1 of the SI, the positive ∇2ρbcp and Hbcp at the BCPs suggest that most of the intermolecular H-bonds are governed by the electrostatic interactions, which can be also reflected from the large H-bonding distances. For the selected H-bonds formed between H2O2 and H2O in MC(2w) and MC(3w), they should possess the partial covalent properties as can be seen from the positive ∇2ρbcp and negative Hbcp at the BCPs.

Figure 3. Reaction profiles for the reaction of TCDF with H2O2. The symbols R and P stand for the separated reactants and products, respectively.

As displayed in Figure 3 and Table 2, the formed MC(0w) has been stabilized by 1.75 kcal/mol relative to TCDF and H2O2. Moreover, the stabilization energy increases upon increasing the number of water molecules. For instance, MC(1w), MC(2w), and MC(3w) have been stabilized by 7.64, 17.14, and 24.81 kcal/mol, respectively. At the same time, the negative enthalpy changes suggest that the formation of the MCs is an exothermic process. Especially, the released reaction heat increases upon

Table 2. Calculated Relative Energy (ΔE), Enthalpy Changes (ΔH), and Gibbs Free Energy Changes (ΔG) for the Available MCs, IMs, TSs, and Products Relative to the Isolated Reactants in the Different Reaction Pathwaysa pathways

parameters

MC

TS1

IM

TS2

Pro

A

ΔE ΔH ΔG ΔE ΔH ΔG ΔE ΔH ΔG ΔE ΔH ΔG

−1.75 −1.10 4.49 −7.64 −7.38 6.82 −17.14 −17.59 7.52 −24.81 −26.14 7.97

51.28 50.81 61.87 39.31 38.15 59.19 29.03 27.13 57.70 20.85 18.46 57.77

−6.46 −6.08 1.19 −12.57 −12.71 2.35 −24.32 −25.64 1.11 −31.05 −33.19 4.37

−1.04 (5.42) −1.45 8.22 −4.91 (7.66) −6.12 13.05 −2.42 (21.90) −3.77 22.32 −2.73 (28.31) −5.06 31.71

−15.28 −14.97 −18.09 −22.67 −23.12 −17.51 −13.59 −14.76 −0.13 −38.33 −40.26 −16.33

B

C

D

a

All of the units are in kcal/mol. The data in parentheses refer to the results relative to the IM(nw). 361

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Figure 4. Variations in the selected bond lengths along with the IRC of the transition states TS1(0w) (left) and TS1(1w) (right).

Table 3. Calculated BDEs of the O−O bond in H2O2 and the Intermediatesa species

H2O2

IM(0w)

IM(1w)

IM(2w)

IM(3w)

BDE

48.62(44.08)

21.09(6.26)

21.41(5.90)

31.16(10.50)

35.48(12.56)

a

All of the units are in kcal/mol. The data in parentheses refer to adiabatic BDEs.

mol for the cleavage of the IM(0w). Moreover, the energy barriers are 7.66, 21.90, and 28.31 kcal/mol with the help of 1, 2, and 3 water molecules, respectively. Here, note that these TSs are lower in energy by 1.04, 4.91, 2.42, and 2.73 kcal/mol than the free reactants, respectively. Therefore, the intermediates IM(nw)n=0−3 can produce two radicals via the homolysis of the O−O bond. Compared with the nucleophilic attack processes, the energy barrier for the cleavage of the O−O bond is relatively small. Therefore, the nucleophilic attack process should be the ratedetermining step in the whole reaction. At the same time, as shown in Table 2, the negative enthalpy changes suggest that the whole reactions are exothermic processes. 2.2. Reaction of TCDF with HO2− Anion. Moreover, given the fact that the anion of H2O2 (HO2−) exists due to the dissociation equilibrium of H2O2, and that the HO2− anion is a better nucleophile relative to the neutral H2O2, so the direct reaction between TCDF and HO2− has also been investigated. As shown in Figure 5, four different modes of nucleophilic attack, i.e., modes a′, b′, c′, and d′, have been considered based on the structural symmetry of TCDF. Similar to the reaction of TCDF with the neutral H2O2, the MCs and TSs for the four modes have also been located. Subsequently, nucleophilic attack by HO2− on TCDF takes place, resulting in the formation of the intermediates containing an O−O bond. As expected, the subsequent process should proceed to produce radicals through the cleavage of the O−O bond homolytically. Unlike the reaction of the neutral H2O2 mentioned above, no explicit water molecules are needed here. Moreover, it was found that the formed four MCs have been stabilized by about 24.34, 22.90, 31.35, and 30.87 kcal/mol relative to the separated reactants, respectively. Similarly, the corresponding TSs are also lower in energy by about 16.69, 15.84, 15.55, and 15.01 kcal/mol relative to the initial reactants. So, the calculated energy barriers for the nucleophilic attack of TCDF by HO2− are 7.65, 7.06, 15.81, and 15.87 kcal/mol relative to the MCs, respectively. In aqueous solution, the energy barriers are 17.71, 17.29, 21.55, and 21.67

decreased by 30.43 to 20.85 kcal/mol with the help of three water molecules, suggesting the positive catalytic role of water molecules. Therefore, it is feasible for the nucleophilic aromatic substitution process to take place with the assistance of H2O molecule. 2.1.3. Cleavage of the O−O Bond. Subsequently, the intermediates IM(nw)n=0−3 containing the O−O bond can be produced after the nucleophilic aromatic substitution process. As displayed in Figure 2, more intermolecular H-bonds can be observed in intermediates IM(nw)n=0−3, which can be verified by the presence of the BCPs, as shown in Figure S1 of the SI. Moreover, as presented in Table S1 of the SI, these intermolecular H-bonds can be divided into two groups in terms of the AIM results. Namely, for the intermolecular Hbonds involving the original TCDF fragment, they should be governed by the electrostatic interactions, as can be seen from the positive ∇2ρbcp and Hbcp at the BCPs of H-bonds. On the other hand, the rest H-bonds formed between the other fragments should possess partially covalent characters as can be seen from the positive ∇2ρbcp and negative Hbcp results at the BCPs, as well as their short H-bonding distances ranging from 1.552 to 1.795 Å. To explore the strength of the O−O bonds, the vertical and adiabatic bond dissociation enthalpies (BDEs) of the O−O bonds in IM(nw)n=0−3 have been calculated as well as that of H2O2. As presented in Table 3, the O−O bond of H2O2 is significantly weakened upon the formation of the IM(nw)n=0−3. For instance, the corresponding vertical BDE of H2O2 (48.62 kcal/mol) has been decreased by about 27.53 to 21.09 kcal/mol in IM(0w). Meanwhile, the significant decreases in the adiabatic BDE have also been observed if the structural relaxation effects are considered. Therefore, from the viewpoint of thermodynamics, it is easy to cleavage for these O−O bonds in IM(nw)n=0−3. Moreover, as displayed in Figure 2, the corresponding TSs involved in the cleavage of the O−O bond have been located. As displayed in Figure 3 and Table 2, the energy barrier is 5.42 kcal/ 362

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Figure 5. Optimized MCs and TSs in the nucleophilic aromatic substitution reaction of TCDF with HO2−.

Figure 6. Optimized MCs, IMs, and TSs in the nucleophilic addition reaction of TCDF with HO2−.

kcal/mol relative to the separated reactants, respectively. Compared with the results in the gas phase, the important

solvent effects should be highlighted for the anionic system. Overall, the calculated energy barrier for the reaction of HO2− 363

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Figure 7. Calculated TSs in the nucleophilic attack of TCDF and its derivatives by hydroperoxides (top) and their anions (middle and bottom), where F-, Br-, and CH3-substitutions are shown from left to right, respectively.

Figure 8. Reaction mechanisms for the reaction of TCDF with ROO− (R = H or alkyl groups).

Here, only the rate-determining step, i.e., the nucleophilic attack process involving three water molecules, has been considered. As shown in Figure 7, the located TSs are similar to the above TS1(3w). For the reactions of F- and Br-substituted TCDF with H2O2, the calculated energy barriers of 19.03 and 18.98 kcal/ mol are lower by 1.82 and 1.87 kcal/mol than those of the results before substitution. As for the reaction of the CH3-substituted H2O2 with TCDF, the energy barrier of 21.76 kcal/mol is higher than that of the result before substitution by 0.91 kcal/mol. Thus, it is feasible for the reaction to take place between the neutral organic hydroperoxides and the TCDF and its derivatives. Moreover, the reactions involving the anions of the organic hydroperoxides have also been investigated. Taking the model a′ for example, the corresponding TSs have been given in Figure 7. For the reactions of HO2− with F- and Br-substituted TCDF, the energy barriers are 3.04 and 8.66 kcal/mol relative to the MCs, which are lower and higher by 4.61 and 1.01 kcal/mol than those of the reactions before substitution, respectively. Similarly, as for the reaction of the CH3-substituted HO2− with TCDF, the energy barrier of 3.83 kcal/mol is lower than the reaction before substitution by about 3.82 kcal/mol. As for the nucleophilic addition reaction of HO2−, as displayed in Figure 7, the energy barriers associated with the

with TCDF is smaller than that of the reaction of the neutral H2O2, exhibiting the dependence of the title reaction on the pH value of the solution. In addition, as shown in Figure 6, another reaction mode has also been observed, i.e., the HO2− can attack the C* atom directly attached to the O atom of TCDF. In detail, first, a MC′ can be formed following by the nucleophilic addition of HO2− to the C* atom via TS1′ to form IM′, where the energy barrier is only 1.08 in the gas phase relative to the corresponding MC, respectively. Then, the C*−O bond in the intermediate IM′ can be dissociated via the transition sate TS2′. Here, the corresponding energy barrier of 1.71 (4.17) kcal/mol in the gas phase (aqueous solution) suggests that the dissociation of the C*−O bond is easy to occur, where the energy barrier of the dissociation of the O−O bond of IM′ is 13.13 kcal/mol. Moreover, the energy barrier of 26.76 kcal/mol is required to overcome the dissociation of the O−O bond after the dissociation of the C*−O bond. Therefore, it is easy to dissociate the C−O bond of TCDF in the presence of HO2−. 2.3. Substitution Effects. To further investigate the reactivity of the organic hydroperoxides with TCDF and its derivatives, the reactions between H2O2 and the F- and Brsubstituted TCDFs have been investigated, as well as the reaction between the TCDF and the CH3-substituted H2O2. 364

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reactant and product properly. In addition, all of the Cartesian coordinates of the studied species have been given in the SI for reference. For comparison, the selected calculations have also been performed at the M06-2X/6-311++G(d,p) level of theory. As presented in Table 1, both the B3LYP and M06-2X methods can give consistent results with each other for the calculated energy barriers in the nucleophilic attack process. Given the compromise between the accuracy and computational cost for the present systems, the results using the B3LYP method have been mainly discussed throughout. To explore the implicit solvent effects on the reactivity of the reaction, the SMD solvation model48 was used. To clarify the catalytic role of water molecules in assisting the PT process, different numbers of water molecules ranging from 1 to 3 have been introduced. To confirm the formation and the nature of the intermolecular H-bonds, atoms-in-molecules (AIM) analyses have been performed. According to the AIM theory,49 the interatomic interactions were indicated by the location of a bond critical point (BCP) and the interaction strengths can be evaluated from the electron density (ρbcp) at the BCP. Similarly, ring structures were characterized by their corresponding ring critical point. What is more, the topological parameters at the BCP, e.g., the Laplacian of ρbcp (∇2ρbcp) and the energy density (Hbcp), can be used to characterize the nature of the H-bonding interaction. In detail, ∇2ρbcp < 0 suggests the existence of a covalent bond and ∇2ρbcp > 0 and Hbcp > 0 suggest the presence of a noncovalent bond, such as H-bonds and van der Waals interactions. In addition, when ∇2ρbcp > 0 and Hbcp < 0, the interactions should have partial covalent properties.50−52 To evaluate the O−O bond strength in the formed intermediate, the adiabatic bond dissociation enthalpies (BDEs) of the O−O bonds have been calculated as the enthalpy difference between the dissociated fragments and the intermediate. As for the vertical BDE, it is calculated as the total energy difference before and after the dissociation without considering the structural relaxation. All of the DFT calculations were done employing Gaussian 09 software package.53

C−O bond dissociation are 1.43, 1.85, and 4.68 kcal/mol for the F-, Br-, and CH3-substituted cases, respectively. On the basis of the above results, one can say that the anions of the organic hydroperoxides are easier to react with the TCDF and its derivatives than those of the neutral hydroperoxides. In summary, the organic hydroperoxides (especially their anions) can react with TCDF and its derivatives. Taking the reactions of ROO− (R = H or alkyl groups) with TCDF as an example, their reaction mechanisms can be outlined as follows. As shown in Figure 8, on the one hand, nucleophilic aromatic substitution reaction occurs between TCDF and ROO− to form an intermediate containing an O−O bond. Then, the O−O bond of the intermediate cleavages homolytically, leading to the production of the different radicals depending on the selected substituents in the ROO−. On the other hand, nucleophilic addition reaction can also easily occur between TCDF and ROO−, resulting in the dissociation of the C−O bond of TCDF. Certainly, more complicated experiments are highly desirable to further verify the present findings.

3. CONCLUSIONS In this study, the reaction mechanisms of the neutral and anionic H2O2 with TCDF and its derivatives have been explored using the DFT. The principal conclusions are summarized below. (1) It is difficult for the reaction to take place between TCDF and neutral H2O2 for the direct nucleophilic aromatic substitution process. However, the explicit water molecules can efficiently promote the proceeding of the reaction via the decrease in the energy barrier. On the contrary, the bulk solvent effects have slight influences on the reaction. (2) The reaction of TCDF and its derivatives with HO2− can occur easily and does not require the assistance of water molecules. On the one hand, nucleophilic aromatic substitution reaction can occur between TCDF and ROO−, leading to the production of the different radicals depending on the selected substituents in the ROO−. On the other hand, nucleophilic addition reaction can also easily occur between TCDF and ROO−, leading to the dissociation of the C−O bond of TCDF. Overall, it is much easier for the anions of the organic hydroperoxides to react with the TCDF than those of the neutral cases, suggesting the high dependence of the reactivity of the title reaction on the pH value of the media. (3) In view of the fact that the formed extremely reactive radicals (e.g., hydroxyl and alkyl radicals) may cause potential damage to organisms, so the title reaction can partially account for the toxicity mechanism of TCDF-like environmental pollutants. Certainly, the related experiments are highly desirable to further confirm the present findings.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.8b00724. Molecular graphs of molecular complexes, intermediates, and transition states; topological parameters for the molecular complexes and intermediates; the Cartesian coordinates of all the species mentioned in this study (PDF)

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4. COMPUTATIONAL DETAILS Geometry optimizations for all of the species in this study have been performed within the framework of the DFT theory using the B3LYP/6-311++G(d,p) level of theory, where the accuracy and reliability of the DFT have been confirmed by many systems.32−45 Then, vibrational frequency analysis has been carried out for the optimized geometries to confirm the stability of the located stationary points. Moreover, intrinsic reaction coordinate (IRC)46,47 calculations were also carried out to ensure that the calculated transition states connected the

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (W. Wang). *E-mail: [email protected] (P. Li.). ORCID

Ping Li: 0000-0003-3469-4912 Notes

The authors declare no competing financial interest. 365

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ACKNOWLEDGMENTS This work is supported by National Natural Science Foundation of China (Nos. 21577076 and 21303093), Natural Science Foundation of Shandong Province (No. ZR2018MB020), and the Doctoral Foundation of Shandong Province (Nos. ZR2016BB20 and ZR2017BB055).

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