English

• 1. INTRODUCTION

  Since it was firstly identified as a chlorocarbon in 1930, hexachlorocyclopentadiene (HCCP) has received much attention due to its chlorinated property, high reactivity and volatility^[1]. And HCCP is also widely used in the building block so as to obtain the biocidal and fire-retarding features, and in the synthesis of pesticides such as Chlordane, Heptachlor, Aldrin and Dieldrin through its Diels-Alder (D-A) condensation with the substrate^[2-5].

  Some studies have shown that HCCP is toxic for organism^[6-8]. In the range-finding study with rats the threshold of toxicity for HCCP was 0.11~0.5 ppm^[4]. Considering its use in the processes of pesticide synthesis, HCCP was qualitatively identified as a contaminant in the discharge of pesticide production plants^[9]. Although the annual United States production of HCCP is not a matter of public record, the yield of HCCP may be annually more than about 50 million pounds by estimating annual production of the cyclodiene insecticides^{[5, 10]}. In addition to a large amount of environmental content, the conversion products of HCCP will also bring potential harm to the environment. Some studies have shown that a chlorine atom can be easily removed from HCCP when the temperature increased. Under oxidative conditions, these chlorinated aromatics can become precursors of PCDD/F, which are some of the most important pollutants due to their carcinogenic, teratogenic, and mutagenic effects^[11-13].

  There have been numerous studies on the chemical reactivity of HCCP. In previous study, an experimental and theoretical investigation of the kinetics of HCCP into the pentachlorocyclopentadienyl radical (PCCP•) was researched^[2]. The results show that the rate constant for Cl fission from HCCP was found to be k = 1.45 × 10¹⁵exp(–222 ± 9 kJ⋅mol^-1⋅RT^-1)⋅s^-1[10]. Moreover, in aqueous phase the HCCP was photo-oxidized very efficiently^{[14, 15]}. The majority of photoproducts in different sensitizer substrates are respectively hexachlorocyclopentenone (C₅Cl₆O), tetrachlorobutadiene (C₄H₂C1₄) or tetrachlorobutyne (C₄Cl₄)^[16]. The diene reaction of HCCP with maleic acid was also researched, and their products can be utilized as a major constituent in the synthesis of polyester resins that are unusually flame resistant^[3]. However, little is known about the degradation mechanism of HCCP and the products in the atmosphere. Thus, in this study we focus on the reactions between HCCP with different oxidants (•OH, •NO₃, •HO₂ radicals and O₃) to gain insight into the understanding of molecular mechanisms of HCCP, which is significant for ecological safety and human health.

  To access the atmospheric behavior of HCCP, it is critical to understand their atmospheric reaction. Some previous studies have shown that quantum chemical calculation has been successfully applied to the study on the chemistry degradation mechanism of pesticides^[17-19], such as DDT and folpet. This study is targeted at investigating the reaction mechanism and rate constants for the reaction of HCCP with the typical atmospheric oxidants by using quantum chemical method. And the kinetic model is constructed based on the rate constants of element reactions within the temperature range of 200~320 K.

  2. COMPUTATIONAL METHOD

  2.1 Mechanism computation

  All the quantum chemical calculations were carried out using the Gaussian 09 program^[20]. In this study, the geometries of the reactants, transition states, intermediates and products, as well as its vibrational frequencies were optimized using the M06-2X method^[21] with a standard 6-311+G(d, p) basis set^[22]. M06-2X has been proven to be a reliable calculation for transformation mechanisms and kinetics of organic pollutants^{[22, 23]}. Meanwhile, in order to verify the transition states connected with the corresponding reactants and products, the intrinsic reaction coordinate (IRC)^[24-26] in both directions from a saddle point was computed. The single point energy of all species was optimized at the M062X/6-311++G(3df, 3pd) level in order to obtain more accurate energy.

  2.2 Kinetics computation

  The transition state theory (TST)^{[27, 28]} is a method to calculate the dynamic parameters of the elementary reaction by using molecular structure and thermodynamic properties. It has been widely applied to calculate the rate processes^[29-31]. In this paper, the KiSThelP program^[32] is used to calculate the rate constants based on TST with Wigner tunneling correction.

  The initial reaction of HCCP with the oxidants can be described as follows:

  ${\text{HCCP + X}}\underset{{{k_{ - 1}}}}{\overset{{{k_1}}}{\rightleftarrows}}{\text{HCCP}} \cdot \cdot \cdot {\text{X}}$

  $ {\text{HCCP}} \cdot \cdot \cdot {\text{X}}\xrightarrow{{k_2}}{\text{post reactive complex}} $

  In which X is the typical atmospheric oxidants, such as •NO₃, •HO₂, •OH and O₃.

  The total rate constant is calculated using the following equation:

  $ k_{total} = K_{eq}{\text{ }}k_2 $

  Where K_eq is the equilibrium constant, K_eq = K₁/K-₁ and K₂ represents the unimolecular reaction rate constants.

  3. RESULTS AND DISCUSSION

  3.1 HCCP + •HO₂/•OH/•NO₃/O₃ initiated reactions

  The initial reactions of •HO₂, •OH, and •NO₃ with HCCP are similar that the oxidants are added to the carbon site of the double bonds of HCCP. Meanwhile, the unsaturated double bonds of HCCP can react with O₃ and form an oxide by the cycloaddition reaction^[33], which is different from its reactions with •NO₃, •HO₂ and •OH. The primary ozonide is unstable, which can further dissociate to produce the Criegee intermediates^[34]. Although there are two double bonds of HCCP, only one of them is taken into consideration owning to the symmetric structure of HCCP. All possible addition pathways are depicted in Fig. S1.

  The potential energy surface (PES) of the initiated transformation pathways of HCCP is shown in Fig. 1. When •HO₂ approaches to HCCP, complexes C1A and C2A can be formed with the energy release of 5.11 and 5.22 kcal⋅mol^-1, respectively. For path 1A, C1A can continue to transform into the intermediate IM1A via a transition state TS1A with the energy barrier of 15.18 kcal⋅mol^-1. The IM2A in path 2A can be produced from C1A, but needs to pass through the transition state TS2A and overcome the energy barrier of 15.34 kcal⋅mol^-1. It can be found that the two pathways have similar features. It is reasonable because only reaction sites are different for the two pathways.

  Figure 1

  

  Figure 1.  Schematic diagram of potential energy for the addition reaction of HCCP with HO₂, OH and NO₃ at the M06-2X/6-311+G(d, p)//M06-2X/6-311++G(3df, 3pd) level

  DownLoad: Full-Size Img PowerPoint

  OH radical is a key oxidant in the initiation of tropospheric degradation of gas-phase organic compounds, typically during daylight hours^[35]. For the •OH-initiated reaction of HCCP, after the •OH is added to the C1 or C2 atom, the post-reactive complexes (C1B and C2B) can be generated through releasing energy of 4.20 kcal⋅mol^-1. The reaction proceeds via the transition state TS1B (or TS2B) with the energy barrier of 3.96 kcal⋅mol^-1 (or 4.42 kcal⋅mol^-1) to produce the final intermediate IM1B (or IM2B).

  The reactions, NO + O₃ → NO₂ + O₂ and NO₂ + O₃ → NO₃ + O₂, lead to the formation of the NO₃ radical^[36]. NO₃ radical concentrations remain low in the daytime but can rise to measurable levels at night^[37]. The reactions of •NO₃ with HCCP are similar with the reactions of •OH and •HO₂. As shown in Fig. 1, it is clear that the O atom of NO₃ can attack the double bonds of HCCP. In this process, a complex (C1C or C2C) compound was formed by releasing energy of 5.76 kcal⋅mol^-1. Furthermore, the energy barrier of the pathways from C1C (or C2C) to IM1C (or IM2C) was determined as 1.22 and 0.84 kcal⋅mol^-1, and the reaction energies for the two pathways are –33.24 and –25.04 kcal⋅mol^-1, respectively. Considering all the pathways of the addition reaction, it is clear that the reaction energies are negative (–13.51~–45.0 kcal⋅mol^-1). The results imply that these processes are exothermic and potentially contribute to the transformation of HCCP from standpoint thermodynamics.

  Ozone is a powerful oxidizing agent, which can oxidize many organic compounds by addition to C=C double bonds. In the troposphere, O₃ plays an important role in the oxidation of olefinic compounds. In recent years, field measurements have indicated very high concentrations of O₃ in or near megacities of China^[38]. The pathways for the O₃-initiated reactions of HCCP are depicted in Fig. 2. The formation processes of IM1D include two steps, which correspond to the HCCP + O₃ → the complex C1D and C1D → IMID. The transformation of HCCP into C1D is exothermic with the reaction energy of 4.46 kcal⋅mol^-1. For C1D → IMID, this process is highly exothermic with reaction energy of 64.44 kcal⋅mol^-1 and needs to overcome the barrier of 10.58 kcal⋅mol^-1. Then a five-membered heterocyclic compound is formed. And the primary ozonide product is unstable. It could open the ring to further the subsequent reactions. As shown in Fig. 2, there are two possible pathways in the ring opening reactions. And the energy barriers in the processes are 24.01 and 25.66 kcal⋅mol^-1, respectively.

  Figure 2

  

  Figure 2.  Schematic diagram of potential energy for the ozonization reaction of HCCP with O₃ at the M06-2X/6-311+G(d, p)//M06-2X/6-311++G(3df, 3pd) level

  DownLoad: Full-Size Img PowerPoint

  Comparing these initiated reactions of HCCP, the energy barrier of NO₃ pathways was lower than those of •OH, •HO₂ and O₃ pathways. These results mean that the NO₃ pathways could be more kinetically favorable than the other pathways.

  3.2 Kinetic calculation

  In order to further assess the dominant subsequent reactions of HCCP in atmosphere, it is necessary to quantitatively the contribution of all initial reactions. The rate constants are calculated with the TST method at the temperature ranging from 200 to 320 K. Because the HCCP structure is symmetric, the rate constants initiated by •NO₃, •OH, and •HO₂ were calculated according to the following formula $ k_{total}^{\text{X}} $= 2(R_add1 + R_add2). As for the rate constants initiated by O₃, it could be calculated with the formula $ k_{total}^{\text{X}} $ = 2R. The rate constants ($ k_{total}^{\text{X}} $) of •NO₃, •OH, •HO₂, and O₃ initiated reactions are 2.49 × 10^-12, 2.46 × 10^-13, 2.44 × 10^-22, and 1.33 × 10^-20 cm³⋅molecule^-1⋅s^-1 at 298 K, respectively, which are listed in Table 1. The initiated reaction rate constants of HCCP can be described as •NO₃ > •OH > O₃ > •HO₂ at 298 K.

  Table 1

  

  Table 1.  Calculated Rate Constants (cm³⋅molecule^-1⋅s^-1) between 200 and 320 K in the Reaction of HCCP and Oxidants

  DownLoad: CSV

  T(K)		200 K	230 K	260 K	298 K	320 K
  NO3	Radd1	8.88×10-12	2.29×10-12	8.30×10-13	3.18×10-13	2.06×10-13
  	Radd2	3.35×10-11	7.73×10-12	2.60×10-12	9.30×10-13	5.86×10-13
  	$ k_{{\text{total}}}^{{\text{N}}{{\text{O}}_3}} $	8.48×10-11	2.00×10-11	6.86×10-12	2.49×10-12	1.58×10-12
  OH	Radd1	1.02×10-13	8.98×10-14	8.33×10-14	7.99×10-14	7.94×10-14
  	Radd2	4.02×10-14	3.99×10-14	4.08×10-14	4.30×10-14	4.46×10-14
  	$ k_{{\text{total}}}^{{\text{OH}}} $	2.84×10-13	2.59×10-13	2.48×10-13	2.46×10-13	2.48×10-13
  HO2	Radd1	1.34×10-26	3.41×10-25	4.25×10-24	5.23×10-23	9.43×10-25
  	Radd2	1.74×10-26	4.47×10-25	5.61×10-24	6.94×10-23	2.32×10-22
  	$k_{{\text{total}}}^{{\text{H}}{{\text{O}}_{\text{2}}}}$	6.16×10-26	1.58×10-24	1.97×10-23	2.44×10-22	4.65×10-22
  O3	R	3.75×10-23	2.80×10-22	1.36×10-21	6.67×10-21	1.44×10-20
  	$ k_{{\text{total}}}^{{{\text{O}}_{\text{3}}}} $	7.49×10-23	5.59×10-22	2.72×10-21	1.33×10-20	2.87×10-20

  Considering the concentrations of •NO₃, •OH, O₃ and •HO₂ in the atmosphere, the degradation rate can be expressed as the formula r = $ k_{total}^{\text{X}} $ [X] [HCCP], where [X] is the concentration of •NO₃, •OH, •HO₂ and O₃. Although the rate constants of $ k_{total}^{{\rm{N}}{{\rm{O}}_{\rm{3}}}}$ and $ k_{total}^{{\text{OH}}} $ are much higher than $ k_{total}^{{{\rm{O}}_{\rm{3}}}}$, the role of O₃ cannot be ignored due to its high concentration (the value of [O₃] is about 7.0 × 10¹¹ molecule⋅cm^-3[39], and [•OH] is about 9.7 × 10⁵ molecule⋅cm^-3[40], [•HO₂] = 1.0 × 10⁸ molecule⋅cm^-3[41], [•NO₃] = 1.2 × 10⁷ molecule⋅cm^-3[42]), and O₃ can participate in atmospheric reactions all day. O₃ is a highly reactive gas and a strong oxidant, which can undergo rapid heterogeneous chemical reactions with organic matters. Thus, the O₃ is indispensable to assess the subsequent reactions of HCCP.

  3.3 Subsequent reactions of HCCP + •NO₃/•OH/O₃

  3.3.1   Subsequent reactions of •NO₃/•OH

  Because O₂ and NO are abundant in the atmosphere and O₂ is the common oxidant, the intermediates can undergo further reactions in the presence of O₂ and NO in the atmosphere after the addition reactions initiated by •NO₃, •HO₂, •OH with HCCP. Owing to the rate constants of •NO₃ and •OH with HCCP to be the fastest, we choose the •NO₃/•OH-initiated reactions as examples to calculate the subsequent degradation reactions.

  The intermediates formed in the •NO₃-initiated reaction could be further oxidized by ubiquitous O₂ in the atmosphere. The subsequent reactions of NO₃ radical are shown in Fig. 3. Obviously, the reaction of IM1C with O₂ is strongly exothermic (43.95 kcal⋅mol^-1). In the troposphere, the reaction of RO₂ + NO → RONO₂ is a radical chain terminating step that can suppress ozone production. Thus, the IM3C peroxy radical can be regarded as a good depressor of photochemical smog. For IM4C → IM5C + NO₂, the NO₂ elimination reaction has a barrier of 28.08 kcal⋅mol^-1, and this process is endothermic with the reaction energy of 9.47 kcal⋅mol^-1. Furthermore, IM5C will open the C–C bond and eliminate a molecule of NO₂ to generate the product P1 after overcoming the energy barrier of 4.59 kcal⋅mol^-1. And this is a high exothermic reaction with 54.09 kcal⋅mol^-1.

  OH radicals are the most important oxidant in the atmosphere. It controls the oxidative removal of most trace gases in the atmosphere, and plays an important role in the generation of photochemical smog. Although the rate constant of •OH is slower than the •NO₃-initiated reaction, we also calculate the subsequent reaction of OH. As shown in Fig. 3, the subsequent reactions of •OH are similar with •NO₃. The reaction of IM1B with O₂ can release energy of 50.07 kcal⋅mol^-1. Then, the IM3B peroxy radical can also react with ubiquitous NO in the troposphere. The pathway can release energy of 24.21 kcal⋅mol^-1. Moreover, the NO₂ elimination reaction occurs, which is exothermic with the reaction energy of 9.59 kcal⋅mol^-1 and needs to overcome the energy barrier of 24.77 kcal⋅mol^-1. This process is the rate determining step in the reaction pathway owning to the high energy barrier. In the end, P2 can be produced through the transition state TS4B and overcomes the energy barrier of 1.27 kcal⋅mol^-1.

  Figure 3

  

  Figure 3.  Schematic diagram of potential energy for the subsequent pathways of IM1C and IM1B at the M06-2X/6-311+G(d, p)//M06-2X/6-311++G(3df, 3pd) level

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  The subsequent reactions of IM2C and IM2B are shown in Fig. S2 and Fig. S3. The subsequent pathway of IM2C also contains four elementary steps: O₂ barrierless reaction, NO addition, NO₂ elimination and C–C bond cleavage. What's more, the C–C cleavage reactions of IM8C are a little different from IM5C maybe owing to the influence of Cl atom. The reaction of IM8C is divided to two steps, in which the C–C bond is broken firstly, and then the NO₂ is removed.

  3.3.2   Subsequent reactions of Criegee intermediate

  The reaction of O₃ with unsaturated hydrocarbons produces short-lived molecules termed Criegee intermediates^[43]. The fate of the Criegee intermediates determines the end products of the ozonolysis reaction and can have a substantial impact on the atmosphere^[44]. It can react with radicals and common gases in atmosphere like O₂, SO₂ and NO₂ because of the active feature^{[45, 46]}. In this study, the subsequent degradation reactions of Criegee intermediate were calculated.

  The intermediate IM2D formed by ozonization reactions is taken as an example to access the subsequent reaction of HCCP. The energy profile for the subsequent reaction of IM2D with SO₂, O₂ and NO₂ and the optimized geometries are shown in Fig. 4. For the O₂ reactions, a five-membered heterocyclic compound IM4D is formed by a barrierless reaction of IM2D with O₂. Then, the transformation of IM4D into P1 releases the energy of 8.28 kcal⋅mol^-1 and needs to overcome the barrier of 27.33 kcal⋅mol^-1.

  Figure 4

  

  Figure 4.  Schematic diagram of the subsequent pathways of Criegee intermediate IM2D initiated by NO₂, O₂ and SO₂

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  In the stratosphere, NO₂ is involved in the conversion of active halogen oxides into less active species. In the troposphere, it is one of the most important O₃ precursors^[47]. In this work, two pathways for the reactions of NO₂ with IM2D are calculated. For the first pathway, the formation processes of P1 include two steps, corresponding to the reactions IM2D + NO₂ → IM5D and IM5D → IMID + NO₃. The transformation of IM2D into IM5D is exothermic with the reaction energy of 10.44 kcal⋅mol^-1 and needs to overcome the energy barrier of 5.22 kcal⋅mol^-1 via a transition state TS4D. Then the O–O bond breaks to form P1 and NO₃, with releasing the energy of 38.52 kcal⋅mol^-1. The second reaction pathway of NO₂ is the N atom added to the terminal C atom of IM2D. In this process, the energy barrier of transition state TS5D is 5.55 kcal⋅mol^-1 and this is an exothermic process with reaction energy 17.67 kcal⋅mol^-1. Then a cyclic intermediate IM7D is formed by the intramolecular reaction of O–O bond, which requires to overcome a high energy barrier of 42.74 kcal⋅mol^-1 and absorb the reaction energy of 41.21 kcal⋅mol^-1. Next, the pentacyclic compound IM7D can react with H₂O in atmosphere, which is an endothermic reaction with the energy barrier of 29.43 kcal⋅mol^-1.

  For the reaction of SO₂ with IM2D, a complex compound C3D was formed by releasing energy of 9.89 kcal⋅mol^-1. The reaction proceeds via the transition state TS8D with the energy barrier of 2.89 kcal⋅mol^-1 to produce the cyclic intermediate IM9D. The pentacyclic intermediate is not stable, and can open the O–O bond simultaneously to form the product P1 and SO₃. In this process, the energy barrier of the pathways from IM9D to P1 and SO₃ is 23.31 kcal⋅mol^-1 with the reaction energy being 51.49 kcal⋅mol^-1.

  4. CONCLUSION

  In this paper, the detailed description of degradation mechanism of HCCP initiated by •OH, •NO₃, •HO₂ and O₃ have been researched with the computational methods. Considering the degradation reactions of HCCP with typical atmospheric oxidants (•NO₃, •OH, •HO₂, and O₃), all the pathways are exothermic, and the maximum energy barrier is 15.34 kcal⋅mol^-1. Moreover, the energy barriers of •NO₃ and •OH initiated reaction are smaller than the reactions with •HO₂ and O₃.

  The kinetic analysis of HCCP is performed with the rate constants. The rate constants of •NO₃, •HO₂, •OH, and O₃ initiated reactions are 2.49 × 10^-12, 2.44 × 10^-22, 2.46 × 10^-13, 1.33 × 10^-20 cm³⋅molecule^-1⋅s^-1 at 298 K, respectively. Obviously, the rate constants of •NO₃, •OH are faster than those of •HO₂ and O₃. But the role of O₃ cannot be ignored in atmosphere owning to the average concentration is high. Once the intermediates are produced in the initial reactions, they can react with O₂ subsequently to generate the peroxy radical isomers, which can further undergo reaction via combination with tropospheric NO. The subsequent reactions can form the open-loop chlorinated products. This research provides an efficient way to investigate the gas-phase atmospheric chemistry transformation of HCCP.

  
  
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  Figure 1  Schematic diagram of potential energy for the addition reaction of HCCP with HO₂, OH and NO₃ at the M06-2X/6-311+G(d, p)//M06-2X/6-311++G(3df, 3pd) level

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  Figure 2  Schematic diagram of potential energy for the ozonization reaction of HCCP with O₃ at the M06-2X/6-311+G(d, p)//M06-2X/6-311++G(3df, 3pd) level

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  Figure 3  Schematic diagram of potential energy for the subsequent pathways of IM1C and IM1B at the M06-2X/6-311+G(d, p)//M06-2X/6-311++G(3df, 3pd) level

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  Figure 4  Schematic diagram of the subsequent pathways of Criegee intermediate IM2D initiated by NO₂, O₂ and SO₂

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  Table 1.  Calculated Rate Constants (cm³⋅molecule^-1⋅s^-1) between 200 and 320 K in the Reaction of HCCP and Oxidants

  T(K)		200 K	230 K	260 K	298 K	320 K
  NO3	Radd1	8.88×10-12	2.29×10-12	8.30×10-13	3.18×10-13	2.06×10-13
  	Radd2	3.35×10-11	7.73×10-12	2.60×10-12	9.30×10-13	5.86×10-13
  	$ k_{{\text{total}}}^{{\text{N}}{{\text{O}}_3}} $	8.48×10-11	2.00×10-11	6.86×10-12	2.49×10-12	1.58×10-12
  OH	Radd1	1.02×10-13	8.98×10-14	8.33×10-14	7.99×10-14	7.94×10-14
  	Radd2	4.02×10-14	3.99×10-14	4.08×10-14	4.30×10-14	4.46×10-14
  	$ k_{{\text{total}}}^{{\text{OH}}} $	2.84×10-13	2.59×10-13	2.48×10-13	2.46×10-13	2.48×10-13
  HO2	Radd1	1.34×10-26	3.41×10-25	4.25×10-24	5.23×10-23	9.43×10-25
  	Radd2	1.74×10-26	4.47×10-25	5.61×10-24	6.94×10-23	2.32×10-22
  	$k_{{\text{total}}}^{{\text{H}}{{\text{O}}_{\text{2}}}}$	6.16×10-26	1.58×10-24	1.97×10-23	2.44×10-22	4.65×10-22
  O3	R	3.75×10-23	2.80×10-22	1.36×10-21	6.67×10-21	1.44×10-20
  	$ k_{{\text{total}}}^{{{\text{O}}_{\text{3}}}} $	7.49×10-23	5.59×10-22	2.72×10-21	1.33×10-20	2.87×10-20

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