Mechanism of OH-initiated atmospheric oxidation of

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Mechanism of OH-initiated atmospheric oxidation of diethyl phthalate

Can. J. Chem. Downloaded from www.nrcresearchpress.com by Shanghai International Studies University on 06/06/13 For personal use only.

Yuan Bao, Xiaoyan Sun, Xiaomin Sun, and Jingtian Hu

Abstract: Diethyl phthalate (1,2-benzenedicarboxylic acid diethyl ester, DEP) is one of a group of widely used plasticizers, which can lead to serious environmental problems. Because of manufacturing and application, DEP can be released into the atmosphere where it can undergo transport and chemical transformation. To assess the atmospheric behavior of pollutants, it is critical to know their atmospheric reactions. In this paper, the reaction mechanism and possible oxidation products for the OH-initiated atmospheric reaction of DEP were theoretically investigated by using the density functional theory (DFT) method. The geometries and frequencies of the reactants, intermediates, transition states, and products were calculated at the MPWB1K/6–31+G(d,p) level, and the energetic parameters were further refined by the MPWB1K/6–311+G(3df,2p) method. The present study shows that H abstractions from the CH3 and CH2 groups, as well as OH addition to the benzene ring, are energetically favorable reaction pathways for the reaction of DEP with OH radicals. Detailed degradation products are provided. Key words: diethyl phthalate, OH radicals, atmospheric oxidation, reaction mechanism, quantum calculation, oxidation degradation. Résumé : Le phtalate de diéthyle (ester diéthylique de l’acide benzène-1,2-dicarboxylique, PDE) est un des plastifiés les plus utilisés et il peut provoquer de sérieux problèmes environnementaux. Lors de sa fabrication et son application de PDE peut se répandre dans l’atmosphère où il peut transporter et subir des transformations chimiques. Afin de pouvoir évaluer le comportement des polluants dans l’atmosphère, il est important de connaître leurs réactions dans l’atmosphère. Dans ce travail, faisant appel à la méthode de la théorie de la fonctionnelle de la densité (TFD), on a étudié d’un point de vue théorique le mécanisme de la réaction et les produits potentiels d’oxydation résultant d’une réaction atmosphérique du PDE, initiée par des radicaux OH. Les géométries et les fréquences des réactifs, des intermédiaires, des états de transition et des produits ont été calculées au niveau MPWB1K/6–31+G(d,p) et les paramètres énergétiques ont ensuite été affinés par la méthode MPWB1K/6–311+G(3df,2p). La présente étude montre que l’enlèvement d’hydrogène à partir des groupes CH3 et CH2, ainsi que l’addition du radical OH sur le noyau benzénique sont des voies réactionnelles énergétiquement favorables pour la réaction du PDE avec les radicaux OH. Des produits de dégradation détaillés sont fournis. Mots‐clés : phtalate de diéthyle, radicaux OH, oxydation atmosphérique, mécanisme réactionnel, calculs quantiques, dégradation par oxydation. [Traduit par la Rédaction]

Introduction Diethyl phthalate (1,2-benzenedicarboxylic acid diethyl ester, DEP) is a commercially important phthalate ester. It is used extensively as an additive in the manufacture of polyvinyl chloride, polyvinyl acetates, plastics, cellulosics, and polyurethanes. It is also used in many other products such as automobile parts, paints, lubricating oils, glues, insect repellents, perfumes, and food-packaging material.1–3 The global production volume of DEP is ~5.2 million tons per year.4 The increasing use of DEP is causing worldwide pollution.5–9 It has been detected in the air, water, soil, and vegetation,10–13 as well as the issues and fluids of wildlife and people.14 DEP may act as endocrine disruption compounds, which interfere with the normal hormone-regulated physiological processes

of people and wildlife.15,16 A response experiment in fiddler crabs showed that 7-day exposure to DEP at 50 mg/L significantly inhibited the activity of chitobiase in the epidermis and hepatopancreas.17 DEP is known not only for its endocrine disruption and toxicity but also for its adverse reproductive effects on people and wildlife,18–20 and its estrogenic activity has recently been documented as well.21,22 Therefore, DEP has been categorized as a priority pollutant by the US Environmental Protection Agency.23 DEP can enter the atmosphere and travel many kilometres.24 The vapor pressure of DEP is 6.48 × 10–2 Pa at 25 °C and it can vaporize quickly.25 It is frequently detected in indoor and outdoor air.24 In a North Sea (German Bright) field test, the atmospheric concentration of DEP was 1.6 ng/m–3 in the gas phase and 0.06 ng/m–3 in the particle phase. The

Received 19 June 2011. Accepted 14 August 2011. Published at www.nrcresearchpress.com/cjc on 26 October 2011. Y. Bao, X. Sun, and J. Hu. Environment Research Institute, Shandong University, Jinan 250100, P. R. China. X. Sun. Environment Research Institute, Shandong University, Jinan 250100, P. R. China; State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy Of Science, Lanzhou 730000, P. R. China. Corresponding author: Jingtian Hu (e-mail: [email protected]). Can. J. Chem. 89: 1419–1427 (2011)

doi:10.1139/V11-128

Published by NRC Research Press


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fraction of DEP in the gas phase reached up to 96%.25 This has greatly increased the potential for human exposure to this highly toxic material. Particle-phase DEP may be removed from the atmosphere through dry or wet deposition. The tropospheric removal of gas-phase DEP involves wet and dry deposition and photolysis, oxidation reactions with OH, NO3, or O3. The wet and dry deposition of gaseous DEP is of relatively minor importance as a removal pathway. Among the various oxidants, OH radicals play the most essential role in determining the oxidation power of the atmosphere. The DEP reaction with OH radicals is considered to be a dominant removal process for gaseous DEP. To assess the atmospheric behavior of pollutants, it is critical to know their atmospheric reactions. The OH-initiated degradation of phthalate esters (PAEs) in water can be found.26,27 Blanco et al.28 investigated the reaction mechanism of methacrylates with OH radicals in the atmosphere. Frankcombe and Smith29 studied the mechanism of OHinitiated oxidation of toluene in the atmosphere. However, the current knowledge of the reactions of DEP in the atmosphere is very limited, and its oxidation mechanism has received little attention in experimental or computational chemistry. Quantum calculation is especially suitable for establishing whether a reaction pathway is feasible or not.30–33 In this paper, we carried out a theoretical study of the application of quantum calculations for the OH-initiated atmospheric oxidation reaction of DEP to find favorable reaction pathways and sites. To find the formation mechanism of secondary pollutants from the OH-initiated atmospheric reactions of DEP, some possible secondary reaction pathways were also investigated. To our knowledge, this is the first study of the OH-initiated atmospheric oxidation of DEP using quantum chemistry.

Computational method High-accuracy molecular orbital calculations for the OHinitiated atmospheric oxidation reaction of DEP were carried out in the presence of O2 and NO. All the calculations were performed on an SGI Origin 2000 supercomputer with the Gaussian 03 package.34 The choice of computational levels and basis sets requires a compromise between accuracy and computational time. The geometries of reactants, intermediates, transition states, and products were fully optimized at the MPWB1K35,36 level with the 6–31+G(d,p) basis set. Zhao and Truhlar37 compared the MPWB1K method with other DFT methods and found that the MPWB1K method gave better results for thermochemistry and gave more excellent saddle-point geometries. The MPWB1K/6–31+G(d,p) structures were employed in single-point energy calculations. The corresponding harmonic vibrational frequencies were also calculated at the MPWB1K/6–31+G(d,p) level to determine the nature of the stationary points, the zero-point energies (ZEP), and the thermal contributions to the free energy of activation. To verify that each transition state actually connected the designated reactants with products, intrinsic reaction coordinate (IRC)38 calculations were performed. A more flexible basis set (6–311+G(3df,2p)) was used to evaluate the energetic parameters of the various species more accurately. The profile of the potential energy surface was constructed at the MPWB1K/6–311+G(3df,2p)//MPWB1K/6–31+G(d,p)

Can. J. Chem. Vol. 89, 2011

level. All the relative energies quoted and discussed in this paper include ZPE corrections with unscaled frequencies obtained at the MPWB1K/6–31+G(d,p) level.

Results and discussion The reaction of DEP with OH radicals Since OH is strongly nucleophilic, H abstraction from DEP by OH radicals should be a possible reaction pathway. In addition, there is a benzene ring in the DEP molecule and, therefore, OH addition to the benzene ring is another possible reaction pathway. Altogether, seven possible reaction pathways (R1–R7) are presented in Fig. 1. H-abstraction pathways Four kinds of hydrogen atoms are found in the DEP molecule: H atoms in the CH3 group, H atoms in the CH2 group, H atoms bonded with the C6 atom in the benzene ring, and H atoms bonded with the C7 atom in the benzene ring. Thus, four primary reaction pathways (R1–R4) were identified. R1 and R2 are H abstractions from the CH3 and CH2 groups. R3 and R4 represent H abstractions from the benzene ring in the DEP molecule. H abstractions from the CH3 and CH2 groups occur via the transition states, TS1 and TS2. The optimized structures of TS1 and TS2 are shown in Fig. 2. The transition vector of TS1 clearly shows the motion of H1 between C1 and O, with an imaginary frequency of 1273i cm–1. The transition vector of TS2 shows the motion of H4 between C2 and O, with an imaginary frequency of 1269i cm–1. H abstraction from the CH3 group has a low potential barrier of 3.05 kcal/mol (1 cal = 4.184 J) and is exothermic by 14.34 kcal/mol at the MPWB1K/6–311+G(3df,2p) level. H abstraction from the CH2 group also has a low potential barrier of 0.83 kcal/mol and is strongly exothermic by 18.14 kcal/mol. In the TS1 and TS2 structures, the breaking C1–H1 and C2–H4 bonds are elongated by 0.125 and 0.119 Å, respectively, whereas the forming O–H1 and O–H4 bonds are 0.328 and 0.338 Å, respectively, longer than the equilibrium value of 0.967 Å in H2O. H abstractions from the benzene ring in the DEP molecule require crossing potential barriers of 3.64 and 5.32 kcal/mol, respectively, via the transition states TS3 and TS4. The transition vector of TS3 shows the motion of H6 between O and C6, and the transition vector of TS4 shows the motion of H7 between O and C7. Comparison of the four H-abstraction pathways shows that H abstractions from the alkyl group have a lower barrier than H abstractions from the benzene ring. Also, H abstractions from the alkyl group are more exothermic than H abstractions from the benzene ring. The H atoms in the alkyl group are more activated than the H atoms in the benzene ring. Therefore, H abstractions from the alkyl group are expected to play an important role for the OHinitiated degradation of DEP in the atmosphere. The intermediates, denoted IM1 and IM2, produced from H abstractions from the alkyl group are open-shell activated radicals and will be further oxidized by O2/NO and removed from the atmosphere. OH addition to the benzene ring Here, we analyze the reaction pathways of OH addition to the C=C bond in the benzene ring. As to the symmetry, the Published by NRC Research Press

Bao et al.

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Fig. 1. Reaction pathways for the reaction of 1,2-benzenedicarboxylic acid diethyl ester (DEP) with OH radicals. Units: kcal/mol (1 cal = 4.184 J). DE, the reaction potential barrier; DH, the reaction enthalpy (0 K). E=3.05 H=-14.34 TS1

9

H8


C3 4 5

8 7

H7

H4

O

H9

O

9

8 7

4 5

H7

+

H3

H 2O

R1

IM1

H9 H8

H2

C1

H5

R

6

H6

E=0.83 H=-18.14 TS2

C2

O

6

C3

H1

C2

O

H5

R

H2

C1

+

H3

H 2O

R2

H6 IM2

H8 8 H7

9

4 C3 5

7 6

H4

O

H9

O

R

C2 H5

H1 C1 H3

E=3.64 H=-2.01

H9 TS3

9

H8

8 7

H2

4 5

H7

+OH

O

H4

C3

C2

O

H2

C1

H5

R

6

H1

H3

+

H 2O

R3

+

H 2O

R4

IM3

H6 DEP E=5.32 H=-3.70

H9 TS4

9

H8

8

4 5

7

6

O

H4

C3

C2

O

H=-17.66

H4

O 4 C3

9

8 7

H7

6

R5

H3

IM5

H9 H=-15.88

H2

C1

H5

OH R

5

H1

C2

O

H6

H8

H3

IM4

H9 H8

H2

C1

H5

R

H6

H1

9

4 5

8 7

H7 6 HO

O

H4

C3

C2

O

H1

H5

R

H2

C1

R6

H3

H6 IM6

H9 H=-16.00

H8 HO H7

9

4 5

8 7 6

O

H4

C3

C2

O

R

H5

H1 C1 H3

H2 R7

H6 IM7

C atoms in the benzene ring fall into three groups: the C4 and C5 atoms are equivalent, the C6 and C9 atoms are equivalent, and the C7 and C8 are equivalent. Therefore, three different OH–DEP adduct isomers can be formed through the addition

of the OH radical to the C4, C6, and C7 atoms in the DEP molecule. The calculations show that OH additions to the C4, C6, and C7 atoms are strongly exothermic and energetically favorable. It can be seen from Fig. 1 that addition of Published by NRC Research Press

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Fig. 2. MPWB1K/6–31+G(d,p) optimized geometries for the transition states. OH H9 H8

9

O

H4

C3

C2

4 5

8 7

H7

O

H1 C1

H5

R

6

OH

H9 H8

H2

9

4 5

8 7

H7

H3

6

H4

C3

C2

O

H8

C3

4 5

8 7

H7

R

6

C2

C1

H5

H2

H8

H3

H7

4 5

8 7

4 5

H7

6

O

H4

C3

C2

O

H1 C1

H5

R

O

R

6

H8

H2

4

8 7

5

H7

6

H6

O

C3 OH

H1

C2

O

C1

H5

8

4 5

C3

C2

H8

4 5

6

O

H6

H4

O

H5

H1 C1

O

HO

4 5

8 7 6

H7

C3 R

H6

O O

C1

H2

H8 H7

9

4

8 7

5 6

C3

H2

H3

H8

9

4

8 7

H7

C2 H5

C1 H3

H2

5

H1 C1

H2

O

H4

C3

C2

H7

HO

4 5 6

H5

R

H4

C3

C2

O

H3

H8

H2

H7

H C1

H2

H3

H1 C1

H2

H3

H6

O

R

TS16

OH to the C4, C6, and C7 atoms is barrierless and is a strongly exothermic processes. The high reaction energies are retained as the internal energy of the adducts. The energyrich OH–DEP adducts (IM5, IM6, and IM7) can further react in the atmosphere.

C2

O

H5

O O

N

TS17

C1

H2

H3

9

C3 4 5

8 7 6

HO

H4 O

R

C2 H5

H1 C1

H2

H3

H6 O TS15

H4

C3

H1

H5

O

H9

H3

O

9

8 7

O

O

O

H9

H8

H5

R

OH 6

C2

O

TS6

H9

O

O

H1

N O

O

O

H9

TS14 H4

O

C1

H5

H2

H3

O

H9

O

N

H3

H1

H3

TS12

H1

C2

TS13

9

H5

R

H4

C2

TS3

OH

TS11

O H4

C2

O

OH

TS10

O

H8

C1

H2

H6

O O

H1 C1

TS9

O C3

N

N

H5

R

H5

R

TS8

8 7

H7

O

C2

O

H6

9

O O

H4

H9

H2

H3

R

9

O 7 6 H6 O N O

H3

H4

O

H9

6

H6

O

H9

TS7

N

9

H5

4 5

8 7

TS5

O

O

C2

O

TS4

9

H8

C3

9

H7

H6

H9

H8

C3

9

8 7

O H4

O

H9

H6

OH

H8

O

H1

H2

H4

O

H9

H3

TS2

H4

O

H9

C1

H5

R

TS1

9

H1

H6

H6


O

H1 C1 H3

H2

H8 HO H7

4

8 7

5 6

H6

H4

O

H9 9

C3 O R

O O O

C2 H5 N

H1 C1

H2

H3

TS18

Secondary reactions The previous discussion shows that H abstractions from the alkyl group and OH additions to the benzene ring are energetically favorable pathways for the reactions of DEP with OH radicals. IM1–IM7 are important intermediates produced Published by NRC Research Press

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1423

NO2

NO2

+

+

H

H5

O

O C3

H9

9

6

H6

TS5

TS6 E=22.15 H=0.03

H7

H2

O

H O

O

6

9

H8

H7

+NO

+NO

H=-15.25

H=-18.51

H7

H2

O

IM8

H5

H9 H6

H7

H7

+O2

H=-71.05

H2 H3

C2

C2

IM1

H5 O

C3

O O

R

C3

9

H8

H6

H9

9

H6

8 7

6

H9

R 4 5

O

H3 H5

6

C

C

H

IM2

H2

H4

H6

H=-71.60

H8

9

H8

8 7

9

R

8 7

6

5

4

H9

O C3

O

6

R

C3

O

H5

4

O

H3

C

C2

IM10

H3

C

C2

H4

H

O

5

H2 O

H6

8 7

6

H9

H6

8 7

H8

R 4

5

4 9

O

H5

C3

O

R

C3

H3

C

C2

IM9

H5 O

O

N O O

IM11

H3

C

C2

H4

H9

H8

H2

N O O

H7

E=29.01 H=0.07

H8

H6

8 7

8 7

6

9

R 4

5

O

5

R

4

H9

H5

P1

O C3

O

H3

C

C2

P2

H3

C

C2

5

O H4

+O2

Atmospheric reaction pathway of IM5 The unimolecular decomposition of IM5 results in the formation of P5 and IM16 via the transition state TS9. This process has a high potential barrier of 19.52 kcal/mol and is strongly endothermic by 9.30 kcal/mol, indicating that the decomposition is energetically unfavorable. In the troposphere, IM5 will mainly be removed by reaction with O2. IM5 has three possible sites to react with O2 and three possible products (IM17, IM18, and IM19) can be formed throughthe addition of O2 to C5, C7, or C9 atoms in the IM5 molecule. The calculated profile of the potential energy surface shows that these addition reactions are barrierless and strongly exothermic by 42.07, 43.76, and 36.50 kcal/mol, respectively, indicating that these addition reactions are energetically favorable and can occur easily in the atmosphere. IM17, IM18, and IM19 have similar structures and will be removed from the atmosphere by a similar reaction mechanism. Therefore, we will just discuss the atmospheric reactions of IM17 here. IM17 could further react with NO to form IM20. IM20 will directly decompose via the transition state TS10 to form IM21 and NO2 with a potential barrier of 15.15 kcal/mol. IM21 further decomposes to form P6 and IM16 via the transition state TS11. The O1–C2 bond in IM16 will rupture through the transition state TS12 with an imaginary frequency of 891i cm–1 to form IM22 and CO2. This process is exothermic by 19.28 kcal/mol and a potential barrier of 15.62 kcal/mol. In the TS12 structure, the O1–C2 bond is elongated to 1.879 Å from 1.446 Å in IM16. IM22 subsequently readily reacts with O2 to form an intermediate, IM23. The calculations show that the reaction of IM22 with O2 is barrierless and strongly exothermic by 72.12 kcal/mol.

H2

4 5

Atmospheric reaction pathways of IM3 and IM4 The intermediates IM3 and IM4, from the H abstraction of the benzene ring, have similar atmospheric reactions. The intermediate IM3 reacts with O2 to yield IM12 in the atmosphere and it is exothermic by 42.11 kcal/mol. Then IM12 reacts with NO to form IM13. The intermediate IM13 immediately decomposes to form product P3 and NO2 via the transition state TS7 and it requires to cross a potential barrier of 15.17 kcal/mol. Similar to IM3, IM4 can also be oxidized by O2/NO in the atmosphere. The reaction pathways are depicted in Fig. 4.

H2

8 7

Atmospheric reaction pathways of IM1 and IM2 IM1 can react with O2 readily to form an intermediate, IM8. The calculated profile of the potential energy surface shows that the reaction of IM1 with O2 is barrierless and strongly exothermic by 71.05 kcal/mol. In the atmosphere, IM8 will further immediately react with NO to form intermediate IM9. IM9 subsequently reacts via a direct decomposition to form P1 and NO2 through the transition state TS5, and this reaction requires a potential barrier of 29.01 kcal/mol and is endothermic by 0.07 kcal/mol. Similarly, IM2 can also be oxidized by O2/NO in the atmosphere. The reaction pathways are depicted in Fig. 3.

Fig. 3. Secondary reaction routes from intermediates IM1 and IM2. Units: kcal/mol (1 cal = 4.184 J). DE, the reaction potential barrier; DH, the reaction enthalpy (0 K).


from the degradation of DEP initiated by OH radicals. IM1– IM7 are open-shell activated radicals and will further react via unimolecular decomposition and (or) with O2 or O2/NO in the atmosphere.

H7

H8

H7



+

+

NO2

NO2

1424

H2

H5

C3

O

R

C3

P4 R

4 5

O

H5 O

P3

O

H3

C

C2

H4

4 5

C2

H4

H

H3

C

H

6

O

E=15.17 H=1.38

E=17.26 H=3.35

TS7

TS8

H7

H2

H2 H

H5

4 5

O

6

9

O

N O

H9

C3

H8

8

+NO

4 5

O O

H9

C3

R

9

8

H7

H=-47.00 +O2

+O2

H2

4 5

R 6

9

H8

C

C2

H5 O

O H9

C3

R

9

8

H7

IM4

O C3

H4

H3

6

IM3

H5

H8

The OH-initiated atmospheric oxidation reaction of DEP has been theoretically investigated in this paper. This study shows that the reaction of DEP with OH radicals can proceed through three energetically favorable channels: H abstraction from the CH3 group, H abstraction from the CH2 group, and OH addition to C4, C5, and C6 atoms in the benzene ring. On the one hand, our quantum calculations can provide a detailed degradation mechanism, which is helpful for understanding the atmospheric-chemistry transformation of the DEP. On the other hand, the quantum chemistry calculations can provide the basic information for direct kinetic studies, such as the energy, force constants, and Hessian matrices of the stationary points. The kinetic studies are planned for future work.

Acknowledgements

H

7

C2

H3

4 5

C

H

H9

H6

O O

H=-42.11

H8

H2

O

IM14

O O

8 7

6

9

H5

6

R

H3

C

C2

7

C3

H4

Atmospheric reaction pathways of IM6 and IM7 The H atom in IM6 can be abstracted by O2 via the transition state TS15. The process has a low potential barrier of 0.26 kcal/mol and is strongly exothermic by 71.14 kcal/mol. TS15 has an imaginary frequency of 1415i cm–1. Similarly, the H atom in IM7 can be abstracted by O2 as well. The two abstraction reactions are energetically favorable and will readily occur in the atmosphere. Similar to IM5, IM6 can react with O2 via an addition mechanism. There are three possible sites in IM6 to react with O2. Three products IM26, IM27 and IM28 can be formed through addition of O2 to C5, C7 and C9 atoms in IM6. The three addition reactions are strongly exothermic by 44.45, 45.12 and 43.43 kcal/mol. IM26, IM27 and IM28 have similar structure and will be removed from the atmosphere by similar reaction mechanism. Therefore, we just discuss the atmospheric reactions of IM26 here. In the atmosphere, IM26 will further react with NO immediately to form an intermediate IM29. IM29 subsequently reacts via a direct decomposition to form oxyl radical P10 and NO2, and this reaction requires crossing a potential barrier of 14.96 kcal/mol and is endothermic by 2.84 kcal/mol. Similarly, IM7 can react with O2 through the addition mechanism. The possible products are IM30, IM31, and IM32. The reaction pathways are described in Fig. 6.

Conclusion

H

IM12

H5

H8

H4

N

H=-16.82

H=-16.03 +NO C2 O

H9

O

H2 H3

C

H

O

H6

O O

H8

H7

H2

H4

R

9

8 7

H9

O O

6

R

H5

7

O C3

O

H3

C

C2

IM13

H4

4 5

C2

IM15

H3

C

H H4

7

H8

8 7

H8

9

8

O

6

9

H6

4 5

H9

H9

8 7

Fig. 4. Secondary reaction routes from intermediates IM3 and IM4. Units: kcal/mol (1 cal = 4.184 J). DE, the reaction potential barrier; DH, the reaction enthalpy (0 K).


H2

In the atmosphere, IM23 will react with NO immediately to form an intermediate, IM24. IM24 subsequently reacts via a direct decomposition to form oxyl radical IM25 and NO2 with a potential barrier of 18.83 kcal/mol, and the process is endothermic by 1.57 kcal/mol. IM25 can react with O2 to yield P7 and HO2 via transition state TS14. The reaction pathways are shown in Fig. 5.

H6

This work was supported financially by the National Nature Science Foundation of China (Nos. 20977059, 20903062, and 20737001), the Natural Science Foundation of Shandong Province (No. Q2008B07), the Independent Innovation Foundation of Shandong University (IIFSDU, Nos. 2009JC016 and 2010TS064), and the Open Project from the State Key Laboratory of Environmental Chemistry and Ecotoxicology, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences (No. KF2009-10). Published by NRC Research Press

Bao et al.

1425

Fig. 5. Secondary reaction routes from intermediate IM5. Units: kcal/mol (1 cal = 4.184 J). DE, the reaction potential barrier; DH, the reaction enthalpy (0 K).


E=19.52 H=9.30

TS9

O

H4

C3

C2

O

H9 H1 C1

H5

H8

H2

+

9

8 7

H7

H3

6

+O2 O

H9 H8 H7

9

8 7 6

H6

H4

4

C3

5

OH R

O

C2 H5

O

H8

H=-36.50

C1

4

8 7

H7 H1

H9

9

5 6

O

H4 C2

OH R

5

R

P5

C3

O

OH

H6

IM16 O

4

H1

H2

C1

H5

H3

H6

H2

IM19

H3 H9

IM5 +O2

H8

H=-43.76

H7

9

8 7

5 6

O

O

4

O

H4

C3

C2

OH

O

H1 C1

H5

R

H2

H3

H6 IM18 H9

+O2

H8

H=-42.07

9

8 7

H7

4

O

H4

C3

C2

H1

O OH 5 H5 R 6 H6 O O IM17

C1

H8 +NO

H3

H=-21.94

9

C3

4

8 7

H6 O

N

C3

C2

O

H9 H1 C1

H5

H8

H2

+

9

E=15.15 H=2.38 TS10

8 7

4 5

H7

H3

6

H6

IM16 -CO2 H4 C2 H5

C1

R

9

+O2

H=-72.12

5 6

H6

O

H1

C2 O

H3 IM22

C1

H5

H2

N +NO

H=-16.05

C3

4

8 7

H7

H4 H2

TS11

H8

P6

E=15.62 H=-19.28 TS12 H1

O

E=10.83 H=1.62

O

OH

H4

H1

H4 O

C1 H3 P7

C1

H5

HO2

E=3.82 H=-66.80

TS14

H2

H3

O

H2

H3

E=18.83 H=1.57 TS13

H4 +

H1

C2

IM24

H2

C1

O

-NO2

C2

H5

H1

IM21

IM23

O

C2

R

O

O

H3

H4

O

H9

OH

H3

IM20

-NO2

H4

H2

O

O

O

C1

H5

R

6

H1

C2

O

OH

5

H7

H4

O

H9

H2

C2 H5

H1 C1

H2

H3

IM25


1426


Fig. 6. Secondary reaction routes from intermediates IM6 and IM7. Units: kcal/mol (1 cal = 4.184 J). DE, the reaction potential barrier; DH, the reaction enthalpy (0 K). H9 E=0.26 H=-71.14 TS15

9

H8

4 5

8 7

H7

O

H4

C3

C2

O

H5

R

6

H1

H2

C1

+

H3

HO 2

HO


P8 O O

H8 H7 H9 9

4 5

8 7

H7 6 HO

O

H4

C3

C2

O

H5

R

H1 C1

C3

4 5

8 7

H=-43.43

H8

9

6

HO

H4

O

H9

H2

C1

H5

R

H6

H1

C2

O

H3

IM28

H2 +O2

H9

H3

O

H9

H6 IM6

H=-45.12

9

H8 O

C3 4 5

8 7

O H7

H4

6

HO

H2

C1

H5

R

H6

H1

C2

O

9

H8 H7

H3

4 5

8 7 6

HO

H6

IM27

H=-44.45

H8

H4

C3

C2

H7

O

4

8 7

5

6

HO

H6

H1

O

+NO

H=-17.19

H3

4 5

8 7

H7

HO

6

H9 9

H8

4 5

8 7

HO

6

O

H4

C3

C2

O

H6

O

H4 C2

R O

H3

H5

O

H1 C1

H2

H3

N IM29

H2

C1

H5

R

O

O

H1

H2

E=14.96 H=2.84 TS17

C3

IM26

E=6.30 H=-66.84 TS16

H1 C1

P10

9

H8

H5

R O

H9

H2

C1

H5

R O

O

H4 C2

-NO2

O

H9 9

O C3

+

H3

HO 2

H6 P9 H9

O O H=-42.41

H9 H8 HO H7

9

4 5

8 7 6

O

H4

C3

C2

O

R

H5

H8 8 HO 7

4 5 6

H7 H1 C1

O

H4

C3

C2

O

H1

H2

C1

H5

R

H3

H6

H2

IM32

+O2

H9

H3 H=-47.30

H8 HO H7

9

4 5

8 7

O O

6

H4

O

H9

H6 IM7

9

C3

H6

C2

O

H5

R

H8 H1 C1

HO

H2

9

4

8 7

5 6

H7

H9 H=-45.31

H8 HO H7

8 7

5

6

O

H3

O

O

H4 C2 H5

R

H6 IM30

H1 C1 H3

H9

H2 +NO

H=-17.45

9

H8

8

HO 7 H7

6

H6

H5

R

H1 C1

H2

H3

P11

-NO2

C3

O 4 O O

H4 C2

H6

IM31

9

O C3

E=15.88 H=2.12 TS18

O

H4

C3

C2

O 4 O O H5 5 N R O

H1 C1

H2

H3

IM33


Bao et al.

1427


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