Quantum Chemical Investigation on the Mechanism and Kinetics of

ARTICLE pubs.acs.org/est

Quantum Chemical Investigation on the Mechanism and Kinetics of PBDE Photooxidation by 3 OH: A Case Study for BDE-15 Jing Zhou,† Jingwen Chen,*,† Chi-Hsiu Liang,‡ Qing Xie,† Ya-Nan Wang,† Siyu Zhang,† Xianliang Qiao,† and Xuehua Li† †

Key Laboratory of Industrial Ecology and Environmental Engineering (MOE), School of Environmental Science and Technology, Dalian University of Technology, Dalian 116024, China ‡ School of Chemistry, University of Leeds, Woodhouse Lane, Leeds, LS2 9JT, United Kingdom

bS Supporting Information ABSTRACT: Computational approaches are crucial to risk assessment and pollution prevention of newly synthesized compounds prior to large-scale production and commercialization. Understanding the kinetics and mechanism of the tropospheric reaction of semivolatile organic compounds with 3 OH is an indispensable component of risk assessment. In this study, we show that the density functional theory (DFT) can be successfully employed to probe the kinetics and mechanism of atmospheric photooxidation of polybrominated diphenyl ethers (PBDEs) by 3 OH, taking 4,40 dibromodiphenyl ether (BDE-15) as a case. The predicted products (HO-PBDEs, brominated phenols and Br2) and overall rate constant (kOH) at 298 K are consistent with the experimental results. Two pathways leading to formation of HO-PBDEs are identified: Br substitution by 3 OH, and abstraction of H gem to 3 OH in BDE-OH adducts by O2. This study offers a cost-effective way for probing the atmospheric indirect photooxidation kinetics and mechanism of PBDEs.

’ INTRODUCTION Semivolatile organic compounds (SOCs) cause extensive environmental contamination1
3 and significant adverse effects on human health.4,5 The reaction with 3 OH in the troposphere strongly determines the environmental persistence and fate of SOCs.6,7 Understanding the mechanism and kinetics of this reaction is an indispensable component of risk assessment.8,9 Several direct and indirect experimental methods have been developed to probe the reactions.10
13 However, almost all the experimental methods are time-consuming, costly, and equipment dependent. Thus, they cannot meet the need of risk assessment and pollution prevention for newly synthesized compounds prior to large-scale production and commercialization.8 Consequently, it is crucial to develop computational approaches to predict the chemical reactivity of SOCs.8 It is worth mentioning that in silico (computer-based) technologies for chemical screening and prioritization have become a research focus in recent years and a topic of subsequent discussion regarding implementation. For example, the U.S. EPA established the National Center for Computational Toxicology in 2005, which aims at providing high-throughput computational tools for screening and assessing chemical exposure, hazard, and risk.14 The new legislation REACH (Registration, Evaluation, Authorization, and Restriction of Chemicals) implemented by the European Union in 2007 also advocates in silico approaches for assessing environmental r 2011 American Chemical Society

risk of chemicals, such as the technology of quantitative structure
activity relationships (QASRs).15 Several QSAR models have been developed to predict the rate constants (kOH) for the reactions of organic chemicals with 16
19 Generally, the QSAR models are attractive as they 3 OH. generate results with minimal computational cost and they are applicable for regulatory purposes. However, the utility of QSARs is constrained as they strongly rely on experimental databases and are only valid for compounds within the applicability domain.20,21 Furthermore, conventional QSAR models cannot provide information on reaction pathways and mechanisms. In recent years, increases in computational capability have enabled quantum chemical computations for molecules or systems comprising up to a few hundred atoms.22,23 The density functional theory (DFT) and ab initio methods allow for accurate predictions of chemical reactivity without any prior knowledge on the substance, and they have been widely applied to identify reaction mechanisms and generate kinetic data.24
27 Some previous studies have shown that quantum chemistry and

Received: January 10, 2011 Accepted: April 26, 2011 Revised: March 18, 2011 Published: May 11, 2011 4839

dx.doi.org/10.1021/es200087w | Environ. Sci. Technol. 2011, 45, 4839–4845

Environmental Science & Technology statistical mechanics pose a viable way forward for investigating radical (e.g., 3 OH and peroxy radicals) oxidation reactions.28
30 The purpose of this study is to show that DFT can be employed to assess the gaseous reactions between a polybrominated diphenyl ether (PBDE) congener and 3 OH. PBDEs are additive-type flame retardants widely used in plastics, textiles, electronic circuitry, and other types of consumer products.5 As there is no chemical bond between the additives and products, PBDEs are susceptible to be released into the environment.31 They have been widely detected in samples of air, water, sediment, bird, fish, marine mammals, and even in humans, and their environmental levels are increasing exponentially.32 As the boiling points of PBDEs range from 310 to 425 °C,33 most PBDE congeners are SOCs. There are only a few experimental data on the reactions of PBDEs with 3 OH. Raff and Hites determined kOH values for 7 PBDEs34 and concluded that 3 OH initiated oxidation was a dominating removal path for PBDEs substituted by 1
2 bromines.34,35 They detected bromophenols and Br2 as primary degradation products, as well as traces of hydroxylated PBDEs (HO-PBDEs).34 HO-PBDEs are of great concern because of their xenoestrogenic36 and thyroid hormone effects.37,38 In addition to biological samples,39,40 HOPBDEs were also detected in surface water and in precipitations,41 suggesting that besides biometabolites, oxidation of PBDEs by 3 OH in the troposphere can be an importance source of HOPBDEs. However, the detailed mechanism for the formation of HO-PBDEs from photooxidation of PBDEs needs further elucidation. Experimental exploration on the mechanism is not easy because of the difficulty in detection of intermediates, whereas quantum chemical calculations can effectively provide information on the reaction intermediates and pathways. BDE-15 (4,40 -dibromodiphenyl ether) was selected as a test case since its molecular C2 symmetry42 could reduce computational cost. Also, experimental kinetic data for this compound are available.34 In this paper, the mechanism and kinetics of single 3 OH-initiated photooxidation in the presence of O2 was investigated. First, activation energies (Ea) and reaction enthalpies (ΔH) were calculated to assess the energetically favorable reaction pathways and sites, and relative stability of the products. Subsequently, the energy-grained master equation (ME)43 was employed to calculate the rate constants of each reaction step, as well as branching ratios and time-dependent product distributions.

’ COMPUTATIONAL METHODS The electronic structure calculations were carried out with the Gaussian 09 program suite.44 Geometry optimizations of reactants, transition states (TS), intermediates and products were performed using the M062X hybrid meta exchange-correlation functional in conjunction with the 6-311þG(d,p) basis set.45 Frequency calculations were also carried out at the M062X/6-311þG(d,p) level to determine the character of the stationary points. The transition state was identified with one imaginary frequency. Intrinsic reaction coordinate (IRC) analysis46 was executed to verify that each TS uniquely connected the designated reactants with the products. As the rate constant calculations are sensitive to the activation energy (Ea), a more flexible basis set 6-311þG(3df,2p) was employed to determine the single point energies. The profile of the potential energy surface (PES) was constructed at the M062X/6-311þG(3df,2p)//M062X/6-311þG(d,p) level. The statistical mechanics calculations were carried out using the energy-grained master equation (ME), a powerful technique for modeling reactions that involve several connected energy wells and multiple product channels.47 The symmetry number of

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BDE-15 was set to 2. All the ME calculations were carried out using the open source program MESMER.48 The selection of functional and basis sets, the computational methods for microcanonical rate constant k(E), branching ratio (R), equilibrium constant (K), and error analysis are detailed in the Supporting Information (SI).

’ RESULT AND DISCUSSION Initial Reactions with 3 OH. BDE-15 has six different substitution positions in the molecule due to its C2 symmetry.42 The possible pathways for the reactions of BDE-15 with 3 OH are depicted in Figure 1. OH-Addition Pathways. OH addition to the aromatic rings results in 5 BDE-OH radical adducts (Figure 1). The computed zero-point corrected Ea and ΔH values are listed in Table 1. The ortho-adducts (BDE-OH2 and BDE-OH6) are the most favorable (Ea < 0, barrierless), followed by the ipso-adduct (BDEOH1) and the two meta-adducts (BDE-OH3 and BDE-OH5). The values of Ea for the two ortho-adducts and the two metaadducts are comparable. The length of the newly formed C
O bonds in the five transition-states (Supporting Information Figure S1) ranges from 1.99 Å to 2.03 Å and is 0.57
0.62 Å longer than those in the corresponding BDE-OH radical adducts. As indicated by the ΔH values, the stability of the two metaadducts (BDE-OH3 and BDE-OH5) is similar (Table 1). However, BDE-OH2 is about 1.32 kcal 3 mol
1 more stable than BDEOH6, which can be due to the different stabilization effect of the intramolecular hydrogen bonds. The H-bond length for BDEOH2 (2.45 Å) is shorter than that of BDE-OH6 by 0.32 Å (Supporting Information Figure S2). Br-Substitution Pathway. The Br atom in the para position can be substituted by 3 OH, forming 40 -OH-BDE-3 (Figure 1). Ea for the substitution reaction is higher than those for OH-addition pathways. However, ΔH for the reaction is much lower than those for the other pathways, implying that the reaction is strongly exothermic, and the products are more stable. The length (1.91 Å) of the C
Br bond to be broken in the TS (40 -OH-BDE-3_TS, Supporting Information Figure S1) is slightly longer than the corresponding bond length (1.90 Å) in BDE-15, while the newly forming C
O bond length is longer than the equilibrium bond length by 48%. Theoretically, the Br atom is a product accompanying the formation of 40 -OH-BDE-3. However, other heterogeneous and secondary reactions are also responsible for the formation of Br atoms, as Br2 was observed in the experiment as a main product.34 Kinetics. The calculated forward reaction rate constants (k1), equilibrium constants (K) and branching ratios (R) at 298 K are listed in Table 1. R values for BDE-OH2 and BDE-OH6 are 0.42 and 0.50, respectively, suggesting a strong preference for the ortho-OH addition. The computed Arrhenius equation is kOH (T) = (2.82  10
13) e958.6/T (250
400 K), which displays a negative temperature dependence and agrees with the experimental results.34 According to the M062X performance test,45 the mean unsigned error for Ea calculation was 1.22 kcal 3 mol
1. We also compared the Ea values from M062X and B2PLYP for 5 species, and the mean absolute error is 0.90 kcal 3 mol
1 (SI). Thus, the computational error of Ea from M062X is estimated to be 1 kcal 3 mol
1. Considering the errors, the calculated overall rate constant at 298
12 cm3molecule
1s
1, which is in reasonable K is 7.02þ1.41
2.27  10
12 cm3 agreement with the experimental value34 of 5.14þ0.98
0.82  10 4840

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12 molecule
1s
1 and the predicted value of 3.47þ5.81 cm3
2.18  10
1
1 18 molecule s using the QSAR model of our group. Subsequent Reactions for the Primary Intermediates. The intermediates produced in the initial reactions can react with atmospheric O2 subsequently. As BDE-OH2 is structurally similar to BDE-OH6, and BDE-OH3 to BDE-OH5, BDE-OH2 and BDE-OH3 were selected in the computations of subsequent reactions, so as to decrease the computational cost. BDE-OH1. The subsequent pathways of BDE-OH1 are depicted in Figure 2. The addition of 3 OH to the ipso-O position can lead to the breaking of the ether bond and formation of bromophenol. In the corresponding TS (OH1b_TS, Supporting Information Figure S3), the two benzene rings are nearly parallel to each other, and greatly different from BDE-OH1. O2 addition to the aromatic rings can form peroxy radical isomers. According to the nodal pattern of the H€uckel theory, the single occupied molecular orbital (SOMO) for the pentadienyl moiety (in BDE-OH1) only has coefficients on the positions 2, 4, and 6. In addition, position 2 is similar to 6, hence only two peroxy radical isomers (OH1_O2 and OH1_O4, Figure S4) were considered. To compare the formation rates, the bimolecular rate constants for the two peroxy radical isomers (OH1_O2 and

Figure 1. Possible pathways for the reactions of BDE-15 with 3 OH.

Table 1. Zero-Point Corrected Activation Energies Ea (kcal 3 mol
1), Zero-Point Corrected Reaction Enthalpies ΔH (kcal 3 mol
1), Forward Reaction Rate Constants k1 (cm3molecule
1s
1), Reverse Reaction Rate constants k
1(s
1), Branching Ratios R and Equilibrium Constants K for Initial Reactions speciesa BDE-OH1

Eab 1.11

ΔH b

k1c


22.09

k
1c

R

Kd

2.84  10
13

2.78  10
5

0.04

9.20


12

4.43  10
3

0.42

0.60

BDE-OH2


0.34


19.91

2.96  10

BDE-OH3

2.72


19.44

7.78  10
14

1.69  10
4

0.01

0.41

BDE-OH5

2.24


19.43

7.84  10
14

3.66  10
4

0.01

0.19

BDE-OH6


0.11


18.59

3.49  10
12

7.79  10
2

0.50

0.04

5.18


30.68

1.28  10
13 7.02  10
12

40 -OH-BDE-3 kOH (overall)

5.14  10
12

kOH (overall, exp) 34 a

0.02

b

The species correspond to the reaction pathways showed in Figure 1. Calculated at the M062X/6-311þG(3df,2p)//M062X/6-311þG(d,p) level. The Bartis
Widom phenomenological rate constants are obtained from eigenvalue-eigenvector analysis at 298 K, 740 Torr. d The OH concentration34 used for the computation is 9  108 molecules cm
3. c

Figure 2. Subsequent reaction pathways for BDE-OH1. 4841

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Table 2. Zero-Point Corrected Activation Energies Ea (kcal 3 mol
1), Zero-Point Corrected Reaction Enthalpies ΔH (kcal 3 mol
1), Forward Reaction Rate Constants k1, and Equilibrium Constants K BDE-OH1

speciesa

Eab

ΔHb

bromophenol

5.32


7.85

OH1_O2 OH1_O4

10.32 4.45


11.88
8.48

2.94  10
22 e 8.06  10
18 e

OH1_B26

17.99


4.72

2.53  10
2 d

OH1_O2C7 OH1_O2C6 BDE-OH2

10.18

22.96

7.34

Kf

8.87  106 d 52.77 0.04 2.06  102


5 d

8.08  10
10


7 d

1.46  10
7

1.02  10 8.65  10


15 e

2-OH-BED-15

7.05


29.96

1.14  10

OH2_O1

6.65


10.01

1.49  10
19 e

OH2_O3

9.75


9.83

7.48  10
22 e

OH2_O5 OH2_O1C6

1.19 22.85


11.24 11.57

5.11  10
15 e 7.08  10
6 d

32.94 5.64  10
10

OH2_O1C5

18.38

6.02

2.16  10
2 d

1.78  10
5

9.69


10.38

1.13  10

9.87  105

OH2_B31_b 3-OH-BED-15

15.18 7.66


10.19
31.68

4.79 6.18  10
15 e

1.42  107

OH3_O2

11.07


9.23

1.26  10
22 e

0.46

OH3_O4

9.06


8.69

2.56  10
21 e

0.11

OH3_O6

3.38


9.62

8.97  10
17 e

0.59

11.56


9.59

8.71  102 d

1.33  106

OH3_B24_b OH3_O2C7

11.78 24.95


9.05 12.35

1.31  10 5.73  10
8 d

3.24  105 4.38  10
11

OH3_O2C6

20.68

7.54

5.28  10
5 d

9.37  10
8

OH2_B31_a BDE-OH3

21.74

k1c

OH3_B24_a

OH3_O6C6 OH3_O6C7

23.45

10.43

22.05

11.09

a

4d

d

3d

0.29 4.24


7 d

9.13  10
10


5 d

1.05  10
9

6.08  10 1.33  10

b

The labeled species corresponds to the reaction pathways shown in Figures 2, 3, and S6. Calculated at M062X/6-311þG(3df,2p)//M062X/6-311þG(d, p) level. c The Bartis
Widom phenomenological rate constants were obtained from eigenvalue
eigenvector analysis at 298 K, 740 Torr. d The unimolecular rate constants are in units of s
1. e The bimolecular rate constants are in units of cm3 molecule
1 s
1. f The O2 concentration34 employed for the calculation is 4.92  1018 molecules cm
3.

OH1_O4, Table 2) were multiplied by the concentration of O2 (4.92  1018 molecules cm
3), and converted to unimolecular rate constants (1.44  10
3 s
1 and 3.97  101 s
1 for OH1_O2 and OH1_O4, respectively). k1 of bromophenol (8.87  106 s
1) is much higher than that of the peroxy radicals. Thus, almost all BDE-OH1 decomposed to form bromophenols. Raff and Hites34 also detected bromophenols as the main products and proved that 3 OH addition to the ipso-O position followed by the cleavage of the ether bond is the dominant pathway. OH1_O2 can cyclize either with the same ring (bicyclization) to form OH1_B26, or with the other ring (cross-cyclization) to form OH1_O2C6 and OH1_O2C7 (Figure 2). Ea for the formation of OH1_B26 is lower than for the formation of OH1_O2C6/OH1_O2C7 by ∼4 kcal 3 mol
1 (Supporting Information Figure S5). Thus, bicyclization of OH1_O2 is more favorable than cross-cyclization. Compared with OH1_O2, OH1_O4 is not apt to form bicyclization radicals, as the bicyclic products of OH1_O4 cannot form the delocalized ally-π system that can lower the energy. The cyclized products may decompose or lead to closed-shell products subsequently. However, we only focused on the primary reaction channels in this study, because of the high work load of computations. BDE-OH2 and BDE-OH3. The subsequent reaction pathways for BDE-OH2 (Figure 3) and BDE-OH3 (Supporting Information Figure S6) are similar to those for BDE-OH1. O2 can also add to the aromatic rings, leading to peroxy radical isomers that can

form cyclic radical isomers subsequently. The calculated PES profiles (Supporting Information Figure S5) show that the crosscyclization radical pathways are not energetically favorable and bicyclization is the predominant exit channel for the peroxy radicals. Abstraction of the H-atom of 3 OH by O2 forming epoxy radicals and HOO 3 radicals is another pathway (Supporting Information Figure S7). However, our calculations show that the barrier height (Ea) of the channel is too high to overcome (Supporting Information Table S4). Therefore, the epoxy radical pathways are not likely to occur. It is worth mentioning that some recent studies also found that atmospheric oxidation of benzene and toluene can form bicyclic radicals and that activation energies for epoxy channels are high.25,49,50 The subsequent reactions of BDE-OH2 and BDE-OH3 can also form HO-PBDEs through abstraction of H gem to the 3 OH by O2 (Figures 3 and S6). The corresponding products are 2-OH-BDE-15 and 3-OH-BDE-15. As indicated by the Ea and ΔH values (Table 2), these processes are strongly exothermic. The k1 values for 2-OH-BDE-15 and OH2_O5 are of the same order of magnitude (Table 2), suggesting that O2-addition to the para-position of the additive 3 OH competes with the formation of 2-OH-BDE-15 (Figure 3). k1 for 3-OH-BDE-15 is much higher than the formation rate of peroxy radical isomers (Table 2). Hence, nearly all BDE-OH3 transforms to 3-OH-BDE-15. Environmental Implications. Overall, we considered 19 reaction channels in the quantum chemical computation, which led to 24 intermediates and 5 closed-shell products. The relative 4842

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Figure 3. Subsequent reaction pathways for BDE-OH2.

Figure 4. Dominant reaction channels for single 3 OH-initiated photooxidation of BDE-15 in the presence of O2 (The width of the arrows indicates the relative importance of the reaction channel. The values on the arrow are the unimolecular rate constants with units of s
1.).

importance of the main channels together with their intermediates/ products is illustrated in Figure 4. The identified closed-shell products, HO-PBDEs (40 -OH-BDE-3, 2-OH-BDE-15, and 3-OHBDE-15), bromophenol and Br2, are consistent with the experimental observations.34 Two pathways were identified to be responsible for the production of HO-PBDEs: Br substitution by 3 OH, and abstraction of H gem to 3 OH in BDE-OH adducts by O2. HOPBDEs and bromophenol are more toxic than the parent compounds.36
38 Under tropospheric or experimental conditions, the intermediates (open-shell radicals) or closed-shell products will react subsequently. The formed HO-PBDEs will continually be oxidized by 3 OH. According to the QSAR model developed by our group,18 the predicted kOH for monohydroxylated PBDEs are generally higher than the parent molecules by a factor ∼1.7 (Supporting Information Table S5). Thus, it is not surprising that only trace amounts of HO-PBDE were detected in the experiment.34 The rate constant kOH is crucial for assessing the fate of SOCs. The computed kOH values are in reasonable agreement with

experimental values. This study poses an effective method combining the statistical mechanics and quantum chemistry to predict kOH and probe the degradation mechanism of SOCs like PBDEs. As the in silico method is cost-effective and independent of the standard chemical samples, it is critical to ecological risk assessment of newly synthesized chemicals and emerging pollutants.

’ ASSOCIATED CONTENT

bS

Supporting Information. Details on the computational methods, modeling conditions, error analysis, Ea and ΔH for epoxy species, rate constants predicted by QSAR model, subsequent reaction pathways of BDE-OH3, formation pathways of epoxy radicals, optimized geometries and corresponding TS, rate constants calculated in different temperatures, PES profile, and time-dependent species profiles. This material is available free of charge via the Internet at http://pubs.acs.org.

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’ AUTHOR INFORMATION Corresponding Author

*Phone/fax: þ86-411-84706269; e-mail: [email protected].

’ ACKNOWLEDGMENT We thank Dr. Cheng Zhong (Wuhan University) and Dr. Renyi Zhang (Texas A&M University) for instructions on the calculations, Dr. David R. Glowacki (University of Leeds) for the help with MESMER, and Prof. W. Peijnenburg (RIVM) for improving the composition. The study was supported by the National Natural Science Foundation of China (20890113, 20977014), the High-tech Research and Development Program of China (2010AA065105), the Fundamental Research Funds for the Central University and the Program for Changjiang Scholars and Innovative Research Team in University (IRT0813) of P. R. China. ’ REFERENCES (1) Cousins, I. T.; Beck, A. J.; Jones, K. C. A review of the processes involved in the exchange of semi-volatile organic compounds (SVOC) across the air
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