OH-Initiated Oxidation Mechanisms and Kinetics of 2,4,4

For each transition state, the intrinsic reaction coordinate (IRC) has been ... for elementary reactions were available using the Gibbs free energies...
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OH-Initiated Oxidation Mechanisms and Kinetics of 2,4,4′-Tribrominated Diphenyl Ether Haijie Cao,† Maoxia He,*,† Dandan Han,† Jing Li,† Mingyue Li,† Wenxing Wang,† and Side Yao‡ †

Environment Research Institute, Shandong University, Jinan 250100, P. R. China Shanghai Institute of Applied Physics, Chinese Academy of Sciences, P. O. Box 800-204, Shanghai 201800, P. R. China

‡

S Supporting Information *

ABSTRACT: 2,4,4′-Tribromodiphenyl ether (BDE-28) was selected as a typical congener of polybrominated diphenyl ethers (PBDEs) to examine its fate both in the atmosphere and in water solution. All the calculations were obtained at the ground state. The mechanism result shows that the oxidations between BDE-28 and OH radicals are highly feasible especially at the less-brominated phenyl ring. Hydroxylated dibrominated diphenyl ethers (OH-PBDEs) are formed through direct bromine-substitution reactions (P1∼P3) or secondary reactions of OH-adducts (P4∼P8). Polybrominated dibenzo-p-dioxins (PBDDs) resulting from o-OH-PBDEs are favored products compared with polybrominated dibenzofurans (PBDFs) generated by bromophenols and their radicals. The complete degradation of OH adducts in the presence of O2/NO, which generates unsaturated ketones and aldehydes, is less feasible compared with the H-abstraction pathways by O2. Aqueous solution reduces the feasibility between BDE-28 and the OH radical. The rate constant of BDE-28 and the OH radical is determined to be 1.79 × 10−12 cm3 molecule−1 s−1 with an atmospheric lifetime of 6.7 days.

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INTRODUCTION Polybrominated diphenyl ethers (PBDEs) have been incorporated into a variety of products such as plastics, electronic devices, building materials, and furniture as fire retardants. They are added to products without covalent bonds and are easily released into the environment. PBDEs have been detected in many samples, such as air,1 water, sediment,2 marine animals,3 and humans.4−6 2,4,4′-Tribromodiphenyl ether (BDE-28), 2,2′,4,4′-tetrabromodiphenyl ether (BDE-47), 2,2′,4,4′,5-pentabromodiphenyl ether (BDE-99), and 2,2′,4,4′,5,5′-hexabromodiphenyl ether (BDE-153) have been detected in most samples.7−11 Today, PBDEs have become ubiquitous worldwide contaminants and have generated deep concern due to their confirmed ability to bioaccumulate12,13 and be transported over long distances.14,15 PBDEs are toxic to the nervous system, immune system, endocrine system, liver, and thyroid.16,17 A prior study on cats that were suffering from feline hyperthyroidisms, and generally exposed to high concentrations of PBDEs, indicates that PBDEs in dust are significantly correlated with serum thyroxin concentration.18 PBDEs can also be metabolized into methoxylated PBDEs and hydroxylated PBDEs (OH-PBDEs) which are more toxic than the parent compounds.19−21 Studies on the concentrations of PBDEs in the environment show an obvious increasing trend in many areas and the trends are expected to continue in the following years22,23 which raises concern over this serious environmental problem and a need for effective removal methods. © 2013 American Chemical Society

Existing studies of the recalcitrant PBDEs have focused primarily on debromination degradation under ultraviolet light and pyrolysis. Pyrolysis of PBDEs leads to the formation of the notorious polybrominated dibenzo-p-dioxins (PBDDs) and polybrominated dibenzofurans (PBDFs) which have been detected in quantitative samples from municipal waste incinerators. The apparent formation of PBDFs was also observed in plastic samples.24 OH-PBDEs are proposed to be another precursor for photochemical formation of PBDDs with high yields.25,26 The transformations of PBDEs and their derivatives have been observed using various catalytic treatments (electrolysis, hydrothermal treatment, catalytic hydrogenation, etc.).27−29 Monoand dibrominated diphenyl ethers are inclined to transform into OH-PBDEs with OH radicals.30 Luo et al. combined reduction and oxidation of BDE-47 in aqueous solution and observed the final degradation products of BDE-47 as phenol and small organic acids.31 DecaBDE treated with zerovalent iron can slowly generate lower brominated PBDEs.32 Ultraviolet photolysis can highly promote the debromination process.33 The authors also pointed out that the reaction with the OH radical predominates in the UV/H2O2/AP system. These experiments have provided evidence supporting the debromination and bimolecular Received: Revised: Accepted: Published: 8238

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oxidation of PBDEs. However, transformation processes under real world conditions are hard to simulate experimentally since the results depend upon many factors (standard samples, catalyzing methods, concentrations, etc). Hence, the precise mechanism for the transformation of PBDEs to PBDDs/PBDFs and the impact of bromine on the property of the PBDEs is vague, and their environmental fates are still unclear. Quantum methods can determine reaction properties precisely and serve as an accurate benchmark for photolysis of PBDEs. Existing theoretical studies about PBDEs have been successful and their results are consistent with the experimental data.34,35 Moreover, the properties of some PBDEs were calculated for both ground and excited states,36 and the results indicate that the structures and activities of PBDEs do vary with the pattern of bromine substitution. To better characterize the atmospheric removal process of PBDEs, BDE-28, which has been detected in many samples including fish, water, and blood plasma,37−39 was selected as the probe compound. BDE-28 has no anthropogenic source and is mostly generated through debromination of BDE-47 and other highly brominated PBDEs. BDE-28 is easily released into the atmosphere and is suggested to be active to react with oxidants. Briefly, the present combined quantum mechanics and molecular dynamics study has been pursued to gain insight into the feasibility of transformation from PBDEs to OH-PBDEs, PBDD/Fs, and other removal processes of PBDEs.

COMPUTATIONAL METHODS The Gaussian 03 program40 was used to perform all the calculations on the geometries, energies, frequencies of stationary points, and transition states (TS). The MPWB1K functional41 was employed in this paper. This method has yielded satisfying results in previous research.42−44 Geometry optimization and energy calculation were obtained under a basis set of 6-31+G(d,p). In order to obtain more accurate energies, a higher basis set of 6-311+G(3df,2p) using the same method was employed. For each transition state, the intrinsic reaction coordinate (IRC) has been calculated to identify the connections between reactants, transition states, and products. Aqueous calculations were performed using the polarizable continuum model (PCM),45−47 which created the solute cavity via a set of overlapping spheres. In this paper, water was selected as a solvent, and its dielectric constant was set at 80.0. To examine the thermodynamic properties in water, both reaction heats and Gibbs free energies, including solvation energies in solution, were calculated. As such, the equilibrium constants for elementary reactions were available using the Gibbs free energies. Master equation calculations were employed to study the kinetic properties using Rice−Ramsperger−Kassel−Marcus (RRKM) theory48 with the recently developed MESMER program.49 The Mesmer program has been successfully applied in previous research.50,51 The microcanonical rate coefficients have been calculated using the RRKM expression: W (E ) hρ(E)

(1)

where W(E) is the rovibrational sum of states at the TS, ρ(E) is density of states of reactants, and h is Planck’s constant. Then, canonical rate coefficients k(T) are determined using the usual equation: k(T ) =

1 Q (T )

∫ k(E) ρ(E) exp(−βE) dE

RESULTS AND DISCUSSION

Reaction Mechanism in the Gas Phase. First, the reliability of several DFT methods and MP2 are checked. For B3LYP, MPWB1K, BB1K, M05, and MP2, the reaction enthalpies of biphenyl + H2 → phenylcyclohexane are −32.40, −48.16, −44.85, −47.10, and −35.24 kcal/mol at the basis of 6-311+G(3df,2p). Apparently, MPWB1K and M05 provide satisfying results compared with experimental values (−47.02 ± 1.2 kcal/mol).52,53 However, the energy enthalpy of M05 shows an abnormal value (−100 kcal/mol) at the basis set of 6-31+G(d,p). In addition, previous research also proves that MPWB1K is one of the best DFT methods in calculation energies.54 Therefore, we have selected the MPWB1K method. Frontier Molecular Orbital Analysis. For BDE-28, the most stable structure (Figure S1) of the three typical conformations of PBDEs are chosen for investigation.55 BDE-28 is anisomerous, and the dihedral formed by two phenyl rings is 74.06°. The ether bonds are 1.357 Å and 1.365 Å, longer than the corresponding ones of BDE-7 (1.355 Å and 1.360 Å) but shorter than that of BDE-47 (1.361 Å and 1.368 Å), indicating an enhanced stabilizing effect caused by bromine atoms. The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the reactants are shown in Figure S3. For BDE-28, the LUMO and LUMO+1 mainly consist of the antibonding orbital of ring 1. The lone electron pair of the Br atom and π electrons of ring 2 contribute most to the HOMO. And the HOMO−1 locates at ring 1 (mainly the lone electron pair of Br atoms and π electrons of phenyl ring 1). From Figure S3, the energy difference between the HOMO of BDE-28 and the LUMO of the OH radical is 4.13 eV, indicating an electron transfer from the HOMO of BDE-28 to the LUMO of the OH radical. The OH radical is inclined to react with the C atoms of ring 2. The phenomenon shows that bromine atoms will decrease the activity of the phenyl ring. OH-Initiated Reactions. The OH radical is chosen as the initial oxidant for its high oxidability and abundance in the air. Our previous studies suggest that OH-addition pathways play a dominant role compared to hydrogen abstraction pathways.34,35,56 Thus, only OH addition pathways are discussed in this paper. The phenyl ring with two bromine atoms of BDE-28 is defined as ring 1, and the other phenyl ring is ring 2 for convenience. The reaction heats and the energy barriers of initial reactions are shown in Figure 1. All the initial reactions are exothermic. Pathways 2, 4, and 10 are direct Br-substitution pathways followed by the formation of 6-hydroxyl-4,4′-dibromodiphenyl ether (6-OH-BDE-15, P1), 4-hydroxyl-2,4′-dibromodiphenyl ether (4-OH-BDE-8, P2), and 4′-hydroxyl-2,4-dibromodiphenyl ether (4′-OH-BDE-7, P3), respectively. The rest of the pathways will generate OH adducts. Obviously, OH radicals prefer to interact with nonbromine substituted carbon atoms, and Br-substitution pathways are not important under natural conditions because of their energy barriers of 5.12−7.15 kcal/mol compared to other hydroxyl addition pathways. Additionally, 2,4-dibromophenoxy (ring 1-O-, R1-O-) shows an ortho-,para-directing effect while 4-bromophenoxy (ring 2-O-, R2-O-) acts as a meta-directing group. Among these reactions, pathway 8 is the most favorable. Overall, ring 2 is more active than ring 1, which has supported our previous conclusions that bromine atoms will deactivate the same ring. Interestingly, an obvious difference is found between BDE-28 and its congener. For BDE-15, the OH additions to the ortho-carbon atoms are barrierless, but the full addition reactions

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k(E) =

Article

(2)

where Q(T) is the reactant partition function. 8239

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Figure 1. Reaction heats (ΔH) and energy barriers (ΔE) of the initial and subsequent elementary reactions. Values within parentheses are respective values considering the solvent effect. Units of ΔE and ΔH are kcal/mol.

have to overcome a high or moderate energy barrier.34 This is attributed to the bromine atom in ring 2. The OH adducts are unstable radical compounds. From Figure 1, IM1 decomposes to 2,4-dibromophenol (2,4-DBP) and the 4-bromophenol radical (4-BPR), while IM5 decomposes to

the 2,4-dibromophenol radical (2,4-DBPR) and 4-bromophenol (4-BP). Intermediates IM2−IM9 will react with an atmospheric O2 molecule as well as other aromatic OH adducts. The O2 molecule has three spin states (two singlet states and one triplet state). The triplet state (the ground state of O2) is the most 8240

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Figure 2. Schemes for the formation of PBDD and PBDFs. Values within parentheses are respective values considering the solvent effect. Units of ΔE and ΔH are kcal/mol.

abundant in the air and is chosen in this paper. The H-abstraction pathways by O2 are barrierless and exothermic, making the OHaddition pathways the rate-determining steps of the formation of OH-BDE-28s. Then, stable products (3-OH-BDE-28 (P4), 5-OH-BDE-28 (P5), 6-OH-BDE-28 (P6), P7, P8, P7b, and P8b)

and HO2 are produced. P7 and P7b are different conformers of 2′-OH-BDE-28, while P8 and P8b are different conformers of 3′-OH-BDE-28. Both OH-DBDEs (P1−P3, resulting from direct bromine-substitution) and OH-BDE-28s (P4−P8b, resulting from the stepwise mechanism) are available products, 8241

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Figure 3. Reactions of IM6 in the presence of O2/NO. Values within parentheses are respective values considering the solvent effect. Units of ΔE and ΔH are kcal/mol.

but 2′-OH-BDE-28 (P7b) is the most favorable one. This is converse to the transformation of BDE-15 for which the authors find that the H-abstraction pathways by O2 are difficult because of high barrier heights.34 One possible explanation is that the electron-donating ability of bromine atoms may decrease the bond strengths of C−H bonds. PBDD/Fs Formation. Since only ortho-hydroxyl PBDEs act as precursors for the formation of PBDDs, P1, P6, and P7, produced via previous pathways, are investigated based on their potential to be transformed to PBDDs (Figure 2). These transformations start with the removal of phenoxyl hydrogen atoms, followed by ring-closing processes. The energy barriers for hydrogen abstraction reactions are definitely different. In particular, the abstraction of phenoxyl hydrogen of P7 is barrierless. The ringclosing reactions are endothermic and proceed with energy barriers of ∼23 kcal/mol for attacking the H-adjacent C atoms. Once produced, OH radicals will subsequently abstract H atoms

in IM11, IM13, and IM15. These processes are barrierless and highly exothermic (∼91 kcal/mol). Additionally, the other ringclosing pathway for IM14 describes the attack at a Br-adjacent C atom and is slightly endothermic by 0.47 kcal/mol, leading to the formation of P9 and a Br atom. As seen from Figure 2, the terminal oxygen is inclined to attack not a Br-adjacent C atom but a H-adjacent C atom. Then, the hydrogen atom in IM11 will be abstracted by the Br atom barrierlessly. Hence, P10 is the preferred product rather than P9. However, this trend is different from our previous results in which the oxygen atom is likely to attack the Br-adjacent atom.57 Although OH-BDE-47 is likely to generate lower-brominated PBDDs, OH-BDE-28 is favorable to transfer to tribrominated dibenzo-p-dioxins (TBDDs). This is consistent with high frequency detection of TBDDs but rare detection of DBDDs in experiments. Recombination of the bromophenols and phenol radicals, which are produced in the early steps, can generate other 8242

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Figure 4. Further reactions of IM10. Values within parentheses are respective values considering the solvent effect. Units of ΔE and ΔH are kcal/mol.

OH-PBDEs or PBDFs. P11 and P12 are formed through the terminal oxygen atom attacking different Br atoms. Apparently, P11 is a more favorable product than P12. That is to say, the ortho-Br of 2,4-DBP is easier to release than the para-Br. The formation of PBDFs, initiated with the recombination of C−C in 2,4-DBP and 4-BPR, is highly endothermic, followed by ringclosing processes. The abstractions of H atoms in IM17 and IM22 are also barrierless but highly endothermic by ∼22 kcal/mol. However, the loss of water molecules in IM18 and IM23 encountered high energy barriers such as 49.48 and 47.41 kcal/mol. The formation of P14 and P15 carries on with modest energy barriers. Hence, dibromodibenzofurans (DBDFs) are more favorable products than tribromodibenzofurans (TBDFs). Comparing the full pathways of the formation of PBDD and PBDFs, PBDDs are the preferred product according to their lower energy barriers. Further Reaction of IM6 Oxidized by O2/NO. Experiments show that many aromatic compounds can completely degrade to small molecules.58,59 In this section, the authors try to ascertain the potential of complete decomposition of BDE-28 in the air. IM6, as the most favorable OH adduct, is selected for the above purpose. Because of the delocalization of the Π bond, IM6 has three active sites (C(1′), C(3′), and C(5′)) to interact with the O2

molecule (Figure 3). All the O2 addition processes of IM6 have positive energy barriers and are exothermic. The terminal oxygen atoms of IM26 and IM27 prefer to attack the phenyl carbon atoms to form the five-member intermediate IM29. IM29 has two active sites (C(4′) and C(6′) atoms) and is supposed to form two peroxy compounds, followed by NO addition to the terminal oxygen atom. The process of O2 attacking the C(5′) atom generates IM28. Both self-decomposition and bimolecular reaction of IM28 produce P16 finally. For IM31, the removal of HNO2 is obviously easier than the removal of NO2. Decompositions of NO Adducts. Both IM34 and IM36 can decompose easily. The reaction heats and the energy barriers of elementary reactions are depicted in Figure 4. A detailed discussion of their decomposition is as follows. IM34 can decompose by the removal of NO2 and generate IM37 as well as general decomposition path of most -O-ONOcompounds. Then intramolecular decomposition of IM37 will proceed moderately via a series of step-by-step bond breaking reactions. During the full process of IM34 decomposition referred to above, the removal of NO2 is the rate-determining step with an energy barrier of 47.83 kcal/mol, which is conceptually unreasonable. Thus, an alternative mechanism is pursued. Our results indicate that the loss of HNO2 of IM34 only needs to 8243

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Table 1. Rate Constants for Elementary Reactions at 298.15 K, 1 atm (Units s−1 for Unimolecular Reactions and cm3 molecule−1 s−1 for Bimolecular Reactions) reaction

rate constant × × × ×

10−16 10−18 10−14 10−17

R+OH→TS1→IM1 R+OH→TS2→P1+Br R+OH→TS3→IM2 R+OH→TS4→P2+Br

5.01 3.38 5.12 1.71

R+OH→TS5→IM3

6.34 × 10−14

R+OH→TS6→IM4 Ring1+OH R+OH→TS7→IM5

−15

6.31 × 10 1.21 × 10−13 1.52 × 10−13

R+OH→TS8→IM6 1.06 × 10−12 R+OH→TS9→IM7 4.76 × 10−14 R+OH→TS10→P3+Br 2.16 × 10−15 × × × ×

−14

R+OH→TS11→IM8 R+OH→TS12→IM9 Ring2+OH IM1→TS1→2,4-DBP +4BPR IM2+O2→P4+HO2

2.34 3.81 1.67 1.15

10 10−13 10−12 106

IM3+O2→P5+HO2

2.50 × 10−15

IM4+O2→P6+HO2 IM6+O2→P7+HO2

4.02 × 10−15 1.02 × 10−16

1.44 × 10−15

reaction IM7+O2→P8+HO2 IM8+O2→P8b+HO2 IM9+O2→P7b+HO2 IM5→TS1→2,4-DBPR +4BP P1+OH→TS13→IM10 +H2O IM10→TS14→IM11 IM11+OH→P9+H2O P6+OH→TS15→IM12 +H2O IM12→TS16→IM13 IM13+OH→P10+H2O P7+OH→TS17→IM14 +H2O IM14→TS18→IM15 IM15+OH→P10+H2O IM14→TS19→P9 2,4-DBP+4BPR→P11 +Br 2,4-DBP+4BPR→P12 +Br 2,4-DBP +4BPR→TS22→IM16 IM16→TS23→IM17 IM18→TS24→P13 2,4-DBP +4BPR→TS25→IM19

rate constant 2.63 4.41 2.10 2.44

× × × ×

reaction

10−15 10−15 10−15 107

IM20→TS27→P14+H2 2,4-DBPR+4BP→P3+Br 2,4-DBPR +4BP→TS29→IM21 IM21→TS30→IM22

2.16 × 10−14 2.13 × 10 6.44 × 10−12 4.49 × 10−18

× × × ×

1.82

IM34→TS46→IM37 +NO2 IM37→TS47→IM38 IM38→IM39

IM6+O2→TS37→IM28 2.54 × 10−20 IM26→TS38→IM29 8.04 × 10−15 IM27→TS39→IM29 1.78 × 10−9

−6

10 10−12 10−8 10−28

IM28→TS40→IM30 1.02 × 10−20 IM30→TS41→P16+OH 1.45 × 1011 IM28+NO→IM31 1.73 × 10−15

7.04 × 10−35 1.39 × 10−21

IM31→TS42→IM32 +NO2 IM32→P16

0.91 1.79 × 10−24 1.78 × 10−23

rate constant

IM33+NO→IM34 2.92 × 10−12 IM28+O2→TS45→IM35 4.00 × 10−19 IM35+NO→IM36 4.69 × 10−12

IM6+O2→TS35→IM26 1.19 × 10−22 IM6+O2→TS36→IM27 7.94 × 10−25

2.20 × 10−5 6.22 × 10−12 1.11 × 10−12

reaction

3.29 × 10−10 6.10 × 10−26 2.32 × 10−21

IM23→TS31→P13+H2O 4.12 × 10−23 2,4-DBPR 1.37 × 10−23 +4BP→TS32→IM24 IM25→TS34→P15+H2 4.74 × 10−11

−5

9.51 6.19 5.91 6.14

rate constant

5.22 × 10−23 3.69 × 10−12

IM31→TS43→P16 3.38 × 10−9 +HNO2 IM29+O2→TS44→IM33 7.52 × 10−21

3.92 × 10−23 7.93 × 106 7.00 × 10−12

IM39→TS48→P17 +IM40 IM40+O2→P18+HO2 IM34→TS49→IM41 +HNO2 IM41→TS50→P19 P19→TS51→P20 IM36→TS52→IM42 +NO2 IM42→TS53→IM43 IM43→IM44 IM44→TS54→P21 +IM45 IM45+O2→P22+HO2

1.73 × 108

IM36→TS56→IM46 +NO2+Br IM46→TS57→P23

1.28 × 10−17 6.65 × 10−24

P23→TS58→P24

3.08 × 10−19

7.61 × 10−15 1.13 × 10−8 7.26 × 10−10 4.64 × 10−17 1.31 × 10−17 6.84 × 107 6.00 × 10−12 1.15 × 1010 4.65 × 10−18

oxygen in water makes the transfer of BDE-28 to OH-BDE-28s easier. Moreover, the water molecule shows various effects on different carbon atoms. Figure 2 shows that phenoxyl hydrogen atoms of o-OH-PBDEs are hard to abstract via OH radicals in water. However, the aqueous solution exhibits a minor effect on other processes for the formation of PBDDs and PBDFs. The energy barrier of the first oxygen molecule addition is decreased, and the O2 addition to the ipso-C atom faces the highest energy barrier. This effect is more obvious in the participation of the second O2 molecule. The aqueous study exhibits basically similar trends with the gas phase analysis except for the above differences. Briefly, degradation of BDE-28 in the wastewater treatment and other places is feasible by oxidant catalysts, which generate OH radicals. However, the pollution control progress must be cautious, out of the consideration of the newly generated OHPBDEs. Kinetic Study. All the rate coefficients are calculated at 298.15 K and 1 atm. For the bimolecular reactions, the rate constants are divided by the concentrations of corresponding excess reactants. For example, the rate constants of the initial reactions have been divided by the atmospheric concentration of OH radicals (9.75 × 105 molecules cm−3).60 Microcanonical rate constants for the barrierless reactions were obtained using the inverse laplace transform method. The rate constants for elementary reactions are presented in Table 1. The total rate constant of BDE-28 and the OH radical is 1.79 × 10−12 cm3 molecule−1 s−1, of which the rate constant of ring 2 and the OH radical accounts for 93.2%. A comparison between our results and experimental values is unavailable because of a lack of experimental data. Alternatively, we have chosen the rate constants

overcome a 28.11 kcal/mol barrier height. Then IM41 is formed. IM41 can decompose with the cleavages of the O−O bond and C(1′)−C(2′) bond, and then P19 is produced. P19 may isomerize to P20 via an energy barrier of 39.38 kcal/mol, and P20 is 0.44 kcal/mol higher than P19. That is to say, P19 is the major product. Similar to IM34, two decomposition pathways are considered for IM36. The energy barriers of the removal of NO2 and NO2 + Br are 40.50 and 40.44 kcal/mol, respectively. Although the following reactions are easy to go on, IM36 is difficult to decompose because of the high energy barriers of rate determining steps. To conclude, the favorable products of the complete oxidation of BDE-28 are 2-bromo-5-(2,4-dibromophenoxy)-4-hydroxycyclohexa-2,5-dien-1-one (P16) and 2,4-dibromophenyl-(3E)-4bromo-6-hydroxy-2,5-dioxohex-3-enoate (P19). These processes have to overcome high barrier heights in the ground state, so efficiency catalysts are required to improve the reaction’s feasibility. Solvent Effect for the Title Reaction. In this section, water is selected as the solvent, since in most experimental research, PBDEs have been explored in soil or solution. To compare with the results in the gas phase, the same processes are calculated in the aqueous solution. The reaction heats and the energy barriers in water are available in Figure 1−4 (values in brackets). The changes of reactions in Gibbs free energy (ΔGgas, ΔGaq), free energies of activation (ΔG≠gas, ΔG≠aq), and the equilibrium rate constants (Kgas, Kaq) for elementary reactions are summarized in Table S1 in the Supporting Information. Water solution obviously increases the energy barriers for most initial steps but stabilizes the products on the whole (Figure 1). The priority of ring 2 is reduced, and the dissolved 8244

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of congeners for indirect comparison. Raff and Hites have determined the rate constant of BDE-7 with OH radicals (3.88−0.71+0.87 cm3 molecule−1 s−1, 298.15 K).30 Previous theoretical results show that the rate constants of BDE-7, BDE-15, and BDE-47 with OH radicals are 3.76 × 10−12 cm3 molecule−1 s−1, 7.02 × 10−12 cm3 molecule−1 s−1, and 8.29 × 10−13 cm3 molecule−1 s−1 at 298.15 K in the atmosphere, respectively.34,35,56 Considering the effect generated by the bromine substituted degree, we think our results are reasonable. Apparently, the rate constants of BDE-28 and the OH radical are smaller than those of BDE-7 and BDE-15, but larger than that of BDE-47. This provides evidence for the negative effect generated by bromine atoms. The association of BDE-28 and OH radicals has shown a similar phenomenon to BDE-7; that is, the phenyl ring with less bromine has higher reactivity. The atmospheric lifetime τ has been calculated through the formula τ = 1/ka[OH]. ka is the total rate constant of the initial reactions. [OH] is the atmospheric concentration of the OH radical (9.7 × 105 molecules cm−3). Thus, the atmospheric lifetime of BDE-28 is 6.7 days. The decompositions of IM1 and IM5 proceed with high efficiency compared to the H-abstraction processes by the O2 molecule. However, we believe the H-abstraction processes are preferred because of the abundance of O2 molecules in the air. The rate constants for the formation of PBDDs are larger than those of PBDFs. Thus, the major product is PBDDs not PBDFs at room temperature, which is consistent with experimental observations.25,26 Decomposition of IM6 shows that O2 prefers to interact with C(5′), leading to the formation of IM28. The kinetic conclusion is consistent with the mechanism. Environmental Relevance. In summary, this study proposes a diverse transformation of BDE-28, determining the major products. The ground-state oxidation mechanism and kinetics indicate that BDE-28 reacts feasibly with the OH radical, especially in the lower-brominated phenyl ring. OH-DBDEs (P1−P3) are formed through a direct bromine-substitution process, and OH-BDE-28s (P4−P8b) are generated via the stepwise mechanism. Among these OH-PBDEs, OH-BDE-28s are preferred thermodynamically. The subsequent O2 additions to the phenyl ring of the OH adduct are difficult, compared to the H-abstraction pathways in a ground state, which is quite different from the actions of BDE-15 in prior work.34 PBDEs are easily transferred to OH-PBDEs, and the subsequent reaction of OH-PBDEs will continue moderately to generate notorious PBDD/Fs. The kinetic results show that BDE-28 has a shorter lifetime than BDE-47 but longer than BDE-7 and BDE-15, providing strong evidence for the deactivation effect of bromine atoms. At room temperature and atmospheric pressure, PBDDs are more favorable than PBDFs both thermodynamically and kinetically. Overall, oxidation of BDE-28 exhibits some differences with its congeners. We expect these results to serve as supplemental data and contribute to the risk assessment of PBDEs.

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AUTHOR INFORMATION

Corresponding Author

*Fax: 86-531-8836 1990. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported financially by the National Natural Science Foundation of China (21077067, 21073220, and 21177076). We thank Dr. Struan H. Robertson for providing the Mesmer program.

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ASSOCIATED CONTENT

S Supporting Information *

Additional information, including the structure, frequencies of stationary points and transition states, and Gibbs free energy of elementary reactions as noted in the text, is available free of charge via the Internet at http://pubs.acs.org. 8245

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