quantum chemical investigation on the mechanism and ... · ortho-adducts (bde-oh2 and bde-oh6) are...

Published: May 11, 2011

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

ARTICLE

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Quantum Chemical Investigation on the Mechanism and Kineticsof PBDE Photooxidation by 3OH: A Case Study for BDE-15Jing 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

’ INTRODUCTION

Semivolatile organic compounds (SOCs) cause extensiveenvironmental contamination1�3 and significant adverse effectson human health.4,5 The reaction with 3OH in the tropospherestrongly determines the environmental persistence and fate ofSOCs.6,7 Understanding the mechanism and kinetics of this reac-tion is an indispensable component of risk assessment.8,9 Severaldirect and indirect experimental methods have been developed toprobe the reactions.10�13 However, almost all the experimentalmethods are time-consuming, costly, and equipment depen-dent. Thus, they cannot meet the need of risk assessment andpollution prevention for newly synthesized compounds prior tolarge-scale production and commercialization.8 Consequently, itis crucial to develop computational approaches to predict thechemical reactivity of SOCs.8

It is worth mentioning that in silico (computer-based) technol-ogies for chemical screening and prioritization have become aresearch focus in recent years and a topic of subsequent discussionregarding implementation. For example, the U.S. EPA establishedthe National Center for Computational Toxicology in 2005, whichaims at providing high-throughput computational tools for screen-ing and assessing chemical exposure, hazard, and risk.14 The newlegislation REACH (Registration, Evaluation, Authorization, andRestriction of Chemicals) implemented by the European Union in2007 also advocates in silico approaches for assessing environmental

risk of chemicals, such as the technology of quantitative struc-ture�activity relationships (QASRs).15

Several QSAR models have been developed to predict the rateconstants (kOH) for the reactions of organic chemicals with

3OH.16�19 Generally, the QSAR models are attractive as they

generate results with minimal computational cost and they areapplicable for regulatory purposes. However, the utility of QSARsis constrained as they strongly rely on experimental databases andare only valid for compounds within the applicability domain.20,21

Furthermore, conventional QSAR models cannot provide infor-mation on reaction pathways and mechanisms.

In recent years, increases in computational capability haveenabled quantum chemical computations for molecules orsystems comprising up to a few hundred atoms.22,23 The densityfunctional theory (DFT) and ab initio methods allow foraccurate predictions of chemical reactivity without any priorknowledge on the substance, and they have been widely appliedto identify reaction mechanisms and generate kinetic data.24�27

Some previous studies have shown that quantum chemistry and

Received: January 10, 2011Accepted: April 26, 2011Revised: March 18, 2011

ABSTRACT:Computational approaches are crucial to risk assess-ment and pollution prevention of newly synthesized compoundsprior to large-scale production and commercialization. Under-standing the kinetics and mechanism of the tropospheric reactionof semivolatile organic compounds with 3OH is an indispensablecomponent of risk assessment. In this study, we show that thedensity functional theory (DFT) can be successfully employed toprobe the kinetics and mechanism of atmospheric photooxidationof polybrominated diphenyl ethers (PBDEs) by 3OH, taking 4,4

0-dibromodiphenyl ether (BDE-15) as a case. The predicted pro-ducts (HO-PBDEs, brominated phenols and Br2) and overall rateconstant (kOH) at 298 K are consistent with the experimentalresults. Two pathways leading to formation of HO-PBDEs areidentified: Br substitution by 3OH, and abstraction of H gem to 3OH in BDE-OH adducts by O2. This study offers a cost-effectiveway for probing the atmospheric indirect photooxidation kinetics and mechanism of PBDEs.

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4840 dx.doi.org/10.1021/es200087w |Environ. Sci. Technol. 2011, 45, 4839–4845

Environmental Science & Technology ARTICLE

statistical mechanics pose a viable way forward for investigatingradical (e.g., 3OH and peroxy radicals) oxidation reactions.28�30

The purpose of this study is to show that DFT can be employedto assess the gaseous reactions between a polybrominated diphen-yl ether (PBDE) congener and 3OH. PBDEs are additive-typeflame retardants widely used in plastics, textiles, electronic circui-try, and other types of consumer products.5 As there is no chemicalbond between the additives and products, PBDEs are susceptibleto be released into the environment.31 They have been widelydetected in samples of air, water, sediment, bird, fish, marinemammals, and even in humans, and their environmental levels areincreasing exponentially.32 As the boiling points of PBDEs rangefrom 310 to 425 �C,33 most PBDE congeners are SOCs.

There are only a few experimental data on the reactions of PBDEswith 3OH. Raff and Hites determined kOH values for 7 PBDEs34 andconcluded that 3OH initiated oxidation was a dominating removalpath for PBDEs substituted by 1�2 bromines.34,35 They detectedbromophenols and Br2 as primary degradation products, as well astraces of hydroxylated PBDEs (HO-PBDEs).34 HO-PBDEs are ofgreat concern because of their xenoestrogenic36 and thyroidhormone effects.37,38 In addition to biological samples,39,40 HO-PBDEs were also detected in surface water and in precipitations,41

suggesting that besides biometabolites, oxidation of PBDEs by

3OH in the troposphere can be an importance source of HO-PBDEs. However, the detailed mechanism for the formation ofHO-PBDEs from photooxidation of PBDEs needs further elucida-tion. Experimental exploration on the mechanism is not easybecause of the difficulty in detection of intermediates, whereasquantum chemical calculations can effectively provide informationon the reaction intermediates and pathways.

BDE-15 (4,40-dibromodiphenyl ether) was selected as a testcase since its molecular C2 symmetry42 could reduce computa-tional cost. Also, experimental kinetic data for this compound areavailable.34 In this paper, the mechanism and kinetics of single

3OH-initiated photooxidation in the presence of O2 was investi-gated. First, activation energies (Ea) and reaction enthalpies (ΔH)were calculated to assess the energetically favorable reactionpathways and sites, and relative stability of the products. Subse-quently, the energy-grained master equation (ME)43 was em-ployed to calculate the rate constants of each reaction step, as wellas branching ratios and time-dependent product distributions.

’COMPUTATIONAL METHODS

The electronic structure calculations were carried out with theGaussian 09 program suite.44 Geometry optimizations of reactants,transition states (TS), intermediates and products were performedusing the M062X hybrid meta exchange-correlation functional inconjunction with the 6-311þG(d,p) basis set.45 Frequency calcula-tions were also carried out at the M062X/6-311þG(d,p) level todetermine the character of the stationary points. The transition statewas identified with one imaginary frequency. Intrinsic reactioncoordinate (IRC) analysis46 was executed to verify that each TSuniquely connected the designated reactants with the products. Asthe rate constant calculations are sensitive to the activation energy(Ea), a more flexible basis set 6-311þG(3df,2p) was employed todetermine the single point energies. The profile of the potentialenergy surface (PES) was constructed at the M062X/6-311þG-(3df,2p)//M062X/6-311þG(d,p) level.

The statistical mechanics calculations were carried out usingthe energy-grained master equation (ME), a powerful techniquefor modeling reactions that involve several connected energywells and multiple product channels.47 The symmetry number of

BDE-15 was set to 2. All the ME calculations were carried outusing the open source program MESMER.48 The selection offunctional and basis sets, the computational methods for micro-canonical rate constant k(E), branching ratio (R), equilibriumconstant (K), and error analysis are detailed in the SupportingInformation (SI).

’RESULT AND DISCUSSION

Initial Reactions with 3OH. BDE-15 has six different sub-stitution positions in the molecule due to its C2 symmetry.42 Thepossible pathways for the reactions of BDE-15 with 3OH aredepicted in Figure 1.OH-Addition Pathways. OH addition to the aromatic rings

results in 5 BDE-OH radical adducts (Figure 1). The computedzero-point corrected Ea and ΔH values are listed in Table 1. Theortho-adducts (BDE-OH2 and BDE-OH6) are the most favor-able (Ea < 0, barrierless), followed by the ipso-adduct (BDE-OH1) and the two meta-adducts (BDE-OH3 and BDE-OH5).The values of Ea for the two ortho-adducts and the two meta-adducts are comparable. The length of the newly formed C�Obonds in the five transition-states (Supporting InformationFigure S1) ranges from 1.99 Å to 2.03 Å and is 0.57�0.62 Ålonger than those in the corresponding BDE-OH radical adducts.As indicated by the ΔH values, the stability of the two meta-

adducts (BDE-OH3 and BDE-OH5) is similar (Table 1). How-ever, BDE-OH2 is about 1.32 kcal 3mol�1 more stable than BDE-OH6, which can be due to the different stabilization effect of theintramolecular hydrogen bonds. The H-bond length for BDE-OH2 (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 3OH, forming 40-OH-BDE-3 (Figure 1). Eafor the substitution reaction is higher than those for OH-additionpathways. However, ΔH for the reaction is much lower thanthose for the other pathways, implying that the reaction isstrongly exothermic, and the products are more stable. Thelength (1.91 Å) of the C�Br bond to be broken in the TS(40-OH-BDE-3_TS, Supporting Information Figure S1) isslightly longer than the corresponding bond length (1.90 Å) inBDE-15, while the newly forming C�O bond length is longerthan the equilibrium bond length by 48%. Theoretically, the Bratom is a product accompanying the formation of 40-OH-BDE-3.However, other heterogeneous and secondary reactions are alsoresponsible for the formation of Br atoms, as Br2 was observed inthe experiment as a main product.34

Kinetics. The calculated forward reaction rate constants (k1),equilibrium constants (K) and branching ratios (R) at 298 K arelisted in Table 1. R values for BDE-OH2 and BDE-OH6 are 0.42and 0.50, respectively, suggesting a strong preference for theortho-OH addition.The computed Arrhenius equation is kOH (T) = (2.82 �

10�13) e958.6/T (250�400 K), which displays a negative tem-perature dependence and agrees with the experimental results.34

According to the M062X performance test,45 the mean unsignederror for Ea calculation was 1.22 kcal 3mol�1. We also comparedthe Ea values from M062X and B2PLYP for 5 species, and themean absolute error is 0.90 kcal 3mol�1 (SI). Thus, the computa-tional error of Ea from M062X is estimated to be 1 kcal 3mol

�1.Considering the errors, the calculated overall rate constant at 298K is 7.02�2.27

þ1.41� 10�12 cm3molecule�1s�1, which is in reasonableagreement with the experimental value34 of 5.14�0.82

þ0.98� 10�12 cm3

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molecule�1s�1 and the predicted value of 3.47�2.18þ5.81 � 10�12 cm3

molecule�1s�1 using the QSAR model of our group.18

Subsequent Reactions for the Primary Intermediates. Theintermediates produced in the initial reactions can react withatmospheric O2 subsequently. As BDE-OH2 is structurallysimilar to BDE-OH6, and BDE-OH3 to BDE-OH5, BDE-OH2and BDE-OH3 were selected in the computations of subsequentreactions, so as to decrease the computational cost.BDE-OH1. The subsequent pathways of BDE-OH1 are de-

picted in Figure 2. The addition of 3OH to the ipso-O positioncan lead to the breaking of the ether bond and formation ofbromophenol. In the corresponding TS (OH1b_TS, SupportingInformation Figure S3), the two benzene rings are nearly parallelto 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, thesingle occupied molecular orbital (SOMO) for the pentadienylmoiety (in BDE-OH1) only has coefficients on the positions 2, 4,and 6. In addition, position 2 is similar to 6, hence only twoperoxy radical isomers (OH1_O2 and OH1_O4, Figure S4)were considered.To compare the formation rates, the bimolecular rate con-

stants for the two peroxy radical isomers (OH1_O2 andFigure 1. Possible pathways for the reactions of BDE-15 with 3OH.

Table 1. Zero-Point Corrected Activation Energies Ea (kcal 3mol�1), Zero-Point Corrected Reaction Enthalpies ΔH(kcal 3mol�1), Forward Reaction Rate Constants k1 (cm

3molecule�1s�1), Reverse Reaction Rate constants k�1(s�1), Branching

Ratios R and Equilibrium Constants K for Initial Reactions

speciesa Eab ΔH b k1

c k�1c R K d

BDE-OH1 1.11 �22.09 2.84� 10�13 2.78� 10�5 0.04 9.20

BDE-OH2 �0.34 �19.91 2.96 � 10�12 4.43� 10�3 0.42 0.60

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

40-OH-BDE-3 5.18 �30.68 1.28� 10�13 0.02

kOH (overall) 7.02� 10�12

kOH (overall, exp) 34 5.14� 10�12

aThe species correspond to the reaction pathways showed in Figure 1. bCalculated at the M062X/6-311þG(3df,2p)//M062X/6-311þG(d,p) level.cThe Bartis�Widom phenomenological rate constants are obtained from eigenvalue-eigenvector analysis at 298 K, 740 Torr. dThe OH concentration34

used for the computation is 9 � 108 molecules cm�3.

Figure 2. Subsequent reaction pathways for BDE-OH1.




OH1_O4, Table 2) were multiplied by the concentration of O2

(4.92 � 1018 molecules cm�3), and converted to unimolecularrate 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 mainproducts and proved that 3OH addition to the ipso-O positionfollowed by the cleavage of the ether bond is the dominantpathway.OH1_O2 can cyclize either with the same ring (bicyclization)

to form OH1_B26, or with the other ring (cross-cyclization) toform OH1_O2C6 and OH1_O2C7 (Figure 2). Ea for theformation of OH1_B26 is lower than for the formation ofOH1_O2C6/OH1_O2C7 by ∼4 kcal 3mol�1 (Supporting In-formation Figure S5). Thus, bicyclization of OH1_O2 is morefavorable than cross-cyclization. Compared with OH1_O2,OH1_O4 is not apt to form bicyclization radicals, as the bicyclicproducts of OH1_O4 cannot form the delocalized ally-π systemthat can lower the energy. The cyclized products may decomposeor lead to closed-shell products subsequently. However, we onlyfocused on the primary reaction channels in this study, because ofthe high work load of computations.BDE-OH2 and BDE-OH3. The subsequent reaction pathways

for BDE-OH2 (Figure 3) and BDE-OH3 (Supporting Informa-tion Figure S6) are similar to those for BDE-OH1. O2 can alsoadd to the aromatic rings, leading to peroxy radical isomers that can

form cyclic radical isomers subsequently. The calculated PESprofiles (Supporting Information Figure S5) show that the cross-cyclization radical pathways are not energetically favorable andbicyclization is the predominant exit channel for the peroxy radicals.Abstraction of the H-atom of 3OH by O2 forming epoxy

radicals and HOO 3 radicals is another pathway (Supporting In-formation Figure S7). However, our calculations show that thebarrier height (Ea) of the channel is too high to overcome(Supporting Information Table S4). Therefore, the epoxy radicalpathways are not likely to occur. It is worthmentioning that somerecent studies also found that atmospheric oxidation of benzeneand toluene can form bicyclic radicals and that activation energiesfor epoxy channels are high.25,49,50

The subsequent reactions of BDE-OH2 and BDE-OH3 canalso form HO-PBDEs through abstraction of H gem to the 3OHby O2 (Figures 3 and S6). The corresponding products are2-OH-BDE-15 and 3-OH-BDE-15. As indicated by the Ea andΔH values (Table 2), these processes are strongly exothermic.The k1 values for 2-OH-BDE-15 andOH2_O5 are of the same

order of magnitude (Table 2), suggesting that O2-addition to thepara-position of the additive 3OH competes with the formation of2-OH-BDE-15 (Figure 3). k1 for 3-OH-BDE-15 is much higherthan 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 reac-

tion channels in the quantum chemical computation, which ledto 24 intermediates and 5 closed-shell products. The relative

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

speciesa Eab ΔHb k1

c K f

BDE-OH1 bromophenol 5.32 �7.85 8.87� 106 d

OH1_O2 10.32 �11.88 2.94� 10�22 e 52.77

OH1_O4 4.45 �8.48 8.06� 10�18 e 0.04

OH1_B26 17.99 �4.72 2.53� 10�2 d 2.06� 102

OH1_O2C7 21.74 10.18 1.02 � 10�5 d 8.08� 10�10

OH1_O2C6 22.96 7.34 8.65� 10�7 d 1.46� 10�7

BDE-OH2 2-OH-BED-15 7.05 �29.96 1.14� 10�15 e

OH2_O1 6.65 �10.01 1.49� 10�19 e 0.29

OH2_O3 9.75 �9.83 7.48� 10�22 e 4.24

OH2_O5 1.19 �11.24 5.11� 10�15 e 32.94

OH2_O1C6 22.85 11.57 7.08� 10�6 d 5.64� 10�10

OH2_O1C5 18.38 6.02 2.16� 10�2 d 1.78� 10�5

OH2_B31_a 9.69 �10.38 1.13� 104 d 9.87� 105

OH2_B31_b 15.18 �10.19 4.79 d 1.42� 107

BDE-OH3 3-OH-BED-15 7.66 �31.68 6.18� 10�15 e

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

OH3_B24_a 11.56 �9.59 8.71� 102 d 1.33� 106

OH3_B24_b 11.78 �9.05 1.31� 103 d 3.24� 105

OH3_O2C7 24.95 12.35 5.73� 10�8 d 4.38� 10�11

OH3_O2C6 20.68 7.54 5.28� 10�5 d 9.37� 10�8

OH3_O6C6 23.45 10.43 6.08 � 10�7 d 9.13� 10�10

OH3_O6C7 22.05 11.09 1.33� 10�5 d 1.05� 10�9

aThe labeled species corresponds to the reaction pathways shown in Figures 2, 3, and S6. bCalculated atM062X/6-311þG(3df,2p)//M062X/6-311þG(d,p) level. cThe Bartis�Widom phenomenological rate constants were obtained from eigenvalue�eigenvector analysis at 298 K, 740 Torr. dThe unimolecular rateconstants are in units of s�1. eThe bimolecular rate constants are in units of cm3molecule�1 s�1. fTheO2 concentration

34 employed for the calculation is 4.92�1018 molecules cm�3.




importance of the main channels together with their intermediates/products is illustrated in Figure 4. The identified closed-shellproducts, HO-PBDEs (40-OH-BDE-3, 2-OH-BDE-15, and 3-OH-BDE-15), bromophenol and Br2, are consistent with the experi-mental observations.34 Two pathways were identified to be respon-sible for the production ofHO-PBDEs: Br substitution by 3OH, andabstraction of H gem to 3OH in BDE-OH adducts by O2. HO-PBDEs and bromophenol are more toxic than the parentcompounds.36�38 Under tropospheric or experimental conditions,the intermediates (open-shell radicals) or closed-shell products willreact subsequently. The formed HO-PBDEs will continually beoxidized by 3OH. According to the QSAR model developed by ourgroup,18 the predicted kOH for monohydroxylated PBDEs aregenerally higher than the parent molecules by a factor ∼1.7(Supporting Information Table S5). Thus, it is not surprising thatonly trace amounts ofHO-PBDEwere 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 combin-ing the statistical mechanics and quantum chemistry to predict kOHand probe the degradation mechanism of SOCs like PBDEs. As thein silico method is cost-effective and independent of the standardchemical samples, it is critical to ecological risk assessment of newlysynthesized chemicals and emerging pollutants.

’ASSOCIATED CONTENT

bS Supporting Information. Details on the computationalmethods, modeling conditions, error analysis, Ea and ΔH forepoxy species, rate constants predicted by QSAR model, sub-sequent reaction pathways of BDE-OH3, formation pathways ofepoxy radicals, optimized geometries and corresponding TS, rateconstants calculated in different temperatures, PES profile, andtime-dependent species profiles. This material is available free ofcharge via the Internet at http://pubs.acs.org.

Figure 3. Subsequent reaction pathways for BDE-OH2.

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




’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 thecalculations, Dr. David R. Glowacki (University of Leeds) for thehelp with MESMER, and Prof. W. Peijnenburg (RIVM) for im-proving the composition. The study was supported by the NationalNatural Science Foundation of China (20890113, 20977014), theHigh-tech Research and Development Program of China (2010-AA065105), the Fundamental Research Funds for the CentralUniversity and the Program for Changjiang Scholars and InnovativeResearch Team in University (IRT0813) of P. R. China.

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quantum chemical investigation on the mechanism and ... · ortho-adducts (bde-oh2 and bde-oh6) are...

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