Theoretical Study of the Reactions of 2-Chlorophenol over the

ARTICLE pubs.acs.org/JPCA

Theoretical Study of the Reactions of 2-Chlorophenol over the Dehydrated and Hydroxylated Silica Clusters Wenxiao Pan, Wenhui Zhong, Dongju Zhang,* and Chengbu Liu Key Lab of Colloid and Interface Chemistry, Ministry of Education, Institute of Theoretical Chemistry, Shandong University, Jinan, 250100, People's Republic of China

bS Supporting Information ABSTRACT: Silica is the main component of combustion-generated fly ash and is expected to have an important impact on the formation of polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) in municipal waste incinerators. In this work, we theoretically studied the reactions of 2-chlorinated phenol (2-CP) over the clusters (SiO2)3 and (SiO2)3O2H4, which mimic the dehydrated and hydroxylated silica structures, respectively. The dehydrated cluster is much more active toward the attack of 2-CP to form highly stable 2-chlorophenolate than the hydroxylated silica cluster. The further dissociation of chlorophenolates to form CP radicals (CPRs) is calculated to be very difficult. The calculated energy barrier of the reaction of 2-CP over the dehydrated (SiO2)3 cluster and IR data are in good agreement with early experimental observations. On the basis of the calculated results, we propose that the formation of PCDD/Fs from CPs over silica surfaces may not involve CPRs, but be relevant to the further conversion of chlorophenolates over silica surfaces. This mechanism is very different from the corresponding reactions mediated by transition metal oxides. The results presented here may be helpful to understand the chemisorption mechanism of CPs on silica surfaces in real waste combustion.

of PCDD/Fs.22,23 However, it is also widely accepted that silica, as the main component of combustion-generated fly ash, must play a significant role in the formation of PCDD/Fs.20 Because the concentration of silica in combustion-generated fly ash is much higher than that of transition-metal species, it is reasonable to imagine that CP precursors of PCDD/Fs would preferentially bind or adsorb at silica surfaces and then are converted to PCDD/Fs through redox reactions catalyzed by transition-metal oxides. Alderman and Dellinger20 have studied the chemisorption of 2-chlorophenol (2-CP), a well-known precursor of PCDD/Fs, on silica surfaces using Fourier transform infrared (FTIR) spectroscopy. They observed the formation of chemisorbed 2-chlorophenolate at silica sites and proposed the relevant formation mechanism. In this work, we present the results of the density functional theory (DFT) calculations in which we studied the nature of adsorption of 2-CP and the formation mechanism of 2chlorophenolate on the cluster models of silica. It should be emphasized that the silica models utilized in this work are only approximations of the real surface. However, the cluster approach for modeling the surface reaction is expected to be a reasonable starting point to understand the complex chemical

1. INTRODUCTION Polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) are highly toxic, and tend to be bioaccumulated, which greatly threaten the natural environment.1
4 Extensive studies5
13 have been carried out on the formation mechanism of PCDD/Fs in the combustion system since they were first discovered in the effluent from a garbage furnace in 1977.14 Among various routes, the surface-mediated formation is believed to account for 70% of all PCDD/Fs.15,16 Two surface-mediated PCDD/F formation mechanisms have been established: one is the de novo pathway in which the macromolecular residual carbon, oxygen, hydrogen, and chlorine combine and react to form PCDD/Fs, and the other refers to the precursor pathway which involves catalytic reactions of various precursors, such as chlorinated phenols (CPs), with other aromatic molecules or radicals in the ash. These two pathways are generally called as CP pathways because CPs have been demonstrated to be key intermediates17 in the former and predominant precursors in the latter.18,19 It has been documented that the initial and key step in the formation of PCDD/Fs from surface-mediated reactions through CP pathways involves the formations of chlorophenolates via the chemisorption of CPs20 and chlorinated phenoxy radicals (CPRs)18,21 on the catalytic surface sites. Generally, transitional metal oxides, especially copper and iron oxides, are considered to present a strong catalytic effect on the surface-mediated formation r 2011 American Chemical Society

Received: September 6, 2011 Revised: December 1, 2011 Published: December 06, 2011 430

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Figure 1. Optimized geometries for (SiO2)3, (SiO2)3O2H4, 2-CP, and 2-CPR with main geometrical parameters (in Å). The values in parentheses are experimental or theoretical results in the literature.29
31

process in the real world. On the basis of the calculated results, we have drawn out the potential energy surface (PES) details of the reaction forming 2-chlorophenolate from 2-CP mediated by silica clusters. The theoretical results, including the activation energy demanded to form chlorophenolate and the IR spectra, are found to be in good agreement with available experimental data.

enough accuracy of the theory level for describing the present CP-silica systems. Furthermore, as will be seen in the following section, the reliability and accuracy of the B3LYP functional for describing the present system is also confirmed by performing benchmark calculations for some structures involved in the present work at the MP2/6-311G(d,p) level.

3. RESULTS AND DISCUSSION 2. MODELS AND COMPUTATIONAL DETAILS It is well-known that the hydroxyl-terminated silica surfaces account for a large proportion of silica interfaces, and that they easily dehydroxylate at moderate temperatures (e450 °C), giving rise to siloxanes. In this work, we used clusters (SiO2)3 and (SiO2)3O2H4 to simulate the adsorption of 2-CP on the dehydrated and hydroxylated silica structures, respectively, as shown in Figure 1. These two silica clusters contain the rhombic Si
O rings, which were confirmed to exist in the surface or the interior of amorphous and crystalline silica.24
26 All calculations were conducted by using Gaussian 03 package.27 The geometries of reactants, intermediates, and products involved in the reactions of 2-CP with silica clusters were optimized using the popular B3LYP functional28,29 with the standard 6-311G(d,p) basis set. The vibrational frequencies were calculated at the same level of theory to determine the nature minima or first-order saddle points of optimized structures and to provide the zero-point vibrational energy (ZPE). Intrinsic reaction coordinate (IRC)30 calculations were performed to verify that each transition state (TS) is connected to two desired minima. No symmetry constrains were used for the optimization calculations. To validate the reliability of the functional and the basis set used, we compared the calculated geometry parameters of (SiO2)3, (SiO2)3O2H4, 2-CP, and 2-chlorophenoxy radical (CPR) with the available experimental data or the theoretical values in literatures, as shown in Figure 1. It is found that the calculated results at the B3LYP/6-311G(d,p) level are in good agreement with the reference values,31
33 implying that there is

3.1. Direct Dissociation of 2-CP into 2-CPR. As mentioned, the formation of CPRs is a key step for the formation of PCDD/ Fs from CP precursors. In order to make a quantitative comparison with the formation of CPRs mediated by silica clusters, we studied the direct dissociation of 2-CP into 2-CPR without the presence of a catalyst, as shown by eq 1.

C6 H4 ClOH f C6 H4 ClO 3 þ H 3

ð1Þ

This process is, in essence, the scission of the O
H bond of the hydroxyl group. The calculated reaction enthalpy at 298 k is 83.1 kcal/mol, indicating a highly endothermic process. In other words, without the assistance of a catalyst, the dissociation of 2-CP into 2-CPR is very difficult. This result is in good agreement with the theoretical value (85.91 kcal/mol) reported by Zhang et al.34 at the MPWB1K/6-311+G(3f,2p) level. 3.2. Reaction of 2-CP with (SiO2)3. The most stable geometry of (SiO2)3 cluster is a silica chain based on two-membered ring (2MR) units,35
37 which have been experimentally confirmed to be more frequently present in nanometer-sized silica particles than in the bulk materials. As shown in panel (a) in Figure 1, the clean (dehydrated) silica cluster contains two three-coordinated and one four-coordinated silicon atoms, and four bridging and two nonbridging oxygen atoms. The reaction of 2-CP with (SiO2)3 cluster is initiated by the adsorption of 2-CP onto active sites of the cluster to form the stable adsorption complex. As seen in Scheme 1, the reaction can proceed through three branches, denoted as paths I, II, and III, which involve the cleavages of 431

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Scheme 1. Three Possible Branches for the Reaction of 2-CP with (SiO2)3 Cluster to Form the 2-Chlorophenolate

Figure 2. Optimized geometries involved in the reaction of 2-CP mediated by (SiO2)3 cluster. The values in parentheses at the bottom of the geometries denote the relative energies of complexes with regard to the isolated reactants, and the values in square brackets are bond lengths calculated at the MP2/6-311G(d,p) level. The bond distances are in angstroms.

Si1
O1, Si2
O1, and Si1
O2 bonds, leading to three isomers of 2-chlorophenolate. The optimized geometries with selected structural parameters for the intermediates and transition states involved in these three branches are given in Figure 2, and the PES profiles are depicted in Figure 3, where the minima connected by TSs were obtained by optimizing the structures from IRC calculations. We first consider the reaction along path I. As shown in Figure 2, initially, 2-CP molecule physically adsorbs on the

Si1
O1 bridge through a van der Waals interaction between the hydroxyl O atom in 2-CP and Si1 atom in (SiO2)3 to form a reactant complex 1 with a Si
O bond distance of 1.916 Å. This step is calculated to be exothermic by 20.4 kcal/mol. The adsorbed 2-CP is then capable of attacking Si1
O1 bond via a fourmembered cyclic transition state, TS1
4, which is 4.8 kcal/mol more stable in energy than the isolated reactants (2-CP + (SiO2)3). In TS1
4, the nucleophilic hydroxyl O atom is attacking the electrophilic Si1 atom and the hydroxyl H atom of 2-CP is 432

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indicates that the defective sites on silica structures are the active centers that convert CPs into chlorophenolates. Our calculated barrier (5.0 kcal/mol) is in the range of experimental values (15 ( 4
22 ( 2 kJ/mol) observed by Alderman et al.20 Furthermore, we calculated the demand for energy for forming 2-CPR from the most stable chlorophenolate 6 by breaking the newly formed Si
O bond. As expected from the well-known fact that the Si
O bond within a tetrahedral structure is very strong, a value as high as 90.0 kcal/mol is obtained. Thus, the overall reaction for forming 2-CPR is still endothermic by 22.7 kcal/mol, as shown in Figure 3. Although the energy demand is much less than that (83.1 kcal/mol) involved in the direct dissociation process of 2-CP into 2-CPR, we do not consider the surfacemediated formation of PCDD/Fs from CPs on silica involve CPR intermediates because the formation of 2-CPR from the chlorophenolate is very difficult. Alternatively, we conjecture that the formation of PCDD/Fs from CPs on silica surfaces may be relevant to the further complex conversion of chlorophenolates. This case is very different from the corresponding reactions mediated by transition metal oxides, where the formation of PCDD/Fs from CPs is believed to involve CPRs.38 To see the reliability and accuracy of the B3LYP functional for describing the present system, taking path II in Figure 3 as an example, we reoptimized the geometries of structures 2, TS2
5, and 5, and calculated their relative energies at the MP2/6-311 (d,p) level. As shown in Figure 3, the differences of the crucial geometrical parameters by these two methods are in the range of 0.001
0.034 Å, indicating the MP2 geometries agree well the B3LYP ones. Furthermore, the calculated barriers using the two methods are also in good agreement (5.6 kcal/mol of MP2 method vs 6.8 kcal/mol of the B3LYP method). These results make us confident for the accuracy of the B3LYP functional in describing the reactions studied in the resent work. Furthermore, we also studied the dependence of the reactivity of 2-CP with silica clusters on the cluster size. Using a bigger cluster (SiO2)8, which was found in previous literatures39,40 has D2d symmetry with four defective oxygen atoms, we recalculated path III shown in Figure 3. The optimized geometries are shown in Figure 4, where structures 3*, TS*3
6, and 6* correspond to 3, TS3
6, and 6 in Figure 3, respectively. The relative energies of 3*, TS*3
6, and 6* with regard to the separated reactants (SiO2)8 and 2-CP are
19.9,
15.5, and
69.3 kcal/mol, which are in good agreement with the corresponding those of 3, TS3
6, and 6,
19.8,
67.3, and
14.8 kcal/mol. This fact indicates that the reactivity of 2-CP with silica clusters does not depend sensitively on the cluster size. 3.3. Reaction of 2-CP with (SiO2)3O2H4. Being different from the dehydrated (SiO2)3 cluster, the hydroxylated silica cluster (SiO2)3O2H4 contains two silanol groups instead. Similar to the reaction of 2-CP with (SiO2)3, 2-CP molecule can form 2chlorophenolate on the hydroxylated cluster via three pathways, which are denoted as paths I0 , II0 and III0 in Scheme 2. These three paths involve the breakages of Si1
O1, Si2
O1, and Si1
O2 bonds, respectively. The optimized geometries of intermediates and transition states are shown in Figure 5, and the PES profiles are given in Figure 6, where connections between the transition states and local minima have been confirmed by performing IRC calculations. Comparing the geometries shown in Figure 5 with those in Figure 2, we find that the reaction mechanism over the hydroxylated silica cluster is similar to that over the dehydrated cluster. Essentially, the reaction along three paths involves also the attack

Figure 3. The potential energy surface for the reaction of 2-chlorophenol with (SiO2)3 along path I (red line), path II (blue line), and path III (black line). The values in quare brackets are relative energies calculated at the MP2/6-311G(d,p) level.

approaching O1 atom, leading to the Si1
O1 bond is elongated to 1.904 Å from 1.679 Å. The IRC calculations indicate that the forward product from TS1
4 is the 2-chlorophenolate, denoted as 4, in which the Si1
O1 bond has cleaved, while the O and H atoms of the hydroxyl group bind to Si1 and O1 atoms, respectively. As is seen in Figure 1, the lengths of the newlyformed Si
O and O
H bonds are 1.625 and 0.960 Å, respectively. The energy barrier for the conversion from 1 to 4 is calculated to be 15.6 kcal/mol, and the released energy by forming complex 1 is enough to overcome the barrier. As seen in Figure 3, the reaction along path I is calculated to be exergonic by 34.8 kcal/mol. The mechanism details of reactions along paths II and III are very similar to that discussed above, and the only difference along three paths is the hydroxyl group attacks different Si
O bonds in the cluster. Along path II, an initial weakly bound complex between 2-CP and (SiO2)3 is formed and denoted as structure 2, in which the hydroxyl group is bound to the Si2
O1 bridge of the cluster. Our calculations show that this complex lies only 2.6 kcal/mol below the separated reactants. Obviously, structure 2 is much less favorable in energy than structure 1. A hydrogen bond between the H atom of the hydroxyl group and the O2 atom of (SiO2)3 is formed in 2 with a distance of 2.081 Å. From 2, the reaction proceeds via TS2
5 with a barrier of 6.8 kcal/mol to form structure 5, a slightly more stable 2-chlorophenolate than structure 4 by 0.6 kcal/mol. In the case of path III, the 2-CP molecule approaches the defective oxygen site in the cluster, i.e., Si1
O2 site to form complex 3 through a van der Waals interaction between the Si1 atom of the cluster and the O atom of the hydroxyl group. The binding energy for complex 3 is depicted to be 19.8 kcal/mol, indicating that structure 3 is almost same stable with structure 1. Structure 3 can be converted into structure 6 via TS3
6 with a barrier of only 5.0 kcal/mol. The energy demand is smaller by 10.6 kcal/mol than that along path I and by 1.8 kcal/mol than that along path II. Moreover, as indicated in Figure 3, the reaction along path III is highly exothermic by 67.3 kcal/mol which is almost twice the value along paths I and II. From the results above, it is clear that path III, where the reaction occurs on the end of cluster, i.e., the defective site of the silica cluster, is the most dynamically favorable and it results in the most thermodynamically stable 2-chlorophenolate. This fact 433

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to path I0 . We only pay attention to the calculated barriers along three paths to find out the most energetically favorable reaction pathway. As shown in Figure 6, the barriers are 22.2, 17.2, and 16.8 kcal/mol along paths I0 , II0 and III0 . Clearly, the reaction preferentially proceeds according to paths III0 , where the 2-CP attacks the Si
O bond at the end of cluster. This case is consistent with the reaction over the dehydrated cluster discussed above. In other words, for both the dehydrated and hydroxylated clusters, the Si
O bonds at the ends are most active toward the attack of 2-CP molecule. Comparing path III0 with path III, it is found that the barrier involved in the former (16.8 kcal/mol) is much higher than that involved in the latter (5.0 kcal/mol), suggesting that hydroxylated silica clusters are less active to the attack of CP molecules than the dehydrated clusters. This fact is because dehydrated clusters contain defective sites at the ends, where the silicon atoms are three-coordinated/unsaturated and so are more active. In contrast, in hydroxylated silica clusters, the dangling bonds on the defective sites have been saturated with hydroxyl groups. So hydroxylated silica clusters are expected to be relative inert toward the reaction with CPs. At room temperature, silica clusters generally exist in their hydroxylated form due to the presence of water in the environment. However, at elevated temperature, hydroxylated clusters are expected to easily dehydrate to form the dehydrated structures. So in a garbage furnace, the combustion-generated fly ash mainly contains dehydrated silica clusters rather than hydroxylated clusters. The dehydrated clusters as catalysts convert CPs into chlorophenolates and finally into PCDD/Fs. It should be stressed that phenoxy radicals are formed readily in the gas phase via the hydrogen abstraction reaction of CPs by radicals in the combustion system such as OH radical, as shown in the previous studies.41,42 In this sense, another important rule of silica in the combustion-generated fly may be serving as a suitable mediate deriving the whole catalytic cycle besides acting a catalyst in the PCDD/F formation from CPs. 3.4. Infrared (IR) Spectra of the Complexes of 2-CP with the Silica Cluster. As mentioned in the Introduction, Alderman and Dellinger20 have monitored the IR spectra of the complexes of 2-chlorophenolates formed on silica surfaces with FTIR. To make a comparison with the experimental results, we here calculated the IR spectra of the complexes of 2-CP with the silica cluster. Complexes 1 and 4 are considered as representatives of physisorbed and chemisorbed complexes of 2-CP on the dehydrated silica clusters, respectively. Figure 7 shows the theoretical results ranging from 1250 to 1700 cm
1. For the physisorbed

of the hydroxyl group in 2-CP to the Si
O bond in the cluster to form 2-chlorophenolate. Taking path I0 as an example, we describe the reaction process. Initially, complex 10 is formed via the hydrogen bonding interaction between the cluster and 2-CP. Two hydrogen bonds in 10 have been located: one is between the hydroxyl O atom of 2-CP and the silanol hydrogen atom of (SiO2)3O2H4, and the other is between the hydroxyl H atom and the silanol O1 atom of the cluster. This complex is more stable by 8.3 kcal/mol than the isolated reactants. 10 is then converted into the ring-opening intermediate 40 via transition state TS1
40 with an energy barrier of 22.2 kcal/mol. TS1
40 is a four-membered transition state with an imaginary frequency of 1162 cm
1, and the corresponding transition vector indicated by vibration analysis as well as the subsequent IRC calculations manifest that the hydroxyl H atom of 2-CP is moving from the hydroxyl group to the O1 atom of (SiO2)3O2H4. In 40 , the distance between Si1 atom and the hydroxyl oxygen atom in 2-CP is shorten to 1.680 Å, clearly indicating the chemisorption of the 2-CP on hydroxylated silica cluster. Paths II0 and III0 respond to paths II and III in the reaction mediated by the dehydrated silica cluster, respectively. The mechanism details are not discussed again due to their similarity

Figure 4. Optimized geometries involved in the reaction of 2-CP mediated by (SiO2)8 cluster along path III. The bond distances are in angstroms.

Scheme 2. Possible Pathways for the Reaction of 2-CP with (SiO2)3O2H4

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Figure 5. Optimized geometries involved in the reaction of 2-CP mediated by (SiO2)3O2H4. The values in parentheses denote the binding energies of these complexes. The bond distances are in angstroms.

Figure 6. The potential energy surface for the reaction of 2-chlorophenol mediated by (SiO2)3O4H8 cluster along path I0 (blue line), path II0 (red line), and path III0 (black line).

Figure 7. Calculated IR spectra of physisorbed complex 1 and chemisorbed intermediate 4.

complex 1, we find that the bending vibration of the phenolic O
H bond occurs at 1324 cm
1, which coincides with the observed value of 1338 cm
1.20 Such a peak is absence in the chemisorbed species 4 due to the dissociation of the phenolic O
H bond. The peak at 1250 cm
1 in complex 1 is attributed to the C
O stretch vibration, which is blue-shifted 10 cm
1 and occurs at 1260 cm
1 in complex 4. The result is also in good agreement with the experimental result with a blue shift value of 8 cm
1. Furthermore, peaks at 1430 and 1581 cm
1 in complex 1 are assigned to the aromatic ring-breathing modes of physisorbed 2-CP, which are red-shifted by 15 and 13 cm
1 for the

chemisorbed species, respectively. However, these red shifts were not reported in the experimental work.20

4. CONCLUSIONS In summary, we have performed a detailed theoretical study for the reactions of 2-CP over the dehydrated and hydroxylated silica clusters. It is found that the dehydrated cluster (SiO2)3 is more active to the attack of 2-CP than the hydroxylated cluster (SiO2)3O2H4. The reaction preferentially occurs at the defective 435

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sites (ends) of the cluster with a barrier of only 5.0 kcal/mol to form the highly stable chlorophenolate, whose further dissociation to form CPR by breaking the strong Si
O bond is expected to be a very difficult process. The calculated barrier and IR data are in good agreement with early experimental observations. On the basis of the present calculated results, we conjecture that the formation of PCDD/Fs from CPs on silica surfaces may be relevant to the further complex conversion of newly formed chlorophenolates, which is different with the corresponding reaction mediated by transition metal oxides, where the formation of PCDD/Fs from CPs are believed to involve CPRs.

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’ ASSOCIATED CONTENT

bS

Supporting Information. Cartesian coordinates and vibrational frequencies for all optimized structures. This information is available free of charge via the Internet at http://pubs.acs. org/

’ AUTHOR INFORMATION Corresponding Author

*Phone: +86-531-88365833; fax: +86-531-88564464; e-mail: [email protected] .

’ ACKNOWLEDGMENT This work is jointly supported by the National Natural Science Foundation of China (No. 20873076) and the Specialized Research Fund for the Doctoral Program of Higher Education (No. 200804220009). We are greatly thankful to the reviewers of this manuscript for their constructive comments which have been used to improve the quality and significance of our manuscript. ’ REFERENCES (1) Choudhary, G.; Keith, L. H.; Rappe, C. Chlorinated Dioxins and Dibenzofurans in the Total Environment; Butterworth Publishers: Boston, 1983. (2) Schecter, A. Dioxin and Health, 2nd ed.; Plenum Press: New York, 1994. (3) Alcock, R. E.; Jones, K. C. Environ. Sci. Technol. 1996, 30, 3133–3143. (4) Fiedler, H. Environ. Eng. Sci. 1998, 15, 49–58. (5) Altarawneh, M; Dlugogorski, B. Z.; Kennedy, E. M.; Mackie, J. C. Prog. Energy Combust. Sci. 2009, 35, 245–274. (6) Tuppurainen, K.; Asikainen, A.; Ruokojarvi, P.; Ruuskane, J. Acc. Chem. Res. 2003, 36, 652–658. (7) Evans, C. S.; Dellinger, B. Environ. Sci. Technol. 2003, 37, 1325–1330. (8) Stanmore, B. R. Combust. Flame 2004, 136, 398–427. (9) Weber, R.; Sakurai, T.; Hagenmaier, H. Chemosphere 1999, 38, 2633–2642. (10) McKay, G. Chem. Eng. J. 2002, 86, 343–368. (11) Huang, H; Buekens, A. Chemosphere 1995, 31, 4099–4117. (12) Tuppurainen, K.; Halonen, I.; Ruokoj€arvi, P.; Tarbanen, J.; Ruuskanen, J. Chemosphere 1998, 36, 1493–1511. (13) Addink, R.; Olie, K. Environ. Sci. Technol. 1995, 29, 1425–1435. (14) Olie, K.; Vermeulen, P. L.; Hutzinger, O. Chemosphere 1977, 6, 455–459. (15) Lanier, W. S.; Von Alten, T. R.; Kilgroe, J. D. Assessment of Trace Organic Emissions Test Results from the Montgomery County South MWC in Dayton, Ohio; Energy Environ. Anal. Inc.: Durham, NC, 1990. (16) Altwicker, E. R. Sci. Total Environ. 1991, 104, 47–72. 436

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