Mechanism and Thermal Rate Constants for the Complete Series

Apr 30, 2009 - Reactions of chlorophenols with atomic H are important initial steps for the formation of polychlorinated dibenzo-p-dioxins and dibenzo...
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Environ. Sci. Technol. 2009, 43, 4105–4112

Mechanism and Thermal Rate Constants for the Complete Series Reactions of Chlorophenols with H Q I N G Z H U Z H A N G , * ,† X I A O H U I Q U , † HUI WANG,‡ FEI XU,† XIANGYAN SHI,† A N D W E N X I N G W A N G * ,† Environment Research Institute, Shandong University, Jinan 250100, P. R. China, and Department of Environmental Science and Engineering, Tsinghua University, Beijing 100084, P. R. China

Received January 19, 2009. Revised manuscript received March 23, 2009. Accepted April 8, 2009.

Reactions of chlorophenols with atomic H are important initial steps for the formation of polychlorinated dibenzo-pdioxins and dibenzofurans (PCDD/Fs) in incinerators. Detailed insight into the mechanism and kinetic properties of crucial elementary steps is a prerequisite for understanding the formation of PCDD/Fs. In this paper, the complete series reactions of 19 chlorophenol congeners with atomic H have been studied theoretically using the density functional theory (DFT) method and the direct dynamics method. The profiles of the potential energy surface were constructed at the MPWB1K/6311+G(3df,2p)//MPWB1K/6-31+G(d,p) level. Modeling of the PCDD/Fsformationrequireskineticinformationabouttheelemental reactions. The rate constants were deduced over a wide temperature range of 600∼1200 K using canonical variational transition-state theory (CVT) with small curvature tunneling contribution (SCT). The rate-temperature formulas were fitted for the first time. This study shows that the substitution pattern of the phenol has a significant effect on the strength and reactivity of the OsH bonds in chlorophenols. Intramolecular hydrogen bonding plays a decisive role in determining the reactivity of the OsH bonds for ortho-substituted phenols.

1. Introduction Polychlorinated dibenzo-p-dioxins (PCDDs) and dibenzofurans (PCDFs) are notorious for their acute and chronic toxicity and their carcinogenic, teratogenic, and mutagenic effects (1). Major sources of PCDD/Fs in the environment are the combustion of waste materials as well as many other high-temperature processes commonly used in industrial settings (2-4). The most direct route to the formation of PCDD/Fs is the gas-phase reaction of chemical precursors (5-7). Chlorophenols (CPs) are the most direct precursors of dioxin and among the most abundant aromatic compounds found in incinerator gas emissions (8-10). The homogeneous gas-phase formation of PCDD/Fs from chlorophenol precursors was suggested to make a significant contribution to the observed PCDD/F yields in full-scale incinerators (11, 12). Chlorophenols are almost ubiquitous in the environment due to their agricultural and industrial * Address corresponding to either author. E-mail: [email protected] (Q. Z.); [email protected] (W. W.). Fax: 86-531-8836 1990. † Shandong University. ‡ Tsinghua University. 10.1021/es9001778 CCC: $40.75

Published on Web 04/30/2009

 2009 American Chemical Society

uses as pesticides, disinfectants, wood preservatives, personal care formulations, and many other products. Five chlorophenols are listed by the U.S. Environmental Protection Agency as priority pollutants, including PCP and 2,4,6-TCP, which are present in the environment in significant quantities. The gas-phase formation of PCDD/Fs from chlorophenol precursors was proposed that involve chlorophenoxy radical-radical coupling, radical-molecule recombination of chlorophenoxy and chlorophenol. The previous researches (13, 14) have shown that the radical-molecule pathway requires chlorine and hydroxyl displacement as first steps, and these steps are not energetically favored. So, the radicalmolecule pathway is not competitive with the radical-radical pathway. The dimerization of chlorophenoxy radicals (CPRs) is the major pathway for the formation of PCDD/Fs (13, 14). Thus, the formation of chlorophenoxy radicals is a crucial elementary step involved in the formation of PCDD/Fs. In municipal waste incinerators, chlorophenoxy radicals can be formed through loss of the phenoxyl-hydrogen via unimolecular, bimolecular, or possibly other low-energy pathways (including heterogeneous reactions). The unimolecular reaction includes the decomposition of chlorophenols with the cleavage of the OsH bond. The bimolecular reactions include attack by H or Cl under pyrolytic conditions and attack by H, OH, O (3P), or Cl under high-temperature oxidative conditions. However, very little work has been done at the high temperatures relevant to these reactions. The kinetic models that account for the contribution of the gaseous route in the production of PCDD/Fs in combustion processes use the rate constants of the elementary reactions (9, 15). Due to the absence of direct experimental and theoretical values, the rate constants of the reactions of chlorophenols with H, OH, O (3P), or Cl were assigned to be the values reported in the literature for the reactions of phenol with H, OH, O (3P), or Cl (9, 15). However, where there are uncertainties, the numerical values have been adjusted somewhat to bias the mechanism in favor of PCDD/Fs formation, i.e., worst case modeling (9, 15). The reactions with atomic H, the simplest free-radical species, are of particular interest since these reactions are desirable not only to provide an uncomplicated probe of chemical reactivity but also to throw light on the formation mechanism of the chlorophenoxy radicals (CPRs). Here, we present the first systematic study on the complete series reactions of 19 chlorophenol congeners with atomic H. At first, we examined the reaction mechanism at high-accuracy of the density functional theory (DFT). In a second step, the rate constants were calculated using canonical variational transition-state theory (CVT) (16-18) with small curvature tunneling contribution (SCT) (19) over a wide temperature range of 600∼1200 K. The calculated values were compared with the available experimental results. The rate-temperature formulas were fitted. Third, the effect of the substitution pattern of the phenol on the strength and reactivity of the OsH bonds in chlorophenols is discussed.

2. Computational Methods By means of the Gaussian 03 programs (20), high-accuracy quantum chemical calculations were carried out on an SGI Origin 2000 supercomputer. The geometrical parameters of reactants, transition states, and products were optimized at the MPWB1K level with a standard 6-31+G(d,p) basis set. The MPWB1K method (21) is a hybrid DFT model with excellent performance for thermochemistry, thermochemical kinetics, hydrogen bonding, and weak interactions. The vibrational frequencies were also calculated at the same level VOL. 43, NO. 11, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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in order to determine the nature of the stationary points, the zero-point energy (ZPE), and the thermal contributions to the free energy of activation. Each transition state was verified to connect the designated reactants with products by performing an intrinsic reaction coordinate (IRC) analysis (22). The minimum energy paths (MEP) were obtained in mass-weighted Cartesian coordinates. The single-point energy calculations were carried out at the MPWB1K/6311+G(3df,2p) level. The kinetic calculations are most sensitive to the energies. The reliability of the MPWB1K/6311+G(3df,2p) level for the potential barriers was clarified in our recent study (23) on the formation of PCDD/Fs from the 2-CP precursor. The profiles of the potential energy surface were constructed at the MPWB1K/6-311+G(3df,2p)// MPWB1K/6-31+G(d,p) level including ZPE correction. By means of the Polyrate 9.3 program (24), direct dynamics calculations were carried out using the canonical variational transition state theory (CVT) (16-18). The level of tunneling calculation is the small curvature tunneling (SCT) (19) method.

3. Results and Discussion 3.1. Reaction Mechanism. Due to the different substitution pattern of phenol, chlorophenols have 19 congeners. They include three monochlorophenols (2-CP, 3-CP and 4-CP), six dichlorophenols (2,3-DCP, 2,4-DCP, 2,5-DCP, 2,6-DCP, 3,4-DCP, and 3,5-DCP), six trichlorophenols (2,3,4-TCP, 2,3,5TCP, 2,3,6-TCP, 2,4,5-TCP 2,4,6-TCP, and 3,4,5-TCP), three tetrachlorophenols (2,3,4,5-TeCP, 2,3,4,6-TeCP, and 2,3,5,6TeCP), and pentachlorophenol (PCP). There are two geometric conformers resulting from the two main orientations of the hydroxyl-hydrogen due to the asymmetric chlorine substitution for 12 chlorophenol congeners (2-CP, 3-CP 2,3DCP, 2,4-DCP, 2,5-DCP, 3,4-DCP, 2,3,4-TCP, 2,3,5-TCP, 2,3,6TCP, 2,4,5-TCP, 2,3,4,5-TeCP, 2,3,4,6-TeCP). The conformer with the hydroxyl--hydrogen facing the closest neighboring Cl is labeled as the syn conformer and otherwise the anti conformer. There exists weak intramolecular hydrogen bonding in the syn conformers. The lengths of the hydrogen bonds are from 2.342 to 2.408 Å, which are slightly longer than that of a typical hydrogen bond. No such intramolecular hydrogen bonding forms in the anti conformers except those with chlorine substitutions at both ortho-positions. The energies of the syn conformers are about 3 kcal/mol lower than that of the corresponding anti forms, suggesting a stabilization effect because of intramolecular hydrogen bonding. So throughout this paper, chlorophenols denote the syn conformers. The structures of chlorophenols along with the structure of phenol are presented in Figure 1.

Figure 1 shows that all the OsH bonds in ortho-substituted phenols are longer that those without ortho substitution. The OsH bond length in chlorophenols with ortho substitution is 0.959 or 0.960 Å at the MPWB1K/6-31+G(d,p) level, whereas the value is 0.955 Å for chlorophenols without ortho substitution. The OsH bond lengths are significantly correlated with the position of the chlorine substitutions, but not with the number of chlorine substitutions. The structures of 19 chlorophenol congeners and phenol were also studied by Han using density functional theory and ab initio molecular calculations (25). The OsH bond lengths calcu4106

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lated by Han at the B3LYP/6-311++G(d,p) and MP2/6311++G(d,p) levels are longer than the corresponding values in our study obtained at the MPWB1K/6-31+G(d,p) level. The OsH bonds in chlorophenols are 0.963-0.968 Å at the B3LYP/6-311++G(d,p) level and 0.962-0.967 Å at the MP2/ 6-311+G(d,p) level (25), whereas the values are 0.955-0.960 Å at the MPWB1K/6-31+G(d,p) level. For phenol, the OsH bond length of 0.955 Å obtained in our study at the MPWB1K/ 6-31++G(d,p) level is in better agreement with the experimental value of 0.956 Å (26) than the value of 0.963 Å calculated by Han at the B3LYP/6-311++G(d,p) and MP2/ 6-311++G(d,p) levels (25). However, the general conclusions from our study that the OsH bond lengths in chlorophenols are strongly influenced by the intramolecular hydrogen bonding and the OsH bonds in ortho-substituted phenols are longer that those without ortho substitution are in accordance with the conclusions obtained from Han’s study (25). The formation of chlorophenoxy radicals from the bimolecular reaction of chlorophenols with atomic H proceeds via a direct hydrogen abstraction mechanism. In order to compare with the formation of chlorophenoxy radicals, we also studied the formation of phenoxy radicals from the reaction of phenol with H. At the MPWB1K/6-31+G(d,p) level, the transition states were located. The structures of the transition states and chlorophenoxy radicals are shown in Figure 2 and Figure 3, respectively. All of the transition states have one and only one imaginary frequency. The values of the imaginary frequencies are large, which implies that the quantum tunneling contribution may be significant and may play an important role in the calculation of the rate constants. The calculated potential barriers and reaction enthalpies at the MPWB1K/6-311+G(3df,2p)//MPWB1K/6-31+G(d,p) level are listed in Table 1. The formation of chlorophenoxy radicals from the reactions of chlorophenols with H is strongly exothermic. The kinetic calculations are most sensitive to the energies, especially to the potential barriers. In order to justify the performance of the MPWB1K method for the potential barriers, we have also carried out additional potential barrier calculations employing the BB1K and B3LYP methods. The values obtained at the BB1K and B3LYP levels are presented in Table 1. The potential barriers calculated at the B3LYP/6-311+G(3df,2p)//B3LYP/6-31+G(d,p) level are 9-11 kcal/mol lower than the corresponding values obtained at the MPWB1K/6-311+G(3df,2p)//MPWB1K/6-31+G(d,p) and BB1K/6-311+G(3df,2p)//BB1K/6-31+G(d,p) levels. It is well-known that the B3LYP method usually underestimates barrier heights (21). Table 1 shows that the two methods, MPWB1K/6-311+G(3df,2p)//MPWB1K/6-31+G(d,p) and BB1K/6-311+G(3df,2p)//BB1K/6-31+G(d,p), produce consistent potential barriers within 0.6 kcal/mol. This study is a subsequent work from our group who had published the two similar works on the formation of PCDD/Fs from the 2-CP and 2,4,5-TCP precursors at the MPWB1K/6311+G(3df,2p)//MPWB1K/6-31+G(d,p) level (23, 27). So, as part of our ongoing work in the field, we chose the energies calculated at the MPWB1K/6-311+G(3df,2p)//MPWB1K/631+G(d,p) level for the following kinetic calculation. The potential barriers for the phenoxyl-hydrogen abstraction from phenol, 2-CP, 3-CP, 4-CP, 2,5-CP, 2,3,5-TCP and TCP by H were also calculated by Altarawneh at the BB1K/6311+G(3df,2p)//BB1K/6-31G(d) (28). The difference in the potential barriers between the MPWB1K/6-311+G(3df,2p)// MPWB1K/6-31+G(d,p) and BB1K/6-311+G(3df,2p) //BB1K/ 6-31G(d) approaches is no more than 1.61 kcal/mol. Table 1 shows that the potential barriers of the phenoxylhydrogen abstraction from chlorophenols except for 4-CP are higher than that from phenol. This means that the phenoxyl-hydrogen abstraction from chlorophenols except for 4-CP is more difficult than from phenol. Since the

FIGURE 1. MPWB1K/6-31+G(d,p) optimized geometries for chlorophenols and phenol. Distances are in angstroms. Gray sphere, C; White sphere, H; Red sphere, O; Green sphere, Cl. influence of the chlorine substitutions in polychlorinated phenols can roughly be described as additive effects of the three different substitutions (ortho, meta, and para), it is of advantage to look at the influence of the chlorine substitution at these three positions upon the reactivity of the OsH bonds at monochlorophenols and then to look at the polychlorinated phenols. At the MPWB1K/6311+G(3df,2p)//MPWB1K/6-31+G(d,p) level, the potential barrier of the phenoxyl-hydrogen abstraction from 2-CP is 1.29 and 2.39 kcal/mol higher than those from 3-CP and 4-CP, respectively. This means that the OsH bond in 2-CP has the lowest reactivity, and the OsH bond in 4-CP has

the highest reactivity. The reactivity of the OsH bonds in monochlorophenols has the order of ortho < meta < para. Chlorine in an aromatic ring is traditionally recognized as an electron-withdrawing group. The inductive effect of the electron-withdrawing chlorine and the intramolecular hydrogen bonding may ultimately be responsible for the reactivity of the OsH bonds in chlorophenols. For dichlorophenols, the potential barriers of the phenoxylhydrogen abstraction from 2,3-DCP, 2,4-DCP, 2,5-DCP, and 2,6-DCP are higher than those from 3,4-DCP and 3,5DCP. For trichlorophenols, the potential barriers of the phenoxyl-hydrogen abstraction from 2,3,4-TCP, 2,3,5VOL. 43, NO. 11, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 2. MPWB1K/6-31+G(d,p) optimized geometries of the transition states for the phenoxyl-hydrogen abstraction from the reactions of chlorophenols with atomic H. Distances are in angstroms. Gray sphere, C; White sphere, H; Red sphere, O; Green sphere, Cl. TCP, 2,3,6-TCP, 2,4,5-TCP, and 2,4,6-TCP are higher than that from 3,4,5-TCP. Obviously, the potential barriers for the phenoxyl-hydrogen abstraction from chlorophenols with intramolecular hydrogen bonding consistently are higher than from other structural conformers for a given number of chlorine substitutions. The relative reactivity 4108

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of the OsH bonds in chlorophenols appears to be assisted and dominated by the effect of intramolecular hydrogen bonding. It appears to reduce the reactivity of OsH bonds. The relatively strong reactivity of the OsH bond in 4-CP can be explained by the inductive effect of the electronwithdrawing chlorine. The induction of chlorine at the

FIGURE 3. MPWB1K/6-31+G(d,p) optimized geometries for chlorophenoxy radicals and phenoxy radical. Distances are in angstroms. Gray sphere, C; White sphere, H; Red sphere, O; Green sphere, Cl. para-position moves the electron density from the hydroxyl group to the chlorine through the aromatic ring, weakening the OsH bond. In order to further investigate the relative strength of the OsH bonds in chlorophenols, we also calculated the OsH bond dissociation energies, D0(OsH), at the MPWB1K/6311+G(3df,2p)//MPWB1K/6-31+G(d,p) level. The values (19 chlorophenols and phenol at 0 K, six dichlorophenols and phenol at 298.15 K) are presented in Table 1 and compared with the data in the literatures. D0(OsH) of six dichlorophenols at 298.15 K was calculated by Gomes at the UB3LYP/ DZVP, ROB3LYP/DZVP and ROB3LYP/6-311++G(2df,2p)

levels (29). D0(OsH) of 2-CP at 0 K was also calculated by Altarawneh at the G3B3, BB1K/6-311+G(3df,2p)//BB1K/631G(d), B3LYP/6-311+G(3df,2p)//B3LYP/6-31G(d) levels (28). The absolute values of D0(OsH) obtained in our study at the MPWB1K/6-311+G(3df,2p)//MPWB1K/6-31+G(d,p) level are 3-4 kcal/mol higher, 1-2 kcal/mol higher and 3-4 kcal/ mol lower than the corresponding values obtained by Gomes at the UB3LYP/DZVP, ROB3LYP/DZVP, and ROB3LYP/6311++G(2df,2p) levels, respectively (29). But, the relative values of D0(OsH), D0(OsH)(dichlorophenol)-D0(OsH)(phenol), obtained in our study are in good agreement with the corresponding relative values obtained by Gomes. Gomes VOL. 43, NO. 11, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 1. Potential Barriers ∆E (in kcal/mol), Reaction Enthalpies ∆H (in kcal/mol), Imaginary Frequencies ν (in cm-1) of the Transition States, and the OsH bond dissociation energies D0(OsH) (in kcal/mol) for the Formation of Chlorophenoxy Radicals from the Reactions of Chlorophenols with Atomic H

phenol 2-CP 3-CP 4-CP 2,3-DCP 2,4-DCP 2,5-DCP 2,6-DCP 3,4-DCP 3,5-DCP 2,3,4-TCP 2,3,5-TCP 2,3,6-TCP 2,4,5-TCP 2,4,6-TCP 3,4,5-TCP 2,3,4,5-TeCP 2,3,4,6-TeCP 2,3,5,6-TeCP PCP

∆Ea

∆Eb

∆Ec

∆Ha

νd

D0(OsH)e

D0(OsH)e

11.73 13.80 12.51 11.41 14.49 13.58 14.44 13.33 12.01 12.99 14.17 14.92 13.88 13.96 12.81 12.40 14.21 13.60 14.11 13.68

11.24 13.37 11.94 10.92 14.06 13.09 13.89 13.19 11.47 12.46 13.57 14.47 13.65 13.47 12.70 11.96 13.89 13.19 13.96 13.46

2.11 3.90 1.00 1.75 4.37 3.53 4.30 3.71 2.21 3.00 4.02 4.74 4.09 3.81 3.29 2.55 4.21 3.81 4.35 3.93

-13.98 -12.01 -12.94 -15.13 -11.22 -13.35 -11.16 -13.26 -14.04 -11.76 -12.56 -10.43 -12.82 -12.56 -14.64 -13.14 -11.79 -14.00 -12.32 -13.55

2166i 2221i 2192i 2176i 2225i 2227i 2221i 2226i 2198i 2202i 2230i 2213i 2238i 2228i 2243i 2210i 2235i 2238i 2238i 2244i

83.95 85.91 84.99 82.80 86.71 84.58 86.77 84.66 83.89 86.17 85.37 87.50 85.11 85.37 83.29 84.79 86.14 83.93 85.61 84.38

83.79

86.61 84.47 86.68 84.56 83.71 86.02

a MPWB1K/6-311+G(3df,2p)//MPWB1K/6-31+G(d,p). b BB1K/6-311+G(3df,2p)// BB1K/6-31+G(d,p). c B3LYP/6-311+G(3df,2p)// B3LYP/6-31+G(d,p). d MPWB1K/6-31+G(d,p). e MPWB1K/6-311+G(3df,2p)//MPWB1K/6-31+G(d,p), 0 K. f MPWB1K/6-311+G(3df,2p)//MPWB1K/6-31+G(d,p), 298 K.

claimed in his paper that the absolute values of D0(OsH) reported in his study for six dichlorophenols may be considered erroneous, but the relative values of D0(OsH) may be interpreted as reliable data (29). D0(OsH) of 2-CP at 0 K was reported by Altarawnch to be 86.8, 83.5, and 80.9 kcal/mol at the G3B3, BB1K/6-311+G(3df,2p)//BB1K/631G(d), B3LYP/6-311+G(3df,2p)//B3LYP/6-31G(d) levels, respectively (28). Obviously, the value of 85.91 kcal/mol obtained in our study at the MPWB1K/6-311+G(3df,2p)// MPWB1K/6-31+G(d,p) level is in excellent agreement with the value of 86.8 kcal/mol calculated at the expensive G3B3 level. Table 1 shows that the OsH bond dissociation energies in chlorophenols except for 4-CP, 3,4-DCP, 2,4,6-TCP, and 2,3,4,6-TeCP are larger than that in phenol. It means that the strength of the OsH bonds in chlorophenols except for 4-CP, 3,4-DCP, 2,4,6-TCP, and 2,3,4,6-TeCP is stronger than that in phenol. D0(OsH) of 2-CP is higher than those of 3-CP and 4-CP. Similarly, D0(OsH) of 2,3-DCP, 2,4-DCP, 2,5-DCP, and 2,6-DCP are higher than that of 3,4-DCP. D0(OsH) of 2,3,4TCP, 2,3,5-TCP, 2,3,6-TCP, and 2,4,5-TCP are higher than that of 3,4,5-TCP. Intramolecular hydrogen bonding appears to increase the strength of the OsH bonds in orthosubstituted phenols. However, the OsH bond dissociation energies in chlorophenols with ortho substitutions are not consistently higher than those without ortho substitution for a given number of chlorine substitutions. For example, D0(OsH) of 2,3-DCP, 2,4-DCP, 2,5-DCP, and 2,6-DCP are lower than that of 3,5-DCP. D0(OsH) of 2,4,6-TCP is lower than that of 3,4,5-TCP. This may indicate the chlorine substitutions at ortho position would decrease the strength of the OsH bonds, and the resulting steric effect and inductive effect may also strongly affect the relative strength of the OsH bonds in chlorophenols. 3.2. Kinetics Calculations. Canonical variational transition state theory (CVT) (16-18) with small-curvature tunneling (SCT) (19) contribution has been successfully performed for the elementary reactions involved in the formation of PCDD/Fs from the 2-CP and 2,4,5-TCP precursors (23, 27) and is an efficient method to calculate the rate constants. In this study, we used this method to calculate the rate constants for the formation of chlorophenoxy radicals from the 4110

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reactions of chlorophenols with atomic H over a wide temperature range of 600∼1200 K, which covers the possible formation temperature of PCDD/Fs in municipal waste incinerators. For the purpose of comparison, we also calculated the rate constants for 2-CP+Hf2-CPR+H2 using the conventional transition state theory (TST) with tunneling effect based on the Wigner method and Miller’s onedimensional Eckart tunneling model and the variational transition state theory (CVT) with the zero-curvature tunneling correction (ZCT). The calculated TST, TST/Wigner, TST/Eckart, CVT, CVT/ZCT, and CVT/SCT rate constants are presented in the Supporting Information. Comparison of the TST rate constants and the CVT, TST/Wigner, TST/Eckart rate constants shows that both the variational effect and the tunneling effect play important roles for the calculation of the rate constants. Due to the absence of the available experimental rate constants, it is difficult to make a direct comparison of the calculated CVT/SCT rate constants with the experimental values for the reactions of chlorophenols with atomic H. To check the validity of our computational scheme, we calculated the rate constants for the formation of phenoxy radicals from the reaction of phenol with H. The CVT/SCT rate constants of C6H5OH+HfC6H5O+H2 are in good agreement with the available experimental values (30) with the maximum relative deviation less than 3 times. For example, at 1000 K, The calculated CVT/SCT rate constant, 1.68 × 10-13 cm3 molecule-1 s-1, perfectly matches the experimental value of 3.72 × 10-13 cm3 molecule-1 s-1 (30). This good agreement confirms that our CVT/SCT results may provide a good estimate for the formation of chlorophenoxy radicals from the reactions of chlorophenols with atomic H. The kinetic properties of 2-CP+Hf2-CPR+H2 were also studied by Altarawneh (28). The CVT/SCT rate constants calculated by Altarawneh are about 5 times higher than our CVT/SCT ones. For example, at 1000 K, the CVT/SCT rate constant calculated by Altarawneh is 7.66 × 10-14 cm3 molecule-1 s-1, whereas the CVT/SCT value in our study is 1.76 × 10-14 cm3 molecule-1 s-1. This may be due to that the potential barrier calculated by Altarawneh at the BB1K/6-311+G(3df,2p)//BB1K/631+G(d,p) level is 0.7 kcal/mol lower than that obtained in

TABLE 2. Arrhenius Formulas (in cm3 molecule-1 s-1) for the Formation of Chlorophenoxy Radicals (CPRs) from the Reactions of Chlorophenols with Atomic H over the Temperature Range of 600∼1200 K reactions C6H5OH+HfC6H5O+H2 2-CP+Hf2-CPR+H2 3-CP+Hf3-CPR+H2 4-CP+Hf4-CPR+H2 2,3-DCP+Hf2,3-DCPR+H2 2,4-DCP+Hf2,4-DCPR+H2 2,5-DCP+Hf2,5-DCPR+H2 2,6-DCP+Hf2,6-DCPR+H2 3,4-DCP+Hf3,4-DCPR+H2 3,5-DCP+Hf3,5-DCPR+H2 2,3,4-TCP+Hf2,3,4-TCPR+H2 2,3,5-TCP+Hf2,3,5-TCPR+H2 2,3,6-TCP+Hf2,4,6-TCPR+H2 2,4,5-TCP+Hf2,4,5-TCPR+H2 2,4,6-TCP+Hf2,4,6-TCPR+H2 3,4,5-TCP+Hf3,4,5-TCPR+H2 2,3,4,5-TeCP+Hf2,3,4,5-TeCPR+H2 2,3,4,6-TeCP+Hf2,3,4,6-TeCPR+H2 2,3,5,6-TeCP+Hf2,3,5,6-TeCPR+H2 PCP+HfPCPR+H2

Arrhenius formulas k(T) ) (1.98 × 10-11) exp(-4767.69/T) k(T) ) (1.10 × 10-11) exp(-6437.64/T) k(T) ) (2.36 × 10-12) exp(-6579.12/T) k(T) ) (4.61 × 10-12) exp(-5602.30/T) k(T) ) (3.82 × 10-12) exp(-7768.93/T) k(T) ) (5.01 × 10-11) exp(-7238.73/T) k(T) ) (2.14 × 10-12) exp(-7005.45/T) k(T) ) (2.78 × 10-12) exp(-6922.27/T) k(T) ) (2.24 × 10-12) exp(-5933.90/T) k(T) ) (2.61 × 10-12) exp(-6678.71/T) k(T) ) (1.91 × 10-11) exp(-7472.63/T) k(T) ) (4.56 × 10-11) exp(-7893.54/T) k(T) ) (3.56 × 10-12) exp(-6881.01/T) k(T) ) (4.36 × 10-12) exp(-6270.24/T) k(T) ) (2.61 × 10-12) exp(-6393.41/T) k(T) ) (1.24 × 10-12) exp(-4592.88/T) k(T) ) (4.99 × 10-12) exp(-7457.09/T) k(T) ) (1.96 × 10-12) exp(-7039.66/T) k(T) ) (1.28 × 10-12) exp(-6776.23/T) k(T) ) (3.18 × 10-12) exp(-7008.45/T)

our study at the MPWB1K/6-311+G(3df,2p)//MPWB1K/631+G(d,p) level. For 19 chlorophenol congeners, the CVT/SCT rate constants vary systematically depending on the substitution pattern of phenol and temperature. Generally, the CVT/SCT rate constants for the phenoxyl-hydrogen abstraction from chlorophenols are lower than that from phenol at a given temperature. For example, at 1000 K, the calculated CVT/ SCT rate constant for 2-CP+Hf2-CPR+H2 is 1.76 × 10-14 cm3 molecule-1 s-1, whereas the value is 1.68 × 10-13 cm3 molecule-1 s-1 for C6H5OH+HfC6H5O+H2. The substitution pattern of phenol strongly affects the rate constants at a given temperature. For example, at 600 K, the CVT/SCT rate constants are 7.45 × 10-17, 8.82 × 10-17, 3.72 × 10-17, 1.26 × 10-16, 6.15 × 10-17 cm3 molecule-1 s-1 for the phenoxylhydrogen abstraction from 2,3,4-TCP, 2,3,5-TCP, 2,3,6-TCP, 2,4,5-TCP, and 2,4,6-TCP, whereas the value is 5.87 × 10-16 cm3 molecule-1 s-1 for the phenoxyl-hydrogen abstraction from 3,4,5-TCP. However, the rate constants for the phenoxyl-hydrogen abstraction from chlorophenols with intramolecular hydrogen bonding are not consistently larger than those from other structural conformers for a given number of chlorine substitutions at a given temperature. For example, at 700 K, the rate constant for the phenoxylhydrogen abstraction from 2,4-DCP is about 3 times larger than that from 3,4-DCP. Regulatory decisions and risk analyses often rely on the use of mathematical models to predict the potential outcomes of contaminant releases to the environment. A better knowledge of the temperature dependence would be useful

for the kinetic models that account for the contribution of the gaseous route in the production of PCDD/Fs in combustion processes (9, 15). So, the calculated CVT/SCT rate constants are fitted over the temperature range of 600∼1200 K and Arrhenius formulas are given in Table 2. The preexponential factor, the activation energy, and the rate constants can be obtained from these Arrhenius formulas.

Acknowledgments This work was supported by NSFC (National Natural Science Foundation of China, project No. 20737001, 20777047), Shandong Province Outstanding Youth Natural Science Foundation (project No. JQ200804) and the Research Fund for the Doctoral Program of Higher Education of China (project No. 200804220046). We thank Professor Donald G. Truhlar for providing the POLYRATE 9.3 program.

Supporting Information Available TST, TST/Wigner, TST/Eckart, CVT, CVT/ZCT, and CVT/SCT rate constants as function of the reciprocal of the temperature (T) over the temperature range of 600∼1200 K for 2-CP+Hf2CPR+H2. This material is available free of charge via the Internet at http://pubs.acs.org.

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