Kinetic Properties for the Complete Series Reactions of Chlorophenols

Environ. Sci. Technol. 2010, 44, 1399–1404

Kinetic Properties for the Complete Series Reactions of Chlorophenols with OH RadicalssRelevance for Dioxin Formation FEI XU,† HUI WANG,‡ Q I N G Z H U Z H A N G , * ,† R U I X U E Z H A N G , † XIAOHUI QU,† AND WENXING WANG† 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 October 20, 2009. Revised manuscript received December 5, 2009. Accepted January 5, 2010.

The chlorophenoxy radical (CPR) is a key intermediate species in the formation of polychlorinated dibenzo-p-dioxins (PCDDs) and dibenzofurans (PCDFs). In municipal waste incinerators, the reactions of chlorophenols with OH radicals play the most central role in the formation of chlorophenoxy radicals. In this paper, molecular orbital theory calculations have been performed to investigate the formation of chlorophenoxy radicals from the complete series reactions of 19 chlorophenol congeners with OH radicals. The single-point energy calculation was carried out at the MPWB1K/6-311+G(3df,2p) level on the basis of the MPWB1K/6-31+G(d,p) optimized geometries. The kinetic modeling of the PCDD/PCDF (PCDD/F for short) formation demands the knowledge of the rate parameters for the formation of chlorophenoxy radicals from chlorophenols. So, the kinetic properties of the reactions of chlorophenols with OH radicals were deduced over a wide temperature range of 600-1200 K using canonical variational transition-state theory (CVT) with small curvature tunneling contribution (SCT). This study shows that the chlorine substitution at the ortho position in chlorophenol not only has a significant effect on the structures of chlorophenols, prereactive intermediates, the transition states, and chlorophenoxy radicals, but also plays a decisive role in determining the rate parameters.

1. Introduction Polychlorinated dibenzo-p-dioxins (PCDDs) and dibenzofurans (PCDFs) are among the most toxic known environmental pollutants (1). Despite international efforts to control and regulate persistent halogenated organic pollutants, total annual global deposition of PCDD/PCDFs (PCDD/Fs for short) from the atmosphere is 13 000 kg/yr (2). Among various PCDD/F emission sources, municipal waste incinerations (MWIs) have been recognized as the most significant sources of dioxins release to the environment. In UK, U.S., and Japan, municipal waste incinerations are responsible for 30-56, 38, and 87%, respectively, of the total PCDD/F emissions (3-5). The total amount of PCDD/Fs emitted from municipal * Corresponding author e-mail: [email protected]; fax: 86-531-8836 1990. † Shandong University. ‡ Tsinghua University. 10.1021/es9031776

 2010 American Chemical Society

Published on Web 01/21/2010

waste incinerations to the atmosphere in China was estimated to be 19.64 g TEQ year-1 in 2006 (6). Chlorophenols (CPs) are structurally similar to PCDD/Fs and among the most abundant aromatic compounds found in MWI exhaust gases (7, 8). CPs have been demonstrated to be the predominant precursors of PCDD/Fs in municipal waste incinerations. It is now known that the potential contributions of the gasphase pathways to the PCDD/F formation were underestimated by the reaction kinetic model proposed by Shaub and Tsang (9-11). 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 (10-13). Recent works have shown that the dimerization of chlorophenoxy radicals (CPRs) is the major pathway in the gas-phase formation of PCDD/Fs from chlorophenol precursors (10, 11, 14-16). The formation of chlorophenoxy radicals is the initial and key step 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 O-H bond. The bimolecular reactions include attack by H, OH, O (3P), Cl, and O2. The reaction kinetic model of the PCDD/F formation indicated that among various formation pathways of chlorophenoxy radicals from chlorophenols, PCDD/F yields are most sensitive to the reaction of phenoxyl-hydrogen abstraction from chlorophenol by OH radicals (11). The reactions of chlorophenols with OH radicals are the dominant propagation pathways for the formation of chlorophenoxy radicals (11). The reaction 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. However, only the theoretical rate constants for the reactions of 2-CP, 2,4,5TCP with OH radicals are on record (17-19). There are no existing experimental or theoretical data on the rate constants in the literature for the reactions of other 17 chlorophenol congeners with OH radicals. This is in spite of the fact that their role in the formation of PCDD/Fs is widely discussed (11). Thus, the rate constants for the formation of chlorophenoxy radicals from the reactions of chlorophenols with OH radicals were assigned to be the values reported in the literature for the formation of phenoxy radicals from the reaction of phenol with OH radicals in the reaction kinetic model of the PCDD/F formation (9, 11, 20, 21). However, where there are uncertainties, the numerical values have been adjusted somewhat to bias the mechanism in favor of the PCDD/F formation, i.e., worst case modeling (9, 20). In a recent contribution from this laboratory, we investigated the formation of chlorophenoxy radicals from the reactions of chlorophenols with atomic H (22). As part of our ongoing work in the field, this paper presents mechanistic and kinetic studies on the formations of chlorophenoxy radicals from the reactions of chlorophenols with OH radicals. To compare with the formation of chlorophenoxy radicals, we also studied the formation of phenoxy radicals from the reaction of phenol with OH. The rate constants were calculated using canonical variational transition-state theory (CVT) (23-25) with small curvature tunneling contribution (SCT) (26) over the temperature range of 600-1200 K, which covers the possible formation temperature of PCDD/Fs in municipal waste incinerators. The effect of the chlorine substitution pattern on the structures of the prereactive VOL. 44, NO. 4, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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intermediates, transition states, and chlorophenoxy radicals is discussed.

2. Computational Methods The quantum chemical computations were performed on an SGI 2000 supercomputer using the Gaussian 03 package (27) for the complete series reactions of 19 chlorophenol congeners with OH radicals. As a reasonable compromise between accuracy and computational time, the geometries of chlorophenols, intermediates, transition states, and chlorophenoxy radicals were fully optimized by employing the MPWB1K method (28) with a standard 6-31+G(d,p) basis set. Harmonic vibrational frequency calculations were made at the same level to determine the nature of the stationary points, the zero-point energy (ZPE), and the thermal contributions to the free energy of activation. Our recently published study on the reactions of chlorophenols with atomic H indicated that MPWB1K/6-31+G(d,p) is an excellent method for prediction of the geometrical parameters and the vibrational frequencies of phenol and chlorophenols. Each transition state was verified to connect the designated reactants with products by performing an intrinsic reaction coordinate (IRC) analysis (29). The minimum energy path (MEP) was constructed starting from the transition state geometry and going downhill to both the asymptotic reactant and product channel with a gradient stepsize of 0.02 amu1/2 bohr. The force constant matrices of the stationary points and selected nonstationary points near the transition state along the MEP were also calculated to do the following kinetic calculation. For a more accurate evaluation of the energetic parameters, a more flexible basis set, 6-311+G(3df,2p), was employed to determine the energies of the various species. The rate constants were calculated by means of canonical variational transition-state (CVT) theory. The CVT theory is based on the idea of varying the dividing surface along a reference path to minimize the rate constant (23-25). The CVT rate constant for temperature T is given by: kCVT(T) ) min kGT(T, s) s

(1)

where kGT(T, s) )

σkBT QGT(T, s) -VMEP(s)/kBT e h ΦR(T)

3. Results and Discussion 3.1. Reaction Mechanism. Due to the different substitution pattern of phenol, chlorophenols have 19 congeners. They 9

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). As introduced in our recently published study (22), 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, 2,3-DCP, 2,4-DCP, 2,5-DCP, 3,4-DCP, 2,3,4-TCP, 2,3,5-TCP, 2,3,6-TCP, 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. For a given chlorophenol, the syn conformer is about 3 kcal/mol more stable than the corresponding anti form. So throughout this paper, chlorophenols denote the syn conformers. The structures of chlorophenols optimized at the MPWB1K/6-31+G(d,p) level were presented in our recent study (22). The effect of the chlorine substitution pattern on the structures of chlorophenols was discussed (22).

(2)

where kGT(T, s) is the generalized transition state theory rate constant at the dividing surface s, σ is the symmetry factor accounting for the possibility of more than one symmetryrelated reaction path, kB is Boltzmann’s constant, h is Planck’s constant, ΦR(T) is the reactant partition function per unit volume, excluding symmetry numbers for rotation, and QGT (T, s) is the partition function of a generalized transition state at s with a local zero of energy at VMEP(s) and with all rotational symmetry numbers set to unity. All the kinetic calculations have been carried out using the Polyrate 9.3 program (30). To include quantum tunneling effects for motion along the reaction coordinate, CVT rate constants were multiplied by a transmission coefficient. In particular, we employed the small curvature tunneling (SCT) method (26), based on the centrifugal-dominant small-curvature semiclassical adiabatic ground-state approximation, to calculate the transmission coefficient. The rotational partition functions were calculated classically, and the vibrational modes were treated as quantum-mechanically separable harmonic oscillators.

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FIGURE 1. MPWB1K/6-31+G(d,p) optimized geometries for the prereactive intermediate, transition state, and phenoxy radical involved in the reaction of phenol with OH radicals. Distances are in angstroms. Gray ) C, white ) H, red ) O.

ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 44, NO. 4, 2010

For phenoxyl-hydrogen abstraction from chlorophenols by OH radicals, a hydrogen bonding intermediate is formed first. Then, the phenoxyl-hydrogen is abstracted via a transition state. The structures of the intermediates for the reactions of phenol and 2-CP with OH are presented in Figures 1 and 2, respectively. The structures of the intermediates for the reactions of other chlorophenols with OH radicals are shown in the Supporting Information. The hydrogen bonds in the intermediates with ortho chlorine substitution are systematically shorter (1.907-1.926 Å) than those without ortho substitution (1.953-1.975 Å). This may be due to the different conformations of the intermediates. In the ortho-substituted intermediates, H(1) atom is at the trans-position of O(2) with respect to the O(1)-H(2) bond. In contrast, H(1) atom is at the cis-position of O(2) in the intermediates without ortho substitution. For the same reason, the ortho substitution also has an effect on other

FIGURE 2. MPWB1K/6-31+G(d,p) optimized geometries of the prereactive intermediate, transition state, and 2-CPR involved in the reaction of 2-CP with OH radicals. Distances are in angstroms. Gray ) C, white ) H, red ) O, green ) Cl.

TABLE 1. Relative Energies of the Intermediates ∆H0o(int.) (in kcal/mol, Including ZPE Correction), Potential Barriers ∆H0*,o (in kcal/mol), Reaction Heats ∆H0o(reaction) (in kcal/mol, 0 K, Including ZPE Correction), and Imaginary Frequencies ν (in cm-1) of the Transition States

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

∆H0o(int.)a

∆H0*,o b

-3.0 -2.6 -3.3 -3.4 -2.4 -2.6 -2.6 -2.7 -3.6 -3.8 -2.5 -2.9 -2.6 -2.9 -2.7 -3.7 -2.8 -2.8 -2.8 -2.9

-0.2 (0.7 ) 3.2d (4.9e) 0.2d (1.4e) -0.8d (0.3e) 3.6d (5.5e) 2.8d (4.6e) 3.5d (5.5e) 2.8d (5.0e) -0.4d (1.1e) 0.6d (2.1e) 3.3d (5.0e) 3.9d (5.9e) 3.4d (5.4e) 3.2d (5.0e) 2.7d (4.5e) -0.0d (1.5e) 3.5d (5.5e) 3.2d (5.0e) 3.7d (5.7e) 3.2d (5.3e) d

e

∆H0o (reaction)c

ν

-28.9 -26.9 -27.8 -30.1 -26.1 -28.2 -26.1 -28.2 -29.1 -26.7 -27.5 -25.3 -27.7 -27.5 -29.5 -28.0 -26.7 -28.9 -27.2 -28.5

1306i 2014i 1575i 1317i 2115i 2060i 2241i 2146i 1595i 1825i 2109i 2277i 2230i 2209i 2187i 1780i 2259i 2183i 2329i 2317i

a ∆H0o(int.), the relative energy of the intermediate with respect to the total energy of the corresponding chlorophenol and OH. b ∆H0*,o, the potential barrier, the relative energy of the transition state with respect to the total energy of the separated reactants. c ∆H0o(reaction), the reaction heats, the relative energy of total energy of the products with respect to the total energy of reactants. d ∆H0*,o including ZPE correction. e ∆H0*,o without ZPE correction.

structural parameters, such as the H(1)-O(1), H(2)-O(2), and O(2)-C(1) bonds. For example, all the ortho-substituted intermediates have relative longer H(2)-O(2) bond distances (0.965-0.966 Å) compared to those without ortho substitution (0.960-0.961 Å). The relative energy (including ZPE correction), ∆H0o(int.), of the intermediate with respect to the total energy of the corresponding chlorophenol and OH is summarized in Table 1. The intermediate involved in the reaction of 2-CP with OH was also studied by Altarawneh (17). The relative energy of -2.6 kcal/mol obtained in our study at the MPWB1K/6311+G(3df,2p)//MPWB1K/6-31+G(d,p) level is in excellent agreement with the value of -2.5 kcal/mol (the relative energy of PR1 in the paper of Altarawneh) calculated by Altarawneh at the BB1K/6-311+G(3df,2p)//BB1K/6-31G(d) level (17). As shown in Table 1, the relative energies of the intermediates

are correlated with the chlorine substitution pattern. ∆Ho0(int.) are -2.9 to -2.4 kcal/mol for the ortho-substituted intermediates, whereas the values are -3.8 to -3.0 kcal/mol for those without ortho substitution. Figures 1 and 2 present the essential structural parameters calculated at the MPWB1K/6-31+G(d,p) level of theory for the transition states corresponding to the reactions of phenol and 2-CP with OH radicals, respectively. The structures of the transition states for the reactions of other chlorophenols with OH radicals are shown in the Supporting Information. Clearly, the conformations of the transition states with ortho chlorine substitution are different from the conformations without ortho substitution. In the ortho transition states, H(1) atom is at the trans-position of O(2) with respect to the O(1)-H(2) bond. This trans-conformation results in a weak intramolecular hydrogen bonding between H(1) and Cl(1). The lengths of the hydrogen bonds are from 2.781 to 2.891 Å, which is slightly longer than that of a typical hydrogen bond. Contrarily, H(1) atom is at the cis-position of O(2) in the nonortho transition states. The cis-structure also leads to a weak intramolecular hydrogen bonding between O(1) and H(3). The lengths of the hydrogen bonds are from 2.720 to 2.804 Å. The hydrogen bond can lower the energy of the transition state, i.e., lower the reaction potential barrier. The increase in length of the O(2)-H(2) bond being broken and the elongation of the O(1)-H(2) bond being formed with respect to its equilibrium value in the reactants and the products are the most important aspect of the geometric structure of the transition state. In the ortho-substituted transition states, the O(2)-H(2) bonds being broken are 7.0%-9.6% longer than the corresponding equilibrium value in chlorophenols, and the O(1)-H(2) bonds being formed are elongated by 39.4%-46.9%. In the transition states without ortho substitution, the O(2)-H(2) bonds being broken are stretched by 10.2%-11.8%, and the O(1)-H(2) bonds being formed are 36.2%-39.6% longer than the corresponding equilibrium value in H2O molecule. All the transition states under study are reactant-like and appear earlier on the reaction path, as can be anticipated for exothermic reactions (31). Additionally, the ortho-substituted transition states are more reactant-like than those without ortho substitution. All of the transition states have one and only one imaginary frequency. The values of the imaginary frequencies are shown in Table 1. It is clear from Table 1 that the values of the imaginary frequencies are strongly influenced by the chlorine substitution pattern. The imaginary frequencies are 2014i2329i cm-1 for the ortho-substituted transition states, whereas the values are 1306i-1825i cm-1 for those without ortho substitution. The large value of the imaginary frequency will narrow the width of the potential barrier. It is known that a narrow barrier is favored in the quantum tunneling effect. VOL. 44, NO. 4, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 2. Parameters C and D Involved in Arrhenius Formula of k = C exp(-D/T) (in cm3 molecule-1 s-1) for the Formation of Chlorophenoxy Radicals (CPRs) from the Reactions of Chlorophenols with OH Radicals over the Temperature Range of 600-1200 K reaction

1402

9

C

D

Phenol + OH f C6H5O + H2O

8.13 × 10-12

1367.37

2-CP + OH f 2-CPR + H2O

3.43 × 10-12

2661.33


3.12 × 10-12

1512.16


3.60 × 10-12

1014.82

2,3-DCP + OH f 2,3-DCPR + H2O

2.48 × 10-13

2030.49


3.43 × 10-12

2948.92


9.93 × 10-13

3472.64


9.54 × 10-13

2168.31


2.32 × 10-12

1214.28


3.36 × 10-12

1254.00

2,3,4-TCP + OH f 2,3,4-TCPR + H2O

9.39 × 10-13

3123.53

2,3,5-TCP + OH f 2,3,5-TCPR + H2O

7.48 × 10-13

3705.89

2,3,6-TCP + OH f 2,4,6-TCPR + H2O

4.27 × 10-13

2149.90

2,4,5-TCP + OH f 2,4,5-TCPR + H2O

1.51 × 10-12

2099.61

2,4,6-TCP + OH f 2,4,6-TCPR + H2O

9.58 × 10-13

2752.80

3,4,5-TCP + OH f 3,4,5-TCPR + H2O

5.20 × 10-12

1225.42

2,3,4,5-TeCP + OH f 2,3,4,5-TeCPR + H2O

5.86 × 10-13

3464.44

2,3,4,6-TeCP + OH f 2,3,4,6-TeCPR + H2O

8.92 × 10-13

3354.83

2,3,5,6-TeCP + OH f 2,3,5,6-TeCPR + H2O

5.88 × 10-13

3571.13

PCP + OH f PCPR + H2O

5.71 × 10-13

3347.88


The potential barriers, ∆H0*, o, and the reaction heats, ∆Ho0(reaction), calculated at the MPWB1K/6-311+G(3df,2p)// MPWB1K/6-31+G(d,p) level are listed in Table 1. In particular, the potential barrier is the relative energy of the transition state with respect to the total energy of the separated reactants (the corresponding chlorophenol and OH), without considering the very shallow prereactive intermediate. The reactions of phenol, 4-CP, 3,4-DCP, and 3,4,5-TCP with OH radicals exhibit negative barriers. This is due to the ZPE (zero-point energy) correction. For example, for phenol + OH f phenoxy + H2O, the energy of the transition state without the ZPE correction is higher by 0.7 kcal/mol than the total energy of the separated reactants (phenol and OH). However, the ZPE of the transition state is 0.9 kcal/mol lower than the total ZPE of the separated reactants. So, the potential barrier without the ZPE correction is positive. In contrast, the barrier including the ZPE correction has negative sign for the reactions of phenol, 4-CP, 3,4-DCP, and 3,4,5-TCP with OH radicals. The potential barrier and reaction heat for 2-CP + OH f 2-CPR + H2O were also calculated by Altarawneh at the BB1K/ 6-311+G(3df,2p)//BB1K/6-31G(d) level (17). The values of 3.0 and -27.1 kcal/mol reported by Altarawneh (17) are in excellent agreement with the values of 3.2 and -26.9 kcal/ mol obtained from this work. It can be seen from Table 1 that the potential barriers for the phenoxyl-hydrogen abstraction from the ortho-substituted chlorophenols consistently are higher than those from chlorophenols without ortho substitution. As discussed in our recently published study (22), the chlorine substitution at the ortho position in chlorophenol increases the strength of the O-H bonds and decreases its reactivity. It is interesting to compare the phenoxyl-hydrogen abstraction from chlorophenol by H and OH radicals. For a given chlorophenol, the potential barrier for the phenoxyl-hydrogen abstraction by OH is about 10 kcal/mol lower than that of H abstraction by H (22). In addition, the phenoxyl-hydrogen abstraction by OH is more exothermic by about 15 kcal/mol than the phenoxyl-hydrogen abstraction by H (22). This indicates that the phenoxylhydrogen abstraction from chlorophenol by OH can occur more readily than the phenoxyl-hydrogen abstraction by H. The structures of chlorophenoxy radicals are displayed in the Supporting Information. Chlorophenoxy radical is delocalized, which is a hybrid of one oxygen-centered and three carbon-centered radicals (two ortho and one para carbon sites). The C-O bonds in chlorophenoxy radicals are longer than the C-O double bond and shorter than the C-O single bond. Additionally, the C-O bond lengths also vary with the positions and the number of chlorine substitutions. The chlorophenoxy radicals without ortho substitution have C-O bond lengths of 1.242-1.247 Å. The C-O bonds in the ortho-substituted chlorophenoxy radicals are consistently shorter than 1.24 Å, and decrease with the increase in the number of the ortho chorine substitutions. The C-O bonds are 1.233-1.237 Å and 1.225-1.229 Å in the chlorophenoxy radicals with one and two ortho chlorine substitutions, respectively. This may be attributed to the fact that the induction of chlorine at the ortho position in the phenolic ring is the most effective (32). 3.2. Kinetic Calculations. The rate constant calculations were carried out using canonical variational transition state theory (CVT) with small-curvature tunneling (SCT) contribution, which has been successfully performed to evaluate the rate constants for the formation of chlorophenoxy radicals from the reactions of chlorophenols with atomic H (22) over the temperature range of 600-1200 K. In particular, direct inspection of the transition state low-frequency mode indicates that the mode of the lowest frequency is a hindered internal rotation instead of a small-amplitude vibration. The

mode was removed from the vibration partition function for the transition state and the corresponding hindered rotor partition function QHR(T), calculated by the method devised by Truhlar (33), was included in the expression of the rate constant. An early experiment studied the reaction of phenol + OH f phenoxy + H2O (34). A relative rate constant of 9.96 × 10-12 cm3 molecule-1 s-1 was reported over the temperature range of 1000-1150 K on the basis of the rate constant for the reaction of CO + OH f CO2 + H (34). The deviation between our CVT/SCT rate constant and the experimental value remains within a factor of less than 5. Due to the great difference in reactivity between carbon monoxide and phenol, this experimental rate constant for the reaction of phenol + OH f phenoxy + H2O has large error limits (34). Taking into account the significant uncertainty for the experimental value of 9.96 × 10-12 cm3 molecule-1 s-1, we think the present CVT/SCT calculations predict reasonable rate constants. The kinetic parameters for the reaction of 2-CP + OH f 2-CPR + H2O were also studied by Altarawneh using the CVT/SCT method (17). The CVT/SCT rate constants calculated by Altarawneh are in excellent agreement with our CVT/SCT values. For example, at 1200 K, the CVT/SCT rate constant calculated by Altarawneh is 2.51 × 10-13 cm3 molecule-1 s-1 (17), and agrees well with the value of 3.73 × 10-13 cm3 molecule-1 s-1 from this study. Due to the absence of the available experimental values, it is difficult to make a direct comparison of the calculated CVT/SCT rate constants with the experimental data for the reactions of other 18 chlorophenols with OH radicals. We hope that our CVT/SCT calculations may provide a good estimate for the formations of chlorophenoxy radicals from the reactions of chlorophenols with OH radicals. The CVT/SCT rate constants are strongly affected by the chlorine substitution pattern. Generally, the CVT/SCT rate constants for the phenoxyl-hydrogen abstraction from the ortho-substituted chlorophenols are smaller than those from chlorophenols without ortho subsitution for a given number of chlorine substitutions at a given temperature. For example, at 1000 K, the calculated CVT/SCT rate constants are 3.26 × 10-14, 1.80 × 10-13, 3.08 × 10-14, and 1.09 × 10-13 cm3 molecule-1 s-1 for the phenoxyl-hydrogen abstraction from 2,3-DCP, 2,4-DCP, 2,5-DCP, and 2,6-DCP, whereas the values are 6.89 × 10-13 and 9.59 × 10-13 cm3 molecule-1 s-1 for the phenoxyl-hydrogen abstraction from 3,4-DCP and 3,5-DCP, respectively. The similar observation also can be found in monochlorophenols as well as in trichlorophenols. For a given chlorophenol, the CVT/SCT rate constants for the phenoxyl-hydrogen abstraction by OH are consistently larger than those of the phenoxyl-hydrogen abstraction by H over the whole studied temperature range. For example, at 600 K, the CVT/SCT rate constant of the phenoxyl-hydrogen abstraction from 2,3,4-TCP by OH is 5.15 × 10-15 cm3 molecule s-1, whereas the value is 7.45 × 10-17cm3 molecule s-1 for the phenoxyl-hydrogen abstraction by H. This is consistent with previous experimental observation: OH is the most reactive center among various oxidative radicals existing in municipal waste incinerations (11). The calculated CVT/SCT rate constants for the phenoxylhydrogen abstraction from chlorophenols by OH radicals are expressed in the Arrhenius form of k ) C exp(-D/T) (in cm3 molecule-1 s-1). The parameters C and D are given in Table 2. The pre-exponential 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 20737001, 20777047, 20977059), VOL. 44, NO. 4, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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Shandong Province Outstanding Youth Natural Science Foundation (project JQ200804), and the Research Fund for the Doctoral Program of Higher Education of China (project 200804220046). We thank Professor Donald G. Truhlar for providing the POLYRATE 9.3 program.

Supporting Information Available MPWB1K/6-31+G(d,p) optimized structures of the prereactive intermediates, transition states, and chlorophenoxy radicals. This material is available free of charge via the Internet at http://pubs.acs.org.

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ES9031776

Kinetic Properties for the Complete Series Reactions of Chlorophenols

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