Atmospheric Oxidation of Trichloroethylene: An Ab Initio Study - The

J. Phys. Chem. A 2010, 114, 9163–9176

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Atmospheric Oxidation of Trichloroethylene: An Ab Initio Study Carrie J. Christiansen and Joseph S. Francisco* Department of Chemistry and Department of Earth and Atmospheric Sciences Purdue UniVersity, West Lafayette, Indiana 47909 ReceiVed: April 26, 2010; ReVised Manuscript ReceiVed: July 13, 2010

The atmospheric oxidation of trichloroethylene has previously been studied experimentally. Phosgene is thought to be the dominant product, although the mechanism of production is not well understood. Additionally, studies omitting a chlorine scavenger show the production of dichloroacetyl chloride. This influence of the chlorine atom on the trichloroethylene oxidation is not well understood. Using ab initio methods, this study presents a comprehensive computational study of both the hydroxyl radical and chlorine atom initiated atmospheric oxidation mechanisms of trichloroethylene (C2HCl3). Potential energy surfaces, including activation energies and enthalpies, are determined. The results from this study, in connection with experimental work, confirm the influence of the Cl-initiated oxidation in determining the product profile of the trichloroethylene oxidation. These products include dichloroacetyl chloride [Cl2CHC(O)Cl], formyl chloride [CH(O)Cl], phosgene [C(O)Cl2], and regeneration of the chlorine atom. 1. Introduction The ability of chlorine atoms to catalyze the destruction of stratospheric ozone has led to numerous studies of the properties of halocarbons in the last 30 years.1-4 Trichloroethylene (TCE) (C2HCl3), which is used as a dry cleaning agent, as well as a degreasing agent and a solvent,5 is one of the most prevalent chloroethenes with an emission rate of 197 × 109 g · y-1.6,7 The atmospheric lifetime of this compound has been calculated from its reaction with OH, varying from 2-5 days up to 9 days.8,9 Experiments have shown that the major products of atmospheric oxidation of TCE are phosgene and dichloroacetyl chloride.9-12 As in the oxidation of tetrachloroethylene, the chloroacetyl chloride is likely produced through the reaction of TCE with chlorine atoms.12 A number of experimental studies examining the hydroxyl radical initiated or the chlorine atom initiated oxidation of TCE have been conducted.7,9-16 The overall rate of the reaction between OH and TCE (reaction 1) is 2.2 × 10-12 cm3 · molecules-1 · s-1,20 averaged at 298 K, and is in good agreement with other literature values.21-23

ClCHCCl2 + OH f products

(1)

Reactions 2 and 3 show the possible additions of the hydroxyl radical to trichloroethylene. The species number, shown in parentheses preceding the species formula, will be used throughout the text to designate each species for clearer identification.

(1) ClCHCCl2 + OH f (2) ClCHC(OH)Cl2

(2)

(1) ClCHCCl2 + OH f (18) ClCHC(OH)CCl2

(3)

A number of studies indicate that the OH radical will add to the less chlorinated carbon, indicating that reaction 3 will dominate.17-19 * To whom correspondence should be addressed. E-mail: francisc@ purdue.edu.

The influence of chlorine atoms in the oxidation of TCE is significant. Although chlorine atom concentration is limited with respect to hydroxyl radicals in the atmosphere, chlorine has been shown to react about 30 times faster with TCE than does OH. The high concentration of OH in the atmosphere makes the OH-initiated reaction with TCE important initially. However, the production of chlorine atoms from the OH-initiated reaction, and the speed with which chlorine reacts with TCE makes the Cl-initiated reaction vitally important further into the degradation.9 Using a gas chromatograph with flame ionization detection, Atkinson and Aschmann13 measured the rate constant of reaction 4 at 298 K to be (8.08 ( 0.10) × 10-11 cm3 · molecules-1 · s-1.

ClCHCCl2 + Cl f products

(4)

Catoire et al.14 used FTIR spectroscopy and calculated a rate constant of reaction 4 at 296 K of (7.2 ( 0.8) × 10-11 cm3 · molecules-1 · s-1. Finally, Garib et al.7 measured the rate constant at (7.7 ( 1.9) × 10-11 cm3 · molecules-1 · s-1. The chlorine will add into the double bond of the tricholoethylene, leaving a carbon-centered radical on one of the carbons. Reactions 5 and 6 show the possible addition reactions of a chlorine atom to TCE.

(1) ClCHCCl2 + Cl f (33) ClCHCCl3

(5)

(1) ClCHCCl2 + Cl f (41) Cl2CHCCl2

(6)

As with the addition of OH, it has been shown that the chlorine will preferentially add to the less chlorinated carbon, shown in reaction 6. Bertrand et al.11 estimates that the rate constant of reaction 6 is at least 8 times greater than the addition according to reaction 5. Although experimental work on TCE is extensive, the reaction mechanisms, including possible intermediates and products, of the chlorine-initiated and the OH-initiated are not clear. An ab initio study of the hydroxyl radical initiated and chlorine atom

10.1021/jp103769z  2010 American Chemical Society Published on Web 08/05/2010

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J. Phys. Chem. A, Vol. 114, No. 34, 2010

Christiansen and Francisco

Figure 1. Hydroxyl radical initiated oxidation mechanism of trichloroethylene.

initiated oxidation of TCE is presented. This work includes analysis of the energetics of previously proposed pathways as well as possible new reaction pathways. In combination with experimental work, this study allows for a more complete understanding of the atmospheric fate of TCE through enhanced understanding of the influence of the chlorine atom initiated oxidation and improved understanding of product formation. 2. Computational Methods All computations are performed using the Gaussian 03 suite of programs.24a Optimized geometries and corresponding energies for reactants, reactive intermediates, products, and transition states are determined using the second-order Møller-Plesset perturbation theory (MP2) with the 6-311+G(2d) basis set. All electrons are correlated in the MP2 optimization. Frequency computations are performed using MP2 and the 6-31G(d) basis set, giving vibrational mode frequencies, as well as the zero point energies. The optimized geometries are used to compute single point energies at the following levels of theory: MP4/6-311+G(2d), MP4/6-311+G(2df), CCSD(T)/6-311+G(2d), CCSD(T)/6-311+G(2df). All energies are corrected with the zero-point energies calculated at the MP2/6-31G(d) level of theory. Finally, using corrected energies, the enthalpy and activation energy barriers are calculated for individual reactions throughout the oxidation. 3. Results 3.1. OH-Initiated Oxidation. The OH-initiated atmospheric oxidation of trichloroethylene is given in Figure 1. Pathways with high energy barriers, which are therefore unlikely, are highlighted in red. Individual pathways of the oxidation are shown in Figures 2a and 2b. The oxidation begins with the additions of OH to one of the two carbons. The least substituted carbon (from the CHCl group) will be referred to as carbon 1, and the carbon from the CCl2 group will be referred to as carbon 2. In pathway 1, addition of the hydroxyl radical to carbon 2 results in Cl2CHC(OH)Cl2 (species 2). This molecule can undergo a number of reactions.

Elimination of Cl and subsequent isomerization gives ClCH2C(O)Cl (species 4). Elimination of HCl from Cl2CHC(OH)Cl2 results in ClCHC(O)Cl (species 5), which follows with the addition of O2, reduction by NO/NO2, and fragmentation results in CH(O)Cl, CO, and Cl. Finally, addition of O2 to Cl2CHC(OH)Cl2 and subsequent reduction gives ClCHOC(OH)Cl2 (species 10). A number of fragmentation reactions can occur from this species including the products ClC(O)C(OH)Cl2 (species 11), CH(O)C(OH)Cl2 (species 12), and C(OH)Cl2 (species 13). Further reaction by C(OH)Cl2 results in the formation of C(O)Cl2 (species 14) and C(O)(OH)Cl (species 17). Pathway 2, addition of OH into carbon 1, forms ClCH(OH)CCl2 (species 18) and follows a similar series of reactions as in pathway 1. Elimination of Cl from ClCH(OH)CCl2 and then isomerization leads to CH(O)CHCl2 (species 20). Elimination of HCl, O2 addition, reduction, and fragmentation results in the formation of CO as well as CH(O)C(O)Cl (species 24). Addition of O2 to ClCH(OH)CCl2 and subsequent reduction by NO gives ClCH(OH)CCl2O (species 27). This molecule can eliminate Cl to end in ClCH(OH)C(O)Cl (species 28) or can fragment to give C(O)Cl2 and CH(OH)Cl (species 29). CH(OH)Cl reacts to give a number of compounds including CH(O)Cl (species 8), C(O)(OH)Cl, and CH(O)(OH) (species 32). It is important to note that many of the systems were checked for spin contamination, and the S2 value was found to be less than 0.79. Therefore, we are confident that no spin contamination exists. Figure 3 gives optimized geometries of all reactants, reactive intermediates, and products in both the OH and Cl-initiated pathways. Transition state geometries are shown in Figure 4. Energies for minimum species are available in Supporting Information Table 1, with vibrational frequencies in Supporting Information Table 3. Supporting Information Tables 2 and 4 give the energies and frequencies for transition state species. All supplementary Tables are available in the Supporting Information. Calculated enthalpies are listed in Table 1. Activation energies are

Atmospheric Oxidation of Trichloroethylene

J. Phys. Chem. A, Vol. 114, No. 34, 2010 9165

Figure 2. Pathways in the hydoxyl radical initiated oxidation mechanism of trichloroethylene. (a) Pathway 1 and (b) pathway 2.

given in Table 2. Figures 5a and 5b depict the potential energy surfaces for pathway 1 and 2 of the OH-initiated oxidation. 3.1.1. Pathway 1. The optimized structure of trichloroethylene, ClCHCCl2 (species 1), is given in Figure 3a. It has a C-C bond length of 1.334 Å. The C-Cl bond lengths range from 1.715 to 1.727 Å. The angles are all near 120°, ranging from 115.8 to 124.0°. The addition of the hydroxyl radical to carbon 2 of ClCHCCl2 (reaction 2), the more substituted carbon, occurs with an enthalpy of -36.3 kcal · mol-1 and an activation energy of 4.2 kcal · mol-1.

(1) ClCHCCl2 + OH f (2) Cl2CHC(OH)Cl2

(2)

The transition state for this reaction is shown in Figure 4a. It has a C-OH bond length of 1.992 Å with an imaginary frequency of 767i cm-1. Following pathway 1a, the product, Cl2CHC(OH)Cl2 (species 2, Figure 3b), undergoes a fragmentation reaction to eliminate a chlorine atom (reaction 7). (2) ClCHC(OH)Cl2 f (3) ClCHC(OH)Cl + Cl

(7)

Although The formed species, ClCHC(OH)Cl (species 3, Figure 3c), is technically a closed shell system, the reactive double bond can allow the species to rearrange (reaction 8).

(3) ClCHC(OH)Cl f (4) ClCH2C(O)Cl

(8)

This reaction has an enthalpy of -19.8 kcal · mol-1 and an activation energy of 53.4 kcal · mol-1. As such, this reaction pathway is unlikely and will not be included in the final potential energy surfaces. The optimized geometries of the transition state for this reaction is shown in Figure 4b, and the product, ClCH2C(O)Cl is given in Figure 3d. Pathway 1b continues from Cl2CHC(OH)Cl2 by elimination of HCl to form ClCHC(O)Cl (reaction 9).

(2) ClCHC(OH)Cl2 f (5) ClCHC(O)Cl + HCl

(9)

Carbon 1 of ClCHC(O)Cl (species 5, Figure 3e) maintains the radical, with a Mulliken atomic spin density of 0.983. The reaction pathway continues with the addition of molecular oxygen to ClCHC(O)Cl to give ClCHO2C-

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Figure 3. Structures of reactants, reactive intermediates, and products. Bond distances given in angstroms and angles in degrees.

(O)Cl (species 6, Figure 3f) (reaction 10). This is followed by reduction of the peroxy by NO, resulting in the formation of ClCHOC(O)Cl (species 7, Figure 3g) (reaction 11).

(5) ClCHC(O)Cl + O2 f (6) ClCHO2C(O)Cl

(10)

(6) ClCHO2C(O)Cl + NO f (7) ClCHOC(O)Cl + NO2 (11) The enthalpies of reactions 10 and 11 are -15.9 and -18.5 kcal · mol-1, respectively.



Figure 4. Transition state structures. Bond distances given in angstroms and angles in degrees.

It is important to note that reactions of peroxy radicals with NO, as in reaction 11, can also result in the formation of organic nitrates, although yields are small.25,26 The final step of pathway 1b is the fragmentation of ClCHOC(O)Cl (species 7) (reaction 12), with an enthalpy of -6.9 kcal · mol-1.

(7) ClCHOC(O)Cl f (8) CH(O)Cl + CO + Cl

(12)

Along with carbon monoxide and chlorine atom, CH(O)Cl (species 8, Figure 3h) is formed in this fragmentation. Pathyways 1c-g start with an O2 addition to ClCHC(OH)Cl2 to form ClCHO2C(OH)Cl2 (species 9, Figure 3i), and then reduction by NO/NO2 to give ClCHOC(OH)Cl2 (species 10, Figure 3j).

(2) ClCHC(OH)Cl2 + O2 f (9) ClCHO2C(OH)Cl2 (13) (9) ClCHO2C(OH)Cl2 + NO f (10) ClCHOC(OH)Cl2 + NO2

(14) ClCHOC(OH)Cl2 has numerous possible reactions. Pathway 1c ends in an abstraction of the hydrogen with O2 (reaction 15).

(10) ClCHOC(OH)Cl2 + O2 f (11) ClC(O)C(OH)Cl2 + HO2 (15)

ClC(O)C(OH)Cl2, formed in reaction 15, is shown in Figure 3k. Pathway 1d ends with an elimination of chlorine from ClCHOC(OH)Cl2 (reaction 16). (10) ClCHOC(OH)Cl2 + O2 f (12) CH(O)C(OH)Cl2 + Cl

(16) This reaction occurs with an enthalpy that is near thermo neutral, at the CCSD(T)/6-311+G(2df) level of theory. The activation energy is 9.6 kcal · mol-1. The transition state structure is shown in Figure 4c; the formed product, CH(O)C(OH)Cl2 is shown in Figure 3l. Pathways 1e-g continue from a C-C bond fragmentation of ClCHOC(OH)Cl2 (reaction 17).

(10) ClCHOC(OH)Cl2 f (8) CH(O)Cl + (13)C(OH)Cl2 (17) The energy barrier of this reaction is 5.0 kcal · mol-1 with an enthalpy of -4.6 kcal · mol-1. The transition structure, shown in Figure 4d, has a C-C bond length of 1.847 Å. The frequency calculations for this transition state show an imaginary vibrational mode at 1010i cm-1. The products include CH(O)Cl (species 8, Figure 3h) and C(OH)Cl2 (species 13, Figure 3m). CH(O)Cl was also a product of pathway 1b. C(OH)Cl2 is near trigonal pyramidal with the radical centered on the carbon, indicated by the Mulliken atomic spin density of 0.905. Pathway 1e involves the removal of the hydrogen and formation of phosgene, C(O)Cl2 (reaction 18).

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TABLE 1: Enthalpy of Reactions (kcal · mole-1)a MP2 6-311+G (2d)

reaction

OH-Initiated Pathway 1 -37.7

(1) ClCHCCl2 + OH f (2) ClCHC(OH)Cl2 (2) ClCHC(OH)Cl2 f (3) ClCHC(OH)Cl + Cl (3) ClCHC(OH)Cl f (4) ClCH2C(O)Cl (2) ClCHC(OH)Cl2 f (5) ClCHC(O)Cl + HCl (5) ClCHC(O)Cl + O2 f (6) ClCHO2C(O)Cl (6) ClCHO2C(O)Cl + NO f (7) ClCHOC(O)Cl + NO2 (7) ClCHOC(O)Cl f (8) CH(O)Cl + CO + Cl (2) ClCHC(OH)Cl2 + O2 f (9) ClCHO2C(OH)Cl2 (9) ClCHO2C(OH)Cl2 + NO f (10) ClCHOC(OH)Cl2 + NO2 (10) ClCHOC(OH)Cl2 + O2 f (11) ClC(O)C(OH)Cl2 + HO2 (10) ClCHOC(OH)Cl2 + O2 f (12) CH(O)C(OH)Cl2 + Cl (10) ClCHOC(OH)Cl2 f (8) CH(O)Cl + (13) C(OH)Cl2 (13) C(OH)Cl2 + O2 f (14) C(O)Cl2 + HO2 (13) C(OH)Cl2 + O2 f (15) C(OH)Cl2O2 (15) C(OH)Cl2O2 f (16) C(OH)Cl2O + NO2 (16) C(OH)Cl2O f (14) C(O)Cl2 + OH (16) C(OH)Cl2O f (17) C(O)(OH)Cl + Cl

MP4 6-311+G (2d)

MP4 6-311+G (2df)

CCSD(T) 6-311+G (2d)

CCSD(T) 6-311+G (2df)

-34.2

-36.9

-33.7

-36.3

18.3

15.7

18.7

15.6

18.3

-21.0

-22.6

-21.8

-20.6

-19.8

-3.8

-7.8

-7.0

-9.0

-8.4

-13.2

-14.2

-15.9

-14.3

-15.9

-25.8

-24.2

-25.0

-17.8

-18.5

-12.0 -19.2

-14.8 -20.4

-9.7 -22.3

-11.5 -22.5

-6.9 -24.3

-24.6

-23.0

-23.8

-16.2

-16.9

-34.0

-33.2

-33.9

-32.3

-33.0

-4.5

-4.8

-2.4

-2.1

0.0

-7.3

-6.8

-6.4

-4.9

-4.6

-25.6

-28.3

-27.9

-28.9

-28.5

-19.8

-21.1

-23.5

-23.2

-25.5

-25.2

-23.9

-24.5

-17.9

-18.4

0.2

-0.5

2.1

3.2

5.7

-28.5

-27.2

-24.8

-23.4

-21.4

OH-Initiated Pathway 2 -37.1

-33.9

-36.2

-33.8

-36.2

21.9

19.5

22.1

19.8

22.2

-11.3

-12.9

-12.6

-11.6

-11.3

7.7

3.3

3.3

0.8

0.6

-13.6

-13.8

-15.7

-12.3

-14.0

-24.8

-23.4

-24.2

-18.0

-18.7

-18.4

-18.0

-15.0

-13.8

-11.3

-24.3

-22.2

-20.5

-17.2

-15.7

-24.9

-28.7

-28.0

-31.7

-31.0

-16.1

-17.0

-19.4

-18.6

-20.8

-25.0

-23.7

-24.4

-17.5

-18.0

-17.1

-17.1

-14.2

-13.8

-11.3

-11.1

-10.1

-9.7

-7.1

-7.0

-25.1

-28.0

-27.5

-29.2

-28.6

-27.4

-28.9

-30.8

-31.6

-33.4

-23.0

-21.6

-22.3

-14.9

-15.6

6.0 -47.2

5.3 -45.0

7.6 -46.3

8.4 -44.1

10.6 -45.4

-22.5

-21.1

-19.1

-17.6

-15.9

-13.5

-11.0

-14.7

-10.6

-13.9

-19.0

-20.0

-22.2

-21.9

-23.9

-23.6

-14.7

-15.5

-8.0

-8.7

-35.4

-41.9

-42.7

-40.9

-41.6

-7.0

-14.7

-12.2

-11.9

-9.8

-11.2

-18.5

-18.0

-17.2

-17.0

-14.8

-16.0

-18.3

-17.6

-19.7

-25.5

-24.1

-24.6

-18.1

-18.5

-26.0

-25.4

-22.0

-21.7

-18.7

-17.8

-15.6

-18.7

-15.8

-18.6

-15.7

-16.5

-19.1

-17.9

-20.3

(1) ClCHCCl2 + (OH) f (18) ClCH(OH)CCl2 (18) ClCH(OH)CCl2 f (19 )CH(OH)CCl2 + Cl (19) ClCHC(OH)Cl f (20) CH(O)CHCl2 (18) ClCH(OH)CCl2 f (21) CH(O)CCl2 + HCl (21) CH(O)CCl2 + O2 f (22) CH(O)CCl2O2 (22) CH(O)CCl2O2 + NO2 f (23) CH(O)CCl2O2 + NO2 (23) CH(O)CCl2O f (24) CH(O)C(O)Cl + Cl (23) CH(O)CCl2O f (25) CH(O) + (14) C(O)Cl2 (25) CH(O) + O2 f CO + HO2 (18) ClCH(OH)CCl2 + O2 f (26) ClCH(OH)CCl2O2 (26) ClCH(OH)CCl2O2 + NO2 f (27) ClCH(OH)CCl2O2 + NO2 (27) ClCH(OH)CCl2O f (28) ClCH(OH)C(O)Cl + Cl (27) ClCH(OH)CCl2O f (29) CH(OH)Cl + (14) C(O)Cl2 (29) CH(OH)Cl + O2 f (8) CH(O)Cl + HO2 (29) CH(OH)Cl + O2 f (30) CH(OH)ClO2 (30) CH(OH)ClO2 + NO f (31) CH(OH)ClO + NO2 (31) CH(OH)ClO f (8) CH(O)Cl + OH (31) CH(OH)ClO + O2 f (17) C(O)(OH)Cl + HO2 (31) CH(OH)ClO f (32) CH(O)(OH) + Cl

Cl-Initiated Pathway 1

(1) ClCHCCl2 + Cl f (33) ClCHCCl3 (33) ClCHCCl3 + O2 f (34) ClCHO2CCl3 (34) ClCHO2CCl3 + NO f (35) ClCHOCCl3 + NO2 (35) ClCHOCCl3 + O2 f (36) ClC(O)CCl3 + HO2 (35) ClCHOCCl3 f (37) CH(O)CCl3 + Cl (35) ClCHOCCl3 f (8) CH(O)Cl + (38) CCl3 (38) CCl3 + O2 f (39) CCl3O2 (39) CCl3O2 + NO f (40) CCl3O + NO2 (40) CCl3O f (14) C(O)Cl2 + Cl (1) ClCHCCl2 + Cl f (41) Cl2CHCCl2 (41) Cl2CHCCl2 + O2 f (42) Cl2CHCCl2O2




TABLE 1: Continued

reaction

MP2 6-311+G (2d)

MP4 6-311+G (2d)

MP4 6-311+G (2df)

CCSD(T) 6-311+G (2d)

CCSD(T) 6-311+G (2df)

-25.2

-23.9

-24.6

-17.8

-18.3

-17.4

-17.6

-14.5

-14.3

-11.6

-13.4

-13.1

-12.6

-10.7

-10.6

-18.6

-19.9

-21.7

-22.1

-23.8

-0.7

0.7

0.0

7.4

6.8

-42.2

-41.6

-38.7

-38.2

-35.6

-66.8

-65.1

-65.6

-64.1

-64.6

MP2 6-311+G (2d)

MP4 6-311+G (2d)

MP4 6-311+G (2df)

CCSD(T) 6-311+G (2d)

CCSD(T) 6-311+G (2df)

(42) Cl2CHCCl2O2 + NO f (43) Cl2CHCCl2O + NO2 (43) Cl2CHCCl2O f (44) Cl2CHC(O)Cl + Cl (43) Cl2CHCCl2O f (14) C(O)Cl2 + (45) CHCl2 (45) CHCl2 + O2 f (46) CHCl2O2 (46) CHCl2O2 + NO f (47) CHCl2O + NO2 (47) CHCl2O f (8) CH(O)Cl + Cl (47) CHCl2O + O2 f (14) C(O)Cl2 + HO2 a

Values corrected for zero point energy.

TABLE 2: Activation Energies for Select Reactions (kcal · mole-1)a

reaction

OH-Initiated Pathway 1

[(1) ClCHCCl2 + OH f (2) Cl2CHC(OH)Cl2]‡

13.1

11.4

10.4

5.1

4.2

[(3) ClCHC(OH)Cl f (4) ClCH2C(O)Cl]‡

50.8

50.3

50.6

53.0

53.4

[(10) ClCHOC(OH)Cl2 + O2 f (12) CH(O)C(OH)Cl2 + Cl]‡

13.9

12.0

12.7

8.9

9.6

9.3

8.4

7.6

5.7

5.0

21.6

19.7

21.7

21.1

23.3

1.6

1.0

1.4

-0.2

0.1

[(1) ClCHCCl2 + (OH) f (18) ClCH(OH)CCl2]‡

10.5

8.6

7.8

2.4

1.7

[(19) ClCHC(OH)Cl f (20) CH(O)CHCl2]‡

60.4

59.1

59.4

60.4

60.8

6.7

5.2

5.9

2.9

3.5

0.7

0.4

0.3

-0.3

-0.4

[(27) ClCH(OH)CCl2O f (28) ClCH(OH)C(O)Cl + Cl]

6.6

5.2

5.9

2.9

3.5

[(27) ClCH(OH)CCl2O f (29) CH(OH)Cl + (14) C(O)Cl2]‡

2.6

2.3

1.8

0.1

-0.5

23.1

21.1

23.2

22.1

24.3

0.8

0.1

0.3

-1.3

-1.1

2.9

3.8

2.0

0.9

-0.7

10.7

1.6

2.3

-1.3

-0.6

11.5

2.9

2.1

-0.1

-0.9

3.8

2.8

3.5

0.8

1.5

-0.1

0.7

-0.6

-2.4

-3.6

5.4

4.1

4.8

1.8

2.5

9.0

8.4

7.6

6.1

5.4

[(10) ClCHOC(OH)Cl2 f (8) CH(O)Cl + (13) C(OH)Cl2]‡ [(16) C(OH)Cl2O f (14) C(O)Cl2 + OH]

‡

[(16) C(OH)Cl2O f (17) C(O)(OH)Cl + Cl]

‡

OH-Initiated Pathway 2

[(23) CH(O)CCl2O f (24) CH(O)C(O)Cl + Cl]

‡

[(23) CH(O)CCl2O f (25) CH(O) + (14) C(O)Cl2]

‡ ‡

[(31) CH(OH)ClO f (8) CH(O)Cl + OH]‡ [(31) CH(OH)ClO f (32) CH(O)(OH) + Cl]‡ Cl-Initiated Pathway 1

[(1) ClCHCCl2 + Cl f (33) ClCHCCl3]‡ [(35) ClCHOCCl3 f (37) CH(O)CCl3 + Cl]‡ [(35) ClCHOCCl3 f (8) CH(O)Cl + (38) CCl3]

‡

[(40) CCl3O f (14) C(O)Cl2 + Cl]

‡


[(1) ClCHCCl2 + Cl f (41) Cl2CHCCl2]

‡

[(43) Cl2CHCCl2O f (44) Cl2CHC(O)Cl + Cl]

‡

[(43) Cl2CHCCl2O f (14) C(O)Cl2 + (45) CHCl2]

‡

a

Values corrected for zero point energy.

(13) C(OH)Cl2 + O2 f (14) C(O)Cl2 + HO2

(18)

The product formed, C(O)Cl2, is a major product of the TCE oxidation, as shown by experimental studies.9-12 However, this is only one source of phosgene in the oxidation. C(O)Cl2 (species 14, Figure 3n), is a symmetric, planar molecule with Cl-C-O bond angles of 124.1° and a Cl-C-Cl bond angle

of 111.9°. The C-O bond length is 1.182 Å, and the C-Cl bond length is 1.751 Å. C(OH)Cl2 can also proceed with an O2 addition and subsequent reduction (reactions 19 and 20).

(13) C(OH)Cl2 + O2 f (15) C(OH)Cl2O2

(19)

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Figure 5. Potential energy surfaces for (a) pathway 1 and (b) pathway 2, in the hydroxyl initiated trichloroethylene oxidation. Energy units in kcal · mol-1. Values calculated using values computed at the CCSD(T)/6-311+G(2df) level.

(15) C(OH)Cl2O2 f (16) C(OH)Cl2O + NO2

(20)

Reaction 19 C(OH)Cl2O2 (species 15), shown in Figure 3o. Reduction of C(OH)Cl2O2 forms C(OH)Cl2O (species 16, Figure 3p). C(OH)Cl2O (species 16) can fragment in two ways. First, pathway 1f ends with the fragmentation of the C-(OH) bond to form another molecule of C(O)Cl2 (reaction 21).

(16) C(OH)Cl2O f (14) C(O)Cl2 + OH

(21)

This reaction transition state (Figure 4e) has a C-(OH) bond length of 1.802 Å. The calculations show an enthalpy of 5.7 kcal · mol-1 and a significant barrier of 23.3 kcal · mol-1. Finally, pathway 1 g ends when C(OH)Cl2O fragments at the C-Cl bond (reaction 22).

(16) C(OH)Cl2O f (17)C(O)(OH)Cl + Cl

(22)

A barrier of 0.1 kcal · mol-1 and an enthalpy of -21.4 kcal · mol-1 make this reaction very likely. The reaction proceeds through the transition state shown in Figure 4f and result in the formation of C(O)(OH)Cl (species 17, Figure 3q). 3.1.2. Pathway 2. Pathway 2 begins with the addition of OH to carbon 1, the less-substituted carbon (reaction 3).

(1) ClCHCCl2 + (OH) f (18) ClCH(OH)CCl2

(3)

Our results concur with the previous literature suggesting addition of the OH to the less substituted carbon.17-19 This is reflected in the lower activation energy barrier, 1.7 kcal · mol-1 for reaction 3, whereas OH addition to carbon 2 (reaction 2) has a barrier of 4.2 kcal · mol-1. The enthalpy for reaction 3 is -36.2 kcal · mol-1, and the imaginary



vibrational frequency is 674i cm-1. The transition state for this reaction is shown in Figure 4g, with a C-(OH) bond length of 2.030 Å. The product, ClCH(OH)CCl2, is depicted in Figure 3r. The C-C bond has lengthened significantly, from 1.334 Å to 1.474 Å, as it achieves more single bond characteristic. As in pathway 1a, in pathway 2a, ClCH(OH)CCl2 can undergo a Cl elimination (reaction 23).

(18) ClCH(OH)CCl2 f (19) CH(OH)CCl2 + Cl

(23) This reaction has an enthalpy of 22.2 kcal · mol-1 and results in the formation of CH(OH)CCl2 (species 19, Figure 3s). According to a proposal by Nolan et al.,16 this species is suggested to undergo an isomerization to form CH(O)CHCl2 (species 20, Figure 3t) (reaction 24).

(19) ClCHC(OH)Cl f (20) CH(O)CHCl2

(24)

However, this reaction has a very large barrier, 60.8 kcal · mol-1. See Figure 4h for the transition state structure. Experimental studies do not observe the formation of CH(O)CHCl2.16 Energetics data given in this work, showing a high activation energy, confirm that this pathway will not contribute significantly. Pathways 2b-c of our scheme, shown in Figure 2b, are analogous to pathway 1b of our scheme (Figure 2a). This set of reactions was also proposed by Nolan et al.16 ClCH(OH)CCl2 can eliminate HCl to give CH(O)CCl2 (species 21) (reaction 25).

(18) ClCH(OH)CCl2 f (21) CH(O)CCl2 + HCl

(25) CH(O)CCl2 is depicted in Figure 3u. Reaction 25 is followed by the addition of molecular oxygen (reaction 26).

(21) CH(O)CCl2 + O2 f (22) CH(O)CCl2O2

(26)

Formation of CH(O)CCl2O2 (species 22, Figure 3v) is followed by a reduction to an alkoxy radical (reaction 27).

(22) CH(O)CCl2O2 + NO2 f (23) CH(O)CCl2O2 + NO2 (27) The destabilizing effect of the oxygen radical in CH(O)CCl2O (species 23, Figure 3w) results in a fragmentation, either of the C-Cl bond (pathway 2b) or of the C-C bond (pathway 2c). Reaction 28 illustrates the final reaction of pathway 2b.

(23) CH(O)CCl2O f (24) CH(O)C(O)Cl + Cl

(28) This reaction occurs with a barrier of 3.5 kcal · mol-1 and an enthalpy of -11.3 kcal · mol-1. The transition state is shown in Figure 4i. The final product of this pathway is CH(O)C(O)Cl (species 24, Figure 3x).

The second fragmentation of CH(O)CCl2O (species 23), along the C-C bond results in the formation of phosgene, C(O)Cl2 (species 14, Figure 3n) and CH(O) (species 25, Figure 3y) (reaction 29).

(23) CH(O)CCl2O f (25) CH(O) + (14) C(O)Cl2

(29) This fragmentation occurs with a minimal barrier, -0.4 kcal · mol-1, and an exothermic enthalpy of -15.7 kcal · mol-1. Figure 4j shows this fragmentation transition state. CH(O) (species 25) can undergo elimination of the hydrogen to form CO through a H abstraction by O2 (reaction 30, pathway 2c).

(25) CH(O) + O2 f CO + HO2

(30)

Reaction 30 has an enthalpy of -31.0 kcal · mol-1. Pathways 2d-h begin with the addition of O2 to ClCH(OH)CCl2 to form ClCH(OH)CCl2O2 (species 26, Figure 3z) (reaction 31), with an enthalpy of -20.8 kcal · mol-1.

(18) ClCH(OH)CCl2 + O2 f (26) ClCH(OH)CCl2O2 (31) Upon the OH addition to form ClCH(OH)CCl2, carbon 2 obtained a radical characteristic; this is indicated by the Mulliken atomic spin density of 0.939 on carbon 2. Addition of O2 to this carbon is therefore expected. Upon addition, the radical is then transferred to the peroxy oxygen, with a Mulliken atomic spin density on the oxygen of 0.939 also. This reaction is followed by reduction to ClCH(OH)CCl2O (species 27, Figure 3 aa) (reaction 32).

(26) ClCH(OH)CCl2O2 + NO2 f (27) ClCH(OH)CCl2O2 + NO2

(32)

Reaction 32 has an enthalpy of -18.0 kcal · mol-1. The radical species, ClCH(OH)CCl2O can undergo two fragmentation reactions. First, in reaction 33, elimination of Cl atom ends pathway 2d.

(27) ClCH(OH)CCl2O f (28) ClCH(OH)C(O)Cl + Cl (33) The transition state for this reaction is given (Figure 4k). Reaction 33 has an activation barrier of 3.5 kcal · mol-1 and an enthalpy of -11.3 kcal · mol-1. ClCH(OH)C(O)Cl is shown in Figure 3bb. Pathways 2e-h continues with the fragmentation of the C-C bond (reaction 34) to form CH(OH)Cl (species 29, Figure 3cc).

(27) ClCH(OH)CCl2O f (29) CH(OH)Cl + (14) C(O)Cl2 (34) This reaction results in yet another source of C(O)Cl2. The fragmentation occurs with a transition bond length of 1.805 Å (Figure 4l), and a vibrational mode associated with the fragmentation of 848i cm-1. The barrier for this reaction is minimal, -0.5 kcal · mol-1, and has an enthalpy of -7.0 kcal · mol-1. One possible reaction from CH(OH)Cl is the

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formation of CH(O)Cl through abstraction of hydrogen using O2 (reaction 35, pathway 2e).

(29) CH(OH)Cl + O2 f (8) CH(O)Cl + HO2

(35) This set of reactions, pathway 2e, is suggested by a number of works to be the dominate oxidation reactions of TCE.9,12,13,16 Additionally, it is important to note that CH(O)Cl is one of the observed products in experiments studying the OH-initiated oxidation of TCE.12,16 CH(OH)Cl also has the potential to undergo O2 addition (reaction 36).

(29) CH(OH)Cl + O2 f (30) CH(OH)ClO2

(36)

The optimized geometry of the formed peroxy radical, CH(OH)ClO2 (species 30) is given (Figure 3 dd). This radical will undergo reduction to form CH(OH)ClO (species 31, Figure 3 ee) (reaction 37).

(30) CH(OH)ClO2 + NO f (31) CH(OH)ClO + NO2 (37)

Figure 6. Chlorine atom initiated oxidation mechanism of trichloroethylene.

Finally, CH(OH)ClO (species 31) can undergo four reactions, all resulting in final products. First, the end of pathway 2f, is the elimination of hydroxyl radical (reaction 38).

Pathway 2 of the Cl-initiated oxidation begins with the addition of chlorine to carbon 1 to give Cl2CHCCl2 (species 41). Again, this addition is followed by the addition of O2 and then reduction to give Cl2CHCCl2O (species 34). Elimination of Cl from this molecule gives Cl2CH(O)Cl (species 44). Cl2CHCCl2O can also fragment at the C-C bond to give C(O)Cl2 and CHCl2 (species 45). CHCl2 will react further with O2 and NO to result in C(O)Cl2 and CH(O)Cl. As mentioned earlier, optimized geometries of minimum species are given in Figure 3, and transition state geometries are given in Figure 4. Supporting Information Tables 1 and 2 contain energies for minimum species and transition states, respectively. Supporting Information Tables 3 and 4 give the frequencies for minimum species and transition state species. These tables are available in the Supporting Information. Table 1 lists enthalpies and Table 2 gives activation energies for reactions involved in the degradation. Potential energy surfaces for the reaction in both pathways of the Cl-initiated oxidation are given in Figure 8. These surfaces allow for a visual description of the energy barriers, as well as the overall thermodynamic driving forces for the diverging pathway. 3.2.1. Pathway 1. Pathway 1 examines the oxidation from the addition of Cl to carbon 2 (reaction 5).

(31) CH(OH)ClO f (8) CH(O)Cl + OH

(38)

This reaction has a significant barrier, 24.3 kcal · mol-1, with an enthalpy of 10.6 kcal · mol-1; the transition state is given (Figure 4m). Once again, the formed product is formyl chloride, CH(O)Cl. Pathway 2g, ending in reaction 39, is the hydrogen abstraction from CH(OH)ClO to form C(O)(OH)Cl with an enthalpy of -45.4 kcal · mol-1.

(31) CH(OH)ClO + O2 f (17) C(O)(OH)Cl + HO2 (39) The last reaction of pathway 2 h is elimination of chlorine (reaction 40).

(31) CH(OH)ClO f (32) CH(O)(OH) + Cl

(40)

This reaction is the most likely of the pathways stemming from CH(OH)ClO, due to its minimal barrier, -1.1 kcal · mol-1. The enthalpy is -15.9 kcal · mol-1. The transition state geometry is shown in Figure 4n, with a C-Cl length of 2.001 Å and a vibrational mode of 694i cm-1. The final product is formic acid, CH(O)(OH) (species 32, Figure 3 ff). 3.2. Cl-initiated Oxidation. The chlorine-initiated oxidation is shown in Figure 6. Individual pathways are represented in Figure 7; pathway 1a-c is given in Figure 7a and pathway 2a-c are shown in Figure 7b. The first pathway, addition of chlorine to carbon 2, forms ClCHCCl3 (species 33). Addition of O2 and reduction by NO gives ClCHOCCl3 (species 35). This species can react in three ways giving ClC(O)CCl3 (species 36), CH(O)CCl3 (species 37), CH(O)Cl, and CCl3 (species 38). CCl3 will continue to react with O2 to eventually give C(O)Cl2.

(1) ClCHCCl2 + Cl f (33) ClCHCCl3

(5)

The optimized transition state is given (Figure 4o) and has a C-Cl bond of 2.267 Å. The enthalpy for reaction 5 is -13.9 kcal · mol-1. At lower levels of theory, for example at the MP2/ 6-311+G(2d) level, the activation energy barrier for reaction 5 is 2.9 kcal · mol-1. At higher levels of theory, using added electron correlations, with the 6-311+G(2d) basis set, the barrier is reduced, going from 3.8 kcal · mol-1 to 0.9 kcal · mol-1 for MP4 and CCSD(T) correlation, respectively. Increasing the basis set from 6 to 311+G(2d) to 6-311+G(2df) using the CCSD(T) correlation method underestimates the barrier, resulting in -0.7 kcal · mol-1 activation energy. This suggests that within the uncertainty of the calculation the reaction is nearly barrierless.



Figure 7. Pathways in the chlorine atom initiated oxidation mechanism of trichloroethylene. (a) Pathway 1 and (b) pathway 2.

Figure 8. Potential energy surfaces for (a) pathway 1 and (b) pathway 2, in the chlorine-initiated trichloroethylene oxidation. Energy units in kcal · mol-1. Values calculated using values computed at the CCSD(T)/6-311+G(2df) level.

9174



The activation energy of reaction 5 is significantly lower than that of the either of the OH addition reactions (reaction 2 and reaction 3). This indicates that chlorine will add to TCE with a lower barrier than occurs will the OH radical addition. The product ClCHCCl3 is shown in Figure 3 gg. The Mulliken atomic spin density of 1.060 on carbon 1 indicates that the addition of the chlorine in reaction 5 created a radical that is now centered on carbon 1. Molecular oxygen adds to this carbon centered radical to form ClCHO2CCl3.

This is followed by reduction of CCl3O2 (species 39) to form CCl3O (species 40, Figure 3 nn) (reaction 47).

(39) CCl3O2 + NO f (40) CCl3O + NO2

CCl3O (species 40) can then undergo chlorine elimination to form C(O)Cl2 as a final product (reaction 48).

(40) CCl3O f (14) C(O)Cl2 + Cl (33) ClCHCCl3 + O2 f (34) ClCHO2CCl3

(48)

(41)

Reaction 41 has an enthalpy of -23.9 kcal · mol-1. As in other addition reactions, the C1-Cl bond length in ClCHO2CCl3 (species 34, Figure 3 hh) increases to accommodate the additional substituent. This addition reaction is followed by reduction (reaction 42).

(34) ClCHO2CCl3 + NO f (35) ClCHOCCl3 + NO2 (42)

This reaction proceeds with a small barrier, 1.5 kcal · mol-1, and an exothermic enthalpy of -18.7 kcal · mol-1. The transition state structure (Figure 4r) shows a C-Cl length of 2.015 Å. The final product, C(O)Cl2, phosgene, as mentioned earlier, has been shown as a major product in the TCE oxidation. 3.2.2. Pathway 2. Addition of chlorine to carbon 1 (reaction 6) is thought to be a more significant route than addition to carbon 2 (reaction 5), following the trend seen in the OH addition reactions.11

(1) ClCHCCl2 + Cl f (41) Cl2CHCCl2 As expected, the product has reduced C-O bond lengths in ClCHOCCl3 (species 35, Figure 3 ii). ClCHOCCl3 can undergo a number of reactions. Pathway 1a ends with removal of the hydrogen to give ClC(O)CCl3 (species 36, Figure 3 jj) (reaction 43).

(35) ClCHOCCl3 + O2 f (36) ClC(O)CCl3 + HO2 (43) Pathway 1b ends in reaction 44, elimination of Cl from ClCHOCCl3.

(35) ClCHOCCl3 f (37) CH(O)CCl3 + Cl

(47)

(44)

This pathway is likely to have a significant contribution, with an exothermic enthalpy, -9.8 kcal · mol-1, and a minimal activation energy, -0.6 kcal · mol-1. The transition state (Figure 4p) has a C-Cl bond of 2.112 Å. The formed product, CH(O)CCl3 is shown in Figure 3 kk. The final part of the first pathway, pathway 1c, continues from ClCHOCCl3 through a fragmentation of the C-C bond (reaction 45).

(35) ClCHOCCl3 f (8) CH(O)Cl + (38) CCl3

(6)

The activation energy of reaction 6, -3.6 kcal · mol-1, compared to the activation of energy of reaction 5, -0.7 kcal · mol-1, confirms that Cl is more likely to add to the lesssubstituted carbon. As in reaction 5, at higher level of theories, the activation energy is underestimated, going from 0.1 kcal · mol-1 at MP2/6-311+G(2d) to -3.6 kcal · mol-1 at CCSD(T)/6-311+G(2df). Again, this indicates that the reaction is nearly barrierless within the uncertainty of the calculation. This reaction is the most energetically favorable of all four initiating reactions, and if chlorine atoms are available, the it will play a considerable role. The enthalpy of reaction 6 is -18.6 kcal · mol-1. The transition state structure is shown in Figure 4s, with a C-Cl length of 2.369 Å and an imaginary frequency associated with the transition of 384i cm-1. The product, Cl2CHCCl2 (species 41, Figure 3 3oo), now has a carboncentered radical on carbon 2, with a Mulliken atomic spin density of 0.955. Molecular oxygen adds to this radical and results in a peroxy radical (reaction 49).

(41) Cl2CHCCl2 + O2 f (42) Cl2CHCCl2O2

(49)

This reaction is followed by a reduction of the peroxy radical to an alkoxy radical, Cl2CHCCl2O (species 43, Figure 3 qq) (reaction 50).

(45) This reaction results in the formation of CH(O)Cl, an observed product of the TCE oxidation, and CCl3 (species 38, Figure 3 ll). This fragmentation occurs with a minimal barrier of -0.9 kcal · mol-1 and enthalpy of -17.0 kcal · mol-1. These energetics indicate that there will likely be competition between reaction 44, reaction 45, and potentially reaction 43. The transition state for reaction 45 is shown in Figure 4q. CCl3 will continue on with O2 adding into the carbon centered radical to form CCl3O2 (species 39, Figure 3 mm) (reaction 46).

(38) CCl3 + O2 f (39) CCl3O2

(46)

(42) Cl2CHCCl2O2 + NO f (43) Cl2CHCCl2O + NO2 (50) Ending pathway 2a, Cl2CHCCl2O (species 43) can now undergo fragmentation of the C-Cl bond (reaction 51).

(43) Cl2CHCCl2O f (44) Cl2CHC(O)Cl + Cl

(51) Reaction 51 has a small activation energy barrier of 2.5 kcal · mol-1; the transition state is given in Figure 4t. The formed product, Cl2CHC(O)Cl, dichloroacetyl chloride, is shown in Figure 3rr. Tuazon et al.12 conducted experimental studies of



the TCE oxidation, investigating the products formed. When chlorine scavengers were included in the experiments, a limited amount of this species, Cl2CH(O)Cl, was observed. However, upon removal of the chlorine scavenger, there was a significant increase in the concentration of Cl2CH(O)Cl, suggesting its formation through a Cl addition reaction such as pathway 2a of this work. A number of experimental studies propose this mechanism (pathway 2a) as one of the most significant pathways of the Cl-initiated TCE oxidation.9,10,12 Cl2CHCCl2O can also undergo fragmentation along the C-C bond.

(43) Cl2CHCCl2O f (14) C(O)Cl2 + (45) CHCl2

(52) The activation energy of this reaction is 5.4 kcal · mol-1, a slightly higher barrier than in reaction 51. The transition state is given in Figure 4u. CHCl2 is another carbon centered radical to which O2 can added, giving CHCl2O2 (species 46, Figure 3tt) (reaction 53).

(45) CHCl2 + O2 f (46) CHCl2O2

(53)

Reaction 54 will follow after, reducing the radical to CHCl2O (species 47, Figure 3 uu).

(46) CHCl2O2 + NO f (47) CHCl2O + NO2

(54)

CHCl2O (reaction 42) can undergo three reactions to end pathways 2b-c. Pathway 2b ends in the chlorine elimination to form another source of CH(O)Cl (reaction 55).

(47) CHCl2O f (8) CH(O)Cl + Cl

(55)

Finally, pathway 2c is a hydrogen abstraction that gives C(O)Cl2 (reaction 56).

(47) CHCl2O + O2 f (14) C(O)Cl2 + HO2

(56)

4. Discussion The initial reactions for the OH-initiated oxidation are given below (reactions 2 and 3).

(1) ClCHCCl2 + OH f (2) Cl2CHC(OH)Cl2

(2)

(1) ClCHCCl2 + OH f (18) ClCH(OH)CCl2

(3)

This study shows that reaction 2 and reaction 3 have activation barriers of 4.2 and 1.7 kcal · mol-1, respectively, suggesting reaction 3 will be a major pathway. This is in agreement with data indicating preferential addition to the lesssubstituted carbon. Since reaction 3 is preferred, oxidation reactions will follow along pathway 2; the potential energy surface for this pathway is shown in Figure 5b. The energetics indicate that HCl elimination (pathway 2b-c) from, and O2 addition (pathway 2d-h) to, ClCH(OH)CCl2 may both occur. However, literature data for a comparable system suggests that the energy barrier for the HCl elimination will be large.27 Additionally, it is well accepted that addition of O2 to form a

peroxy radical has almost no significant energy barrier.28-30 Therefore, it is expected that the HCl elimination will not contribute considerably, whereas the O2 addition reaction will be significant. Experimental studies, using chlorine scavengers to investigate the OH-initiated oxidation in isolation, indicate the major products to be C(O)Cl2 and, to a lesser degree, CH(O)Cl.12,16 As CH(O)Cl yield is minimal (5-7%), Nolan et al.16 propose that the HCl elimination pathway (pathway 2b-c) is dominant. Additionally, they suggest that the branching percentage of the C-Cl cleavage (pathway 2b) and the C-C fragmentation (pathway 2c) is 15-85%, respectively.16 This would result in significant formation of phosgene (from pathway 2c), and only a small amount of chlorine atoms (from pathway 2b). However, work by Tuazon et al.12 clearly shows that the OHinitiated oxidation of TCE results in the formation of chlorine atoms and subsequent initiation of the reaction between TCE and Cl. Detailed experiments show a significant change in product formations from the TCE + OH reaction when Cl scavengers are introduced.12 Reactions of TCE + OH in the absence of a chlorine scavenger indicate formation of significant amounts of CH(O)Cl and C(O)Cl2 with the primary product being dichloroacetyl chloride, Cl2CHC(O)Cl. Upon addition of a chlorine scavenger, the CH(O)Cl concentration drops and the C(O)Cl2 concentration increases. But most importantly is the significant drop in the production of Cl2CHC(O)Cl. This influence of absence of the chlorine scanvenger on the production of chloroacetly chlorides is demonstrated with a number of other chloroethylenes. Clearly, the reaction of TCE with OH results in the formation of chlorine atoms, which then go on to react with TCE, altering the product profile. Understanding the oxidation mechanism of OH reacting with TCE is important; the crucial point is the generation of chlorine atoms that will begin the Cl-initiated oxidation of TCE. This work shows a number of energetically favorable routes for the production of chlorine atoms from the reaction of TCE with OH including: 2b, 2d, and 2 h. The HCl elimination proposed as the primary route of OH-initiated oxidation by Nolan et al.16 does not indicate the production of a significant concentration of chlorine atoms. It is possible that the small concentration of CH(O)Cl that is formed upon addition of OH to TCE, in the presence of a Cl scavenger, may indicate that pathway 2d is not significant. But, other pathways after the O2 addition to ClCH(OH)CCl2, pathway 2d or 2 h, may contribute, and are sources of Cl atoms. It is also possible that, whatever the favored mechanism, pathways 2b, 2d, and 2 h combined contribute enough Cl atoms to initiate the Cl addition to TCE. The chlorine-initiated reactions have barriers much lower than the OH addition reactions. The formed Cl atoms will react with TCE with a much lower barrier than in the OH addition. Therefore, the products formed from the reaction of OH with TCE will have contributions from the products formed from the reaction of Cl with TCE. Reaction 6, addition of Cl to the least substituted carbon, and therefore pathway 2, will have an important contribution in the Cl-initiated oxidation. The potential energy surface for pathway 2 of the Cl-initiated oxidation is 15% and 85%, respectively. The lowest energy pathway, pathway 2a, results in the formation of dichloroacetyl chloride [Cl2CHC(O)Cl] and regeneration of the Cl atom. Tuazon et al.12 showed that when OH is allowed to react with TCE in the absence of a chlorine scavenger, the yield of dichloroacetyl chloride is as much as 50%. The results of Tuazon et al.12 are consistent with the findings of this work.

9176



Pathway 2b-c will also contribute significantly. The C-C bond breakage that occurs in reaction 52, results in the formation of phosgene C(O)Cl2, as well as CHCl2.

(43) Cl2CHCCl2O f (14) C(O)Cl2 + (45) CHCl2

(52) CHCl2 will lead to production of additional phosgene [C(O)Cl2], formyl chloride [CH(O)Cl], and also regeneration of the Cl atom. These product formations are mirrored in experimental studies.12 When Cl scavengers are removed, the addition of OH to TCE leads to a significantly increased formation of CH(O)Cl.12 Additionally, the removal of Cl scavengers results in the decreased yield of phosgene.12 This further indicates the importance of the TCE + Cl reaction pathway once it has been initiated by the TCE + OH reactions. However, to sustain the TCE + Cl reaction pathways, there must be a constant source of chlorine atoms. As discussed above, initially these Cl radicals are formed from the reactions of TCE + OH. Upon addition of chlorine to TCE, Figure 6 shows that two of the three major pathways (pathway 2a and 2b) result in the regeneration of the Cl atom. The addition of Cl to TCE is a low energy barrier process and is self-catalyzing, and therefore plays a major role in the oxidation of TCE. 5. Conclusions The reactions of the hydroxyl radical initiated oxidation include a number of low energy paths that lead to the formation of chlorine atoms. Activation energy calculations show that the barrier for the addition of a chlorine atom to TCE is significantly lower than the addition of the hydroxyl radical to TCE. Additionally, most of the energetically favorable pathways of the chlorine-initiated oxidation result in the reformation of the chlorine atom, constituting a self-catalyzing reaction. In combination with experimental work, this study suggests that the chlorine-initiated oxidation plays a major role in the oxidation of TCE. This ultimately results in the formation of dichloroacetyl chloride [Cl2CHC(O)Cl], phosgene [C(O)Cl2], formyl chloride [CH(O)Cl], and chlorine atoms, which are consistent with observed products from the experimental studies. Acknowledgment. The authors express thanks to the U.S. Department of Energy, Global Change Education Program for financial support through the Graduate Research Environmental Fellowship awarded to Carrie J. Christiansen. Supporting Information Available: Zero point corrected energies and vibrational frequencies of all species are available as supplementary tables. This information is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Molina, M. J.; Rowland, F. S. Nature 1974, 249, 810. (2) Rowland, F. S. Angew. Chem., Int. Ed. 1996, 35, 1786.

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