Atmospheric Oxidation of Tetrachloroethylene: An ... - ACS Publications

J. Phys. Chem. A 2010, 114, 9177–9191

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Atmospheric Oxidation of Tetrachloroethylene: 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 28, 2010; ReVised Manuscript ReceiVed: July 5, 2010

A number of experimental studies have been conducted to determine the atmospheric oxidation of tetrachloroethylene, many indicating phosgene as the major product. Although various mechanisms have been suggested, the mechanism of phosgene production is unclear. Additionally, confusion has arisen over the role chlorine atoms may play in the oxidation of tetrachloroethylene and the products produced. To clarify these points, this study presents a comprehensive computational study of both the hydroxyl radical and the chlorine atom initiated atmospheric oxidation mechanism of tetrachloroethylene. The energetics for the oxidation of tetrachloroethylene (C2Cl4) are computed using ab initio methods. Potential energy surfaces of the reaction pathways are determined from the computations. This study clarifies the involvement of the Cl-initiated reaction pathways in the oxidation of tetrachloroethylene. Results from this work suggest that the final products are primarily from the Cl-initiated oxidation and include: trichloroacetyl chloride [ClC(O)CCl3], phosgene [C(O)Cl2], and regeneration of the initiating chlorine atom. 1. Introduction The study of chlorinated compounds has been of particular interest since the 1970s, when Rowland and Molina presented the work detailing the ability of chlorine atoms to catalyze the destruction of stratospheric ozone.1 The depletion of ozone by halogenated hydrocarbons, particularly cholorofluorocarbons (CFCs), has since been well documented.1–4 Tetrachloroethylene (C2Cl4), also known as perchloroethylene (PERC), has been used extensively in dry cleaning, for metal degreasing, and in industrial uses.5,6 During the 1980s, emission rates averaged as much as 600 ktons · yr-1.5 Although they have decreased, levels are still high. More recently, the global emission rates have been estimated between 200 and 300 ktons · yr-1.7,8 As hydroxyl radical addition is thought to be the primary route of oxidation of PERC, the lifetime of C2Cl4 has been calculated primarily on the basis of its reaction with OH and is determined to be 0.4 years.5–7,9,10 A number of experimental studies examining the hydroxyl radical initiated oxidation have been conducted.11–17 Reaction 1 shows the initial step of this oxidation mechanism. This reaction has a rate constant of 3.5 × 10-12e(-920/T) cm3 · molecules-1 · s-1.18

(1) Cl2CCCl2 + OH f (2) Cl2C(OH)CCl2

(1)

A number of proposed reaction mechanisms detailing PERC oxidation, initiated by the addition of OH, are given in the literature. However, the identity of the resulting products are not fully understood. A number of studies do indicate that PERC is a major contributor to tropospheric phosgene.19–21 One recent study determined a 70% yield of phosgene from the OH-initiated oxidation of PERC.17 It is important to note that this determination was done in the presence of a chlorine scavenger. Although yields were not given, the presence of trichloroacetyl chloride was observed when the scavenger was absent. Another study indicates a yield of 0.47 phosgene upon the addition of OH into PERC.16 Trichloroacetyl chloride yield was low, less

than 0.15, when in the presence of a chlorine scavenger.16 The yield rose to as much as 0.41 when the chlorine scavenger was removed, indicating the formation of trichloroacetyl chloride through reactions involving chlorine atoms.16 This interaction of chlorine atoms, likely with the parent species, significantly complicates the degradation pathway. Kinetics data further implicate the importance of the chlorine atom initiated oxidation. The rate of addition of Cl to PERC, reaction 2, is 200-300 times faster than the reaction with OH.22,23

(1) Cl2CCCl2 + Cl f (27) Cl2CCCl3

(2)

The fundamentals of the reaction mechanism of C2Cl4 + Cl were proposed over 40 years ago.24 Since then, a number of studies have considered this mechanism experimentally.23–27 Experimentally determined rates for a number of reactions within this oxidation scheme are compiled from the literature by Olkhov and Smith.27 The rate of the chlorine addition to PERC (reaction 2) has been observed at 2.2 × 10-11 cm3 · molecule-1 · s-1.28 The fast rate of Cl addition, as well as the influence chlorine scavengers have on product production in past experimental studies, requires that a complete understanding of the oxidation of PERC in the atmosphere include an investigation of the Clinitiated oxidation as well as the OH-initiated oxidation pathways. Therefore, a computational study of previously proposed and new reaction pathways resulting from the addition of OH and Cl into PERC is presented. This study, in combination with experimental work, allows for a better understanding of the influence of the chlorine atom, origin of products, and better understanding of the mechanism of production of the observed products, phosgene and trichloroacetyl chloride. 2. Computational Methods All computations are performed using the Gaussian 03 suite of programs.29 Optimized geometries and corresponding energies

10.1021/jp103845h  2010 American Chemical Society Published on Web 07/29/2010

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Christiansen and Francisco

Figure 1. Hydroxyl radical initiated oxidation mechanism of tetrachloroethylene. Reactions highlighted in red are unlikely to occur due to high energetic barriers.

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 thermochemistry, including the zero point energy (ZPE) correction. The MP2/6-311+G(2d) optimized geometries are used to compute single point energies with the following methods and basis set combinations: MP4/6-311+G(2d), MP4/6-311+G(2df), CCSD(T)/6-311+G(2d), and CCSD(T)/6-311+G(2df). All energies are corrected with the MP2/6-31G(d) level of theory ZPE correction to obtain total energies. Finally, using corrected energies, the enthalpy and activation energy barriers are calculated for individual reactions throughout the oxidation pathways. 3. Results 3.1. OH-Initiated Oxidation. The complete OH-initiated atmospheric oxidation of PERC is given in Figure 1. Pathways with high energy barriers are highlighted in red. The overall reaction mechanism has been separated into four major reaction pathways. These pathways are depicted individually in Figure 2a-d. The OH oxidation of PERC starts by addition of a hydroxyl radical onto one of the two equivalent carbons of PERC, Cl2CCCl2 (species 1), to form the radical Cl2C(OH)CCl2 (species 2). Pathway 1 (Figure 2a) begins with the chlorine shift isomerization of Cl2C(OH)CCl2 (species 2) to form ClC(OH)CCl3 (species 3). ClC(OH)CCl3 (species 3) can undergo a hydrogen abstraction to form trichloroacetyl chloride (species 4). ClC(OH)CCl3 (species 3) can also continue on to ClC(OH)OCCl3 (species 6). ClC(OH)OCCl3 (species 6) has three possible branches: (1) OH elimination to form ClC(O)CCl3 (species 4); (2) chlorine elimination to create C(O)(OH)CCl3 (species 7); (3) C-C bond fragmentation that leads to ClC(O)(OH) (species 8) and CCl3 (species 9). CCl3 (species 9) will eventually lead to the production of phosgene (species 12).

Pathway 2 (Figure 2b) comprises two branches from Cl2C(OH)CCl2 (species 2). First, a chlorine elimination from Cl2C(OH)CCl2 (species 2) and subsequent isomerization leads to the product ClC(O)CHCl2 (species 14). Second, an isomerization of Cl2C(OH)CCl2 (species 2) creates Cl2COCHCl2 (species 15). Pathway 3, seen in Figure 2c, begins with the elimination of an HCl molecule from Cl2C(OH)CCl2 (species 2) to form ClC(O)CCl2 (species 16). Upon O2 addition and reduction, ClC(O)CCl2 (species 16) is converted into ClC(O)CCl2O (species 18). Through the elimination of both chlorine and carbon monoxide or a chlorine atom, ClC(O)CCl2O (species 18) will form phosgene and ClC(O)C(O)Cl (species 19). Pathway 4, shown in Figure 2d, starts with the addition of O2 to Cl2C(OH)CCl2 (species 2) and subsequent reduction leading to Cl2C(OH)CCl2O (species 21). This radical can undergo two reactions. First, the elimination of a chlorine atom eventually leads to ClC(O)C(O)Cl (species 19). The second reaction undergone by Cl2C(OH)CCl2O (species 21) is fragmentation to create Cl2C(OH) (species 24) and phosgene (species 12). Cl2C(OH) (species 24) can undergo a hydrogen abstraction to form phosgene. Additionally, Cl2C(OH) (species 24) can undergo O2 addition, reduction, and OH elimination, leading to another molecule of C(O)Cl2 (species 12). Optimized geometries of all reactant, reactive intermediates, and products in both the OH and Cl-initiated pathways are shown in Figure 3. Figure 4 shows optimized geometries for the computed transition states. Energies for all stable species involved in the oxidation mechanism are given in Supplementary Table 1 (Supporting Information). All values are corrected using the MP2/6-31G(d) zero point energy. Vibrational frequencies for these species are shown in Supplementary Table 2 (Supporting Information). Corresponding values for transition state species are given in Supplementary Tables 3 and 4 (Supporting Information). Using these values, enthalpies for all reactions are given in Table 1. Finally, Table 2 shows activation energies for selected reactions in the degradation pathways.

Figure 2. Various pathways in the hydoxyl radical initiated oxidation mechanism of tetrachloroethylene: (a) pathway 1; (b) pathway 2; (c) pathway 3; (d) pathway 4.

Atmospheric Oxidation of Tetrachloroethylene J. Phys. Chem. A, Vol. 114, No. 34, 2010 9179

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Figure 3. Continued.


Atmospheric Oxidation of Tetrachloroethylene

J. Phys. Chem. A, Vol. 114, No. 34, 2010 9181

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

Using these calculated enthalpies and activation energies, the potential energy surfaces for the OH-initiated oxidation are shown in Figure 5. 3.1.1. Initial Reaction. The optimized geometry of tetrachloroethylene, Cl2CCCl2 (species 1, Figure 3a) is a planar, symmetric molecule. The C-C bond length is 1.344 Å and all C-Cl bonds are 1.717 Å. All C-C-Cl angles are 122.1°, and the Cl-C-Cl angles are 115.7°. The oxidation of PERC begins with the addition of OH to Cl2CCCl2 (species 1) to create Cl2C(OH)CCl2 (species 2) (reaction 1).


(1)

Throughout this work, the carbon to which the OH (or Cl) is added, will be referred to as carbon 1. The neighboring carbon will be designated carbon 2. As mentioned previously, the kinetics of this reaction have been studied experimentally.18 Additionally, theoretical studies of similar systems, such as the addition of an OH radical to propene and ethene, have been conducted.30,31 The activation energy of reaction 1 is 3.6 kcal · mol-1 with an enthalpy of -39.0 kcal · mol-1. The product, Cl2C(OH)CCl2 (species 2, Figure 3b), has a C-C bond length of 1.490 Å, approximately 0.15 Å longer than in Cl2CCCl2 (species 1), indicating a shift to single C-C bond character as a result of the added substituent. The corresponding transition state for this reaction is given in Figure 4a. The imaginary vibrational frequency corresponding to the C-OH bond stretch is 755i cm-1, with the C-OH bond length at 2.004 Å. Zhou et al. report a comparable C-OH bond length of 2.071 Å.31 Sosa et al. reported a transition state C-OH bond of 2.2 Å for addition to ethene.30 3.1.2. Pathway 1. The reaction mechanism for pathway 1 is given in Figure 2a. Following addition of the OH to create Cl2C(OH)CCl2 (species 2), an isomerization can occur. A chlorine shifts from carbon 1 to carbon 2, creating ClC(OH)CCl3 (species 3, Figure 3c) (reaction 3).

(2) Cl2C(OH)CCl2 f (3) ClC(OH)CCl3

(3)

Sosa et al. proposed and computationally investigated an analogous isomerization reaction, and subsequent steps, in the C2H4 system.30 Our calculated enthalpy of reaction 3 is 4.4

kcal · mol-1 with a barrier of 13.5 kcal · mol-1. The transition state for this reaction is given in Figure 4b and has a C-Cl bond length of 2.336 Å and a imaginary frequency of 476i cm-1. In species 2, carbon 2 has a nonplanar geometry, approaching tetrahedral. The Mulliken atomic spin density of carbon 2 is 0.953, indicating that a large portion of the radical resides on this carbon. After isomerization of the chlorine, the electron distribution has shifted to carbon 1. Carbon 1 now has a Mulliken spin density of 0.934. It is also important to note the change in carbon-chlorine bond lengths. The chlorines bonded to carbon 2 have bond lengths of 1.702 and 1.700 Å in Cl2C(OH)CCl2 (species 2). Upon isomerization, these chlorines now have bond lengths of 1.777 and 1.772 Å. With the additional substituent, electron repulsion requires the lengthening of these bonds. Conversely, the bond length of the chlorine bonded to carbon 1 goes from 1.801 to 1.736 Å. From this point, ClC(OH)CCl3 (species 3) has two possible reactions. Branch a of pathway 1, reaction 4, is the hydrogen abstraction of ClC(OH)CCl3 (species 3) by O2, to form ClC(O)CCl3 (species 4).

(3) ClC(OH)CCl3 + O2 f (4) ClC(O)CCl3 + HO2

(4) Reaction 4 has an enthalpy of -21.7 kcal · mol-1. The next reaction from ClC(OH)CCl3 (species 3) is the addition of molecular oxygen to form the peroxy radical ClC(OH)O2CCl3 (species 5, Figure 3e), reaction 5.

(3) ClC(OH)CCl3 + O2 f (5) ClC(OH)O2CCl3

(5)

Reaction 5 has an enthalpy of -27.4 kcal · mol-1. The addition of O2 transforms the geometry on carbon 1 to tetrahedral and shifts the electron density of the radical to the outermost oxygen. O2 addition is followed by a reduction by NO forming ClC(OH)OCCl3 (species 6, Figure 3f), with an enthalpy of -16.5 kcal · mol-1 (reaction 6).

(5) ClC(OH)O2CCl3 + NO f (6) ClC(OH)OCCl3 + NO2 (6)

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

ClC(OH)OCCl3 (species 6) has three possible reaction branches. Pathway 1b ends in an OH group elimination from ClC(OH)OCCl3 (species 6) to form another molecule of ClC(O)CCl3 (species 4) (reaction 7).

(6) ClC(OH)OCCl3 f (4) ClC(O)CCl3 + OH

(7)

The energy barrier of this reaction is 21.6 kcal · mol-1 with an enthalpy of 12.4 kcal · mol-1. The transition state has a frequency of 919i cm-1 corresponding to the C-OH stretch. The optimized

structure for this elimination shows that a C-OH stretch at 1.743 Å with flattening bond angles on carbon 1 (Figure 4c). Ending pathway 1c, a chlorine elimination from ClC(OH)OCCl3 (species 6) creates C(O)(OH)CCl3 (species 7, Figure 3g), a halogenated organic acid (reaction 8).

(6) ClC(OH)OCCl3 f (7) C(O)(OH)CCl3 + Cl

(8)

The C-Cl stretch in the transition state, shown in Figure 4d, has an imaginary frequency of 749i cm-1 and a length of 2.010



TABLE 1: Enthalpy of Reactions (kcal · mol-1) Corrected with Zero Point Energy 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)

-39.0

-36.2

-39.1

-36.0

-39.0

OH-Initiated Pathway 1

(2) Cl2C(OH)CCl2 f (3) ClC(OH)CCl3 (3) ClC(OH)CCl3 + O2 f (4) ClC(O)CCl3 + HO2 (3) ClC(OH)CCl3 + O2 f (5) ClC(OH)O2CCl3 (5) ClC(OH)O2CCl3 + NO f (6) ClC(OH)OCCl3 + NO2 (6) ClC(OH)OCCl3 f (4) ClC(O)CCl3 + OH (6) ClC(OH)OCCl3 f (7) C(O)(OH)CCl3 + Cl (6) ClC(OH)OCCl3 f (8) ClC(O)(OH) + (9)CCl3 (9) CCl3 + O2 f (10) CCl3O2 (10) CCl3O2 + NO f (11) CCl3O + NO2 (11) CCl3O f (12) C(O)Cl2 + Cl (2) Cl2(OH)CCl2 f (13) ClC(OH)CCl2 + Cl (13) ClC(OH)CCl2 f (14) ClC(O)CHCl2 (2) Cl2C(OH)CCl2 f (15) Cl2COCHCl2

4.1

3.9

4.5

4.4

-21.5

-21.6

-21.7

-21.7

-22.3

-23.3

-25.9

-25.0

-27.4

-23.2

-21.9

-22.7

-15.8

-16.5

7.9

6.6

9.0

10.1

12.4

-19.6

-19.4

-17.2

-16.0

-14.3

-21.1

-20.2

-20.1

-18.7

-18.9

-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



(2) Cl2C(OH)CCl2 f (16) ClC(O)CCl2 + HCl (16) ClC(O)CCl2 + O2 f (17) ClC(O)CCl2O2 (17) ClC(O)CCl2O2 + NO f (18) ClC(O)CCl2O + NO2 (18) ClC(O)CCl2O f (12) C(O)Cl2 + Cl + CO (18) ClC(O)CCl2O f (19) ClC(O)C(O)Cl + Cl


(2) Cl2C(OH)CCl2 + O2 f (20) Cl2C(OH)CCl2O2 (20) Cl2C(OH)CCl2O2 + NO f (21) Cl2C(OH)CCl2O + NO2 (21) Cl2C(OH)CCl2O f (22) Cl2C(OH)C(O)Cl + Cl (22) Cl2C(OH)C(O)Cl + OH f (23) Cl2COC(O)Cl + H2O (23) Cl2COC(O)Cl f (19) ClC(O)C(O)Cl + Cl (21) Cl2C(OH)CCl2O f (24) Cl2C(OH) + (12) C(O)Cl2 (24) Cl2C(OH) + O2 f (12) C(O)Cl2 + HO2 (24) Cl2C(OH) + O2 f (25) Cl2C(OH)O2 (25) Cl2C(OH)O2 + NO f (26) Cl2C(OH)O + NO2 (26) Cl2C(OH)O f (12) C(O)Cl2 + OH (1) Cl2CCCl2 + Cl f (27) Cl2CCCl3 (27) Cl2CCCl3 + O2 f (28) Cl2CO2CCl3 (28) Cl2CO2CCl3 + NO f (29) Cl2COCCl3 + NO2 (29) Cl2COCCl3 f (4) ClC(O)CCl3 + Cl (29) Cl2COCCl3 f (9) CCl3 + (12) C(O)Cl2 (9) CCl3 + O2 f (10) CCl3O2 (10) CCl3O2 + NO f (11) CCl3O + NO2 (11) CCl3O f (12) C(O)Cl2 + Cl

3.9 -18.3

Cl-Initiated Pathway 1


Å. The enthalpy of the reaction is -14.3 kcal · mol-1. At lower levels of theory, for example, at the MP2/6-311+G(2d) level, the activation energy barrier for reaction 8 is 1.9 kcal · mol-1. The barrier is reduced to 1.1 kcal · mol-1 for MP4/6-311+G(2d). Increasing the basis set and level of theory to the CCSD(T)/6311+G(2df) correlation method underestimates the barrier, resulting in -0.2 kcal · mol-1 activation energy. This suggests that within the uncertainty of the calculation the reaction is nearly barrierless. As such, reaction 8 likely has the lowest barrier of all three reactions stemming from ClC(OH)OCCl3 (species 6).

19.3

17.2

20.4

17.3

20.1

-18.3

-20.4

-19.9

-18.8

-18.2

18.3

14.4

15.0

12.8

13.5

-2.5

-6.5

-5.6

-8.0

-7.3

-11.7

-12.6

-14.7

-11.8

-13.7

-25.2

-23.8

-24.5

-17.6

-18.2

-19.9

-22.6

-17.2

-19.4

-14.6

-18.6

-18.3

-15.3

-14.9

-12.4

-15.6

-16.4

-18.9

-17.7

-20.0

-24.9

-23.5

-24.2

-17.4

-17.9

-17.2

-17.6

-14.5

-14.4

-11.7

-3.5

-4.6

-4.5

-6.4

-6.2

-17.8

-17.4

-14.4

-14.1

-11.5

-15.1

-14.8

-13.8

-12.8

-12.1

-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

-14.5

-12.5

-16.6

-12.2

-15.9

-15.9

-16.7

-19.3

-17.7

-20.1

-25.2

-23.9

-24.6

-17.9

-18.5

-17.1

-17.6

-14.4

-14.4

-11.6

-15.4

-15.6

-14.6

-14.8

-14.3

-16.9

-18.1

-20.2

-19.4

-21.2

-25.5

-24.1

-24.6

-18.1

-18.5

-26.0

-25.4

-22.0

-21.7

-18.7

Finally, in pathway 1d, ClC(OH)OCCl3 (species 6) can fragment at the C-C bond, as shown in reaction 9.

(6) ClC(OH)OCCl3 f (8) ClC(O)(OH) + (9) CCl3

(9) This fragmentation has an activation energy of 5.8 kcal · mol-1. The enthalpy is -18.9 kcal · mol-1. The transition state (Figure 4e) has an imaginary frequency of 1167i cm-1 corresponding to the C-C stretch. This stretch has a bond

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length of 1.838 Å. Upon fragmentation, the C-O bond length decreased from 1.356 to 1.194 Å, resulting in a planar molecule, carbonochloridic acid, ClC(O)(OH) (species 8, Figure 3h). Also formed in the fragmentation is CCl3 (species 9, Figure 3i). CCl3 (species 9) undergoes further reactions; O2 adds to produce a peroxy radical, CCl3O2 (species 10) (reaction 10).

(9) CCl3 + O2 f (10) CCl3O2

(10)

This product (Figure 3j) is a tetrahedral peroxy radical. The carbon-substituent angles range from 103.0° to 111.6°. The enthalpy for this reaction is -21.2 kcal · mol-1. The CCl3O2 (species 10) species then undergoes an oxygen abstraction (reaction 11).

(10) CCl3O2 + NO f (11) CCl3O + NO2

(11)

The peroxy radial is converted into an alkoxy radical. This reaction has an enthalpy of -18.5 kcal · mol-1. The formed CCl3O (species 11, Figure 3k) does not change in geometry significantly from the reactant. Again, the major difference comes in the C-O bond length, which decreases from 1.449 to 1.333 Å. The final step of the oxidation mechanism is the extrusion of a chlorine atom from CCl3O (species 11), resulting in the formation of a second phosgene molecule (reaction 12).

(11) CCl3O f (12) C(O)Cl2 + Cl

(12)

The optimized geometry of the transition state is shown in Figure 4f. The C-Cl stretch has a vibrational frequency of 903i cm-1. The transition state geometry shows a C-Cl bond length of 2.015 Å. This elimination has a small activation energy of 1.5 kcal · mol-1. The enthalpy of the reaction is calculated to be -18.7 kcal · mol-1. The product, phosgene (Figure 3l), 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. Phosgene has been observed experimentally as a major product in the oxidation of PERC,16,17 but this pathway is just one of many sources of phosgene in this degradation mechanism. Reaction 12 was considered previously by Li and Francisco.32 Using the UMP2/6-31G(d) level of theory, they calculated the enthalpy of the reaction to be -25.9 kcal · mol-1 with an activation energy of 10.2 kcal · mol-1. Although values discussed from the present study are at the CCSD(T)/6-311+G (2df) level of theory, MP2/6-31G(d) were calculated. The enthalpy at this level of theory is -26.0 kcal · mol-1, consistent with the Li and Francisco results. 3.1.3. Pathway 2. Pathway 2 (Figure 2b) is made up of two small branches. The first branch of this pathway, 2a, was proposed and explored experimentally by Nolan et al.17 Additionally, Sosa et al. computationally investigated a set of analogous reactions in the C2H4 system.30 The first reaction is a chlorine elimination from carbon 1 of Cl2C(OH)CCl2 (species 2) to form ClC(OH)CCl2 (species 13, Figure 3m) (reaction 13).

TABLE 2: Activation Energies for Select Reactions (kcal · mol-1) Corrected with Zero Point Energy reaction

[(1) Cl2CCCl2 + OH f (2) Cl2C(OH)CCl2]‡

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)

13.7

11.6

10.4

4.6

3.6

14.7

14.0

15.3

12.3

13.5

24.3

21.8

22.7

20.8

21.6

1.9

1.1

1.2

-0.3

-0.2

9.7

9.0

8.0

6.9

5.8

3.8

2.8

3.5

0.8

1.5

56.2

54.6

54.9

56.4

56.8

48.9

45.0

44.9

43.6

43.6

6.1

4.7

5.5

2.0

2.6

6.7

5.3

6.1

2.9

3.6

8.6

8.0

8.4

6.2

6.9

6.8

5.6

6.3

2.8

3.4

5.7

5.0

4.4

2.3

1.7

21.6

19.7

21.7

21.1

23.3

3.3

3.8

1.9

0.4

-1.3

6.0

4.6

5.4

2.3

3.0

9.0

8.0

7.4

5.3

4.6

3.8

2.8

3.5

0.8

1.5


[(2) Cl2C(OH)CCl2 f (3) ClC(OH)CCl3]‡ [(6) ClC(OH)OCCl3 f (4) ClC(O)CCl3 + OH]‡ [(6)ClC(OH)OCCl3 f (7)C(O)(OH)CCl3 + Cl]‡ [(6) ClC(OH)OCCl3 f (8) ClC(O)(OH) + (9) CCl3]‡ [(11) CCl3O f (12) C(O)Cl2 + Cl]‡ OH-Initiated Pathway 2

[(13) ClC(OH)CCl2 f (14) ClC(O)CHCl2]‡ [(2) Cl2C(OH)CCl2 f (15) Cl2COCHCl2]‡ OH-Initiated Pathway 3

[(18) ClC(O)CCl2O f (19) ClC(O)C(O)Cl + Cl]‡ OH-Initiated Pathway 4

[(21) Cl2C(OH)CCl2O f (22) Cl2C(OH)C(O)Cl + Cl]‡ [(22) Cl2C(OH)C(O)Cl + OH f (23) Cl2COC(O)Cl + H2O]‡ [(23) Cl2COC(O)Cl f (19) ClC(O)C(O)Cl + Cl]‡ [(21) Cl2C(OH)CCl2O f (24) Cl2C(OH) + (12) C(O)Cl2]‡ [(26) Cl2C(OH)O f (12) C(O)Cl2 + OH]‡ Cl-Initiated Pathway 1

[(1) Cl2CCCl2 + Cl f (27) Cl2CCCl3] [(29) Cl2COCCl3 f (4) ClC(O)CCl3 + Cl]‡ ‡


[(29) Cl2COCCl3 f (9) CCl3 + (12) C(O)Cl2]‡ [(11) CCl3O f (12) C(O)Cl2 + Cl]‡



Figure 5. Potential energy surfaces for pathways in the OH-initiated tetrachloroethylene oxidation: (a) pathway 1; (b) pathway 3; (c) pathway 4. Energy units in kcal · mol-1. Values calculated using values computed at the CCSD(T)/6-311+G(2df) level.

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(2) Cl2C(OH)CCl2 f (13) ClC(OH)CCl2 + Cl

(13) The enthalpy of this reaction is significantly high at 20.1 kcal · mol-1. This planar species has a double C-C bond characteristic, with a bond length of 1.343 Å. The C1-Cl bond length decreases from 1.801 to 1.712 Å upon elimination of the neighboring chlorine. Next, the hydrogen of the OH group transfers to carbon 2 in an isomerization of ClC(OH)CCl2 (species 13) to give ClC(O)CHCl2 (species 14, Figure 3n) (reaction 14).

(13) ClC(OH)CCl2 f (14) ClC(O)CHCl2

(14)

The transition state for this isomerization (Figure 4g) shows an O-H bond at 1.200 Å and a C-H bond of 1.609 Å with an O-H-C angle of 103.6°. Additionally, the C-C bond length has begun to stretch, at 1.446 Å in the transition state. As expected, the C-O bond length of the product decreases, from 1.346 to 1.192 Å. The Cl-C1-C2 bond angle has decreased from 122.4° to 114.6°. Additionally, the C-C bond length increases from 1.343 to 1.519 Å. Although the enthalpy of this reaction is exothermic, -18.2 kcal · mol-1, the activation barrier is large, 58.6 kcal · mol-1. With a significant endothermic enthalpy in the first step and a large barrier in the second step, this reaction pathway is unlikely. This pathway was proposed by Nolan et al.,17 but no ClC(O)CHCl2 (species 14) was observed experimentally. Given these energetics, this result is understandable. Pathway 2b is an isomerization, reaction 15, of Cl2C(OH)CCl2 (species 2) to form Cl2COCHCl2 (species 15, Figure 3o).

(2) Cl2C(OH)CCl2 f (15) Cl2COCHCl2

(15)

producing ClC(O)CCl2O2 (species 17, Figure 3q) (reaction 17), with an enthalpy of -13.7 kcal · mol-1.

(16) ClC(O)CCl2 + O2 f (17) ClC(O)CCl2O2

(17) Bond lengths of neighboring chlorines increase from 1.687 and 1.685 Å to 1.751 and 1.766 Å to accommodate the O2 substituent. As shown in reaction 18, a reduction of the peroxy radical to an alkoxy radical, ClC(O)CCl2O (species 18, Figure 3r) (reaction 18), occurs with an enthalpy of -18.2 kcal · mol-1.

(17) ClC(O)CCl2O2 + NO f (18) ClC(O)CCl2O + NO2 (18) Two possible reactions can occur from ClC(O)CCl2O (species 18). First, the elimination of both chlorine and carbon monoxide produces another molecule of phosgene (species 12, Figure 3l) as the final product of pathway 3a (reaction 19).

(18) ClC(O)CCl2O f (12) C(O)Cl2 + Cl + CO

(19) This fragmentation has an enthalpy of -14.6 kcal · mol-1. Pathway 3b ends as shown in reaction 20, the elimination of a chlorine atom from ClC(O)CCl2O (species 18) to form ClC(O)C(O)Cl (species 19).

(18) ClC(O)CCl2O f (19) ClC(O)C(O)Cl + Cl

(20)

A reaction pathway in the C2H4 system, analogous to this one, was proposed by Sosa et al.30 The geometry of the produced molecule, Cl2COCHCl2 (species 15), is only slightly changed from the reactant. Most significant is the shift of electron density from carbon 2 to the alkoxy radical oxygen. In Cl2C(OH)CCl2 (species 2) the Mulliken atomic spin density of carbon 2 is 0.953 and virtually zero on the oxygen; whereas the oxygen has a spin density of 0.974 in Cl2COCHCl2 (species 15). The transition structure (Figure 4h), with vibrational frequency of 2514i cm-1, has a O-H bond length of 1.321 Å and the H-C bond is 1.282 Å with an O-H-C angle of 103.2°. The enthalpy of this reaction is 13.5 kcal · mol-1 with a large energetic barrier of 43.6 kcal · mol-1. Due to this very large barrier, it is unlikely that this pathway will have any significant competition with the other pathways stemming from Cl2C(OH)CCl2 (species 2). 3.1.4. Pathway 3. The reactions in pathway 3 (Figure 2c) were presented and studied experimentally by Nolan et al.17 The mechanism begins with the elimination of an HCl molecule from Cl2C(OH)CCl2 (species 2) to form ClC(O)CCl2 (species 16, Figure 3p) (reaction 16).

The corresponding transition state (Figure 4i) has an imaginary vibrational frequency of 872i cm-1. The energetics of this reaction show a small barrier of 2.6 kcal · mol-1 and an enthalpy of -12.4 kcal · mol-1. This structure has a C-Cl elimination bond length of 2.051 Å and the angles surrounding carbon 2 are flattening, approaching trigonal planar. ClC(O)C(O)Cl (species 19, Figure 3s) is a planar, symmetric molecule with a C-C bond length of 1.546 Å, C-O bond lengths of 1.190 Å, and C-Cl bond lengths of 1.755 Å. It has O---C angles of 124.3°, Cl-C-C angles of 111.7°, and O-C-Cl angles of 124.0°. A brominated analog of this molecule has been identified as a product in the atmospheric degradation of 1,2-dibromoethane by Christiansen and Francisco.33 The geometry of this species is quite similar to the geometry of the brominated analog, the only significant difference being in the bond lengths of the halogen substituents. Additionally, this product, as well as the brominated analog, has been presented as a possible precursor to the formation of atmospheric oxalic acid, a secondary organic aerosol precursor.34 3.1.5. Pathway 4. Pathway 4, shown in Figure 2d, starts with the addition of O2 onto carbon 2 of Cl2C(OH)CCl2 (species 2) to form Cl2C(OH)CCl2O2 (species 20, Figure 3t) (reaction 21).

(2) Cl2C(OH)CCl2 f (16) ClC(O)CCl2 + HCl

(2) Cl2C(OH)CCl2 + O2 f (20) Cl2C(OH)CCl2O2

(16)

(21)

The enthalpy of this reaction is -7.3 kcal · mol-1. This planar molecule has the radical centered on carbon 2, indicated by a Mulliken atomic spin density of 0.868. ClC(O)CCl2 (species 16) will undergo an exothermic O2 addition onto carbon 2,

This oxygen addition is exothermic with an enthalpy of -20.0 kcal · mol-1. Cl2C(OH)CCl2O2 (species 20) now has a tetrahedral geometry on each carbon and a radical electron centered on the outer oxygen of the peroxy radical. As expected, this species



can undergo reduction, through an NO/NO2 conversion to create Cl2C(OH)CCl2O (species 21, Figure 3u) (reaction 23), with an enthalpy of -17.9 kcal · mol-1.

(20) Cl2C(OH)CCl2O2 + NO f (21) Cl2C(OH)CCl2O + NO2 (22) Other than the shortening of the C2-O bond, from 1.443 to 1.346 Å, the geometry of this species is quite similar to that of Cl2C(OH)CCl2O2 (species 20). This alkoxy radical can undergo two reactions. The first reaction path stems from Cl2C(OH)CCl2O2 (species 20), comprising pathway 4a, was proposed in 1976 by Howard as a method of ClC(O)C(O)Cl production from C2Cl4.14 It begins, as shown in reaction 23, with the elimination of a chlorine atom from Cl2C(OH)CCl2O (species 21) to create Cl2C(OH)C(O)Cl (species 22, Figure 3v).

(21) Cl2C(OH)CCl2O f (22) Cl2C(OH)C(O)Cl + Cl (23) The transition state of this elimination is depicted in Figure 4j, showing a C-Cl bond length of 2.056 Å. The vibrational frequency associated with the C-Cl stretch is 864i cm-1. The enthalpy of the reaction is -11.7 kcal · mol-1, and the activation energy is 3.6 kcal · mol-1. The formed product, Cl2C(OH)C(O)Cl (species 22), is a closed shell system with a hydroxyl group on one carbon and a carbonyl on the opposite carbon. Although Cl2C(OH)C(O)Cl (species 22) is a somewhat stable, closed shell system, it can undergo a hydrogen abstraction from the OH group, to create Cl2COC(O)Cl (species 23, Figure 3w) (reaction 24).

(22) Cl2C(OH)C(O)Cl + OH f (23) Cl2COC(O)Cl + H2O (24) The enthalpy of this abstraction is -6.2 kcal · mol-1 with an activation energy of 6.9 kcal · mol-1. The transition state is shown in Figure 4k. Cl2COC(O)Cl (species 23) shows an increased C-C-O angle: from 108.9° in Cl2C(OH)C(O)Cl (species 22) to 113.7° in Cl2COC(O)Cl (species 23). The final reaction in this pathway is another chlorine elimination and production of ClC(O)C(O)Cl (species 19, Figure 3s) (reaction 25).

(23) Cl2COC(O)Cl f (19) ClC(O)C(O)Cl + Cl

(25) The enthalpy of this reaction is -11.5 kcal · mol-1 with an activation energy of 3.4 kcal · mol-1. The transition state shows a C-Cl bond length of 2.050 Å (Figure 4l), with a corresponding imaginary frequency of 872i cm-1. The product, ClC(O)C(O)Cl (species 19), has been discussed as a product of pathway 3. The second possible reaction that Cl2C(OH)CCl2O (species 21) undergoes is a fragmentation to create Cl2C(OH) (species 24, Figure 3x) and phosgene, C(O)Cl2 (species 12, Figure 3l) (reaction 26).

(21) Cl2C(OH)CCl2O f (24) Cl2C(OH) + (12) C(O)Cl2 (26)

The activation energy of this reaction is 1.7 kcal · mol-1 and has an enthalpy of -12.1 kcal · mol-1. The imaginary frequency is 970i cm-1. The fragmentation transition state (Figure 4m) shows the C-C bond length at 1.836 Å and a flattening of the geometry surrounding each carbon. Cl2C(OH) (species 24) has three possible reaction pathways. Pathway 4b results in the formation of another molecule of phosgene. Cl2C(OH) (species 24) can undergo a hydrogen abstraction from Cl2C(OH) (species 24) to form C(O)Cl2 (reaction 27), with an enthalpy of -28.5 kcal · mol-1.

(24) Cl2C(OH) + O2 f (12) C(O)Cl2 + HO2

(27)

Finally, O2 can add to Cl2C(OH) (species 24) (reaction 28), forming Cl2C(OH)O2 (species 25, Figure 3y), followed by reduction (reaction 29) to the alkoxy radical, Cl2C(OH)O (species 26, Figure 3z).

(24) Cl2C(OH) + O2 f (25) Cl2C(OH)O2

(28)

(25) Cl2C(OH)O2 + NO f (26) Cl2C(OH)O + NO2 (29) Reactions 28 and 29 have enthalpies of -25.5 and -18.4 kcal · mol-1, respectively. Pathway 4c ends in reaction 30, the elimination of an OH radical from Cl2C(OH)O (species 26), giving phosgene once more.

(26) Cl2C(OH)O f (12) C(O)Cl2 + OH

(30)

Although the barrier of this reaction is not small, 23.3 kcal · mol-1, it is likely to proceed using energy released in the previous step. The enthalpy of reaction 30 is 5.7 kcal · mol-1, and the imaginary frequency is 744i cm-1. The transition state for this reaction is also given (Figure 4n). The C-OH bond length is 1.802 Å. 3.2. Cl-Initiated Oxidation. Upon the formation of Cl atoms from the C2Cl4 + OH oxidation reactions, chlorine atoms can add to the parent species, CCl2CCl2 (species 1). Figure 6 depicts the complete reaction scheme for the chlorine-initiated oxidation of tetrachloroethylene. Following addition of the chlorine atom, this oxidation proceeds with O2 addition and subsequent reduction to form Cl2COCCl3 (species 28). This alkoxy radical then diverges to give ClC(O)CCl3 (species 4) in pathway 1 and two molecules of phosgene in pathway 2. Optimized geometries of all intermediates and transition states are shown in Figures 3 and 4, respectively. As with the OHinitiated oxidation, zero-point corrected energies and associated vibrational frequencies of all species are found in Supplementary Tables 1-4 (Supporting Information). Table 1 gives enthalpies, and Table 2 lists the activation energies. Using these calculated enthalpies and activation energies gives the potential energy surface for the Cl-initiated oxidation shown in Figure 7. 3.2.1. Pathway 1. The first step in this oxidation is the addition of the chlorine atom to either of the equivalent carbons of PERC (reaction 2).


(2)

9188


Christiansen and Francisco at -20.1 kcal · mol-1. A reaction similar to this one, reaction 32, has been considered computationally.35

ClCCCl2 + O2 f ClCO2CCl2

(32)

The next reaction is conversion of the peroxy radical to an alkoxy radical though an NO to NO2 conversion (reaction 33).

(28) Cl2CO2CCl3 + NO f (29) Cl2COCCl3 + NO2 (33) The geometry of the produced species, species 29 (Figure 3cc), does not change significantly except for the shortening of the C-O bond from 1.441 to 1.345 Å. The enthalpy of this reaction is -18.5 kcal · mol-1. At this point in the oxidation, there are two possible reactions in which the Cl2COCCl3 (species 29) radical can participate. The first, pathway 1, is the extrusion of a chlorine to form a stable product, trichloroacetyl chloride (reaction 34).

(29) Cl2COCCl3 f (4) ClC(O)CCl3 + Cl

Figure 6. Chlorine atom initiated oxidation mechanism of tetrachloroethylene.

The transition state for this reaction, shown in Figure 4o, shows a C-Cl bond length of 2.302 Å. An imaginary frequency of 540i cm-1 corresponds to the addition of the chlorine to the carbon. The radical resulting from this transition state, Cl2CCCl3 (species 27), is shown in Figure 3aa. One-half of this species has now adopted a tetrahedral geometry while the other half remains trigonal planar. The C-C bond length has increased to 1.494 Å while the C-Cl bond lengths have stayed relatively constant, ranging from 1.705 to 1.800 Å. The activation energy of this reaction is -1.3 kcal · mol-1 with an enthalpy of -15.9 kcal · mol-1. As in other reactions, the activation energy of this elimination is underestimated when using the CCSD(T) /6311+G(2df) method, indicating that within the uncertainty of the calculation the reaction is nearly barrierless. The oxidation proceeds with an O2 addition into the Cl2CCCl3 radical (species 27) (reaction 31).

(27) Cl2CCCl3 + O2 f (28) Cl2CO2CCl3

(31)

The produced peroxy radical (Figure 3 bb) has a geometry close to that of the reactant. The most noticeable change, once again, is the length of the C-C bond. At 1.555 Å, the bond has increased 0.05 Å from the previous radical and is more than 0.2 Å longer than the parent species, PERC. With the addition of the O2, the C-C bond now has more single bond character. Additionally, the electron density is now distributed among the substituents. This can be seen in a shift of the Mulliken atomic spin densities. The spin density on the radical carbon of Cl2CCCl3 (species 27) is +0.944 and is -0.016 on the same carbon of Cl2CO2CCl3 (species 28), indicating a shift of the density away from the carbon and to the substituents, primarily to the oxygen group. The enthalpy for this reaction is calculated

(34)

With a vibrational frequency of 870i cm-1, the transition state associated with the reaction (Figure 4p) shows a C-Cl bond length of 2.049 Å and a relaxation of the carbon-substituent angles, moving closer to a planar geometry in that half of the species. This planar geometry is realized in the formed product (Figure 3d). The angles surrounding this carbon now range from 113.1° to 124.2°. This product has a C-C bond length of 1.547 Å. C-Cl bonds range from 1.775 to 1.763 Å, and the C-O bond length is 1.187 Å. Carbon-substituent angles surrounding the CCl3 group are all near 109°, a tetrahedral geometry. This reaction has an activation energy of 3.0 kcal · mol-1 and an enthalpy of -11.6 kcal · mol-1. 3.2.2. Pathway 2. The second reaction that can proceed from Cl2COCCl3 (species 29) is a C-C bond breakage to form phosgene, a final product of this oxidation, and CCl3 (species 9) (reaction 35).

(29) Cl2COCCl3 f (9) CCl3 + (12) C(O)Cl2

(35)

Figure 4q depicts the transition state for this reaction. The C-C bond of this transition state is 1.879 Å. This C-C bond stretch has an associated vibrational frequency of 1094i cm-1. The activation energy of this reaction is 4.6 kcal · mol-1 with an enthalpy of -14.3 kcal · mol-1. The stable product formed, phosgene, is a known product of other chlorinated compounds as well.36 Most phosgene is produced in the troposphere and removed though rain.37 Additionally, a significant portion of the phosgene that is produced in the stratosphere is transported to the troposphere, although some does undergo photolysis in the stratosphere and therefore can ultimately contribute to ozone depletion.37 Recent measurements show phosgene levels in the upper troposphere at 20-25 pptv as well as 30 pptv in the lower stratosphere.38 Figure 3l shows the optimized geometry of phosgene. The C-Cl bond length is 1.751 Å, while the C-O bond length is 1.182 Å. O-C-Cl angles are 124.1°, and the Cl-C-Cl angle is 111.9°. The other species produced from this reaction, CCl3 (species 9), is shown in Figure 3i. The Cl-C-Cl angles are all 120.0°, and the C-Cl bond lengths are 1.696 Å.



Figure 7. Potential energy surface in the Cl-initiated tetrachloroethylene oxidation. Energy units in kcal · mol-1. Values calculated using values computed at the CCSD(T)/6-311+G(2df) level.

The further oxidation of CCl3 (species 9), has been previously detailed as part of the hydroxyl initiated oxidation, in pathway 1e. Molecular oxygen adds to the carbon centered radical of CCl3 (species 9) producing a peroxy radical, which is subsequently reduced to an alkoxy radical. Cl elimination results in the formation of another molecule of phosgene. See the previous discussion of pathway 1e for details. 4. Discussion The oxidation of PERC begins with the addition of OH, giving Cl2C(OH)CCl2 (species 2), with five possible reactions stemming from this species (reactions 3, 13, 15, 16, and 21).

(2) Cl2C(OH)CCl2 f (3) ClC(OH)CCl3

(3)

(2) Cl2C(OH)CCl2 f (13) ClC(OH)CCl2 + Cl

(13) (2) Cl2C(OH)CCl2 f (15) Cl2COCHCl2

(15)

(2) Cl2C(OH)CCl2 f (16) ClC(O)CCl2 + HCl

(16) (2) Cl2C(OH)CCl2 + O2 f (20) Cl2C(OH)CCl2O2

(21) Reactions 13 and 15 have already been deemed unlikely due to very large energetic barriers. Reaction 3 has an activation energy

of 13.5 kcal · mol-1 and an enthalpy of 4.4 kcal · mol-1. Figure 5b and 5c show the potential energy surfaces for pathway 3 (reaction 16) and pathway 4 (reaction 21), respectively. Literature data for a comparable system suggest that the energy barrier for the HCl elimination will be large.39 Additionally, it is well accepted that addition of O2 to form a peroxy radical has almost no significant energy barrier.40–42 Therefore, it is expected that the HCl elimination will not contribute considerably, while the O2 addition reaction will be significant. Work by Tuazon et al.16 clearly shows that the OH-initiated oxidation of PERC results in the formation of chlorine atoms, which then initiates the reaction between PERC and Cl. When Cl scavengers are excluded, a significant change in products formed from the PERC + OH reaction is noted.16 Reactions of TCE + OH in the absence of a chlorine scavenger result in the formation of phosgene, C(O)Cl2 (species 12), and trichloroacetyl chloride, ClC(O)CCl3 (species 4). Upon removal of a chlorine scavenger, the yield of phosgene remains about the same (approximately 50%) while the yield of trichloroacetyl chloride, which was minimal before, is now around 40%.16 The formation of chloroacetly chlorides, resulting from the removal of chlorine scavengers, is demonstrated with a number of other chloroethylenes. Tuazon et al.16 demonstrate that the reactions of PERC with OH produce chlorine that then go on to react with PERC. Nolan et al.17 studied the OH-initiated oxidation of PERC and trichloroethylene and confirmed the major product of PERC + OH (when in the presence of a chlorine scavengers) to be phosgene. They noted a relatively low yield (about 50%) of phosgene from the PERC + OH reaction and minimal production of formyl chloride [CH(O)Cl] from TCE + OH (which

9190



primarily comes from the O2 addition to the carbon centered radical formed after OH addition to TCE). These observations, lead them to propose that the HCl elimination pathway (pathway 3) is dominant. Additionally, they suggest that the branching percentage is 10% for the C-Cl cleavage (pathway3a) and 90% for the C-C fragmentation (pathway 3b).17 This proposal might be most applicable to the trichloroethylene reactions, as phosgene can be formed from other pathways in the PERC oxidation. However, in this system, their proposed dominant pathway, 3b of this work, results in significant formation of C(O)Cl2 (species 12) as well as CO and Cl. The crucial point in understanding the oxidation mechanism of OH reacting with PERC is the generation of chlorine atoms that will begin the Cl-initiated oxidation of PERC. A number of energetically favorable pathways producing chlorine atoms from the reaction of PERC with OH are suggested in this work; they include 3a, 3b, and two sources in 4a. The HCl elimination proposed as the primary route of OH-initiated oxidation by Nolan et al.17 (pathway 3) does indicate the production of a significant concentration of chlorine atoms. It is also possible that pathway 4a contributes chlorine. As formed, these Cl atoms can add to PERC with much lower activation energy than the addition of OH. Reaction 1 and reaction 2 show the possible additions of Cl and OH.

reaction pathways. These Cl radicals are initially formed from the reactions of PERC + OH. Upon addition of chlorine to PERC, Figure 6 shows both pathways result in the regeneration of the Cl atom. The addition of Cl to PERC has a low energy barrier and is self-catalyzing, and therefore a significant channel in the oxidation of PERC.


(1)


(2)

Acknowledgment. We express thanks to the U.S. Department of Energy, Global Change Education Program, for financial support through the Graduate Research Environmental Fellowship awarded to C.J.C.

Reaction 1, addition of the hydroxyl radical, has an activation energy of 3.6 kcal · mol-1, while reaction 2, addition of chlorine, has little or no barrier, at -1.3 kcal · mol-1. This is validated by experimental rate data that shows the addition of Cl to PERC is 200-300 times faster than the reaction of OH to PERC.22,23 Because of the production of chlorine atoms from the OHinitiated reactions, and the favorable addition of Cl to PERC, the products formed from the reaction of OH with PERC will likely be the products produced from the reaction of Cl with PERC. The potential energy surface of the Cl-initiated oxidation of PERC is given in Figure 7. At Cl2COCCl3 (species 29), pathway 1 reacts to eliminate a Cl and results in the formation of trichloroacetyl chloride (reaction 34). Reaction 35 shows the progress of pathway 2, the C-C bond fragmentation, eventually resulting in the formation of two molecules of phosgene.

(29) Cl2COCCl3 f (4) ClC(O)CCl3 + Cl

(34)

(29) Cl2COCCl3 f (9) CCl3 + (12) C(O)Cl2

(35)

These pathways will be in close competition; the activation barrier of reaction 34 is 3.0 kcal · mol-1, and 4.6 kcal · mol-1 for reaction 35. The products of these reactions and their relative amounts, closely align with the experimental results of Tuazon et al.16 A crucial point in this oxidation is the regeneration of Cl atoms that occurs at both the end of pathways 1 and 2, creating a self-catalyzing reaction. The Cl addition reaction favored over the OH addition reaction because of the significantly lowered activation barrier for the addition of Cl to PERC (reaction 2) compared with the addition of OH to PERC (reaction 1). However, there must be a constant source of chlorine atoms to sustain the PERC + Cl

5. Conclusions This study has greatly expanded the understanding of the oxidation reactions of tetrachloroethylene, particularly the influence of the chlorine-initiated oxidation. 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 PERC is significantly lower than the addition of the hydroxyl radical to PERC. Additionally, both pathways of the chlorine-initiated oxidation result in the reformation of the chlorine atom, constituting a self-catalyzing reaction. This results in the Cl atom-initiated oxidation pathways and ultimately results in the formation of trichloroacetyl chloride [ClC(O)CCl3 (species 4)], phosgene [C(O)Cl2 (species 12)], and chlorine atoms. Combined with past experimental studies, this work allows for a more comprehensive understanding of the oxidation chemistry of tetrachloroethylene in the atmosphere.

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