Ab Initio Study of Carbon−Chlorine Bond Cleavage in Carbon

Environ. Sci. Technol. 2005, 39, 612-617

Ab Initio Study of Carbon-Chlorine Bond Cleavage in Carbon Tetrachloride NIANLIU ZHANG, PAUL BLOWERS,* AND JAMES FARRELL Department of Chemical and Environmental Engineering, The University of Arizona, Tucson, Arizona 85721-0011

Chlorinated solvents in groundwater are known to undergo reductive dechlorination reactions with Fe(II)containing minerals and with corroding metals in permeablebarrier treatment systems. This research investigated the effect of the reaction energy on the reaction pathway for C-Cl bond cleavage in carbon tetrachloride (CCl4). Hartree-Fock, density functional theory, and modified complete basis set ab initio methods were used to study adiabatic electron transfer to aqueous-phase CCl4. The potential energies associated with fragmentation of the carbon tetrachloride anion radical (•CCl4-) into a trichloromethyl radical (•CCl3) and a chloride ion (Cl-) were explored as a function of the carbon-chlorine bond distance during cleavage. The effect of aqueous solvation was investigated using a continuum conductor-like screening model. Solvation significantly lowered the energies of the reaction products, suggesting that dissociative electron transfer was enhanced by solvation. The potential energy curves in an aqueous medium indicate that reductive cleavage undergoes a change from an innersphere to an outer-sphere mechanism as the overall energy change for the reaction is increased. The activation energy for the reaction was found to be a linear function of the overall energy change, and the Marcus-Hush model was used to relate experimentally measured activation energies for CCl4 reduction to overall reaction energies. Experimentally measured activation energies for CCl4 reduction by corroding iron correspond to reaction energies that are insufficiently exergonic for promoting the outer-sphere mechanism. This suggests that the different reaction pathways that have been observed for CCl4 reduction by corroding iron arise from different catalytic interactions with the surface, and not from differences in energy of the transferred electrons.

Introduction Groundwater contaminated by chlorinated solvents can be detoxified via reductive dechlorination reactions that transform chlorinated solvents into their nonchlorinated analogues and chloride ions (Cl-) (1). Reductive dechlorination can occur via solvent reaction with Fe(II)-bearing minerals in aquifer sediments (2-4), or it can be promoted in engineered remedial systems using corroding iron (5-9) or metal cathodes (10-12) as electron donors. Because of its ubiquity as a groundwater contaminant, dechlorination * Corresponding author phone: 520-626-5319; fax: 520-621-6048; e-mail: [email protected]. 612

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reactions of carbon tetrachloride (CCl4) have received considerable research attention. The products of CCl4 dechlorination have been found to depend on the reducing agent and the reaction conditions (12, 13). In most studies of CCl4 dechlorination by corroding metals and metal cathodes, complete reduction to methane (CH4) was observed, but in some cases, formate (HCOO-), carbon monoxide (CO), and carbon dioxide (CO2) were the terminal reaction products (13, 14). In some instances, CH4 was the first stable species observed from CCl4 reduction (11, 15), whereas in other instances, chloroform (12), methylene chloride, and chloromethane were produced as intermediate species. Sequential dechlorination is not desirable in permeable-barrier treatment systems because the intermediate species are also toxic and react much more slowly than CCl4 (5, 12). Three parallel pathways are believed to be involved in the reductive dechlorination of CCl4 (16). In most instances, sequential dechlorination is observed via a hydrogenolysis pathway (1). Hydrogenolysis begins with a dissociative electron-transfer reaction to produce a trichloromethyl free radical (•CCl3) and Cl- (14, 17). The first stable species from this pathway is CHCl3, which can be produced from abstraction of a hydrogen atom by •CCl3 (18) or via reduction and protonation of •CCl3 (19). Two other pathways for CCl4 reduction involve formation of a dichlorocarbene (:CCl2) intermediate. The dichlorocarbene can either hydrolyze, producing CO, or undergo further reduction through a series of reactions, eventually producing CH4 (16). In reactions at metal and metal oxide surfaces, electrons are transferred one at a time (20). Therefore, the first step in CCl4 reduction by iron and Fe(II)-containing oxides begins with the transfer of a single electron. This reaction can occur via an inner-sphere mechanism in which a concerted electron transfer and bond-breaking reaction occurs

CCl4 + e- f •CCl3 + Cl-

(1)

or via an outer-sphere mechanism that involves only the transfer of a single electron

CCl4 + e- f •CCl4-

(2)

Several studies using different reducing agents have concluded that the pathway producing CH4 without intermediate CHCl3 production does not involve •CCl3 formation (16, 21). This suggests that production of a stable tetrachloromethyl anion radical (•CCl4-) might allow the transfer of a second electron prior to loss of the first Cl- ion. This would prevent •CCl3 formation, thereby promoting the :CCl2 pathway. Prior ab initio calculations suggest that reduction of CCl4 (22) and other methyl halides in the gas phase or in organic solvents is initiated by an inner-sphere electron-transfer mechanism (23). However, no computational studies of CCl4 reduction in aqueous systems have been done. The high polarity of water might offer sufficient stability to a •CCl4species to promote an outer-sphere electron transfer. This might lead to the pathway that produces CH4 without intermediate production of CHCl3. The goal of this study was to elucidate electronic factors affecting the reaction pathway from reduction of CCl4. This information is important for understanding the products produced from CCl4 reduction in permeable barrier treatment systems using corroding iron as a reducing agent. Toward that end, potential energies for CCl4 and •CCl410.1021/es049480a CCC: $30.25

 2005 American Chemical Society Published on Web 12/03/2004

TABLE 1. Experimentally Measured and Optimized Bond Lengths and Bond Angles in CCl4 Calculated Using the MP2 Method with Different Basis Sets parameter

6-31+G*

6-311+G**

expt

C-Cl ∠Cl-C-Cl

1.7714 109.4702

1.7718 109.4888

1.76 109.5

TABLE 2. Optimized Bond Lengths and Bond Angles in •CCl4Calculated Using the MP2 Method with Different Basis Sets, Where C‚‚‚Cl Designates the Cleaving Carbon-Chlorine Bond and C-Cl Represents the Remaining Bonds

FIGURE 1. Optimized geometries of CCl4 and •CCl4- calculated under the C3v symmetry constraint. species were determined as a function of the C-Cl bond length during cleavage. The effect of aqueous solvation was incorporated using a continuum solvation model. The empirical Marcus-Hush model for describing the relationship between activation and reaction energies was validated and was used to interpret previously published experimental data.

Computational Methods All ab initio calculations were carried out with the Gaussian 98 program (24) using a personal computer equipped with a Pentium IV processor operating at 1.5 GHz. The choice of basis sets and computational methods required a compromise between accuracy and computational time. The appropriate basis sets for these reactions must include diffuse functions for describing the anions (25-28). The effect of the basis set on the calculation accuracy was investigated by comparing results using the 6-31+G* and 6-311+G* basis sets to experimental measurements of molecular geometries. Geometry optimizations were performed using a correlated Hartree-Fock method incorporating a second-order

parameter

6-31+G*

6-311+G**

C‚‚‚Cl C-Cl ∠Cl-C-Cl

2.4898 1.7639 107.9914

2.4474 1.7687 108.303

Moller-Plesset (MP2) correction (29, 30) and also using density functional theory (DFT) (31). Unrestricted methods were used for all open-shell systems. The default criteria for optimization in Gaussian 98 were used, as were the default temperature and pressure of 298.15 K and 1 atm for frequency calculations. Correlation and exchange effects were taken into account in the DFT calculations using the B3LYP functional (29). The B3LYP method was chosen because DFT methods have been validated for the reductive dechlorination of other halocarbons (32, 33). A modified CBS-RAD (complete basis set-radical) method was also employed because of its speed and accuracy for obtaining thermochemical properties (34). The energy minima of all species were corrected through the use of scaling factors for the zeropoint energies (35). Aqueous solvation was incorporated by using the conductor-like screening model (COSMO) (36) as implemented in Gaussian 98. Compared with other continuum solvation model (CSM) methods, COSMO offers a good balance between computational speed and accuracy (36). Hartree-Fock and B3LYP methods were used to optimize the geometries in water, and the MP2 and B3LYP methods were used to calculate the single-point energies for the solvated systems. On the assumption that the cleaving C-Cl bond length approximates the dissociation reaction coordinate, potential energy profiles for CCl4 and •CCl4- were determined by calculating single-point energies of structures optimized under C3v symmetry constraints. In these calculations, one C-Cl bond length was fixed at values between 1.5 and 3.5 Å in increments of 0.25 Å. Reaction energies were calculated by subtracting the reactant energies from the sum of the energies of the product species. Mulliken population analysis (37, 29) was used to determine the partial atomic charges on the atoms in •CCl4- during bond cleavage.

Results and Discussion Vacuum Results. Calculations of all species in vacuo were used to validate the computational methods. The optimized geometries of CCl4 and •CCl4- were determined at the MP2 and B3LYP levels using 6-31+G* and 6-311+G* basis sets. Calculated bond lengths and bond angles, along with experimentally measured values are reported in Tables 1 and 2. Both basis sets produced structures that were in close agreement with experimental measurements. Comparison of the 6-31+G* and 6-311+G** results with experimental values indicates that increasing the size of the basis set did not result in more accurate calculations. Therefore, all further calculations were performed with the 6-31+G* basis set. The minimum energy MP2 and B3LYP structures for CCl4 and •CCl4- obtained using the 6-31+G** basis set are shown VOL. 39, NO. 2, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 3. Experimental and Calculated Properties of CCl4and •CCl4- Calculated with the 6-31+G** Basis Set at Different Levels of Theory other calculations parameter lengtha

CCl4 C-Cl bond •CCl - C-Cl bond 4 •CCl - C-Cl bond 4

lengtha energyb adiabatic electron affinityb vertical electron affinityb CCl4 heat of formationc •CCl - heat of formationc 4

expt 1.76d

2.0g >0h -93.35i -174 ( 33i

this work

MRD-CI

MRSDCI

MP2

B3LYP

CBS

1.766e

0.74d -1.53d -

1.771 2.490 0.561 0.962 -0.68 -8.05 -73.64

1.794 2.565 0.526 1.5 0.48 27.30 -137.43

1.01 -0.21 -136.54 -234.30

2.48f 0.74d 0.38d -1.69d -

a Bond lengths in Å. b Bond energies and electron affinities in eV. c Heats of formation in kJ/mol. d Reference 42. e Reference 39. f Reference 40. g Reference 41. h Reference 43. i Reference 35.

in Figure 1. The CCl4 molecule had a Td-symmetric structure (38) at the global minimum, and the •CCl4- anion had a C3vsymmetric structure, with a longer cleaving C-Cl bond at its global minimum. Bond lengths, bond angles, and heats of formation for CCl4 and •CCl4- calculated using the MP2, B3LYP, and modified CBS-RAD methods are reported in Table 3. Also included in Table 3 are the adiabatic and vertical electron affinities. The adiabatic electron affinity is the energy released by reaction 2, where the •CCl4- species is at its energetically optimized geometry and the free electron is assigned an energy of zero. The vertical electron affinity is the energy released by reaction 2, where •CCl4- retains the optimized geometry for CCl4. Experimental and calculated parameter values are also listed in Table 3. The C-Cl bond lengths and bond energies calculated here are in close agreement with experimental and previously calculated values. The adiabatic and vertical electron affinities are within a factor of ∼2 of experimental values and more closely approximate experimental values than those calculated in previous investigations. Calculated electron affinities within a factor of 2 of experimental values are consistent with the known errors associated with the computational methods (42, 43). The more accurate electron affinities calculated in this study, as compared with prior investigations, indicate that the methods employed here were appropriate for these compounds. For heats of formation, only the modified CBS-RAD method gave results that were within 50% of experimental values for CCl4 and •CCl4-. Although the heats of formation calculated using the B3LYP and MP2 methods had large absolute errors, the experimental adiabatic and vertical electron affinities were reproduced quite well by the calculations. Composite energy methods are sometimes known to systematically underestimate heats of formation, but they are still more accurate than MP2 and B3LYP methods (44). Although there were systematic errors in the calculated energies, the relative energies, like heats of reaction calculated via ab initio methods, were normally much closer to the experimental values than were the heats of formation (34). Solvent Effects. The electron-transfer and bond-breaking mechanisms were investigated by calculating the energies of CCl4 and •CCl4- as a function of the C-Cl bond distance during cleavage. Vacuum and aqueous-phase energy profiles for CCl4 and adiabatic •CCl4- as a function of the C-Cl bond elongation during cleavage are shown in Figures 2 and 3. Figure 2 shows that solvation of CCl4 did not significantly alter its energy profile, except when the C-Cl bond was nearly broken. In contrast, Figure 3 shows that the energy for •CCl4was significantly lowered by solvation. This finding is consistent with previously observed solvation effects on charged species in polar solvents (23, 33). Comparison of the •CCl - potential energy profiles in the gas and aqueous phases 4 in Figure 3 shows that the solvation energy of the anion decreased progressively along the reaction coordinate. This 614

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FIGURE 2. Potential energy profiles for CCl4 in vacuo and in water calculated using the B3LYP method.

FIGURE 3. Potential energy profiles for •CCl4- in vacuo and in water calculated using the MP2 and B3LYP methods. indicates that dissociation of •CCl4- was enhanced by the solvent. Assuming that the electron-transfer step is adiabatic, the crossing point of the potential energy profiles represents the transition state, and the energy difference between the transition state and energy minimum of CCl4 approximates the activation energy for the electron transfer. Whether CCl4 reduction occurs via an inner-sphere or outer-sphere reaction mechanism depends on the overall energy change for the reaction (∆E). The overall energy changes for reactions 1 and 2 depend on the initial energy of the electron, which also determines the relative positions of the potential energy profiles for CCl4 and •CCl4-. Aqueous phase energy profiles for CCl4 and •CCl4- along the reaction coordinate are shown in Figure 4 for ∆E ) 0. The energy of product fragments at infinite separation was obtained by adding the energies of Cl- and •CCl3 in solution using the COSMO method. The

FIGURE 4. Potential energy profiles for the reactants (CCl4 + e-) and products (•CCl4- or •CCl3 + Cl-) in water for ∆E ) 0 kcal/mol calculated using the B3LYP method.

FIGURE 6. Charge in units of elementary charge (1.602 19 × 10-19 C) on the breakaway Cl atom in solvated •CCl4- calculated using the B3LYP method and Mulliken charge analysis.

TABLE 4. Comparison of ab Initio and Empirical Marcus-Hush Model Results for Relating the Activation Energya to the Overall Reaction Energya

FIGURE 5. Potential energy profiles for the reactants (CCl4 + e-) and products (•CCl4- or •CCl3 + Cl-) in water for ∆E ) -21 kcal/mol calculated using the B3LYP method. CCl4 and •CCl4- energy profiles intersect at a C-Cl bond length of 2.3 Å, and an activation energy of 18.8 kcal/mol is required to reach the transition state. In the •CCl4- energy profile, there are no stationary points for bond elongations greater than 2.0 Å. This indicates that the •CCl4- species is not stable and that it will fragment into •CCl3 and Cl-. The absence of a stable •CCl4- species is consistent with an innersphere electron-transfer mechanism in which electron transfer and bond breaking are concerted. The shape of the •CCl4- energy profile in Figure 4 indicates that a stable •CCl4- species can form if the electron is transferred when the C-Cl bond length is less than 2.0 Å. Figure 5 shows the energy profiles for CCl4 and •CCl4- for an initial electron energy of 21 kcal/mol. Comparison of Figures 4 and 5 shows that increasing the potential energy of the electron from 0 to 21 kcal/mol shifts the crossing point of the product and reactant energy profiles to a shorter C-Cl bond. Crossing of the energy profiles at a shorter bond distance decreases the activation energy for the reaction to 0.61 kcal/mol and also changes the reaction from an innersphere to an outer-sphere mechanism. A similar observation of a change in mechanism with driving force has been reported for the reduction of aryldialkyl sulfonium cations (45). The data in Figure 5 suggest that, if electrons with sufficiently high energy are available for CCl4 reduction, a stable CCl4-• radical can form that can then facilitate the :CCl2 reaction pathway. When only low-energy electrons are available, electron transfer occurs only when the C-Cl bond length is sufficiently activated. In this case, CCl4-• is not stable and will dissociate into CCl3• + Cl-, which favors the pathway leading to CHCl3 production. Further insight into the nature of the electron-transfer reaction can be gained from changes in the charge distribution on •CCl4- as the bond-breaking process occurs. Figure 6 shows the partial atomic charges for the atoms in •CCl4as a function of the C-Cl bond distance. For a C-Cl bond length of 1.75 Å, the charge on the breakaway Cl atom was only -0.065, whereas the charge on the other Cl atoms was -0.32. This can be attributed to the C-Cl bond distance of 1.75 Å being shorter than the bond distances for the three

reaction energy

empirical modelb

MP2 calculation

empirical modelc

B3LYP calculation

-20 -10 0 10 20

11.915 16.010 20.708 26.010 31.915

10.434 15.100 20.708 25.499 31.608

9.202 13.148 17.797 23.148 29.201

6.677 12.263 17.797 22.437 28.027

a Activation energies and driving forces in kcal/mol. b Based on the MP2-calculated value of ∆E. c Based on the B3LYP-calculated value of ∆E.

FIGURE 7. Relationship between the activation energy and the overall reaction energy calculated using the B3LYP and MP2 methods. other C-Cl bonds. As the C-Cl bond length was increased, more of the excess charge was transferred to the breakaway Cl atom. By the time the C-Cl bond length reached 3.0 Å, the breakaway Cl atom carried almost all of the negative charge. This indicates that the extra electron was gradually transferred from a diffuse orbital to a σ/CCl orbital during the dissociation process. The applicability of the empirical Marcus-Hush model (46) for describing the relationship between the activation and overall reaction energies can be evaluated from the data used to produce Figures 4 and 5. Although Marcus-Hush theory is not strictly applicable to inner-sphere electrontransfer reactions, after some modifications (46, 47) it has been found to be applicable to the electrochemical reduction of aliphatic halides (48, 23). After modification, the MarcusHush model can be expressed as

(

Ea ) Eoa 1 +

∆E 4Eoa

)

2

(3)

where Eoa is the activation energy at equilibrium where ∆E ) 0. Activation energies calculated using eq 3 are listed in Table 4 along with simulation results in Figure 7. Reasonable agreement between Marcus-Hush theory and the ab initio VOL. 39, NO. 2, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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results indicates that the model is valid even for inner-sphere electron transfer to CCl4, at least for driving forces ranging from -20 to +20 kcal/mol. Results from these simulations appear to corroborate experimental evidence showing that the products of CCl4 reduction depend on the energy of the electrons involved in the reaction. Balko and Tratnyek reported that reduction of CCl4 via photogenerated electrons in the oxide coating iron metal produced a different product distribution than was observed for reaction in the dark (49). They proposed that reduction by higher energy electrons in the oxide film favored the :CCl2 pathway over the CHCl3-producing pathway. In most permeable barrier remedial systems, corroding iron is used as the reducing agent. The potential energies of electrons in corroding iron depend on the rate of corrosion (50), which depends on the overall concentration of oxidants in the system and on the nature of the oxides coating the iron surface. Thus, the overall reaction energies associated with CCl4 reduction in iron remedial systems is unknown and likely varies over time. However, it is possible to estimate reaction energies for CCl4 reduction by iron cathodes. Two studies of CCl4 reduction by nearly oxide-free iron cathodes reported activation energies for CCl4 reduction between 10 and 16 kcal/mol (51, 52). The data in Figure 7 indicate that activation energies that are greater than 10 kcal/mol correspond to reactions that are less exergonic than -21 kcal/ mol. As indicated by the data in Figure 5, reactions less exergonic than -21 kcal/mol are not sufficiently energetic for promoting outer-sphere electron transfer. This indicates that CCl4 reduction by iron electrodes must occur via the inner-sphere, concerted electron-transfer mechanism. Experiments with iron cathodes were conducted at potentials below those associated with freely corroding iron. Thus, the higher free corrosion potentials associated with corroding iron systems will result in reactions that are less exergonic than those on iron cathodes. This indicates that the concerted mechanism, favoring CHCl3 production, should also predominate for CCl4 reduction by corroding iron. The fact that CCl4 reduction by corroding iron and iron oxides often occurs via pathways other than the hydrogenolysis pathway suggests that the :CCl2 pathway might be surface catalyzed. Recently, McCormick and Adriaens presented evidence that the pathway producing CH4 from reduction of CCl4 by magnetite is surface catalyzed in that chlorinated carbene and carbanion intermediates are stabilized by interactions with surface cations (16). There is substantial experimental evidence that oxides do affect the CCl4 reduction pathway at iron surfaces. Magnetite-coated iron cathodes were found to yield near-stoichiometric production of CH4 without intermediate production of CHCl3 (11). In contrast, on iron surfaces with minimal oxide coatings, such as polished iron cathodes (51, 52) and acid washed filings (5), near-stoichiometric production of CHCl3 has been observed from CCl4 reduction.

Acknowledgments This research was supported by the National Science Foundation Directorate for Bioenvironmental Systems through Grant BES-0083070 and by Grant 2P42ES04940-11 from the National Institute of Environmental Health Sciences of the National Institute of Health, with funds from the U.S. Environmental Protection Agency.

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Received for review April 5, 2004. Revised manuscript received October 5, 2004. Accepted October 13, 2004. ES049480A

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