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Free Energies for Degradation Reactions of 1,2,3-Trichloropropane from ab Initio Electronic Structure Theory. Eric J. Bylaska* and Kurt R. Glaesemann ...
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J. Phys. Chem. A 2010, 114, 12269–12282

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Free Energies for Degradation Reactions of 1,2,3-Trichloropropane from ab Initio Electronic Structure Theory Eric J. Bylaska* and Kurt R. Glaesemann William R. Wiley EnVironmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, P.O. Box 999, Richland, Washington 99352, United States

Andrew R. Felmy Fundamental Sciences DiVision, Pacific Northwest National Laboratory, P.O. Box 999, Richland, Washington 99352, United States

Monica Vasiliu and David A. Dixon Department of Chemistry, The UniVersity of Alabama, P.O. Box 870336, Tuscaloosa, Alabama 35487-0336, United States

Paul G. Tratnyek OGI School of Science and Engineering, Oregon Health & Science UniVersity, 20000 NW Walker Road, BeaVerton, Oregon 97006-8921, United States ReceiVed: June 21, 2010; ReVised Manuscript ReceiVed: September 18, 2010

Electronic structure methods were used to calculate the gas and aqueous phase reaction energies for reductive dechlorination (i.e., hydrogenolysis), reductive β-elimination, dehydrochlorination, and nucleophilic substitution by OH- of 1,2,3-trichloropropane. The thermochemical properties ∆H°(298.15 K), S°(298.15 K, 1 bar), and f ∆GS(298.15 K, 1 bar) were calculated by using ab initio electronic structure calculations, isodesmic reactions schemes, gas-phase entropy estimates, and continuum solvation models for 1,2,3-trichloropropane and several likely degradation products: CH3-CHCl-CH2Cl, CH2Cl-CH2-CH2Cl, C•H2-CHCl-CH2Cl, CH2Cl-C•H-CH2Cl, CH2dCCl-CH2Cl, cis-CHCldCH-CH2Cl, trans-CHCldCH-CH2Cl, CH2dCH-CH2Cl, CH2Cl-CHCl-CH2OH, CH2Cl-CHOH-CH2Cl, CH2dCCl-CH2OH, CH2dCOH-CH2Cl, cis-CHOHdCH-CH2Cl, trans-CHOHdCHCH2Cl, CH(dO)-CH2-CH2Cl, and CH3-C(dO)-CH2Cl. On the basis of these thermochemical estimates, together with a Fe(II)/Fe(III) chemical equilibrium model for natural reducing environments, all of the reactions studied were predicted to be very favorable in the standard state and under a wide range of pH conditions. The most favorable reaction was reductive β-elimination (∆G°rxn ≈ -32 kcal/mol), followed closely by reductive dechlorination (∆G°rxn ≈ -27 kcal/mol), dehydrochlorination (∆G°rxn ≈ -27 kcal/mol), and nucleophilic substitution by OH(∆G°rxn ≈ -25 kcal/mol). For both reduction reactions studied, it was found that the first electron-transfer step, yielding the intermediate C•H2-CHCl-CH2Cl and the CH2Cl-C•H-CH2Cl species, was not favorable in the standard state (∆G°rxn ≈ +15 kcal/mol) and was predicted to occur only at relatively high pH values. This result suggests that reduction by natural attenuation is unlikely. I. Introduction 1,2,3-Trichloropropane (TCP) occurs as a groundwater contaminant from agricultural use of fumigants that contain TCP as an impurity, cleaning/degreasing operations with solvent formulations containing TCP, and chemical manufacturing where TCP was a precursor, for example, in the synthesis of epichlorhydrin. Recently, improved monitoring capabilities have led to more frequent detection of TCP in groundwater samples, and reassessment of its toxicity has resulted in more stringent regulation. Together, these developments have placed TCP among the emerging contaminants that currently are being considered by a number of organizations as a possible priority for future research and regulation (e.g., U.S. Environmental Protection Agency1). * To whom correspondence should be addressed. Phone: 1-509 371 6164. Fax: 1-509 371 6354. E-mail: [email protected].

Prior experimental studies on the pathways and kinetics of TCP degradation suggest several reaction pathways that might contribute to the attenuation of TCP contamination under environmental conditions.2 These include reductive dechlorination (i.e., hydrogenolysis), reductive β-elimination, dehydrochlorination, and nucleophilic substitution, especially by H2O/OH- (i.e., hydrolysis). This combination of degradation pathways, summarized for TCP in Figure 1, covers the entire range of degradation pathways that determine the fate of most chlorinated solvents under environmental conditions. In this sense, TCP is a prototypical groundwater contaminant, which makes it a useful compound for experimental and theoretical studies of the environmental chemistry of halogenated organic chemicals. For many chlorinated hydrocarbons, the principle degradation pathway in anaerobic groundwater environments is reductive dechlorination (i.e., hydrogenolysis). This is one

10.1021/jp105726u  2010 American Chemical Society Published on Web 11/01/2010

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Figure 1. Possible degradation pathways of TCP for a range of environmental conditions. •

of the most studied reactions in environmental science because in situ remediation strategiessespecially those that are microbiological (i.e., bioremediation)smainly involve reductive dechlorination of chlorinated solvents.3-16 Reductive dechlorination of TCP utilitizes two electrons and a proton to make chloride ion and either 1,2-dichloroethane or 1,3-dichloroethane.

CH2Cl-CHCl-CH2Cl + 2e- + H+ f CH3-CHCl-CH2Cl CH2Cl-CH2-CH2Cl

{

}

+ Cl-

(1)

While nominally a two electron-transfer reaction, the mechanism likely occurs in two one-electron steps. The first electron-transfer results in the loss of a chloride anion and the formation of either the 2,3-dichloropropyl or 1,3dichloropropan-2-yl radicals.

CH2-CHCl-CH2Cl + e- + H+ f CH3-CHCl-CH2Cl (1c) •

CH2Cl-CH-CH2Cl + e- + H+ f CH2Cl-CH2-CH2Cl (1d)

For a chlorinated hydrocarbon with chlorines on adjacent carbons, like TCP, alkenes can be formed by a direct twoelectron reductive β-elimination reaction.2,8,17-22 The reaction for this alternative reduction pathway involves the addition of two electrons, resulting in two chlorides and a double bond

CH2Cl-CHCl-CH2Cl + 2e- f CH2dCH-CH2Cl + 2Cl-

(2)

•

CH2Cl-CHCl-CH2Cl + e- f CH2-CHCl-CH3 + Cl(1a) •

CH2Cl-CHCl-CH2Cl + e- f CH2Cl-CH-CH3 + Cl(1b) The second electron-transfer leads to the addition of a hydrogen atom to the radical to form either 1,2-dichloropropane or 1,3-dichloropropane.

In the presence of base, several chloro-alkenes can also be formed by the more traditional dehydrochlorination reaction to form either 2,3-dichloroprop-1-ene, (1E)-1,3-dichloroprop-1-ene, or (1Z)-1,3-dichloroprop-1-ene.

CH2Cl-CHCl-CH2Cl + OH- f CH2dCCl-CH2Cl + H2O + Cl-

(3a)

Degradation Reactions of 1,2,3-Trichloropropane

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CH2Cl-CHCl-CH2Cl + OH- f -

cis-CHCldCH-CH2Cl + H2O + Cl

(3b)

CH2dCOH-CH2Cl f CH3-C(dO)-CH2Cl

CH2Cl-CHCl-CH2Cl + OH- f trans-CHCldCH-CH2Cl + H2O + Cl-

(3c)

Dehydrochlorination is generally believed to occur by a bimolecular E2 elimination in which OH- extracts the proton away from either the primary (1°) or secondary (2°) carbon atom coupled with concerted loss of chloride from the neighboring carbon atom.23,24 When the proton is extracted from the primary carbon atom, either a trans- or a cis- alkene can be formed. In addition to the reduction and elimination reactions, nucleophilic substitution reactions can lead to dechlorination by substitution with a variety of anions: predominantly OH- in the aqueous environment, but SH-, HCO3-, F-, etc. may also react. Previous work has shown that a wide variety of dechlorinations by nucleophilic substitution are thermodynamically favorable for other chlorinated solvents.25,26 The rates of nucleophilic substitution reactions vary widely with the structure of the chlorinated hydrocarbon and environmental conditions,27 and even some of the slower hydrolysis reactions (e.g, CCl4), which may be catalytically enhanced by absorption to sediments,28 can be important on time scales relevant to transport in groundwater systems.23,28,29 Substitution of chlorine atoms by OH- at the primary carbon atoms of TCP is expected to proceed by an SN2 reaction.

CH2Cl-CHCl-CH2Cl + OH- f CH2Cl-CHCl-CH2OH + Cl- (4a) resulting in a 2,3-dichloropropan-1-ol. Substitution at the secondary carbon atom may proceed by either an SN2 or an SN1 reaction,

CH2Cl-CHCl-CH2Cl + OH- f CH2Cl-CHOH-CH2Cl + Cl-

(4b)

resulting in a 1,3-dichloropropan-2-ol. Both of these alcohols can further undergo dehydrochlorination by an E2-elimination,

CH2Cl-CHCl-CH2OH + OH- f CH2dCCl-CH2OH + H2O + Cl-

(4c)

CH2Cl-CHOH-CH2Cl + OH- f CH2dCOH-CH2Cl + H2O + Cl- (4d) CH2Cl-CHCl-CH2OH + OH- f cis-CH2Cl-CHdCHOH + H2O + Cl- (4e) CH2Cl-CHCl-CH2OH + OH- f trans-CH2Cl-CHdCHOH + H2O + Cl-

1-en-1-ol, or (1Z)-3-chloroprop-1-en-1-ol may subsequently tautomerize to a ketone.

(4f)

resulting in either a 2-chloroprop-2-en-1-ol, 3-chloroprop-1en-2-ol, (1E)-3-chloroprop-1-en-1-ol, or (1Z)-3-chloroprop1-en-1-ol. The 3-chloroprop-1-en-2-ol, (1E)-3-chloroprop-

(4g)

cis-CH2Cl-CHdCHOH f CH2Cl-CH2-CH(dO) (4h) trans-CH2Cl-CHdCHOH f CH2Cl-CH2-CH(dO) (4i) The diversity of possible reaction pathways and mechanisms for a compound such as TCP makes it difficult to obtain a comprehensive understanding of the competition among these pathways from experimental studies alone. We and others have been applying the methods of computational chemistry to study the environmental degradation of simple and larger organochlorine compounds.25,26,30-41 In the present study, we use electronic structure methods to investigate the thermochemical properties (in the gas phase and in aqueous solution) for the likely degradation pathways of TCP. We report thermochemical K), S°(298.15 K, 1 bar), ∆GS(298.15 propertiess∆H°(298.15 f K, 1 bar)scalculated using isodesmic reactions schemes, gasphase entropy estimates, and continuum solvation models for the possible degradation products of TCP shown in Figure 1. From these thermochemical data we estimate the relevant energetics of reactions 1, 2, 3, and 4. Future work will focus on the barriers for the detailed reaction mechanisms to fully characterize the kinetics. In Section II, the computational methods we used are described. Calculations for the enthalpies of formation in the gas-phase of species are reported in Section III. The difficulties associated with calculating absolute heats of formation from atomization energies are avoided by using isodesmic reactions. The choice of this method is based on results from many studies, which show that using isodesmic reactions leads to excellent agreement with experiment.42,43 Section IV reports the results of the calculations of the gasphase entropies using standard statistical mechanical expressions for the vibrational, rotational, and translational entropy contributions. Section V reports the calculations of the solvation energies using the COSMO continuum solvation model.44 Such a treatment of solvation is more computationally efficient than performing supermolecule calculations with explicit water molecules coupled with continuum simulations. It has been shown to give solvation energies within a few kcal/mol, which for this study is adequate considering the errors in the gas phase enthalpies of formation. In Section VI, we provide estimates to aqueous reaction energetics for the reductive dechlorination, reductive β-elimination, dehydrochlorination, and nucleophilic substitution reactions. Concluding remarks are given in Section VII. II. Ab Initio and Continuum Solvation Calculations All of the electronic structure calculations in this study were performed with the NWChem program suite45 unless noted below. The gas phase geometries and frequencies for all of the neutral and radical compounds were calculated using electronic structure calculations. Tables SM-1 and SM-2 of the Supporting Information contain the electronic energies and thermal vibration energies at 298.15 K for all of the compounds studied. Most of the electronic structure calculations in this study were performed at the density functional

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theory (DFT)46 and second-order Møller-Plesset perturbation theory (MP2)47 levels. The Kohn-Sham equations of DFT48 were solved using the local density approximation (LDA)49 and the gradient-corrected PBE96,50 B3LYP,51,52 and PBE053 exchange-correlation functionals. DFT calculations and MP2 calculations were performed using the 6-311++G(2d,2p) basis set (all d-orbitals were Cartesian 6d). The 6-311++G(2d,2p) basis sets were obtained from the Extensible Computational Chemistry Environmental Basis Set Database.54 A number of the calculations performed in this study required higher accuracy and were done at the G3(MP2) level55 with the Gaussian 03 program suite.56 G3(MP2) calculations are slightly more accurate for compounds than the G2 and G2(MP2) levels,57,58 which preceded it due to the inclusion of core-valence and relativistic effects, and it is comparable in efficiency to the G2(MP2) level. The accuracy of G3(MP2) is quite good for the computational expense, and it has reproduced experimental atomization energies to within a few kcal/mol for a large number of organic molecules.55 All openshell calculations used an unrestricted wave function. Solvation energies, ∆GS, for rigid solutes that do not react strongly with water can be approximated as a sum of noncovalent electrostatic, cavitation, and dispersion energies. The electrostatic contributions to the solvation energies were estimated by using the self-consistent reaction field theory of Klamt and Schu¨u¨rmann (COSMO),44 with the cavity defined by a set of overlapping atomic spheres with radii suggested by Stefanovich and Truong (H ) 1.172 Å, C- ) 2.096 Å, C ) 1.635 Å, O ) 1.576 Å, and Cl ) 1.750 Å). The dielectric constant of water used for all of the solvation calculations was 78.4.44 This continuum model can be used with a variety of ab initio electronic structure calculations in the NWChem program suite including LDA, PBE96, B3LYP, and PBE0. Calculated gasphase geometries were used to perform these calculations. The solvent cavity discretization was generated from the surface of nonoverlapping spheres that were discretized by an iterative refinement of triangles starting from a regular octahedron. Three refinement levels, which are equivalent to 128 points per sphere, were used to define the solvent cavity in these calculations. Previous calculations have shown that a reasonable convergence ( 15 kcal/mol). Equations 1a-b are thought to be the first-electron transfer step in reductive dechlorination and probably also to some degree in reductive β-elimination. As shown in Figure 2, the results of the chemical equilibrium simulations show that these reactions only become thermodynamically favorable under unrealistically high pH conditions. These results, that the reactions are not favored thermodynamically, are consistent with experimental results that there is no detectable dechlorination of TCP by mild reductants2 such as those found naturally and associated with natural attention of contaminants (FeII, Fe3O4, etc.). Even Fe0, a thermodynamically much stronger reductant that is used in engineered schemes for groundwater remediation,87 gives only slow dechlorination of TCP.2 Current investigations of options for remediation of TCP by reductive pathways are now focused on Zn0, which is an even stronger reductant than Fe0 and dechlorinates TCP rapidly enough to have potential for field-scale applications to remediation of TCP contaminated groundwater. VII. Conclusion Ab initio electronic structure theory, canonical ensemble entropy formulas, and self-consistent reaction field theory were

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used to estimate the thermochemical properties, ∆H°(298.15 K), f S°(298.15 K, 1 bar), and ∆GS(298.15 K, 1 bar) of TCP and its likely degradation productssCH3-CHCl-CH2Cl, CH2ClCH2-CH2Cl, C•H2-CHCl-CH2Cl, CH2Cl-C•H-CH2Cl, CH2dCCl-CH2Cl, cis-CHCldCH-CH2Cl, trans-CHCld CH-CH2Cl, CH2dCH-CH2Cl, CH2Cl-CHCl-CH2OH, CH2Cl-CHOH-CH2Cl, CH2dCCl-CH2OH, CH2dCOHCH2Cl, cis-CHOHdCH-CH2Cl, trans-CHOHdCH-CH2Cl, CH(dO)-CH2-CH2Cl, and CH3-C(dO)-CH2Cl. The most difficult computational step in our thermodynamic estimations was determining the gas phase enthalpies of formation, K). For this, a strategy based on isodesmic reactions ∆H°(298.15 f from different electronic was used to determine ∆H°(298.15K) f structure methods. This strategy was found be reliable, especially for predicting overall reaction energies which were found to be consistent within a few kcal/mol between the various electronic structure methods used. Using these thermochemical free energies and a Fe(II)/Fe(III) chemical equilibrium model for natural reducing environments the aqueous reaction energies for reductive dechlorination, reductive β-elimination, dehydrochlorination, and nucleophilic substitution by OH- of 1,2,3-trichloropropane were estimated. It was found that all the reactions were found to have very favorable thermodynamics in the standard state as well as under a wide range of pH conditions. The most favorable reaction was reductive β-elimination (∆G°rxn ≈ -32 kcal/mol), followed closely by reductive dechlorination (∆G°rxn ≈ -27 kcal/mol), dehydrochlorination (∆G°rxn ≈ -27 kcal/mol), and nucleophilic substitution by OH- (∆G°rxn ≈ -25 kcal/mol). For both the reduction reactions studied it was found that the first electrontransfer step, yielding the intermediate C•H2-CHCl-CH2Cl, and CH2Cl-C•H-CH2Cl species, was not favorable in the standard state (∆G°rxn ≈ +15 kcal/mol) and was predicted to occur only at relatively high pH values. This result suggests that reduction from natural attenuation will be unlikely. Finally, the results of this study demonstrate that ab initio electronic structure methods can be used to calculate the reaction energetics of a potentially large number of organic compounds in solution, including radical and anionic compounds for which experimental data are unavailable, and can be used to help identify the potentially important environmental degradation reactions. Appendix

TABLE A1: Experimental Enthalpies of Formation (kcal/ mol) Used in Computations compd

∆H°f

compd

∆H°f

H2(g) H2O(g) e-(g) H•(g) C•(g) Cl•(g) Cl-(g) OH•(g) OH-(g) C2H6(g) C2H5Cl(g) C2H5OH(g)

0.0 -57.83 ( 0.01d 0.0b 52.1b 171.3b 28.992 ( 0.002c,f -55.9a 9.03 ( 0.01d -33.12d,e -20.04 ( 0.07c,g -26.8 ( 0.2c -56.12 ( 0.12c

1,2,3-C3H5Cl3(g) 1,2-C3H6Cl2(g) 2,3-C3H5Cl2-1-OH(g) 2-C3H7Cl(g) C3H7Cl-2-OH(g) 2-C3H5Cl(g) cis-1-C3H5Cl(g) trans-1-C3H5Cl(g) 3-C3H5Cl(g) 2-C3H5OH(g) C3H6O(g) C3H8(g) r1-C3H7(g) r2-C3H7(g)

-43.8a -38.91a -75.6a -34.0a -65.19a -5.90a -2.8a -3.7a -1.3a -42.1a -52.23a 25.02a 23.9a 22a

a NIST experimental ref 89. b Experimental ref 88. c Experimental ref 90. d Experimental refs 91, 92, 93, 94. e Experimental refs 95, 96. f Experimental ref 97. g Experimental ref 98.

TABLE A2: Experimental and High Quality ab Initio Gibbs Free Energies of Formation (kcal/mol) Used in Computations compd -

e (aq) H2(g) H+(aq) Cl-(aq) OH-(aq) H2O(l)

∆G°( f 298.15 K) 64.0a,b 0.00 0.00 -31.36c -37.58c -56.675c

∆Gs(e-) ) -34.6 kcal/mol from refs 99 and 100. b E°(H) ) 98.6 kcal/mol calculated from ∆Gs(H+) ) -263.98 kcal/mol101 and + 88 c ∆G°(H ∆G°(aq) obtained in experimental (g)) ) 362.58 kcal/mol. f f ref 88. a

TABLE A3: Standard States for Entropies (cal mol-1 K-1) atomic standard states 1/2H2 C - graphite 1/2O2 1/2Cl2 a

15.617a 1.372a 24.515a 27.845a

Experimental ref 88.

Acknowledgment. This research was supported by BES Nanoscale Science, Engineering, and Technology program and BES Geosciences program under the BES Division of Chemical Sciences, Geosciences, and BioSciences of the U.S. Department of Energy, Office of Science, under Grant No. DE-AC0576RL01830. Some of the calculations were performed on the Spokane and Chinook computing systems at EMSL, a national scientific user facility sponsored by the Department of Energy’s Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory. The Pacific Northwest National Laboratory is operated by Battelle Memorial Institute. We also wish to thank the Scientific Computing Staff, Office of Energy Research, and the U.S. Department of Energy for a grant of computer time at the National Energy Research Scientific Computing Center (Berkeley, CA). Supporting Information Available: Tables of ab initio total energies, and enthalpy corrections for the gas-phase compounds determined from LDA/6-311++G(2d,2p), PBE96/6-311++G(2d,2p), B3LYP/6-311++G(2d,2p), PBE0/6-311++G(2d,2p), and MP2/6-311++G(2d,2p) total energy, vibration, and hindered rotor calculations. Also, tables containing of the total enthalpies from G3(MP2) ab initio calculations, and gas-phase Gibbs free K,gas) are given. This energies of formation ∆G°(298.15 f information is available free of charge via the Internet at http:// pubs.acs.org. In addition to the Supporting Information available, the optimized structures for all the molecules calculated can be obtained by correspondence with E. J. B. (Eric.Bylaska@ pnl.gov). References and Notes (1) US EPA. Federal Register; 2009; Vol. 74, p 51850. (2) Sarathy, V.; Tratnyek, P. G.; Salter, A. J.; Nurmi, J. T.; Johnson, R. L.; Johnson, G. O. B. EnViron. Sci. Technol. 2010, 44, 787. (3) Balko, B. A.; Tratnyek, P. G. J. Phys. Chem. B 1998, 102, 1459. (4) Amonette, J. E.; Workman, D. J.; Kenedy, D. W.; Fruchter, J. S.; Gorby, Y. A. EnViron. Sci. Technol. 2000, 34, 4606. (5) Vogel, T. M.; Criddle, C. S.; Mccarty, P. L. EnViron. Sci. Technol. 1987, 21, 722. (6) Wade, R. S.; Castro, C. E. J. Am. Chem. Soc. 1973, 95, 226. (7) Criddle, C. S.; McCarty, P. L. EnViron. Sci. Technol. 1991, 25, 973. (8) Curtiss, G. P.; Reinhard, M. EnViron. Sci. Technol. 1994, 28, 2393.

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