Reductive Dechlorination of Hexachloroethane in the Environment: Mechanistic Studies via Computational Electrochemistry Eric V. Patterson, ² Christopher J. Cramer,* ,‡ and Donald G. Truhlar* ,‡ Contribution from the DiVision of Science, Truman State UniVersity, 100 East Normal Street, KirksVille, Missouri 63501, and Department of Chemistry and Supercomputer Institute, UniVersity of Minnesota, 207 Pleasant Street Southeast, Minneapolis, Minnesota 55455-0431 ReceiVed September 28, 2000. ReVised Manuscript ReceiVed December 27, 2000 Abstract: Ab initio and density functional levels of electronic structure theory are applied to characterize alternative mechanisms for the reductive dechlorination of hexachloroethane (HCA) to perchloroethylene (PCE). Aqueous solvation effects are included using the SM5.42R continuum solvation model. After correction for a small systematic error in the electron affinity of the chlorine atom, theoretical predictions are accurate to within 23 mV for four aqueous reduction potentials relevant to HCA. A single pathway that proceeds via two successive single-electron transfer/barrierless chloride elimination steps, is predicted to be the dominant mechanism for reductive dechlorination. An alternative pathway predicted to be accessible involves trichloromethylchlorocarbene as a reactive intermediate. Bimolecular reactions of the carbene with other species at millimolar or higher concentrations are predicted to potentially be competitive with its unimolecular rearrangement to form PCE. Introduction Small, polychlorinated organic compounds such as hexachlo- roethane (HCA) are widespread trace-level contaminants in drinking water supplies. 1-4 As many of these species are known or suspected human carcinogens, considerable effort has gone into the development of technologies for the in situ transforma- tion of these environmental contaminants to less dangerous products. One such method is reductive dehalogenation via zero- valent iron, 5 where oxidation of Fe 0 to Fe(II) drives the reduction of halogenated hydrocarbons in aqueous solutions that are in contact with the metal. 6-8 Several recent papers have reported zero-valent metal- mediated reductive dechlorination of substituted methanes, 9,10 larger alkanes, 7 and ethylenes. 11,12 In the case of HCA, the use of sulfidic 6,8,13-15 and other 16 reducing agents has also been extensively examined. In addition, decomposition of chlorinated hydrocarbons with alternative sources of reducing power (e.g., TiO 2 /UV, alternative electrochemical couples, autotrophic en- zyme activity) have been reported. 7,17,18 Most of these reports have focused on how reaction conditions affect the kinetics of disappearance of HCA, with some additional analysis of product distributions. A number of possible pathways have been suggested for the zero-valent iron reductive elimination of chlorine from hexachlo- roethane. These are summarized in Chart 1. All share in common the observation that perchloroethylene (PCE) is the major product of reductive dehalogenation of HCA. Butler and Hayes 6 have observed small amounts (no more than 1% of initial HCA concentration) of pentachloroethane (PCA) as an intermediate when sulfide is used as a reductant; transformation of PCA to PCE by dehydrohalogenation in aqueous systems has been previously studied. 19 As accurate thermochemical data are not available for many of the reactive intermediates in pathways (a-d), the mechanism is not firmly established. We present here high-level quantum chemical calculations having the goal of accurately describing the thermochemistry for different possible microscopic steps in the reductive dechlo- rination pathway. Electron correlation is included by coupled- cluster theory and density functional theory (DFT). The effects of aqueous solvation are included in the quantum mechanical treatment using the SM5.42R continuum model. 20 We first validate the computational models for polychlorinated species ² Truman State University. ‡ University of Minnesota. 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