Ab Initio Electronic Structure Study of One-Electron Reduction of Polychlorinated Ethylenes Eric J. Bylaska* and Michel Dupuis Fundamental Sciences DiVision, Pacific Northwest National Laboratory, P.O. Box 999, Richland, Washington 99352 Paul G. Tratnyek OGI School of Science & Engineering, Oregon Health & Science UniVersity, 20000 NW Walker Road, BeaVerton, Oregon 97006-8921 ReceiVed: December 8, 2004; In Final Form: April 1, 2005 Polychlorethylene radicals, anions, and radical anions are potential intermediates in the reduction of polychlorinated ethylenes (C 2 Cl 4 ,C 2 HCl 3 , trans-C 2 H 2 Cl 2 , cis-C 2 H 2 Cl 2 , 1,1-C 2 H 2 Cl 2 ,C 2 H 3 Cl). Ab initio electronic structure methods were used to calculate the thermochemical properties, ΔH° f (298.15 K), S°(298.15 K,1 bar), and ∆G S (298.15 K, 1 bar) of 37 different polychloroethylenyl radicals, anions, and radical anion complexes, C 2 H y Cl 3-y • ,C 2 H y Cl 3-y - , and C 2 H y Cl 4-y •- for y ) 0-3, for the purpose of characterizing reduction mechanisms of polychlorinated ethylenes. In this study, 8 radicals, 7 anions, and 22 radical anions were found to have stable structures, i.e., minima on the potential energy surfaces. This multitude of isomers for C 2 H y Cl 4-y •- radical anion complexes are π*, σ*, and -H‚‚‚Cl - structures. Several stable π* radical anionic structures were obtained for the first time through the use of restricted open-shell theories. On the basis of the calculated thermochemical estimates, the overall reaction energetics (in the gas phase and aqueous phase) for several mechanisms of the first electron reduction of the polychlorinated ethylenes were determined. In almost all of the gas-phase reactions, the thermodynamically most favorable pathways involve -H‚‚‚Cl - complexes of the C 2 H y Cl 4-y •- radical anion, in which a chloride ion is loosely bound to a hydrogen of a C 2 H x Cl 2-x • radical. The exception is for C 2 Cl 4 , in which the most favorable anionic structure is a loose σ* radical anion complex, with a nearly iso-energetic π* radical anion. Solvation significantly changes the product energetics with the thermodynamically most favorable pathway leading to C 2 H y Cl 3-y • + Cl - . The results suggest that a higher degree of chlorination favors reduction, and that reduction pathways involving the C 2 H y Cl 3-y - anions are high energy pathways. I. Introduction The widespread use of polychlorinated ethylenes as solvents has resulted in their ubiquitous presence in the environment. Due to their volatility, these toxic compounds are widely dispersed at low concentrations in the atmosphere. 1 In the subsurface, the immiscibility of the polychlorinated ethylenes leads to pools and ganglia of nonaqueous-phase liquid below a spill site, which then becomes a source for dissolved-phase contamination that can form a very large plume of contaminated groundwater. 2 This sort of contamination is difficult to remediate using extraction technologies such as conventional “pump and treat”, and therefore, there is great interest in situ remediation strategies that degrade polychlorinated ethylenes. 3 Most of these technologies, whether they be chemical and microbiological, rely mainly on reductive reactions for dechlorination of the contaminants. 4 Two likely reductive pathways for polychlorinated ethylenes (as shown in Figure 1) in anaerobic and reducing groundwater environments are hydrogenolysis (eq 1) and elimination (eq 4 below). 5 The hydrogenolysis mechanism involves two electrons and a proton and results in the formation of a chloride ion and of a new C-H bond. This two electron transfer (ET) process is assumed to occur in two sequential steps: the first electron transfer to the polychlorinated ethylene is a dissociative electron attachment reaction leading to the formation of a polychloro- ethylen-1-yl radical and a chloride ion, eq 2. The second ET allows the newly formed radical to bind to a proton to form a neutral compound, eq 3. The other mechanism, an elimination reaction, involves two electrons and results in the release of two chloride ions and the formation of a double or triple C-C bond (eq 4). This two ET process is also assumed to occur in C 2 H x Cl 4-x + 2e - + H + f C 2 H x+1 Cl 3-x + Cl - (1) C 2 H x Cl 4-x + e - f C 2 H x Cl 3-x • + Cl - (2) C 2 H x Cl 4-x + 2e - f C 2 H x Cl 2-x + 2Cl - (4) 5905 J. Phys. Chem. A 2005, 109, 5905-5916 10.1021/jp0407526 CCC: $30.25 © 2005 American Chemical Society Published on Web 06/11/2005