Reductive dechlorination of hexachloroethane in the environment: mechanistic studies via computational electrochemistry.

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.