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 dechlorina tion of hexachloroethane (HCA) to perchloroethylene (PCE). Aqueous solvatio n effects an included using the SM5.42R continuum solvation model. After co rrection for a small systematic error in the electron affinity of the chlor ine atom, theoretical predictions are accurate to within 23 mV for four aqu eous reduction potentials relevant to HCA. A single pathway that proceeds v ia two successive single-electron transfer/barrierless chloride elimination steps, is predicted to be the dominant mechanism for reductive dechlorinat ion. An alternative pathway predicted to be accessible involves trichlorome thylchlorocarbene as a reactive intermediate. Bimolecular reactions of the carbene with other species at millimolar or higher concentrations are predi cted to potentially be competitive with its unimolecular rearrangement to f orm PCE.