Ab initio electronic structure study of one-electron reduction of polychlorinated ethylenes

Polychlorethylene radicals, anions, and radical anions are potential intermediates in the reduction of polychlorinated ethylenes (C2Cl4, C2HCl3, trans-C2H2Cl2, cis-C2H2O2, 1,1-C2H2O2, C2H3Cl). Ab initio electronic structure methods were used to calculate the thermochemical properties, Delta H-f(o)(298.15 K), S degrees(298.15 K, 1 bar), and Delta G(S)(298.15 K, 1 bar) of 37 different polychloroethylenyl radicals, anions, and radical anion complexes, C2HyCl3-y*, C2HyCl3-y-, and C2HyCl4-y center dot- 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 C2HyCl4-y center dot- radical anion complexes are pi*, sigma*, and -H center dot center dot center dot Cl- structures. Several stable pi* 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 center dot center dot center dot Cl- complexes of the C2HyCl4-y center dot- radical anion, in which a chloride ion is loosely bound to a hydrogen of a C2HxCl2-x center dot radical. The exception is for C2Cl4, in which the most favorable anionic structure is a loose sigma* radical anion complex, with a nearly iso-energetic pi* radical anion. Solvation significantly changes the product energetics with the thermodynamically most favorable pathway leading to C2HyCl3-y center dot + Cl-. The results suggest that a higher degree of chlorination favors reduction, and that reduction pathways involving the C2HyCl3-y- anions are high energy pathways.