Ab Initio Electronic Structure Study of One-Electron Reduction of Polychlorinated Ethylenes

Abstract

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, (298.15 K), S degrees (298.15 K,1 bar), and DeltaG(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 pi*, sigma*, and -H...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...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 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 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.