PNNL

Abstract

Polychlorethylene radicals, anions, and radical anions are potential intermediates in the reduction of polychlorinated ethylenes (C2Cl4, C2HCl3, trans-C2H2Cl2, cis-C2H2Cl2, 1,1-C2H2Cl2, C2H3Cl). Ab initio electronic structure methods were used to calculate the thermochemical properties, ?Hof(298.15K), So(298.15K,1 bar),??GS(298.15K, 1 bar) of 37 different polychloroethylene-yl radicals, anions, and radical anion complexes: C2HyCl3-y•, C2HyCl3-y-, and C2HyCl4-y-• for y = 0,1,2,3 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-• radical anion complexes are ?*, ?*, and -H…Cl- structures. Several stable ?* 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 C2HyCl4-y-• radical anion, in which a chloride ion is loosely bound to a hydrogen of a C2HxCl2-x• radical. The exception is for C2Cl4, in which the most favorable anionic structure is a loose ?* radical anion complex, with a nearly iso-energetic ?* radical anion. Solvation significantly changes the product energetics with the thermodynamically most favorable pathway leads to C2HyCl3-y• + 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.