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 (C ₂C1 ₄, C ₂HCl ₃) trans-C ₂H ₂Cl ₂, cis-C ₂H ₂Cl ₂, 1,1-C ₂H ₂Cl ₂, C ₂H ₃Cl). Ab initio electronic structure methods were used to calculate the thermochemical properties, ΔH _f°(298.15 K), S°(298.15 K,1 bar), and ΔG _S(298.15 K, l bar) of 37 different polychloroethylenyl radicals, anions, and radical anion complexes, C ₂H _yCl _3-y C ₂H _yCl _3-y, ^-, and C ²H _yCl _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 ₂H _y,Cl _4-y radical anion complexes are π*, σ*, and -H⋯CI ^-structures. Several stable π* 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 ₂H _yCl _4-y ^• radical anion, in which a chloride ion is loosely bound to a hydrogen of a C ₂H _xCl _2-x ^• radical. The exception is for C ₂Cl ₄, in which the most favorable anionic structure is a loose σ* radical anion complex, with a nearly iso-energetic π* radical anion. Solvation significantly changes the product energetics with the thermodynamically most favorable pathway leading to C ₂H _yCl _3-y ^• + Cl ^-. The results suggest that a higher degree of chlorination favors reduction, and that reduction pathways involving the C ₂H _yCl _3-y ^- anions are high energy pathways.