Reductive Dechlorination of Trichloroethylene:  A Computational Study

Vitamin B 12 catalyzes the reductive dechlorination of several ubiquitous pollutants including the conversion of trichloroethylene (TCE) to ∼95% cis -1,2-dichloroethylene (DCE) and small amounts of trans -DCE and 1,1-DCE. The origins of this unexpected selectivity were investigated using density functional and coupled-cluster theory. At all levels of theory considered, the initially formed trichloroethylene radical anion is an unstable species. Breakage of one of the three C−Cl bonds during the dissociative process gives the most stable ion complex when the two remaining chlorines occupy a cis geometry. Once formed, the cis -1,2-dichloroethen-1-yl radical is about 6 kJ/mol more stable than the corresponding trans radical and 21 kJ/mol more stable than the 1,1-dichloroethen-2-yl radical. The calculated relative energies can be rationalized by delocalization of the unpaired electron over the nonbonding orbitals of the α-chlorine. The computed geometries of the radicals suggest significant interactions between the orbital occupied by the unpaired electron and the σ* orbital of the β C−Cl bond trans to the radical. The barrier for interconversion of the two 1,2-dichlorinated vinyl radicals lies between ∼30−40 kJ/mol depending on the level of theory. The reactivities of the three radicals with respect to hydrogen atom abstraction from methanol (C−H or O−H) as well as chlorine elimination were investigated. All three radicals show a strong preference for abstraction of the α-hydrogen atom of methanol (17−25 kJ/mol), with a significant positive reaction energy for chlorine elimination (60−80 kJ/mol). These results are discussed further in relation to the experimentally observed product distribution.