Reductive Dechlorination of Trichloroethylene:  A Computational Study

Abstract

Vitamin B12 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.

Chlorine Isotope Effects from Isotope Ratio Mass Spectrometry Suggest Intramolecular C-Cl Bond Competition in Trichloroethene (TCE) Reductive Dehalogenation

“…The parameter ε PCE , in contrast, relates to changes in the four chemically equivalent positions of PCE (average of primary and secondary isotope effects) and is the same in fits of δ 37 Cl PCE [Equation (9)], δ 37 Cl Cl− [Equation (20)] and δ 37 Cl TCE [Equation (21)], for the same reasons as discussed above for carbon.…”

“…Previous studies, however, considered that selective formation of cis-DCE may be caused by cleavage of either C-Cl bond in the α-positions followed by selective interconversion of cis-/trans-dichloroethene intermediates, as illustrated in Scheme 2 [20,37] With x = 1 or x = 0, the reaction would follow a position-specific cleavage; however with 1 > x > 0 two positions would be involved. According to the derivation shown in the Supporting Information, the enrichment trends in chlorine isotope values of cis-DCE and chloride can be expected to be described by individual enrichment factors according to:…”

“…Respective anionic intermediates have been investigated in a computational study by Nonnenberg et al, which suggest that these intermeditates are not susceptible to interconversion [20], which would be necessary to explain the observed selectivity to form the cis-conformation. Based on our results, the scenario of a direct nucleophilic substitution can, therefore, be ruled out for reduction of TCE by Geobacter lovleyi.…”

“…The behaviour of chlorovinyl-radical species was investigated in the computational study by Nonnenberg et al [20]. From their calculations, the dichlorovinyl-radicals may indeed be susceptible to a selective interconversion from the trans-into the energetically preferred cis-conformation of DCE (Scheme 4).…”

“…Specifically, it remains unclear whether the transformation of TCE to cis-DCE is stereoselective at the geminal chlorine substituent in E-position-as one would presume for a nucleophilic substitution mechanism [19]-or whether both geminal E-and Z-positions are involved-as brought forward for SET by computational results from Nonnenberg et al resulting in the formation of radical intermediates [20]. Involvement of both germinal chlorine substituents would also be expected for the pathway of nucleophilic addition, as suggested by a computational study by Pratt and van der Donk [21].…”