Ab initio study of carbon-chlorine bond cleavage in carbon tetrachloride

Chlorinated solvents in groundwater are known to undergo reductive dechlorination reactions with Fe(ll)-containing minerals and with corroding metals in permeable-barrier treatment systems. This research investigated the effect of the reaction energy on the reaction pathway for C-Cl bond cleavage in carbon tetrachloride (CCl4). Hartree-Fock, density functional theory, and modified complete basis set ab initio methods were used to study adiabatic electron transfer to aqueous-phase CCl4. The potential energies associated with fragmentation of the carbon tetrachloride anion radical (CCl4-) into a trichloromethyl radical (CCl3) and a chloride ion (Cl-) were explored as a function of the carbon-chlorine bond distance during cleavage. The effect of aqueous solvation was investigated using a continuum conductor-like screening model. Solvation significantly lowered the energies of the reaction products, suggesting that dissociative electron transfer was enhanced by solvation. The potential energy curves in an aqueous medium indicate that reductive cleavage undergoes a change from an inner-sphere to an outer-sphere mechanism as the overall energy change for the reaction is increased. The activation energy for the reaction was found to be a linear function of the overall energy change, and the Marcus-Hush model was used to relate experimentally measured activation energies for CCl4 reduction to overall reaction energies. Experimentally measured activation energies for CCl4 reduction by corroding iron correspond to reaction energies that are insufficiently exergonic for promoting the outer-sphere mechanism. This suggests that the different reaction pathways that have been observed for CCl4 reduction by corroding iron arise from different catalytic interactions with the surface, and not from differences in energy of the transferred electrons.