Understanding environmental reactions of carbon tetrachloride, trichloroethylene, perchloroethylene, and arsenic applying computational chemistry methods

In recent years, as the progress of computational abilities has increased, computational chemistry has become an interesting tool for researchers for investigating mechanisms of environmental reactions when results of experimental explorations are not sufficiently clear. In this research, several environmental reactions have been investigated using ab initio and density functional theory (DFT) methods.;This research investigated the effect of the reaction energy on the reaction pathway for C-Cl bond cleavage in carbon tetrachloride (CT). Ab initio and density functional theories were used to study adiabatic electron transfer to aqueous phase CT. The potential energies associated with fragmentation of the CT anion radical into a trichloromethyl radical and a chloride ion were explored as a function of the C-Cl bond distance during cleavage. The effect of aqueous solvation was simulated using a continuum conductor-like screening model. Solvation significantly lowered the energies of the reaction products, indicating that the dissociative electron transfer was enhanced by solvation. It was found that reductive dissociation electron transfer of CT undergoes a change from an inner-sphere to an outer-sphere mechanism as the reaction energy is increased. The results showed a liner relationship between the activation energy for the dissociation and the overall energy change, which is in good agreement with the results of the Marcus model.;This research also investigated the thermodynamic favorability and resulting structures for chemisorption of trichloroethylene (TCE) and perchloroethylene (PCE) on iron surfaces using periodic DFT with the non-local Perdew-Burke-Enzerhof (PBE) functional. Chemisorption structures were obtained for four physically adsorbed initial configurations. An initial configuration with two carbons (C-bridge) physically adsorbed at bridge sites between adjacent iron atoms was shown to be the most stable configuration for TCE, while the mode with two carbons (C-hollow) physically adsorbed at hollow site was verified to be the most stable configuration for PCE.;Finally, in this research, we evaluated the accuracies and costs of several DFT methods including Harris, PWC LDA, and BLYP GGA functionals for interaction of arsenite with ferric hydroxides by comparison to calculated and experimental properties of surface complexes. It was found that the approach of using low-level structures coupled with high level single-point energies was much less expensive than the approach of using high level functionals for both structures and energies and could obtain similar computed binding energies. Further work has been done to investigate the appropriate models for interaction of arsenite with ferric hydroxide between pH values of -4 through +4. (Abstract shortened by UMI.).