The Energetics of the Hydrogenolysis, Dehydrohalogenation, and Hydrolysis of 4,4‘-Dichloro-diphenyl-trichloroethane from ab Initio Electronic Structure Theory

Abstract

Most approaches that have been proposed for the remediation of groundwater contaminated with carbon tetrachloride (CCl 4) produce chloroform (CHCl 3) as the major product and methylene chloride (CH 2 Cl 2) as a minor product. Both of these products are nearly as persistent and problematic as the parent compound, but competing reaction pathways produce the more desirable products carbon monoxide (CO) and/or formate (HCOO-). Results scattered throughout the chemical and environmental engineering literature show that the branching between these reaction pathways is highly variable, but the controlling factors have not been identified. If we understood the fundamental chemistry that controls the branching among these, and related, product-formation pathways, we could improve the applicability of a host of remediation technologies (both chemical and biological) to the large plumes of CCl 4 that contaminate DOE sites across the country. This project will provide the first complete characterization of the mechanisms and kinetics of competing degradation reactions of CCl 4 through laboratory experiments in simple model systems closely coordinated with theoretical modeling studies. The results provide strategies for maximizing the yield of desirable products from CCl 4 degradation, and the most promising of these will be tested in column model systems using real site waters and matrix materials. Under Task 1, we have developed and validated the necessary experimental protocols and analytical methods for routinely determining all of the reaction products of CCl 4 reduction over an experimental time course. We have applied these protocols on a numerous batch experiments to test the effects of various experimental conditions (pH, buffer type, buffer composition, iron type, initial concentration of CCl 4 , etc.) We fit these data with an improved kinetic model-which we developed for the current purpose-that allows us to rigorously quantify the yield of CHCl 3. The most notable result is that one particular form of nano-sized iron metal apparently produces much less CHCl 3 than other types of Fe 0. Currently, we are focusing on the fundamental (Task 2) and practical (Tasks 4) implications of this result.

One-Electron-Transfer Reactions of Polychlorinated Ethylenes: Concerted and Stepwise Cleavages

“…Several groups have been interested in applying the methods of computational chemistry to study the environmental degradation of simple and larger organochlorine compounds. 2,21,22,[25][26][27][28][29][30][31][32][33][34][35][36][37][38][39][40][41] In the present study, we extend a previous study in which we reported the thermochemical properties of chlorinated ethylenes, 21 to now provide estimates of the activation barrier as a function of the strength of the reducing agent for the concerted and stepwise reaction mechanisms for all the chlorinated ethylenes in the gas phase and in aqueous solution.…”

Free Energies for Degradation Reactions of 1,2,3-Trichloropropane from ab Initio Electronic Structure Theory

“…The diversity of possible reaction pathways and mechanisms for a compound such as TCP makes it difficult to obtain a comprehensive understanding of the competition among these pathways from experimental studies alone. We and others have been applying the methods of computational chemistry to study the environmental degradation of simple and larger organochlorine compounds. ,,− In the present study, we use electronic structure methods to investigate the thermochemical properties (in the gas phase and in aqueous solution) for the likely degradation pathways of TCP. We report thermochemical propertiesΔ H f ° (298.15 K), S °(298.15 K, 1 bar), Δ G S (298.15 K, 1 bar)calculated using isodesmic reactions schemes, gas-phase entropy estimates, and continuum solvation models for the possible degradation products of TCP shown in Figure .…”

Ab Initio Electronic Structure Study of One-Electron Reduction of Polychlorinated Ethylenes

“…We have been interested in applying the methods of computational chemistry to study the environmental degradation of simple and larger organochlorine compounds. − In the present study, we use electronic structure methods to investigate the thermochemical properties (in the gas phase and in aqueous solution) for the reaction pathways of polychlorochloroethylenes under reducing conditions. We report thermochemical properties, (298.15 K), S °(298.15 K,1 bar), Δ G S (298.15 K, 1 bar) calculated through isodesmic reactions schemes, gas-phase entropy estimates, and continuum solvation models for the polychloroethylenyl radicals and anions shown in Figure , and the open shell polychloroethylene radical anions as shown in Figure .…”