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A COMPUTATIONAL CHEMISTRY STUDY OF THE ENVIRONMENTALLY IMPORTANT ACID-CATALYZED HYDROLYSIS OF ATRAZINE AND RELATED 2-CHLORO-S-TRIAZINES
Citation:
Sawunyama, P. AND G W. Bailey. A COMPUTATIONAL CHEMISTRY STUDY OF THE ENVIRONMENTALLY IMPORTANT ACID-CATALYZED HYDROLYSIS OF ATRAZINE AND RELATED 2-CHLORO-S-TRIAZINES. PEST MANAGEMENT SCIENCE 58(8):759-768, (2002).
Impact/Purpose:
Elucidate and model the underlying processes (physical, chemical, enzymatic, biological, and geochemical) that describe the species-specific transformation and transport of organic contaminants and nutrients in environmental and biological systems. Develop and integrate chemical behavior parameterization models (e.g., SPARC), chemical-process models, and ecosystem-characterization models into reactive-transport models.
Description:
Many chlorine-containing pesticides, for example 2-chloro-s-triazines, are of great concern both environmentally and toxicologically. As a result, ascertaining or predicting the fate and transport of these compounds in soils and water is of current interest. Transformation pathways for 2-chloro-s-triazines in the environment include dealkylation, dechlorination (hydrolysis), and ring cleavage. This study explored the feasibility of using computational chemistry, specifically the hybrid density functional theory method, B3LYP, to predict hydrolysis trends of atrazine (2-chloro-4-ethylamino-6-isopropylamino-s-triazine) and related 2-chloro-s-triazines to the corresponding 2-hydroxy-s-triazines. Gas-phase energetics are described on the basis of calculations performed at the B3LYP/6-311++G(d,p)//B3LYP/6-31G* level of theory. Calculated free energies of hydrolysis (DeltahG298) are nearly the same for simazine (2-chloro-4-6-di(ethylamino)-s-triazine), atrazine, and propazine (2-chloro-4-6-di(isopropylamino)-s-triazine), indicating that hydrolysis is not significantly affected by the side chain amine-nitrogen alkyl substituents. Aqueous solvation effects were examined by means of Self-Consistent Reaction Field methods (SCRF). Molecular structures were optimized at the B3LYP/6-31G* level using the Onsager model, and solvation energies were calculated at the B3LYP/6-311++G(d,p) level using the isodensity surface polarizable continuum model (IPCM). The extent of solvent stabilization was greater for ionic species than neutral species. Likewise, solvation reduced the hydrolysis barrier for protonated atrazine by 36.0 kJ mol-1.