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Computational chemistry study of the environmentally important acid‐catalyzed hydrolysis of atrazine and related 2‐chloro‐ s ‐triazines
Author(s) -
Sawunyama Phillip,
Bailey George W
Publication year - 2002
Publication title -
pest management science
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.296
H-Index - 125
eISSN - 1526-4998
pISSN - 1526-498X
DOI - 10.1002/ps.522
Subject(s) - chemistry , solvation , isopropyl , hydrolysis , computational chemistry , triazine , alkyl , diamine , atrazine , substituent , organic chemistry , medicinal chemistry , molecule , pesticide , agronomy , biology
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‐ N 4 ‐ethyl‐ N 6 ‐isopropyl‐1,3,5‐triazine‐2,4‐diamine) 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 (Δ h G 298 ) are nearly the same for simazine (2‐chloro‐ N 4 , N 6 ‐diethyl‐1,3,5‐triazine‐2,4‐diamine), atrazine, and propazine (2‐chloro‐ N 4 , N 6 ‐di‐isopropyl‐1,3,5‐triazine‐2,4‐diamine), suggesting that hydrolysis is not significantly affected by the side‐chain amine‐nitrogen alkyl substituents. High‐energy barriers also suggest that the reactions are not likely to be observed in the gas phase. 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). Although the extent of solvent stabilization was greater for cationic species than neutral ones, the full extent of solvation is underestimated, especially for the transition state structures. As a consequence, the calculated hydrolysis barrier for protonated atrazine is exaggerated compared with the experimentally determined one. Overall, the hydrolysis reactions follow a concerted nucleophilic aromatic substitution (S N Ar) pathway. Published in 2002 for SCI by John Wiley & Sons, Ltd