Pesticide interaction with environmentally important cations: A molecular dynamics and DFT study of metamitron and fenhexamid

Abstract

Metamitron (Meta), 4-amino-3-methyl-6-phenyl-1,2,4-triazin-5-one, and fenhexamid (Fen), N-(2,3-dichloro-4-hydroxyphenyl)-1-methylcyclohexanecarboxamide, are active substances of herbicide and fungicide products, respectively, employed in agriculture. When these molecules are spread on crops, an important amount reaches the soil and diffuse through water to deep layers of the soil structure and thus to groundwater tables. To get some insight into the interaction of pesticides with soil, in particular clay surfaces, we have run theoretical calculations. This paper presents preliminary results of this wide and challenging work. Metamitron, fenhexamid and their complexes with one or two Ca2+ and Na+, which are among the most abundant cations into soils, are under investigation. Conformational searches have been performed thanks to Car-Parrinello molecular dynamics (CPMD) simulations from which geometries as well as relative, complexation and interaction energies have been extracted. Thermodynamical complexation values have been calculated for complexes of Meta or Fen with one or two cation(s) in order to understand the complex stability. We found that complexation free energies are comparable for Meta and Fen: around −32kcal/mol with one sodium cation, ≈−130kcal/mol with one Ca2+, and −7kcal/mol with two Na+. With two Ca2+, complexation free energy is four times larger in absolute value for Fen (−40kcal/mol) than for Meta due to longer inter-cation distances in Fen-(Ca2+)2 complexes. The larger values obtained with Ca2+ in the gas phase suggest a strong adsorption of Meta and Fen with Ca2+-montmorillonite clay surfaces.