A DFT mechanistic study of two possible hydrolytic evolution pathways of thiamethoxam; implications in food and environmental safety

Abstract

A detail density functional theoretical mechanistic study of two possible hydrolytic evolution pathways for the degradation of the popular neonicotinoid based pesticide: thaimethoxam, is conducted. Hydrolysis of this compound under conditions of high humidity and neutral pH is highly feasible, occurring via two successive asynchronous cyclic transition states of moderate free energies: 178.15 and 11.57 kJ; energy barriers allowing rapid hydrolysis under warm humid conditions. Alternately, the photo-induced hydrolytic transformation, initiated by excitation of ground state thimethoxam (So→S1:236.6 nm), leads to an overall exergonic degradation process, involving two free-radical followed by two successive non-radical steps; the latter processes occurring via activation free energy barriers of 147.01 and 81.13 kJ, respectively. Though both enthalpy driven pathways occur rapidly, indicating low environmental residence time for this pesticide, the highly stable final product is of unknown toxicity.