A computational mechanistic study of the hydrolytic degradation of three common pyrethroid insecticides

Abstract

The hydrolytic evolution mechanisms of three pyrethroid insecticides: Imiprothrin, deltamethrin and tetramethrin, commonly applied in bug sprays, were investigated via density functional theoretical (DFT) modelling. Whereas for deltamethrin, hydrolysis terminates after cleavage of the ester linkage in the starting compound via an asynchronously formed four membered cyclic transition state, for the other two compounds, the final transition state result in the release of formaldehyde and a cyclic amide via a five membered cyclic activated complex. Furthermore, though the mechanisms for all compounds are limited by the ester hydrolysis step; the first for deltamethrin and tetramethrin, the hydrolytic evolution for imiprothrin commences with a water catalysed tautomerization process. The limiting activation free energy barrier, for all three compounds: ΔGǂ = 221–290 kJ mol−1, is such that their residence time in air should be several months; the shortest of which would be that for deltamethrin followed by tetramethrin.