Theoretical Study of Pyrethroids Type I and II

G.S. OTERO; B. PASCUCCI; P.G. BELELLI; F. ILLAS; M.M. BRANDA

Granada

Congreso; XXXIX Congreso Internacional de Químicos Teóricos de Expresión Latina; 2013

XXXIX Congreso Internacional de Químicos Teóricos de Expresión Latina (QUITEL)

Compounds known as pyrethroids, derived from the family of pyrethrins, are typically employed as constituents of insecticides. Their high lipophilicity, relatively short lifetime in the environment and with low impact on terrestrial vertebrates have led to their wide acceptance for use as insecticides. Both pyrethrins and pyrethroids (like permethrin of the type I and the cypermethrin of type II) are widely used for different types of formulations for agricultural use, domestic use, public health and food preparation stages [1, 2]. Pyrethroids are formed by two structural units, one acidic and one alcoholic. The alcoholic unit can be a primary or a secondary alcohol. Some of the compounds possess a cyano substituent on the position of aa-methylene which increases the toxicity of these compounds. Those who are grouped as type I pyrethroids, have no a-cyano group, but it is present in type II pyrethroids. In order to obtain information on the structural and energetic properties of this family of molecules a computational study was carried out on cypermethrin, permethrin, tetramethrin, allethrin and phenothrin. Calculations were performed using the density functional theory with functional hybrid B3LYP, B3PW91, B98 and B97-2, using in all cases the basis set 6-31G (d, p) for analysis of neutral forms of these molecules. A similar study was carried out on the different ions, a protonated and a deprotonated forms, adducts and fragments which can be detected in chromatographic studies in the gas phase. The B3LYP hybrid functional was selected as recommended in the literature for the study of gas-phase organic molecules [3]. Bond lengths and angles of the optimized structure of cypermethrin obtained from computational analysis were compared with an experimental X-ray diffraction study of the crystal structure of this molecule [4]. For other pyrethroid molecules there are no experimental data in the literature. Calculation of various physicochemical and energetic properties of these molecules allows the prediction of the primary processes of fragmentation associated with the experimental technique of mass spectrometry.