Chemical reactivity indices for the organophosphorus compounds in gas and aqueous phases by DFT

Organophosphorous compounds (OP) constitute an important class of pesticides. Organophosphorous pesticides are widely used worldwide in agriculture, though they have a short term degradation and fewer residues. These pesticides are characterized by their high toxicity, low chemical stability and practically void accumulation in tissue. The increasing amount of use overtime is significantly increasing the risks of environmental contamination of groundwater, food, plants, water resources and human beings. These compounds are released in to the enviroment and due to their physicochemical properties, they can disperse in various enviromental media, provoking serious health problems. Therefore, there is a need for certain molecular descriptors to predict the toxicity of OPs. In this study, twentyone organophosphorous pesticides were modeled for both gas and aqueous phases. The most stable conformer for each of the molecules was determined by carrying out a confermer analysis by means of Molecular Mechanics MMFF calculations. Geometry optimizations of the most stable confermers were performed with Density Functional Theory method. The DFT calculations were carried out as implemented in GAUSSIAN 03, using the exchange-correlation functional B3LYP, which combines HF and Becke exchange terms with the Lee?Yang?Parr correlation functional, in combination with the 6-31G* basis set. The solvation effects were computed using COSMO as the solvation model. Based on the quantum mechanical calculations, the charge densities, frontier orbitals, energetic parameters and thermodynamic properties of molecules were determined. These results were used to calculate the chemical reactivity indicies ; hardness, chemical potential, electrophilic indicies and Fukui functions of the molecules. The relationship between the LD50 of the molecules and the calculated descriptors were examined through lineer regression in order to determine the best descriptor showing the toxicity of the molecule. Keywords : Organophosphorus pesticides, quantum mechanical methods, Density Functional Theory, chemical ractivity indices, toxicity, solvent effect.