DFT Studies on Hydrolyses of Dimethylchlorothiophosphate

Abstract

Many thiophosphates are widely used as substrate analogues in studies of enzymatic reactions as well as organic insecticides in agricultural pest control. These organophosphorus compounds yield toxic metabolites by several reactions before hydrolysis occurs. Therefore, the rates of hydrolyses of these compounds are important for green chemistry. More sluggish degradations of thiophosphate pesticides are often observed as compared with the corresponding phosphate compound: for example, the hydrolysis rate of diazinon (I, X=S) is slower than that of the diazoxon (I, X=O), where the sulfur atom is replaced by oxygen atom, by about 10 times at pH of 7.4, corresponding to the difference in the free energy of activation by about 1.1 kcal/ mol at 20 °C. However, an inverse thio effect (kS/kO > 1) was found occasionally for the hydrolyses of organophosphorus compounds including parathion (II, X=S) and paraoxon (II, X=O), and it was rationalized by the higher electronegativity of oxygen as compared to sulfur. Although, the differential electronic structure including the bond order and charge localization of P=O and P=S bond are discussed in detail, the relative bond order and charge localization are varied for the phosphorothioates and the origin of differential reactivity of those compounds is not clear.