Quantum Chemical Estimation of Environmental and Metabolic Fates of Pesticides

Abstract

Recent progress in computer hardware and programs of molecular orbital (MO) calculations provides an opportunity for us to examine electronic properties of chemicals and reaction mechanisms at a molecular level. The semiempirical and ab initio MO methods have been applied to theoretically estimate the various transformation processes of pesticides in the environment. The electron distribution in the frontier MO's was used as the most convenient index to explain a reactivity of pesticides together with its regioselectivity. Mulliken charge at an individual atom and electrostatic interaction with a hydroxide ion were used as an index to explain the relative ease of hydrolysis for organophosphorus pesticides and hydrolysis pathways of carbamates. The reaction coordinate technique was further applied to seek out transition states and hydrolysis pathways could be estimated together with a substituent effect. Electronic properties in the excited states were useful to estimate photooxidation and photosensitization of pesticides. The photoinduced decarboxylation and isomerization of pyrethroids were successfully explained by the new photolysis index ΔM which was defined as a change of electron population at each bond via molecular excitation. In the case of metabolic oxidation catalyzed by cytochrome P-450, the detailed analysis using reaction indices suggested the triplet mechanism for aliphatic hydroxylation. The relative ease of aromatic hydroxylation at an individual carbon atom was qualitatively estimated by the electron distribution of the highest occupied MO, but the steric constraint at an enzyme active site was found to be an important factor to determine its regioselectivity.