Prediction of Soil Sorption (Koc) of Herbicides Using Semiempirical Molecular Properties

Abstract

Relationships between soil sorption normalized to organic carbon (Koc) and molecular properties of 71 herbicides were examined. The Koc values were obtained from the literature. Various molecular properties were calculated by quantum mechanical methods using molecular modeling software. The quantitative structure activity relationship (QSAR) models based on four molecular properties, van der Waals volume (VDWv), molecular polarizability (α), dipole moment (μ), and energy of highest occupied molecular orbital (eHOMO), together accounted for 70% of the variation in Koc. Herbicides were broadly divided into six families based on structural similarities, and separate equations were established for each group. The three descriptors, VDWv, α, and μ, along with either energy of lowest unoccupied molecular orbital (eLUMO), or electrophilic superdelocalizability (SE), or eHOMO appeared to be determinants and accounted for 82 to 99% of the variation in Koc. Applicability of these models was tested for one herbicide analogue and 10 metabolites. The QSAR models appear to be specific to structurally similar chemicals. The QSAR models could be developed to predict Koc of structurally similar compounds even before they are synthesized or for some of the metabolites of existing herbicides. Models of this type can also be developed to create priority lists for testing, so that time, money, and efforts can be focused on the potentially most hazardous chemicals.