Application of QSARs to Predict Transformations of Polyhalogenated Aliphatic Compounds in Homogeneous and Heterogeneous Systems. R. Venkatapathy, NCEA-US Environmental Protection Agency, OH; and J. A. Perlinger, Dept. of CEE, Michigan Technological University, MI
Polyhalogenated aliphatic compounds (PHAs) represent important classes of environmental pollutants because of their high production volumes and widespread use. Under reductive environments, PHAs may be reductively dehalogenated, yielding a variety of products that may be less or more toxic than the parent compounds. The toxicity of the parent compounds and their respective dehalogenated products depend on their concentration in the environment, which in turn depends on the kinetics of the transformation reaction. Quantitative Structure Activity Relationships (QSARs) were developed to predict rates of transformation of PHAs in homogeneous and heterogeneous systems. The descriptors for predicting rates depended on the rate-limiting step in the compounds’ reaction with a reductant. Statistical techniques were used to examine rate-limiting steps such as those due to diffusion, sorption or dissociative electron-transfer in the transformation of PHAs in various systems. Descriptors that were chosen to describe electron-transfer were calculated using quantum chemistry while diffusion and sorption of reactants and products to/from the surface were described using physical-chemical properties of the PHAs. While the descriptors for predicting transformations in homogeneous systems was explained using Marcus theory, descriptors for transformations in heterogeneous systems had no theoretical basis. Results from QSARs for homogeneous systems support earlier hypotheses based on PHA kinetic results that the rate-limiting step in the homogeneous systems is a dissociative one-electron transfer. Experiments were carried out for three compounds that were not in the original training set to verify the QSARs for two different homogeneous systems. The difference between measured and predicted rate constants were within the uncertainties at 95% confidence, thus demonstrating the predictive ability of QSARs for transformation of PHAs in homogeneous systems.
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