Computational chemistry applied to studies of organic contaminants in the environment: Examples based on benzo[a]pyrene

The topics of DNA adduct structure, biodegradation mechanisms, photo-chemistry, and adsorption of polycyclic aromatic hydrocarbons (PAHs) are discussed using benzo[a]pyrene and its partial oxidation products as an example. The use of classical mechanical, semi-empirical, and ab initio computational techniques are discussed in terms of their ability to answer important questions regarding the environmental fate of this important carcinogenic compound. The role of H-bonding variation with computational technique was analyzed and significant errors are likely when this parameter is predicted by classical force field simulations or semi-empirical calculations. The stability of the conformations of benzo[a]pyrene also change with computational method although the density functional theory (DFT) and second-order Moller-Plessett (MP2) methods used here converge to similar relative energies. Analysis of the relationship among HOMO-LUMO gaps, triplet and singlet excitation energies, and phototoxicity suggests that the HOMO-LUMO gaps are not the true parameter related to phototoxicity but a correlation between HOMO-LUMO gaps and excitation energies gives the appearance of a cause-and-effect relationship. Methods for predicting the water-soot partition coefficient of PAHs and the complexation of partial oxidation products with cations and metals in the environment are also discussed.