Molecular models of benzene and selected polycyclic aromatic hydrocarbons in the aqueous and adsorbed states

Energy gaps between the highest‐occupied molecular orbital and lowest‐unoccupied molecular orbital (ΔE HOMO‐LUMO ) for a suite of common polycyclic aromatic hydrocarbons (PAHs) in the gas‐phase were calculated with three different molecular modeling methods: semiempirical, ab initio Hartree‐Fock, and density functional calculations. Results indicate that semiempirical, Hartree‐Fock, and density functional calculations may provide useful relative HOMO‐LUMO gap information, but these methods overestimate the actual ΔE HOMO‐LUMO . Based on vibrational frequency analyses, density functional calculations reliably produce dynamically stable structures that can be used to predict model ΔE HOMO‐LUMO values. Both the semiempirical and ab initio Hartree‐Fock methods were unreliable in predicting dynamically stable structures; hence prediction of ΔE HOMO‐LUMO values was not possible for several PAHs. Changes in the HOMO‐LUMO gap of benzene and selected PAHs due to solvation effects were calculated using self‐consistent reaction field methods and explicit solvation. Self‐consistent isodensity polarized continuum model calculations modeling water and octanol solvation do not change calculated ΔE HOMO‐LUMO values enough to affect predicted phototoxicities; thus, gas‐phase values may be used for PAHs in solution and in vivo. Energetics of PAH bonding to mineral surface groups were also modeled. In some cases, interaction of PAHs with model aluminate surface defects suggests that ΔE HOMO‐LUMO values may be lowered significantly by adsorption that would lower chemical stabilities. Significant increases in calculated ΔE HOMO‐LUMO that would increase chemical stability of the compounds were not predicted.