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-unocc upied molecular orbital (Delta EHOMO-LUMO) for a suite of common polycyclic aromatic hydrocarbons (PAHs) in the gas-phase were calculated with three d ifferent 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 relati ve HOMO-LUMO gap information, but these methods overestimate the actual Del ta EHOMO-LUMO. Based on vibrational frequency analyses, density functional calculations reliably produce dynamically stable structures that can, be us ed to predict model Delta EHOMO-LUMO values. Both the semiempirical and ab initio Hartree-Fock methods were unreliable in predicting dynamically stabl e structures; hence prediction of Delta EHOMO-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 he ld methods and explicit solvation. Self-consistent isodensity polarized con tinuum model Calculations modeling water and octanol solvation do not chang e calculated Delta EHOMO-LUMO values enough to affect predicted phototoxici ties; thus, gas-phase values may be used for PAHs in solution and in vivo. Energetics of PAH bonding to mineral surface gropes were also modeled. In s ome cases, interaction of PAHs with model aluminate surface defects suggest s that Delta EHOMO-LUMO values may be lowered significantly by adsorption t hat would lower chemical stabilities. Significant increases in calculated D elta EHOMO-LUMO that would increase chemical stability of the compounds wer e not predicted.