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

Abstract

Energy gaps between the highest‐occupied molecular orbital and lowest‐unoccupied molecular orbital (ΔEHOMO‐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 ΔEHOMO‐LUMO. Based on vibrational frequency analyses, density functional calculations reliably produce dynamically stable structures that can be used to predict model ΔEHOMO‐LUMO values. Both the semiempirical and ab initio Hartree‐Fock methods were unreliable in predicting dynamically stable structures; hence prediction of ΔEHOMO‐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 ΔEHOMO‐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 ΔEHOMO‐LUMO values may be lowered significantly by adsorption that would lower chemical stabilities. Significant increases in calculated ΔEHOMO‐LUMO that would increase chemical stability of the compounds were not predicted.

Light‐induced cytotoxicity of 16 polycyclic aromatic hydrocarbons on the US EPA priority pollutant list in human skin HaCaT keratinocytes: Relationship between phototoxicity and excited state properties

“…Betowski et al (2002) reported the excited singlet state energy at the CIS/6-31G and CIS/6-311G(d,p) level. However, the CIS excitation energies, as well as the energy obtained through the semiempirical and ab initio Hartree-Fock methods, are always higher than the experimental values (Kubicki et al, 1999). The TD-DFT excitation energies have been proved to be in close agreement with the experimental values for other molecules .…”

Characterization of Cation–π Interactions in Aqueous Solution Using Deuterium Nuclear Magnetic Resonance Spectroscopy

“…P olycyclic aromatic hydrocarbons (PAHs) are a group of ubiquitous hydrophobic organic contaminants (HOCs) that continue to receive considerable attention because of their toxicity, persistence, and extensive distribution in the environment (e.g., Means et al, 1980; Kubicki et al, 1999; Schwarzenbach et al, 2003). In general, the chemical state in which PAHs are present in the environment determines their fate, transport, and overall environmental effects (e.g., Schnoor, 1996; Kubicki et al, 1999; Schwarzenbach et al, 2003). At many contaminated sites, a complex suite of organic chemicals and inorganic elements are commonly present together (United States Department of Energy, 1992; National Research Council, 1994; Spiro and Stigliani, 1996).…”

“…The importance of cation–π interactions from the perspective of environmental science and engineering, however, has gone largely unexplored. For example, it is reasonable to expect that cation–π interactions may affect the distribution of PAHs between water and mineral surfaces in low organic carbon environments (Kubicki et al, 1999; Zhu et al, 2004). Cation–π interactions may also influence the relative toxicities of PAHs and heavy metals that co‐exist in aqueous solution, because their association with one another would probably alter their transport across cell membranes.…”

“…Based on their calculations, the ordering K + > Rb + ≫ Na + , Li + was predicted for the overall binding energies of cation–benzene complexes when the cation desolvation energies were considered. More recently, a molecular modeling study of benzene and selected PAHs in the aqueous and adsorbed states identified the cation–π interaction as a potentially important environmental process (Kubicki et al, 1999). Microscopic characterization of weak solute interactions in aqueous solution has been technically challenging.…”

Editorial: Molecular modelling in soil research

“…During the last 10 years, a wide range of smaller and larger systems has been investigated. This includes the complexation of aluminium in the soil solution (Tunega et al , 2000), molecular models of organic acids (Trout et al , 2005), interactions of organic chemicals with aluminium oxides (Vermohlen et al , 2000), benzene and aromatic hydrocarbons in the adsorbed state (Kubicki et al , 1999), the interaction of herbicides with clay surfaces (Tunega et al , 2004) and organic moieties (Haberhauer et al , 2001), as well as the interactions of aromatic hydrocarbons and soil organic matter (Kubicki & Apitz, 1999). The various studies involved, for example, water as a natural solvent in the contact with surfaces of kaolinite minerals, intercalation mechanisms for different clay minerals, adsorption of phenoxyacetic acid derivatives on kaolinite and montmorillonite, and interaction of organic functional groups with phenoxyacetic acid derivatives.…”