A comparison of calculated and experimental geometries for crowded polycyclic aromatic hydrocarbons and their metabolites.

It has become useful to consider the subclass of PAHs with a crowded bay re gion because of similar biological activity within the subclass. Crowding i n the bay region of a polycyclic aromatic hydrocarbon results in a twisted molecular geometry. The purpose of this study is to help gauge the utility of various computational methods for determining the molecular geometry of molecules in this subclass and their metabolites. The results from serm-emp irical methods AM1 and PM3, ab initio Hartree-Fock methods and density func tional methods will be compared to experimentally determined geometries for crowded PAHs. It will be seen that excellent geometries for all local mini mum energy structures are obtained from semi-empirical methods. More exact and computationally extensive methods yield equivalent or somewhat better r esults only with good basis sets. However, methods disagree on the relative energies of the isomers of diol-epoxides.