Conformations of Benzene- and Dibenzo[a,l]pyrene Diol Epoxides Studied by Density Functional Theory:  Ground States, Transition States, Dynamics, and Solvent Effects

The (−)-anti- and (+)-syn-diol epoxides of dibenzo[a,l]pyrene (DBPDE, 11,12-dihydroxy-13,14-epoxy-11,12,13,14-tetrahydrodibenzo[a,l]pyrene) and the stereochemically corresponding benzene diol epoxides (BDE, 1,2-dihydroxy-3,4-epoxy-1,2,3,4-tetrahydrobenzene) have been studied by density functional theory (DFT) to determine the structures, energies, dynamics, thermal properties, and solvent effects on the different conformers. The smaller BDE is used as a model compound for studies of transitions between diequatorial and diaxial conformations of the hydroxyl groups. It was found that DBPDE is distorted due to overcrowding in the fjord region and that the arene oxide prefers to be on the same side of the saturated ring as the distal ring (“in”) in most stereoisomeric states. For the anti-diastereomer, a diequatorial orientation of the hydroxyl groups is preferred, while the orientation preference in the syn-diastereomer seems to depend on the solvent and the in/out conformation. Transition states for the interconversions between in and out conformations of DBPDE as well as between diequatorial and diaxial conformations on BDE have been found, and transition rates have been estimated by transition state theory. The barriers are found to be moderate, the highest being 9.6 kcal/mol. Solvent effects as well as zero-point vibrational energy and thermal effects were included and found to be significant in some cases. The results presented here are in agreement with previous experimental studies.