Properties of some condensed aromatic systems

The structures, energies, and other properties of a series of linearly annelated arenes, and of a corresponding series annelated in the fashion of phenanthrene, were examined at the B3LYP/6-311G* level of theory. The compounds in the latter series were uniformly found to have a lower energy than their isomers in the former series. In both series, the C-C bond lengths were found to be correlated with the charge density at the bond critical point, the ellipticity at that point, and with the pi-bond order. This indicates that the pi-electron distribution is the dominant factor in determining the bond lengths. The resonance energies were estimated and were found to increase linearly with the number of rings with the [n]phenacenes, but at a slower rate with the linearly annelated benzenes. The ionization potentials were calculated, and the change in electron density on ionization was examined. It was found that the radical cations formed via ionization have the positive charge mainly located at the pi-orbitals that had the highest bond order in the parent compounds and at the hydrogens. The carbons tended to increase their electron populations in the a-bonds. The difference in the UV spectra of the two series of compounds also has been studied.