Theoretical studies on stabilities and spectroscopy of C84O

The equilibrium geometries and relative stabilities of the 32 possible isomers of C84O have been studied using density functional theory (DFT) at the B3LYP/6-31G level. At the ground state, the most stable geometry of C84O has been found to be 12,13-C84O. In 12,13-C84O, the oxygen atom is added to the 6/6 bond, and an annulene-like structure is formed. Based on the optimized geometries at the B3LYP/6-31G level, the electronic absorption spectra and C-13 NMR spectra of the stable isomers of C84O have been calculated with the INDO/CIS and B3LYP/6-31G methods, respectively. Compared with those of C-84(D-2), the main absorptions in the electronic spectrum of 12,13-C84O are red-shifted, and the chemical shifts of the bridged carbon atoms are changed upfield. The aromaticities of the 32 isomers of C84O characterized by the nucleus independent chemical shifts (NICS) have been investigated at the B3LYP/3-21G level. The aromaticities of the isomers with the annulene-like structures of C84O are better than those of the isomers with the epoxy structures. The dependence of the NICS values in 12,13-C84O on the distance of the helium atom from the cage center has also been explored. (C) 2008 Elsevier B.V. All rights reserved.