Department of Chemistry, Faculty of Science, Yadegar-e-Imam Khomeini (RAH) Shahre Rey Branch, Islamic Azad University, P.O. Box 18155-144 Tehran, I.R. IRAN
Hartree-Fock (HF) and Density Functional Theory (DFT) methods employed to study the effect of conformational change on the 13C chemical shifts of 6-mono axial and equatorial substituted and 6,7-di axial-axial, equatorial-equatorial and axial-equatorial substitutedderivatives of 5,6,7,8-tetrahydrodibenzo[a,c]cyclo-octene. The geometry of the conformers have been optimized in the gas phase employing the 6-311G(d,p) basis set. The 13C chemical shifts were calculated by Gauge Including Atomic Orbitals (GIAO) method. The correlation between calculated 13C chemical shifts in the gas phase and experimental ones in CDCl3 solvent is linear with squared regression coefficient of 0.96. Also the calculated 13C chemical shift in the gas phase by the HF method shows better correlation with experimental ones compared with DFT method. Calculations of both the paramagnetic and diamagnetic shielding of carbon atoms demonstrate that the difference between experimental 13C chemical shifts of the axial and equatorial substituted carbon atoms are more due to the paramagnetic than the diamagnetic shielding.