Thermal and Magnetic Properties of Coronene and Related Molecules

Ab initio calculations of coronene and other benzenoid aromatic hydrocarbons at the SCF, MP2, and density-functional levels are used to estimate the gas-phase Δ H f ° of coronene. The results obtained show that the best improvement over the SCF Δ H f ° is obtained using the correlation-corrected B3LYP density-functional method, while the MP2 results appear to make agreement with the experimental value worse. The B3LYP/6-311G** Δ H f ° of coronene and benzo[ ghi ]perylene are 71.8 and 73.2 kcal/mol. The computed proton and 13 C NMR chemical shifts of coronene are in good agreement with experiment. The calculated nucleus-independent chemical shifts (NICS) suggest that the central ring of coronene has diminished aromatic character, but the peripheral rings are fully aromatic. For benzo[ ghi ]perylene, the NICS values reveal that only three of the five peripheral rings are fully aromatic. The magnetic susceptibility of coronene shows a large exhaltation, Λ, relative to a nonaromatic model system. Thermal and magnetic properties show a small additional aromatic character in coronene over and above model systems.