PLANAR VIBRATIONS OF BENZENOID HYDROCARBONS - COMPARISON OF BENZENE FORCE-FIELDS AND APPLICATION OF A SIMPLE PREDICTIVE MODEL TO KEKULENE

A simple force field model (MO/8) designed for predictive calculations of aromatic planar vibrations has been compared with recent theoretical and experimental force fields for benzene and applied to polycyclic aromatic hydrocarbons (PAHs). The rms frequency error for benzene is ca. 20 cm(-1) in the MO/8 model, which is better than those for unsealed ab initio calculations in the Hartree-Fock level (ca. 100-130 cm(-1)), the MP2 method (ca. 50 cm(-1)), and local density functional (LDF) and coupled cluster singles and doubles (CCSD) methods (ca. 30 cm(-1)). The MO/8 model requires only topological connections of benzenoid rings for its application to any size of polycyclic benzenoid hydrocarbons. Without any knowledge except for the structure formula the MO/s model was applied to kekulene (C48H24), and on this basis spectral intensities were calculated. The results were found to be in good agreement with available experimental data including IR, fluorescence, and phosphorescence spectra. The essential nature of the benzenoid force field was proved to be governed by the topology of the carbon network, which was effectively incorporated in the MO/8 model for describing highly correlated motions of CC bonds in conjugated systems.