SEMIEMPIRICAL CALCULATIONS OF THE POLARIZABILITY AND 2ND-ORDER HYPERPOLARIZABILITY OF C-60, C-70, AND MODEL AROMATIC-COMPOUNDS

The polarizabilities (alpha) and the second-order hyperpolarizabilities (gamma) of a number of aromatic hydrocarbons and the two fullerenes C60 and C70 have been calculated by using a finite-field approach with the PM-3 parametrization of the MNDO Hamiltonian. The geometries were optimized. The calculations on the acenes, pyrene, and coronene predict that the linear extension of six-membered aromatic rings is less effective for increasing gamma as compared to extension of the pi-system by addition of ethylene groups and that two-dimensional extension of aromatic six-membered rings is less effective than the one-dimensional extension of these rings. The gamma values for C60 and C70 are larger than expected based on the calculated HOMO energy, whereas they are smaller than expected based on the number of pi-centers. A balance between the number of pi-centers and the degree of distortion apparently dominates the values of gamma for C60 and C70. The calculated values for gamma show a good correlation between the HOMO-LUMO energy difference for the compounds considered in this study. Three-dimensional extension of the pi-system to compounds such as C60 and C70 is less effective for increasing gamma as compared to the two-dimensional extension.