ATOM CHARGE-TRANSFER IN MOLECULAR POLARIZABILITIES - APPLICATION OF THE OLSON-SUNDBERG MODEL TO ALIPHATIC AND AROMATIC-HYDROCARBONS

The atom monopole-dipole interaction model of Olson and Sundberg is used to calculate molecular dipole polarizability tensors of aliphatic and aromatic hydrocarbons for light in the visible region. Atom monopole and dipole polarizabilities are optimized to fit experimental mean polarizabilities and anisotropies for seven alkanes and five planar aromatic hydrocarbons. For alkanes the best fit is obtained with vanishing atom monopole polarizabilities; i.e., for the model in which no atom charge transfer occurs. For the planar aromatic molecules the best fit is obtained by a model which includes both monopole and dipole atom polarizabilities for the horizontal (in-plane) components and only dipole polarizabilities in the vertical (out-of-plane) components. Only the ring carbon atoms have nonzero monopole polarizabilities in this model. The vertical polarizabilities are treated by an approximation which neglects dipole coupling among atoms in the same conjugated system in order to overcome a violation of energy conservation when dipole coupling is included. The parameters so obtained are used to calculate polarizability tensors for an additional 12 aromatic hydrocarbons, including methylbenzenes and polycyclic aromatic compounds. The calculated mean polarizabilities and anisotropies lie within the ranges of available experimental data in all cases, and the relative magnitudes of principal polarizability components agree with those assigned from experimental data.