A SIMPLE NUMERICAL PROCEDURE TO ESTIMATE RAMAN INTENSITIES OF CONJUGATED ORGANIC-MOLECULES

The analysis of vibrational spectra on the basis of classical mechanic s is an adequate procedure for large molecules with an extended pi-ele ctron system. The theoretical estimates of the intensity of the vibrat ional bands facilitate the assignment of lines to normal modes. For th is aim, several semiempirical procedures have been developed applying effective atomic charges, polarizabilities, dipole increments, or pola rizabilities for chemical bonds' It is characteristic for these method s that the increments that are considered a quality of the pure molecu le are taken equal for a specific type of bonds, e.g., for all the fiv e-ring bonds of the C-60 fullerene. This treatment ingnores the influe nce of symmetry reduction upon the parameters in the molecule by the e xciting radiation field in the case of Raman scattering. However, this influence is very significant for molecules with extended electron de localization, especially for the pi-electronic system in conjugated or ganic molecules. The distribution of these electrons is very sensitive to an external electric field, a mechanism which is the origin of the strong Raman signals from organic molecules. Vice versa, this depende nce on the pi-electrons makes the Raman technique a very informative m ethod to analyse the molecular electron distribution and its alteratio n by substituents. In this way, the fundamental relationship between m olecular structure and electric polarizability and hyperpolarizability can be investigated which is very important in material sciences,espe cially for the applications of organic molecules in nonlinear optics a s was pointed out recently by Zerbi(2).