Calculation of vibrational Raman intensities of organic molecules with a delocalized π system

The intensity distribution in vibrational Raman spectra of conjugated organic molecules is numerically calculated by a novel approach. The π-electron system is treated in the Hückel molecular orbital approximation. The effect of the external (static) field upon the molecular orbitals is treated in a closed form. Electron-phonon coupling is treated by taking the resonance integrals as a function of the π-bond distortion. Applying normal coordinates derived from a semiempirical vibrational analysis, Raman intensities are calculated for some polymethines, polyenes and the Ag mode vibrations of fullerene C60 and compared to experimental spectra.