A relationship between Raman and infrared spectra: the case of push–pull molecules

Vibrational spectra of push–pull polyenes show a peculiar feature namely, in infrared and Raman spectra strong, coincident bands appear, arising from vibrations localised on the polyene bridge. A simple model, based on the introduction of an effective internal field due to the charge transfer between end groups allows the infrared and Raman intensities of these bands to be related.