Manifestation of dipole-dipole intermolecular forces in VI-brational spectra of organic compounds

The temperature dependence of line width in the Raman spectra of acetonitrile and benzonitrile in the liquid state was investigated experimentally by photographic and photoelectric methods. By excluding the effect of dipole-dipole broadening from the observed line width, the temperature dependence of the component width due to rotational Brownian motion of the molecules in the liquid was revealed. It was found that the parameters of the rotational Brownian motion undergo sharp changes near the crystallization points of the liquids. To explain this effect it is suggested that molecular association occurs in the liquid at low temperatures under the influence of dipole-dipole forces. By simple calculations, an assignment is made of the frequencies of benzonitrile in the low-frequency region, and the forms of the molecular vibrations are established.