Non-coincidence effect of aromatic ring vibrations

The non-coincidence of the maxima of the isotropic and the anisotropic profiles in Raman spectra has always been considered an efficacious probe of the structure and dynamics of polar liquids. An anomalous, negative non-coincidence effect has been observed in ring breathing modes of aromatic molecules, without dipole moment (p -xylene, mesitylene) or with a weak permanent dipole (toluene, o -xylene, m -xylene). Temperature effects and isotopic dilution have also been performed in toluene and o -xylene, confirming the presence of a resonant coupling mechanism. Previous analogous results of other aromatic non-polar liquids (benzene, hexafluorobenzene) have been confirmed. It has been hypothesized that quadrupolar intermolecular forces and steric repulsive interactions balance each other giving final negative values of non-coincidence effect.