Non-coincidence effect of aromatic ring vibrations

The non-coincidence of the maxima of the isotropic and the anisotropic prof iles in Raman spectra has always been considered an efficacious probe of th e structure and dynamics of polar liquids. An anomalous, negative non-coinc idence effect has been observed in ring breathing modes of aromatic molecul es, without dipole moment (p-xylene, mesitylene) or with a weak permanent d ipole (toluene, o-xylene, m-xylene). Temperature effects and isotopic dilut ion have also been performed in toluene and o-xylene, confirming the presen ce of a resonant coupling mechanism. Previous analogous results of other ar omatic 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 n oncoincidence effect.