The negative Raman non-coincidence effect of ring vibrations

The totally symmetric ring modes of benzene, benzene-d 6 , pentafluorobenzene, hexafluorobenzene, cyclopentane, cycloheptane, cyclopentanone, cyclohexanone were investigated with customary Raman devices. For the first time it is shown that a negative or anomalous noncoincidence effect seems to be a general property of aromatic and aliphatic ring vibrations. In agreement with the results of isotopic dilution experiments carried out with benzene and benzene-d 6 this new effect is explained in terms of the pair excitation model as a result of a dominant repulsive resonant intermolecular vibrational coupling.