THE NEGATIVE RAMAN NON-COINCIDENCE EFFECT OF RING VIBRATIONS

The totally symmetric ring modes of benzene, benzene-d6, pentafluorobe nzene, hexafluorobenzene, cyclopentane, cycloheptane, cyclopentanone, cyclohexanone were investigated with customary Raman devices. For the first time it is shown that a negative or anomalous noncoincidence eff ect seems to be a general property of aromatic and aliphatic ring vibr ations. In agreement with the results of isotopic dilution experiments carried out with benzene and benzene-d6 this new effect is explained in terms of the pair excitation model as a result of a dominant repuls ive resonant intermolecular vibrational coupling.