Evidence for resonant intermolecular coupling in liquid benzene and pyridine from Raman difference spectroscopy of isotopic mixtures

Raman difference spectroscopy (RDS) affords a powerful tool for measuring very small frequency differences between similar samples. This technique has been used to measure the changes upon isotopic dilution in band frequencies and bandwidths for a number of vibrations of liquid benzene and liquid pyridine. Both blue and red shifts of the vibrational frequencies were observed, and these were interpreted in terms of additive resonant intermolecular coupling and volume effects. Both effects may provide either positive or negative contributions to the frequency shifts. The frequency shifts observed in mixtures containing both benzene and pyridine have also been interpreted in terms of resonance and volume effects.