Internal field correction in far-infrared absorption of highly polar liquids

The total integrated intensity of the far-infrared absorption of highly polar liquids is analysed. It is shown that the discrepancy already observed between the theoretical result given by the Gordon sum rule corrected by the Polo Wilson internal field factor, and the experimental result, can be explained by taking into account the shape and the polarisability tensor of the molecule. New experimental results on pure liquid C5H5N and CH3CN and dilutions of C5H5N into benzene are presented. The experimental integrated absorption of these compounds and of some previously studied liquids agree well with our theoretical determination.