Rotation-translation spectra of polar molecules in liquids

Abstract

Polar liquids and polar solutes in non polar solvents show a strong absorption in the far infrared region of the spectrum below 150 cm-l [l-5]. This absorption cannot be accounted for by intra molecular vibrations of the molecules. It has been described by some authors as a pure rotational spectrum of the polar molecules [3], by others as a “liquid lattice” absorption [5]. It has also been suggested that dimer formation might be responsible for absorption in this frequency region. In order to check these assignments, a more quantitative approach seems necessary. We therefore examined the temperature dependence of these spectra and the appearance of the absorption for different polar molecules in a variety of solvents. All spectra were recorded on a Perkin-Elmer 301 grating spectrometer with an experimental limitation on the low frequency side of 16 or 12.5 cm-l depending on the quality of the detecting system. Fig. 1 shows the far infrared spectra of nitrobenzene, nitromethane, acetonitrile and benzonitrile in dilute solutions in n-heptane. The strong absorption at frequencies of 150 cm-l and higher as found in the benzene derivatives is due to internal vibrations of the phenyl group and is not characteristic for the absorption in the low frequency region. In this low region there is an almost uniform behaviour of all the solutes examined. The absorption starts around 170 cm-l and rises continuously towards longer wavelengths, in most cases till a maximum is reached between 70 end 20 cm-l, in some cases till the frequency limit posed by the instrument is reached. The most striking feature of fig. 1 is the much lower absorption intensity (per molecule) for the phenyl compounds as compared with the methyl compounds. Fig. 2 shows the influence of the solvent on the position of the band. In the more polarizable solvents the entire band shifts to shorter wavelengths. Finally in fig. 3 the influence of the ---CYCN \