Very Low Frequency Far-Infrared Spectra of Mixtures of Dioxan with the Halogens and Related Molecules

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Abstract
There has been considerable interest recently in the study of the submillimetre (or very far-infrared) absorption shown by both polar1–4 and non-polar molecules.2,5–8 Such absorption, in the ∼5–100 cm−1 region, for non-polar molecules is thought2,6 to be caused by fluctuating dipoles associated with classical quadrupole-induced dipole interactions (in general multipole-induced dipole interactions). These effects are said to be “collision-induced” and can be thought of as being due to “collision” complexes in the liquid phase. For polar molecules the absorption is now thought1–4 to be due to libration of the dipole in a “cage” of surrounding molecules (either molecules of the same species or of solvent). This so-called Poley-Hill model9,10 is not inconsistent with the idea1,2 of a residual rotation of the molecular dipole in the liquid phase. This similarity is underlined by the fact that some simple polar molecules11,12 have a liquid phase Poley-Hill absorption the general shape and position of which follow quite closely that of the pressure broadened, gas phase spectrum. Our interest in the far-infrared region stems from our studies on halogen complexes with various n and bπ donors.13,14 The pyridine-I2 system (in cyclohexane) for example shows two bands in the far in frared not present in either component. The band at -183 cm is thought to be the stretching mode of the iodine, perturbed in the complex. The band at ∼96 cm to the v(1-I) band, has so far been interpreted molecular″ stretching modebetween donor and acceptor, v(D-A).