Simple models of the far infra-red absorption of polar molecules in liquid and rotator phases. Part 2.—Application to systems of non-spherical molecules

Abstract

Two simple models of relaxational and librational absorption of polar molecules in the far infrared (f.i.r.) and microwave regions are applied to the absorption spectra of the condensed phases of non-spherical polar molecules which, in some instances, exhibit molecular association. The systems analysed are the geometrical isomers of dihalogenobenzenes in the liquid state, HCl in the rotator phase at 100 K, and dimethyl sulphoxide (DMSO) and [²H₆]DMSO in the liquid state. Lifetimes of molecular association in the latter systems will be ≲τ_r or ζ/2kT; a good fit to the observed spectra is obtained with reasonable values for the parameters of molecular motion. The parameters so obtained provide a picture of the proposed molecular motion in rotator solids and liquids and an understanding of both temperature and isomeric effects. The librational frequencies of this work are larger by a factor of two than those obtained from dielectric increment data and it is suggested that collisionally-induced dipole absorption may be important in the spectra of the liquid dihalogenobenzenes.