Application of the confined rotator model for microwave and far-infrared absorption of some polar liquids

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Abstract
The confined-rotator model of librational and relaxational absorption of polar molecules in the microwave and far-i.r. regions is applied to some dipolar liquids. An acceptable fit to observed data was obtained by using only two model parameters, viz. the mean time between collisions, τ, and the halfwidth of the potential well, β. With increasing temperature 2β becomes broader, while τ decreases. The absorption band of water near ca. 200 cm–1, which has no analogue among “simple” polar liquids, seems to be attributed to an “almost free” rotational motion of a small proportion of water molecules. From this point of view the presence of the second absorption band near ca. 680 cm–1 results from librations of molecules of an “ice-like” framework and allows us to consider water as an anomalous liquid with two clearly defined “structures”. The new c.p.-method is used for calculations of the complex permittivity, ε=ε′–iε″. It comprises the determination of a complex power for rotators interacting with radiation. Using the frequency dependence of ε(ω) a dipole autocorrelation function is calculated. Comparison with other models is made.

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