This paper proposes a model of the thermal vibrational excitation of a mean-statistical cluster of a liquid, which reveals a possible mechanism for the formation of an absorption band in the low-frequency IR region, caused by quasi-lattice translational vibrations of the liquid inside the cluster. It is shown how the frequency corresponding to this band is related to the thermodynamic characteristics of the elastic properties of the liquid -the isobaric expansion and isothermal compressibility coefficients (the generalized susceptibilities.) The concept of thermal quantum excitation of clusters in liquids is formulated as quasi-phase transitions of the first kind.
V. N. Demidov, "An expression for the frequencies of the spectral bands of quasi-lattice translational vibrations of liquids in terms of a new thermodynamic model," J. Opt. Technol. 70, 623-627 (2003)