Simple models of the far infra-red absorption of polar molecules in liquid and rotator phases

Abstract

Two simple theoretical models of librational and relaxational absorption of polar molecules in the microwave and far infra-red regions are discussed. The first model is an auto-correlation function treatment of a librator in a multi-well potential, while the second is an itinerant oscillator model of a damped librator within a temporary cage of its nearest neighbours which undergo co-operative diffusional reorientation. These models are compared and applied successfully to the absorption spectra of the rotator and liquid phases of (CH₃)₃CCl and CH₃CCl₃, and to liquid CH₂Cl₂ and C₆H₅Cl, giving reasonable values of the parameters of molecular motion (e.g. barrier heights V∽2.2 to 10 kJ mol^–1, 2π/ω;₀∽τ_i∽τ_j∽0.3 to 1 ps, ξ/2kT∽τ_r∽τ_D5 to 20 ps), comparison being made with other calculations where possible. The temperature dependence of the librational frequency (2π/ω₀) and the apparent collision frequency (1/τ_i) is discussed, as is the simplifying assumption of a single librational frequency.