As the density of a gas increases, free rotation of the molecules is gradually transformed into rotational diffusion of the molecular orientation. After ‘unfreezing’, rotational motion in molecular crystals also transforms into rotational diffusion. Although a phenomenological description of rotational diffusion with the Debye theory is universal, the gas-like and solid-like mechanisms are different in essence. In a dense gas the change of molecular orientation results from a sequence of short free rotations interrupted by collisions. In contrast, reorientation in solids results from jumps between various directions defined by a crystal structure, and in these orientational ‘sites’ libration occurs during intervals between jumps. We consider these mechanisms to be competing models of molecular rotation in liquids. The only way to discriminate between them is to compare the theory with experiment, which is mainly spectroscopic.

Line-shape analysis of the absorption or scattering spectra supplies us with normalized contours Gℓ(ω) which are the spectra of orientational correlation functions Kℓ = 〈Pℓ; [u(t)·u(0)]〉. The full set of averaged Legendre polynomials unambiguously defines the orientational relaxation of a linear or spherical rotator whose molecular axis is directed along the unit vector u(t). Unfortunately, only the lowest few Kℓ are available from spectroscopic investigation. The infrared (IR) rotovibrational spectroscopy of polar molecules gives us G1(ω – ωυ) which is composed of some rotational branches around vibrational frequency ωυ.