Hindered rotation and oscillation of molecules in liquids and in crystals

Abstract

The extended portion of rotation wings in liquids and the separate lines which take their place in the corresponding solids are explained as due to small oscillatory motions of molecules about their equilibrium positions. The motion is properly quantised in solids on account of the perfect crystalline arrangement and results in distinctly separated lines possessing definite frequency shifts. In liquids, there is only an imperfect quantisation on account of the quasi-crystalline arrangement and this results in a broadening of the lines into a continuous wing. A rough computation of the oscillation frequency in benzene from the specific heat data on the lines indicated by Pauling gives 72 cms.^-1 which is of the expected order of magnitude. These oscillations, as pictured by Pauling, may be regarded as incomplete rotations and such a picture involves a direct relationship between the optical anisotropy of the molecule and the intensity of the scattered line. This is in accordance with the experimental fact that only substances like benzene and naphthalene which have a marked degree of anisotropy give rise to extended wings in the liquid state and lines in the solid state.