Translation–rotation of rough spheres. Far infrared–microwave estimation of the binary collision approximation

Abstract

The rotation–translation of rough spheres, developed recently by Berne and Montgomery, is considered in relation to microwave and far infrared data on 2-chloro-2-nitropropane in the liquid phase. When there is no momentum transfer between the incoming radiation and the molecules in the liquid, the B-M model reduces to rotational J-diffusion. This fails to match the characteristic frequency shift between gas and liquid of the far infrared power absorption peak. By considering the coupled Langevin equations for translation and rotation in the diffusion limit it becomes clear that for mixed translation/rotation it is not possible for any type of autocorrelation function associated with observable power spectra to remain a single exponential. The results of Berne and Montgomery's calculations on rough sphere fluids should therefore be regarded with caution when dealing with liquids made up of dipolar molecules.