Intramolecular interference effects in dynamic light scattering from rigid rings

A formula for the apparent diffusion coefficient [D_app(κ)] of a rigid ring is derived from the exact first cumulant of its dynamic structure factor. D_app(κ) is expressed in terms of the ring radius, the diffusion coefficients (D_zz and D_xx) for translation parallel and perpendicular, respectively, to the symmetry axis, and the diffusion coefficients (D and D) for rotation around the symmetry and transverse axes, respectively. D_app(κ) exhibits oscillations as a function of the scattering vector k , which depend on D and the anisotropy of translational diffusion (D_zz ? D_xx). The maxima in D_app(κ) are associated with minima in the static structure factor S(κ, 0), which are due to destructive intramolecular interference. The oscillations in D_app(κ) result from periodic variations in the relative intensities of scattered light from different orientations of the ring, which manifest the various motions to different extents. The orientations contributing most to the scattered intensity are those that exhibit the least destructive interference and consequently contribute most to the decay of the dynamic structure factor. © 1993 John Wiley & Sons, Inc.