Theory of intramolecular spin relaxation by translational diffusion in locally ordered fluids

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Abstract
The theory of spin relaxation in locally ordered fluids presented in part I of this series is further developed. General expressions are obtained for the zero-frequency spectral density associated with translational diffusion in fluids bounded by planar interfaces. Consisting of spatial integrals involving the potential of mean force and the (possibly spatially varying) diffusivity, these expressions circumvent the need to solve the Smoluchowski diffusion equation. The new results enable direct physical interpretations of the correlation times involved. The effect on the relaxation behaviour of surface-induced dynamic perturbations, in the form of a reduced local diffusivity or a narrow potential barrier, is examined. It is shown that the predictions of the continuous diffusion model agree with those of the traditional discrete exchange model when the local diffusivity is very low or when the barrier is very high.