Pulsed-field-gradient (PFG) NMR is a valuable tool for
In the case of completely restricted diffusion, when molecular species move in confined spaces, the echo attenuation measuring self-diffusion processes in molecular systems. Stimulated by the potential of PFG NMR to provide struc-becomes independent of time for sufficiently long observation times t ӷ t cr . The cross-over time t cr is the time in which tural information over space scales much smaller than those accessible by conventional NMR imaging (1), in the past the species travel over a linear distance of the order of the confinement length d: t cr Å d 2 /2D with D denoting the self-few years the interest of researchers has again concentrated on the investigation of the phenomenon of restricted diffu-diffusion coefficient of free diffusion. Now, the propagator P(z 2 , z 1 , D) becomes simply p(z 2 ), the (a priori) probability sion. The potential of PFG NMR for providing structural information may be rationalized on the basis of its analogy density for finding the species at z 2 after time t. It is no longer dependent on t. For S inc (q, t), one thus obtains with scattering experiments. This analogy has repeatedly been emphasized in the literature (1-5) and has turned out to be a most helpful tool for the design of PFG NMR mea-C å S inc (q, t r ϱ) surements and for the interpretation and correlation of the Å S inc (q) results.
In pulsed-field-gradient NMR, the attenuation of the spin