THE VALIDITY OF THE SHORT-GRADIENT-PULSE APPROXIMATION IN NMR-STUDIES OF RESTRICTED DIFFUSION - SIMULATIONS OF MOLECULES DIFFUSING BETWEEN PLANES, IN CYLINDERS AND SPHERES

The so-called short-gradient-pulse approximation, in which the field-gradient pulses are described as delta functions, is most useful in the description of the pulsed held gradient (PFG) experiment when applied to systems showing restricted diffusion, In this contribution, the validity of this approximation is tested by performing Brownian dynamic simulations for molecules entrapped in planar, cylindrical, and spherical geometries, in all cases with reflecting walls, A rather broad variation in the parameters determining the echo decays is covered and, from the data presented, it is possible to chose suitable parameters when applying the PFG experiment to systems showing restricted diffusion, Some limiting behavior of the echo decays are discussed and compared with theoretical results. (C) 1995 Academic Press, Inc.