Local magnetic field distribution in a polycrystalline sample exposed to a strong magnetic field

A model for a polycrystalline material is proposed to describe the local magnetic field distribution that gives rise to susceptibility broadening in nuclear magnetic resonance. The following contributions to the broadening have been accounted for: (i) broadening due to various orientations of the crystallites containing the resonating spin; (ii) broadening due to different orientations of neighboring crystallites; and (iii) broadening due to variations in the distribution of neighboring crystallites. The effect of each contribution on the NMR spectra taken with and without magic angle spinning has been studied for axially symmetric susceptibility tensors. Numerical simulations of the overall magnetic field distribution have been carried out for different packing densities and anisotropies Delta(chi) of the susceptibility tensor. The overall field distribution appeared to be symmetric with a first moment that is not affected by the presence of neighboring crystallites. The MAS line-narrowing factor was found to decrease for higher packing density and increasing Delta(chi). The proposed method also allows the calculation of local gradients associated with the local magnetic fields, H-1 NMR experiments on adsorbed molecules in NaX zeolite have been used to test the proposed model and to examine the limitations that magnetic susceptibility forces upon spectral resolution. (C) 1996 Academic Press, Inc.