Measurement of Heteronuclear Bond Distances in Polycrystalline Solids by Solid-State NMR Techniques

Abstract

High resolution dipolar/chemical shift NMR experiments for solids are critically reviewed and used to measure15N-!H bond distances in a series of compounds, many of which have also been studied by neutron diffraction. The results demonstrate that when recorded carefully, with attention paid to the experimental procedures described, two-dimensional dipolar/chemical shift spectra can yield bond distances accurate to within 0.005 A and mutual orientations of dipolar and chemical shift tensors accurate to within 3°. A comparison of the NMR distances with similar data from neutron diffraction experiments shows the NMR distances to be uniformly ~0.035 A longer, a result that is consistent with some vibrational averaging of the15N-'H dipolar interaction. Collectively, the experimental data and procedures described here demonstrate for the first time that high resolution dipolar/chemical shift NMR experiments are a viable method for locating protons in polycrystalline or amorphous solids. Previously, such determinations have been possible only with single crystal neutron diffraction techniques. © 1987, American Chemical Society. All rights reserved.