Approaches to high-resolution 1H NMR in solids

Abstract

Principles and selected applications of solid-state 1H NMR are reviewed, with emphasis on the role of local fields and particularly the magnetic dipole-dipole couplings. The strong dipolar fields of the hydrogen nuclei compound the usual difficulties of solid-state NMR, notably the line broadening due to spatial anisotropy, and largely determine what information can be obtained from a spectrum and how. Methods designed to defeat both the 1H- 1H interactions and the orientational broadening, such as a combination of multiple-pulse averaging and magic-angle spinning, may yield highly resolved spectra displaying the isotropic chemical shifts. Although still far from routine, techniques to eliminate homonuclear dipole-dipole couplings have been applied successfully to polymers, zeolites, coals, and surface systems, among others. Yet these same dipolar interactions, if used to advantage, have spectroscopic value as well precisely because they do establish definite relationships among the coupled nuclei according to a well-defined geometrical dependence. Potential uses include the determination of cluster sizes and the measurement of H-X internuclear distances in powders to high accuracy. Some examples of these techniques, plus related methods such as zero-field NMR and proton imaging, are mentioned to suggest the wide range of possible chemical applications.