High resolution 1H spectra of powdered solids observed by Hahn echo pulse sequence with magic-angle spinning.

The 1H magic-angle spinning (MAS) spectrum for a typical powdered solid is composed of a high resolution component and a broadline component. The high resolution component can be well isolated from the broadline component by the Hahn echo sequence with a long echo time. Compared with CRAMPS experiment, which measures the proton system as a whole, the high resolution echo-MAS method measures only a fraction of the solid, which is usually small at room temperature and quite different from the majority of the solid in both molecular motion and chemical environment. It is shown that for a sample of fumaric acid monoethyl ester, the chemical shifts of the high resolution component are apparently distinguishable from the isotropic chemical shifts of the broadline component in the CRAMPS spectrum as the temperature approaches the melting point. In addition, for a sample of malonic acid, the echo-MAS spectrum is sensitive to moisture and temperature, while its corresponding CRAMPS spectrum is not. It is suggested that the molecules which produce the high resolution component are related to the lattice defects in a solid, including the surface disorder of the polycrystallites, while the molecules that generate the broadline component are located on the rigid lattice of the solid.