CROSS-POLARIZATION WITH LONG DELAYED CONTACT IN ROTATING SOLIDS

The cross polarization NMR spectra and cross relaxation rates of polyc rystalline hexamethylbenzene and adamantane were obtained by employing a long delay before contact for samples spinning at, and off, the mag ic angle with respect to the static magnetic field. The results are co mpared with those obtained by using the conventional cross polarizatio n pulse sequence. The results support the memory effect explanation of origin of the SPEDA spectra in contrast with the assumptions that the high resolution SPEDA spectra arising either, from the nearly isotrop ic motion of a very small fraction, or a pool of highly thermo-activat ed molecules, or from special spin pairs within the crystallites which are orientated at the magic angle with respect to the static magnetic field.