FormulaC n.m.r. in Organic Solids: Limits to Spectral Resolution and to Determination of Molecular Motion [and Discussion]

The history of high-resolution n.m.r. in solids has been, inter alia, a quest for narrow spectral lines. Yet, with few exceptions, solid state resonances have not been sharpened to the degree of liquid resonances. To aid in the appraisal of the status of n.m.r. in solids, we identify and summarize, for the particular case of $^{13}$C n.m.r. in organic solids, those effects that can degrade resolution. Some of these mechanisms are under the experimenter's control; for example, certain are exacerbated at high magnetic field. Others, however, represent fundamental limitations imposed by the specimen and are valid reflections of the complexity of a solid as contrasted to a liquid. In solids, magnetic dipolar spin-spin couplings can not only degrade resolution but also complicate, hopelessly in some cases, the determination of spin-lattice relaxation rates from which one seeks information about molecular motions. The consequences of this competition between spin-spin and spin-lattice effects are examined, as well as criteria and strategies to isolate the motional contributions to relaxation rates.