Nuclear magnetic resonance motional narrowing

The application of nuclear magnetic resonance (NMR) spectroscopy to polymer systems has contributed to significant advances in understanding of their structure and dynamical properties at the molecular level. From the analytical point of view, NMR spectroscopy is particularly suitable for a determination of the polymer structure by direct observation of the protons and carbons in different structural moieties. However, until the mid-1970s the application of this technique was limited to polymer solutions and to some elastomers in the solid state with a relatively high degree of the molecular mobility which allows the observation of the motionally narrowed absorption signals. 

Broad-line nuclear magnetic resonance has been used to study melting in stearic acid and a mesomorphic crystalline to waxy) phase transition in lithium stearate. Extensive motion, liquidlike, though less extensive than that in an isotropic free-flowing liquid, takes place within the system below the melting point of stearic acid or the crystalline to waxy phase transition of lithium stearate. The amount of liquid-like character, as measured by the intensity of a narrow component in the NMR spectrum relative to the total intensity of the whole spectrum, depends on the presence of impurities in the system and even more significantly on whether and how many times the sample has been melted. 

This anisotropy also poses a challenge to the analysis of crystals by nuclear magnetic resonance spectroscopy, which is covered in Section 5.5. Each change in orientation of a crystal in the spectrometer corresponds to a different angle between the external magnetic field and the chemical bond axes, and a different spectrum. The rapid and random motions of molecules in a liquid average over these differences to yield narrow lines in the spectrum. To duplicate the same effect in crystals, the crystal is powdered and the sample is then rotated at rates of 10 kHz or faster. 

These two results will be analysed below in terms of the model of motional or exchange narrowing, which was originally developed by Anderson [12] as well as by Kubo and Tomita [13]. This model is the basis of many methods in magnetic resonance, in particular in nuclear-spin resonance. It was treated in detail theoreti-... 

The broad lines obtained for solid-state NMR spectra without applying any line-narrowing improvements are due to the different behaviour of nuclear spin interactions in solids compared to liquids. These interactions are averaged to zero or reduced to the isotropic values in liquids by the fast molecular motions, whereas the fixed (and different) orientations (with respect to the external magnetic field Bq) of the local environments of NMR active isotops in the rigid lattice of a solid cause line broadenings. The recorded broad NMR line patterns are superpositions of resonances from randomly oriented individual nuclei due to a random distribution of different orientations, since zeolitic materials usually are microcrystalline powders. Table 1 summarizes the nuclear spin interactions and their behaviour in liquids versus solids (17). 

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