Solid-state NMR techniques

The influence of the homonuclear magnetic dipole-dipole interaction on can be reduced either by an increase of the sample spinning frequency, Vjot, (Eq. (20)) or by the application of a multiple-pulse sequence causing an additional averaging of this interaction (combined rotation and multiple-pulse spectroscopy, CRAMPS 19-21 ). With today s instruments, sample spinning frequencies of up to 40 kHz can be reached using MAS NMR rotors with an outer diameter of 2.0 mm. 

application of the MAS technique for the investigation of quadrupole nuclei leads to a decrease of the linewidth of the CT by a factor of about 3.6. A further improvement of the resolution of the NMR spectra of quadrupole nuclei requires the application of more sophisticated techniques, such as the DOR (27) or the more recently developed MQMAS NMR spectroscopy (26,28). Until now, however, neither of these techniques has been applied for investigations of working solid catalysts, and they are therefore not considered in this review. 

Techniques for Signal Enhancement and Discrimination in Solid-State 

Signal enhancement, if performed as a selective method, is very helpful for the assignment of NMR lines. In a more general manner, the application of signal enhancement techniques may improve the signal-to-noise ratio of the NMR spectrum, which is important for investigations of samples at elevated temperatures (Section II.D). 

A suitable way to enhance NMR signals of nuclei with small magnetogyric ratios or low concentrations (rare spins S) interacting with abundant spins I is the polarization transfer from the spin I to the spin S ensemble via a cross polarization (CP) experiment. This experiment is based on a double resonance technique, which can be applied in combination with MAS for the characterization of surface sites of solid catalysts and rigidly bound surface complexes. The pulse sequence used for CP experiments is demonstrated in Fig. 4. 

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Also, new areas for applications are opening up. A most recent development has been the successful demonstration of three-dimensional imaging of ceramic and polymeric materials by solid state NMR techniques. This area is most likely to expand considerably. 

Different solid-state NMR techniques CPMAS NMR, the second moment of the signal, the spin-lattice relaxation time in the rotating frame T p) were combined to reach the conclusion that in the case of por-phine H2P the double-proton transfer is followed by a 90° rotation within the crystal (see Scheme 2). 

The solid-state NMR technique may also be used in cellulose derivatives to follow the degree of substitution and degradation of the chain e.g. as found for cellulose nitrate 16). Investigations on the composition of copolymers may also been done as examplared by celluloseacetate-butyrate given in Fig. 6, 20). Here, owing to relaxation differences the spectra cannot be used for elementary analyses, but for estimating the relative number of the components. 

One of the most important areas of application of the solid-state NMR technique is the investigation of the structures of cross-linked amorphous materials in cases where X-ray diffraction technqiues are not applicable. Polymeric resins are one such important class of materials. A lot of work has been done in this area by several investigators 36,37 38 since the beginning of the 80. Some solid-state NMR data of phenolic resins are presented in Fig. 10. Comparison with liquid state data for... 

This technique is the most widely used and the most useful for the characterization of molecular species in solution. Nowadays, it is also one of the most powerful techniques for solids characterizations. Solid state NMR techniques have been used for the characterization of platinum particles and CO coordination to palladium. Bradley extended it to solution C NMR studies on nanoparticles covered with C-enriched carbon monoxide [47]. In the case of ruthenium (a metal giving rise to a very small Knight shift) and for very small particles, the presence of terminal and bridging CO could be ascertained [47]. In the case of platinum and palladium colloids, indirect evidence for CO coordination was obtained by spin saturation transfer experiments [47]. 

In the 1970s the structure and dynamics of lipid bilayer membranes were extensively investigated by NMR. The principles of the NMR spectroscopy applied to the study of biomembranes are reviewed in Ref. 5, together with the fruitful achievements in the early stage. In the 1980s the NMR biomembrane research was carried out mainly by applying the solid-state NMR techniques [6-11]. Generally, the solid-state spectra are of low reso-... 

Rotational-echo double-resonance (REDOR)(75,79) is a new solid-state NMR technique which is sensitive to through-space carbon-nitrogen interactions between selectively 13C and 15N-enriched sites separated by up to 5A (20-22). The parameter directly measured in a REDOR experiment is the heteronuclear dipolar coupling constant DCN, which is in itself proportional to the inverse third power of the intemuclear distance, rCN. It is this dependence on (icn)3 which accounts both for REDOR s ability to accurately measure short distances and its insensitivity to longer-range interactions. As a technique which can probe, in detail, intermolecular interactions over a distance range of 5A, REDOR is well suited to studying the distribution of small selectively-labeled molecules in polymer delivery systems. 

Solid-State NMR Techniques for the Structural Determination of Amyloid Fibrils... 

Numerous new developments and applications of solid state NMR techniques have emerged. Multidimensional NMR methods are able to probe connectivity patterns of zeolite framework structures and solve ambiguities in line assignments [27], high-resolution techniques for quadrupolar nuclei have been developed [31-34], and powerful double-resonance methods permit the study of spatial... 

This chapter provides a basis of theory for solid state NMR and a brief review of experimental aspects. The main focus of the discussion section is on the application of solid state NMR techniques to the study of polymorphism. 

Crosslinked polymer networks formed from multifunctional acrylates are completely insoluble. Consequently, solid-state nuclear magnetic resonance (NMR) spectroscopy becomes an attractive method to determine the degree of crosslinking of such polymers (1-4). Solid-state NMR spectroscopy has been used to study the homopolymerization kinetics of various diacrylates and to distinguish between constrained and unconstrained, or unreacted double bonds in polymers (5,6). Solid-state NMR techniques can also be used to determine the domain sizes of different polymer phases and to determine the presence of microgels within a poly multiacrylate sample (7). The results of solid-state NMR experiments have also been correlated to dynamic mechanical analysis measurements of the glass transition (1,8,9) of various polydiacrylates. 

FIGURE 5. Comparison of 1SN- H bond distances obtained by solid state NMR techniques with distances obtained by single-crystal neutron-diffraction techniques. Reprinted with permission from Reference 108. Copyright (1987) American Chemical Society... 

Solid state NMR has been used to study polymers of various classes over the past several years. In particular, the technique has been used to study curing reactions in epoxies (12). polyimides (1), and acetylenic terminated sulfones (13). The ability to observe the evolution of the carbons of the reacting species has been clearly shown to provide valuable information which has been difficult or impossible to obtain with other techniques. The use of 13C solid state NMR techniques is essential for the understanding of curing reactions in high temperature polymers in order to be able to correlate the reaction chemistry with the structural and resulting physical properties. 

Kokotailo, G.T. and Fyfe, C.A. (1995) Zeolite structure analysis with powder X-ray diffraction and solid-state NMR techniques. Rigaku J., 12, 3-10. 

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