Line narrowing cross-polarization

The combination of cross polarization (basically a pulse sequence) and MAS is sufficient to drastically reduce the linewidths of spin-Vi nuclei. Liquid-state proton NMR spectra, as we have seen, are characterized by extremely narrow lines and complex multiplets due to spin-spin coupling in addition, the normal chemical shift range is only around 10 ppm. 

Also, the high mobility present in elastomers creats a weak dipolar coupling so that the cross polarization is inefficient and results in weak enhancement compared to standard free induction decay spectra. As far as material identification is concerned, the spectrum resulting from acquiring a standard pulsed free induction decay at an elevated temperature is adequate. Further research will probably show the narrow lines from the magic angle spectra of natural rubber may allow assignments to lesser components. ... 

However, it is found that a combination of techniques, such as proton dipolar decoupling (removes the dipolar interactions), magic angle spinning (reduces the chemical shift tensor to the isotropic chemical shift value), and cross-polarization (increases the sensitivity of rare spins, like 13C) applied to a solid state material, results in sharp lines for 13C nuclei in the solid state10). Thus, the observation of narrow lines or high resolution NMR in the solid state is possible. 

Solid-state experiments use a dry sample that is packed into a rotor and spun at high frequency inside the spectrometer s magnetic field. This approach is termed cross-polarization magic angle spinning (CPMAS), and is the standard protocol for solids. It relies on transfer of magnetization from protons to C (or other nuclei) in order to achieve rapid analyses with reasonably narrow spectral lines. Cody et al. 

Usually, NMR techniques allow for a clear cut identification of the phase state of the observed component while fluid components show narrow lines preferably observed by direct excitation, the solid constituents yield wide lines under static conditions and are most easily detectable by cross-polarization. However, several exceptions of this mle have to be taken into account ... 

Fig. 26. Experimental ( H)- C cross-polarization spectra of an aqueous dispersion of poly-n-butylcyanoacrylate nanocapsules at different mixing periods tc. The spectra are measured at a resonance frequency of Wc 100 MHz after contact times of cp O.l, 0.25 and 0.5 ms under full proton decoupling. The wide lines derive from the polymer forming the capsule membrane while the narrow lines are assigned to the triglyceride oil. At cp = 0.25 ms, the broad signals of the polymer have almost developed to their full intensity, whereas the signals of the oil still gain amplitude. ...

Fig. 36. Comparison between the ( H)- C cross-polarization spectra of the dispersed nanocapsules after 3 h storage at (a) 50°C and (b) 100°C. For better comparability, the amplitude of the spectrum b has been multiplied by two. The underlying solid-state spectra show almost identical line shapes, indicating that the rotational diffusion and hence the size of the particle remains unchanged.No additional narrow contributions corresponding to polymer particles of decreased size are observed in the cp spectrum.

The cross polarization, or CP, process may be used with any or all of the line narrowing techniques to obtain NMR spectra of solids with resolution approaching that of liquids ). A combination of cross polarization (CP), magic angle spinning (MAS) and dipolar decoupling were used to obtain the spectrum of a very insoluble polyphenylene sulfide (Ryton) as shown in Fig. 6. 

The spin-lattice relaxation time for in solids is very long (several minutes). Since the nuclei have to relax before another excitation pulse can be sent, this requires hours of instrument time in order to collect a spectrum of reasonable intensity. A pulse technique called cross-polarization can be used to reduce this effect by having the protons interact with the carbon nuclei, causing them to relax more rapidly. FTNMR systems for sohd samples include the hardware and software to produce narrow line spectra from solid samples in a reasonable amount of time using high-power dipolar decoupling, MAS, and cross-polarization. 

Exposure of the sample to the atmosphere enables carbon dioxide and water to co-adsorb with the pyridine. The chemical shifts indicate that the carbon dioxide may have reacted to give a carbonate species(, ) whereas, the pyridine spectrum now resembles more closely that of liquid pyridine in that the linewidths are narrower (. 2 ppm) and the intensities are nearly 2 2 l(Figure 4b), The water appears to have altered the surface in such a way as to cause the pyridine to be more loosely bound, or it may be competing with the pyridine for the chemisorbed sites on the surface. However, lengthening the cross-polarization contact time from 1 ms to 3 ms alters the line intensities in favor of the y carbon( Figure 4c) implying that the pyridine maintains a preferential C2 rotation. 

The Si NMR [19] cross-polarization magic-angle spinning (CP MAS) and H NMR magic-angle spinning with multiple-pulsed line narrowing (CRAMPS for combined rotation and multiple-pulse spectroscopy) techniques of Maciel et al. [20,21] indicate that FS-662... 

Between crossed polars these defects appear as dark lines or brushes with curved or irregular shapes that correspond to extinction positions of the director and molecular long axes. Thus, the director can be either parallel or perpendicular to the polarizer and analyzer. The brushes tend to cover the specimen in rather a continuous way, indicating the liquid-like nature of the mesophase. The points where the brushes meet are called singularities in the texture (see Figure 3A). For nematic phases two forms of schlieren defect are found, one where two brushes meet at a point and one where four brushes meet. All tilted smectic phases (C, I, F, and ferrielectric C), except for the antiferroelectric phase, exhibit four brush singularities. Therefore, this provides a simple way of distinguishing between smectic and nematic phases. It should be noted that phases such as smectics A and B(hexatic) and crystal phases B(crystal), E, G, H, J, and K do not exhibit schlieren textures and so this narrows down the possibilities for phase identification. 

In general, crystalline materials, including membrane proteins as the subject of this article, are an ideal target to be studied by solid-state NMR owing to the expected narrow line widths and efficient cross-polarization dynamics. From this... 

Multidimensional NMR provides especially interesting information about polymer dynamics. A long-standing method for qualitative characterization of molecular mobility is H wideline NMR spectroscopy. There, large-amplitude motions are detected through the reduction of the dipolar line width. However, ID proton lineshapes leave many questions open, as they typically represent superpositions of broad and narrow lines, and their relation to different structural units is often not obvious. In a straightforward combination of wideline NMR, cross polarization (CP) and MAS spectroscopy in a 2D experiment [23], it is possible to separate the dipolar patterns for the different structural units (Wideline SEparation, WISE). This is demonstrated in Figure 5.4, where a WISE... 

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