Cross-polarization from

Figure 8.1. (a) Spherulites growing in a thin film of isotactic polystyrene, seen by optical microscopy with crossed polars (from Bassett 1981, after Keith 196.3). (b) A common sequence of forms leading to sphertililic growth (after Bassett 1981). The fibres consist of zigzag polymer chains. 

In addition to sample rotation, a particular solid state NMR experiment is further characterized by the pulse sequence used. As in solution NMR, a multitude of such sequences exist for solids many exploit through-space dipolar couplings for either signal enhancement, spectral assignment, interauclear distance determination or full correlation of the spectra of different nuclei. The most commonly applied solid state NMR experiments are concerned with the measurement of spectra in which intensities relate to the numbers of spins in different environments and the resonance frequencies are dominated by isotropic chemical shifts, much like NMR spectra of solutions. Even so, there is considerable room for useful elaboration the observed signal may be obtained by direct excitation, cross polarization from other nuclei or other means, and irradiation may be applied during observation or in echo periods prior to... 

Fig. 10.22. Diagram showing the cross-polarization from protons, H, to a heteronucleus, X, such as carbons. Heteronuclear dipolar coupling enables the transfer of magnetization from H to X, such as protons to carbons. Homonuclear dipolar coupling between the abundant protons enables the redistribution of proton spin energy through spin diffusion.

Fig. 11.8 (a) REDOR pulse sequence for the determination of dipolar couplings between 13C and 15N. Initially 13C polarization is generated by cross polarization from the protons. During the following evolution period n pulses are used to prevent the averaging of the heteronuclear dipolar... 

De Paul, S.M., Ernst, M., Shore, J.S., Stebbins, J.F., and Pines, A. (1997) Cross-polarization from quadrupolar nuclei to silicon using low-radio-frequency amplitudes during magic-angle spinning. /. Phys. 

Recently, various heteronuclear X—Y double resonance approaches have helped to increase the informational content of static and MAS NMR spectra of such systems. Frank and coworkers136 demonstrate, for the first time, that cross-polarization from 31P to insensitive 29Si nuclei is possible in conjunction with MAS results in significant sensitivity enhancements, and can provide important insights into the structure of inorganic... 

Analysis of CP kinetics and the parameters obtained are a source of important information about host-guest interactions. From the classic I-S model of cross polarization, it was found that the values of T pH relaxation times for guest and host molecules are considerably different. This suggests that DMF carbons cross-polarize from the DMF protons, not from the remote protons, of calixarene. Moreover, such a result proves that the DMF is located outside the calices and between the calixarene molecules. The simple I-S model is not sufficient to describe the cross-polarization of the complex of tetra (C-undecyl)calix[4]resorcinarene with DMA. The best fit of experimental data was seen when the more advanced I-F-S model of cross polarization was adopted [57]. Analysis of CP kinetic parameters provides important information on molecular mobility (Fig. 19, Table 5). 

Kaflak-Hachulska A, Slosarczyk A, Kolodziejski W (2000) Kinetics of NMR cross-polarization from protons to phosphorus-31 in natural brushite. Solid State NMR 15 237-238... 

This 2D experiment is well suited for imaging, because the indirectly detected spectroscopic dimension requires stepping of the spectroscopic evolution time t from an arbitrary but fixed rotor phase in only 16 steps through one rotor cycle tg. This dimension introduces the information about molecular order into the spectrum. Following t, magnetization is cross-polarized from H to C, and the response is acquired by frequency... 

Fig. 6.1.10. Comparison of H CRAMPS and C high-resolution spectra of a blend of PEI (U) and PEEK (FI), under cross-polarization from H to " C, before and after, a chemical shift filter (see text) selectively destroys the magnetization of protons in the aromatic region between 5 and 10 ppm [51].

To understand the varied chemistry of the aluminas, techniques need to be developed for studying the surface independent of the bulk. Structural and dynamical aspects of the surface do have their origins in the bulk, but the specific details delineating the surface will be different. Clearly it would be advantageous to apply the same surface selective CP methodology developed for the silicas [12-16] to the surface of the aluminas. Before addressing this particular point, however, we need to consider the feasibility of the experiment. Are the aluminum atoms at the surface indeed observable by NMR methods If surface aluminum atoms are observable, we must then recognize that the spin of interest, Al, is not a spin-1/2 nuclide (/ for Al is 5/2) hence Al has a nonzero nuclear electric quadrupole moment. Cross-polarization from protons to a quadrupolar nucleus presents the experimenter with another layer of complication in compari-... 

Figure 13. NMR spectra (28 MHz) of polycrystalline deuterated dimethoxy-benzene taken with magic angle spinning at 1.07 kHz, with (S ) and without (S°) cross polarization from protons. (Reproduced, with permission, from Ref. 33. Copyright 1980, North Holland Press.)...

Fig. 13. Chevron texture with typical transition bars observed upon cooling from the nematic state for polyester prepared from di-n-propyl-p-terphenyl-, carboxylate and tetrame-thylene glycol. Crossed polarizers. From ref. 20.

A potential problem in obtaining solid-state NMR spectra involves the longitudinal relaxation times which tend to be long, thus compromising sensitivity. To overcome this, cross-polarization from abundant nuclei, such as H, to the dilute spin, S, under observation may be employed. Recycle delays are then determined by the Ti of rather than of S. In addition, the magnetization of S can be increased up to a maximum of 7h/7s-... 

V. Vitzthum, P. MieviUe, D. Camevale, M.A. Caporini, D. Gajan, C. Coperet, M. LeUi, A. Zagdoun, A.J. Rossini, A. Lesage, L. Emsley, G. Bodenhausen, Dynamic nuclear polarization of quadrupolar nuclei using cross polarization from protons surfece-enhanced aluminium-27 NMR, Chem. Commun. 48 (2012) 1988—1990. 

Figure 5.2. Monolayers of the amphiphile 1-monopalmitoyl-( )-glycerol at the air-water interface assemble in domains in which the molecular tilt azimuth is organized in star-shaped patterns. It is possible to preserve this order during the transfer on to a solid support. LB monolayers of this material have been utilized for the anchoring of nematic liquid crystals. The order within the monolayer determines the order within the bulk phase of the nematic liquid crystal (LC). The image here shows the LC cell between crossed polarizers. (From J. Fang, U. Gehlert, R. Shashidar and C. Knobler, Langmuir (1999), 15, 297)...

Fig. 25. Photomultiplier tube (PMT) outputs (upper traces) for 633 nm He Ne light with negative high voltage pulses (lower traces) applied to a PMMA sample 1.09 m long and 0.635 cm thick with aligned and crossed polarizers. From these measurements,...

The excitation of H and Na quadrupole order using adiabatic demagnetization in the rotating frame has been described. Double-quantum crosspolarization between half-integer quadrupolar spin systems, B Na and B-> A1, has been described. Spin-locking of / = nuclei in static and spinning samples has been described and applied to cross polarization from Na to N at a distance of more than 2.6 A. H and Na NMR studies have been... 

G. (2012) Dynamic nuclear polarization of quadrupolar nuclei using cross polarization from protons surface-enhanced aluminium-27 NMR. Chem. Commun., 48,1988-1990. 

Experimentally, the amount of sample required is determined by the sensitivity of the nucleus to be observed. Typically, mM of the sample (5-10 mg of lipid 0.5-1 mg of a small peptide 1-4 mg of a large protein) in a volume of about 0.7 mL or on 10-50 glass plates, may be needed for less sensitive ( H, i N) nuclei. For iR, 3ip and i C in MAS NMR, somewhat less material is required. Cross-polarization from the abundant proton magnetization may improve sensitivity, and decoupling (5-10 kHz for 80-100 kHz for H) is routinely used to improve spectral shape and reduce spectral widths. 

Recently there have been attempts to use magic angle spinning (6) in combination with multiple quantum transitions (21) and cross polarization from (22) to achieve spectral simplification. Such techniques promise to add new dimensions to NMR of anisotropic systems. 

Fig. 5.62. iPP, development of shish kebabs in a strained melt followed by measurements of the Hght intensity after the passage of a beam through a sample (crossed polarizers). The direction of strain is 45° against the crossed polarizers. From Korn-held et al. [62]... 

The polarization transfer from protons to low-gamma nuclei leads to a polarization enhancement of Therefore, in almost all experiments, cross polarization from protons to low-gamma nuclei is used for excitation instead of a 90° pulse on... 

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