Cross polarization —See

Si NMR studies of solutions are difficult because of the long spin-lattice relaxation times of the nucleus and its negative nuclear Overhauser enhancement. The 29Si-1H dipole-dipole relaxation is inefficient because in most compounds the intemuclear distance is large. Fortunately, the problem of relaxation can often be overcome by resorting to cross-polarization (see Section II,E). 

We have seen that it can be difficult to reach the critical concentration required to observe an isotropic-anisotropic transition because concentrated suspensions of colloids are not always stable. However, orientation of flexible polymers as well as of anisotropic particles in suspension can be induced by flow, a phenomenon that has long been observed, reported, and studied. This phenomenon is especially strong when a pretransitional effect exists, which can be easily observed by the naked eye on a sample that is shaken between crossed polarizers (see for example the section on clays). In these systems, birefringence is induced via mechanical forces, like the shear stresses in a laminar flow ( Maxwell-dy-namo-optic effect ). 

PLM is most often used qualitatively as a contrast enhancement tool. For simple observation, the orientations of the polarization states transmitted by the polarizer and the analyzer are set to be perpendicular, and so ideally, if there is no change in polarization due to the specimen, the field is dark. This (0 and 90°) condition is generally called crossed polars (see Figure 1(f)). An anisotropic specimen rotated between crossed polars has four positions of minimum intensity or extinction (condition 2 in Section 2.17.3.1) and four positions of maximum intensity at 45°. In white light, anisotropic stmctures with k>200nm appear brightly colored when viewed in crossed (or in parallel) polars. 

Fig. 14 are the simulated distributions including the different parent rotational levels. An interesting observation from these distributions is that the shape of the multiplet peak corresponding to each 011 (/I) rotational level for the perpendicular polarization is not necessarily the same as that for the parallel polarization see for example the peak labelled v = 0, N = 22. From the simulations, relative populations are determined for the OH (A) product in the low translational energy region from H2O in different rotational levels for both polarizations. The anisotropy parameters for the OH product from different parent rotational levels are determined. Experimental results indicate that the ft parameters for the 011 (/I) product from the three parent H2O levels Ooo, loi, I11, are quite different from each other. Most notably, for the 011 (/I, v 0, N = 22) product the ft parameter from the foi H2O level is positive while the ft parameters from the Ooo and In levels are negative, indicating that the parent molecule rotation has a remarkable effect on the product anisotropy distributions of the OH(A) product. The state-to-state cross-sections have also been determined, which also are different for dissociation from different rotational levels of H2O. 

Figure 2, Cross-polarization timing diagram ( see text for discussion), (Reproduced with permission from Ref, 40, Copyright 1982, Royal Society of London,)...

Fig. 2 Optical microscopy image of a small section of a poly(ethylene oxide) (PEO) droplet dispersion sample, see text (1000-mm wide) obtained at Tc = - 2.6 °C. Amorphous droplets appear dark and semicrystalline droplets appear white under nearly crossed polarizers. The plot shows the fraction of crystallized droplets as a function of temperature upon cooling (0.4 °C min-1) for homogeneous nucleation. (Reprinted with permission from [84], Copyright 2004 by the American Physical Society)...

I would have thought that a reasonable fraction of the polymer was very much immobilized with the carbon black. If so why do you not see it when cross polarization experiments with spinning are carried out ... 

C. J. Carman We did try to look at the bound rubber. It turns out that there still is too much motion in such rubber to get cross polarization. You can see it by bound rubber measurements. On the time scale of this experiment it does not contribute to the spectrum. 

The former is a protein of 14.7 kDa involved in the multienzyme nucleotide excision repair (NER) pathway with a determined NMR solution structure . In this protein, the Zn + possesses rather a structural than a catalytic role. Zn NMR spectra were acquired using a rather sophisticated probe (for details, see Reference 87) and operating at temperatures 5-250 K. Data acquisition was performed with the application of spin-echo methods for enhanced sensitivity . Specifically, experiments were carried out at 25 K using a combination of CP (cross-polarization) and spikelet echo pulse sequences which provide a considerable increase in signal-to-noise ratio (of the order of 30) relative to a classical quadrupole echo pulse sequence. The proton field strength applied to the above measurements was 60 kHz with a matching field of 20 kHz for zinc and a contact time... 

Optical properties of twinned crystals. Each individual in a twin exhibits its own optical characteristics. If a gypsum twin is seen along its b axis and examined between crossed polarizers, it can be seen that each individual extinguishes independently. The twin plane (100) is a plane of symmetry of the composite whole, and the vibration directions of the two individuals, like all the other properties, are related to each other by this plane of symmetry (see Rig. 57 a). 

---