Crossed polarizers example

The first example of the deliberate separation of optically active molecules is appropriate as an example of physical separation in the clearest sense of the term. The molecules are referred to as optically active because polarized light interacts differently with right- and left-handed molecules. In the case of simple diastereomers the RR and SS forms are enantiomers while the RS and SR forms are not. The separation of the latter and former was first done under a microscope using crossed polarizers and the crystals which were seen were separated from those that caused little or no rotation of plane-polarized light by hand using tweezers. A truly physical separation of chemical species using a physical property of chemical origin ... 

In the following, we will discuss heteronuclear polarization-transfer techniques in four different contexts. They can be used as a polarization-transfer method to increase the sensitivity of a nucleus and to shorten the recycle delay of an experiment as it is widely used in 1H-13C or 1H-15N cross polarization. Heteronuclear polarization-transfer methods can also be used as the correlation mechanism in a multi-dimensional NMR experiment where, for example, the chemical shifts of two different spins are correlated. The third application is in measuring dipolar coupling constants in order to obtain distance information between selected nuclei as is often done in the REDOR experiment. Finally, heteronuclear polarization transfer also plays a role in measuring dihedral angles by generating heteronuclear double-quantum coherences. 

In recent years new NMR techniques offering broad applications in stereochemical analysis have come into use. A prominent example is 2D-NMR (both 2D-resolved and 2D-correlated spectroscopy), which has been extensively applied to biopolymers (149-151). Its use with synthetic polymers has, until now, been limited to but a few cases (152, 153). A further technique, cross-polarization magic-angle spinning spectroscopy (CP-MAS NMR) will be discussed in the section on conformational analysis of solid polymers. 

In addition to providing motional information there is another benefit to be derived from CP studies which will be exemplified further in Section 8 and which depends on the fact that the effectiveness of the CP phenomenon is proportional to r 3, where r is the distance between the / and S nuclei. In other words, the efficiency of transfer of magnetization falls off extremely rapidly as the distance between / and S increases. Bulk Si nuclei in Si02, for example, will be relatively distinct from any proton species, whereas Si nuclei at the surface will be relatively close to surface OH groups (or any chemically bound organic species). The use of cross-polarization Si n.m.r. will therefore enable one to examine Si nuclei at the surface relative to those in the bulk material. 

Cross-polarization Facility. - This was perhaps considered a luxury a few years ago, but cross-polarization is gradually gaining in importance as a technique for the study of surface species, for example, on Si02, and as such may well become an essential feature of solid-state instrumentation. Certainly, if the user is planning to study C, Si, or N, then it is well worth the investment in the necessary spectrometer hardware. 

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