Anisotropic exchange symmetric

As well as providing a means of measuring 1 H/2H-exchange in proteins, NMR is a most powerful technique for studying the mobility of individual amino acids. For example, the rotational freedom of the aromatic side chains of tyrosine and phenylalanine about the C 3—Cy bond is readily studied by various NMR methods. ]H NMR can detect whether or not the aromatic ring is constrained in an anisotropic environment. In an isotropic environment or where there is rapid rotation on the NMR time scale, the 3 and 5 protons of phenylalanine and tyrosine are symmetrically related, as are the 2 and 6 (structures 1.12). The resultant spectrum is of the AA BB type, containing two pairs of closely separated doublets. But if there is slow rotation in an anisotropic environment, the symmetry breaks down to give four separate resonances (an ABCD spectrum), since the 5 and 6 protons are in different states from the 2 and 3. At an intermediate time... 

Cross-section structure. An anisotropic membrane (also called asymmetric ) has a thin porous or nonporous selective barrier, supported mechanically by a much thicker porous substructure. This type of morphology reduces the effective thickness of the selective barrier, and the permeate flux can be enhanced without changes in selectivity. Isotropic ( symmetric ) membrane cross-sections can be found for self-supported nonporous membranes (mainly ion-exchange) and macroporous microfiltration (MF) membranes (also often used in membrane contactors [1]). The only example for an established isotropic porous membrane for molecular separations is the case of track-etched polymer films with pore diameters down to about 10 run. All the above-mentioned membranes can in principle be made from one material. In contrast to such an integrally anisotropic membrane (homogeneous with respect to composition), a thin-film composite (TFC) membrane consists of different materials for the thin selective barrier layer and the support structure. In composite membranes in general, a combination of two (or more) materials with different characteristics is used with the aim to achieve synergetic properties. Other examples besides thin-film are pore-filled or pore surface-coated composite membranes or mixed-matrix membranes [3]. 

So far, only isotropic or symmetric exchange coupling has been discussed. Moriya proposed that anisotropic superexchange results when the effects of spin-orbit coupling... 

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