Anisotropy magnetic field

The concept of an anisotropy magnetic field HA, which is referred to simply as the anisotropy field , is also introduced to describe magnetic anisotropy. The anisotropy energy is considered to be that of the saturation magnetization moment Ms in an induction /r0//A, i.e. K= -/r0//AMs cos 9, where 9 is the angle between IIA and Ms. 

Chemical shift anisotropy (CSA) 2 Reorientation of the CSA principal axis Increases with the square of the magnetic field [13]... 

Even for applied magnetic fields below 1 T, the Zeeman energy may be larger than the anisotropy energy. Above the blocking temperature application of a... 

In the light of what we have said above, we might expect that satellites due to platinum-element coupling would be useful in structure determination. However, because of chemical shift anisotropy they are in fact often not visible, and experience (and theory) suggest that the chance of seeing them decreases as the magnetic field of the spectrometer increases. 

We have referred to the various interactions which can cause line broadening in the solid state. One of these, which is normally not a problem in liquid state NMR, is due to the fact that the chemical shift itself is a tensor, i.e. in a coordinate system with orthogonal axes x, y and z its values along these axes can be very different. This anisotropy of the chemical shift is proportional to the magnetic field of the spectrometer (one reason why ultra-high field spectrometers are not so useful), and can lead in solid state spectra to the presence of a series of spinning sidebands, as shown in the spectra of solid polycrystalline powdered triphenylphosphine which follows (Fig. 49). In the absence of spinning, the linewidth of this sample would be around 75 ppm ... 

Anisotropy Non-uniform distribution of electrons about a group which can lead to non-uniform localised magnetic fields within a molecule. The phenomenon leads to unexpected chemical shifts -particularly in 1H NMR - in molecules where steric constraints are present. 

NMR spectroscopy is a powerful technique to study molecular structure, order, and dynamics. Because of the anisotropy of the interactions of nuclear spins with each other and with their environment via dipolar, chemical shift, and quadrupolar interactions, the NMR frequencies depend on the orientation of a given molecular unit relative to the external magnetic field. NMR spectroscopy is thus quite valuable to characterize partially oriented systems. Solid-state NMR... 

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