Chemical shift anisotropies magnetic field dependency

Chemical shift anisotropy (CSA)—the dependence of the chemical shift on the orientation of the nuclear spin with respect to the magnetic field a nuclear spin relaxation mechanism is important for C, N, and F. 

As is seen from Eq. (4), chemical shift (unlike dipolar coupling) is field dependent. This has important consequences for nuclei with large chemical shift anisotropies, such as13C. While chemical shift anisotropy (CSA) for sp3 carbons is 15-50 ppm, it is 100-200 ppm for sp and sp2 carbons. In an applied field of 1.41 T, 200 ppm translates as 3.02 kHz, but in the very large field of 11.74 T (in a 500 MHz magnet), it translates as 25.2 kHz. Since the sample must be spun at a rate comparable with the size of the CSA (in hertz) it is... 

In C NMR spectroscopy, deviations from a Lorentzian lineshape, which is usually obtained in liquids, can be caused by a chemical shift anisotropy (CSA). If a CSA is present, the position of the resonance line depends on the relative orientation of the molecule with respect to the direction of the magnetic field applied (27,22). The superposition of the individual resonance lines results in typical lineshape patterns that can be described by two parameters the chemical shift anisotropy, AS, and the asymmetry parameter, Tj, respectively. In the case of an axially symmetric CSA tensor, i.e., 17 = 0, the relation between the resonance frequency, w, and the orientation of the molecule is given by... 

For chemical shift anisotropy, the screening constant is not a simple arithmetic value but a tensor, which expresses itself in the fact that it depends on the orientation of the molecule in the magnetic field. This is not a problem for a liquid, in which Brownian movement results... 

In solids, the chemical shift is a directional property and varies with the crystal s orientation in the magnetic field. This orientation dependence, the chemical shift anisotropy (CSA), can be described by a symmetric second-rank tensor that can be diagonalized and reduced to three principal values 5n, 822,833, where the isotropic chemical shift, 8i, is the average of these values 8i = l/3-(8n + >22 + 833). For a single crystal, or any particular nucleus in a powdered sample, the frequency varies according to ... 

Relaxation is a complex phenomenon, depending on many factors [2], such as dipole-dipole relaxation, chemical shift anisotropy, spin-rotation relaxation, quad-rupolar relaxation etc. Examples for macroscopic factors are sample and magnetic field homogenity and the viscosity of the NMR solvent. When running NMR experiments on ionic liquids, we have to deal with three main challenges ... 

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