Chemical shift anisotropy . See

Only the anisotropy of the magnetic susceptibility influences the isotropic mean value, whereas the complete susceptibility tensor contributes to the chemical shift anisotropy [see Eq. (36)]. The influence of the susceptibility of a spherical symmetric charge distribution on the isotropic chemical shift of a nearby nucleus will be zero, but there is a contribution to the chemical shift tensor. Especially in solid state proton chemical shift investigations this effect is quite remarkable and can be observed when studying proton chemical shift anisotropies. 

Chemical shielding anisotropy see Relaxation, chemical shielding anisotropy Chemical shift anisotropy see Relaxation, chemical shielding anisotropy Chemical shifts... 

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. 

As we shall see, all relaxation rates are expressed as linear combinations of spectral densities. We shall retain the two relaxation mechanisms which are involved in the present study the dipolar interaction and the so-called chemical shift anisotropy (csa) which can be important for carbon-13 relaxation. We shall disregard all other mechanisms because it is very likely that they will not affect carbon-13 relaxation. Let us denote by 1 the inverse of Tt. Rt governs the recovery of the longitudinal component of polarization, Iz, and, of course, the usual nuclear magnetization which is simply the nuclear polarization times the gyromagnetic constant A. The relevant evolution equation is one of the famous Bloch equations,1 valid, in principle, for a single spin but which, in many cases, can be used as a first approximation. 

As a consequence of the small quadrupole moment of Li, the quadrupolar interaction in solid state NMR spectra is much smaller for Li than for Li. This has been used to advantage for the determination of the Li chemical shift anisotropy from the Li static solid state powder spectrum of 2,4,6-tris(isopropyl)phenyllithium (see below) . Applying MAS up to 10 kHz, the CSA contributions to the lineshape can be completely ehminated in most Li spectra of organolithium compounds. If the measurement of the quadrupolar... 

Fig. 4. Quadrupolar powder patterns (a) Spin NMR powder pattern showing that the central -)<- ) transition is broadened only by dipolar coupling, chemical shift anisotropy, and the second-order quadrupolar interactions, (b) Spin 1 NMR powder pattern for a nucleus in an axially symmetric electric field gradient (see text). The central doublet corresponds to 6 = 90° in Eq. (10). The other features of low intensity correspond to 6 = 0° and 6 = 180°. (c) Theoretical line shape of the ) - -) transition of a quadrupolar nuclear spin in a powder with fast magic-angle spinning for different values of the asymmetry parameter t (IS) ...

Benzene was the first molecule studied by NMR within liquid crystals, that is oriented in nematic liquids.81 This opened up much research, using benzene,82 leading to information about the chemical-shift anisotropy and selected spin-spin couplings. Isotope substitution too played a major role for example see Ref. 83. The 1H NMR powder spectrum at ca. 225 K gave principal values of the proton-shift parameter matrix.84 Various isotopically labelled versions of benzene... 

In MAS, the sample is rotated rapidly by mechanical means around an axis that makes a prescribed angle with the static field. If the sialic spectrum (at rotation speed zero) has contributions from second-order tensor interactions such as the chemical shift anisotropy, then MAS breaks these contributions into a series of sidebands, separated from the average resonance frequency by multiples of the rotation frequency (/). The envelope of these sidebands traces out the original second-order tensor. At high rotation speeds, all sidebands are outside the static spectrum and therefore have zero amplitude so that only the average resonance survives. It is not always technically possible to reach such speeds in such cases the intensity of the sidebands can be diminished by applying certain pulse sequences [total suppression of sidebands (TOSS)] (see section IV.D). 

For molecules adsorbed on colloidal metals, the tumbling motion of the metal particles can be rapid enough to average out the chemical shift anisotropy so that the lines are narrow enough for a simple liquids NMR experiment. Using this method, the resonance has been observed in colloidal solutions of Pd and Pt particles (see Sections IV.C and IV.D). 

Chemical shift anisotropy is caused by the interaction of the nuclear spin with the field arising from the perturbation of the shells of molecular electrons by an external field. It is modulated by the rotational tumbling of molecules in liquids and solutions and the anisotropy in chemical shifts [4]. At the extreme narrowing conditions (See Appendix A.2), we have ... 

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