Anisotropy chemical shielding

However, because of chemical shielding anisotropy (CSA) and quadnipolar and dipolar effects, the Lamior... 

When l l, the above gives the so-called cross-correlation functions and the associated cross-correlation rates (longitudinal and transverse). Crosscorrelation functions arise from the interference between two relaxation mechanisms (e.g., between the dipole-dipole and the chemical shielding anisotropy interactions, or between the anisotropies of chemical shieldings of two nuclei, etc.).40 When l = 1=2, one has the autocorrelation functions G2m(r) or simply... 

When r s, one has interconversion between operators Br and Bs, and Rrs is a cross-relaxation rate. Note that the cross-relaxation may or may not contain interference effects depending on the indices l and /, which keep track of interactions Cyj and C,. Cross-correlation rates and cross-relaxation rates have not been fully utilized in LC. However, there is a recent report41 on this subject using both the 13C chemical shielding anisotropy and C-H dipolar coupling relaxation mechanisms to study a nematic, and this may be a fruitful arena in gaining dynamic information for LC. We summarize below some well known (auto-)relaxation rates for various spin interactions commonly encountered in LC studies. 

In order to discuss the origin of these terms we need to allow the spins to have anisotropic shielding tensors. Molecular tumbling in solution makes the chemical shielding in the direction of the external magnetic field a stochastic function of time and acts therefore as a relaxation mechanism, called the chemical shielding anisotropy (CSA) mechanism. The Hamiltonian for each of the two spins, analogous to Eq. (5), contains therefore two... 

The chemical shift of a particular atom varies with the orientation of the molecule to the field. In a solid this gives a range of values, an effect known as the chemical shielding anisotropy, which broadens the band. 

Skibsted, J., Vosegaard, T., Bildsoe, H., and Jakobsen, H. J. (1996). Cs chemical shielding anisotropies and quadrupole coupling constants from magic angle spinning NMR of cesium salts. J. Phys. Chem. 100, 14872-81. 

Ab Initio Calculations of 31P NMR Chemical Shielding Anisotropy Tensors in Phosphates The Effect of Geometry on Shielding... 

The determination of the NMR chemical shift or chemical shielding anisotropy (CSA) tensor from quantum mechanical methods remains a crucial... 

The chemical shielding anisotropy (CSA) tensors for the simple phosphate structures listed in Table I, were calculated using ab initio methods. The isotropic tensor value (aiso), the three principal components (aa), the CSA anisotropy (Act) and asymmetry parameter (rj) were evaluated and are given in Table II. 

Influence of Geometry on Chemical Shielding Anisotropy Tensor... 

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