Shielding in Isotropic Media

The terms shielding a and chemical shift S are often used side-by-side. They are related to each other by Equation (3.1), in which is the shielding of the reference, VOCE in our case  

In this section, the reader will be confronted with and introduced to some comparatively elemental facts on the theory underlying interpretation of the shielding parameter accessible for normal molecules in isotropic solutions, where normal refers to molecules which are not oversized (such as vanadium bound to proteins), and were we therefore are in the so-called extreme narrowing limit , characterised by the condition 2TruQT 1, where vq is the measuring frequency and the molecular correlation time, a measure of the mobility of a solute molecule in a solvent. Extreme narrowing simply means that the molecule is freely mobile and the frequency applied to obtain NMR information does not influence the respective parameters. Although the term contains the component extreme , we are well in the domain of normal conditions. 

The anisotropic shielding constant is a 3 x 3 tensor, and its principal (diagonal) 

The term (7 i stands for non-local contributions, i.e. for contributions which do not stem from the immediate coordination sphere of vanadium. This term, mainly reflecting the influence of counfer-ions and other matrix effects, is usually negligibly small in NMR. The local term the diamagnetic contribution, which governs variations in proton shielding, also accounts for substantial shielding variations in lithium NMR. In other nuclei, including is dominated by the core electrons, i.e. is a sizable 

The term thus depends on the strength of the ligand field - via AE 

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