Chemical-shielding tensor, effect

These effects of changes in geometry on the shielding tensor in PH3 provide the framework for discussion of trends observed in the simple phosphate systems described below. In theory, the difference in the variation for the isotropic and individual principal components of the chemical shielding tensor can be used... 

The methods listed thus far can be used for the reliable prediction of NMR chemical shifts for small organic compounds in the gas phase, which are often reasonably close to the liquid-phase results. Heavy elements, such as transition metals and lanthanides, present a much more dilficult problem. Mass defect and spin-coupling terms have been found to be significant for the description of the NMR shielding tensors for these elements. Since NMR is a nuclear effect, core potentials should not be used. 

It can be seen that, in all cases, relaxation rates are directly proportional to (Aa). Because Aa reflects the anisotropy of the shielding tensor and because the chemical shift originates from the shielding effect, the terminology Chemical Shift Anisotropy is used for denoting this relaxation mechanism. Dispersion may be disconcerting because of the presence of Bq (proportional to cOq) in the numerator of and R2 (Eq. (49)). Imagine that molecular reorientation is sufficiently slow so that coo 1 for all considered values of coo from (49), it can be seen that R is constant whereas R2 increases when Bq increases, a somewhat unusual behavior. 

It is apparent from the calculations in the model pyridine-methanol system, that various principal components of the chemical shift tensor are affected differently by the HB. Consequentely, the principal values perpendicular to the direction of the interaction will exhibit a greater effect than the shielding components along the direction of the interaction. This general argument is in agreement with the qualitative observations in purine. A full 3D study of the interaction in the pyridine-methanol complex is required to quantitatively interpret the results in purine. 

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

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