Diamagnetic effects in H NMR

Diastereotopic protons As diastereotopic protons reside in different chemical environments, different chemical shifts and splitting patterns are observed. We will discuss this in more detail in the part dedicated to spin-spin coupling constants. 

Furthermore, symmetry of orbital interactions in cyclic systems can lead to either diamagnetic (aromatic) or paramagnetic (antiaromatic) ring currents. Such effects have been discussed extensively in chemical textbooks and will not be analyzed here. 

Particularly strong paramagnetic deshielding is observed when the HOMO orbitals are orthogonal to LUMO orbitals (e.g. when a lone pair is orthogonal to a it-system). It is the contribution from n - excitation that 

Additional information can be obtained in the solid state where conversion of the isotropic chemical shift into a tensor opens additional opportunities for steieoelectronic insights inherent to the directionality components of the tensor. Calculations indicate that the orientation of the chemical shielding tensor of the carbene is such that the most shielded component is perpendicular to the molecular plane, whereas the least shielded component is perpendicular to the directions of both the lone pair and the p-orbitas. 

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