Shielding-deshielding mechanism

In fact, theoretical calculations on the conformations (molecular mechanics, semiempirical and ab initio) and shielding/deshielding contributions (aromatic ring current increments) were performed in conjunction with variable-temperature NMR and dynamic NMR experiments that allowed the full understanding of the fundamentals of the MTPA action and of the correlation between NMR spectra and absolute configuration. ... 

In general -substituent effects are small, but a marked exception exists when the substituent and the carbon in question have a syn-diaxial relation. While the steric interactions suggest a shielding of the carbon, in fact an appreciable deshielding is found. Thus, the C-l9 methyls in the 2/7- and 4/ hydroxy-5cc-cholestanes are 2-5 and 2-4 ppm to high frequency of the value in 5 cc-cholestane itself. This type of evidence has also been advanced against the operation of the steric polarization mechanism. 

The observed large values of the carbon contact shifts (rel to H) indicate that the pseudocontact interactions are probably of secondary importance (data for these complexes is in Table LVI). A comparison of the Ni(2+) (acac)2 complexes with the Co(2-l-) derivatives shows that the mode of delocalization is probably different. The shift at the y carbon (shielded carbon for both Ni and Co, deshielded proton Ni, shielded Co) was interpreted in terms of a dominant n delocalization mechanism due to the close agreement of the aicjffiu fatio with that predicted by theory. Both the pC and are deshielded (/ C > fJH). The observed ratio is too large (relative to theory)... 

The three factors determining the nuclear shielding of any atom are introduced and their relative importance discussed. The dependence of shiftability on the inverse cube radius for valence shell electrons is identified for Group III atoms. The deshielding effects of ligand substitution and their correlation with the nephelauxetic effect are discussed. The dependence of relaxation rate and mechanisms available upon quadrupole moment is explored. 

---