Carbon-13 shielding anisotropy

The neighbor anisotropy term <7 of eq. (3.2) plays an important role in proton shielding, permitting, for example, a distinct differentiation between aromatic and olefinic protons due to the ring current effect. However, this contribution is small in 13C NMR (<2 ppm). A comparison of the methyl carbon shieldings in methylcyclohexene and toluene shows that the ring current effect often cannot be clearly separated from other shielding contributions ... 

Table 5.14. Calculated C NMR shielding components average diamagnetic (o-J ) and paramagnetic (cjpJ components, overall average (a ), and shielding anisotropy (ct - Oj ) as a function of carbon-oxygen distance, R(C-O), for CO/-...

The correlation between theoretical and experimental shielding anisotropies has been most important for 13 C shielding results. Several previously discussed experimental techniques including the liquid crystal and cross-polarization techniques have recently supplied a large amount of data. The theoretical approach, in progressing from diatomics to small polyatomics has also placed strong emphasis on carbon-containing molecules. 

By applying polarization functions, ab initio shielding calculations for some polyenals and their Schiff bases reproduce the experimental values well even on the carbonyl and the imine carbons using the LORG theory without including correlation effects. In addition, there is a trend that the calculation with polarization functions yields smaller anisotropies of chemical shieldings than those without polarization functions. 

The shielded chemical shift is in turn due to the strong magnetic anisotropy of the sp -hybridized carbons. All the above dications are definitively classical, nonbridged cations, as also shown by the chemical shift additivity criterion of Schleyer and coworkers. 

Carbon-13 shifts of cyano groups in nitriles are found between 110 und 125 ppm (Table 4.46) [77 a, 352]. Shift values close to 125 ppm are characteristic of nitriles with a branched alkyl groups (Table 4.46). Similiar to the isoelectronic ethynyl group, the nitrile function shields the oc carbon owing to the anisotropy effect. Alkenyl carbon shifts of... 

In aromatic compounds carbon-13 shifts are largely determined by mesomeric (resonance) and inductive effects. Field effects arising from through-space polarization of the n system by the electric field of a substituent, and the influences of steric (y) effects on the ortho carbon nuclei should also be considered. Substituted carbon (C-l) shifts are further influenced by the anisotropy effect of triple bonds (alkynyl and cyano groups) and by heavy atom shielding. 

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