Carbon shielding

Regarding carbon spectra, an alcohol, ether, or ester function on the adjacent carbon shields the carbons of a CF3 group (Scheme 5.13). 

Para carbon shieldings, however, clearly follow the pattern described by the canonical formulae. They may be correlated well with the total charge densities22 and with the Hammett constants. 

In summary, it may be said that theoretical calculations, both ab initio and semiempirical, are not yet feasible, as they are too coarse a tool for the quantitative interpretation of subtle changes in carbon shieldings caused by slight molecular modifications, at least for aliphatic compounds. Nevertheless, they... 

In the difluoro case a significant C-F double-bond contribution (nir interaction, 149) was shown, by ab initio calculations of carbon shieldings in fluor-inated methanes (280) to occur if at least one pair of fluorine atoms is present... 

The AB and AX systems of all 13C —13C bonds appear in one spectrum when the INADEQUATE pulse sequence (Fig. 2.48) is applied. Complete interpretation usually becomes difficult in practice due to signal overlapping, isotope shifts and AB effects (Section 2.9.4). A separation of the individual 13C— 13C two-spin systems by means of a second dimension would be desirable. It is the frequency of the double quantum transfer (d e) in Fig. 2.48 which introduces a second dimension to the INADEQUATE experiment. This double quantum frequency vDQ characterizes each 13CA — I3CX bond, as it depends on the sum of the individual carbon shieldings vA and vx in addition to the frequency v0 of the transmitter pulse located in the center of the spectrum if quadrature detection is applied [69c, 71] ... 

On the other hand, 13C chemical shifts may reflect the contribution of polar species to the bonding state of a molecule. For example, the methylene sp2 carbon shielding in diazomethane and ketene indicates that the carbanionic polar states of both molecules are significant [81]. 

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 ... 

The calculated shift of C-3 is 39.1 ppm as compared to an observed value of 39.45 ppm. Prediction of carbon chemical shifts using the Grant-Paul relation (4.1) is a practical aid in assigning the carbon signals of larger alkyl groups, e.g. in cholestane derivatives (Section 5.2.2). - Other increment systems have been proposed [201, 202], as well as an absolute scale for carbon shielding [203],... 

Table 4.7. (b) Increments A, for Prediction of Axial and Equatorial Methyl Carbon Shieldings [87]. 

Carbon-13 shifts of alkynes (Table 4.13) [246-250] are found between 60 and 95 ppm. To conclude, alkyne carbons are shielded relative to olefinic but deshielded relative to alkane carbons, also paralleling the behavior of protons in proton NMR. Shielding relative to alkenes is attributed to the higher electronic excitation energy of alkynes which decreases the paramagnetic term according to eq. (3.4), and to the anisotropic effect of the triple bond. An increment system can be used to predict carbon shieldings in alkynes... 

Thioacetal and thioketal carbon shieldings (45-65 ppm) [328, 329] also reflect the heavy atom influence when compared with acetal and ketal carbon shifts (100 + 5 ppm). 

In the following sections, substituents, increments for carbon shielding in basic organic skeletons are summarized. Their practical use according to eq. (4.21) is illustrated by a few representative examples. Most of the tabulated increments are not based on regressional analyses, and deviations may be as large as +0.5 ppm. 

Carbon-shielding tensors, alkene and alkyne transition metal complexes, 1, 472 Carbon—silicon bond formation... 

Regarding carbon spectra, an alcohol, ether, or ester function on the adjacent carbon shields the carbons of a CF3 group (Scheme 5.15). Again, l,l,l-trifluoro-2-propanol is used to provide an example of a carbon NMR spectrum (Figure 5.7). Three signals are seen, with the CF3 carbon appearing as a quartet at 125 ppm with a one-bond C-F... 

E. Tarroni and C. Zannoni, Order parameters and carbon shielding tensors of some anthracene derivatives from C-13 NMR experiments, J. Phys. Chem., 100 (1996) 17157-17165. 

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