Carbon isotropic shieldings

The NMR magnetic shielding for atoms like carbon is affected greatly by what it is bonded to and the type of bond to its neighbor. Use the inner carbon atoms of normal butane as the reference atom and calculate the shift in C isotropic shielding for 2-butene and 2-butyne. Can you explain these shifts as a function of the changing molecular environments ... 

In-plane components (a22 and a33). The feature of conformation dependence of the g22 and g33 components is better understood by taking p-IONONE as an example. Here, the conformation-dependent shifts of C5 and C8 are examined. As can be seen from Figure 6, the isotropic shieldings of both carbons exhibit clear periodic dependence on the twist angle about the C6-C7 bond,cp6.7. For example, the a-cp6.7 curve of C5 appears to be approximated by a sine wave whose period is 180°. The curve for C8 shifts in phase by a half period relative to that for C5. These curves are expressed by using the Fourier series of expansion as follows ... 

Table 6 Isotropic shielding constant (in ppm) for X being oxygen or carbon and the shift compared to the gas-phase value. QM denotes the system that is treated quantum mechanically, and the water part describes the approach for treating the solvent. For the latter, PCMl and PCM2 denotes polarizable continuum models and TIP3P and SPC two different force helds. - marks gas-phase calculations. All results are from ref. 33... 

Another group of experiments correlates carbon isotropic shifts with anisotropic contributions from the same nuclei.122-125 The assignment power of these experiments stems from the fact that different types of carbons can have similar isotropic shielding but different anisotropic chemical shifts. Therefore, based on the magnitude of the chemical shift anisotropy, the carbons in question can be assigned. As an added advantage, the chemical shift anisotropy could, in many cases, be more sensitive than isotropic chemical shifts to structural variations between polymorphs. 

The author examined the correlation between the calculated and experimental isotropic shieldings. The 6-31G shielding data are in qualitative agreement with the experimental data and completely reproduce the relative order of all the carbon shieldings studied. The 6-31G shieldings for the carbonyl carbons shift are about 20 ppm downfield of the experimental values. If the experimental data are converted to the methane reference using the data reported by Jameson and Jameson, this discrepancy still remains large (about 16 ppm). 

Figure 1 Calculated versus observed isotropic shieldings for carbon in the perturbed Hartree-Fock GIAO approach. The 45° line represents exact agreement between theory and experiment.

The parameter (p,p) measures the effective completeness of the basis set and is equal to the number of electrons for a complete basis. The calculated isotropic shielding for carbon, o, is given for gauge origins chosen at the center of mass (c.m.) and at the location of one of the carbon atoms (C). 

The problem was revisited once more following the publication of the powder data of Yannoni et al., which yielded values for the principal values of the carbon shielding tensor. Again the nuclear origin (at carbon) was chosen and the same extrapolation procedure yielded results in moderately good agreement with experiment as shown in Table 4. Fowler and co-workers note that the discrepancy between the isotropic shielding observed in solution and that obtained from the powder data (6 ppm) may result from errors inherent in the line shape analysis required in the evaluation of powder data. 

The third method of determining order parameters uses carbon-13 shielding constants a- and for this interaction, which has a finite isotropic average, eqn (3) for a rigid molecule gives... 

Fig.3 The dependences on the dihedral angles(< >,i /), of the isotropic chemical shielding constant for the L-alanine residue Cp- (a)and Ca-(b) carbons in peptides. Chemical shielding calculations were carried out using the GIAO-CHF method with 4-31G ab initio MO basis set. The 4-31G optimized geometries for the model molecules, N-acetyl-N -methyl-L-alanineamide, were employed.

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