Carbon chemical shielding

Gu, Z., Zambrano, R., and McDermott, A., Hydrogen bonding of carboxyl groups in solid-state amino acids and peptides Comparison of carbon chemical shielding, infrared frequencies, and structures, J. Am. Chem. Soc. 116, 6368-6372 (1994). 

Table 4 Comparison of the Calculated and Observed Carbon Chemical Shielding Tensor in C q for Two Different Gauge Origins ...

Figure 6 Fragments of the peptide 34-42 showing the cis and trans conformations. Also indicated is the orientation of the carbonyl carbon chemical shielding tensor, with perpendicular to the plane. Note the different orientations of the C-C inter-nuclear vector with respect to the CS tensor components. Reprinted with permission of the American Chemical Society from Costa PR, Kocisko DA, Sun BQ, Lansbury PT Jr and Griffin RG (1997) Journal of the American Chemical Society, 119 10487-10493)...

H and 13C NMR Data. A ketone or aldehyde carbonyl group bound to a CF2H group shields its proton slightly (0.1 ppm), and even more surprisingly, it also has a shielding effect upon its carbon chemical... 

Csaszar, A. G. 1992. Conformers of Gaseous Glycine, J. Am. Chem. Soc. 114,9568-9575. de Dios, A. C., J. G. Pearson, and E. Oldfield. 1993. Chemical Shifts in Proteins An Ab Initio Study of Carbon-13 Nuclear Magnetic Resonance Chemical Shielding in Glycine, Alanine, and Valine Residues, J. Am. Chem. Soc. 115, 9768-9773. 

Facelli, J. C., Molecular Structure and Carbon-13 Chemical Shielding Tensors Obtained from Nuclear Magnetic Resonance, 19, 1. 

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. 

Schneider modified the calculation of the Grant-Cheney force that evaluates shielding by steric perturbations. The force F exerted at the C,.—H bond (C, being the carbon atom whose chemical shielding is to be computed) is calculated by an equation similar to that for a nonbonded potential ... 

Several 1,4-disubstituted derivatives of the dication, 63, were successfully prepared. The 1,4-diphenyl, 1,4-dimethyl, 1,4-dicyclopropyl-substituted derivatives of carbodication 63 (65, 66, and 67) are exceptionally stable. The 1,4-dimethyl-1,4-cyclohexyl dication, devoid of the adjacent cyclopropyl groups, could not be prepared. The carbocationic center in all these dications are somewhat shielded as compared to their monocations. The C NMR chemical shifts of the carbocationic centers for the dications 65, 66, and 67 are 235.4, 293.4, and 260.8 ppm, respectively. The diprotonated ann -tricyclo(5.1.0.0 )octa-2,6-dione, 68, may be treated as a dicarboxonium ion, instead of dihydroxy dicarbenium ion, since the cabronyl carbon is shielded by only 25.2 ppm, much smaller than that observed for the protonated cyclohexanedione (34 ppm). The para carbons of the phenyl substituents in carbodication 65 are relatively shielded by about 5 ppm from that of the parent l,4-diphenylcyclohexane-l,4-diyl dication showing relatively less delocalization of the charge into the aromatic rings. 

Table 4.1 chemical shifts of methylene groups in substituted ethanes in which the methylene carbon is shielded... 

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],... 

The 13C NMR spectmm of the cation 581 shows substantial shielding of both the C(7) cationic and vinylic carbon chemical shifts (813C 34.0 and 125.9, respectively).1011 A similar shielding phenomenon is observed for ion 582. Interestingly, ion 582... 

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.

Table 2 Calculated l3C chemical shielding for the CP carbon of the L-alanine residue in peptides and polypeptides by the 4-31G GIAO-CHF method...

This trend is consistent with those observed for both proton and carbon chemical shifts, with the proton on the most highly substituted carbon, and the carbon with the most alkyl substituents being the most highly deshielded. Computational work by Wiberg and Zilm has allowed identification of the factors that lead to the observed shielding trends that are observed for alkyl fluorides.1... 

As was the case for the monofluoro series, halogens attached directly to the CF2 carbon deshield the fluorine nuclei (Tables 4.2 and 4.3). Iodine has the greatest deshielding effect on fluorine chemical shifts I > Br > Cl > F. In contrast, iodine has its usual shielding effect upon carbon chemical shifts. When considering proton chemical shifts for the fluoromethanes, again one must keep in mind the significant solvent effects observed for all di- and trihalomethanes. 

An indication of the NCH3 conformation was provided by the chemical shift of C-4. In compounds where the NCH3 is axial such as 14-epicorynoline (99), which has a trans B/C junction, this carbon was shielded relative to those alkaloids with an equatorial NCH3. 

The data in Table 21 indicate that C(2) is shielded, and C(3) is deshielded, in the Z-isomer relative to the Eav-isomer. It has been found113 that in compounds of general formula XCH=CHY, where X and Y are, respectively, an electron-withdrawing and an electron-donating group, the olefinic carbon chemical shifts of isomeric E,Z pairs are simply related by an equation bz = bE + A. For enamino ketones 343 (and other structurally related compounds113), the relationship for C(2) is... 

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