13C chemical shift tensor

Figure 3 shows several examples of 13C chemical shift tensors of carbe-nium ions oriented in the molecular frame as indicated. If the molecule or ion has a rotational axis of C3 or higher order that passes through the nucleus in question, symmetry demands that the two components perpendicular to the C3 or higher order rotational axes be identical in such cases the identical components are designated SL. The unique component that lies along the... 

The semi-empirical bond polarization theory provides a fast method for the calculation of 13C chemical shift tensors with an accuracy level comparable to ab initio methods for most molecules. There are no restrictions in the size of the molecular system, so that the method can be applied to large molecules or crystal lattices. Chemical shift calculations were carried out for several small molecules and compared to ab initio results. Additionally the calculated chemical shift tensor for the fullerence Qo is presented. 

The authors of this work proposed a semi-empirical scheme for the calculation of 13C chemical shift tensors based on the bond polarization theory (5). This method can reproduce 13C chemical shift tensors with deviations from experiment comparable to the errors of the ab initio methods. One major advantage is that the calculations can be performed for large molecular systems with hundreds of atoms even on a PC computer. In contrast to the ab initio method a set of empirical parameters is needed for the calculations. In the case of the bond polarization theory these parameters can be estimated directly from experimental chemical shifts solving a set of linear equations. 

Figure 1. Correlation between calculated and experimental principal values of 13C chemical shift tensors...

Table I. Calculated principal values of 13C chemical shift tensors compared to experimental values and ab initio results (the reference U.F. denotes a private communication from Ulrich Fleischer, Ruhr University Bochum) ...

The resulting principal values of the 13C chemical shift tensors of the C60 carbons are 8n = 228 ppm, 822 = 178 ppm, and 833 = -3 ppm. Tycko et al reportet the experimental values 8n = 213 ppm, S22 = 182 ppm, and 833 = 33 ppm obtained from low temperature measurements of a powder pattern spectrum (18). However, the spectra have a low signal to noise ratio and a wide slope so that a larger error for the experimental value can be assumed. The chemical shift anisotropy of 217 ppm corresponds quite well with the spectral range of about 200 ppm reported by Kerkoud et al for low temperature single crystal measurements (19). 

Figure 2. Model of a Qo molecule with principal axes of the calculated 13C chemical shift tensor...

The similarity between the 13C chemical shift tensors of the N7-H and N9-H forms can be rationalized by inspecting the optimized structures of both conformers. Figure 2 shows the optimized structures for the N7-H and N9-H tautomeric forms. It is apparent that most of the interatomic distances of equivalent atoms agree within a few thousands of an Angstrom. The bond angles, not shown in the figure, also are quite similar. 

Sastry DL, Thkegoshi K, McDowell CH (1987) Determination of the 13C chemical shift tensors in a single crystal of methyl a-D-glucopyranoside. Carbohydr Res 165 161-171... 

Figure 3.2.21 The orientation of the 13C chemical shift tensors with respect to the molecular frame of the carboxylic group of a peptide.

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