Carbon chemical shift anisotropy patterns

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). 

Fig. 2. The MAT experiment applied to poly(2-hydroxypropyl ether of bisphenol A)5 (top) to examine the 180° ring flips affecting 13C 4 and 5. (a) The complete two-dimensional MAT spectrum.5 The projection in f2 is effectively the lineshape that would be recorded for a powder sample. As this spectrum clearly shows, the chemical shift anisotropy powder patterns from the nine 13C sites in this polymer are extensively overlapped and would not be resolved without the aid of this MAT experiment, (b) The powder lineshapes for each 13C site taken from the two-dimensional spectrum in (a).5 Those for carbons 4 and 5 show distortions of the lineshape shoulders typical of motional averaging, in this case from 180° phenyl ring flips.

Unfortunately, for systems with a number of different carbon nuclei, there is usually extensive overlap of the CSA signals, and the analysis of the powder pattern is difficult, if not impossible. Therefore, other techniques that partially narrow the lines must be used. Three different experiments can be used to determine chemical-shift anisotropies for powders ... 

NMR spectroscopy has been the most useful tool in cephalosporin C chemistry. In cephalosporins the carbons are unsaturated or highly substituted with heteroatoms, and the protons are usually widely separated in chemical shift and have simple coupling patterns. Recently, solvent induced chemical shifts, nuclear Overhauser effects, and the anisotropy of the sulfoxide bond have been utilized in chemical studies of cephalosporin C derivatives. Analytical information may be derived from NMR spectra of cephalothin by observing the contribution of the 0-lactam protons, thiophene protons, methylene groups, and methyl protons (from acetate). 

Fig. 8.16. The change in chemical shift with rotation is shown for a uniaxial chemical shift. In this case, an average isotropic shift of 1.67 will be measured when the anisotropy range is 1-2. The top figure is the powder pattern for a CSA going from 2 to 1. The bottom figure shows the specific shift as a function of rotation angle. These measurements should agree very closely with the value measured for the corresponding carbon in the liquid or solution state if other solid-state effects are neglected. (Reproduced with permission from Ref. [62]. 1983 Plenum Publishing Corporation.)...

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