Carbon isotropic shifts

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. 

This paper presents a study of the i5N and, 3C chemical shifts in both tautomeric forms of purine. All the principal values of the, 5N and, 3C chemical shifts tensors were measured in a sample in which only the N7-H tautomeric form was present. In another sample, containing both tautomeric forms, the principal values of the quaternary carbons, the isotropic shifts of the remaining carbons and isotropic shifts of the nitrogens could also be obtained for the N9-H tautomeric form. The experimetal procedures are described in detail elsewhere. 

It is well known [89] that, in solution state 13C NMR, the 13C chemical shift for C=0 carbons is shifted to a higher value by hydrogen bonding. In general, this is also observed for the isotropic 13C chemical shift in solid state 13C NMR. A classic illustration is a-diacetamide, the crystal structure [90] of which contains dimers, with only one of the two carbonyl groups involved in hydrogen bonding ... 

In contrast to pyridine adducts, those of pyridine-iV-oxide produce isotropic shifts which support a dominant -delocalization mechanism. Spin density distributions over the aromatic carbons have been determined from H and spectra of several Ni(acac)2(py-NO)2 complexes. (79) relaxation measurements indicate that Tj values arise mainly from hyperfine dipolar interaction induced by spin density localized on Ni(ii) and on the C-centred 2p orbital. Adducts of Ni(acac)2 with aniline, (80) fluoroanilines, (80,81) alkylanilines, (81-83) aniline derivatives, (541, 542) and nitrogen heterocycles (543) have been extensively studied. The results are consistent with a dominant n-spin delocalization mechanism. 

Lanthanide Shift Reagents.—The effects of random co-ordinate error in analysis of lanthanide-induced axial pseudocontact shifts have been discussed, " and the contributions of contact and pseudocontact shifts in the n.m.r. spectra of isoquinoline and of endo-norbornenol have been evaluated. An experimental and computational approach to the use of lanthanide-induced shifts as a rigorous method for structure determination has been described. The method was used to predict the lanthanide-induced shift behaviour of a substrate. The recording of experimental data in excellent agreement with the molecular structure was reported. Contact shift contributions to lanthanide isotropic shifts have been found to be important for organic compounds even where the carbon atom is five bonds away from the lanthanide. 

Fig. 18. Contour plot of the central portion of the 1 3-trimethoxybenzene 2D MAT spectrum obtained by shearing the experimentally acquired spectrum by 45°. The contour interval is 4% and the lowest contour plotted is 4% of the maximum peak height. The isotropic shifts of the carbons can Ik read from the vertical axis and the anisotropic shifts can be read from the corresponding horizontal slices. (Reproduced from Hu et al. with permission.)...

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