Isotropic chemical shifts conformational analysis

The use of solid state NMR for the investigation of polymorphism is easily understood based on the following model. If a compound exists in two, true polymorphic forms, labeled as A and B, each crystalline form is conformationally different. This means for instance, that a carbon nucleus in form A may be situated in a slightly different molecular geometry compared with the same carbon nucleus in form B. Although the connectivity of the carbon nucleus is the same in each form, the local environment may be different. Since the local environment may be different, this leads to a different chemical shift interaction for each carbon, and ultimately, a different isotropic chemical shift for the same carbon atom in the two different polymorphic forms. If one is able to obtain pure material for the two forms, analysis and spectral assignment of the solid state NMR spectra of the two forms can lead to the origin of the conformational differences in the two polymorphs. Solid state NMR is thus an important tool in conjunction with thermal analysis, optical microscopy, infrared (IR) spectroscopy, and powder... 

The peptide cvclo(Glv-Pro-GlvK presents a quite different situation. The analysis of its NMR spectrum leads to the conclusion that it adopts a Cp-symmetric conformation in solution, at least on NMR timescales. The solution NMR spectrum (Figure 2B) shows the minimum number of resonances expected (one per carbon in the repeating trlpeptlde unit). By contrast, in the solid-state spectrum there Is clear indication of asymmetry since there are two Pro Cg resonances in Figure 2A, X-ray diffraction analysis has revealed an asymmetric molecular conformation for the crystalline peptide (17), with two different types of 3-turns, only one of which is intramolecularly hydrogen-bonded. Analysis of the solid state isotropic chemical shifts in terms of local conformation yields a picture of the molecule which is consistent with the X-ray data,... 

An analysis of the CS tensors for the doublet peaks of alkoxy carbon sites (a/a ) is expected to be important for deducing torsion angles between Planes II and III and between Planes iP and III. It can be seen from Table 1 that the and S22 components as well as the isotropic shifts are almost similar to each other, but a clear difference is found for the S32 components. To understand such a difference, as shown in Fig. 32A, we constructed a simple model of the banana molecule in which the torsion angle between Planes II and III (0) was artificially changed from 0° to 360° in steps of 30°, and quantum chemical calculations for the CS tensors were performed at each step. In this model, the conformation in which the two planes are coplanar and carbon sites (a/a ) are located on the same side of the C=N bond... 

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