Applications in Crystal Structure Refinement

In Fig. 12 the superposition of the X-ray and the H-refined structure (5 = 1000) is shown. The spheres at the proton positions are the isotropic 50% probability ellipsoids. It is thus possible to refine crystal structures using chemical shifts as target functions and thereby resolve structural features that are not well represented in diffraction investigations. These new H-refined crystal structures will now fulfil the requirements of both the diffraction investigation and the NMR. 

The lack of crystalline order in polymers such as cellulose or silk significantly reduces the number of interference spots in diffraction investigations hence the patterns often cannot be analysed unambiguously. In these cases a crystal structure refinement using NMR chemical shifts will be of great value, because NMR spectroscopy does not require long-range crystalline order. In our second example of cellulose, the situation occurred that the C4 values in BPT C chemical shift calculations deviated seriously from 

The question whether there exist any other structural models that fulfil all 

X-ray and NMR criteria still remains open to discussion. Additionally, the 

There are specific diiferences between structure data obtained by NMR methods and diffraction methods. Not only the time scale of the NMR experiment and therefore the time average is different, but chemical shifts depend mostly on the valence electrons whereas X-ray diffraction looks mainly at the inner shell electrons. It is well conceivable that these differences become a valuable source of new information. The same is true when neutron and X-ray data are comparatively analysed to produce density difference distributions. As far as NMR methods are concerned, the investigations reported here represent the first steps towards crystal structure refinement. 

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