Conformation and crystal structure

There is a variety of methods for the determination of the structure of oligosaccharides. Different techniques are available for the determination of solution conformations and crystal structures respectively. The following section will deal with the most frequently used techniques. 

Polyamides are an important example of polymers which do not contain pseu-doasymmetric atoms in their main chains. The chain conformation and crystal structure of such polymers is influenced by the hydrogen bonds between the carbonyls and NH groups of neighboring chains. Polyamides crystallize in the form of sheets, with the macromolecules themselves packed in planar zigzag conformations. 

CP/MAS NMR approach has been described to characterize molecular chain conformations and crystal structures of native and regenerated celji lose samples in the dry and hydrated states. First, C isotropic chemical shifts in the solid state are correlated to torsion angles cj) and vp in the 3"l 4-glycosidic linkage and X about the exo-cyclic C5-C6 bond, respectively. 

Also crystallizing in space group P2 ]c, uranocene has two molecules per unit cell, so that the U(CsH8)2 molecule occupies a special position of site symmetry 1. In other words, the molecule has an eclipsed conformation, and it may be assigned to special position 2(a). Similarly, the two halves of the [Re2Clg]2- dianion in K2 [Rc2C lmolecular dimensions (indicating that the symmetry of the dianion is Z>4h within experimental error) and crystal structure are shown in Fig. 9.6.5. 

Suck D, Saenger W (1972) Molecular and crystal structure of 6-methyluridine. A pyrimidine nucleoside in the syn conformation. J Am Chem Soc 94 6520-6526... 

The molecular and crystal structure of (81) is simpler than that of ferrocene as only one polymorph featuring eclipsed conformation of the cyclopentadienyl rings, has been found at ambient and low (100 K) temperature. The larger metal-carbon distance (2.186 A in (81) vs. 2.03 or 2.06 A in ferrocene) corresponds to the larger metal covalent radius see Covalent Radii) and may also be responsible for the fact that an eclipsed conformation is found for the solid-state structure of decamethylruthenocene (82), as opposed to decamethylferrocene where more closely spaced methyl groups impose the staggered Dsd conformation. 

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