Single crystals oscillation photograph

Fig. 2 Single-crystal x-ray diffraction oscillation photograph for an incommensurate tunnel inclusion compound (the 1,9-diiodononane/urea inclusion compound), recorded for a single crystal oscillating about its tunnel axis. The layer lines (horizontal) from the host component are indicated (h) on the left-hand side. The layer lines from the guest component are indicated (g) cn the right-hand side. In this case, the guest layer lines contain discrete scattering (sharp spots) and diffuse scattering. The fact that separate sets of layer lines are observed for the host and guest components is a consequence cf the incommensurate relationship between C h and Cg.

Monochromatic x-rays are used and the diffracted x-rays are detected by their action on photographic flues or plates. They can also be detected by means of a radiation counter and electronic equipment feeding data to a computer. Since x-rays of a given wavelength are diffracted only for a certain specific orientations of the sample so if the sample is a single crystal it should be placed in all possible orientations during experiment. For this the sample is rotated or oscillated about one of its axes. This can also be achieved by using a sample made as a powder of a very small crystal, in such a powder the minute particles are randomly oriented and all possible orientations are included. 

In fibres of some polymers, made under certain conditions, the crystalline regions are found to be tilted with respect to the fibre axis in a well-defined crystallographic direction. This is a very valuable feature, because the diffraction patterns of specimens in which this type of orientation occurs are of precisely the same form as tilted crystal diffraction patterns of single crystals rotated round a direction inclined to a principal axis. The unit cell cannot be obtained directly, for 90° oscillation tilted crystal photographs are required for direct interpretation, but unit cells obtained by trial can be checked by the displacements of diffraction spots from the layer lines this is a severe check, and consistent displacements would leave no doubt of the correctness of a unit cell. This procedure played an effective part in the determination of the unit cell of polyethylene terephthalate (Daubeny, Bunn, and Brown, 1954). 

Vriend, G., and Rossmann, M- G. Determination of the orientation of a randomly placed crystal from a single oscillation photograph. J. Appl. Cryst. 20, 338-343 (1987). 

It was later possible to extract single crystals of the specimen, and from Weissenberg and oscillation photographs they were found to be PbCl2, which has in the equation for sin2 Q the constants A = 0.0073,... 

Figure 8.3 Single ciystal synchrotron X-ray diffraction patterns recorded from a variety of macromolecular crystals illustrating the variety of diffuse scattering features, (a) aPP, resolution limit 1.8A, wavelength 1.49A, 4.9° oscillation photograph. (b) RNAse, resolution limit 1.9 A, wavelength 1.488 A, still exposure.

Fig. 7.5. The oscillation photograph of a single crystal monnted along its c-axis (By courtesy of L.S. Dent Glasser)...

Fig. 4.2 Single-crystal X-ray oscillation photograph obtained from a polydiacetylene single crystal. The chain direction is vertical.

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