Crystal rotated single

Rotating single-crystal measurements also permitted the extraction of the orientation of the magnetic tensor in the molecular reference frame and the experimental easy axis was found to coincide with the idealized tetragonal axis of the coordination dodecahedron of Dy. Crystal field calculations assuming idealized tetragonal symmetry permitted the reproduction of magnetic susceptibility data for gz = 19.9 and gxy 0 [121]. More elaborated calculations such as ab initio post Hartree-Fock CASSCF confirmed this simple analysis [119]. 

When the gradient of the electric field does not have axial symmetry another parameter must be introduced to describe the magnitude of the electric field gradients in the solid. This is the asymmetry parameter j , which is a measure of the deviation from axial symmetry (5, 94). In addition, the direction of the principal axis of the electric field gradient tensor must be specified. From single crystal rotation patterns, the values of these parameters may be deduced (94). 

Hosomi H, Ohba S, Tanaka K, Toda F (2000) The first in situ observation of intramolecular rotation and cyclization of anilide by an X-ray study partial single-crystal to single-crystal photocyclization of A-methyl-A- ( )-methylmethacrolyl anilide in inclusion crystals. J Am Chem Soc 122 1818-1819... 

If the spacings of the arcs on a powder photograph do not lead to identification, the determination of unit cell dimensions from the powder photograph may be attempted the methods are described in Chapter VI. If crystals large enough to be handled individually can be picked out of the specimen, single-crystal rotation photographs may be taken and used for identification this also is dealt with in Chapter VI. 

Fig. 74. Arrangements for taking single-crystal rotation photographs (a) on flat films, (6) on cylindrical films.

Single crystal rotation photographs. Above potassium nitrate (orthorhombic rotation axis, c). Centre gypsum (monoclinic rotation axis, c). Below benzil (hexagonal ... 

When an X-ray beam passes through such a fibre perpendicular to its length, the pattern produced is of the same type as that given by a single crystal rotated about a principal axis. All orientations perpendicular to the fibre axis are already present in the specimen, so that the effect of rotation is produced. Examples are shown in Plate X. The reflections are less sharp than those produced by single crystals, for two reasons firstly, the orientation of the crystals in the fibre is not perfect, so that each spot is drawn out to the form of a short arc, and secondly, in most polymer fibres the crystals are so small that the reflections are inevitably more diffuse than those of la rge crystals (see p. 437). 

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). 

For normal-beam single-crystal rotation photograph, using crystal of volume V completely bathed in X-rays ... 

Zero layer on single-crystal rotation photographs. The spacings d... 

Single-crystal rotation photographs. The method found most convenient for finding the unit cell dimensions of crystals of low symmetry is to send a narrow monochromatic X-ray beam through a single... 

Figure 6. Top single-crystal rotation diagram recorded on Weis-senherg camera from a single crystal inside a glass capillary. Bottom second layer line of Weissenberg photograph of a single...

Oriented, polycrystalline specimens. If, for example, all Az = A = 0 the array would be a three-dimensionally ordered domain i.e. a "single crystal". The most highly organized fibers are ones in which many such domains are oriented with only the helix axes parallel. As I indicated above, such fibers provide X-ray diffraction patterns like those from a rotated single crystal. The not-quite-perfect parallelism of the domains causes the intensity to be distributed along arcs instead of concentrated in spots (see Fig. 2). 

Crystalline Particles. Usually, one of the unit cell edges is preferentially oriented parallel to the fiber axis and the particle intensity transform corresponds to a single-crystal rotation pattern and the reflections are confined to layer lines spaced at intervals of Z/o where a is the dimension of the unit cell parallel to the fiber axis (3). 

X-ray crystallographic analysis of two complexes, 87a 1 and 87c l, before and after irradiation, were performed at low temperature (— 170°C). Fortunately, these reactions were revealed to proceed via a single crystal-to-single crystal transformation without decomposition of the initial crystal structures. Furthermore, the reaction pathway was successfully traced by x-ray analyses of crystals irradiated for different times up to 67 days. These revealed that the ketyl group rotated around the C(ketyl)-C5 (phenyl) bond and finally coupled with the steroid C5 position to produce the adduct. 

The relative orientation of monomer and polymer crystals was determined by rotating single crystals of photopolymerizable diolefins on photoirradiation. 

Bernal, J. D. On the interpretation of X-ray single-crystal rotation photographs. Proc. Roy. Soc. (London) A113, 117-160 (1926). 

The single crystal rotation pattern of Li3N has also been determined, showing evidence of CSA (Differt and Messer 1980). 

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