Weissenberg photographs

After noting down the indices of reflecting planes, the next task is to find whether there are any systematic absences. The absences due to centering of the lattice (f.c.c., b.c.c., etc.) occur throughout the reciprocal space and a general survey will reveal the lattice type which is to be determined. The presence of glide planes and screw axis will affect the absences of certain layers or rows of points in the reciprocal lattice. 

To verify and understand how these absences result due to the presence of these microsymmetries, let us consider the following derivation of crystal structure factor in the case of their presence. 

The presence of a-glide perpendicular to c-axis means that for any atom at xyz there must be an identical atom at a + 1 /2, y, —z (movement of half of a cell along a which makes x into x + 1/2 and reflected across the glide plane which tmus z into —z). The structure factor will then be n/2 of such pair of atoms 

The situation will be simpler to follow for set of reflections for which I = 0, i.e., for zero layer Weissenberg. Equation (8.1) will take a simpler form 

By the same argument, there will be reflections from hkO planes for which h is even and the no reflection when h is odd. 

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Fig. 3 X-ray diffraction photographs of poly-2,5-DSP. (a) Rotation photograph along the c-axis (oscillation angle 42°) (b) Weissenberg photograph of hkQ zone.

Weissenberg Photograph of o)-amino Acid Pyruvate Aminotransferase. 137... 

Fig. 15. A Weissenberg photograph of (o-amino acid pyruvate aminotransferase (mercury derivative). A single bent triangular Si(lll) monochromator and a Weissenberg camera for macromolecular crystallography were used. The oscillation angle 30° the radius of the camera 287 mm, exposure time 4.7 min...

The interpretation of Weissenberg photographs is quite simple. Con -sider first the zero layer of reflections, the X-ray beam being perpendicu -lar to the axis of rotation of the crystal—in other words, the reflections... 

Weissenberg photographs the reflections are often short streaks owing to such imperfections.)... 

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...

Normally, if the assumed model for a crystal structure has an R value of 0.5 and resists attempts to refine to a lower residual, then the model structure is rejected as false, and a new model is tried until a fit between the observed and calculated structure factors yields an acceptable residual (R < 0.25). (Other models were tried for this complex, but they either gave Fourier maps which were uninterpretable or they converged to the present model). However, the normal crystal structure is solved with data obtained from crystals which have dimensions of the order of 0.1 mm. In the crystals available for this experiment, two of the dimensions were of the order of 0.01 mm. Thus, long exposures were required to give a small number of relatively weak diffraction spots. (Each Weissenberg photograph was exposed for five days with Cuka radiation 50 kv., 20 ma. loading, in a helium atmosphere). 

From the oscillations and Weissenberg photographs, all the polymers have been found to be oriented three-dimensionally, while oligomers of m-PDA Me and CVCC Me are amorphous. 

V= 534.9 A Z = 2,d = 3.98 g cm- andF(000) = 559.86. Single-crystal precession and Weissenberg photographs indicated that the space group is triclinic. A Delaunay reduction of the cell chosen failed to show additional symmetry. The structure was successfully refined in the space group FI. 

Although single crystals of IrFs have been obtained, none were of suitable size and shape to yieid an accurate structure and when it became clear that the structure would be no more precise than that reported for RuFs, the analysis was abandoned. Nevertheless, the precession and Weissenberg photographic data have established the space group P2ila and indicate a close structural relationship to the other platinum-metal pentafluorides. We, therefore, believe that these pentafluorides will all show essentially the same tetrameric unit as detailed for RhFs tB Figures 1 and 2 and Table III. 

The X-ray examination was carried out on a sample of levynite from Breiddalur, Iceland, and involved the taking of Weissenberg photographs for which a small crystal was rotated respectively about the c and the b hexagonal axes. The intensities of reflection within the c-zone were then found to be similar to those of... 

Buerger, M. J. The Weissenberg reciprocal lattice projection and the technique of interpreting Weissenberg photographs. Z. Krist. 88, 356-380 (1934). 

Furthermore, it was shown that Weissenberg photographs of twinned a-Tel crystals give the suggestion of the Laue group mmm. Thus, it appears that Te I was erroneously determined as orthorhombic and is the the triclinic phase a-Tel instead. 

To verify (or disprove) the polymeric structure of the series of In (III) and Tl (HI) porphyrins, the X-ray crystal structure of T1(TPP)F was determined. A suitable crystal of T1(TPP)F was obtained, by slow diffusion of pentane into a solution of the compound in methylene chloride. Preliminary Weissenberg photographs along the c axis revealed a four-fold symmetry, and from the systematic absences of hOl (h+1 = 2n), OkO (k=2n), and from the subsequent least-squares refinement, the space group was determined to be P2j/n. 

According to Paccard et al. (1982) YjRhjSij crystallizes with the HfjNijSij type of structure [Cmcm, a = 4.174(2), b = 10.598(4) and c = 13.726(5)]. Crystal symmetry has been checked by single crystal Weissenberg photographs. Samples were prepared by arc melting from 99.9 Y, 99.99% Pd and 99.999% Si. 

Fig. 7.9. Replica of zero layer Weissenberg photograph and the undistorted weighted reciprocal lattice (By the courtesy of L.S. Dent Glasser)...

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