Band 117, Heft 4

The monoclinic crystals of the isomorphous compounds with general formula [ Me (II)(N 2 H 4 ) 2 ]Cl 2 , where Me (II) = Cd, Zn, Mn, form twins according to different laws. The twin law (001) was observed in crystals of [Cd(N 2 H 4 ) 2 ]Cl 2 , of [Zn(N 2 H 4 ) 2 ]Cl 2 and of [Mn[N 2 H 4 ) 2 ]Cl 2 ; the twin law (110) in crystals of [Cd(N 2 H 4 ) 2 ]Cl 2 and of [Mn(N 2 H 4 ) 2 ]Cl 2 ; the twin law (100) only in crystals of [Mn(N 2 H 4 ) 2 ]Cl 2 . Twinning was deduced from Weissenberg photographs, taking into account the symmetries of the reciprocal layers of the twins. A large number of crystals was examined and their statistical distribution over the twin laws was calculated. The compounds can be assigned to the possible space groups C 2 h 3 – C 2/ m ; C 2 3 – C 2; C s 3 – Cm . From the dimensions of the unit cells it can bo inferred that molecules of hydrazine form bridge-complexes between two metal-ions, as in the compounds with general formula [ Me (II)(N 2 H 4 ) 2 ](NCS) 2 . Obliquities of the twins and twin indices were calculated. Due to the geometrical rather than structural character of the quasi-normal axis, no sure relation can be found between the obliquities, or the twin indices, and the statistical frequencies with which the twin laws appear. More likely the formation of the twins is due to chains of bridge complexes [unk], which form “clusters“ opposing the adjustment of single atoms in the previous structure.

Using the method of the convolute molecule with the structure of triphenylene as an example, the conditions which make the appearance of “nearly homometric” structures probable are discussed.

The lattice constants, space groups and linear coefficients of thermal expansion in the various axial directions were determined for the selenates and chromates of lead, strontium and barium. The lead and strontium compounds are monoclinic, while barium Chromate and selenate are orthorhombic (barite type).

The paper reports the detailed x-ray analysis of the structure of echitamine iodide. This substance and the chloride which are isomorphous belong to the orthorhombic space group P 2 1 2 1 2 1 with four molecules in the unit cell. The axial dimensions of the iodide are a = 18.45 Å, b = 13.83 Å, c = 8.48 Å. The structure has been solved from the hk 0, h 0 l and 0 kl data by Fourier-projection methods, use being made of the ‘isomorphous replacement’ technique and the ‘heavy atom’ method to obtain the phases of the reflections. The final R index for 379 reflections of measurable intensity is 0.145. The molecular configuration of the quarternary echitamine ion has been established, and it has been shown that this dihydro-indole alkaloid is unique in having two five-membered rings fused to a ‘boat’-shaped substituted cyclohexane ring along a common bond. The positions of all the side-groups are well explained by postulating weak intramolecular hydrogen bonds.

Now data obtained from Weissenberg photographs of single crystals of metallic gallium have been refined by projections and generalized projections of the electron density on (100). The revised positional parameters thus obtained are y = 0.1549 ± 0.0008 and z = 0.0810 ± 0.0006. The closest neighbors of a gallium atom are one at 2.484 Å, and two each at 2.691 Å, 2.730 Å, and 2.788 Å. These distances are discussed in terms of the metallic bond theories of Pauling. It is concluded that in gallium metal each atom forms one single bond and six additional bonds of order between 0.3 and 0.5.

It is shown how the orientation of absorbing dichroitic crystals may be determined by using the dichroism of reflection.

An x-ray study of cadmium-bromide crystals grown in solutions has shown that 6 R is by far the most common polytype (92 out of 130 structures were this type). Also encountered were 4 H , 12 H (or 12 R ), ∼ 40 H (or ∼ 120 R ), and disordered structures. No structures other than 6 R were formed by sublimation. Twinning in which (0001) is both a twin plane and composition plane is common in 6 R crystals. Some relationships between screw dislocations and the results of this study are considered.

m - chloronitrobenzene crystallizes in the orthorhombic system, with unit-cell dimensions a = 6.00 Å, b = 21.40 Å and c = 5.35 Å. The space group is Pbn 2 1 – C 2 v 9 .