Volume 139, Issue 6

Crystals of μ -N,N′-bis-(2-dimethylaminoethyl)-oxamidato-diisothiocyanato copper(II) with dimethylformamide are monoclinic, a = 15.312 Å, b = 7.445 Å, c = 12.227 Å and β = 99°4′ containing two molecules in the unit cell of symmetry P 2 1 / a . The structure was solved by the heavy-atom method using three-dimensional Fourier and difference Fourier syntheses. The refinement was carried out by block-diagonal matrix least-squares calculations including anisotropic temperature factors for all non-hydrogen atoms and isotropic temperature factors for hydrogen atoms. The final R index for 1507 non-zero reflections was 0.060. The molecule is centrosymmetric. The copper atom is surrounded by four atoms in a square planar environment with three Cu–N bonds, 1.937 Å, 1.909 Å, 2.092 Å and one Cu–O bond, 2.016 Å. The longer out-of-plane Cu–O distance, 2.293 Å links the oxygen atom of dimethylformamide.

Plagionite, Pb 5 Sb 8 S 17 , is monoclinic, space group C 2/ c , with a = 13.4857(8), b = 11.8656(4), c = 19.9834(7) Å, β = 107.168(4)°, ϱ meas = 5.54 g cm −3 and ϱ cal = 5.55 g cm −3 for Z = 4. The mineral is the second member of an homologous series Pb 3+2 n Sb 8 S 15+2 n which differ from other lead sulfantimonides in having tabular rather than acicular habits. Intensity data were recorded with an equi-inclination counter diffractometer. The structure was solved with the symbolic addition procedure and refined, by least-squares techniques, to R = 18.2% for 2302 observable reflections. The asymmetric unit contains 16 atoms. Two Pb atoms are coordinated by six and seven S atoms, respectively, in octahedral-like configuration; a third Pb atom has an irregular eightfold coordination which may be described either as a square antiprism, or as a trigonal prism with neighbors along two face normals. Three of four independent Sb atoms have square pyramidal coordination ; a fourth forms a trigonal pyramidal group. Of nine independent sulfur atoms, four have square pyramidal coordination, three have distorted tetrahedral coordination and two have threefold coordination. The structure is composed of slabs of PbS-like structure which are repeated by b . Stacks of such slabs exist parallel to (112) and (1[unk]2) alternately along c and extend indefinitely along [[unk]10] and [110] respectively. It is postulated that structures of other members in the homologous series contain similar slabs which differ in the width of the PbS-like unit.

An attempt is made to correlate some of the observed features in the layer stacking sequences of the polytypes of SiC, ZnS and CdI 2 based on energetics at the atomic and molecular level. Semiempirical potential functions have been used for the atomic interactions and the difference in energy of the cubic and hexagonal packing has been estimated for the different cases. The observed relative proportions of these two types of packings is shown to be governed by the above difference in energy. The results indicate that the occurrence of numbers such as 2, 3 in the Zhdanov symbol of SiC and larger numbers in the Zhdanov symbol of ZnS are a consequence of the slightly lower energy favouring the cubic packing. In the case of CdI 2 the occurrence of numbers 1 and 2 in the Zhdanov symbol is explained by the almost equal energy of packing of the sandwiched layers. The possibility of observing polytypism in the structure of diamond is pointed out. These results indicate that among various possible factors governing the phenomenon of polytypism, the energetics at the atomic and molecular level are important to be considered.

Abstraft The crystal structure formed by the unusual molecules C 17 H 18 O 4 , a diketo acetate derivative of peristylane, itself a key molecule in the proposed synthesis of dodecahedrane, has been solved by x-ray diffraction methods using the symbolicaddition procedure. The crystals are monoclinic with space group P 2 1 / c – C 2 h 5 , having cell dimensions a = 11.6449, b = 10.9957, c = 14.2992 Å and β = 133.31°. There are four molecules per unit cell giving a calculated density of 1.427 g/cm 3 . The refinement by block-diagonal matrix least squares with anisotropic temperature factors for non-hydrogen atoms led to a final R value of 0.057 for the 1946 observed reflections. The molecule has a bowl shaped structure whose base is a planar, regular carbon pentagon. One of the side pentagons bearing one of the carbonyl oxygen atoms is distorted significantly from planarity and causes a deformation of the molecule from complete regularity.

Diebenzofuran, C 12 H 8 O, forms orthorhombic crystals with four molecules in the unit cell of dimensions a = 7.65 ± 0.02 Å, b = 5.90 ± 0.01 Å, c = 19.32 ± 0.02 Å. Diffraction effects permit space groups Pnam and Pna 2 1 . The intensity-statistics test favored Pnam . The parameters were refined by the three-dimensional, least-squares method utilizing 539 observed reflection intensities obtained photographically with Cu Kα . radiation, to an R of 0.092. The bond lengths and bond angles of the molecule are consistent with the standard values in other similar aromatic compounds. A crystallographic mirror plane normal to c axis passes through the oxygen atom. Some physical properties of the compound are discussed by comparing its structure with those of fluorene and carbazole.