Band 211, Heft 9

We developed a small vacuum chamber for very low background single crystal diffraction experiments. The chamber has been designed for a large Eulerian cradle. The φ and χ rotations are not limited by the chamber, while the ω rotation is limited to a cone of 20° opening angle. A crucial part of the design is the position of the beam stop inside the vacuum, eliminating any scattering at the exit windows. The chamber effectively eliminates any air scattering of the primary beam, creating a very low background for single crystal diffraction experiments.

An attempt has been made to study the effect of atomic heterogeneity of various degrees on the probability distribution of the normalised structure factors. This treatment provides a method of studying separately the effect of two factors affecting heterogeneity, namely p = N H / N L and g = f H / f L where N H and N L are numbers per asymmetric unit of heavy and light atoms, respectively and f H and f L are atomic scattering factors of the heavy and light atoms, respectively. A number of real examples of various p and g values indicating different degrees of heterogeneity have been studied. We have shown that in cases of atomic heterogeneity a modified structure factor E ′ may be defined which is Gauss distributed irrespective of whether E is Gauss distributed or not. Depending on the parameters p and g , the agreement between the two distributions P ( E ′) vs E ′ and P ( E ) vs E vary from being poor to excellent. In cases where the agreement is excellent it should be possible, in principle, to obtain the heavy atom position.

Fe 2 O(SeO 3 ) 2 , frequently obtained in various chemical systems by low-hydrothermal treatment of SeO 2 and FeC 2 O 4 · 2 H 2 O, has been investigated using single-crystal X-ray diffraction. Crystal data (at room temperature): space group Pccn , a = 6.639(2) Å, b = 12.839(4) Å, c = 13.266(4) Å, V = 1130.8(6) A 3 , Z = 8; R ( F ) = 0.031, R w ( F 2 ) = 0.061. The crystal structure is composed of two crystallographically different types of [FeO 6 ] octahedra, a [FeO 4 ] tetrahedron and two types of trigonal pyramidal [SeO 3 ] groups, which are linked by common edges and corners to a three-dimensional framework. Mean bond lengths are 2.015 Å and 2.027 Å for Fe [6] –O, 1.870 Å for Fe [4] –O, and 1.703 Å and 1.707 Å for Se(IV) [3] –O. The oxo-oxygen is nearly planar-coordinated to three Fe atoms.

Crystals of the orthorhombic phase of strontium germanate showing superstructure have been obtained. Crystal data are: a = 5.8520(4) Å, b = 14.2991(8) Å, c = 9.9165(6) Å, space group Pbn 2 1 and Z = 8 × Sr 2 GeO 4 . The structure has been determined based on a set of 3567 diffraction intensities measured with a single crystal diffractometer giving a final value of R ( F ) = 6.4% for 1513 reflections used. The superstructure of our crystal is twice as large as the average structure ( a = 5.85 Å, b = 7.15 Å, c = 9.92 Å). The dominant rule of differentiating two-layers superstructure type like our crystal and three-layers type like other crystals of the compounds α ′ L -(Ca,Sr) 2 SiO 4 is the size effect of cations at the tetrahedral positions. In addition, this rule leads to the results that the two-layers type shows the space group Pbn 2 1 and the three-layers type shows the space group P 2 1 nb on account of the geometrical restriction.

The crystal and molecular structures of two polymorphic forms of propyl-[3-(pentamethyldisiloxanyl)-propyl]dimethylammonium bromide (C 13 H 34 N + OSi 2 · Br − ) have been determined by X-ray analysis using direct methods of phase determination. Both forms have the monoclinic space group P 2 1 / c with Z = 4. At room temperature form A with a = 19.051(5) Å, b = 8.390(2) Å, c = 13.542(5) Å, β = 104.00(3)°, V = 2100(1) Å 3 , D x = 1.127 Mgm −3 , μ (Cu K α ) = 3.696 mm −1 was found. After cooling the crystal form B appeared, which was still stable after reheating. Form B has a = 18.42(2) Å, b = 8.390(3) Å, c = 13.57(1) Å, β = 95.98(6)°, V = 2085(3) Å 3 , D x = 1.136 Mgm −3 , μ (Cu K α ) = 3.723 mm −1 . In both polymorphs the molecules form bilayers which are held together mainly by N + –Br − contacts. Form A is related to form B by shifting every bilayer parallel to the c -axis by c /2 relative to its neighboring bilayers. Because of the strong relation between both lattices the phase transformation can be regarded as topotactic reaction.

The molecular and crystal structures of the terminally blocked homo-tri- and homo-pentapeptides from 1-amino-1-cyclopentane-carboxylic acid, Z -(Ac 5 c) 3 -O t Bu ( 1 ) and Z -(Ac 5 c) 5 -O t Bu ( 2 ), were determined by X-ray diffraction. The two compounds exhibit the following parameters: ( 1 ) orthorhombic, P 2 1 2 1 2 1 , a = 11.539(7) Å, b = 16.185(2) Å, c = 30.925(4) Å and Z = 8; ( 2 ) triclinic, P [unk], a = 10.835(5) Å, b = 11.631(9) Å, c = 18.212(5) Å, α = 92.20(7)°, β = 94.79(10)°, γ = 107.6(3)° and Z = 2. The crystal structures were solved by direct methods. The least-squares refinements led to R values of 0.080 and 0.060 for 4061 and 4018 reflections with I ≥ 2 σ ( I ) (according to the SHELXL-93 computer program weighting scheme) for ( 1 ) and I ≥ 3 σ ( I ) (Structure Determination Programs, SDP crystallographic package weighting scheme) for ( 2 ), respectively. The two independent molecules, A and B , in the asymmetric unit of the tripeptide ( 1 ) are folded into a type-I/III and a type-I′/ β -bend conformation, respectively, each stabilized by an intramolecular C=O…H–N H-bond. The pentapeptide molecules adopt a 3 10 -helical structure, stabilized by three consecutive, intramolecularly H-bonded type-III (III′) β -bend conformations.

The structure of the title compound, crystallized from benzene, in the monoclinic system, space group P 2 1 / c , with a = 9.032(3) Å, b = 10.111(1) Å, c = 14.625(1) Å, β = 121.3(1)°, Z = 4, has been solved from diffraction data collected on 2D imaging plate detector. This method allowed successful integration of non-uniform reflection profiles with short exposure time. Structural results revealed correct reaction path in the synthesis.