Rhombohedral pyroaurite (Rgm, a=3.1094+2, c=23.4117+9 A, at 21°) consists, like hexagonal sj6grenite (P6/mmc, a=3.113+3, c=15"61+1/~), of positively charged brucite-like layers [Mg6Fe~(OH)16] z+ alternating with disordered negatively charged interlayers [CO3.4H20]E-(Z= ~t for pyroaurite). Mg and Fe are randomly distributed among the octahedral positions. The OH layer sequence in pyroaurite is -BC-CA-AB-BC-. The water molecules of the (liquid-like) interlayer prefer positions about 0.56 A off the threefold axis connecting two OH groups of adjacent brucite-like layers. The following distances were found: Me-OH 2.06, OH-OH 2.72 (3 x ) and 3"11 (6 x ), OH-HEO 2.93 A; /_ OH-HzO-OH 158 °. During the refinement with 283 reflections the residual dropped to R= 6.1%. The interlayer only contributes to the 001 and 101 reflections.

Both the approach used and the progress made in the assignment of structure types to the crystal structures contained in the ICSD database are reported. Extending earlier work, an hierarchical set of criteria for the separation of isopointal structures into isoconfigurational structure types is used. It is shown how these criteria, which include the space group (number), Wyckoff sequence and Pearson symbol, c/a ratio, ranges, ANX formulae and, in certain cases, the necessary elements and forbidden elements, may be used to uniquely identify the representative structure types of the compounds contained in the ICSD database.

No abstract

No abstract

No abstract

No abstract

Ammonium paratungstate, (NH4)10[H2W12042].10H20 crystallizes in space group Pbca: a=19. 07, b=24.42, c= 10.915 .~, Z=4, D,,,[4][5][6][7][8][9][10][11][12][13] Dx=4.23 g.cm -3. The W atoms were located by a Patterson map, and the lighter atoms by a AF-map. Using 3002 visually estimated reflexions, including 609 unobserved ones, the structure was refined to R=8.5% (8.1% for observed reflexions only; isotropic temperature factors).

Synthesis and Structure of Alkyl N-AcylimidatesAlkyl N-acylimidates 1 are prepared more easily and in higher yields than before by reaction of alkyl imidate hydrochlorides 5 with acyl halides 7 in the presence of 2.2 mol of base (14 examples). The X-ray analysis of a crystalline derivative (ldbd) shows, that the C=N-and the C = 0 parts are twisted significantly (torsional angle 77.6"). The stereochemical, dynamic, and electronic properties of the compounds 1 are interpreted by means of ab initio 3-21 G calculations on conformers of the parent system HO -CH = N -CH = 0 (8). Low rotational (ca. 6 kcal/mol) and inversional (max. 8 kcal/mol) barriers indicate the many favourable electronic interactions between the C = N-and the C = 0 groups in such N-functionalised imine dcrivatives. The compounds 1 are significantly higher in energy than bisacylamines and are therefore suggested to be superior, more reactive synthetic C -N -C building blocks. The spectroscopic properties (IR, 13C-and 'H NMR, MS) arc given and discussed.I N-Acylimidsaureester 1, die durch die Atomreihung -0 -C = N -= 0 gekennzeichnet sind, lassen auf Grund ihrer Haufung an funktionellen Zentren zahlreiche Reaktionsmoglichkeiten erwarten. So bieten sich z. B. fur nucleophile Ang r s e die Kohlenstoffatome der Carbonyl-und der Imidfunktion an. Auf diese Weise konnten eine Reihe fiinf-und sechsgliedriger Heterocyclen synthetisiert 0 VCH Verlagsgesellschaft mbH, D