Volume 216, Issue 7

In the past decade, new theoretical approaches have been developed to determine, predict and understand the struc-ture of chemical compounds. The central element of these methods has been the investigation of the energy landscape of chemical systems. Applications range from extended crystalline and amorphous compounds over clusters and molecular crystals to proteins. In this review, we are going to give an introduction to energy landscapes and methods for their investigation, together with a number of examples. These include structure prediction of extended and mo-lecular crystals, structure prediction and folding of proteins, structure analysis of zeolites, and structure determination of crystals from powder diffraction data.

To compare the molecular structures between colorless and colored forms of fluoran basic dyes, we developed a computer program COMPARE to compare two sets of isooriented orthogonal coordinates of atoms, intramolecular distances less than 4.65 Å and anisotropic displacement parameters by using half-normal probability plots. As a result the differences in the molecular structure between colorless and colored forms were observed with high sensitivity on all moieties of molecules, especially in hydrogens.

Occurrence, chemistry, crystal growth, technical applications, structure and polytypism of xonotlite Ca 6 Si 6 O 17 (OH) 2 are reviewed. Atomic coordinates of the three simplest ordered polytypes in modified Gard notation: Ma 2 bc (space group P 2/ a , a = 17.032, b = 7.363, c = 7.012 Å, β = 90.36°), Ma 2 b 2 c (space group A 2/ a , a = 17.032, b = 7.363, c = 14.023, β = 90.36°), and M 2 a 2 bc (space group P 1̅, a = 8.712, b = 7.363, c = 7.012 Å, α = 89.99° β = 90.36°, γ = 102.18°) were modeled from geometric principles based on the known structure of the M 2 a 2 b 2 c polytype (space group A1̅, a = 8.712, b = 7.363, c = 14.023, α = 89.99 β = 90.36, γ = 102.18°). Unique reflection arrangements in the reciprocal lattice, characteristic of each polytype, were defined as criteria to identify xonotlite polytypes on X-ray single-crystal photographs. Precession- and Weissenberg-photographs of a xonotlite from the Kalahari manganese field (Republic of South Africa) indicated the predominance of the (100) twinned M 2 a 2 b 2 c polytype, followed by the Ma 2 b 2 c polytype, and very low concentrations of the Ma 2 bc polytype. The M 2 a 2 bc polytype could not be identified which agrees with previous electron diffraction experiments on xonotlites from other localities. Diffuse streaks parallel to a * and less intensive ones parallel to c * on single-crystal photographs suggest the presence of additional disordered polytypes.

Rietveld refinement and the final model for pure κ -alumina are presented. The starting model was obtained through total energy calculations based on first principles. The resulting geometry parameters are within intervals of expected values for octahedral and tetrahedral coordina-tions. This result thus serves as a validation of the compu-tational approach as a useful alternative to classical methods in the first step of structure determination.

The structure of the title compound was determined at 10 K using the Fddd diffractometer equipped with a Mo-target rotating anode. The object of the study was to determine the geometrical parameters of the cation and anion at what we assume is their resting state geometry, given the temperature of data collection. The cation is es-sentially unchanged in geometry from 294 K to 10 K. The anion is considerably distorted, having changed from a trigonal bipyramid (site symmetry 32) to a “T-shaped” or “seesaw” structure, sitting at a two-fold axis. During the cooling process, the space group changed from Fd 3̅ c to F 4 1 / adc . The shape of the anion is, interestingly, similar to that of XeO 2 F 2 .