The structure of three polymorphic forms of glycine in the crystal phase, α-glycine (P21/n), β-glycine (P21), and γ-glycine (P32), was completely optimized with standard gradient procedures using a point charge model and using a model constituted of a 15-molecule cluster surrounded by point charges. The calculations were performed with a 6-31G basis set and in the SCF step of the calculations the MIA approach was used. The results for the 15-molecule cluster are in better agreement with the experimental results than the results obtained with the point charge model.

Using ab &itio calculations at the 4-21G level and procedures for extrapolation to rg geometry as well as using the electrostatic crystal field (ECF-MO) approach, the geometry and torsion potential were calculated for 1,1'-biphenyl in the gas phase, in the

The wavelength dispersion delta lambda/lambda in a graphite (002) monochromated Mo K alpha beam was analyzed. A wavelength window was found with 0.68 < lambda < 0.79 A, i.e. delta lambda/lambda = 0.14. The very large dispersion leads to systematic errors in Iobserved(H) caused by scan-angle-induced spectral truncation. A limit on the scan angle during data collection is unavoidable, in order that an omega/2 theta measurement should not encompass neighboring reflections. The systematic intensity errors increase with the Bragg angle. Therefore they influence the refined X-ray structure by adding a truncational component to the temperature factor: B(X-ray) = B(true) + B(truncation). For an Mo tube at 50 kV, we find B(truncation) = 0.05 A2, whereas a value of 0.22 A2 applies to the same tube but operated at 25 kV. The values of B(truncation) are temperature independent. The model bias was verified via a series of experimental data collections on spherical crystals of nickel sulfate hexahydrate and ammonium hydrogen tartrate. Monochromatic reference structures were obtained via a synchrotron experiment and via a 'balanced' tube experiment.

242ChemInform Abstract The complexes (I)-(V) are synthesized by 1:1 and 1:2 molar reaction of the corresponding Cu(II) salts with the ligand L1 or L2 in ethanol/methanol with 60-75% yields. For comparison the Zn(II) halide compounds (VI) and (VII) are prepared and characterized. The structures of the title compounds (I) and (IV) (both space group P21/c, Z=4) are determined by X-ray analysis. In complex (I), the tridentate ligand L1 and the acetonitrile ligand coordinate in a square-planar geometry. The perchlorate ligands are in axial positions; the second perchlorate is considered to be semi-coordinating. In complex (IV), the bis(methylbenzimidazolyl)pyridines L2 act as tri-and bidentate ligands. The sixth donor atom, again from a perchlorate O atom, is considered to be semi-coordinating. On the basis of IR, ligand-field, and ESR spectra, analogous structures are discussed for the other Cu(II) compounds.

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