April 1996 issue
Cover illustration: Reaction cavities for 2-cyanoethyl groups in cobaloxime complexes. Courtesy of K. Sawada, D. Hashizume, A. Sekine, H. Uekusa, K. Kato, Y. Ohashi, K. Kakinuma and Y. Ohgo.
Neutron diffraction, using FeAlO3 polycrystalline samples at T = 30 K, reveals a classical Néel ferrimagnetism with strong `180 degrees Fe-O-Fe' superexchange antiferromagnetic interactions and an extremely weak spontaneous magnetization (0.38 ± 0.17 μB). Piezoelectricity originates in the bond arrangement on the tetrahedral sites.
The structure of the x = 0.1 member of the (1-x)Ta2O5.xWO3 solid solution is described and compared with the structures of the x = 0.14 and x = 0.267 members.
The anharmonic thermal vibration of atoms of 13Cx12C1-x and the isotope dependence of their lattice constant have been examined after κ refinement.
Lattice-dynamical calculation of atomic displacement parameters, IR and Raman spectra, and thermodynamic functions for natural garnets is shown. Evidence for disordered Mg and Fe atoms in pyrope and almandine, respectively, is given.
The deformation densities of d electrons in K2[PdCl6] and K2[PtCl6] crystals were analyzed by theoretical calculations using embedded cluster models.
The crystal structure of Mg6SO2(OH)14 has been solved from single-crystal diffraction data collected on an extremely small crystal of dimensions ca 0.5 x 100 x 2.5 μm3 with synchrotron radiation. The structure is based on a stack of zigzag sheets composed of MgO6 octahedra.
The disordered room-temperature structure and ordered low-temperature structure of [Pt2(μ-H)(μ-S)(dppe)2](PF6) are reported, together with the room-temperature structure of the corresponding PtPd complex. NMR spectroscopy confirms the genuine heteronuclear nature of the mixed-metal complex.
For the first time the J aggregation in the crystal of merocyanine dye is determined. The structure and stability of layered organic-inorganic structures with internal surfaces are discussed.
Studies on the mechanism of ferro-paraelectric phase transitions in DMACA are described. Temperature changes of Sb-Cl bond lengths of up to 0.3 Å were observed and attributed to the deformation of the lone-electron pair on the Sb''' atom caused by N-H⋯Cl hydrogen bonds.
Modification of the crystal structure of ammonium Rochelle salt owing to the replacement of NH4 with K ions is studied at room temperature. Differences in the accommodation ratios and the environment of the two types of NH4 ion sites are given.
A quantitative relation has been obtained between the void space around the reactive group and the reaction rate in the photoisomerization of four crystal forms of a cobalt complex.
Multi-temperature X-ray diffraction analysis of the vanadocene Cp2V single crystal at 108, 170, 297 and 357 K shows that crystal disorder in this compound has a dynamic nature, and at 108 K the crystal structure of Cp2V is ordered. A model of the disordering at higher temperatures is supposed.
No ordering of the H-atom positions in the short hydrogen bonds between hydrogen succinate ions occurs by the phase transition.
The monoclinic cell assigned to the twins is not primitive (leading to overlap of hkl with h = ±3n) as originally supposed, but B-centred. This was discovered using a partial data set from which both h = ±3n and (h + l) = 2n + 1 had been removed. Reflexions h = ±3n do not in fact overlap and were restored for full refinement.
Structural studies of natural chlorinated auxin, 4-chloroindole-3-acetic acid and other phenyl-substituted halogen derivatives, based on X-ray structure analysis and computational chemistry methods, are presented.
Structure solution with the crystal structure prediction program MOLPAK, starting with a molecular mechanics geometry-optimized model, is described. An example of a polynitrodiazabicyclo[3.3.0]octane ring system is studied.
From neutron diffraction data collected at 15 K, mean-square nuclear displacements are obtained for the intramolecular H and D vibrations. The isotope effect is highly significant. For the hexanoate anion, normal-mode analysis using the force field from ab initio quantum mechanical calculations gives mean-square displacements for H and D which do not agree with the neutron diffraction results.
4-Cyclohexylamino-5-hydroxy-1,5-diphenyl-Δ3-imidazolin-2-one crystallizes as a racemate, whereas the corresponding 4-tert-butylamino derivative occurs optically resolved.
The structural comparison of two trans-syn thymine photodimers with their respective parent compounds shows that the dimerization results in a strained geometry about the cyclobutane ring and a disturbed planarity of the thymines. The degree of such stereochemical changes, consequent to dimerization, depends on the nature of the linker backbone.
The charge-transfer salt TSeF-TCNQ has the same segregated stack structure as TTF-TCNQ. Replacement of S by Se changes the relative importance of cation-cation and anion-anion stack interactions in the crystal.