Powder neutron diffraction of SrNbO2N at room temperature and 1.5 K

The structure of strontium niobium dioxygen nitride, SrNbO₂N, has been solved by powder neutron diffraction at room temperature and 1.5 K. SrNbO₂N crystallizes in the tetragonal space group I4/mcm, with a = 5.7056 (4) and c = 8.1002 (9) Å at room temperature, and a = 5.6938 (4) and c = 8.0974 (8) Å at 1.5 K. The crystal structure is derived from the cubic perovskite archetype by a slight rotation of the Nb(O,N)₆ octahedra with respect to the tetragonal axis. A partially ordered distribution of oxygen and nitrogen on the anionic sites was found.     

A new crystal phase of barium nitroprusside trihydrate studied by neutron diffraction at 20 K

The crystal of barium penta­cyano­nitro­syl­ferrate trihydrate {barium nitro­prusside trihydrate, Ba[Fe(CN)₅(NO)]·3H₂O} has been studied by neutron diffraction at 20 K. The study was performed to characterize the structural phase generated by the phase transition undergone by the crystals at 80 K, at which temperature the unit‐cell volume doubles. This crystal phase still exists at 20 K. The crystal structure, in space group P1, is completely ordered. The positional changes of the water mol­ecules in the present structure with respect to those of the compound at 105 K are presented.     

Na2Fe(CN)5(NO)·2D2O at 11 and 293 K by X‐ray,and at 15 K by neutron diffraction

The crystal structure of Na₂Fe(CN)₅(NO)·2D₂O, disodium penta­cyano­nitro­syl­ferrate(III) bis­(dideuterium oxide), has been determined by X‐ray diffraction at 11 and 293 K, and by neutron diffraction at 15 K. The accurate and extensive data sets lead to more precise determinations than are available from earlier work. The agreement in atomic positional and displacement parameters between the determinations at low temperature is very good.     

MEM(TCNQ)2 at room temperature and 10 K,and the absence of a spin‐Peierls transition

Precise X‐ray determinations of the crystal structure of the 1:2 complex of N‐ethyl‐N‐methyl­morpholinium and 7,7,8,8‐tetra­cyano‐p‐quinodi­methanide, abbreviated as MEM–TCNQ or MEM(TCNQ)₂ (C₇H₁₆NO⁺·2C₁₂H₄N₄^0.5?), have been performed at 293 and at 10 K. Evidence for the expected spin‐Peierls transition at 19 K is not found, and this may follow from radiation damage to the crystal or from insufficient equipment sensitivity.     

Low‐temperature study of 3‐acetylindole at 110 K

The title compound, C₁₀H₉NO, contains an acetyl group that is nearly coplanar with the indole ring system, with an angle between the planes of the heterocyclic ring and the acetyl group of 1.75 (17)°. The planes of the benzene and pyrrole rings in the indole system make a dihedral angle of 2.05 (11)°. Each molecule in the unit cell is linked through N—H...O hydrogen bonds to two other molecules, forming hydrogen‐bonded chains in the [101] direction with graph set C(6). The significance of this study lies in the analysis of the interactions occurring via hydrogen bonds in this structure, as well as in the comparison drawn between the molecular structure of the title compound and those of several other indole derivatives possessing a 3‐carbonyl group. The correlation between the IR spectrum of this compound and the structural data is also discussed.     

Two polymorphs of anhydrous 4‐pyridone at 100 K

Two polymorphs of the title compound, C₅H₅NO, (I), have been obtained from ethanol. One polymorph crystallizes in the monoclinic space group C2/c [henceforth (I)‐M], while the other crystallizes in the orthorhombic space group Pbca [henceforth (I)‐O]. In the two forms, the lattice parameters, cell volume and packing motifs are very similar. There are also two independent molecules of 4‐pyridone in each asymmetric unit. The molecules are linked by N—H...O hydrogen bonds into one‐dimensional zigzag chains extending along the b axis in the (I)‐M polymorph and along the a axis in the (I)‐O polymorph, with the graph set C₂²(12). The structures are stabilized by weak C—H...O hydrogen bonds linking adjacent chains, thus forming a ring with the graph set R₆⁵(28). The significance of this study lies in the analysis of the hydrogen‐bond interactions occurring in these structures. Analyses of the crystal structures of the two polymorphs of 4‐pyridone are helpful in elucidating the mechanism of the generation of spectroscopic effects observed in the IR spectra of these polymorphs in the frequency range of the N—H stretching vibration band.     

4‐Ethynyl‐1,2‐methylenedioxybenzene at 150 K

The title compound, C₉H₆O₂, contains two moderate C—H?O hydrogen bonds. That involving the terminal alkyne gives rise to chains along the b axis. The other hydrogen bond occurs over a centre of symmetry, leading to dimers. The combination of the two interactions gives rise to rings, each comprising six mol­ecules, which are part of infinite sheets in the bc plane.     

The low‐temperature phase of 1,3‐di­bromo‐2,4,6‐tri­methyl­benzene: a single‐crystal neutron diffraction study at 120 and 14 K

In the low‐temperature phase of di­bromo­mesityl­ene (1,3‐di­bromo‐2,4,6‐tri­methyl­benzene), C₉H₁₀Br₂, the mol­ecule deviates significantly from the C_3h molecular symmetry encountered in tri­bromo­mesityl­ene (1,3,5‐tri­bromo‐2,4,6‐tri­methyl­benzene), even for the endocyclic bond angles. An apparent C_2v molecular symmetry is observed. The angle between the normal to the molecular plane and the normal to the (100) plane is ∼20°. The overall displacement was analysed at 120 K with rigid‐body‐motion tensor analysis. The methyl group located intermediate between the two Br atoms is rotationally disordered at both temperatures. This disorder was treated using two different approaches at 14 K, viz. the conventional split‐atom model and a model using the special annular shapes of the atomic displacement parameters that are available in CRYSTALS [Watkin, Prout, Carruthers & Betteridge (1999). Issue 11. Chemical Crystallography Laboratory, Oxford, England], but only through the latter approach at 120 K. The disorder locally breaks the C_2v molecular symmetry at 14 K only. Intra‐ and intermolecular contacts are described and discussed in relation to this methyl‐group disorder. The bidimensional pseudo‐hexagonal structural topology of tri­halogeno­mesityl­enes is altered in di­bromo­mesityl­ene insofar as the (100) molecular layers are undulated and are not coplanar as a result of an alternating tilt angle of ∼34° propagating along the [011] and [01] directions between successive antiferroelectric molecular columns oriented roughly along the a axis.     

Acetone‐4‐methyl­thio­semicarbazone at 220 K

In the title compound, C₅H₁₁N₃S, the trans conformation is stabilized by a weak intramolecular N—H?N hydrogen bond. Unusually, one N—H bond is not involved in any hydrogen‐bond interactions and instead the mol­ecules form a one‐dimensional polymer via N—H?S intermolecular hydrogen bonds.     

Dielectric properties of polystyrene and some polychlorostyrenes from 4°K to room temperature

Dielectric measurements of carefully purified specimens of polystyrene and poly(2,3,4 or 3,4-chlorostyrene) have been obtained at audio frequencies ranging from 0.1 to 20 kHz and at temperatures between 4 and 300°K. Each of the samples exhibits a dielectric loss maximum in the range 15–50°K. The temperature of the maximum loss decreases with the addition of a substituent which lowers the symmetry of the pendant phenyl group. The results are explained by a model which invokes a coupling mechanism between two distinct modes of side group motions. This same model also explains some results of previously reported measurements of mechanical losses in similar polymers.     

MnCl2‐Promoted synthesis of quinoxaline derivatives at room temperature

MnCl₂ efficiently catalyzes the condensation of o‐phenylenediamine derivatives with 1,2‐diketones at room temperature to afford the corresponding quinoxaline derivatives in high yields. © 2008 Wiley Periodicals, Inc. Heteroatom Chem 19:218–220, 2008; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/hc.20401     

A low‐temperature neutron diffraction study of Mn12‐acetate

In the low‐temperature region, where the dodecanuclear mixed‐valence manganese carboxyl­ate hexa­deca­acetatotetra­aqua­dodeca­oxo­dodeca­manganese bis­(acetic acid) tetra­hydrate, [Mn₁₂O₁₂(C₂D₃O₂)₁₆(H₂O)₄]·2C₂HD₃O₂·4H₂O, displays unusual magnetic properties, its structure is similar to that previously determined at room temperature [Lis (1980). Acta Cryst. B 36 , 2042–2046], differing only by a small change in the configuration of one of the coordinated acetate groups, related to the formation of additional hydrogen bonds, and by the orientation of the methyl groups. Since most of the magnetization density of this system resides on the Mn atoms, the consequences of these rearrangements for the magnetic properties of the compound are small.     

Monoclinic AlPO4 tridymite at 473 and 463 K from X‐ray powder data

Upon cooling from its hexagonal high‐temperature modification, AlPO₄ (aluminium phosphate) tridymite successively transforms to several displacively distorted forms, including a normal structure–incommensurate–lock‐in phase transition sequence. The space‐group symmetries in this series are P112₁, P112₁(αβ0) and P2₁2₁2₁, respectively. The distortion pattern of the intermediate P112₁ phase can be described as alternate shifts of adjacent layers of tetrahedra coupled with tilting of the tetrahedra. The symmetry and direction of the shifts are different from the analogous SiO₂ tridymite modification. The atomic displacement parameters of the O atoms are strongly anisotropic due to thermal motions of the rigid tetrahedra. Condensation of a lattice vibration mode results in the formation of an incommensurate structural modulation below 473 K. The 3+1 superspace‐group symmetry of the modulated phase is P112₁(αβ0).     

Powder neutron diffraction of α-UB2C (α-UB2C-type)

The crystal structure of α-UB₂C (low temperature modification below T = 1675(25)°C) was determined from powder X-ray data (RT) and powder neutron diffraction data (at 29 K) employing the Rietveld-Young-Wiles profile analysis method. α-UB₂C crystallizes in the orthorhombic space group Pmma with a = 0.60338(3), B = 0.35177(2), C = 0.41067(2) nm, V = 0.0872 nm³, Z = 2. The residuals of the neutron refinement were R₁ = 0.032 and R_F = 0.043. The crystal structure of α-UB₂C is a new structure type where planar nonregular 6³-U-metal layers alternate with planar nonmetal layers of the type (B₆C₂)³. Boron atoms are in a typical triangular prismatic metal surrounding with a tetrakaidekahedral coordination B[U₆B₂C₁], whereas carbon atoms occupy the center points of rectangular bipyramids C[U₄B₂]. The crystal structure of α-UB₂C derives from the high temperature modification β-UB₂C (ThB₂C-type, ), which reveals a similar stacking of slightly puckered metal layers 6³, alternating with planar layers B₆ · (B₆C₃)². The phase transition from β-UB₂C to α-UB₂C is thus essentially generated by carbon diffusion within the B₆ · (B₆C₃)² layers to form (B₆C₂)³ layers.     

Partial ordering of tripivaloylmethane at 110 K

Tripivaloylmethane [systematic name: 4‐(2,2‐dimethylpropanoyl)‐2,2,6,6‐tetramethylheptane‐3,5‐dione], C₁₆H₂₈O₃, is known to crystallize at room temperature in the space group R3m with three molecules in the unit cell. The molecules are conformationally chiral and pack so that each molecular site is occupied with equal probability by the two enantiomers. Upon cooling to 110 K, the structure partially orders; two molecules in the unit cell order into two different conformations of opposite chirality, while the third remains disordered. The symmetry of the resulting crystal is P3, with each of the molecules lying about a different threefold rotation axis. This paper describes an unusual case of order–disorder phase transition in which the structure partially orders by changes of molecular conformation in the single crystals. Such behaviour is of interest in the study of phase transitions and molecular motion in the solid state.     

4‐(Piperidin‐1‐yl)pyridinium hexafluorophosphate at 150 K

Structural characterization of the title compound, C₁₀H₁₅N₂⁺·PF₆⁻, shows it to be ionic, with the pyridine rather than the piperidine N atom being protonated and forming hydrogen bonds to the counter‐ions, resulting in two independent ion pairs. A number of unusual features are noted, in particular the remarkably close inter‐ring hydrogen contacts [1.97 (3)–2.00 (3) Å] and the considerable differences in the pair of cations, in respect of the torsion angles within the piperidine ring involving the bonds to either side of the N atom.