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  • 1
    Electronic Resource
    Electronic Resource
    Copenhagen : International Union of Crystallography (IUCr)
    Acta crystallographica 57 (2001), S. o719-o720 
    ISSN: 1600-5368
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The crystal structure of the title compound, C6H7N3O, exhibits packing typical of amides, with N—H...O hydrogen-bond dimers forming a corrugated tape and N—H...N bonds connecting the tapes.
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  • 2
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 91 (2002), S. 2818-2826 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The positron work function of 6H-SiC was determined to be −2.1±0.1 eV from an analysis of the energy spectrum of positrons reemitted from the surface. The positron reemission yield, highest in the sample inserted into vacuum after atmospheric exposure and cleaning with ethanol, was significantly reduced after sputtering with 3 keV, 125 μA min Ne+ ions. The yield was not recovered even after annealing at 900 °C, presumably due to the stability of sputter induced defects. Sputtering at lower energies caused a smaller decrease in the reemission yield that was largely recovered after annealing at 850 °C. Analysis using electron induced Auger electron spectroscopy and positron-annihilation-induced Auger electron spectroscopy indicated that the surface was Si enriched after sputtering and C enriched after subsequent annealing. Values of positron diffusion length and mobility in the unsputtered material were extracted from the dependence of the reemission yield on the beam energy. The application of SiC as a field-assisted positron moderator is discussed. © 2002 American Institute of Physics.
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  • 3
    Electronic Resource
    Electronic Resource
    Oxford [u.a.] : International Union of Crystallography (IUCr)
    Acta crystallographica 56 (2000), S. e259-e259 
    ISSN: 1600-5759
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The crystal structure of the title compound, C28H26, in the monoclinic space group C2/c has a columnar packing arrangement with a 7.20 Å axis, a feature common to several tetraphenylmethanes.
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  • 4
    Electronic Resource
    Electronic Resource
    Copenhagen : International Union of Crystallography (IUCr)
    Acta crystallographica 56 (2000), S. 512-525 
    ISSN: 1600-5740
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The crystal structures of six synthetic 2-oxa-steroids (A-ring lactone steroids) have been determined by single-crystal X-ray diffraction. The conformation and hydrogen bonding in these oxa-steroids is compared with packing motifs in the natural steroids and the anabolic agent, Anavar®. O—H...O hydrogen bonding with lactone carbonyl O is the preferred arrangement in molecules with a C—OH group. The donor H atoms of A, B and D rings participate in C—H...O interactions with lactone carbonyl O and D-ring hydroxyl/ketone O acceptor atoms. The conformation of the lactone ring in these analogues is different from the natural androgens because replacement of the C2-methylene group by an O atom changes the geometry of the A ring. Two structurally related lactone steroids provide the first example of O—H...O/C—H...O interaction mimicry and furthermore the two components form a binary solid solution. The O—H...O and C—H...O hydrogen bonds in 2-oxa-steroid crystal structures are analysed and the observed preferences discussed in terms of geometric and chemical factors.
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  • 5
    Electronic Resource
    Electronic Resource
    Oxford [u.a.] : International Union of Crystallography (IUCr)
    Acta crystallographica 56 (2000), S. 1512-1514 
    ISSN: 1600-5759
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: In the crystal structure of the title diamide, C6H6N4O2, linear tapes of carboxamide N—H...O and pyrazine C—H...N hydrogen-bond dimers are connected by N—H...O bonds to form a staircase-like pattern.
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  • 6
    ISSN: 1600-5740
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The title complex has been studied using low-temperature X-ray (150 K) and neutron (100 K) diffraction. Molecules of the triazine host form a two-dimensional hexagonal network mediated by trigonally symmetric Cl3 synthons having Cl...Cl interactions of 3.441 (3) Å, a C—Cl...Cl angle of 165° and a Cl...Cl—C angle of 105°, close to the ideal values of 180 and 90°, respectively. The guest molecules are of an appropriate size to fit the hexagonal networks and interact with the host via C—H...π (phenyl) and C—Br...π (phenyl) interactions which stabilize the overall structure. Both C-donor bond vectors are directed more closely towards the mid-point (X) of an individual aromatic bond, rather than the ring centroid, with H...X 2.817 (9) Å and C—H...X 174.0 (9)°, and Br...X 3.353 (4) Å and C—Br...X 158.1 (2)°.
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  • 7
    ISSN: 1600-5740
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The structures of four para-substituted derivatives of diphenylethynylmethanol have been determined [ditolylethynylmethanol, di(4-chlorophenyl)ethynylmethanol, di(4-bromophenyl)ethynylmethanol and bis(4,4′-biphenylyl)ethynylmethanol]. The dimethyl, dichloro, dibromo and diphenyl compounds have been analysed using X-ray diffraction at 150 K, and the dichloro compound has also been studied using neutron diffraction at 150 K. In common with the parent diphenylethynylmethanol [Garcia, Ramos, Rodriguez & Fronczek (1995). Acta Cryst. C51, 2674–2676], all four derivatives fail to form the expected strong O—H...O hydrogen bonds due to steric hindrance. Instead, the supramolecular structural organization in this family of gem-alkynols is mediated by a variety of weaker interactions. The two most acidic protons, O—H and C[triple-bond]C—H, participate in weak hydrogen bonds to π-acceptors, forming synthons that stabilize all five structures. These primary interactions are reinforced by a variety of other weak hydrogen bonds involving C—H donors and the hydroxy-O as an acceptor, and by halogen...halogen interactions in the dichloro and dibromo compounds.
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  • 8
    Electronic Resource
    Electronic Resource
    Oxford [u.a.] : International Union of Crystallography (IUCr)
    Acta crystallographica 56 (2000), S. 123-124 
    ISSN: 1600-5759
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The title compound, C36H26O2Se2, displays crystallographic twofold symmetry. The packing involves corrugated linear ribbons mediated through C—H...O and C—H...Se interactions. The ribbons are connected through C—H...π interactions.
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  • 9
    Electronic Resource
    Electronic Resource
    Oxford [u.a.] : International Union of Crystallography (IUCr)
    Acta crystallographica 56 (2000), S. 360-361 
    ISSN: 1600-5759
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The crystal structure of the title compound, C32H28O2, (I), confirms the erythro stereochemistry of the aldol adduct. In the crystal, (I) forms centrosymmetric O—H...O=C hydrogen-bonded dimers which in turn are connected by C—H...O and C—H...π interactions.
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  • 10
    Electronic Resource
    Electronic Resource
    Oxford [u.a.] : International Union of Crystallography (IUCr)
    Acta crystallographica 57 (2001), S. 97-99 
    ISSN: 1600-5759
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: 3-Acetyl-1,6,7,12b-tetrahydroindolo[2,3-a]quinolizin-2(12H)-one, C17H16N2O2, consists of two symmetry-independent molecules and each forms a layered structure stabilized by N—H...O and C—H...O hydrogen bonds. In 3-acetyl-6,7-dihydroindolo[2,3-a]quinolizin-4(12H)-one monohydrate, C17H14N2O2·H2O, the structure is stabilized by O—H...O, N—H...O and C—H...O hydrogen bonds, with the ordered water molecule playing a crucial role in the self-assembly. Contribution from the weak interactions to the strong hydrogen-bonded network is a common feature in both structures.
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