Showing papers in "Acta Crystallographica Section B Structural Crystallography and Crystal Chemistry in 1974"

The crystal structure of the 1:1 radical cation–radical anion salt of 2,2'-bis-l,3-dithiole (TTF) and 7,7,8,8-tetracyanoquinodimethane (TCNQ)

Abstract: The salt of the radical cation of 2,2'-bis-l,3-dithiole (TTF) and the radical anion of 7,7,8,8-tetracyanoquinodimethane (TCNQ), C18HaN4S4, crystallizes in the monoclinic system, space group P2~/c, with cell constants: a= 12.298 (6), b=3\"819 (2), c= 18\"468 (8) /~, fl= 104\"46 (4) °, Z=2, Dm= 1\"62 (1) and Dc=1\"615 g cm -3. Intensities for 1373 independent reflections were collected on an automated diffractometer. The structure was solved by standard heavy-atom methods and has been refined by fullmatrix least-squares calculations to an R value of 0\"044. The TTF radical cations and the TCNQ radical anions form homologous columnar stacks with interplanar spacings of 3.47 and 3.17 A, respectively. The dihedral angle between the least-squares planes of the cations and the anions is 58.5 ° and is approximately bisected by [010].

Crystal structure analyses of 1,4,7,10,13,16-hexaoxacyclooctadecane and its complexes with alkali thiocyanates

Abstract: The results of crystal structure analyses of 1,4,7,10,13,16-hexaoxacyclooctadecane (CH2CH20)6 and its complexes with NaNCS, KNCS, RbNCS, CsNCS [and Ca(NCS)2] are discussed. In the K*, Rb*, Cs* and Ca 2+ complexes the unsubstituted hexaether adopts a conformation with virtual Ds~ symmetry although the larger Rb + and Cs + ions are displaced by more than 1 ,~, from the mean plane of the ligand. In the Na* complex, the ring is strongly distorted from its symmetrical conformation to accommodate the smaller cation. The uncomplexed hexaether has a centrosymmetric conformation containing three different types of monomeric subunit. The shortening of the C-C bonds found in these and related complexes is discussed and judged to be mainly an artificial effect arising from inadequate treatment of curvilinear vibrations.

Refinements of the crystal structures of dl‐serine and anhydrous l‐serine

Abstract: The crystal structure of DL-serine has been refined on the basis of 964 independent reflections measured on an X-ray diffractometer. The refinement has converged to an R index of 0.032, with anisotropic temperature factors for all atoms including hydrogens. The resulting coordinates of the heavy atoms are in good agreement with those reported from an earlier neutron-diffraction study, and there is no indication of systematic differences in the C-C, C-N or C-O bond lengths. The structure of L-serine was refined to an R index of 0.059 for 953 reflections; the absolute configuration was confirmed from the anomalous scattering effects of C, N, and O. The final parameters include isotropic temperature factors for the H atoms. The conformations of the L-serine molecule in three different crystal structures are compared; differences in conformation are directly related to differences in the hydrogen-bond arrangements.

The crystal and molecular structures of thiosemicarbazones; an antitumor agent 5-hydroxy-2-formylpyridine thiosemicarbazone sesquihydrate and the inactive acetone thiosemicarbazone

Abstract: The crystal and molecular structures of two thiosemicarbazones have been determined by X-ray diffraction techniques. The antitumor agent 5-hydroxy-2-formylpyridine thiosemicarbazone sesquihydrate forms pale-yellow, orthorhombic crystals, with a= 13.261 (10), b=11.159 (5) and c= 13.699 (4) ,~, space group Pbcn, Z = 8, 0(calc) = 1.466 g cm-3 and 0(obs) = 1 "46 g cm -3. Crystals of acetonethiosemicarbazone are colorless, with a=7.854 (4), b=7.791 (4), c= 6.307 (2) ,~, c~= 111.52 (2), fl= 109.82 (2) and y=85.12 (2) °, space group PI indicated by the intensity statistics. With two molecules per unit cell 0(calc)= 1.282 g cm -3 compared to o(obs)= 1.27 g cm -3. In both molecules the sulfur atom is trans to the nitrogen atom relative to the C-N bond. The most significant feature is the longer N-N bond [1-398 (6) ,~] in acetonethiosemicarbazone compared to the N-N bond [1.379 (6) A] in the 5-hydroxy2-formylpyridine thiosemicarbazone. The shorter bond length appears to be related to an interaction with the pyridine ring.