July 1999 issue
cif-access (inorganic compounds)
cif-access (organic compounds)
The two crystallographically independent conformations of the title compound differ only by the rotational positions adopted by their methoxymethyl substituents. Intermolecular contacts are dominated by hydrogen bonding involving the amine H atoms.
The title compound, ethyl 2-(1,2,3,4,5,6,11,11a-octahydro-3- methyl-1\,4-dioxopyrazino[1\,2-b]isoquinolin-2-yl)-4-phenylbutanoate, is formed due to cyclization of quinapril in solution. The configurations at the three chiral centres are all S. The six-membered B and C rings assume half-chair and boat confirmations, respectively. The molecules in the lattice are stabilized by van der Waals contacts.
The title compounds, C15H20O2, (I), and C15H22O3.CH3OH, (II), were obtained by intramolecular cyclization of the 5- and 3-acetonyl derivatives of 4-methylbicyclo[6.3.0]undecane-2,6-dione, respectively. Dione (I) is an ophiobilin-like C5–C8–C5 fused-ring compound and diol (II) is a C5–C6–C5–C5 tetracyclic compound.
Biphenylene is one of the key compounds in the discussion of bond localization in aromatic systems, usually addressed as the Mills–Nixon effect [Mills & Nixon (1930). J. Chem. Soc. p. 2510]. A pronounced bond localization due to the Mills–Nixon effect in the direction of one of the Kekule forms is established. Compared with the previous structure determination by Fawcett & Trotter [Acta Cryst. (1966), 20, 87-93], the significance of bond-length alternation in relation to the s.u.'s in the six-membered rings has been dramatically increased.
The title compound crystallizes with two nearly identical molecules in the asymmetric unit. The molecules essentially exhibit mirror symmetry.
In the title compounds, C10H8C12N4 (R= H), (I), and C18H20C12N4O2 (R = COCHMe2), (II), the dihedral angle between the imidazolidine and phenyl rings are 67.7(1) and 70.34(9)°, respectively. In (I), the imidazolidine ring adopts a half-chair conformation, whereas in (II), it is in a flattened envelope conformation.
The structure of Na2Ca2Nb4O13 consists of perovskite-type layers with a thickness corresponding to four perovskite cubes. The interlayer structure is of the rock-salt-type, composed of Na and O atoms. The atomic configuration suggests a ferroelectric nature along the b axis.
β-Ba5Sb4 displays a more symmetrical arrangement than α-Ba5Sb4 (Pnma). It contains two anionic moieties, Sb24− and Sb3−, lying in tricapped and distorted monocapped trigonal prisms of Ba atoms, respectively.
KNa(VO3)2 adopts the pyroxene structure with K+ and Na+ cations, which lie on twofold axes, occupying the dodecahedral and octahedral cavities, respectively.