J. Weiler

(Z)-α,β-dimethoxystilbene

C16H16O2, M(r) = 240.30, orthorhombic, P2(1)2(1)2(1), a = 7.569 (1), b = 9.308 (1), c = 18.502 (2) angstrom, V = 1303.5 (3) angstrom3, Z = 4, D(x) = 1.22 g cm -3, lambda(Cu Kalpha) = 1.5418 angstrom, mu = 6.39 cm-1, F(000) = 512, T = 291 K, R = 0.056 for 2123 observed reflections. The central skeleton of the molecule exhibits a marked deviation from planarity. The dihedral angle between the two O-C(vinyl)-C(aromatic) planes is 11.6 (7)-degrees. The phenyl rings are twisted out of the ethylenic bond plane by 34.2 (6) and 49.8 (6)-degrees, respectively.

1:2 complex of 1,4,7,10,13,16-hexaoxacyclooctadecane (18-crown-6) with N-m-bromophenylurea

C7H7BrN2O.1/2C12H24O6, M(r) = 347.2, monoclinic, P2(1)/c, a = 8.910 (3), b = 24.924 (8), c = 8.391 (4) angstrom, beta = 122.00 (1)-degrees, V = 1580.3 (10) angstrom 3, Z = 4, D(x) = 1.46 Mg m-3, lambda-(Mo K-alpha) = 0.71069 angstrom, mu = 2.77 mm-1, F(000) = 712, T = 293 K, final R = 0.067 for 1229 observed reflections. The host 18-crown-6 molecule lies about a crystallographic centre of symmetry. This centre relates the two guest bromophenylurea molecules which lie on either side of the host molecule almost perpendicular to the macrocyclic ring. Each guest molecule is hydrogen bonded to two adjacent O atoms of the crown ether by means of one H atom from each amino group. The complex adopts the biangular conformation of the macrocycle and is evidence for the formation of a discrete 1:2 crown ether-urea molecular complex.

Effets de substituants et conformation de stilbenes

Simultaneous study of para nitrostilbenes by X-ray diffraction and /sup 13/C nuclear magnetic resonance shows a quantitative relationship between the effect of a para nitro substituent on the chemical shift of ethylenic carbon not adjacent to the substituted phenyl ring and the dihedral angle between this phenyl ring and the central double bond. The alternative introduction of substituents in each phenyl ring discloses at the position of carbon beta a certain pi -polarization which extends to the double bond and to an insaturated group carried by carbon alpha.