On the Application of Phase-relationships To Complex Structures .29. Choosing the Large-es

The set of large Es through which a structure is solved by direct methods is usually chosen by a convergence or convergence-divergence process. This process aims to give a strong phase-extension pathway starting from a small set of Es whose phases are known or allocated in some way. Sometimes sets of reflexions thus obtained are poorly conditioned and under tangent-formula refinement even initially correct phases will degenerate to randomness. A simple new algorithm has been developed which improves the conditioning of the complete set of reflexions and their relationships and is more appropriate to current trends to start refinement from a complete set of random phases. A particular feature of this algorithm is that it maximizes the minimum number of relationships for any reflexion

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