Doyle Britton

A complete catalogue of polyhedra with eight or fewer vertices

All non-isomorphic convex polyhedra with 4, 5, 6, 7 and 8 vertices are listed. The relationships within each class are described.

The crystal structure of silver cyanate

are both impor tant . The second form could contr ibute to the intermolecular interaction. I t is not apparen t why the external bond in I C N should be so much stronger t han in this molecule. Excep t for the N I there are no unusual ly short distances between the molecules. I n par t icular the hydrogen a toms on one chain of molecules do not appear to be interfering with the format ion of N I bonds on the next chain. One possibility for the decrease in s t rength is t h a t the a bonds in I C N are affected, t h a t the external interact ion involves a d orbital which then contributes to the I C a bond and causes some rehybridizat ion of the C and N a orbitals, and tha t all this rehybridizat ion is necessary for the format ion of the external bond. This would not be possible with the benzene ring in between, and the bond would be weakened. We t h a n k Miss Jud i th Krudener for her help with the intensi ty measurements and with the prepara t ion of the data , and Mr D. Grgurich for preparing the compound and making some prel iminary X r a y measurements . We thank the Nat ional Aeronautical and Space Agency for a fellowship to E. 0. S., and the Nat ional Science Foundat ion for their suppor t of this work. Most of the computer programs used were supplied by the Princeton Univers i ty crystal lographic group under the direction of Prof. Rober t Jacobson. We would like to t h a n k them, and especially Mr Byron Gorres, for their cooperation and help.

Recognition and use of systematically aberrant phase relationships

Conventional application of direct methods sometimes fails because of systematically aberrant triples. The structure of 8-nitro-l-naphthonitrile, C11H6N2O2, provides an example. Here the procedure of Thiessen & Busing [Acta Cryst. (1974), A30. 814-821] could be used to identify such aberrant triples. The trial structure could then be found straightforwardly. Some situations where aberrant triples may cause trouble are discussed.