High-resolution,quasi-Fabry—Perot X-ray interferometer

An extremely simple X-ray interferometer is described that should provide resolutions (λ/Δλ) of more than 10⁵. It relies on the use of nonlocalized, quasi-Fabry—Pérot interference fringes, which are detectable when a widely divergent beam of X-rays from a point source is incident on a good quality crystal. The crystal has to have a suitable lattice spacing, and in the basic method described, must not exceed a certain thickness (or coherence length).     

Crystal structure of 1,4,8,12-tetraazacyclopentadecane copper(II) tribromocuprate(I)

The title compound is a mixed valence compound obtained as one of the products of the reaction of Cu(15-ane N₄)Br₂ with CoBr₂·6H₂O in water/acetone solvent (15-ane N₄=1,4,8,12-tetraazacyclopentadecane). The compound is tetragonal, space group P4₂/mbc witha=16.675(2)Å,c=13.185(3)Å,V=3666(1)ų, withZ=8, for p_calc=2.106 g/cc. Refinement of 674 unique observed reflection yielded final values ofR=0.088 andR _w=0.083. The compound contains chains of alternating Cu(15-ane N₄)²⁺ cations and CuBr ₃ ^2– anions. The chains run perpendicular to thec axis, and are arranged in alternate layers running parallel toa andb. The chains lie athwart the mirror planes atz=0 andz=1/2, with disorder observed for both the (15-ane N₄) rings and the CuBr ₃ ^2– anions. The Cu(II) ion is coordinated by the tetradentate macrocycle to yield an approximate square planar coordination. The CuBr ₃ ^2– anions are nearly planar with Cu–Br(ave.)=2.37Å. Long semi-coordinate Cu(II)...Br bonds of 3.0Å link the cations and anions.     

Similarity searching in files of three-dimensional chemical structures: Representation and searching of molecular electrostatic potentials using field-graphs

This paper reports a method for the identification of those molecules in a database of rigid 3D structures with molecular electrostatic potential (MEP) grids that are most similar to that of a user-defined target molecule. The most important features of an MEP grid are encoded in field-graphs, and a target molecule is matched against a database molecule by a comparison of the corresponding field-graphs. The matching is effected using a maximal common subgraph isomorphism algorithm, which provides an alignment of the target molecule's field- graph with those of each of the database molecules in turn. These alignments are used in the second stage of the search algorithm to calculate the intermolecular MEP similarities. Several different ways of generating field-graphs are evaluated, in terms of the effectiveness of the resulting similarity measures and of the associated computational costs. The most appropriate procedure has been implemented in an operational system that searches a corporate database, containing ca. 173,000 3D structures.     

ESR linewidths and g-values of some two-dimensional copper bromide salts

The angular variation of the g factor and linewidths of several (RNH₃)CuBr₄ salts are reported. The square planar CuBr^2?₄ anions present in the structures have spin-hamiltonian parameters of g=2.047 and g=2.153. The linewidths show a (3 cos²₀?1) dependence. The temperature dependence of the linewidths is also discussed.