A.McL. Mathieson

The influence of allylic oxygen on the CD of skew dienes

Abstract The CD of eight compounds each containing a conjugated diene system is discussed. In five of these the sign of the Cotton effect attributable to the lowest energy diene absorption band is at variance with the sign predicted by the rules relating sign to diene helicity. Each of the five compounds incorporates one or more oxygen substituents (hydroxyl, methoxyl, (1-carboxyvinyl)oxy) allylic to the diene system. If its is assumed that the helicity of the system OCCC may have a larger influence on the CD than the helicity of the diene system itself, these results are consistently explained.

The absolute molecular structure of (+) kreysiginine

Abstract X-ray analysis of crystals of the acetone adduct of kreysiginine methiodide, C 21 H 27 O 5 N.CH 3 I.CH 3 COCH 3 , has established the absolute structure of kreysiginine, an alkaloid from Kreysigia multiflora Reichb. (order Liliaceae). The crystals are orthorhombic, with unit cell dimensions, a =8·366, b =16·349, c =18·606A, Z=4,, the space group being P2 1 2 1 2 1 . Intensity data for 2058 reflexions were measured with CuKα radiation on a single-crystal diffractometer. The crystal structure was solved by the heavy atom method and refined by difference and least-squares procedures. The absolute chirality was defined by Bijvoets technique. Kreysiginine (I) was found to be a member of the class of homo-morphine alkaloids based on the I-phenylethylisoquinoline system. Because it is opposite in absolute configuration to that of the main group of morphine alkaloids based on the benzylisoquinoline system, the absolute configuration of morphine was reconfirmed by application of Bijvoets technique to the crystal data for morphine hydriodide dihydrate.

An analysis of electron diffraction patterns of thin vapour-grown carbon filaments

Abstract The electron diffraction patterns of carbon filaments with hollow cores and external diameters of the order of 10 nm have been analysed to obtain information about their structure. These filaments were grown by catalytic decomposition on iron particles of hydrocarbon-hydrogen mixtures at about 1100°C. The unusually thin walls of these filaments resulted in considerable detail along the equatorial lines of their diffraction patterns, and this detail has been closely reproduced by a theoretical model.

Crystal structures of the dihydrogen hexafluorosilicate monohydrates ofp-bromoaniline andp-toluidine

Analysis of X-ray counter data for the dihydrogen hexafluorosilicate monohydrates ofp-bromoaniline, BrC6H4NH2.1/2H2SiF6.H2O, andp-toluidine, CH3C6H4NH2.1/2H2SiF6.H2O, has shown that the two compounds are isostructural and that the orthorhombic crystals belong to the space groupPbca, witha = 7·80,b = 24·45,c = 9·48 Å anda = 7·89,b = 23·87,c = 9·53 Å, respectively, there being eight molecules in each unitcell. Structure analysis was initiated by the heavy atom method, and subsequent difference syntheses, carried out on thep-bromoaniline compound, then on thep-toluidine analogue, revealed the entire structure apart from the hydrogen atoms. From a difference synthesis on data for thep-toluidine compound refined by full-matrix least-squares procedures, the hydrogen atom sites were located, a reliability index,R = 0·052, being attained. The structures are sheet-like with bi-polar grouping of amino groups on either side of sheets of SiF62− groups and are cross-linked by ionic forces and hydrogen bonds. The SiF62− ion is octahedral, with an average Si-F distance of 1·69 ± 0·02 Å.

Crystal structure of dinitro(2,9-dimethyl-1,10-phenanthroline)-palladium(II)

Dinitro(2,9-dimethyl-1,10-phenanthroline)-palladium(II) crystallizes in the orthorhombic system, space groupP21nb, with cell dimensionsa = 6·790,b = 19·807,c = 10·625 Å andZ = 4. The structure was determined from three-dimensional Fourier syntheses and refined by full-matrix least squares, the finalR for 1155 measured reflexions being 0·07.

The absolute molecular structure of podopetaline

Abstract X-ray analysis of crystals of podopetaline hydrobromide, C 20 H 33 N 3 ·HBr, has established the absolute structure of podopetaline, an alkaloid from Podopetalum ormondii F. Muell. The crystals are orthorhombic, with unit cell dimensions, a = 6·619, b = 10·907, c = 26·806 A, Z = 4, the space group being P2 1 2 1 2 1 . The intensity data were measured with CuKα radiation on a single-crystal diffractometer. The crystal structure was solved by the heavy atom method and refined by difference and least-squares procedures to a final R index of 0·069 for the 849 observed terms. The absolute chirality has been defined by Bijvoets technique. Podopetaline (I) therefore provides an absolute chirality reference for Ormosia alkaloids which was not previously available.

The crystal structure of coclaurine hydrobromide monohydrate and the absolute configuration of coclaurine

Abstract The structure and absolute configuration of coclaurine has been determined by X-ray analysis of its hydrobromide monohydrate. The (+)-enantiomer has the D-configuration. The chirality of the molecular structure is correlated with circular dichroism measurements.

Extinction-free measurements in crystallography

OVER the past fifty or more years extinction has remained a major experimental problem in crystallography1–3. Many strategies have been devised to reduce its magnitude for the accurate determination of physically-significant structure factors (F). Its elimination, as a practical possibility, has appeared remote because extinction is a concomitant of the production of diffraction intensity in the standard techniques. Phenomenologically, extinction is a manifestation of multiple diffraction (see ref. 4), and this being so, an obvious conclusion is that the experimental disposition to be sought is one where the recorded intensity must arise solely from single-scattering events5–7. Then, by definition, the diffracted beam is not ‘extinguished’ and simple kinematical theory is applicable. Starting with the design of a ‘defocusing’ X-ray monochromator8, I have been investigating the intensity of asymmetric reflections from extended-face crystals over the regions of both positive and negative asymmetry, first with an abraded surface9 and then with an optically flat one. The results from this latter study, with an extrapolation of the work of Hirsch and Ramachandran5, show that it is feasible to establish values of integrated intensity which involve only single-scattering events.

ThePoranthera corymbosa alkaloids, I. Crystal and absolute molecular structure of porantherine hydrobromide

The alkaloid porantherine, C15H23N, has been isolated fromPoranthera corymbosa Brogn. (family Euphorbiaceae). It forms a stable hydrobromide which crystallizes in the orthorhombic space groupP212121, with cell dimensionsa=11·689,b = 15·190 andc = 7·721 Å. Its structure has been solved by the heavy-atom method and refined to anR of 0·063. The molecular skeleton has the form of a tertiary amine, with three fused six-membered rings centred on the nitrogen atom and a two-carbon atom bridge which holds the whole molecule rigid. The absolute configuration has been determined from the differences between Bijvoet pairs of reflexions.

ThePoranthera corymbosa alkaloids, II. Molecular structure and absolute configuration of porantheridine hydrobromide

Crystals of the hydrobromide of the alkaloid designated porantheridine, C15H27NO, isolated from the plantPoranthera corymbosa Brogn. (family Euphorbiacaea), belong to the space groupP212121, with cell dimensionsa= 10·059,b = 16·704 andc = 9·629 Å. The crystal structure was solved by the heavy-atom method and refined with 624 reflexions to a residual of 0·078. The molecule consists of three six-membered rings fused on to a common central nitrogen atom; one ring contains an oxygen atom with ann-propyl group and a methyl group substituted on the vicinal carbon atoms. The rings have thechair conformation, two being fusedtrans to each other andcis to the third. The absolute configuration was determined using the anomalous scattering power of the bromine atom.