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The crystal and molecular structures of cyanodi-methylarsine (cacodyl cyanide), diiodomethylarsine, and "cacodyl disulphide" (dimethylarsi.no dimethylditbioarsinate) have been determined by x-ray diffraction of single crystals, in order to investigate the stereochemistry of arsenic. The structure of cyanodimethylarsine was determined from projections along the three crystallographic axes. The central arsenic bond angle of 105° agrees well with that found in the halogendiphenylarsines, and is considerably larger than the value assumed in an electron-diffraction investigation of the corresponding dimethyl derivatives. There is an unusually short As….N inter-molecular separation, which is indicative of charge-transfer bonding involving donation of nitrogen lone pair electrons to vacant arsenic 4d-orbitals, which probably accounts for the solidity of the compound whereas the halogendimethylarsines are all liquid at room temperature. Diiodomethylarsine, one of the few simple arsenic derivatives which are solid at room temperature, was structurally determined from partial three-dimensional data in normal and generalized projections. Though severe absorption corrections had to be applied, values of the bond distances, valency angles, and intermolecular separations were obtained. The molecular structure of "cacodyl disulphide" was determined precisely, utilizing the full three-dimensional data, and the analysis revealed the compound to be dimethylarsino dimethyldithioarsinate, an unusual structure, having one trivalent and one pentavalent arsenic atom in the molecule. The former atom has a trigonal-pyramidal configuration, while the latter is roughly tetrahedral. There is an unusually short intermolecular As[superscript III].. .As[superscript III] separation which suggests charge-transfer bonding involving donation of lone pair electrons on each As[superscript III] to vacant 4d-orbitals on the other As[superscript III]. The structure of the alkaloid cleavamine, a product of the acidification of the Vinca rosea Linn alkaloid catharanthine, was determined from a three-dimensional x-ray analysis of cleavamine methiodide. The indole system is retained in cleavamine but the rest of the molecule undergoes rearrangement via bond fission, the result: being a tetracyclic alkaloid structurally resembling the known alkaloid quebrachamine. The presence of the iodide ion prevented accurate measurements of the bond distances and angles, but, the measurements were sufficiently precise to differentiate between the different types of bonds and angles. The absolute configuration of cleavamine was also established. The crystal and molecular structure of 5-iodo-2'-deoxy-uridine, the first anti-viral agent to have proven clinical chemotherapeutic value, was elucidated by a three-dimensional x-ray investigation. The bond lengths and angles in the nucleoside were determined accurately, and indicated that, the pyrimidine base is in the di-keto form. The deoxyribose ring is puckered with C2' displaced 0.59 Å from the plane of the other four atoms; the distances and angles in the sugar ring are all normal. The most significant intermolecular distance is an I....0 (carbonyl) separation of 2.96 Å; this is considerably shorter than the usual van der Waals contact (3.55 Å) and suggests charge-transfer bonding involving donation of oxygen lone pair electrons to vacant 5d-orbitals of the iodine atom. A strong intermolecular attraction of this type may be the cause of the increase in "melting temperature" observed when 5-iodo-2'-deoxyuridine is substituted for thymidine in DNA, and hence may be the molecular basis for the compound's anti-viral activity. A preliminary x-ray investigation was carried out: on crystals of sodium thymidylyl-(53')-thymidylate-(51). The cell constants and space group of the dinucleotide were determined, and the intensities of the three-dimensional diffraction maxima were recorded, but the absence of a relatively heavy atom made attempts at: a complete structural elucidation unsuccessful. A number of crystallographic computer programmes have been written for the IBM 1620 and 7040 computers; a list of these is given in an appendix. / Science, Faculty of / Chemistry, Department of / Graduate

The zoning of plagioclase feldspars was studied with the four-axis universal stage. The composition, in terms of the extinction angle X’ʌ010, ⊥a, was measured for each zone in several crystals in each of nearly fifty rocks. The values of the extinction angles for each zone were plotted against the distance of the zone from the rim of the crystal, and the resulting zone-curves for each rock compared. . Three degrees of correlation between zone-curves are recognizable, and the rocks studied may be arbitrarily grouped on this basis as follows: 1. Rocks in which the correlation is poor or absent. These include lavas and some intrusive granites. 2. Rocks in which the correlation is good or perfect. These include hornfelses and, the foliated and gneissic metamorphic rocks. 3. Rocks in which the correlation is indefinite. This group, which contains only twelve percent of all the rocks studied, includes some intrusive granites and some foliated metamorphic rocks. The degree of mobility of magmas may be estimated. The mobility appears to be severely restricted by the continued growth of its feldspar phenocrysts, and by the confining walls of intruded country rock. The author concludes that extreme complexity and non-correlation of zoning are due to relative motion between growing crystals, and that these features are most common in magmatic rocks which have been thoroughly mixed during the fluid stage. He concludes further that simplicity and close correlation of zoning are due to the growth of crystals in a quiet, relatively undisturbed environment in which the crystals do not move relatively to one another. Finally, it is concluded that cyclic super saturation, as proposed by Hills, (1936), is a less important cause of plagioclase zoning than changing pressure-temperature environments due to mixing or turbulence. / Science, Faculty of / Earth, Ocean and Atmospheric Sciences, Department of / Graduate

Experiments were performed on bicrystals of high-purity (99.986% and 99.9987%) tin to investigate the dependence of the melting behaviour of grain boundaries, particularly those of the "equiaxed" type, on applied stress, orientation difference, boundary orientation, growth conditions, and purity. By a comparison with previous work on columnar boundaries it was concluded that there was no structural difference between equiaxed and columnar boundaries having the same geometry. Any difference in behaviour was attributed to the build-up of impurities at the equiaxed boundary due to the zone-refining effect on growth. Calculations on the diffusion of Pb, the major impurity in Sn, show that the impurity content does not decrease appreciably in the duration of the test, and consequently could be considered constant from specimen to specimen. Thus, the melting behaviour is a property of the atomic structure of a grain boundary, quite apart from any effect due to impurities. Tilt boundaries exhibit no dependence of melting behaviour on orientation difference above 16°, whereas twist boundaries do. A peak of minimum disorder appears at 41°. The dependence is predictable from coincidence plots. / Applied Science, Faculty of / Mining Engineering, Keevil Institute of / Graduate

Single crystals of the CsCl type intermetallic compound β’AuZn were prepared and tested in tension over a wide range of temperatures, strain rates and orientations for three compositions, Au-rich (51.0 at.% Au), stoichiometric and Zn-rich (51.0 at. % Zn). Slip surfaces are generally non-crystallographic planes in the zone of the slip direction [001], and are temperature, strain rate and orientation sensitive. A model based on thermally activated sessile-glissile transformations of screw dislocations has been proposed to explain non-crystallographic slip. Multi-stage work-hardening is observed over the temperature range 0.2 ⪝T/T𝘮⪝ O.35. In stage I the work-hardening rate is low[(formula omitted) /lOOO to (formula omitted)/5000] but rises sharply during stage II [Q₁₁ ~ (formula omitted)/5OO]. Stage III is characterized by a rapidly decreasing hardening rate coincident with the onset of profuse large-scale cross-slip. Surface slip line studies revealed that the end of easy glide is coincident with the onset of localized slip on non-crystallographic planes in the [100] zone. Thin foil electron microscopy was carried out on critically chosen crystallographic sections from annealed and deformed crystals. At the beginning of stage I clusters of edge dislocation dipoles were revealed, forming walls perpendicular to the glide plane. The dislocation density of the walls increases during easy glide. During testing at intermediate temperatures ( ~ .3 to .4 T𝘮 ) serrated yielding was detected in non-stoichiometric crystals and was attributed to dislocation-solute atom interactions. Under special testing conditions (77°K or near < OOl> orientations) slip occurs in <111> directions. The associated work-hardening rates are very high and ductility is low. Thermal activation studies were made to determine the dislocation mechanism responsible for the temperature sensitivity of yield in stoichiometric crystals below 〜220°K. Activation volume measurements are consistent with both the Peierls-Nabarro and cross-slip mechanisms below 〜150°K. / Applied Science, Faculty of / Materials Engineering, Department of / Graduate

The crystal structures or a number of molecular complexes formed by 4:4'-dinitrodiphenyl with various diphenyl derivatives are described. The general type of structure is undoubtedly the same for all the complexes examined and the typical arrangement may be taken as that in the complex of 4:4'-dinitrodiphenyl with 4-hydroxydiphenyl, the structure of which has been fully determined. As shown in fig. 7 the dinitrodiphenyl molecules form layers in face-centred array and lie one above the other with a spacing of about 3.7 A. The arrangement of these molecules alone is such that a set of tubular cavities, also in face-centred array, run through the structure. In the complexes these cavities are occupied by the other component molecules, the hydroxydiphenyl molecules in the case considered; which thus lie nearly normal to the planes containing the dinitrodiphenyl molecules and are seen end-on in fig. 7. In the other structures examined geometrical and symmetry conditions require that the individual molecules should be tilted in varying degrees, but the type or structure still remains essentially the same. It is shown that the ratio of the components in these complexes is determined by the length of the molecule other than dinitrodiphenyl, and that in no case is it necessary for the intermolecular distances to be shorter than those found in crystals or ordinary aromatic nitro-compounds. A bonding mechanism in terms of dipole attraction between the component molecules is shown to be consistent with all the observed data. Of interest are the periodic distortions which appear to occur in the crystal lattices of the complexes or 4:4'-dinitrodiphenyl with 4-iodo-, 4-bromo- and 4-chlorodiphenyl.

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