The thesis is devoted to the analysis by X-rays of the crystal and molecular structure of the organic compound 4-bromo 4'-nitro- diphenyl, and to automatic contouring of maps from spot-height data - a technique which finds much application in crystallography. In an initial X-ray investigation, crystal data for two other organic compounds were obtained. These were 4-phenylbenzaldehyde and 4-hydroxydiphenyl. All three compounds are examples of non-ortho- substituted diphenyls. This family and its analogues are interesting theoretically, and are used in agriculture and industry as herbicides and chemical reagents. The most important feature of their molecular configuration seems to be the coplanarity or non-coplanarity of the two six-membered rings. 4-brorao 4-nitrodiphenyl (4B4NDP) was chosen from the three as being the most suitable for structure analysis by X-ray diffraction. It is isomorphous with the 4,4'-halogenodiphenyls (fluoro, chloro and bromo), so that its crystal structure is characteristic of the whole isomorphous group. Over 3000 intensities were measiired by integrated precession photographs. A trial structure was arrived at by the "heavy atom" method after the coordinates of the two independent bromine atoms had been revealed by three-dimensional Patterson synthesis. The structure was refined by the full-matrix least-squares method to an agreement factor of 0.10, which indicates a satisfactory solution of the structure. The crystal structure could thus be regarded as correct, although it was clear that the atomic parameters and bond lengths obtained were not of the highest accuracy. The molecules were found to be nearly straight and parallel with each other in the structure. The rings are not coplanar, but twisted relative to each other by 35° around the long molecular axis. Some useful computer programmes were written to assist the crystal lattice and structure determinations. They include a programme for reciprocal lattice geometry and reflection generation, and another for the geometrical interpretation of a three-dimensional. Patterson function. Finally, about one third of the thesis is devoted to the automatic contouring of crystallographic maps. Through practical experiments on the Elliott 803 computer, a thorough analysis of all known publications, and personal contacts, a fund of technical knowledge was collected. From this, it was possible to select those methods and ideas which best suited the particular aim of the contouring project : to produce a computer programme which would calculate contours with maximum speed and efficiency. A discontinuous plotting technique was employed, which with two or three valuable short-cut algorithms resulted in a concise programme suitable for drawing contours on a digital incremental plotter or cathode ray tube plotter, or any other device not requiring contours to be plotted continuously. The programme found its greatest usefulness in the form of a FORTRAN subroutine at the S.R.C. Atlas Computer Laboratory, where it cut the time required for a three dimensional crystallographic map to 30 minutes, when the existing contouring routine in the Atlas FORTRAN library would have taken ten hours of computing time. ACL have now incorporated the programme in the FORTRAN library.
|Date of Award||1969|