The Crystal Structures of Some Organic Molecules: An X-ray Study

The crystal structures of some organic molecules have been examined by X-ray diffraction methods. The thesis has been divided into four sections which are summarised by the following four paragraphs. Calycanthine dihydrochloride, C22H28N4Cl2.2H2O crystallises in the space group D2 4 -P 2 1 2 1 2 1 with four molecules in the unit cell. Its isomorphism with calycanthine dihydrohromide has been established and a reliable electron-density projection obtained by the isomorphous replacement method. A benzenoid ring could be correctly picked up but extensive overlap prevented fuller interpretation. Similar difficulties were encountered in other projections and with Dr. T. A. Hamor taking up the analysis of the hydrobromide by three-dimensional methods, work on the hydrochloride was terminated. A joint communication giving the structure finally elucidated is appended. Quaterrylene, C40H20 crystallises in the monoclinic system; space group P 2 1/a with four molecules per unit cell. Its crystal structure has been determined by two-dimensional methods. A dimorph is found to occur. The relation between crystal structure and morphology is of the usual type. In the polymorph examined, the lengths of the peri-bonds connecting the naphthalenic residues have been determined with moderate accuracy. The mean value is assessed at 1.53 +/- 0.01 A which is significantly larger than current theory predicts. Possible reasons for this difference are discussed. The hydrogen bond: The major part of this section is a review and original comments are confined to the last three chapters,in one of which the structural features of some anomalous short hydrogen bonded structures have been examined. Potassium hydrogen-di-p-nitrobenzoate, K-H(C7H4NO4)2, crystallises in the triclinic system; space group P 1, with two molecules per unit cell. Its structure has been determined via Patterson projection and Buerger minimum function maps. The third co-ordinates were obtained by trial and error methods and initially refined by generalised projections. Finally, a partial three- dimensional least squares refinement incorporating 992 reflexions has been carried out. This structure reveals a short hydrogen bond of 2.492+/-0.02 A. Since the I. R. spectrum of this acid salt is of the superposition type, comparison of its structural features with those of the acid salts giving anomalous I. R. spectra reveals that the cause of the anomaly is not the short hydrogen bond but is, in some way, connected with the extensive networks which the formally symmetrical hydrogen bonds, together with carboxyl groups and alkali metal ions, invariably form in the acid salts of the latter type. The benzenoid rings are substantially planar but the nitroxyl and the carboxyl groups are twisted the same way by 5 to 10 about the C-N and the C-C bonds. The oxygen atoms surrounding a potassium ion form a considerably distorted octahedron. Mathematical derivations and explanatory notes have been included as appendices to the relevant sections. The unit cell parameters of Rb-H-di-o-nitrobenzoate are included as Appendix to Section IV. References are given at the end of each section