The X-ray Structural Study of Some Organic Crystals

Isoclovene, C15H24, is a liquid sesquiterpene of hitherto virtually unknown constitution. It forms two isomorphous hydrohalide derivatives, isoclovene hydrochloride and isoclovene hydrobromide, which crystallise in the monoclinic space group, P2, with two molecules per unit cell. An attempt was made initially to carry through a structural analysis of the hydrohromide derivative using the heavy atom technique and two dimensional Fourier methods. Only the (010) electron-density projection, when calculated on the basis of the heavy atom phases, gave an unambiguous representation of the molecule, hut the prohibitive length of projection (13.8 A) prevented any significant resolution being achieved. The (001) electron-density projection provided a representation of the molecule and its superimposed mirror image due to the existence of the well known false symmetry centre which exists between two phase-determining atoms in the non-centric projections of space group P2 . Further spurious symmetry was present in the (100) electron-density projection because the heavy atom phases could only be coupled with about one half of the observed terms, those satisfying the condition k + l = 2n. Attempts to correlate these three projections of the molecule on the basis of tentative trial structures were not successful. The solution of the structure resulted from the application of superposition methods to a three dimensional sharpened Patterson function computed with data derived from the hydrochloride derivative. A three dimensional representation of the molecule and its superimposed mirror image was thereby obtained. This map was compared with the essentially analogous distribution given by a triple Fourier synthesis phased solely on the chlorine atom phases. The corresponding prominent peaks on both maps were assumed to indicate the most likely atomic sites. Thereafter, there existed the interpretationsl problem of selecting ''true peaks" from their ''mirror image" counterparts. Seven carbon atoms were located and included with chlorine in the initial phasing calculations. Four successive triple Fourier syntheses sufficed to reveal the structure. Two further Fo syntheses and twelve successive least squares cycles reduced the discrepancy over 976 observed terms to 12.5%. This analysis has led to a determination of the molecular structure and relative configuration of isoclovene hydrochloride. From these results the structure of the parent compound, isoclovene, has been deduced. Photo-irradiated acridizinium bromide, C26N2H20Br2, has been obtained in a crystalline form belonging to the monoclinic space group P2 1/m with two molecules per unit cell. The projections of the Patterson function on the three main zones have been examined but it has not yet proved possible to derive an entirely satisfactory trial structure for the most favourable (100), projection