Investigations of the molecular arrangement of uniaxial optically active crystals

Abstract

The present paper deals with the X-ray investigation of certain optically active uniaxial crystals. The investigation was undertaken for several reasons. In the first place it was hoped to obtain evidence on the disputed question as to whether their optical activity must be ascribed to pseudo-symmetrical intergrowths of biaxial lamellæ (their rotatory power thus being produced in a similar way as that exhibited by definite spiral piles of thin mica plates) or whether special arrangements of the atoms within the unit cells proved to be present in the structures of such crystals. It was therefore necessary to discuss in how far X-rays are able to reveal the presence and the true nature of a lamellar structure. The cases of both relatively large lamellæ and of lamellæ which are too small to give individual X-ray reflections must be discussed. The latter case has been dealt with especially in connection with recent state­ments of G. Friedel with regard to the value of X-ray analysis of structures of this type. The crystals discussed in this paper have been investigated with particular reference to this difficulty. The conclusions arrived at are given in the Summary at the end of the paper. They suggest that the rotatory power of the crystals is really due to some special arrangement of atoms within a unit of structure. For the crystals investigated these arrangements have been considered more in detail. The principal points discussed are put together in the Summary.