THE INCOMMENSURATELY MODULATED PHASES OF HEXAMINE DERIVATIVES. Valérie Bussien Gaillard, Gervais Chapuis, and Wlodek Paciorek, Institut de Cristallographie, Université de Lausanne, BSP, 1015 Lausanne, Switzerland

Hexamine suberate, azelate and sebacate form layer structures with alternating sheets of hexamine and alkanedioic acid linked by H-bonds between N and O atoms. The acid molecules are totally extended with 6, 7 and 8 carbon atoms and an acid group at each end. They form a two-dimensional array with their axis inclined by 30deg. with respect to the layer normal.

At room temperature, hexamine suberate and sebacate (even number of carbons) are strongly modulated, as witnessed by satellite reflections up to the fourth, respectively sixth order. The modulations are very stable between 120 and 300K. Hexamine azelate (odd number of carbons) is commensurate. However, strong diffuse scattering observed on precession photographs reveals some disorder in the structure.

The two incommensurate compounds have been refined using the superspace group formalism. Displacive atomic modulations including up to eight harmonics lead to satisfactory models. The analysis of the refinements indicates that in a layer, the zig-zag planes of the acid chains take essentially two orientations, forming an angle of approximately 60deg.. In addition, intermediate orientations lying within the two extremes are also observed. In the commensurate structure, the disorder originates from the acid chains which can take two possible orientations, with zig-zag planes forming an angle of 180deg..

The origin of the incommensurability is most probably caused by the incompatible intrinsic packing of hexamine and acid molecules. In this hypothesis, the acid molecules must reorient themselves in order to fit in the extremely stable layers of hexamines. Molecular mechanic methods have been performed in a rigid framework of hexamines. A layer of suberic acid composed of parallel chains has been placed within this frame. The optimisation of the system leads to the features observed in the incommensurate structures. Simulations with a layer of azelaic acid molecules generates, however, a single set of orientations of the chains which are all parallel. This strongly support our hypothesis on the origin of the incommensurability.