Hexamethylenetetramine Suberate, a Strongly Anharmonic Modulated Structure

Hexamethylenetetramine suberate is an organic compound presenting a one-dimensional modulation with strong satellite reflections between 123 and 300 K. Single-crystal X-ray intensities of main reflections and satellites up to fourth order have been measured and used for refining a model based on displacive atomic modulation functions. Harmonic terms up to eighth order were necessary to describe the strongly anharmonic modulation. The structure consists of alternating sheets of hexamethylenetetramine and suberic acid connected by hydrogen bonds. Each layer of suberic acid chains consists of alternating areas, each of which is characterized by a common orientation for all chains. These orientations subtend an angle of roughly 60 degrees. At the interfaces, neighbouring chains in adjacent areas adopt intermediate orientations. The two carboxyl groups participate in hydrogen bonds, the character of which depends on the internal coordinate t. In one group and for all values of t, the same oxygen forms a double bond with the alpha-carbon and the other a single bond, and in the other the carboxylate form is observed for all values of t. The packings of both moieties are incompatible and are conjectured to be responsible for the incommensurability of the structure. [References: 22]