Hexamethylenetetramine Sebacate, an Incommensurate Structure With Large Nonsinusoidal Modulations - Comparison of Two Refinement Strategies

The organic salt hexamethylenetetramine sebacate presents a 1D modulation with strong satellite reflections. Single-crystal X-ray intensities of main reflections and satellites up to sixth order have been measured at room temperature and used to refine this incommensurate structure. Two incommensurate structure models of hexamethylenetetramine (hexamine) sebacate are proposed and compared. The first is based on displacive atomic modulation functions and the second one uses displacive modulation and occupational step functions. Both models require high-order harmonic terms to describe the strongly nonsinusoidal modulation components. The structural characteristics of hexamine sebacate are similar in the two models. The structure is a layer structure consisting of alternating sheets of hexamine and sebacic acid connected by H bonds. Each layer of sebacic acid molecules consists of close-packed aliphatic chains with their axis slightly tilted with respect to the layer normal. The chains form alternating areas, each of which is characterized by a common orientation of the chains. These characteristics have also been observed in hexamethylenetetramine suberate. [References: 18]