HEXAMETHYLENETETRAMINE SUBERATE, A STRONGLY ANHARMONIC MODULATED STRUCTURE

Hexamethylenetetramine suberate is an organic compound presenting a on e-dimensional modulation with strong satellite reflections between 123 and 300 K. Single-crystal X-ray intensities of main reflections and s atellites up to fourth order have been measured and used for refining a model based on displacive atomic modulation functions. Harmonic term s up to eighth order were necessary to describe the strongly anharmoni c modulation. The structure consists of alternating sheets of hexameth ylenetetramine and suberic acid connected by hydrogen bonds. Each laye r of suberic acid chains consists of alternating areas, each of which is characterized by a common orientation for all chains. These orienta tions subtend an angle of roughly 60 degrees. At the interfaces, neigh bouring chains in adjacent areas adopt intermediate orientations. The two carboxyl groups participate in hydrogen bonds, the character of wh ich depends on the internal coordinate t. In one group and for all val ues of t, the same oxygen forms a double bond with the alpha-carbon an d the other a single bond, and in the other the carboxylate form is ob served for all values of t. The packings of both moieties are incompat ible and are conjectured to be responsible for the incommensurability of the structure.