Acta Crystallographica Section B

Structural Science

Volume 59, Part 3 (June 2003)

research papers

Acta Cryst. (2003). B59, 416-427 [ doi:10.1107/S0108768103007754 ]

Structural changes of hexamethylenetetramine and undecanedioic acid co-crystal (HMT-C11) as a function of the temperature

C. B. Pinheiro, M. Gardon and G. Chapuis

Abstract: HMT-C11 belongs to the family of adducts formed by the co-crystallization of N₄(CH₂)₆ molecules (hexamethylenetetramine or HMT) and aliphatic dicarboxylic acids HOOC(CH₂)_{n - 2}COOH with 5 n 13 (Cn). The adducts exhibit a layered structure in which the packing between HMT and Cn is determined by strong hydrogen bonds. The compounds in this family studied so far present thermotropic structural phase transitions and, depending on the chain length, disordered, twinned and modulated phases. The structure refinement of HMT-C11 based on X-ray diffraction experiments indicates three distinct phases from the melting point down to liquid nitrogen temperature: phase I is not crystalline; phase II is disordered (stacking fault) and its average structure is described in space group Bmmb; phase III is partially disordered and its symmetry is P2₁/c. The systematic study of the structure evolution of phase III upon cooling revealed that the disorder has a dynamic character (anharmonicity). The main structural change observed from 293 K down to 93 K is the increase of the tilting angle of the C11 chains relative to the layer plane and the rotation of the HMT molecules. Both HMT and C11 behave like rigid bodies in the temperature range investigated. The quality of the refinements leads to a conclusive model for the O-HN hydrogen bonds linking HMT and C11.

Keywords: hexamethylenetetramine; chain aliphatic carboxylic acids; phase transition; disorder.


	Structure factor file (CIF format) (266.3 kbytes) Contains datablock 93K

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