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

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 − 2COOH 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—H...N hydrogen bonds linking HMT and C11.

Structural phases of hexamethylenetetramine–pimelic acid (1/1): a unified description based on a stacking model

“…Phase I is orthorhombic and disordered. As already observed in Phase II of HMT-C9 (Bonin et al, 2003) and Phase II of HMT-C11 (Pinheiro et al, 2003), regular diffuse rods characterize its diffraction pattern and its average structure is described in the space group Bmmb. Phase II, stable between 313 and 285 K, possesses an ordered structure described in the space group Pccn, with a doubling of one lattice parameter with respect to Phase I.…”

“…In view of the similarities already evoked between HMT-C7, HMT-C9 and HMT-C11, it is also possible to describe the structures of Phase II and III of HMT-C9 and that of Phase II of HMT-C11 by the stacking model. Indeed the thermotropic phase sequences of both HMT-C9 and HMT-C11 do not ®t completely in the descriptive scheme proposed for HMT-C7; they both possess a lowtemperature phase (Birkedal et al, 2003;Pinheiro et al, 2003) which cannot be deduced by a stacking of the elementary layers de®ned in our model. The basic entity of the model is the elementary layer which possesses some symmetry constraints, like the alternation of the two chain orientations A/B along both b I and c I directions.…”

“…3). The diffractograms display a strong similarity with those of HMT-C9 Phase II (Bonin et al, 2003) and those of HMT-C11 Phase II (Pinheiro et al, 2003). The diffuse scattering and the process for modelling it will be discussed in the subsequent sections.…”

“…The initial attempts to re®ne Phase II and Phase III structures using the program SHELXL (Sheldrick & Schneider, 1997) indicated unacceptably large ADP's for the O atoms of the carboxyl groups. Following the results obtained for HMT-C11 (Pinheiro et al, 2003), we proceeded with the structural re®nement considering the anharmonicity in the displacement parameters of the O atoms. In these two phases we also considered the HMT molecule as a rigid unit (Terpstra et al, 1993;Kampermann et al, 1995;Bu È rgi et al, 2000).…”

“…Only the lock-in phase of HMT-C10 has been reported up to now (Gardon et al, 2001). For n odd, two compounds have already been investigated: HMT-C9 (Bonin et al, 2003;Hostettler et al, 1999) and HMT-C11 (Pinheiro et al, 2003). These crystals exhibit a high-temperature disordered phase, as revealed by the well organized diffuse rods observed in their diffraction patterns.…”