Urotropin azelate: a rather unwilling co-crystal

Abstract

Urotropin (U) and azelaic acid (AA) form 1:1 co-crystals (UA) that give rise to a rather complex diffraction pattern, the main features of which are diffuse rods and bands in addition to the Bragg re¯ections. UA is characterized by solvent inclusions, parasite phases, and high vacancy and dislocation densities. These defects compounded with the pronounced tendency of U to escape from the crystal edi®ce lead to at least seven exotic phase transitions (many of which barely manifest themselves in a differential scanning calorimetry trace). These involve different incommensurate phases and a peritectoid reaction in the recrystallization regime (T h b 0X6). The system may be understood as an OD (order±disorder) structure based on a layer with layer group Pcc2 and cell a o 9 4.7, b 9 26.1 and c 9 14.4 A Ê . At 338 K the layer stacking is random, but with decreasing temperature the build-up of an orthorhombic MDO (maximal degree of order) structure with cell a 1 = 2a o , b 1 = b, c 1 = c and space group Pcc2 is begun (at $301 K). The superposition structure of the OD system at T = 286 (1) K with space group Bmmb and cell a = 2a o , b = b and c = ca2 owes its cohesion to van der Waals interactions between the AA chains and to three types of hydrogen bonds of varied strength between UÐU and UÐAA. Before reaching completion, this MDO structure is transformed, at 282 K, into a monoclinic one with cell a m = Àa o c/4, b m = b, c m = À2a o ca2, space group P2 1 ac, spontaneous deformation $2, and ferroelastic domains. This transformation is achieved in two steps: ®rst a furtive triggering transition, which is not yet fully understood, and second an improper ferroelastic transition. At $233 K, the system reaches its ground state (cell a M = a m , b M = b, c M = c m and space group P2 1 ac) via an irreversible transition. The phase transitions below 338 K are described by a model based on the interaction of two thermally activated slip systems. The OD structure is described in terms of a three-dimensional Monte Carlo model that involves ®rst-and second-neighbour interactions along the a axis and ®rst-neighbour interactions along the b and c axes. This model includes random shifts of the chains along their axes and satisfactorily accounts for most features that are seen in the observed diffraction pattern.

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.…”

“…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.…”

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

“…Phase II, stable in the temperature range 282 T 350 K, is disordered. Its average structure is described in the non-conventional orthorhombic space group Bmmb (alternate setting of the Cmcm; Bonin et al, 2003). This phase is disordered and characterized by the presence of strong and well organized rods of diffuse scattering in its diffraction pattern.…”

“…In the re®nements of the HMT-Cn structure performed so far (Bussien Gaillard et al, 1996, 1998Hostettler et al, 1999;Gardon et al, 2001;Bonin et al, 2003) the COOH groups are disordered or present large anisotropic displacement parameters (ADPs). Considering the key role played by the O atoms linking the HMT and Cn molecules, it is particularly interesting to understand the behaviour of the COOH groups as a function of temperature.…”

Stacking disorder: the hexagonal polymorph of tris(bicyclo[2.1.1]hexeno)benzene and related examples

“…Stacking disorder manifests itself by flocks of parallel diffuse streaks of scattering density along a specific direction of reciprocal space [5]. This kind of disorder is found among inorganic substances and minerals [7][8][9][10][11], molecular crystals [12][13][14][15][16][17][18], protein crystals [19][20][21], e.g. Rubisco [22] and t-RNA [23], and even in a virus crystal [24].…”