Monoclinic-to-orthorhombic phase transition of the hexamethylenetetramine–2-methylbenzoic acid (1/2) cocrystal with temperature-dependent dynamic molecular disorder

Abstract

As a function of temperature, the hexamethylenetetramine-2-methylbenzoic acid (1/2) cocrystal, CHN·2CHO, undergoes a reversible structural phase transition. The orthorhombic high-temperature phase in the space group Pccn has been studied in the temperature range between 165 and 300 K. At 164 K, a t phase transition to the monoclinic subgroup P2/c space group occurs; the resulting twinned low-temperature phase was investigated in the temperature range between 164 and 100 K. The domains in the pseudomerohedral twin are related by a twofold rotation corresponding to the matrix (100/0-10/00-1. Systematic absence violations represent a sensitive criterium for the decision about the correct space-group assignment at each temperature. The fractional volume contributions of the minor twin domain in the low-temperature phase increases in the order 0.259 (2) → 0.318 (2) → 0.336 (2) → 0.341 (3) as the temperature increases in the order 150 → 160 → 163 → 164 K. The transformation occurs between the nonpolar point group mmm and the nonpolar point group 2/m, and corresponds to a ferroelastic transition or to a t structural phase transition. The asymmetric unit of the low-temperature phase consists of two hexamethylenetetramine molecules and four molecules of 2-methylbenzoic acid; it is smaller by a factor of 2 in the high-temperature phase and contains two half molecules of hexamethylenetetramine, which sit across twofold axes, and two molecules of the organic acid. In both phases, the hexamethylenetetramine residue and two benzoic acid molecules form a three-molecule aggregate; the low-temperature phase contains two of these aggregates in general positions, whereas they are situated on a crystallographic twofold axis in the high-temperature phase. In both phases, one of these three-molecule aggregates is disordered. For this disordered unit, the ratio between the major and minor conformer increases upon cooling from 0.567 (7):0.433 (7) at 170 K via 0.674 (6):0.326 (6) and 0.808 (5):0.192 (5) at 160 K to 0.803 (6):0.197 (6) and 0.900 (4):0.100 (4) at 150 K, indicating temperature-dependent dynamic molecular disorder. Even upon further cooling to 100 K, the disorder is retained in principle, albeit with very low site occupancies for the minor conformer.

Temperature-induced order–disorder structural phase transitions of two-dimensional isostructural hexamethylenetetramine co-crystals

“…Polymorphs are multiple crystal forms of a given compound and are categorized as either monotropic or enantiotropic in which the higher melting form is or is not (former versus latter) thermodynamically stable at all temperatures below the melting point (Carlton, 2011). The order-disorder enantiotropic phase transitions are triggered by molecular rearrangement in the crystal structure associated with or without a space group change (Asghar et al, 2016;Chia & Quah, 2016;Khan et al, 2015;Quah et al, 2012). The molecules of positional or conformational disordered crystals at the hightemperature phase are transformed into a stable state with single position or conformation at the low-temperature phase (Suzuki et al, 2014).…”