Phase-transition-induced twinning in the 1 : 1 adduct of hexamethylenetetramine and azelaic acid

The title compound, C6H12N4. C9H16O4, undergoes several thermotropic phase transitions. The crystalline structure is layered, with sheets of azelaic a cid linked to sheets of hexamethylenetetramine by hydrogen bonds. In the ro om-temperature phase, the azelaic acid molecules are disordered. By lowerin g the temperature, this disorder partially disappears. The ordering is clea rly observed in reciprocal space where on the rods of diffuse scattering, p resent in the room-temperature phase, a series of superstructure reflection s emerges. This phase transition leads to twin-lattice quasisymmetry (TLQS) twinning. The structure of this twinned phase is explored in this paper. T here are two orientational domains linked by a mirror plane which relates d isordered orientations of the acid molecules above the phase transition. A single domain has space group P2(1)/c. The structure has been solved and re fined on the complete set of data to R-1 = 0.0469. The chains remain partia lly disordered, showing two acid groups with unequal population: the major form corresponding to a carboxylic acid and the minor to a carboxylate. The ordering of the structure, when going through the phase transition, is int erpreted in terms of stabilization by C-H ... O hydrogen bonding. A least-s quares estimator of the twinning volume ratio is developed that gives an ex pression for the twinning ratio in terms of the intensities of nonoverlappi ng reflections. The twinning ratio obtained in the structure refinement com pares very well with that obtained from this estimator.