Structural Phase Transition in Adipic Acid

The structure of adipic acid undergoing phase transition at ~136 K has been investigated using single-crystal x-ray diffraction. On lowering the temperature, the a parameter of the room temperature monoclinic cell decreases by ~0.25 ?. Accompanying the transition, the b parameter nearly triples while a reverts back to the room temperature value. Two-thirds of the molecules in the structure loses center of symmetry. Packing diagrams of the high- and low-temperature phases on superposition reveal that there exist alternating domains perpendicular to the b direction, which contain molecules with and without symmetry. Unsymmetric molecules show displacements along the ac diagonal. Intermolecular hydrogen contacts involving carboxylic oxygens and the methylene hydrogens seem to increase interchain interactions in the low-temperature phase.