STEREOCHEMISTRY AND TRANSFORMATION OF -OH ... O = HYDROGEN-BONDS .1. POLYMORPHISM AND PHASE-TRANSITION OF 1,3-CYCLOHEXANEDIONE CRYSTALS

The properties of substances or crystals which may depend on the stereochemical configuration of bistable -OH...O = hydrogen bonds are analysed for 1,3-cyclohexanedione crystals and compared with other simple cyclic beta-diketoalkane structures (enolized forms). 1,3-Cyclohexanedione (CHD) is known to form monoclinic crystals, space group P2(1)/c, built up of chains of hydrogen-bonded enol molecules (polymorph CHD1). When cooled to below 287(1)K or subjected to elevated pressures of about 10 MPa, the CHD1 crystal undergoes a phase transition involving an H-atom transfer in the hydrogen bond (structure CHD2). The mechanism of this hydrogen-bond transformation is explained. A new polymorph of 1,3-cyclohexanedione (CHD3) is reported and its structure has been determined by X-ray diffraction. The CHD3 crystals are monoclinic, space group P2(1), with two different stereochemical configurations of the hydrogen bonds. The CHD structures, as well as other simple cyclic beta-diketoalkanes and squaric acid, illustrate the interdependence between the stereochemical configuration of bistable -OH...O = hydrogen bonds and the crystal structure, polymorphism, formation of inclusion compounds, phase transitions and other structural transformations, which affect the physical properties of the crystals.