Reversible phase transition of the 1: 1 co-crystal of 1,4-diazabicyclo-[2.2.2]octane (DABCO) and hydroquinone

The temperature-dependent polymorphic crystal structure of the 1 : 1 co-crystal of 1,4-diazabicyclo-[2.2.2]octane (DABCO) and hydroquinone in the low-temperature phase was determined at 93( 2) K. DSC measurement confirms that the co-crystal undergoes a reversible phase transition at about 158 K from monoclinic C2/c to monoclinic P2(1)/n without distinctly changing the cell parameters. The crystal structural analysis of data collected at 298( 2) and 93(2) K shows that in both structures, in addition to van der Waals' forces, conventional intermolecular N-H/center dot center dot center dot O hydrogen bonds are the key molecular interactions, and the hydrogen bonding interactions show no notable changes. The lack of the two-fold axis in the low temperature structure is the most important difference between the structural forms. Contrary to the equal orientation of the molecules in RT phase, the multiplicity of the orientation in LT phase suggests that the LT structure is a supper-lattice for the RT structure void of the particular two-fold axis. The LT phase transition is simply the ordering of dynamically disordered molecules about this two-fold axis at RT, the increase in intramolecular bond distances is also attributed to a reduction in tilting motional disorder and the phase transition is the disorder-order type. In the specific system with narrow hysteresis and minor structural changes across the T(c), no distinct dielectric anomaly was observed in frequency ranges of 10-1000 kHz.