Reversible Phase Transition of 1,4-Diazoniabicyclo[2.2.2]octane-1-acetate-4-acetic Acid Chloride Trihydrate

1,4-Diazoniabicyclo­[2.2.2]­octane-1-acetate-4-acetic
acid forms a complex (1) with chloride ion and water
molecule in the ratio 1:1:3. Differential scanning calorimetry (DSC)
measurement shows a pair of reversible peaks at 210.7 K (heating)
and 180.3 K (cooling) with a large heat hysteresis about 30.4 K, indicating
this compound undergoes a reversible structural phase transition.
Dielectric measurement further confirms the phase transition. The
DSC and dielectric measurements results of its deuterated compound
(2) exhibit obvious change compared to those of 1. The crystal structures of these two compounds, determined
at 153 and 298 K, are all monoclinic in P21/n, suggesting the phase transition is isosymmetric.
Structural analysis reveals that the changes of the relative location
of water molecules and chloride ions affect the formation of different
modes of hydrogen-bonded anionic chains, leading to the reversible
structural phase transition.