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

Abstract

1,4-Diazoniabicyclo­[2.2.2]­octane-1-acetate-4-acetic acid forms a complex (<b>1</b>) 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 (<b>2</b>) exhibit obvious change compared to those of <b>1</b>. The crystal structures of these two compounds, determined at 153 and 298 K, are all monoclinic in <i>P</i>2₁/<i>n</i>, 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