Phase Transition of Quaternary Alkyl Halide Salts of 1,4-Diazabicyclo[2.2.2]octane

A series of quaternary alkyl halide salts of 1,4-diazabicyclo[2.2.2]octane (DABCO) exhibited first-order phase transitions in the temperature range of 62–100 °C Abrupt changes of halide anion conductivities were observed by two to three orders of magnitude at the transition temperatures. The transition temperature and the heat of transition tend to increase with the increase of the alkyl chain length. The temperature dependence of an IR spectrum showed that the band progressions due to long alkyl chain disappeared above the phase transition. This fact suggests that the trans-zigzag conformation of alkyl chains in the low temperature phase is destroyed by the internal rotation of the alkyl chain above the transition temperature.