Conformations of 1-butyl-3-methylimidazolium chloride probed by high pressure Raman spectroscopy

The behavior of 1-butyl-3-methylimidazolium chloride has been investigated by Raman spectroscopy as a function of hydrostatic pressure. Under ambient pressure two rotational isomers (GA and AA forms) of the 1-butyl-3-methylimidazolium cation coexist in the ionic liquid state. As the supercooled liquid was compressed from ambient to 0.9 GPa, the contribution of the GA conformer decreases in intensity as the pressure was elevated. A new high pressure phase is formed above the pressure of 1.5 GPa. This new high pressure phase of 1-butyl-3-methylimidazolium chloride arises from perturbed GA conformer, i.e., distorted Crystal 2. Crystal 1 form ( AA form) is known as a thermodynamically stable form under ambient pressure. Nevertheless, crystal 1 form is switched to a metastable state under the condition of high pressure.