Stepwise Conformational Disorder in an Ionic Plastic Crystal

Area of Science:

• Physical Chemistry
• Materials Science
• Spectroscopy

Background:

• Ionic liquids (ILs) exhibit unique phase transitions.
• Understanding structural dynamics in ILs is crucial for their applications.
• Choline bis(trifluoromethanesulfonyl)imide ([Chol][NTf2]) is a representative IL.

Purpose of the Study:

• To investigate the structural changes and conformational dynamics of [Chol][NTf2] during phase transitions.
• To correlate spectroscopic observations with differential scanning calorimetry (DSC) data.

Main Methods:

• In situ Raman spectroscopy coupled with DSC measurements.
• Analysis of vibrational modes to identify conformers ([Chol]: gauche/anti, [NTf2]: transoid/cisoid).
• Low-frequency Raman and inelastic neutron scattering (INS).
• Infrared spectroscopy to probe hydrogen bonding.

Main Results:

• The low-temperature crystal phase contains gauche [Chol] and transoid [NTf2] conformers.
• In the plastic crystal phase, [Chol] gains conformational flexibility, while [NTf2] remains conformationally rigid.
• Decoupled conformational dynamics between [Chol] and [NTf2] were observed.
• [NTf2] gains flexibility only upon melting.
• A cation-cation hydrogen bond motif, present in the liquid, is already observed in the plastic crystal phase.

Conclusions:

• The plastic crystal phase of [Chol][NTf2] exhibits distinct structural and dynamic properties compared to the low-temperature crystal and liquid phases.
• The decoupling of conformational dynamics between the cation and anion is a key feature of the plastic crystal phase.
• The early emergence of hydrogen bonding in the plastic crystal phase influences its properties.