Striking temperature-dependent molecular reorganization at the C-2 position of [EMIM][BF4]

Highlights: • Observed anomalous temperature-dependent infrared characteristics for the ionic liquid, 1-ethyl-3-methylimidazolium tetrafluoroborate ([EMIM][BF₄]) using a C-D vibrational probe. • Infrared studies indicated the presence of a liquid–liquid transition which was later confirmed by thermoanalytical studies. • Analysis suggest ordered structures with stronger hydrogen bonding at a higher temperature. • Existence of similar hidden and ordered transitions for other imidazolium-based ionic liquids at higher temperatures cannot be ruled out. Understanding the temperature-dependent structural evolution of imidazolium-based ionic liquids can facilitate their high-temperature applications. In this manuscript, we report an anomalous redshift and line-narrowing of the C-D vibration at the C-2 position of the imidazolium cation of the ionic liquid, 1-ethyl-3-methylimidazolium tetrafluoroborate ([EMIM][BF₄]), when compared to other investigated ionic liquids upon heating suggesting the possibility of structural ordering of this ionic liquid upon heating. Computational studies show that this ordering could arise due to the formation of strong hydrogen bonding conformers. Further infrared studies indicate the existence of a possible hidden transition in this liquid which was subsequently confirmed by calorimetric measurements.