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Ultraslow Phase Transitions in an Anion-Anion Hydrogen-Bonded Ionic Liquid.

A Raman spectroscopy study of 1-ethyl-3-methylimidazolium hydrogen sulfate, [C2C1im][HSO4], as a function of temperature, has been performed to reveal the role played by anion-anion hydrogen bond on the phase transitions of this ionic liquid. Anion-anion hydrogen bonding implies high viscosity, good glass-forming ability, and also moderate fragility of [C2C1im][HSO4] in comparison with other ionic liquids. Heating [C2C1im][HSO4] from the glassy phase results in cold crystallization at ∼245 K. A solid-solid transition (crystal I → crystal II) is barely discernible in calorimetric measurements at typical heating rates, but it is clearly revealed by Raman spectroscopy and X-ray diffraction. Raman spectroscopy indicates that crystal I has extended ([HSO4]-)n chains of hydrogen-bonded anions but crystal II has not. Raman spectra recorded at isothermal condition show the ultraslow dynamics of cold crystallization, solid-solid transition, and continuous melting of [C2C1im][HSO4]. A brief comparison is also provided between [C2C1im][HSO4] and [C4C1im][HSO4], as Raman spectroscopy shows that the latter does not form the crystalline phase with extended anion-anion chains.