1H NMR study on reorientational dynamics of an ionic liquid, 1-butyl-3-methylimidazolium bromide, accompanied with phase transitions

We measured longitudinal and transverse relaxation times (T1 and T2) of 1-butyl-3-methylimidazolium bromide ([bmim]Br) from 203 to 393 K, using pulse 1H NMR. T1 and T2 data show the existence of liquid, super-cooled liquid and ‘coagulated state’. The coagulated state appears only in the cooling process at the temperature where [bmim]Br will crystallize in the heating process. The activation energy for rapid reorientational motions of the alkyl groups in the crystalline state was determined to be 13.3 kJ mol−1. By the T2 measurement, it was confirmed that very slow reorientational molecular motions of the overall [bmim]+ begin near the crystallizing point.