Heat capacities of liquid, stable crystal, and liquid-quenched glass of a room-temperature ionic liquid (RTIL), 1-hexyl-3-methylimidazolium bis(trifluromethylsulfonyl)imide were measured between 5 and 310 K by adiabatic calorimetry. Heat capacity of the liquid at 298.15 K was determined for an IUPAC project as (631.6 +/- 0.5) J K(-1) mol(-1). Fusion was observed at T(fus) = 272.10 K for the stable crystalline phase, with enthalpy and entropy of fusion of 28.34 kJ mol(-1) and 104.2 J K(-1) mol(-1), respectively. The purity of the sample was estimated as 99.83 mol % by the fractional melting method. The liquid could be supercooled easily and the glass transition was observed around T(g) approximately 183 K, which was in agreement with the empirical relation, T(g) approximately ((2)/(3)) T(fus). The heat capacity of the liquid-quenched glass was larger than that of the crystal as a whole. In the lowest temperature region, however, the difference between the two showed a maximum around 6 K and a minimum around 15 K, at which the heat capacity of the glass was a little smaller than that of crystal.

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Heat capacities of liquid, crystal(s) and liquid-quenched glass (LQG) of room-temperature ionic liquid 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide ([bmim][Tf(2)N]) and N-butyl-N-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide ([bmp][Tf(2)N]) were measured by adiabatic calorimetry. The melting points of [bmim][Tf(2)N] and [bmp][Tf(2)N] were 270.42 and 265.82 K, respectively. Heat capacity anomalies depending on thermal history after crystallization were observed above 200 K in both compounds. Two thermal anomalies due to glass transitions in crystalline [bmim][Tf(2)N] were observed at 59 and 73 K. One thermal anomaly independent of thermal history was observed in a metastable crystalline [bmp][Tf(2)N]. Thermal properties related to LQG of [bmim][Tf(2)N] and [bmp][Tf(2)N] are similar to those of other glass formers, and they are classified as fragile liquids. Heat capacities of [bmim](+) and [bmp](+) due to normal modes of the intramolecular vibration were evaluated through DFT calculations. The comparison between experimental and calculated heat capacity differences shows that the trends in the liquid phase are consistent with each other, while those in the crystal phase deviate further as the temperature is increased. This result supports the authors' previous conclusion that the origin of the low melting point is not a curious property of the liquid but primarily related to properties of the crystal.

Heat capacity of [C4mim][Tf2N] (1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide) was measured by adiabatic calorimetry. Broad heat capacity anomaly and change in spontaneous endothermic effect were observed for crystal phase. The relation between these anomalies and thermal history after crystallization is described.