We measured longitudinal and transverse relaxation times (T(1) and T(2)) for (1)H, (13)C-T(1) and (13)C spectra for room-temperature ionic liquids of 1-alkyl-3-methylimidazolium bromide [C(n)mim]Br (n = 2, 3, 4) as a function of temperature. Their values and spectra reveal close relationships between their unique phase behaviours and the dynamics of carbons constituting the cations. Carbons in these cations are classified into groups according to their dynamics, namely imidazolium carbons, an N-methyl carbon, a terminal methyl carbon of the alkyl group and others of the alkyl group. The temperature dependences of T(1) values for these groups differ greatly, resulting in a variation in the characteristic thermal behaviours of the salts. Values of (1)H-T(1) and (13)C-T(1) suggest that some carbons continue to move even in the crystalline and/or solid states. Using (13)C-T(1) data, we also estimated the temperature dependences of the correlation times for the segmental motions of carbons in the liquid states.

We investigate the cation rotational dynamics of a room temperature ionic liquid (RTIL) 1-butyl-3-methylimidazolium hexafluorophosphate ([C(4)mim]PF(6)) in its three crystalline states by (1)H NMR spectroscopy. Spin-lattice and spin-spin relaxation time (T(1) and T(2), respectively) measurements as a function of temperature confirm the presence of three polymorphic crystals of [C(4)mim]PF(6): crystals α, β, and γ, which we previously discovered using Raman spectroscopy and calorimetry. Second moment calculations of (1)H NMR spectra reveal that certain segmental motions of the butyl group in addition to the rapid rotation of the two methyl groups in the cation occur in all the crystals. The trend in the mobility of the segmental motions is γ < β ≤ α, which is consistent with the strength of cation-anion interactions (or crystal packing density) estimated from high-frequency Raman scattering experiments. T(1) measurements demonstrate two types of rotational motions on the nanosecond time scale in all three crystals: fast and slow motions. The three crystals have similar activation energies of 12.5-15.1 kJ mol(-1) for the fast motion, which is assigned to the rotation of the methyl group at the terminal of the butyl group. These observed activation energies were consistent with that estimated by quantum chemical calculations in the gas phase (11.9 kJ mol(-1)). In contrast, the slow motions of crystals α and γ are attributed to different segmental motions of the butyl group and that of crystal β to either a little segmental motion or a certain PF(6)(-) rotational motion. These nanosecond rotational motions obtained from the T(1) measurements do not appear to be affected by crystal packing density because local interactions in the crystalline state rather than packing density govern such nanosecond motions. With respect to the segmental motions, the mobility is likely to change significantly with the conformation of the butyl group. On the basis of these findings, crystal γ, which is the only crystalline phase previously determined using single-crystal X-ray diffraction, is considered to be the most stable phase because of the slowest segmental motions and the strongest cation-anion interactions.

The structural flexibility and conformational variety of the ions in room-temperature ionic liquids (RTILs) have significant effects on their physicochemical properties. To begin a systematic study of the thermodynamic properties of nonaromatic RTILs, 1-methyl-1-butylpiperidinium bis(fluorosulfonyl)amide ([Pip1,4][FSA]) was selected as the first sample. In addition to the rotational flexibility of the alkyl group, the [Pip1,4](+) cation has characteristic ring-flipping flexibility, which is very different from the behavior of the well-studied imidazolium-based cations. Calorimetry investigations using laboratory-made high-sensitivity calorimeters and Raman spectroscopy revealed that [Pip1,4][FSA] has two crystalline phases, Cryst-α and Cryst-β, and that every phase change is linked to conformational changes of both the cation and anion. Each phase change is also governed by very slow dynamics. The phase changes from supercooled liquid to Cryst-α and from Cryst-α to Cryst-β, which were observed only during heating, are not in fact phase transitions but structural relaxations. Notably, the temperatures of these structural relaxations exhibited heating rate dependences, from which the activation energy of the ring-flipping was estimated to be 38.8 kJ/mol. It is thought that this phenomenon is due to the associated conformational changes of the constituent ions in viscous surroundings.

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