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ABSTRACT: The rotational dynamics of the hexafluorophosphate anion (PF(6)(-)) in the crystalline and liquid states of the archetypal room temperature ionic liquid (RTIL) 1-butyl-3-methylimidazolium hexafluorophosphate ([C(4)mim]PF(6)) are investigated using (31)P NMR spectroscopy line shape analyses and spin-lattice relaxation time measurements. The PF(6)(-) anion performs isotropic rotation in all three polymorphic crystals phases α, β and γ as well as in the liquid state with a characteristic timescale that ranges from a few ps to a few hundred ps over a temperature range of 180 - 280 K. The rotational correlation time τ(c) for PF(6)(-) rotation follows the sequence γ-phase < α-phase ≈ liquid < β-phase. On the other hand, in the liquid state, all local motions in the cation as well as its global rotational reorientation are characterized by timescales that are slower compared to that for the PF(6)(-) anion rotation. The timescale τ(c) and the activation energy of PF(6)(-) rotation in this RTIL are found to be comparable with those observed in ordinary alkali and ammonium salts despite the large counterion size and low melting point of the former. The high sphericity of the PF(6)(-) ion is hypothesized to play an important role in the decoupling of its rotational dynamics that appear to be practically independent of the averaged cation-anion interaction.
The Journal of Physical Chemistry B 12/2012; · 3.70 Impact Factor
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ABSTRACT: We studied the crystallization process of 1-butyl-3-methylimidazolium bromide ([C(4)mim]Br) using measurements of supersensitive scanning calorimetry, free induction decay (FID) signals of (1)H NMR, and direct observation. These three methods provided consistent, complementary results, which showed extremely slow dynamics at crystallization. This sample does not crystallize during the cooling process, loses mobility, and changes to a coagulated state, which is not the thermodynamic glass state. The FID signals and direct observation in the heating process indicate that the coagulated sample liquefies just before crystallization. The crystallization of [C(4)mim]Br does not occur from specialized locations such as the surface or wall of the sample tube but randomly in the liquid. The calorimetric measurements show that it takes 150 min for approximately 3 mg of this sample to crystallize perfectly. Conformational changes of the butyl group continue for approximately 330 min after crystallization. Such slow dynamics are thought to be due to the cooperative linking of crystallization and complex conformational changes in dense fields with high viscosity.
The Journal of Physical Chemistry B 03/2012; 116(13):3991-7. · 3.70 Impact Factor
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ABSTRACT: 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 Journal of Physical Chemistry B 03/2012; 116(12):3780-8. · 3.70 Impact Factor
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ABSTRACT: We investigated the rotational dynamics of two imidazolium-based ionic liquids, 1-butyl-3-methylimidazolium bromide ([C(4)mim]Br) and 1-butyl-2,3-dimethylimidazolium bromide ([C(4)C(1)mim]Br), to reveal the effects of methylation at position 2 of the imidazolium ring (C(2) methylation). The rotational correlation time (τ(local)) for each carbon in the cations is derived from the spin-lattice relaxation time of (13)C nuclear magnetic resonance. The τ(local) results obtained here provide three principle insights into the rotational dynamics of ionic liquids. First, all τ(local) values for [C(4)C(1)mim]Br are greater than those for [C(4)mim]Br owing to a viscosity increase due to C(2) methylation. Second, the rate of change in τ(local) on C(2) methylation differs among the carbons in the cation, which indicates that each carbon has a different microviscosity. Third, the τ(local) increase in the (13)C at the root of the butyl group on C(2) methylation is very small compared to both intuitive prediction and the results from quantum chemical calculations. This indicates that the motion of the butyl group root in [C(4)C(1)mim]Br is not significantly inhibited by the methyl group at the position 2 of the imidazolium ring. The finding provides conclusive information on the origin of the increases in the melting point on C(2) methylation. Hunt previously found through calculation that decreases in entropy are caused by two factors, namely, reductions in the rotational mobility of the butyl group and in the number of stable anion interaction sites with C(2) methylation, resulting in an increase in melting point and viscosity. Our finding experimentally illustrates that the origin of the increases in melting point is not the inhibition of butyl group motion and that the reduction in stable anion interaction sites plays a major role in the increases. Additionally, it is suggested that the viscosity increase on C(2) methylation can be interpreted in the same manner.
The Journal of Physical Chemistry A 03/2011; 115(14):2999-3005. · 2.95 Impact Factor
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ABSTRACT: The proton at the 2 position of the cation ring in imidazolium-based ionic liquids (ILs) strongly interacts with anions; therefore, the methylation at this position (C(2) methylation) causes significant changes in the physicochemical properties of these liquids. We investigated the C(2) methylation effects on the phase behaviors and cation conformations of ILs by calorimetric and Raman spectroscopic measurements, focusing on the pairs of 1-butyl-3-methylimidazolium salt ([C(4)mim]X) and 1-butyl-2,3-dimethylimidazolium salt ([C(4)C(1)mim]X), where X(-) is Cl(-), Br(-), I(-), BF(4)(-), and PF(6)(-). The melting and freezing points of all pairs increased after the C(2) methylation, as reported previously, and the reason for the increase was the overcompensation of the DeltaS(trans) decrease for the DeltaH(trans) decrease. The C(2) methylation also affected the phase behaviors of the ILs. With Raman spectroscopic measurements, all cation conformations in crystalline phases were assigned to trans-trans (TT), gauche-trans (GT), or gauche'-trans (G'T) conformers of the butyl group. Except in [C(4)C(1)mim]BF(4), all crystal-crystal phase transitions of the present samples occurred accompanied by conformational changes among TT, GT, and G'T. For the gas states of [C(4)mim](+) and [C(4)C(1)mim](+), DFT calculations showed that there were hardly any differences in the structures of the butyl group for each set of paired conformers or in the energetic orders among the conformers. On the other hand, the conformer adopted in the crystalline phase differed between [C(4)mim]X and [C(4)C(1)mim]X. In addition, the population of the conformers in the liquid state also differed in each pair. The data from higher frequency Raman spectra suggested that the difference in cation conformation in each pair, for the crystalline and liquid states, was due to the shift in the position of the anion relative to that of the cation. By C(2) methylation, the relative distance between the anion and cation decreased for Cl(-), Br(-), and I(-) salts, but it increased for BF(4)(-) and PF(6)(-) salts.
The Journal of Physical Chemistry B 07/2010; 114(28):9201-8. · 3.70 Impact Factor
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ABSTRACT: We have investigated phase-transition behaviors of a typical room temperature ionic liquid, 1-butyl-3-methylimidazolium hexafluorophosphate ([C(4)mim]PF(6)), using calorimetric and Raman spectroscopic techniques. Although there was some confusion on its phase behaviors in previous reports, our measurements with a laboratory-made calorimeter at a slow scanning rate (5 mK/s) have definitely revealed that [C(4)mim]PF(6) has three crystalline phases. From the Raman spectroscopic study, the conformations of the butyl group for these crystalline phases are assigned to gauche-trans, trans-trans, and gauche'-trans conformations in lower-energy order. It has been also shown that these three conformers coexist in the liquid, supercooled liquid, and glass states. It is concluded that all of the phase transitions of [C(4)mim]PF(6) except the glass transition are associated with conformational changes of the butyl group.
The Journal of Physical Chemistry B 12/2009; 114(1):407-11. · 3.70 Impact Factor
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ABSTRACT: Most ionic liquids (ILs) are characterized by the presence of some kind of isomer for component ions. We investigated the liquid and crystalline structures of 1-isopropyl-3-methylimidazolium halides ([i-C3mim]X, X = Br, I) by means of Raman scattering measurements backed up by density functional theory (DFT) calculations. The DFT calculations indicated that there are two stable rotational isomers of [i-C3mim](+), one of an asymmetric form (Asym) and the other a symmetric one (Sym). For both salts, it was revealed that Asym and Sym coexist in liquid states, in contrast to the presence of only Asym in crystalline states. We determined the isomer populations and showed that Asym is dominant in the liquid states of both salts (approximately 7/3). Conformational changes in [i-C 3mim](+) during the crystallizing and melting processes were observed by conducting simultaneous measurements using the techniques of Raman spectroscopy and calorimetry. We have revealed that the conformational change from Asym to Sym occurs gradually, with linking to melting in the premelting region. This is a conclusive observation of the structural changes of ILs in the premelting region and demonstrates the conformational changes between isomers during the crystallizing and melting processes.
The Journal of Physical Chemistry A 08/2008; 112(33):7543-50. · 2.95 Impact Factor
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ABSTRACT: The rotational dynamics of cations and anions in the room temperature ionic liquid, 1-butyl-3-methylimidazolium hexafluorophosphate ([C4mim]PF6) have been investigated in the supercooled liquid and glassy states using 13C, 31P, and 19F NMR spectroscopy. The α-relaxation process of the supercooled liquid corresponds well with that of the isotropic rotational reorientation of the [C4mim]+ cations. The timescale of the reorientational motion of the butyl chain in the cations, which is reminiscent of the conformational isomerization, is found to be slower than that of the imidazolium ring. This counterintuitive result can be attributed to the presence of local structures in the form of polar and nonpolar nanodomains in the liquid and significant steric and coulombic interactions between the rings or chains in the cations and such domains. On the other hand, the dynamics of the constituent PF6− anions is dominated by free rotational diffusion at temperatures above ∼230 K, while a restricted rotational or librational motion dominates at lower temperatures. This transition temperature can be identified with the mode coupling critical temperature Tc where the anion rotational timescale decouples from that of the [C4mim]+ cations. The librational motion of the anions has a characteristic timescale on the order of 10−10 s with an activation energy of ∼0.16 eV typical of a β-relaxation process. This dynamical process continues below Tg, well into the glassy state of this ionic liquid.
Phys. Rev. B. 85(5).
