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• W2324804553 endingPage "3788" @default.
• W2324804553 startingPage "3780" @default.
• W2324804553 abstract "We investigate the cation rotational dynamics of a room temperature ionic liquid (RTIL) 1-butyl-3-methylimidazolium hexafluorophosphate ([C4mim]PF6) in its three crystalline states by 1H NMR spectroscopy. Spin–lattice and spin–spin relaxation time (T1 and T2, respectively) measurements as a function of temperature confirm the presence of three polymorphic crystals of [C4mim]PF6: crystals α, β, and γ, which we previously discovered using Raman spectroscopy and calorimetry. Second moment calculations of 1H 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. T1 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 PF6– rotational motion. These nanosecond rotational motions obtained from the T1 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." @default.
• W2324804553 created "2016-06-24" @default.
• W2324804553 creator A5010851451 @default.
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• W2324804553 creator A5050440083 @default.
• W2324804553 creator A5060293337 @default.
• W2324804553 creator A5075168064 @default.
• W2324804553 date "2012-03-19" @default.
• W2324804553 modified "2023-10-05" @default.
• W2324804553 title "NMR Study of Cation Dynamics in Three Crystalline States of 1-Butyl-3-methylimidazolium Hexafluorophosphate Exhibiting Crystal Polymorphism" @default.
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