Pressure-Induced Spectral Changes of Room-Temperature Ionic Liquid, N,N-Diethyl-N-methyl-N-(2-methoxyethyl)ammonium Bis(trifluoromethylsulfonyl)imide, [DEME][TFSI]

Yukihiro Yoshimura1, Takahiro Takekiyo1, Yusuke Imai2 and Hiroshi Abe2


1Department of Applied Chemistry, National Defense Academy, Yokosuka, Kanagawa 239-8686, Japan
 2Department of Materials Science and Engineering, National Defense Academy, Yokosuka, Kanagawa 239-8686, Japan

J. Phys. Chem. C 116 (2012) 2097-2101.

Abstract
We have investigated pressure-induced Raman spectral changes of ionic liquid, N,N-diethyl-N-methyl-N-(2-
methoxyethyl)ammonium bis(trifluoromethylsulfonyl)imide ([DEME][TFSI]) up to 5.5 GPa. We found that the CH stretching spectrum of [DEME] cation changes significantly above 4 GPa together with the large shift of conformational equilibrium between C1 and C2 conformers of [TFSI] anion toward C2 one. The results along with the visual observations show that no crystallization occurs even at 5.5 GPa. We suppose that structural organization in [DEME][TFSI] might significantly change at 4 to 5 GPa region.

Fig. 1
Raman CH stretching spectral changes of [DEME][TFSI] at room temperature as a function of pressure.


Fig. 2
Raman-frequency shifts of [DEME] cation against the pressure. The numbers (1-4) correspond to the peaks of [DEME] cation in Figure 1.
Fig. 3.
Photomicrographs in the sequence of elevated pressures. The p-T conditions were 0.1 MPa to 5.5 GPa and 298 K.
Fig. 4.
Optimized structures of C1 and C2 conformers of [TFSI] anion by B3LYP/6-311G+(d) level.
Fig. 5
Raman spectral changes due to the [TFSI] anion as a function of pressure at room temperature. The spectra can be represented by several mixed GaussianLorentzian curves. Representative band component analyses of Raman bands are shown as dashed curves.
Fig. 6
Pressure dependence of the intensity ratio between the C1 and C2 conformers of [TFSI] anion estimated fromthe results of Figure 5.
Fig. 7
Comparison of the Raman spectra of [DEME][TFSI] at various p-T values. (a) The lattice vibrational region and (b) the CH stretching region.
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