Conformational preferences of two imidazolium-based ionic liquids at high pressures

Takahiro Takekiyoa, Yusuke Imaib, Naohiro Hatanoa, Hiroshi Abeb, Yukihiro Yoshimuraa


aDepartment of Applied Chemistry, National Defense Academy, Yokosuka, Kanagawa 239-8686, Japan
bDepartment of Materials Science and Engineering, National Defense Academy, Yokosuka, Kanagawa 239-8686, Japan

Chemical Physics Letters 511 (2011) 241-246.

Abstract
Conformational stabilities of 1-ethyl-3-methylimidazolium tetrafluoroborate ([emim][BF4]) and 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF4]) at high pressures have been investigated by Raman spectroscopy. The trans (planar) conformer for the CNCC angle of the [emim]+ is predominant at high pressures, but the conformational change of the CNCC angle of the [bmim]+ is independent of pressure. The gauche.trans conformer for the butyl cation of the [bmim]+ is predominant at high pressures. Thus, the difference in the alkyl chain length between the cations causes the respective conformational preferences of the two imidazolium-based ionic liquids at high pressures.

Fig. 1
Optimized structures of the conformers of (a) [emim]+ and (b) [bmim]+ calculated by B3LYP/6-311G+(d) level.


Fig. 2
Raman spectra of pure (a) [emim][BF4] and (b) [bmim][BF4] in the region from 290 to 900 cm-1. The ‘CH2(N) b’ and ‘CH2 r’ represent the CH2(N) bending and CH2 rocking modes, respectively. The ‘BF4- str’ and ‘BF4- sc’ represent the BF4- stretching and scissoring modes, respectively.
Fig. 3.
Raman spectral changes of pure (a) [emim][BF4] and (b) [bmim][BF4] as a function of pressure.
Fig. 4.
Intensity fractions of the conformers of (a) [emim][BF4] and (b) [bmim][BF4] as a function of pressure. The open and closed circles in (a) represent the gauche (I430) and trans (I448) conformers of [emim]+. The open and closed triangles in (a) represent the gauche (I387) and trans (I448) conformers of [emim]+. The open and closed circles in (b) represent the gt and tt conformers of [bmim]+. The open and closed triangles in (b) represent the gauche and trans conformers of [bmim]+. The straight lines represent the results of the least-squares fit.
Fig. 5.
Pressure dependences of the intensity ratio between the conformers of [emim][BF4](O: I430/I448 and O: I387/I448) and [bmim][BF4](s).The straight lines represent the results of the least-squares fit.
Fig. 6.
Definitions of the volume contributions.
Fig. 7.
Raman BF4- stretching spectra of pure (a) [emim][BF4] and (b) [bmim][BF4] as a function of pressure.
Fig. 8.
Changes in the BF4- stretching (GBF4) frequencies and the band widths (GBF4) of [emim][BF4](d) and [bmim][BF4](s) as a function of pressure. The straight lines represent the results of the least-squares fit.
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Last Modified: April 1, 2011