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International Journal of Spectroscopy
Volume 2011 (2011), Article ID 648245, 5 pages
Research Article

Effect of Counteranions on the Conformational Equilibrium of 1-Butyl-3-methylimidazolium-Based Ionic Liquids

1Department of Applied Chemistry, National Defense Academy, 1-10-20 Hashirimizu, Yokosuka, Kanagawa 239 8686, Japan
2Department of Materials Science and Engineering, National Defense Academy, 1-10-20 Hashirimizu, Yokosuka, Kanagawa 239-8686, Japan

Received 13 July 2011; Accepted 13 September 2011

Academic Editor: Rolf W. Berg

Copyright © 2011 Naohiro Hatano et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.


We have investigated the nonspherical anion effect on the trans-trans (TT) and gauche-trans (GT) equilibrium in pure 1-butyl-3-methylimidazolium ([bmim]+)-based room temperature ionic liquids (RTILs) by the Raman spectroscopy. The intensity ratio of the [bmim]+ cation in [bmim]+-based RTILs having nonspherical anions changes with nature of the anions. However, the enthalpy change of the [bmim]+ cation is approximately −1.0 kJ/mol for all [bmim]+-based RTILs used in this study and is independent of the anionic species. The present results indicate that the conformational stability of the [bmim]+ cation in [bmim]+-based RTILs including nonspherical anions is driven by the entropic contribution associated with the orientation and configuration of the [bmim]+ cation with respect to the counteranion.