JOURNAL OF RAMAN SPECTROSCOPY J. Raman Spectrosc. 2007; 38: 551–558 Published online 21 December 2006 in Wiley InterScience (www.interscience.wiley.com) DOI: 10.1002/jrs.1680 Raman and ab initio study of the conformational isomerism in the 1-ethyl-3-methyl-imidazolium bis(trifluoromethanesulfonyl)imide ionic liquid J. C. Lass ` egues, 1∗ J. Grondin, 1 R. Holomb 2,3 and P. Johansson 2 1 Laboratoire de Physico-Chimie Mol ´ eculaire, UMR 5803, CNRS, Universit ´ e Bordeaux I, 351 Cours de la Lib ´ eration, 33405 Talence Cedex, France 2 Department of Applied Physics, Chalmers University of Technology, SE-41296, G ¨ oteborg, Sweden 3 Department of Solid State Electronics, Uzhhorod National University, Uzhhorod, Ukraine Received 5 October 2006; Accepted 19 November 2006 The calculated and experimental Raman spectra of the (EMI + )TFSI − ionic liquid, where EMI + is the 1-ethyl-3-methylimidazolium cation and TFSI − the bis(trifluoromethanesulfonyl)imide anion, have been investigated for a better understanding of the EMI + and TFSI − conformational isomerism as a function of temperature. Characteristic Raman lines of the planar (p) and non-planar (np) EMI + conformers are identified using the reference (EMI + )Br − salt. The anion conformer of C 2 symmetry is confirmed to be more stable than the cis (C 1 ) one by 4.5 ± 0.2 kJ mol −1 . At room temperature, the population of trans (C 2 ) anions and np cations is 75 ± 2% and 87 ± 4%, respectively. Fast cooling quenches a metastable glassy phase composed of mainly C 2 anion conformers and p cation conformers, whereas slow cooling gives a crystalline phase composed of C 1 anion conformers and of np cation conformers. Copyright  2006 John Wiley & Sons, Ltd. KEYWORDS: ionic liquid; conformation; Raman spectroscopy; 1-ethyl-3-methylimidazolium; bis(trifluoromethanesulfonyl)imide INTRODUCTION The ⊲EMI C ⊳TFSI  ionic liquid, where EMI C is the 1-ethyl-3- methylimidazolium cation and TFSI  the bis(trifluorometha- nesulfonyl)imide anion, has recently been investigated using Raman spectroscopy and DFT calculations to characterize the conformational states of the cation 1 and the anion. 2 A number of papers have also been devoted either to other properties of this ionic liquid, 3–5 or to the conformational dynamics of the TFSI  anion in a variety of situations. 6–9 Further theoretical and experimental studies deal with the modelling of ionic liquid salts in which EMI C is associated with other anions than TFSI  . 10 – 16 In the present work, the DFT calculations of the cation and anion are revisited and additional Raman results on the ⊲EMI C ⊳TFSI  system are presented for a better understanding of EMI C and TFSI  conformational isomerism as a function of temperature. Raman spectra of ⊲EMI C ⊳Br  are also recorded for comparison, as the crystal structure of this salt has been determined. 10 L Correspondence to: J. C. Lass` egues, Laboratoire de Physico-Chimie Mol´ eculaire, UMR 5803, CNRS, Universit´ e Bordeaux I, 351 Cours de la Lib´ eration, 33405 Talence Cedex, France. E-mail: jc.lassegues@lpcm.u-bordeaux1.fr EXPERIMENTAL The ionic liquids ⊲EMI C ⊳TFSI  (99C%) and ⊲EMI C ⊳Br  (99C%) were purchased from Solvionic. 17 They are claimed to have melting temperatures of 257 and 326 K, respectively, 17 but they can very easily be obtained in a supercooled liquid state. A solution of 1 mol LiTFSI (Sigma-Aldrich 99.99%) in 40 mol H 2 O was also prepared. All these samples were sealed under vacuum in glass tubes for the Raman measurements. The Raman spectra were recorded, as previously described, 8 with a Labram HR800 Jobin-Yvon spectrometer equipped with a krypton ion laser (752.45 nm), an air-cooled CCD detector (ANDOR) and a 600 grooves mm 1 grating giving a spectral resolution of 2 cm 1 . First-principles calculations of minimum energy geome- try, symmetry and vibrational properties of the planar (p) and non-planar (np) conformers of EMI C and of the cis (C 1 ) and trans (C 2 ) conformers of TFSI  have been carried out using the Gaussian03 quantum-chemical package. 18 Initially, the mod- els were geometry-optimized by the HF/6-311 C G L method and the subsequent calculations of geometry and vibrational spectra were performed using the hybrid (HF C DFT) B3LYP functional consisting of a linear combination of the pure corrected exchange functional proposed by Becke and the Copyright  2006 John Wiley & Sons, Ltd.