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2006, 110, 8179-8183 Published on Web 04/04/2006
Conformational Equilibrium of Bis(trifluoromethanesulfonyl) Imide Anion of a Room-Temperature Ionic Liquid: Raman Spectroscopic Study and DFT Calculations Kenta Fujii,† Takao Fujimori,† Toshiyuki Takamuku,‡ Ryo Kanzaki,† Yasuhiro Umebayashi,† and Shin-ichi Ishiguro*,† Department of Chemistry, Faculty of Science, Kyushu UniVersity, Hakozaki, Higashi-ku, Fukuoka 812-8581, Japan, and Department of Chemistry and Applied Chemistry, Faculty of Science and Engineering, Saga UniVersity, Honjo-machi, Saga, 840-8502, Japan ReceiVed: February 27, 2006; In Final Form: March 22, 2006
The structure of bis(trifluoromethanesulfonyl) imide (TFSI-) in the liquid state has been studied by means of Raman spectroscopy and DFT calculations. Raman spectra of 1-ethyl-3-methylimidazolium (EMI+) TFSIshow relatively strong bands arising from TFSI- at about 398 and 407 cm-1. Interestingly, the 407 cm-1 band, relative to the 398 cm-1 one, is appreciably intensified with raising temperature, suggesting that an equilibrium is established between TFSI- conformers in the liquid state. According to DFT calculations followed by normal frequency analyses, two conformers of C2 and C1 symmetry, respectively, constitute global and local minima, with an energy difference 2.2-3.3 kJ mol-1. The wagging ω-SO2 vibration appears at 396 and 430 cm-1 for the C1 conformer and at 387 and 402 cm-1 for the C2 one. Observed Raman spectra over the range 380-440 cm-1 were deconvoluted to extract intrinsic bands of TFSI- conformers, and the enthalpy of conformational change from C2 to C1 was evaluated. The enthalpy value is in good agreement with that obtained by theoretical calculations. We thus conclude that a conformational equilibrium is established between the C1 and C2 conformers of TFSI- in the liquid EMI+TFSI-, and the C2 conformer is more favorable than the C1 one.
Introduction Bis(trifluoromethanesulfonyl) imide anion (TFSI-) was first reported as H+TFSI- and Cs+TFSI- by Foropoulos and DesMarteau,1 and it has been revealed that series of 1-alkyl3-methylimidazolium and N-alkyl-N-methyl-pyrolidinium TFSIsalts give ionic liquids at room temperature. Since then, TFSIhas been attracting attention as an anion of electrolytes for polymer batteries2 and lithium ion second batteries.3 Ion-pair formation between Li+ and TFSI- ions in room-temperature ionic liquids (RTILs) has also been studied with regard to the lithium-ion conductivity.4 Watanabe et al. reported some physicochemical properties, such as density, ion conductivity, and self-diffusion constant, of RTILs at various temperatures5 and also reported high-level quantum chemical calculations for ionpair formation of various RTILs.6 Johansson et al. theoretically predicted the presence of conformers of TFSI- from ab initio calculations for the first time.7 Rey et al. suggested the presence of conformational equilibrium for TFSI- in a polyoxyethylene solution of Li+TFSI- by infrared and Raman spectroscopy.8 Holbrey et al. indicated that TFSI- adopts an unusual cis geometry in 1,3-dimethylimidazolium bis(trifluoromethanesulfonyl) imide (DMI+TFSI-) crystals.9 However, no direct evidence was obtained for the conformation of TFSI- in the liquid state so far. * Corresponding author. E-mail:
[email protected] † Kyushu University. ‡ Saga University.
10.1021/jp0612477 CCC: $33.50
We have studied solvent conformation and its effect on the metal-ion solvation and complexation by means of Raman spectroscopy and DFT calculations.10 The technique may be also useful for RTILs. The conformational flexibility is particularly important and essential property of RTILs, because it may relate to the melting point of electrolytes. Here, we report the conformational equilibrium of TFSI- in the liquid 1-ethyl3-methylimidazolium (EMI+) TFSI- studied by means of Raman spectroscopy at varying temperature and DFT calculations. Experimental Section Materials. 1-Ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl) imide (EMI+TFSI-) of spectroscopic grade (Japan Carlite Co. Ltd.) were used without further purification. Water content was checked by a Karl Fischer test to be less than 60 ppm for the salt examined. All materials were treated in a high performance glovebox (Miwa), in which water and oxygen contents were kept less than 1 ppm. Raman Spectroscopy. Raman spectra were obtained using an FT-Raman spectrometer (Perkin-Elmer GX-R) equipped with an Nd:YAG laser operating at 1064 nm. The laser power at 800-1000 mW was kept constant throughout measurements. Spectral data were obtained at optical resolution of 4 cm-1 and accumulated 512-1024 times to obtain data with a sufficiently high signal-to-noise ratio. The sample liquid in a quartz cell was stirred and thermostated at a given temperature within (0.3 K. The sample room was filled with dry N2 gas to avoid © 2006 American Chemical Society
8180 J. Phys. Chem. B, Vol. 110, No. 16, 2006
Letters
Figure 1. Raman spectrum of EMI+TFSI- over the range 200-1600 cm-1 at 298 K.
Figure 2. Temperature dependence of Raman spectra in the range 360-480 cm-1.
condensation of moisture on the surface of the cell. No appreciable damage of the sample was detected after irradiation. Raman spectra obtained were deconvoluted to extract single Raman bands. A single Raman band is assumed to be represented as a pseudo-Voigt function, fV(ν) ) γfL(ν) + (1 - γ)fG(ν), where fL(ν) and fG(ν) stand for the Lorentzian and Gaussian components, respectively, and the parameter γ (0 < γ < 1) is the fraction of the Lorentzian component. To avoid uncertainty in obtaining the γ value of the peaks, the value was fixed to that obtained at the lowest temperature. The intensity I of a single Raman band is evaluated according to I ) γIL + (1 - γ)IG, where IL and IG denote integrated intensities of the Lorentzian and Gaussian components, respectively. A nonlinear least-squares curve-fitting program, based on the MarquardtLevenberg algorithm,11,12 was developed in our laboratory and used throughout the analyses. DFT Calculations. The geometry optimization and normal coordinate analyses for the isolated single TFSI- ion were carried out on the basis of density functional theory according to Becke’s three-parameter hybrid method13 with LYP correlation (B3LYP).14 DFT calculations were carried out using Gaussian03 program package.15
ab initio calculations for M2+-TFSI- (M ) Mg, Ca, Sr, and Ba) ion pairs in polyoxyethylene.19 Arnand et al. carried out HF/6-31+G**//HF/3-21+G* ab initio calculations for TFSIand Li+TFSI- ion pair and concluded that the geometry with the C1 symmetry gives the global minimum.4 In contrast, Johansson et al. pointed out on the basis of HF/6-31G* and HF/6-31G+(d) calculations that two conformers with the C1 and C2 symmetries might be present in equilibrium, as the C2 conformer is more favorable only 2.3 kJ mol-1 than the C1 conformer.7 Recently, Lopes et al. performed ab initio calculations and concluded that the C2 conformer gives the global minimum.20 Indeed, according to MacFarlane et al.,21 TFSIwas found to have the C2 geometry in the crystalline state. It is thus plausible that TFSI- is present mainly as the C2 conformer, together with the C1 conformer as a minor species. TFSI- has the molecular structure F3C-S(O2)-N--S(O2)CF3, and the terminal CF3 group can rotate along the S-N bond to give rotational isomers. Therefore, the torsion potential energy surface has been examined for two dihedral C-S-N-S angles. According to Johansson et al.,7 one C-S-N-S dihedral angle among the two is 92.7° for both C1 and C2 conformers, and another is 99.8° for C2 conformer and -168.2° for C1. We also carried out HF/6-31G(d) and B3LYP/6-31G(d) calculations for TFSI-, in which a C-S-N-S dihedral angle θ was fixed at 90° and another was varied over the range -180°
