12662
J. Phys. Chem. A 2010, 114, 12662–12666
Ion-Pair Structure of Vaporized Ionic Liquid Studied by Matrix-Isolation FTIR Spectroscopy with DFT Calculations: A Case of 1-Ethyl-3-methylimidazolium Trifluoromethanesulfonate Nobuyuki Akai,* Akio Kawai, and Kazuhiko Shibuya* Department of Chemistry, Graduate School of Science and Engineering, Tokyo Institute of Technology, 2-12-1 H57 Ohokayama, Meguro-ku, Tokyo 152-8551, Japan ReceiVed: August 29, 2010; ReVised Manuscript ReceiVed: September 30, 2010
The matrix-isolation infrared spectrum of a thermally evaporated ionic liquid, 1-ethyl-3-methylimidazolium trifluoromethanesulfonate ([Emim][OTf]), was measured by FTIR spectroscopy and analyzed with the aid of DFT calculations. The main chemical species in the observed IR spectrum was mainly identified as the 1:1 cation-anion pair, which corresponds to the second stable ion-pair structure bonded through five hydrogen bonds between three O atoms of the anion side and four H atoms of the cation. 1. Introduction Room-temperature ionic liquids (RTILs) have unique properties such as high ionic conductivity, nonvolatility, and high viscosity. Many researchers have been trying to understand the relationship between the properties and the anion-cation structure from the viewpoint of molecular science. For example, spectroscopic and thermodynamic studies have revealed that RTILs make a localized domain structure instead of a random structure in the liquid phase.1-5 RTILs were originally believed to be nonvolatile, because they could not be distilled in the range below their thermal decomposition temperature. However, in 2006, some RTILs were found to be distillated without thermal decomposition.6,7 Gas-phase experiments then started to provide spectroscopic information on vaporized RTILs.8-19 The mass8,9 and photoelectron10 spectra of chemical species vaporized from RTILs were reported in 2007. It was found that the vaporized species are not large clusters but rather are predominantly the smallest 1:1 cation-anion pairs. Theoretical results also support the experimental findings.20 Review articles have described the present situation regarding physicochemical research on RTILs.21,22 There is much theoretical information on their geometrical structures.23-28 Some experimental data are available,12-14 but more experimental information is needed to understand the nature of the smallest cation-anion pairs. We recently reported the matrix-isolation infrared spectrum of the evaporable RTIL 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide ([Emim][Tf2N]), which is composed of the cation and anion pair illustrated in parts a and b, respectively, of Figure 1.12,13 The spectra calculated at the B3LYP and MP2/6-31G* levels do not agree well with the observed spectrum, possibly because of limitations of the basisset size. It will be required to use basis sets higher than the 6-311++G(3df,3pd) level29-34 to calculate the vibrational frequencies of molecules containing sulfur atoms such as the [Tf2N] anion. Because the IR band intensities of [Tf2N] anions dominate over those of [Emim] cations, the observed IR spectrum of [Emim][Tf2N] corresponds essentially to that of the [Tf2N] anion. Thus, the vibrational analysis in the previous study was performed with uncertainty. Another analytical * Corresponding authors. Fax: +81-3-5734-2224. E-mail: kshibuya@ chim.titech.ac.jp (K.S.),
[email protected] (N.A.).
Figure 1. Molecular structures and their abbreviations: (a) 1-ethyl3-methylimidazolium cation, (b) bis(trifluoromethanesulfonyl)imide anion, (c) trifluoromethanesulfonate anion, and (d) trifluoromethanesulfomic acid.
problem originates from the fact that large RTILs have more possible conformations of their alkyl chains and alignments of their anion-cation pairs. In the present study, we reduced the molecular size of the anion part and performed higher-level calculations. The RTIL used was 1-ethyl-3-methylimidazolium trifluoromethansulfonate ([Emim][OTf]), which is composed of the cation and anion pair illustrated in Figure 1a,c. The geometrical structures and IR spectra were expected to be calculated at the B3LYP/6-311++G(3df,3pd) level. 2. Experimental and Calculation Methods The sample of [Emim][OTf] (Merck) was vaporized at ca. 500 K after being purified at 450 K for 10 h under a vacuum (
