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J. Phys. Chem. B 2009, 113, 4756–4762
Cryogenic Neon Matrix-isolation FTIR Spectroscopy of Evaporated Ionic Liquids: Geometrical Structure of Cation-Anion 1:1 Pair in the Gas Phase Nobuyuki Akai,* David Parazs, 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: December 7, 2008; ReVised Manuscript ReceiVed: January 23, 2009
Low-temperature infrared spectra of thermally evaporated ionic liquids, 1-ethyl- and 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide and bis(trifluoromethanesulfonyl)amide have been measured in a cryogenic Ne matrix. The experimental IR spectrum of bis(trifluoromethanesulfonyl)amide can be reproduced theoretically by not B3LYP/6-31G* but MP2/6-31G* calculation, which suggests that the vibrational analysis for ionic liquids composed of bis(trifluoromethanesulfonyl)imide anion would be more successfully performed using the MP2 calculation. By comparison of the matrix-isolation spectra of the ionic liquids with the MP2 calculation, their geometrical structures in the gas phase are determined to be of C(2-position)-H+ · · · Ninteraction structure, which corresponds to the geometry of the energetically second-lowest ion-pair structure. The present study may provide a valuable clue to understand a vaporization mechanism of ionic liquid. Introduction Recently, room-temperature ionic liquids (RTIL) used as new solvents and industrial materials have been widely investigated because they have some specific properties such as flameproofness, reasonable conductivity, and nonvolatility. RTILs had been believed to be decomposed below the temperatures they are evaporated at. The common view has been revoked by Earle et al. in 2006.1 They have shown that some RTILs such as 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide, [Bmim][TFSI], can be distillated under a vacuum condition without thermal decomposition. At almost the same time, Zaitsau et al. have succeeded in measuring vapor pressures of several RTILs including [Bmim][TFSI].2 The discovery of evaporable RTILs has affected various basic investigations for RTILs. Especially, the vaporization mechanism and their geometrical structures interest many experimental and theoretical researchers. In the past few years, several experimental reports for vaporization mechanism of RTILs have been published.3-7 Armstrong et al. have reported several imidazolium-based RTILs evaporated into the gaseous 1:1 ion pairs by mass spectrometry with electron ionization.3 Leal et al. have shown that discrete cation-anion pairs exist in the gas phase by Fourier transform ion cyclotron mass spectrometry,4 and Strasser et al. have measured photoelectron spectra of RTILs.5 In addition, several remarkable theoretical studies have also been published in these years.8-16 A classical simulation has also shown that the enthalpy of vaporization for overall neutral 1:1 ion pair is the lowest among expected candidates such as anion, cation, and ion clusters.16 An excellent review of the physicochemical properties of RTILs has been recently published by Weinga¨rtner.17 It is well-known that vibrational spectra provide important information to determine molecular structures with aids of quantum chemical calculations, such as a density functional theory (DFT) method. A lot of structural investigations on RTILs have been published on pure liquids, mixture solutions, and interfacial surfaces using various vibrational spectroscopic methods.18-26 RTILs are suggested to make unique structures in such bulky phases, and the alkyl group of 1-alkyl-3methylimidazolium cation has several conformations. Some * Corresponding authors. Fax: +81-3-5734-2231; e-mail: akai.n.ab@ m.titech.ac.jp (N. A.),
[email protected] (K. S.).
SCHEME 1: Molecular structures and their abbreviations; (a) 1-alkyl-3-methylimidazolium cation (X ) E for ethyl and X ) B for butyl), (b) bis(trifluoromethanesulfonyl)imide anion, and (c) bis(trifluoromethanesulfonyl)amide
theoretical studies on stable ion-pair structures have been reported,27-29 which requires accurate spectral data in the gas phase to confirm the most stable geometries of RTILs. However, the vibrational spectra of RTILs are not easy to be measured in the gas phase, because the vapor pressures are not high enough to obtain the well-resolved vibrational/rotational spectra. Then, we have been measuring the infrared spectra of such RTILs using a low-temperature matrix-isolation method, which provides some information on the ion pairs under a gas-like environment. We have recently reported the matrix-isolation infrared spectrum of 1-ethyl-3-methylimidozolium bis(trifluoromethanesulfonyl)imide ([Emim][TFSI]), where the spectrum is found to be quite different from the theoretical spectra estimated at the B3LYP/6-31G* level and the experimental spectrum under neat conditions.30 In the present paper, we report the matrix-isolation IR spectra of chemical species evaporated from [Emim][TFSI], [Bmim][TFSI], and bis(trifluoromethanesulfonyl)amide (H-TFSI), which are shown in Scheme 1. We employed H-TFSI as a prototype molecule to characterize RTILs, including [TFSI] anion, and considered an appropriate method to reproduce the experimental vibrational spectra of RTILs. We will discuss the stable structures of evaporated RTILs in the gas phase and speculate the vaporization mechanism. Experimental and Calculation Methods The [Emim][TFSI] sample was obtained from Tokyo Kasei Kogyo Co., Ltd., and [Bmim][TFSI] and H-TFSI were purchased from Kanto Chemical Co., Inc. The samples were thermally
10.1021/jp8107478 CCC: $40.75 2009 American Chemical Society Published on Web 03/12/2009
RTIL Cation-Anion 1:1 Pair in the Gas Phase
J. Phys. Chem. B, Vol. 113, No. 14, 2009 4757
TABLE 1: Optimized Geometrical Parameters of H-TFSI Obtained at the B3LYP/6-31G* and MP2/6-31G* levels parametera N-H N-S S-C S-O1 S-O2 C-F1 C-F2 C-F3 H · · · O1 S-N-S H-N-S N-S-O1 N-S-O2 N-S-C S-N-S-C H-N-S-O1 H-N-S-O2 N-S-C-F2 a
B3LYP
MP2
Bond Length (Å) 1.019 1.022 1.705 1.691 1.879 1.845 1.455 1.453 1.450 1.448 1.332 1.338 1.329 1.334 1.325 1.331 2.578 2.586 Bond Angle (°) 129.8 128.9 115.1 115.6 104.8 105.2 109.2 109.2 101.8 100.6 86.7 88.6 17.8 19.1 153.1 155.8 177.0 178.1
Exp.b
TABLE 2: Observed Wavenumbers (cm-1) with Relative Intensities and Calculated Wavenumbers (cm-1) with IR Intensities (km mol-1) of H-TFSI calca
obs 0.99 1.664 1.840 1.401 1.417 1.330 1.307 1.298
128.4 115.8 106.0 109.1 102.2
matrix
b
gas phase
solid
B3LYP
MP2
cm-1 int. cm-1 int. cm-1 cm-1 int. cm-1 int. 3392 1466 1457 1297 1242 1233 1224d
12.9 3394 15 26.7 1463 78 9.3 1442 6 1326 2 8.7 1301 21 47.7 1241 100 5.8 19.8 1225 66 1200
1141 857
611
See Figure 1 for numbering of O and F atoms. b From ref 36.
37.2 1140 100
61.6
859 768 647 615 570 503 460
assignmentc
3185 3554 102 3568 127 V(N-H) 1436 1413 93 1467 117 Va(SO2) 1406 356 1467 340 Va(SO2)
1331 1305 33 1250 1286 387 1283 28 1199 1270 114 1270 299 1219 0 10 1197 169 1119 1112 435 70 1058 1110 0 807 428 80 862 798 18 756 101 0.5 750 12 0.5 603 4 55 592 590 288 5 565 0 14 557 20 83 547 0 543 1 513 1
1341 1302 1299 1289 1288 1272 1255 1163 1161 850 822 775 767 619 605 576 566 553 549 519
69 432 15 286 78 33 144 0 391 514 25 13 7 2 298 0 28 0 1 1
V(CF3) V(CF3) V(CF3) V(CF3) + V(SO2)
Vs(SO2) Va(SNS)
σ (SNS)
a
Both calculations were performed using the same basis set of 6-31G*. Estimated wavenumbers are not adjusted by any scaling factor, and the wavenumbers lower than 500 cm-1 are omitted. All the calculated wavenumbers are tabulated in Appendix Table A1. b From ref 37. c The symbols have their usual meanings; V, stretching; σ, bending; subscript “a”, asymmetrical; subscript “s”, symmetrical modes. d Doublet band. Figure 1. Optimized structure of H-TFSI.
spectrophotometer (JEOL, SPX200ST). The spectral resolution was 0.5 or 0.25 cm-1, and the accumulation times were 100. Energies, optimized structures, and vibrational wavenumbers were predicted by DFT at the B3LYP functional32,33 and secondorder Møller-Plesset perturbation theory (MP2) calculations34 with 6-31G* and 6-31++G** basis sets, which were performed using the Gaussian 03 program.35 Results and Discussion
Figure 2. Infrared spectra of H-TFSI recorded in the Ne matrix (a) and in the solid phase (b) and calculated at the levels of MP2 /6-31G* (c) and B3LYP/6-31G* (d).
evaporated at 450 or 490 K and mixed with pure Ne gas in the nozzle, after each sample was placed in a stainless steel pipe nozzle with a heating system and purified at 400 K for a day under high vacuum conditions (
