Structure of the Room-Temperature Ionic Liquid 1-Hexyl-3

May 28, 2010 - Ekaterina I. Izgorodina , Zoe L. Seeger , David L. A. Scarborough , and Samuel ... George E. Romanos , Lawien F. Zubeir , Vlassis Likod...
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J. Phys. Chem. A 2010, 114, 6713–6720

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Structure of the Room-Temperature Ionic Liquid 1-Hexyl-3-methylimidazolium Hydrogen Sulfate: Conformational Isomerism Johannes Kiefer*,†,‡ and Cory C. Pye§ School of Engineering, UniVersity of Aberdeen, Aberdeen, Scotland, U.K., Erlangen Graduate School in AdVanced Optical Technologies, UniVersity Erlangen-Nuremberg, Erlangen, Germany, and Department of Chemistry, Saint Mary’s UniVersity, Halifax, NoVa Scotia, Canada ReceiVed: April 7, 2010; ReVised Manuscript ReceiVed: May 19, 2010

The acidic room-temperature ionic liquid 1-hexyl-3-methylimidazolium hydrogen sulfate has recently been identified to have beneficial properties for practical applications in catalysis and electrochemistry. In the present work, the conformational isomerism of this ionic liquid is studied by means of density functional theory calculations and experiments in terms of infrared absorption and Raman scattering spectroscopy. For the hydrogen sulfate anion, the trans conformer is found to be the favored isomer in the ionic liquid. For the 1-hexyl-3-methylimidazolium cation, three different low-energy conformations were obtained, differing only in the orientation of the hexyl chain. The comparison of vibrational frequencies with IR and Raman data showed good agreement for all three conformations, indicating their presence in the ionic liquid. Beyond revealing the conformational information, the experimental spectra indicate strong interionic interactions. Vibrations of sulfuric acid could be observed, indicating possible proton transfer from the cation to the anion. This is further supported by the appearance of modes around 2000 cm-1 in the IR spectrum, which could tentatively be assigned to C2-H stretching vibrations red-shifted as a result of strong interionic hydrogen bonds as a prerequisite of proton transfer. 1. Introduction Within the last two decades, room-temperature ionic liquids (RTILs) have evolved from being a matter of academic interest to a class of materials for numerous practical applications. Their history from the very beginning to this day has been summarized in a recent review article.1 RTILs are salts with a melting point below 100 °C and hence are liquid at moderate temperature.2 Owing to their ionic nature, many RTILs exhibit characteristics such as nonvolatility, nonflammability, high electro-conductivity, and good thermal stability. This makes them interesting for applications in many areas, for example, in catalysis,3,4 in biotechnology,5,6 in separation technology,7,8 in electrochemistry,9,10 and even for use as lubricants.11 In principle, the macroscopic characteristics in terms of chemical and thermophysical properties can be tailor-made by combining suitable cations and anions resulting in a huge number of possible compounds,12 but to do this in reality requires a full understanding of the molecular structure and how it affects the macroscopic properties. However, this is still at an early stage, and much effort has been put into analyzing the structure of RTILs on a molecular scale employing theoretical as well as experimental methods. Among the experimental approaches, vibrational spectroscopy in terms of infrared (IR) absorption and Raman scattering is frequently applied.13 As concerns theoretical methods, density functional theory (DFT)-based simulations and ab initio calculations are established tools.14,15 However, the most promising approach is to combine theory and experiment, e.g., to predict the spectra * Corresponding author. Address: School of Engineering, University of Aberdeen, Fraser Noble Building, Aberdeen AB24 3UE, United Kingdom. Tel: +44 (0)1224 272495. E-mail: [email protected]. † University of Aberdeen. ‡ University Erlangen-Nuremberg. § Saint Mary’s University.

Figure 1. Chemical structure of the 1-hexyl-3-methylimidazolium cation.

of possible molecular conformations and compare them with the experimental data.16–21 Herein we study the structure of the ionic liquid 1-hexyl-3methylimidazolium hydrogen sulfate [HMIM][HSO4] using theoretical and experimental methods. The chemical structure of the imidazolium-based [HMIM]+ cation is shown in Figure 1. Generally speaking, [HMIM][HSO4] is the simplest homologue of the [HMIM] alkylsulfates. This particular RTIL has gained increasing interest in recent years, as its hydrogen sulfate anion is acidic, which makes the liquid a suitable candidate to act as a catalyst for homogeneous catalysis. Fraga-Dubreuil et al.22 tested [HMIM][HSO4] and other RTILs as catalysts for esterification of acetic acid with four different alcohols. Depending on the specific alcohol, they reported acid conversion between 50 and 77%, and ester yields from 80 to 92%. Using concentrated sulfuric acid resulted in significantly shorter reaction times but was accompanied by the formation of ether side products. Frizzo et al.23 studied how the cyclocondensation reaction of β-enaminones and tert-butylhydrazine is affected in the presence of ionic liquids in comparison to the standard environment ethanol. They tested a series of 10 RTILs with the result that using [HMIM][HSO4] the pyrazole yield was slightly improved from 77 to 81%. For the sake of completeness, it should be noted that the best performance was reported for 1-butyl-3-methylimidazolium tetrafluoroborate (96%). Li et al.24

10.1021/jp1031527  2010 American Chemical Society Published on Web 05/28/2010

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TABLE 1: Relative Energies of the Hydrogen Sulfate Anion at Different Levels in kJ/mola,b level

relative energy

HF/6-31G* HF/6-31+G* MP2/6-31G* MP2/6-31+G* MP2/6-31G** MP2/6-31++G** B3LYP/6-31G* B3LYP/6-31+G* B3LYP/6-31G** B3LYP/6-31++G**

0.86 1.62 -0.19 (-0.20) 1.34 -0.80 1.08 -0.69 0.95 -1.24 0.67

a

Electronic-only, relative to CHS#2. b The CHS#3 is given in parentheses. Note that the negative numbers mean the cis conformation is more stable.

used [HMIM][HSO4] as a catalyst for ring-opening reactions of N-tosyl aziridines to synthesize β-amino ethers. A comparison with other acidic RTILs revealed that [HMIM][HSO4] is superior, providing very short reaction times and a yield as high as 97%. Further benefits reported were mild reaction conditions (at room-temperature) and an easy reuse of the catalyst. A completely different kind of application in the field of electrochemistry has been published by Zhang and Hua25 who investigated the influence of [HMIM][HSO4] and other additives on the performance of a zinc electro-deposition process. Here the addition of [HMIM][HSO4] resulted in the most significant reduction of power consumption among all tested substances. Therefore [HMIM][HSO4] is of increasing interest in many respects and merits further and fundamental investigation. In the present work we analyze the conformational isomerism of the ionic liquid [HMIM][HSO4] by means of DFT calculations and vibrational spectroscopy in terms of Fourier-transform IR and Raman spectroscopy. The computational method is used to identify lowest energy conformers of the cation and anion, and to predict the corresponding vibrational spectra. These spectra are compared to the experimental data. For the sake of completeness, it should be noted that other groups have performed conformational studies of related RTILs incorporating 1-alkyl-3-methylimidazolium cations with ethyl and butyl chains (see, e.g., refs 17 and 26–28). 2. Analytical Methods 2.1. Computational Approach. The ab initio calculations were carried out with Gaussian 03.29 Geometry optimizations, followed by harmonic vibrational frequency calculations to characterize the stationary points, were sequentially performed at the HF/STO-3G, HF/3-21G, HF/6-31G*, and HF/6-31+G* levels. For the cation conformers, the calculation was continued at B3LYP/6-31G* and B3LYP/6-31+G*. For the anion conformers, the calculation was continued at the MP2/6-31G*, MP2/6-31+G*, MP2/6-31G**, MP2/6-31++G**, B3LYP/631G*, B3LYP/6-31+G*, B3LYP/6-31G**, and B3LYP/631++G** levels. The relative energies for the different levels are reported in Table 1, but detailed results are discussed later only for the higher level calculations. For MP2 calculations, the frozen core approximation was utilized. Z-matrix coordinates constrained to the appropriate symmetry were used for efficiency, as any problems would manifest themselves by an imaginary mode orthogonal to the spanned Z-matrix space. The Hessian was also evaluated at the starting STO-3G geometry to aid convergence. Previous work by one of the authors14 reported studies on the dimethyl-, ethylmethyl-, propylmethyl-, and butylmethylimi-

Kiefer and Pye dazolium cations, including a full conformational analysis. It was shown that the three conformers with an all-trans arrangement of the butyl chain were the lowest in energy at the Hartree-Fock level. The three conformations considered here (out of a possible 81) are derived from these three stable butylmethylimidazolium conformations upon extending the chain length by two methylene units. 2.2. Experimental Methods. Chemical. The ionic liquid [HMIM][HSO4] was prepared according to the literature.22 Infrared Spectroscopy. The IR spectrum from 500 to 4000 cm-1 was recorded with a Nicolet Model 360 FTIR at 2 cm-1 nominal resolution. The device operates in attenuated total reflection (ATR) mode. For this purpose it is equipped with a diamond crystal at whose surface the evanescent electromagnetic field interacted with the sample. The number of reflections is 1, and the penetration depth of the system is approximately 1/5 of the wavelength. In the IR spectrum shown later, the wavelength dependence of the penetration depth is taken into consideration. Raman Spectroscopy. The experimental Raman setup consisted of a grating stabilized diode laser operating at 785 nm, which was focused into a quartz glass cuvette filled with the ionic liquid. The scattered light was collected by an achromatic lens, filtered by a long-pass filter (785 nm cutoff wavelength) and dispersed in a spectrograph with an integrated charge coupled device chip for signal detection. The spectral resolution of the system was approximately 2-3 cm-1 varying with wavelength. The described system allows recording Raman spectra in the range 100-1800 cm-1. 3. Results and Discussion The experimental spectra are displayed in Figure 2a,b. To allow a convenient comparison, both spectra are plotted on the same spectral scale. Enlarged spectra showing the individual features in greater detail are included in the Supporting Information available online. In order to extract data in terms of line position and peak intensity, the IR and Raman spectra have been deconvoluted (c.f. Figure 2c).30 The dotted line indicates the experimental spectrum; the thin solid lines show the individual Voigt profiles, and the thick solid line is their summation in agreement with the experiment. Due to a careful alignment of the Raman experiment, it was possible to achieve good signal-to-noise ratio and negligible interference from elastically scattered light, even in the low wavenumber region between 100 and 500 cm-1, which is not accessible by the IR device. On the other hand, the IR spectrometer delivered data between 1800 and 4000 cm-1, which is not possible with the employed Raman setup. Combining both methods allows recording of vibrational spectra from 100 to 4000 cm-1 while the fingerprint region is covered by both techniques. The detailed analysis and discussion of the experimental data will be done in the following subsections in context with the computational results. 3.1. HSO4- Anion. For hydrogen sulfate, the quantum chemical computations revealed that two principal conformations are possible (with Cs point group symmetry), i.e., the cis (CHS#1) and the trans (CHS#2), as illustrated in Figure 3, but at most one of these is stable at a given level. For the sake of completeness it should be noted that, when employing the MP2/ 6-31G* level, a third possible isomer CHS#3 (rightmost molecule in Figure 3) was suggested with minimum energy, but differing only marginally from the cis configuration CHS#1 by reduction in symmetry to C1. Which conformer is favored depends on the basis set employed for the calculation. The potential energy

Structure of [HMIM][HSO4]

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Figure 4. Potential energy as a function of dihedral angle calculated with different basis sets. Further details are given in the Supporting Information material available online.

Figure 2. (a) Experimental Raman scattering spectrum (full spectral range 100-1800 cm-1). (b) experimental IR absorption spectrum (full spectral range 500-4000 cm-1). (c) Deconvoluted IR spectrum (spectral range 2000-4000 cm-1). Enlarged spectra are given in the Supporting Information available online.

Figure 3. Lowest energy conformers of the hydrogen sulfate anion.

as a function of the H-O-S-O dihedral angle is plotted in Figure 4 and reveals this dependence (further details are given in the Supporting Information available online). In the following, the discussion focuses on the isomers CHS#1 and CHS#2. There is very little difference in energy between the two structures, although the levels that include diffuse functions generally favor the trans isomer. Adding diffuse functions to sulfur and oxygen tends to lengthen the S-O distances in the third decimal place, whereas adding polarization functions to hydrogen tends to decrease the O-H distance in the third decimal place. The obtained bond lengths are summarized in Tables 2 and 3. As concerns spectroscopy, adding diffuse functions to sulfur and oxygen tends to decrease all vibrational frequencies by tens of cm-1 except the OH twisting and stretching modes, whereas adding polarization functions to hydrogen tends to decrease the OH bending frequency and increase the OH stretching fre-

quency. In general, there is extensive coupling between the HOS bending and the S-O stretching. The vibrational frequencies obtained from the quantum chemical calculations are summarized in Tables 4 and 5 for the conformers #1 and #2, respectively, together with the wavenumbers extracted from the vibrational spectra. For the following discussion, only the results from 6-31++G** level calculations are considered. Owing to their similar structure, the frequencies of the main conformers #1 and #2 are similar as well. In general, there is no obvious trend that the computational method provides data that agrees to a higher degree with the experimental data. A significant difference between both conformers is found for the SO4 deformation. For CHS#1, a lower frequency (368 cm-1 and 360 cm-1 for MP2 and B3LYP) is predicted than for CHS#2 (403 cm-1 and 394 cm-1, correspondingly). Here the Raman mode at 400 cm-1 favors the existence of CHS#2. The same holds for the 506 cm-1 Raman frequency, which agrees better with the CHS#2 calculations. The signals at 527 and 528 cm-1 in the IR and Raman spectra, respectively, may be assigned to either conformer. The same holds for the 683 cm-1 Raman mode. The calculated frequencies in the range 900-1300 cm-1 do not give a clear picture in comparison with the experimental data as the Raman and IR modes in this part of the spectrum may be assigned to vibrations of either HSO4- conformer. A highly interesting feature, however, that can be observed in the experimental spectra are the lines at 903 cm-1 (IR), 959 cm-1 (Raman), 1157/1158 cm-1 (Raman/IR), 1364 cm-1 (Raman) and 2963 cm-1 (IR). Comparing these frequencies with values reported by Horn et al.31,32 for sulfuric acid, we can find excellent agreement. They assigned signals in the IR spectrum to the following modes: νs(S-(OH)2) at 902 cm-1, νas(S-(OH)2) at 957 cm-1, νs(OdSdO) at 1158 cm-1, νas(OdSdO) at 1359 cm-1, and νasS(O-H)2 as well as νsS(O-H)2 at 2960 cm-1. Of particular interest are the similarities concerning the modes from moieties containing (OH)2. This indicates that our experimental spectra show signals from sulfuric acid H2SO4. This can either originate from a residual from the RTIL synthesis, or there must be a proton transfer taking place in the ionic liquid. Considering the latter, the added proton may have three possible origins: a neighboring anion, a cation, or water. The first option would mean that two HSO4- ions interact with each other to form H2SO4 and SO42-. However, this is not supported by our data, which do not show characteristic signals from sulfate (see, e.g., Rudolph33,34 who studied HSO4- in aqueous solutions). The second option is proton transfer from the HMIM+ cation to the anion. Similar behavior has been predicted by Dhumal et al.20 employing DFT calculations to study interionic interactions of the 1-ethyl-3-methylimidazolium acetate ion pair. The simulations suggested proton transfer of the

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TABLE 2: CHS#1 Structural Parameters in Terms of Bond Length in Å (CHS#3 in Parentheses). MP2 6-31G* S-O(H) S-O(σ) S-O(non-σ) H-O

1.7023 1.4859 1.4748 0.9741

B3LYP

6-31+G*

6-31G**

6-31++G**

6-31G*

6-31+G*

6-31G**

6-31++G**

1.7074 1.4761 1.4866 0.9777

1.7013 1.4859 1.4748 0.9670

1.7110 1.4916 1.4794 0.9691

1.7117 1.4887 1.4774 0.9711

1.7177 1.4923 1.4799 0.9717

1.7090 1.4889 1.4773 0.9679

1.7161 1.4924 1.4799 0.9683

(1.7020) (1.4852) (1.4736) (0.9743)

TABLE 3: CHS#2 Structural Parameters in Terms of Bond Length in Å MP2 S-O(H) S-O(σ) S-O(non-σ) H-O

B3LYP

6-31G*

6-31+G*

6-31G**

6-31++G**

6-31G*

6-31+G*

6-31G**

6-31++G**

1.7000 1.4712 1.4803 0.9754

1.7112 1.4919 1.4795 0.9765

1.6996 1.4711 1.4801 0.9678

1.7079 1.4762 1.4864 0.9702

1.7099 1.4740 1.4827 0.9724

1.7138 1.4767 1.4866 0.9728

1.7075 1.4739 1.4825 0.9688

1.7128 1.4768 1.4866 0.9693

TABLE 4: CHS#1 Vibrational Frequencies (cm-1, CHS#3 in Parentheses). MP2 assignment

6-31G*

OH twist A′′ SO4 def A′′ SO4 def A′ SO4 def A′ SO4 def A′ SO4 def A′′ SO(H) A′ SO A′ HOS bend A′ SO A′′ SO A′ OH str A′

31i (53) 370 (365) 380 (392) 538 (540) 546 (544) 561 (562) 735 (735) 1049 (1051) 1151 (1156) 1307 (1301) 1325 (1330) 3762 (3759)

6-31+G*

B3LYP

6-31G**

158i 363 369 523 530 541 693 1019 1119 1251 1266 3735

80 371 381 538 546 562 734 1048 1130 1307 1317 3874

6-31++G** 153i 364 368 523 529 541 690 1019 1100 1252 1259 3850

6-31G* 72 360 368 521 530 546 713 1005 1115 1257 1283 3749

6-31+G* 127i 357 361 512 518 530 681 984 1094 1213 1237 3750

6-31G** 113 361 370 522 530 547 715 1005 1104 1257 1277 3813

experiment 6-31++G** 117i 357 360 512 518 530 681 983 1081 1213 1229 3818

IR

Raman

506 527 1085 1095 1230

528 683 1000 1022 1074 1094 1207 1229 1268

TABLE 5: CHS#2 Vibrational Frequencies (cm-1). MP2 assignment OH twist A′′ SO4 def A′′ SO4 def A′ SO4 def A′ SO4 def A′ SO4 def A′′ SO(H) A′ SO A′ HOS bend A′ SO A′′ SO A′ OH str A′

6-31G* 72i 377 413 536 549 563 737 1057 1185 1284 1341 3739

6-31+G* 107 370 405 520 531 545 697 1025 1152 1220 1285 3713

6-31G** 116i 377 411 536 548 563 735 1057 1160 1285 1335 3854

B3LYP 6-31++G** 81 370 403 520 531 544 692 1025 1133 1221 1279 3829

C2-H to the acetate anion, but their experimental spectra did not support this option. For the present study we can assume that, in the case of [HMIM][HSO4], this proton transfer might take place, although HSO4- is not a strong base, and, as a consequence, the equilibrium of the corresponding reaction forming sulfuric acid and a carbene would lie to the reactants’ side. The third option is proton transfer between hydrogen sulfate and water molecules. Although the ionic liquid was dried before use, the experimental procedure did not provide an inert atmosphere. Therefore it is likely that a small amount of water was present in the ionic liquid owing to its hygroscopic nature. However, considering the involved reaction where HSO4- plus H2O forms H2SO4 plus OH-, we must again stress the low basicity of hydrogen sulfate. In summary, the origin of the sulfuric acid can not be fully resolved at this point, and, as it is not the main scope of this work, it requires further investigation to be performed in a future study.

6-31G* 107i 367 398 517 534 548 714 1013 1152 1236 1294 3723

6-31+G* 101 364 395 508 521 533 689 989 1129 1185 1252 3727

6-31G** 137i 367 397 517 535 548 715 1014 1136 1237 1289 3792

experiment 6-31++G** 74 364 394 507 521 533 682 989 1114 1185 1246 3797

IR

Raman

527

400 506 528

1230 1283

683 1000 1134 1175 1229 1268 1300

3.2. HMIM+ Cation. For the 1-hexyl-3-methylimidazolium cation, three minimum energy conformations, CIm#1, CIm#2, and CIm#3, were calculated. They are illustrated in Figure 5. The energies of these three conformations are within 4 kJ/mol of each other, with CIm#1 < CIm#3 < CIm#2. Further details can be found in the Supporting Information. Obviously the imidazolium ring and the methyl moiety are identical in all conformers; hence only changes in the hexyl chain occur. All three options are relatively similar in structure: CIm#1, CIm#2, and CIm#3 show straight hexyl chains where all CH2 groups are in staggered (anti) conformation relative to each other. The only difference is the orientation of the chain around the CR-Cβ axis of rotation. The vibrational frequencies derived from the DFT calculations are summarized in Table 6 together with the IR and Raman data from the experiments. Interestingly, although exhibiting very similar structure, there are several distinct differences in the vibrational spectra of the individual HMIM+ conformers.

Structure of [HMIM][HSO4]

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TABLE 6: HMIM+ Vibrational Frequencies (cm-1)a B3LYP/6-31G* twisting N-CR twisting CR-Cβ twisting N-CR + chain twisting twisting N-CMe twisting Cβ-Cγ twisting N-CMe twisting Cβ-Cγ + Cδ-Cε twisting Cγ-Cδ bend chain oop bend at N + bend Cγ twisting Cδ-Cε oop bend at NMe + oop bend at Nchain + twisting Cγ-Cδ twisting Cβ-Cγ + Cδ-Cε twisting Cβ-Cγ + oop bend at NMe oop bend at NMe + bend Cγ oop bend at NMe + bend Cβ+ bend Cε twisting Cε-Cω ip bend at N + ip bend at N oop bend at N + oop bend at N+ bend chain oop bend at NMe + oop bend at NChain oop bend at NMe - oop bend at NChain ip bend at NMe + ip bend at Nchain ip bend at NMe - ip bend at Nchain ip bend at NMe - ip bend at Nchain bend chain ip bend at NMe - ip bend at Nchain + bend chain ip bend at NMe - ip bend at Nchain + bend chain bend chain bend chain ip ring def oop ring def + twisting Cβ-Cγ oop ring def oop ring def oop ring def CH2 rocking + ip ring def + N-C stretch CH2 rocking ring CH oop bend CH2 rocking CH2 rocking ring CH oop bend CH2 rocking ring CH oop def. CH2 rocking CH2 rocking+twisting CωH3 rocking C-C stretching C-C stretching CH2 twisting C-C stretching ring str def ring str. + CH2 wagging + C-C stretch C-C stretch + ring str. ring stretch C-C stretch C-C stretch C-C stretch CMeH3 rock CringH ip bend C-C stretch CMeH3 rock CH2 twisting CringH ip bend CRH2 twisting + CH2 rocking CH2 twisting CH2 wagging CH2 twisting CH2 twisting + CringH ip bend CH2 wagging + CringH ip bend CH2 twisting + CringH ip bend CH2 twisting + CringH ip bend CH2 twisting

CIm#1

CIm#2

CIm#3

29 49

21 161 48

26 44

54 67 78 125

55 70

IR

Raman

74 36

90 124

96 124

136

135 174

206

210

247

246

133 149 215 238 248 280 295

234

256 257 290 408 438 450

301 415 340 404 501 593

268 307

342 395 423

344 400 425 455 506 590

505 598

627 631 677 739 740 748 754 807 825 871 903 905 1007 1013 1037 1040 1044

624 631 658 701 740 748 772

631 659 697 739 748 772

824 847 871 880

824 849 871 881

906 967 1017

906 967 1015

970 1006

1035 1037 1038

1030

1035

663 697 754 821 860

763 806 831 856 891

903 1000

1038 1051

1052 1068 1107 1132 1141 1158 1162 1181 1230 1271 1281

1068 1073 1107 1132 1138 1158 1161 1177 1227 1250

1072 1074 1107 1132 1140 1159 1149 1183 1229 1255

1074 1134 1158 1225 1230

1157 1175 1229 1268

1283 1289

1311 1329

1310 1316

1289 1309 1320 1339

1300 1312 1316

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TABLE 6: Continued

a

B3LYP/6-31G*

CIm#1

CH2 wagging CH2 twisting CH2 twisting CRH2 twisting + C-N ring stretch CH2 wagging + CRH2 twisting CRH2 wagging CH2 wagging CH2 wagging C-N ring stretch + CRH2 wagging + CMeH3 bending CωH3 bending CRH2 twisting+ C-N ring stretch CMeH3 bending CβH2 bending CMeH3 bending CH2 bending CδH2 bending CH2 bending CRH2 bending CH2 bending CωH3 bending CωH3 bending CMeH3 bending CH2 bending CH2 bending ring stretching ring stretching CγH2 sym str CδH2 + CεH2 sym str CδH2-CεH2 sym str CγH2 sym str - CεH2 sym str CγH2 sym str + CεH2 sym str CδH2 asym str - CγH2 asym str - CεH2 asym str CβH2 sym str CωH3 sym str CβH2 sym str CγH2 asym str - CεH2 asym str CβH2 - CγH2 - CδH2 - CεH2 asym str CβH2 + CγH2 asym str CMeH3 sym str CRH2 sym str CωH3 asym str CωH3 asym str CRH2 asym str CMeH3 asym str CMeH3 asym str CH ring asym CH ring sym CH ring sym

1336 1345 1350 1353

CIm#2

CIm#3

IR

Raman

1334 1343 1346 1354 1364 1395 1409 1417 1424 1440 1449 1474 1500 1504

1390 1408 1417 1426 1440 1447 1475 1504 1507 1509 1512

1509 1512 1513 1523 1524 1527

1520 1524 1527 1529 1537 1607 1613

1346 1354 1362 1396 1409 1418 1424 1438 1449 1475 1501 1504

1418 1428 1457

1508 1513 1514

1445 1457 1484

1509 1520

1521

1523 1524 1527

1534 1605 1614 3011 3017 3024

1533 1607 1614 3008 3017 3025

3021 3029 3038 3040 3043

3035 3041 3044

3033 3043 3044 3047

3052 3068 3084 3092 3092 3105 3116 3143 3175 3189 3300 3305 3317

3046 3067 3083 3092 3097 3106 3117 3145 3175 3190 3302 3304 3319

3016

1364

3068 3085 3092 3095 3107 3117 3142 3175 3190 3300 3307 3317

1535 1611 3025

3051 3072

3114 3146 3208

oop: out-of-plane; ip: in-plane.

In general, the spectrum below 200 cm-1 is dominated by the twisting modes; in the range 200-900 cm-1, the deformation/ bending vibrations appear; the region 900-1150 cm-1 incorporates mainly C-C stretches; and between 1150 and 1600 cm-1, further twisting, wagging, and bending modes dominate the spectrum. The region beyond 3000 cm-1 is well known as the CH stretching region.

Bending modes of the hexyl chain occur at 450 cm-1 (CIm#1), 340, 404, and 501 cm-1 (CIm#2), and 342, 395, 423, and 505 cm-1 (CIm#3). They are in good agreement with the Raman lines at 344, 400, 425, 455, and 506 cm-1, indicating that all three conformers are present in the ionic liquid. The IR signal at 663 cm-1 shows agreement with the out-of-plane ring deformation modes of CIm#2 (658 cm-1) and CIm#3 (659 cm-1) rather than

Figure 5. Low-energy conformers of the 1-hexyl-3-methylimidazolium cation.

Structure of [HMIM][HSO4] with CIm#1 (677 cm-1). The same holds for the 697 cm-1 IR mode in comparison with the in-plane ring deformation and N-C stretching vibrations of CIm#2 (701 cm-1) and CIm#3 (697 cm-1) in contrast to CIm#1 (739 cm-1). However, the CH2 rocking vibrations of CIm#1 at 754, 807, and 903 cm-1 are well recognized in the experimental data at 754 cm-1 (IR), 806 cm-1 (Raman), and 903 cm-1 (IR), respectively. Concerning C-C stretching, the 970 cm-1 IR line agreed well with the 967 cm-1 modes from CIm#2 and CIm#3. On the other hand, a further C-C stretch predicted at 1007 (CIm#1), 1017 (CIm#2), and 1015 cm-1 (CIm#3) shows the best agreement with the 1006 cm-1 (IR) for CIm#1. The Raman line at 1074 cm-1 again meets the predicted frequencies of CIm#2 and CIm#3. Further differences can be found in the twisting and wagging region. The calculated CH2 wagging and CH2 twisting frequencies at 1271 cm-1 and 1281 cm-1, respectively, are in concert with the 1268 cm-1 Raman and 1283 cm-1 IR signals. The Raman band at 1364 cm-1, however, agrees again with wagging/ twisting modes of CIm#2 and CIm#3. Most of the other modes are relatively similar in frequency when the different conformers are compared and do show reasonable agreement with the experimental data. In the previous subsection we discussed the hydrogen sulfate conformers. Recalling that the experimental data suggest strong interactions between anion and cation indicating a proton transfer, such behavior should exert influence on the HMIM+ vibrational structure as well. For interactions of the cation with anions and cosolvent molecules, the acidic hydrogen atom at the C2 position in imidazolium-based ionic liquids (cf. Figure 1) was identified as the predominant hydrogen bonding donor site.35,36 Dhumal et al.20 have shown that strong hydrogen bonding interactions at the C2-H can result in a substantial frequency shift of the C2-H stretching vibration. When hydrogen bonding takes place, a redistribution of charge occurs and eventually leads to a weakening of the covalent C2-H bond. This in turn results in a lower vibrational frequency, which can be observed as a red-shifted line in an experiment. The redshift was predicted to be about 1200 cm-1 from the 3202 cm-1 of the isolated cation to 2005 or 2024 cm-1 depending on the exact interaction geometry.20 In our IR spectra analyzed herein, there are bands at 1912 cm-1 and in the range 2000-3000 cm-1, which may be attributed to such shifted C-H stretching vibrations. A detailed investigation of the ion-ion interactions, however, is not the scope of the present study. 4. Summary and Conclusion In the present paper we have studied the conformational isomerism of the ionic liquid [HMIM][HSO4]. For this purpose, DFT calculations as well as vibrational spectroscopy in terms of infrared absorption and Raman scattering have been employed. For the hydrogen sulfate anion, the computational methods predicted two principal lowest energy conformations, i.e., cis and trans configuration. A comparison of the vibrational frequencies derived from the DFT calculations and the experimental data revealed that the trans conformer is the favored isomer in the ionic liquid under investigation. For the 1-hexyl-3-methylimidazolium cation, three different low-energy conformations were studied. They differed in the orientation of the hexyl chain only, while the ring structure remained the same. The comparison of vibrational frequencies with IR and Raman data showed good agreement for all three conformations, indicating their presence in the ionic liquid. A predominant isomer could not be identified.

J. Phys. Chem. A, Vol. 114, No. 24, 2010 6719 Beyond revealing the presence of different conformational states of the ions, the experimental spectra indicate strong interionic interactions. Vibrations of sulfuric acid could be observed, indicating a possible proton transfer from either the cation to the anion or from water molecules to the anion. The first option is further supported by the appearance of vibrational modes around 2000 cm-1 in the IR spectrum, which could tentatively be assigned to C2-H stretching vibrations red-shifted as a result of strong interionic hydrogen bonds as a prerequisite of proton transfer. This phenomenon, however, requires further investigation using theoretical as well as experimental methods, which is the scope of ongoing work in our groups. Acknowledgment. Part of the experimental work was financed by the German Research Foundation (DFG), which funds the Erlangen Graduate School in Advanced Optical Technologies in the framework of the German Excellence Initiative and the Priority Program SPP-1191. J.K. thanks Katharina Obert and Peter Wasserscheid for providing the ionic liquid, and Kristina Noack and Markus Wangler for supporting the Raman experiment and data analysis. C.C.P. thanks ACENet for computational resources. Supporting Information Available: Enlarged IR and Raman spectra, detailed diagrams of the hydrogen sulfate potential energy as a function of the HOSO torsion angle for all employed basis sets, and a table containing electronic energies. This information is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Plechkova, N. V.; Seddon, K. R. Applications of ionic liquids in the chemical industry. Chem. Soc. ReV. 2008, 37, 123. (2) Wasserscheid, P.; Keim, W. Ionic liquids - New “solutions” for transition metal catalysis. Angew. Chem., Int. Ed. 2000, 39, 3773. (3) Welton, T. Ionic liquids in catalysis. Coord. Chem. ReV. 2004, 248, 2459. (4) Zhao, D. B.; Wu, M.; Kou, Y.; Min, E. Ionic liquids: Applications in catalysis. Catal. Today 2002, 74, 157. (5) Sheldon, R. A.; Lau, R. M.; Sorgedrager, M. J.; van Rantwijk, F.; Seddon, K. R. Biocatalysis in ionic liquids. Green Chem. 2002, 4, 147. (6) Roosen, C.; Mu¨ller, P.; Greiner, L. Ionic liquids in biotechnology: Applications and perspectives for biotransformations. Appl. Microbiol. Biotechnol. 2008, 81, 607. (7) Zhao, H.; Xia, S. Q.; Ma, P. S. Use of ionic liquids as ‘green’ solvents for extractions. J. Chem. Technol. Biotechnol. 2005, 80, 1089. (8) Han, X.; Armstrong, D. W. Ionic liquids in separations. Acc. Chem. Res. 2007, 40, 1079. (9) Armand, M.; Endres, F.; MacFarlane, D. R.; Ohno, H.; Scrosati, B. Ionic-liquid materials for the electrochemical challenges of the future. Nat. Mater. 2009, 8, 621. (10) Galinski, M.; Lewandowski, A.; Stepniak, I. Ionic liquids as electrolytes. Electrochim. Acta 2006, 51, 5567. (11) Bermudez, M. D.; Jimenez, A. E.; Sanes, J.; Carrion, F. J. Ionic liquids as advanced lubricant fluids. Molecules 2009, 14, 2888. (12) Rogers, R. D.; Seddon, K. R. Ionic liquids - Solvents for the future. Science 2003, 302, 792. (13) Kiefer, J.; Fries, J.; Leipertz, A. Experimental vibrational study of imidazolium-based ionic liquids: Raman and infrared spectra of 1-ethyl3-methylimidazolium bis(trifluoromethylsulfonyl)imide and 1-ethyl-3-methylimidazolium ethylsulfate. Appl. Spectrosc. 2007, 61, 1306. (14) Turner, E. A.; Pye, C. C.; Singer, R. D. Use of ab initio calculations toward the rational design of room temperature ionic liquids. J. Phys. Chem. A 2003, 107, 2277. (15) Kirchner, B. Theory of complicated liquids: Investigation of liquids, solvents and solvent effects with modern theoretical methods. Phys. Rep. 2007, 440, 1. (16) Berg, R. W. Raman spectroscopy and ab-initio model calculations on ionic liquids. Monatsh. Chem. - Chem. Mon. 2007, 138, 1045. (17) Holomb, R.; Martinelli, A.; Albinsson, I.; Lasse`gues, J. C.; Johansson, P.; Jacobsson, P. Ionic liquid structure: The conformational isomerism in 1-butyl-3-methyl-imidazolium tetrafluoroborate ([bmim][BF4]). J. Raman Spectrosc. 2008, 39, 793.

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