Electronic Structure and Normal Vibrations of the 1-Ethyl-3

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Electronic Structure and Normal Vibrations of the 1-Ethyl-3-methylimidazolium Ethyl Sulfate Ion Pair Nilesh R. Dhumal,† Hyung J. Kim,*,†,‡,^ and Johannes Kiefer§,|| †

Department of Chemistry, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, United States School of Computational Sciences, Korea Institute for Advanced Study, Seoul 130-722, Korea § School of Engineering, University of Aberdeen, Fraser Noble Building, Aberdeen AB24 3UE Scotland, United Kingdom Erlangen Graduate School in Advanced Optical Technologies, University Erlangen-Nuremberg, D-91058 Erlangen, Germany

)

‡

bS Supporting Information ABSTRACT: Electronic and structural properties of the ion pair 1-ethyl-3-methylimidazolium ethyl sulfate are studied using density functional methods. Three locally stable conformers of the ion pair complex are considered to analyze molecular interactions between its cation and anion. Manifestations of these interactions in the vibrational spectra are discussed and compared with experimental IR and Raman spectroscopy data. NBO analysis and difference electron density coupled with molecular electron density topography are used to interpret the frequency shifts of the normal vibrations of the ion pair, compared to the free anion and cation. Excitation energies of low-lying singlet excited states of the conformers are also studied. The density functional theory results are found to be in a reasonable agreement with experimental UV/vis absorption spectra.

1. INTRODUCTION Room-temperature ionic liquids (RTILs), often referred to as a “new” class of materials, have been the object of intensive scrutiny recently. Even though the first study of RTILs dates back to early last century,1 it is only in the last couple of decades that their potential for use in such diverse areas as chemical synthesis, catalysis,2
4 separation technology,5,6 and electrochemistry7,8 was recognized. During this period, RTILs have witnessed a rapidly growing interest from both academia and industry. A great deal of theoretical and experimental attention has been paid to the determination of structural, dynamic, and thermodynamic properties of RTILs.9 Our own effort has been directed toward, among others, quantitative understanding of molecular interactions of RTIL cations and anions by employing density function theory (DFT) combined with experimental vibrational spectroscopies. This has provided useful insight into the influence of molecular interactions on ion pair geometry and vibrational spectra. For example, according to our recent study on 1-ethyl-3-methylimidazolium acetate, charge transfer through interionic hydrogen bonds plays an important role in vibrational frequency shifts of ion pair complexes, both their direction and magnitude, with respect to free ionic species.10,11 In the present work we extend our prior study to investigate 1-ethyl-3-methylimidazolium ethyl sulfate. This particular RTIL was one of the first produced on a technical scale and commercially available. Its synthesis was described by Holbrey et al.12 and aspects of reactor design were discussed by Jess et al.13 In addition to its use as a green solvent, this RTIL has a potential application in the field of separation technology, in particular, separation of azeotropic mixtures.14 As such, a considerable r 2011 American Chemical Society

amount of effort has been focused on the determination of the macroscopic thermophysical and transport properties in neat 1-ethyl-3-methylimidazolium ethyl sulfate and its mixtures with other chemicals.15
18 Spectroscopic studies were also conducted in recent years with the aim of obtaining a proper understanding of its molecular and microscopic structure. Kiefer et al. analyzed the vibrational structure experimentally by means of IR and Raman spectroscopies.19 Infrared and NMR spectroscopy combined with DFT were used by Zhang et al. to study hydrogen bond interactions in binary mixtures of 1-ethyl-3-methylimidazolium ethyl sulfate and water.20 Sarkar et al. examined molecular interactions of ions and cosolvent molecules of similar binary mixtures using time-resolved fluorescence techniques.21 Molecular orientations of 1-ethyl-3-methylimidazolium ethyl sulfate at the liquid surface were investigated via vibrational spectroscopy with the sum frequency generation method.22 By employing DFT, we study the electronic and vibrational structures of the ion pair 1-ethyl-3-methylimidazolium ethyl sulfate. We examine ion pair geometry and related vibrational spectra in detail and compare with experimental IR absorption and Raman scattering measurements. The directions of frequency shifts of ion pair complexes with respect to free ions are analyzed using difference electron density coupled with electron density topography. Low-lying excited electronic states of ion pairs are also studied and compared with UV/vis absorption spectroscopy measurements. While the present work bears Received: December 23, 2010 Revised: March 8, 2011 Published: March 29, 2011 3551

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Figure 1. Optimized geometries of the different conformers of the 1-ethyl-3-methylimidazolium ethyl sulfate ion pair.

some similarity to our earlier study on 1-ethyl-3-methylimidazolium acetate,11 it is the first detailed study of the interionic interactions in 1-ethyl-3-methylimidazolium ethyl sulfate, which are more complicated than those in the former. The outline of this paper is as follows: Brief descriptions of the computational and experimental methods are given in section 2 and 3, respectively. In section 4, the DFT results on ion pair geometry and vibrational structures are discussed and compared with the measurements. NBO analysis and difference electron density calculations are also presented there. Section 4 concludes.

2. COMPUTATIONAL METHOD Geometry optimization calculations were performed on isolated 1-ethyl-3-methylimidazolium and ethyl sulfate ions, and their ion pair using the GAUSSIAN-03 program23 by employing the internally stored 6-31G(d,p) and the hybrid density functional theory that incorporates Becke’s three-parameter exchange with Lee, Yang, and Parr’s (B3LYP) correlation functional method.24,25 The stationary point geometries were confirmed to be local minima on the potential energy surface. Specifically, all of their vibrational frequencies were found to be real with no imaginary frequency component. The normal modes were assigned by visualizing the displacement of atoms around their equilibrium positions by using the visualization package UNIVIS-2000.26 The molecular electron density topography was studied and critical points were identified.27 Four types of nondegenerate critical points of rank 3 were identified in the three-dimensional space. These include: maxima (3, 3), e.g., nuclear positions; minima (3,
3) generally known as cage critical points; and two types of saddle points, (3,
1) and (3, þ1), respectively referred to as the bond critical and ring critical points. Here R and σ of (R, σ) represent, respectively, the number and algebraic sum of eigenvalues of the Hessian matrix. Natural bond orbital analysis was performed within the B3LYP/ 6-31G(d,p) framework of theory.28 Difference electron density

ΔF was calculated via ΔF = Fcomplex
(Ffree anion þ Ffree cation), where the subscripts free cation and anion denote that the individual ions comprising the ion pair complex are in vacuo. Difference electron density maps showing contours from (0.001 to (0.0009 au were derived (au: atomic units).

3. EXPERIMENTAL METHOD Chemical. 1-Ethyl-3-methylimidazolium ethyl sulfate was purchased from Solvent Innovation GmbH, Germany (ECOENG 212, 99%). Infrared Spectroscopy. The IR spectrum from 500 to 4000 cm
1 was recorded on an attenuated total reflection (ATR) module with a Nicolet Model 360 FTIR at 2 cm
1 nominal resolution. The number of reflections at the diamond crystal surface is 1, and the penetration depth of the system is approximately one-fifth of the wavelength. For the measurement, a droplet of the IL was placed on the ATR crystal and covered with a glass cap to avoid water absorption from the surrounding air. Raman Spectroscopy. Raman spectra were recorded with two different experimental setups. First, the spectrum from 500 to 3800 cm
1 was recorded with a spectral resolution of about 8 cm
1 using a frequency doubled, continuous-wave Nd:YAG laser at 532 nm. Elastically scattered light was blocked by an OG550 color glass filter. Second, the spectrum between 300 and 1800 cm
1 was recorded with a spectral resolution of 2
3 cm
1 using a grating stabilized diode laser operating at 785 nm. Elastically scattered light was suppressed by a long-pass filter (790 nm cutoff wavelength). Further details can be found in ref 29. For the measurements, the IL was filled into a 10 mm fused silica cuvette. UV/Vis Spectroscopy. The UV/vis spectrum from 200 to 1000 nm was recorded by a Shimadzu UV-3600 spectrophotometer with a spectral resolution of 2 nm. For the measurement the IL was filled into a fused silica cuvette providing a defined absorption path length of 10 mm. 3552

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Table 1. Relative Stabilization (kJ mol
1) Energies of Three Lowest Energy Conformers of 1-Ethyl-3-methylimidazolium Ethyl Sulfate Ion Paira conformer

ΔE

anion

cation

C1

C2

C3

N1
C2

1.338

1.337

1.339

1.341

C2

46.81 (45.62)

C2
N3

1.339

1.338

1.343

1.341

C3

54.31 (51.89)

N3
C4

1.383

1.385

1.385

1.385

C4
C5

1.364

1.363

1.363

1.364

N1
C5 N3
C6

1.382 1.470

1.385 1.468

1.381 1.461

1.383 1.463

N1
C7

1.483

1.479

1.476

1.472

C7
C8

1.526

1.526

1.528

1.527

C2
H9

1.079

1.088

1.078

1.078

C4
H10

1.079

1.078

1.078

1.100

C5
H11

1.078

1.078

1.095

1.079

C6
H12

1.089

1.092

1.090

1.091

C6
H13 C6
H14

1.091 1.091

1.092 1.092

1.092 1.092

1.091 1.090

C7
H15

1.092

1.090

1.094

1.093

C7
H16

1.093

1.094

1.094

1.093

C8
H17

1.093

1.092

1.094

1.093

C8
H18

1.093

1.094

1.094

1.094

C8
H19

1.093

1.095

1.094

1.094

C1

a

Table 2. Calculated Bond Lengths (Å) in Free Cation and Anion and C1
C3 Conformers of 1-Ethyl-3-methylimidazolium Ethyl Sulfate Ion Pair

0.0 (0.0)

Zero point corrected energies are displayed in parentheses.

4. RESULTS AND DISCUSSION 4.1. Geometric Analysis. The DFT results for optimized

geometries of three lowest energy conformers of the 1-ethyl-3methylimidazolium ethyl sulfate ion pair are shown in Figure 1, and their relative SCF energies are presented in Table 1. Interionic interactions via C
H---O hydrogen bonds are denoted as dashed lines in Figure 1. We note that the same criteria for intermolecular hydrogen bond interactions as in ref 13, based on Popelier’s criteria30,31 were employed in the present study. To be specific, the separation between the H atom of a C
H group of the cation and an O atom of the anion is less than 2.5 Å and a bond critical point (BCP), i.e., critical point of type (3,
1), is present between the two atoms. In this description, the lowest energy conformer C1 exhibits five interactions between the cation and anion, whereas four and three interactions are predicted for the C2 and C3 conformers respectively. We notice that C2
H9, C5
H11, and C4
H10 bonds of, respectively, the C1, C2, and C3 conformers show bifurcated interactions. It is interesting to note that the cation
anion interactions of 1-ethyl-3-methylimidazolium ethyl sulfate in Figure 1 differ markedly from those of a similar ion pair, 1-ethyl-3-methylimidazolium acetate, we studied previously.11 Specifically, two oxygen atoms O21 and O22 of the ethyl sulfate anion interact not only with the C2 and methyl group hydrogen atoms of the cation but also with those of the ethyl group in the most stable C1 conformer. Furthermore, C2 hydrogen as well as O21 and O22 of the anion shows bifurcated interactions. In the case of 1-ethyl-3methylimidazolium acetate, however, hydrogen atoms of the cation ethyl group do not participate in interactions with the anion oxygen atoms in its most stable conformer.11 In addition, no bifurcated interactions were found there.11 Another interesting difference is that while C2 hydrogen atom of the cation does not interact with the anion in C2 and C3 conformers of the present 1-ethyl-3-methylimidazolium ethyl sulfate, it actively participate in the corresponding conformers of 1-ethyl-3-methylimidazolium acetate. Thus despite the apparent similarity of these two ion pairs, i.e., their ion-pair interactions are mainly effected via two oxygen atoms of the anions, their details are quite different. The results for the geometries of the three different conformers in Figure 1 are compiled in Table 2. For comparison, the bond lengths of isolated ions are also presented there. It should be noticed that the molecular interactions induce significant changes in ion geometries. As mentioned above, a different number of C
H---O interactions are predicted for the C1, C2, and C3 conformers. The strength of O---H interactions can be correlated with the weakening, i.e., elongation, of C
H bonds participating in the interactions.32 If we adopt this view, the interionic interactions involving the C4
H10 bond in the C3 conformer are the strongest, while those associated with

S20-O21

1.482

1.491

1.480

1.481

S20-O22 S20-O23

1.482 1.473

1.496 1.467

1.503 1.469

1.499 1.471

S20-O24

1.715

1.666

1.673

1.671

O24
C25

1.418

1.439

1.435

1.440

C25
C26

1.523

1.519

1.519

1.523

C25
H27

1.100

1.096

1.098

1.096

C25
H28

1.100

1.097

1.096

1.096

C26
H29

1.095

1.094

1.094

1.092

C26
H30 C26
H31

1.095 1.097

1.094 1.095

1.095 1.094

1.094 1.095

O21----H10

2.427

1.824

O21----H11 O22----H10

2.432

O22----H11

1.832

O22----H17

2.495

O21----H9

1.920

O21----H14

2.285

O22----H9

2.183

O22----H15

2.396

O22----H19

2.658

2.653 2.168

C2
H9 of C1 are the weakest. This indicates that C4
H10 and O21 of C3 form the strongest interionic H-bond among all conformers considered here. The result of their O---H separation being the shortest lends further support to this interpretation. It should be remarked here that the stabilization of a conformer is governed not only by the strength of individual C
H---O interactions but also by the total number of these interactions between the cation and anion. This explains why the C1 conformer characterized by five interactions is more stable than C2 (four interactions) and C3 (three interactions) even though C4
H10---O21 of C3 is the strongest interionic H-bond. 3553

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Figure 2. Experimental vibrational spectra: Raman spectrum (upper diagram) and infrared spectrum (lower diagram). In the inset, the fingerprint regions are enlarged with higher spectral resolution.

Analogous to the cation case, the anion’s SO bonds participating in the interactions become elongated compared to an isolated anion. In contrast, the bonds that do not participate in the molecular interactions become shortened with respect to those of the free ions. Examples include the S20
O23, S20
O24 and C25
C26 bonds of the anion, which exhibit significant bond contraction in all conformers. 4.2. Molecular Electron Density Topography Analysis. Molecular electron density (MED) topography is widely used to study both the intra- and intermolecular interactions in molecular systems. Some of the important criteria proposed by Popelier30,31 for the existence of X
H 3 3 3 Y type molecular interactions, viz., the presence of a BCP along the bond path H 3 3 3 Y and a proper value of the electron density at the BCP, are based on MED topography. (Other criteria include: penetrations of H and Y atoms, increase in the net charge of the hydrogen atom, energetic destabilization of hydrogen, decrease of dipolar polarization and decrease of hydrogen atomic volume.) These topographical properties, when correlated with interaction energy and internuclear distances of the complex, show a linear relationship. To gain quantitative insight into molecular interactions in the present ion pair system, we have analyzed the molecular electron density topography of its three lowest energy conformers in Figure 1 as well as the isolated ions. We note that the Fbcp value can serve as a good measure for the strength of interactions.33 Among all interionic molecular interactions indicated in Figure 1,

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the maximum BCP electron density is obtained for C4
H10--O21 of C3 with Fbcp = 0.0363. This further confirms the conclusion above that this interionic bond is the strongest. Also Fbcp of the anion SO bonds that do not interact with the cation was found to increase in the ion pair complex, compared to the free ethyl sufate anion. This indicates that these noninteracting SO bonds become stronger in the former than in the latter. This trend is in line with the contraction of these bonds upon ion pair formation observed above. A similar result was obtained for the anion C
C bond. For detailed information on the electron density Fbcp at BCPs associated with various interactions, the reader is referred to the Supporting Information. We add a cautionary remark: While the strength of strongest X
H 3 3 3 Y type interaction plays an important role in the determination of the conformer stability, i.e., SCF energy, many other factors, such as the number of molecular interactions as well as electrostatic interactions between moieties that do not form such bonds, also contribute. Therefore it is not inconsistent that C3 is higher in energy than C1 even though C4
H10---O21 of the former is the strongest interionic hydrogen bond among all conformers we considered. 4.3. Vibrational Frequency Analysis. The spectroscopic analysis of the normal vibrations of the system also provides useful information about its molecular interactions in that the latter often manifest as frequency shifts in the vibrational spectrum. Here we make direct comparison of DFT and experimental results to obtain quantitative understanding of the ion pair interactions of 1-ethyl-3-methylimidazolium ethyl sulfate. In the upper and lower panels in Figure 2, the experimental Raman and IR spectra are displayed, respectively. The fingerprint region is enlarged to provide further details. B3LYP derived vibrational frequencies of the lowest energy ion pair complex C1 and free cation and anion in the region 500
3200 cm
1 are compiled in Table 3. The results there are the harmonic vibrational frequencies, scaled by a factor of 0.97.34 The stretching vibration of C2
H9 shows a red shift of ∼128 cm
1 from 3202 cm
1 in the free cation to 3074 cm
1 in C1 due to its weakening in the ion pair state. This seems to be in good accord with the measurements that show bands at 3103/3108 cm
1 (IR/Raman) and 3151/3161 cm
1 but no signal around 3200 cm
1. It also suggests that there are no free cations in the ionic liquid. In other words, all 1-ethyl-3methylimidazolium cations are involved in molecular interactions with anions via their C2
H9 moiety. In contrast, a blue shift is predicted for C
H stretching vibrations of the anionic CH3 groups compared with the isolated anion. This is ascribed to the contraction and thus strengthening of these bonds in the ion-pair complex. Another interesting result in that the doublet, separated by 8 cm
1 (1570 and 1562 cm
1) in the free cation, is also present in the lowest energy conformer at 1568 and 1553 cm
1. In the experimental vibrational spectra only a single band at 1574/1570 cm
1 (IR/Raman) is observed. A closer look at the 1574 cm
1 IR band, however, shows that its line shape is not symmetric. A deconvolution yields two individual lines at 1575 and 1567 cm
1 with a difference of 8 cm
1 in perfect match with the predicted doublet. A new vibration at 1471 cm
1 is assigned to CH2 scissor and can be correlated to the 1470 cm
1 mode in the experimental IR spectrum. The CH2 scissor vibrations of the free cation at 1473 cm
1 shifts to 1461 cm
1 in C1. This accounts for the mode at 1460/1456 cm
1 observed in the IR/Raman experiments. 3554

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Table 3. Selected Vibrational Frequencies (cm
1) of Free Cation, Anion, and C1 Conformer of 1-Ethyl-3-methylimidazolium Ethyl Sulfate Ion Paira experiment vibration C2
H9 stretch

cation

anion

3202 (30)

3074 (395)

C7
H15 stretch C2
H9, C6
H14 stretch H12
C6
H13 stretch

Raman

3151, 3103

3161, 3108

3073 (16) 3062 (9)

3066 (244) 3058 (11)

H29
C26
H31 stretch

3021 (52)

H18
C8
H19 stretch

3038 (34) 3035 (16)

H29
C26
H30 stretch

3005 (57)

3027 (36)

2944 (31)

2992 (12)

symm H15
C7
H16 stretch

2991 (29)

H27
C25
H28 stretch symm H13
C6
H14 stretch

2973 (39)

symm H18
C8
H19 stretch symm H29
C26
H31 stretch

2964 (20) 2958 (26)

symm H27
C25
H28 stretch

2995 (30)

H27
C25
H28 stretch

2982 2949, 2901

2944 (31)

2938

N3
C4
H10 rock

1570 (32)

1568 (13)

N1
C2
H9 rock

1562 (54)

1553 (59)

H13
C6
H14 scissor

1574

1471 (12)

1470

H15
C7
H16, H18
C8
H19 scissor

1473 (15)

1461 (13)

1460

H12
C6
H13 rock N1
C7
H15, N3
C6
H13 rock

1448 (16) 1425 (9)

1431 (10) 1416 (11)

1435

H15
C7
H16, H18
C8
H19 rock H15
C7
H16 rock

1349 (14)

N1
C7
H16 rock

1311 (11)

1456

1390

1388 (11)

1387

1317 (10)

1328, 1302

1337

H27
C25
H28

1263 (36)

H29
C26
H30 wag

1250 (25)

N1
C2
H9 rock þ S20
O23 stretch O21
S20
O22 stretch

1238 (89)

1242

1230 (297)

S20
O23 stretch N1
C2
H9 þ O21
S20
O22 asym stretch

1570

1422

1392 (11)

H29
C26
H31, H27
C25
H28 rock

1195 (261) 1148 (108)

H29
C26
H30 twist

1117 (29)

1225 (304)

1215

1167 (341)

1169

1171, 1154

1104 (11)

1109

1112

H13
C6
H14 twist

1097 (11)

1096

1093

O24
C25 stretch

1037 (66)

1061

1060

1014 960, 912

1020 958, 915

C25
C26 stretch

1050 (70)

C2
N1
C5 stretch O21
S20
O22 symm stretch

973 (152)

1006 (22) 969 (320)

890 (43)

893 (96)

C25
C26 stretch þ C25
C26
H30 wag N1
C2
H9 wag

805 (38)

849 808

H15
C7
H16 wag

788 (16)

S20
O24 stretch

a

IR

C1

684 (291)

764

725 (238)

731

734 700

C4
C5
H11 wag

729 (21)

716 (25)

756, 708

CN bond oscillation

584 (15)

651 (23)

648

585 (16)

617, 600 577, 565

N1
C7 stretch O21
S20
O23 wag

573 (50)

SO bond oscillation

526 (31)

O21
S20
O22 bend

517 (15)

535 (52), 517 (15)

596 570 413, 339

The intensity of vibrations is reported in parentheses (kM/mol).

For the anion SO bonds participating in the molecular interactions with the cation, our calculations predict that their stretching frequency is red-shifted by 63 cm
1 in C1, compared

to the free anion frequency 1230 cm
1. In contrast, a blue shift of 30 to 1225 cm
1 results for S20
O23, which does not participate in molecular interactions. In the vicinity of this frequency, the 3555

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Table 4. Electronic Excitation Energies, Eex (eV), and the Corresponding Wavelength of the Absorption Maximum, λmax (in nm)a C1a

C2

C3

b

expt Eex

λmax

λmax

Eex

λmax

Eex

λmax

4.12 (0.001)

301

320

3.28 (0.00)

378

3.15 (0.000)

394

4.59 (0.003)

270

288

3.78 (0.001)

328

3.54 (0.002)

350

4.69 (0.001)

264

260

4.15 (0.003)

299

4.04 (0.002)

307

a

The oscillator strengths are given in parentheses. b Experimental results for λmax.

Figure 4. Difference electron density maps of the C1 conformer. Contours in the range between (0.001 and (0.0009 au are shown. The red and blue lines represent contours for ΔF < 0 and ΔF > 0, respectively. The positions of the bond critical points are marked as filled circles in black. Figure 3. Experimental UV/vis absorption spectrum. The enlarged spectrum shows the visible and near-infrared range in detail.

experimental IR spectrum shows a band at 1215 cm
1, which we tentatively assign to this mode. We found that, in general, the intensity of red-shifted vibrations is higher than that of blueshifted vibrations, consistent with the analysis by Joseph and Jemmis.35 DFT calculations yield another doublet at 1104 and 1097 cm
1 separated by 7 cm
1. This agrees well with a doublet at 1109 and 1096 cm
1 in the experimental spectra. For the O24
C25 stretch vibration, a blue shift of 39 cm
1 results in C1 relative to the free anion frequency 684 cm
1. This mode at 725 cm
1 can be correlated to the 731 cm
1 band in the experimental spectra. In addition to vibrational analysis, we have studied energies of the three lowest electronically excited singlet states of the ion pair using the TDDFT method. In Table 4, the TDDFT predictions for the C1
C3 conformers are compared with experimental data extracted from the UV/vis spectrum illustrated in Figure 3. We first notice that there is no absorption in the visible range; the RTIL is colorless to the human eye. In the UV region, however, the spectrum shows a broad absorption band with a maximum at around 226 nm. In the wing of this band on the longer wavelength side, we observe several shoulder structures at 320, 288, 260, and 234 nm. Owing to the limited spectral resolution, we do not exclude the possibility that more individual spectral lines may be present there. Nonetheless, the calculated excitation energies, in particular, those of C1, show a relatively good agreement with the shoulder bands in Figure 3. This appears

to provide further evidence that C1 is the predominant interaction configuration for the anions and cations in the ionic liquid. Due to the absence of the solvation effect in our ab initio calculations, however, care should be taken in the interpretation of their results. For the sake of completeness we note that there is a weak absorption band in the near-infrared spectral range at 902 nm, which is shown enlarged in Figure 3. This can be attributed to the third overtone modes of the stretching vibrations of aliphatic CH-groups. 4.4. Natural Bond Orbital Analysis. To gain a deeper understanding of the molecular interactions in terms of electron density, NBO analysis has been carried out using the B3LYP/ 6-31G(d,p) theory. Results for the electron density in the antibonding orbital of different bonds in the cation, anion, and three conformers of the 1-ethyl-3methylimidazolium ethyl sulfate ion pair are compiled in Table S2 of Supporting Information. The NBO analysis clearly indicates the weakening of the bonds caused by the hydrogen bonded interactions, which tend to increase electron density in the localized antibonding orbital via charge transfer from the proton acceptor to proton donor.36,37 Such a charge transfer is facilitated by the lone pairs of the proton acceptors through donation of their electron density to the antibonding orbitals of the proton donors. Thus the increase of electron density in the C
H antibonding orbitals of the conformers compared to those in the free cation and anion leads to the elongation of C
H. Hobza38,39 has pointed out the important difference between the red and blue shifts of vibrational frequencies from the NBO analysis. The transfer of a small portion of electron density to the antibonding orbital is 3556

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The Journal of Physical Chemistry A responsible for a red shift while a blue shift occurs via transfer of a larger portion of electron density to the nonparticipating part of the proton donor. We note that our results here agree well with Hobza’s point. For example, a small increase in electron density in the antibonding orbitals of the C2
H9 bond in C1 arising from the interionic interaction leads to a significant red-shift in its stretching vibrations. Finally, we consider the reorganization of electron density induced by hydrogen-bonded interactions by analyzing the difference electron density. As mentioned in section 2 above, the difference electron density, ΔF, was computed by subtracting the sum of electron densities of the individual anion and cation in their free state from that of the complex. ΔF along a cross section of the plane bisecting the O21, O22, and O23 atoms is shown in Figure 4. The ΔF contours in the range of (0.001 to (0.0009 au, are displayed. The red and blue contours denote, respectively, ΔF < 0 and ΔF > 0. It is transparent from the figure that the BCPs of the S
O and C
H bonds participating in the O 3 3 3 H interactions are located in a red region, where the electron density is depleted upon ion pair formation. This explains the red shift of their stretching frequencies caused by bond weakening. Two remaining S
O bonds are characterized by BCP in the ΔF > 0 region, viz., the electron-density is enhanced compared to free ions, indicating strengthening of these bonds. These results are also in concert with the vibrational analysis above; i.e., their frequencies are blue-shifted.

4. CONCLUDING REMARKS In the present article we have studied electronic and vibrational structures of the ion pair 1-ethyl-3-methylimidazolium ethyl sulfate. This RTIL compound is of broad interest in both academia and industry due to its potential applications in many areas, including chemical synthesis and separation. By employing a combined approach of DFT and experimental IR and Raman spectroscopies, we have investigated the influence of molecular interactions of the ion pair on its geometry, vibrational and UV/ vis spectra, and electron density topography. We have obtained a generally good agreement between the DFT results and experimental spectroscopy measurements. B3LYP theory predicts that the ion pair is characterized by several interionic C
H---O hydrogen bonds, some of which exhibit bifurcated interactions. The latter aspect contrasts with 1-ethyl-3-methylimidazolium acetate ion pair, which does not show any bifurcated interionic interactions.11 The vibrational analysis reveals that stretching vibrations of its C
H and S
O bonds participating in these interactions shift to lower frequencies, while the corresponding shifts for nonparticipating S
O and C
C are in the opposite direction. These findings are supported by the experimental data obtained by IR absorption and Raman scattering spectroscopies. The NBO analysis was employed to distinguish and understand the red and blue shifts of the vibrational modes. The electron density topographical considerations and difference electron density maps presented in this work provide additional bearing on vibrational frequency shifts, lending further support to the NBO analysis results. In addition to the vibrational analysis, the excitation energies for the first three electronically excited singlet states have shown reasonable agreement with experimental UV/vis spectroscopic data. One of the main aspects of RTILs not addressed in the present study is solvation. According to computer simulation studies, RTIL environments have a significant influence on, e.g., structure

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and free energetics of reaction systems.40 This suggests that ion pair conformations would also be affected by solvation. Though challenging, it would thus be very desirable in the future to include solvation effects in the electronic structure calculations.

’ ASSOCIATED CONTENT

bS

Supporting Information. Detailed DFT results for electron density at bond critical points and in antiboniding orbitals of C1
C3 conformers of the 1-ethyl-3-methylimidazolium ethyl sulfate ion pair and its free cation and anion (Tables S1 and S2). This material is available free of charge via the Internet at http:// pubs.acs.org.

Notes ^

Permanent address: Carnegie Mellon University.

’ ACKNOWLEDGMENT J.K. gratefully acknowledges support from the British Council (ARC 1378) and the German Research Foundation (DFG) through funding of the Erlangen Graduate School in Advanced Optical Technologies and the Priority Program SPP-1191. J.K. also thanks Alexander Douplik for providing access to the UV/vis spectrometer. ’ REFERENCES (1) Plechkova, N. V.; Seddon, K. R. Chem. Soc. Rev. 2008, 37, 123–150. (2) Welton, T. Coord. Chem. Rev. 2004, 248, 2459–2477. (3) Zhao, D. B.; Wu, M.; Kou, Y.; Min, E. Catal. Today 2002, 74, 157–189. (4) Wasserscheid, P.; Keim, W. Angew. Chem., Int. Ed. 2000, 39, 3773–3789. (5) Han, X.; Armstrong, D. W. Acc. Chem. Res. 2007, 40, 1079–1086. (6) Zhao, H.; Xia, S. Q.; Ma, P. S. J. Chem. Technol. Biotechnol. 2005, 80, 1089–1096. (7) Armand, M.; Endres, F.; MacFarlane, D. R.; Ohno, H.; Scrosati, B. Nat. Mater. 2009, 8, 621–629. (8) Galinski, M.; Lewandowski, A.; Stepniak, I. Electrochim. Acta 2006, 51, 5567–5580. (9) Acc. Chem. Res. 2007, 11, 40, full issue and references therein. Berg, R. W. Monatsh. Chem. 2007, 138, 1045–1075. Kempter, V.; Kirchner, B. J. Mol. Struct. 2010, 972, 22–34. Lassegues, J. C.; Grondin, J.; Holomb, R.; Johansson, P. J. Raman Spectrosc. 2007, 38, 551–558. Katsyuba, S. A.; Zvereva, E. E.; Vidis, A.; Dyson, P. J. J. Phys. Chem. A 2007, 111, 352–370. Kiefer, J.; Pye, C. C. J. Phys. Chem. A 2010, 114, 6713–6720. Hunt, P. A.; Gould, I. R.; Kirchner, B. Aust. J. Chem. 2007, 60, 9–14. Wulf, A.; Fumino, K.; Michalik, D.; Ludwig, R. ChemPhysChem 2007, 8, 2265–2269. Fumino, K.; Wulf, A.; Ludwig, R. Angew. Chem. Int. Ed. 2008, 47, 3830–3834. (10) Dhumal, N. R. Chem. Phys. 2007, 342, 245–252. (11) Dhumal, N. R.; Kim, H. J.; Kiefer, H. J. Phys. Chem. A 2009, 113, 10397. (12) Holbrey, J. D.; Reichert, W. M.; Swatloski, R. P.; Broker, G. A.; Pitner, W. R.; Seddon, K. R.; Rogers, R. D. Green Chem. 2002, 4, 407–413. (13) Grosse B€ owing, A.; Jess, A. Chem. Eng. Sci. 2007, 62, 1760–1769. (14) Pereiro, A. B.; Deive, F. J.; Esperanca, J. M. S. S.; Rodriguez, A. Fluid Phase Equilib. 2010, 294, 49–53. (15) Sumartschenkowa, I. A.; Verevkin, S. P.; Vasiltsova, T. V.; Bich, E.; Heintz, A.; Shevelyova, M. P.; Kanbo, G. J. J. Chem. Eng. Data 2006, 51, 2138–2144. 3557

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