Electronic Structure and Spectroscopic Analysis of 1-Ethyl-3

Article pubs.acs.org/JPCA

Electronic Structure and Spectroscopic Analysis of 1‑Ethyl-3methylimidazolium Bis(trifluoromethylsulfonyl)imide Ion Pair Shubham Vyas,†,‡ Christopher Dreyer,§ Jason Slingsby,† David Bicknase,§ Jason M. Porter,§ and C. Mark Maupin*,† †

Chemical and Biological Engineering Department, ‡Chemistry and Geochemistry Department, and §Mechanical Engineering Department, Colorado School of Mines, Golden, Colorado 80401, United States S Supporting Information *

ABSTRACT: Electronic and structural properties of the room temperature ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulonyl)imide are studied using density functional theory (DFT) methods in addition to infrared and UV−vis spectroscopy. The DFT methods were conducted for both gas phase and solution phase using the integral equation formalism polarizable continuum model, while optical absorption experiments were conducted using neat and dilute methanol solutions. Three energetically similar conformers were obtained for each of the gas phase and solution phase DFT calculations. These multiple configurations were considered when analyzing the molecular interactions between the ion pair and for a molecular-level interpretation of the experimental IR and UV−vis spectroscopy data. Excitation energies of low-lying singlet excited states of the conformers were calculated with time-dependent DFT and experimentally with UV−vis absorption spectra. Difference density plots and excited-state calculations in the gas phase are found to be in good agreement with the experimental findings, while the implicit solvation model calculations adversely impacted the accuracy of the predicted spectra.

1. INTRODUCTION Room temperature ionic liquids (RTILs) were discovered nearly a century ago and have received considerable attention from both industry and academia.1−5 Within the last two decades industrial scale synthesis of RTILs has become viable,6 which has enhanced the use of these versatile liquids in a wide range of applications including energy storage7 (e.g., batteries and supercapacitors), chemical synthesis and catalysis,8−16 CO2 sequestration and separation,17−21 solvents for lignocellulosic biomass,22−26 and increasingly in various industrial processes1 (e.g., paint additives, antistatic cleaning agents, and production of pharmaceutical intermediates). The remarkable properties of RTILs is due, in part, to the bulky nature of the organic cation and organic or inorganic anion, which results in the RTIL remaining in a liquid state at ambient conditions. Additional favorable characteristics of RTILs include very low vapor pressure, thermal stability, high ionic conductivity, and miscibility with other solvents. The various favorable properties of RTILs have opened the possibility of these ILs to replace volatile organic solvents. The vast possible combination of anion and cation pairs allows for the tailoring of RTIL properties (e.g., hydrophobic or hydrophilic) for specific tasks (i.e., task specific RTILs, designer solvents). There have been numerous studies both experimental and computational to characterize the molecular structure, thermodynamic and physical characteristics, as well as chemical and spectroscopic properties of many RTILs. The vast number of cation and ion combinations, and the considerable experimental effort required for the characterization of RTILs, © 2014 American Chemical Society

has hampered the discovery of novel RTILs for specific applications. Computational techniques, such as time-independent and time-dependent density functional theory (i.e., DFT and TD-DFT, respectively), are promising methods for the discovery and characterization of task specific RTILs. However, these computational methods need to be validated and their accuracy quantified, as many RTILs are utilized in photochemical and photophysical experiments that require accurate characterization of their spectroscopic properties. We recently reported absorption and infrared (IR) properties of the contaminant methylimidazolium ([MIM]) in 1-ethyl-3methylimidazolium bis(trifluoromethylsulonyl)imide ([EMIM+][NTf2−]).27 In this manuscript, we present a computational and experimental characterization of the [EMIM+][NTf2−] ion pair (Figure 1). The computational characterization was conducted using DFT and TD-DFT calculations to evaluate the structural features of [EMIM+][NTf2−], IR signatures and UV−vis absorption features in both the gas phase and the solution phase models. Previously published calculated IR spectra were compared with the computational work in this manuscript to gain confidence in our methodology and configurational sampling. Implicit solvation models have not been utilized in several of the past studies which is a key component of this paper. However, recently reported work from Kiefer and co-workers had Received: April 11, 2014 Revised: July 28, 2014 Published: August 1, 2014 6873

dx.doi.org/10.1021/jp5035689 | J. Phys. Chem. A 2014, 118, 6873−6882

The Journal of Physical Chemistry A

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representative of the multiple configurations. In addition to the IR spectrum, UV−vis spectra were created by conducting a total of 20 vertical excitations using the TD-DFT formalism with the B3LYP/aug-cc-pVTZ level of theory and basis set. The resulting excitations were then Boltzmann weighted and combined to form a single UV−vis spectrum. A Gaussian broadening of 0.333 eV was applied to the UV−vis data to produce the simulated spectrum. To gain insight into the excited state character, difference density plots were also calculated at the same level of theory and basis set. In order to evaluate the impact of the solvent environment on the geometry, IR spectra, and UV−vis spectra, the methodology outlined for the gas phase system was repeated using the integral equation formalism variant of the polarizable continuum implicit solvation model (IEFPCM) with 1-hexanol (ε = 12.51). 1-Hexanol was chosen to represent the solvent model because it has been suggested the dielectric constant of ionic liquids is around 13.33,34 2.2. Experimental Setup. The optical absorption spectra were collected using a number of transmission cells with path length and window materials suitable to the ultraviolet, visible, and infrared absorption features of [EMIM+][NTf2−]. A list of the different cell types and window materials can be found in Table 1. Fixed path length cells were used for UV−vis measurements and adjustable path length cells were used for the quantitative IR measurements of neat [EMIM+][NTf2−]. High purity [EMIM+][NTf2−] was obtained as a generous gift by Boulder Ionics. The supplied [EMIM+][NTf2−] had a water content of less than 20 ppm, a chloride content of less than 1 ppm as measured by ion chromatography, and no other halides above a 5 ppm detection limit. The electrochemical window of the [EMIM+][NTf2−] was measured to be 4.50 V at 0.1 mA/cm2 limits using a glassy carbon working electrode, a platinum counter electrode, and a scan rate of 5 mV/s. Methylimidazole content in the [EMIM+][NTf2−] was estimated to be less than 25 ppm by electrochemical techniques. All samples were stored and handled in a controlled atmosphere, low humidity, low oxygen glovebox to prevent water and oxygen contamination. A syringe and tubing manifold system was used to isolate the samples from the atmosphere during spectroscopic measurements. Infrared absorption measurements were collected using a Nicolet 6700 Fourier Transform Infrared Spectrometer (FTIR) with adjustable path length flow cells. The UV−vis spectra were collected using a Varian Cary 5G UV−vis-NIR spectrometer. All measurements were made at room temperature (∼23 °C) and ambient pressure. Quantitative absorption spectra of [EMIM+][NTf2−] were collected in two ways: by dilution in spectroscopic grade methanol (UV−vis), and by correcting for reflection losses (IR). The strong absorption of [EMIM+][NTf2−] at wavelengths between 180 and 240 nm prevented measurements of

Figure 1. Ionic liquid pair [EMIM+][NTf2−] (left), cation (right top), and anion (right bottom).

inclusion of solvation model in their work to interpret the IR signatures of [EMIM+][NTf2−].28 Difference density plots have been utilized to characterize the nature of the excited state similar to our recent report on the absorption and IR properties of methylimidazolium in [EMIM+][NTf2−]. In addition to the computational characterization, experimental IR and UV−vis were conducted using neat and dilute methanol solutions.

2. METHODS 2.1. Computational Methodology. Density functional theory (DFT), as implemented in the Gaussian 09 software, was used to conduct geometry optimizations for the 1-ethyl-3methylimidazolium ([EMIM+]) cation, bis(trifluoromethylsulfonyl)imide ([NTf2−]) anion, and the [EMIM+][NTf2−] ion pair at the Becke’s three-parameter hybrid exchange functional with the Lee−Yang−Parr correlation functional (B3LYP) level of theory and the 6-31G(d,p) basis set (Supporting Information).15,29,30 In order to obtain the lowest energy complex for the [EMIM+][NTf2−] ion pair, a configurational search consisting of several unique anion and cation geometries were optimized. The resulting ion pair configurations, and the previously optimized anion and cation were reoptimized using very tight convergence criteria at the B3LYP/aug-cc-pVTZ level of theory and basis set. All optimized geometries were subjected to frequency calculations where the absence of imaginary eigenvalues indicates an energy minimum. The IR spectrum was created from the normal modes generated by the frequency calculations for minimum energy configurations (i.e., within 2 kcal/mol of the lowest energy configuration). The IR spectrum for each configuration was generated in GaussView v 5.0.9,31 the frequencies were scaled by a factor of 0.970 as suggested in the literature,32 and a Gaussian broadening of 4 cm−1 was applied to produce the simulated spectrum. The spectra were then Boltzmann weighted and combined to form a spectrum that is

Table 1. Transmission Cells Used for UV−vis and IR Measurements cell UV−vis UV−vis UV−vis UV−vis IR #1 IR #2

cell type (path length/flow) #1 #2 #3 #4

fixed/no flow fixed/flow fixed/flow fixed/no flow adjustable/flow adjustable/flow

window material fused fused fused fused CaF2 KBr 6874

silica silica silica silica

transmission 180 180 180 180 120 200

nm−2700 nm nm−2700 nm nm−2700 nm nm−2700 nm nm−10 μm nm−35 μm

path length(s) 4 mm 5 mm 100 μm 10 μm 12 μm−3 mm 12 μm−3 mm



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neat liquids at these wavelengths. Measurements were instead made by mixing a small amount of [EMIM+][NTf2−] with spectroscopic-grade methanol. Spectroscopic-grade methanol is largely transparent to ultraviolet light above 180 nm, allowing for quantitative absorption measurements in this wavelength region after correcting for the small background absorption of the methanol. There are established methods for quantitative infrared absorption measurements of neat liquids35 and therefore only a brief summary of the methods is included here. When measuring optical absorption in liquid samples, it is difficult to obtain an accurate baseline transmission measurement. The transmission of a liquid sample is related to its absorbance by Beer’s law: ⎛I⎞ −ln⎜ ⎟ = αL ⎝ I0 ⎠

Table 2. FTIR Infrared Absorption Measurements of Neat [EMIM+][NTf2−]

(1)

where I is the transmitted light intensity, Io is the incident (or baseline) light intensity, α is the absorption coefficient, L is the path length, and the product of αL is the absorbance. Quantitative spectroscopy requires the fractional transmission, I/Io, through a known thickness of the sample to be measured. For gas phase absorption measurements, Io is obtained by measuring transmission through the empty cell. This introduces negligible error because most gases have similar real refractive indices (i.e., similar reflection losses). When measuring liquids, the reflection losses in the empty cell are quite different from reflection losses in the filled cell due to the large change in the refractive index between gases and liquids. Because of this challenge, experimental procedures for correcting for reflection losses have been developed. To correct for reflection losses in the FTIR absorption spectra of [EMIM+][NTf2−] we measured the absorption spectrum at a number of different path lengths. Two window materials were used depending upon the spectral region being measured, with KBr windows extending to longer wavelengths in the mid infrared. Longer path length cells were used to measure “anchor points”, which serve to correct for baseline variation. The shortest path length cells were used to measure the strongest absorption peaks. Several of the absorption peaks of [EMIM+][NTf2−] are so strong that even using the thinnest cell spacers, ∼12 μm, the liquid was optically thick at these wavelengths. To overcome this, the spacer was removed altogether and a drop of the ionic liquid was placed between two windows and compressed until the peak absorbance dropped below 2. These shorter path lengths are listed in Table 2, and were determined by extrapolating from longer path length cells. The shortest path length absorbance spectrum in Table 2 is used to calculate an approximate real refractive index spectrum using a Kramers−Kronig transformation. The real refractive index spectrum is then used to calculate reflection losses. Each spectrum is then corrected for reflection losses and for baseline shifts (using the anchor points). This process is repeated, iteratively, until convergence. Each of the corrected absorbance spectra are then piecewise averaged into one final composite absorbance spectrum.

pathlength [μm]

duplicates

3120 1040

2 2

488

2

52.4 28.3

2 2

13.4

2

12.7 11.8 4.2 1.5 1.2

2 2 1 1 1

spectral range [cm−1]

window type

5000−7000 1822−2626 3686−7000 1822−2626 3686−7000 1666−2626 715−980 1530−3686 715−980 1530−3686 1530−3686 1530−3686 480−3686 480−1415 480−1415

CaF2 CaF2 CaF2

anchor points [cm−1] 480, 543, 586, 641, 689, 715, 775, 814, 900, 980, 1100, 1282, 1415, 1530, 1666, 1823, 1902, 2077, 2151, 2218, 2626, 3686, 4973, 5409, 6500, 6995

CaF2 KBr KBr CaF2 CaF2 KBr KBr KBr

ion pair in configurational space for both gas phase and solution phase. Indeed, previous computational work by Tsuzuki et al. at the Møller−Plesset (MP) level of theory concluded the existence of 17 configurations for the ion pair, all of which were within 2.4 kcal/mol, and only 3 configurations were within 0.5 kcal/mol.36 Furthermore, a recent detailed report using the same theoretical manifold as Tsuzuki et al., but with a larger basis set, obtained 23 possible configurations of the ion pair [EMIM+][NTf2−], and found only 3 configurations within 0.5 kcal/mol.37 Since the overall aim of our study is to interpret the IR and UV−vis properties of the ion pair at the molecular level, we used the B3LYP functional, which is known to do an excellent job at computing both IR and UV−vis properties.38−41 In the gas phase, we identified several configurations at the B3LYP/6-31+G(d,p) level of theory, which were subjected to further optimization at B3LYP/aug-cc-pVTZ level of theory. We located 23 unique minima for the ion pair that were within 10 kcal/mol (see Supporting Information) and 6 configurations within 0.5 kcal/mol at the B3LYP/aug-cc-pVTZ level of theory. Although these six ion pair configurations are energetically similar they are very different geometrically (Figure 2). A common interaction found in all ion pair configurations is the presence of tight hydrogen bonding between the N(Me)− CH−N(Et) moiety in the EMIM cation and the sulfonyl oxygen atoms in the NTf2 anion. In the solution phase, however, three unique minima were located using the implicit solvation (IEFPCM of 1-hexanol) as shown in Figure 3. However, these three conformers of the ion pair were significantly different geometrically than what was found in the gas phase (Figure 2). Lassegues and co-workers42,43 carried out an extensive conformational sampling of NTf2 anion where they found that the transoid rotamer of the anion is more stable than the cisoid rotamer by about 0.5 kcal/mol, which was also the conclusion of Ishiguro et al.44 However, in all of our calculations, we found that the NTf2 anion exists in between cisoid and the transoid rotamers in order to optimize the interactions with EMIM cation. In order to test if the gas phase and solution phase calculations actually resulted in different geometries or if this was an artifact of the initial conditions, the gas phase geometries and solution phase geometries were swapped and the high level optimization recomputed. Swapping the gas and

3. RESULTS AND DISCUSSION 3.1. Computed Geometry. Due to the ion pair formation and several possible hydrogen bonds in [EMIM+][NTf2−], there are likely several energetically similar local minima for the 6875



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Table 3. Geometric Analysis of the Lowest Energy Configuration in the Gas Phase at the B3LYP/aug-cc-pVTZ Level of Theorya [EMIM+] (Å) C1-H2 C1-H3 C1-H4 C1-C5 C5-H6 C5-H7 C5-N8 N8-C9 C9-H10 C9-C11 C11-H12 C11-N15 N8-C13 C13-H14 C13-N15 N15-C16 C16-H17 C16-H18 C16-H19 C20-F21 C20-F22 C20-F23 C20-S24 S24-O25 S24-O26 S24-N27 N27-S28 S28-O33 S28-O34 S28-C29 C29-F30 C29-F31 C29-F32 O25---H7 O26---H18 O34---H14 O34---H17

Figure 2. Optimized geometries of [EMIM+][NTf2−] ion pair in the gas phase using B3LYP/aug-cc-pVTZ level of theory. The configuration index is at the top while the relative gas phase energy is reported at the bottom. Configuration geometry is provided in the Supporting Information.

Figure 3. Optimized geometries of [EMIM+][NTf2−] ion pair in condensed phase using IEFPCM and B3LYP/aug-cc-pVTZ level of theory. Configuration index is at the top while the relative gas phase energy is reported at the bottom. Configuration geometry is provided in the Supporting Information.

solution phase geometries and the subsequent optimization resulted in a geometric rearrangement of the ion pair yielding one of the previously obtained minimum energy structures. For example, if Figure 2A geometry was subjected to a minimization in the solution phase, the geometry converged to that found in Figure 3A. As a result, we concluded that the ion pair, [EMIM+][NTf2−], has different geometric arrangements in the gas phase and in the solution phase as represented by IEFPCM. It has been reported45 that the ethyl group of the EMIM cation prefers to stay out of the imidazolium ring plane, which is the case in all of the configurations obtained for the ion pair in the gas phase as well as in the solution phase (Figures 2 and 3). Regarding the conformation of the NTf2 anion, a myriad of computational and experimental investigations46−49 have been reported for various systems. These studies indicated that both cis and trans conformations are possible for −CF3 groups with respect to the O2S−N−SO2 backbone; however, the trans conformation is slightly more stable. Minimum energy structures of [EMIM+][NTf2−] obtained in this study also support the notion that both cis and trans conformations can exist for the NTf2 anion, which is also in excellent agreement with previously published work on geometries of RTILs. Detailed geometrical analysis of the lowest energy optimized structures for [EMIM+], [NTf2−], and the ion pair are shown in Table 3. The geometric structure of the EMIM cation has C−H bond length values very close to the typical 1.09 Å C−H value while the NTf2 anion has bond lengths similar to the expected

a

[NTf2−] (Å)

1.089 1.089 1.089 1.522 1.089 1.088 1.480 1.378 1.074 1.358 1.075 1.379 1.333 1.075 1.335 1.467 1.086 1.087 1.087 1.345 1.340 1.337 1.892 1.454 1.454 1.603 1.603 1.454 1.454 1.892 1.340 1.337 1.345

[EMIM+][NTf2−] (Å) 1.088 1.090 1.089 1.520 1.089 1.088 1.477 1.379 1.074 1.357 1.074 1.378 1.333 1.078 1.333 1.464 1.085 1.087 1.087 1.331 1.330 1.339 1.882 1.465 1.451 1.597 1.593 1.445 1.471 1.887 1.339 1.331 1.337 2.453 2.708 2.007 2.575

Shown in Figure 2A.

CF (1.34 Å) and SO (1.45 Å) bond distances. The optimized geometric structure of the ion pair results in the presence of four hydrogen bonds. While there is no change in the CH bond distances of the atoms participating in hydrogen bonding, an increase in bond length can be seen in all of the participating SO interactions. This elongation of the bond is due to the character of the SO bond, which is a double bond, and the polarization induced by the hydrogen bonding. The ion pair complex energy for [EMIM+][NTf2−] in the gas phase was calculated to be −76.6 kcal/mol at B3LYP/ aug-cc-pVTZ level of theory, which is in excellent agreement with the previously described range of −77.4 to −78.8 kcal/mol by Tsuzuki et al. at MP2/6-311G** level of theory.36 In contrast to our gas phase results and previous work, the solution phase results show that the complexation energy is merely −6.3 kcal/mol at B3LYP/aug-cc-pVTZ level of theory. 3.2. Computed and Observed IR Spectrum. Infrared spectrum of [EMIM+][NTf2−] has been reported by Kiefer et al.28,50,51 before and been revisited a few times. The goal of 6876



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investigating the infrared signatures of the ion pair is aimed at gaining confidence in the configurational sampling and accuracy of the level of theory utilized in this study. The observed IR spectrum of the neat [EMIM+][NTf2−] ion pair is shown in Figure 4, which is in excellent agreement with the previously

Figure 4. Measured infrared absorption spectrum of neat [EMIM+][ NTf2−].

published spectrum by Kiefer et al., with a strong N−C−H scissor vibration at 1555 cm−1 compared to 1574 cm−1 obtained by Kiefer and co-workers, which is experimentally observed at 1575 cm−1. Furthermore, SO2 asymmetric stretch at 1225 cm−1 in our calculation is also in excellent agreement with the reported value of 1221 cm−1 by Kiefer et al. and the experimental number of 1227 cm−1. Other important infrared signatures include C−F stretching which is calculated to be at 1156 and 1151 cm−1 and experimentally observed at 1192 cm−1. Additionally, the SO2 symmetric stretches were calculated at 1063 and 1056 cm−1, which were observed at 1094 cm−1 in our measurements. In addition to the experimental spectrum an overlay of the computed and experimental spectra is shown in Figure 5. The computed spectra were obtained by Boltzmann weighting the contributions from all of the low energy conformers of the ion pairs that are shown in Figure 2 and Figure 3. When comparing the computed and experimental IR spectra of the ion pair, an excellent match is seen with the gas phase calculations while significantly red-shifted peaks (vs experimental IR spectrum) are observed for the solution phase IR spectrum. The differences between the gas phase and solution phase results will be discussed in the subsequent section. Further discussion on the vibrational signatures is limited to the gas phase calculations and only with the characterization of important computed infrared peaks. These peaks, when appropriate, are compared with the experimental spectrum and summarized in Table 4. From Table 4 it is clear that most of the intense peaks are due to the NTf2 anion, specifically the sulfonyl, −CF3, and N−SO2 groups. The strong signatures include peaks at 492, 586, 649, 1020−1091, 1098, 1167−1199, 1261−1310, and 3217 cm−1, which are attributed to S−CF3, NSO2, SO2, S−N−S, NS, SO coupled with NS units, CF3, SO, and C−H vibrations, respectively. These signatures are in good agreement with experimental spectra and previously reported data. Thus, we conclude that the computational

Figure 5. Infrared spectra of [EMIM+][NTf2−] as determined by the B3LYP/aug-cc-pVTZ level of theory (black) for (A) gas phase and (B) solution phase. The experimentally observed spectrum is shown in gray color in both (A) and (B).

protocol and configurational sampling of the ion pair is able to predict the molecular properties accurately. 3.3. Computed and Observed UV−vis Spectrum. The absorption spectrum of [EMIM+][NTf2−] ion pair was recorded both in neat conditions as well as in methanol (Figure 6 and Supporting Information). A very prominent absorption band was located around 210 nm while a very weak band was observed at 260 nm. Samanta et al.52 also observed the peak around 260 nm in their UV−vis measurements, although their 260 nm band possessed significant intensity. One reason for the discrepancy in intensity is possibly that their measurements were carried out using a 1 cm cuvette while we collected our spectra using a 10 μm cell. This low energy electronic transition was found to exist both in the neat conditions and in the dilute methanol solution. The use of very dilute condition with a small cell path length evaluated if the electronic excitation around 260 nm was due to some larger assembly of ion pairs or an inherent property of single [EMIM+][NTf2−] ion pairs. Since the 260 nm peak also exists in the dilute conditions using methanol, we attribute this peak to a single [EMIM+][NTf2−] ion pair. It is possible that 6877



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Table 4. Calculated Frequencies for [EMIM+], [NTf2−], and [EMIM+][NTf2−] at the B3LYP/aug-cc-pVTZ Level of Theory and Comparison with the Experimentally Observed Frequenciesa computational

a

vibrations

[EMIM+] (cm−1)

C13−H14 str C9−H10, C11−H12 str H6−C5−H7 sym str C16−H17, C16−H18, C16−H19 sym str C1−H2, C1−H3, C1−H4 sym str N15−C11−H12 scissor, N8−C9−H10 scissor H6−C5−H7 scissor H17−C16−H18 scissor H2−C1−H3 twist H17−C16−H19 scissor H6−C5−H7 wag O25−S24−O26, O33−S28−O34 asym str H6−C5−H7 twist, O33−S28−O34 asym str O25−S24−O26, O33−S28−O34 asym str H6−C5−H7 twist O25−S24−O26 asym str O25−S24−O26, O33−S28−O34 sym str C29−S28, C20−S24 str N8−C13−H14 scissor C20−F22 str C29−F31 str C23−F21, C29−F32 str H6−C5−H7 rocking H17−C16−H19 twist O25−S24−O26, O33−S28−O34 sym str O33−S28−O34 sym str O25−S24−O26 sym str O25−S24−O26, O33−S28−O34 sym str O25−S24−O26, O33−S28−O34 wag H10−C9−C11−H12 wag O25−S24−O26, O33−S28−O34 scissor N8−C13−H14 twist O33−S28−O34 wag O25−S24−O26, O33−S28−O34 wag O25−S24−O26, O33−S28−O34 scissor F22−C20−F23 scissor O25−S24−O26 scissor

3171 (30) 3169 (16)

1554 1545 1462 1462 1447 1439 1343

[NTf2‑] (cm−1)

(24) (51) (15) (13) (13) (14) (12)

experimental [EMIM+][NTf2−] (cm−1)

[EMIM+][NTf2−] (cm−1)

3114 (265)

3165, 3126

2972 2967 2948 1555 1550

(17) (19) (11) (24) (34)

2993 2951

1461 1452 1446 1333

(15) (21) (12) (22)

1472 1457 1432 1352

1575

1281 (467) 1268 (389)

1298

1260 (74)

1165 (13) 1150 (375)

1246 1221 1164 1161

(70) (284) (11) (193)

1156 1151 1132 1130 1073

(115) (306) (399) (65) (11)

1192 1171 1154

1063 (391) 1056 (206) 987 (428)

1135 1094 1056

724 703 629 567

740 699 615 570

1227 1192

1139 (105) 1133 (631) 1124 (143) 1117 (205)

1072 (92)

986 (596) 736 (10) 738 (25) 645 (15) 579 (352) 485 (72)

(39) (16) (129) (152)

544 (37) 534 (10) 493 (19)

538

The intensities are provided in the brackets.

calculations, which is in good agreement with the experimental peak observed at 260 nm. Overlaid UV−vis spectra obtained experimentally and computationally are shown in Figure 6. These spectra were obtained by Boltzmann weighting the UV−vis contributions from the six lowest energy configurations of the ion pair as shown in Figure 2 and the three lowest energy conformers of the ion pair in Figure 3. Both the gas phase and solution phase calculations are able to predict the strong electronic transition around 200 nm, while only the gas phase calculations are able to predict the low energy transition around 260 nm. Calculations with the implicit solvation did not predict the experimentally observed lower energy transition. Since our system of [EMIM+][NTf2−] ion pair is in the liquid state, the implicit solvation models should have yielded better results than the gas phase results, which is contrary to what we observe. Similar observations were also made while discussing the infrared signatures of the ion pair in the previous section.

methanol might interact with the ionic liquid in dilute conditions to impact the UV−vis signatures. However, there was no difference between the spectra in neat and dilute conditions (see Figure S2, Supporting Information). We calculated the vertical excitations on all of the [EMIM+][NTf2−] configurations using time-dependent density functional theory both in the gas phase as well as in the solution phase. Computed results for the lowest energy configurations are summarized in Table 5. Vertical excitations for all of the six configurations shown in Figure 2 are very similar to that of the lowest energy conformer (see Supporting Information). In principle, the solution phase geometry should be able to mimic the experimental results more accurately as experiments were conducted in solution; however, solution phase calculations are not able to predict the absorption feature around 260 nm. On the other hand, the results obtained in the gas phase calculations are much closer to the experimental spectra. A very small peak at 245 nm is predicted by the gas phase 6878



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Figure 6. UV−vis spectrum of [EMIM+][NTf2−] as calculated in the gas phase (A and B) and in the solution phase (C and D) at the B3LYP/aug-ccpVTZ level of theory. The computed spectrum is indicated by the black curve where the vertical lines represent the electronic transitions. The experimental spectrum is indicated by the gray curve as obtained in spectroscopic grade methanol with 0.342 mol % of [EMIM+][NTf2−] and a 10 μm cell.

Table 5. Calculated Vertical Excitations of [EMIM+][NTf2−] Ion Pair Both in the Gas Phase and in the Solution Phase at B3LYP/aug-cc-pVTZ Level of Theory gas phase

conformers are not the same in the gas phase versus the solution phase as shown in Figures 2 and 3; thus, the properties predicted are also significantly different. Calculations of vertical excitations were carried out with different solvation models; however, no improvements in the UV−vis signatures were observed. Additionally, changes in the dielectric constants of the solvation model did not produce the observed UV−vis properties of the RTIL under study (results not shown). We anticipate that due to the nature of ionic liquids, the implicit solvation models are not able to predict the geometries and the UV−vis properties of the [EMIM+][NTf2−] ion pair accurately. On the other hand, the UV−vis properties yielded by the gas phase calculations are in good agreement with the experimental findings despite the fact that the dielectric constant term is not included in these calculations. As a result, we also conclude that there is only a very small effect of the dielectric environment on the ion pair. It has been shown that the dispersion interactions in the RTILs can play a significant role in predicting the molecular properties.55 Since B3LYP density functional does not take into account the dispersion interactions, we carried out similar conformational searches and properties calculations using recently implemented APF and APFD functionals56 with the 6-31G(d,p) basis set. APFD is the APF functional with dispersion interactions included. The use of APF and APFD

solution phase

excitation number

wavelength (nm)

oscillator strength

wavelength (nm)

oscillator strength

1 2 3 4 5 6 7 8

245 220 209 205 200 197 197 192

0.0002 0.0000 0.0001 0.0281 0.0008 0.0004 0.0808 0.0014

200 195 186 184 182 177 175 173

0.1787 0.0004 0.0009 0.0181 0.0043 0.0029 0.0002 0.0078

The reason behind these controversial results is believed to be the nature of the ionic liquids. The implicit solvation model used in this manuscript does well in mimicking the dielectric environment around the molecules under investigation. Such an approach may not work for predicting the properties of a molecule that has significant ionic strength since implicit solvation models only take into account the continuous dielectric environment and neglect the other solvent properties.53,54 As a result, even the geometries of low energy 6879



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functionals gave us the ability to directly compare the effect of dispersion interactions on the molecular properties of the RTIL under consideration. The results from APF and APFD indicate no noticeable changes in either the IR or UV−vis spectra. The conclusion that the gas phase results are in agreement with the experimental data while the solution phase calculations are not able to reproduce observed properties remained unchanged (see Supporting Information, Figures S3 and S4). To gain additional insight into the differences in the excited state manifolds both in the gas phase as well as in the solution phase, and to evaluate the nature of the excited states, we calculated the difference density plots for the first four excited states of the ion pair. Such a method has been used in the past for organic and inorganic molecules as well as for imidazolium cations.57−59 One can compute the electronic density on the ground and the Franck−Condon excited state and take the difference to obtain the gain and loss on the electronic density upon vertical excitation. Computed difference density plots for [EMIM+][NTf2−] ion pair are shown in Figures 7 and 8 using the gas phase and implicit solvation calculations, respectively.

Figure 8. Calculated difference density plots utilizing the implicit solvation model for [EMIM+][NTf2−] ion pair’s excited states at the B3LYP/aug-cc-pVTZ level of theory. The blue-gray and gold color contour shows the accumulation and depletion of the electronic density upon vertical excitation from the ground state, respectively. The iso-contour value for these plots is ±0.004 au.

intimate transition and/or the screening incorporated in the solvation model is damping out this transition. Kiefer and coworkers60 recently reported the observation of vibrational energy transfer via H-bond in [EMIM+][NTf2−] using timeresolved spectroscopy. Any conclusions from the current study regarding this effect is not feasible; nevertheless, extension of this work into the excited state optimizations followed by excited state surface exploration would be very useful.

4. CONCLUSIONS UV−vis absorption and IR properties of the commonly used ionic liquid [EMIM+][NTf2−] were investigated both experimentally and computationally. Contrary to the common belief, the implicit solution phase calculations could not be used to calculate the properties of the RTIL under investigation, presumably, due to the nature of the ionic liquids. However, results from the gas phase calculations were in excellent agreement with the experimental data. The lowest energy peak was located around 260 nm experimentally, which is in agreement with the previous studies, and only arises due to the intimate interactions between the [EMIM+] cation with [NTf2−] anion. The nature of this peak was identified as a transition from the [NTf2−] to the [EMIM+] moiety. Most of the intense IR peaks were identified as originating from the strong IR groups in the [NTf2−] anion. We conclude that the gas phase calculations are more appropriate in computing the ground state properties of the RTILs.

Figure 7. Calculated difference density plots in the gas phase for the [EMIM+][NTf2−] ion pair’s excited states at the B3LYP/aug-cc-pVTZ level of theory. The blue-gray and gold color contour shows the accumulation and depletion of the electronic density upon vertical excitation from the ground state, respectively. The iso-contour value for these plots is ±0.004 au.

From Figure 7 it is clear that the S0 to S1 state transition (λ = 245 nm) in the gas phase involves transfer of the electronic density from the NTf2 anion to the EMIM cation moiety while this transition is absent in the solution phase calculations (Figure 6). The solution phase S1 state is found to be completely localized on the EMIM cation with no indication of an intimate transition between the cation and anion. Thus, one can conclude that the solution phase geometry is not able to predict the intimate transition between EMIM and NTf2, while the gas phase calculations are able to predict the lowest energy transition in addition to predicting that this transition has very small oscillator strength, in good agreement with the experimental results. It is noted that the computed difference density results in the gas phase indicate that both the S2 and S3 states are delocalized over the entire ion pair, further supporting the hypothesis that the solvation model induces a change in the optimized geometry that is not conducive to the

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ASSOCIATED CONTENT

S Supporting Information *

Optimized coordinates of all conformers utilized in this study as well as from the configuration searches, as well as the calculated UV−vis and IR spectral properties of individual conformers and the full citation information for refs 7 and 30. This material is available free of charge via the Internet at http://pubs.acs.org. 6880



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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected], Phone: 303-273-3197, Fax: 303-273-3730. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the National Science Foundation (NSF grant CBET-1337044). The authors acknowledge computer resource support from Colorado School of Mines Campus Computing, Communications and Information Technology and Boulder Ionics for their generous gift of the RTILs utilized in this study. This research was supported by the US Department of Energy, Office of Electricity (Dr. Imre Gyuk, Program manager), via a contract with Sandia National Laboratories. This work also made use of the Renewable Energy Materials Research Science and Engineering Center shared facilities funded by National Science Foundation grant DMR- 0820518. S.V. gratefully acknowledges the postdoctoral fellowship from Camille and Henry Dreyfus Foundation.

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ABBREVIATIONS EMIM, 1-ethyl-3-methylimidazolium; NTf2, bis(trifluoromethylsulfonyl)imide or bistriflimide REFERENCES

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Electronic Structure and Spectroscopic Analysis of 1-Ethyl-3

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