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Characterization of the Bridged Proton Structure in HTFSI Acid Ionic Liquid Solutions Kyle T. Munson, Jason Vergara, Lei Yu,* and Timothy D. Vaden* Department of Chemistry and Biochemistry, Rowan University, 201 Mullica Hill Road, Glassboro, New Jersey 08028, United States S Supporting Information *

ABSTRACT: Acidic ionic liquid (AIL) solutions were prepared by dissolving bis(trifluoromethanesulfonyl)imide (HTFSI) acid in the ionic liquid (IL) 1-butyl-3methylpyrrolidinium bis(trifloromethanesulfonyl)imide (PyrrTFSI). The HTFSI/ PyrrTFSI solutions were investigated by conductivity measurements, optical spectroscopy, and DFT calculations in order to understand the ionization/solvation mechanism of HTFSI in the solutions. The HTFSI/PyrrTFSI solution conductivities first increased at lower concentrations and then decreased when the concentration of HTFSI is higher than ∼1.5 M. The spectroscopic results indicate that the solvation structure may evolve from lower to higher concentrations to make protonated TFSI− motifs. Both spectroscopic and DFT simulation results support the observation of proton-sharing [H(TFSI)2]− dimers, which may form through a bridged hydrogen in the format of either a N−H−N connection or a N−H−O connection. Both configurations may exist in the AIL solution. The proton-sharing mechanism implied by these structures confirms that the TFSI− ion can be a proton acceptor and a Brønsted base as well in IL solutions. However, the IL molecular cations such as imidazolium and (in this work) pyrrolidinium do not contribute significantly to the proton solvation and transportation in the solutions. ions can conceivably protonate the TFSI− ions at the imide nitrogen sites. However, in aqueous and nonpolar solvents, TFSI− is considered a noncoordination anion and is very weak Brønsted base. Further, the dimeric [H(TFSI)2]− has not been observed.5 Room-temperature ILs have recently become a very dynamic basic and applied scientific research topic because these chemicals possess a unique combination of properties such as negligible vapor pressures, low melting points, nonflammability, good solvation of many organic and inorganic chemicals, and high ionic conductivities.14 ILs are advanced candidates for numerous applications including both solvent and electrolyte for electrochemical processes such as those used in fuel cells, lithium batteries, and solar cells.11,15−37 ILs offer excellent hightemperature performance, long life, and safety as a result of their negligible volatility, thermostability, and nonflammability. Meanwhile, AILs with HTFSI as the acidic molecule and proton source have been developed and used as proton conductive electrolytes and fuel cell components.28−34,38−42 As examples, AILs can swell into Nafion or other polymer membranes that are used as the proton exchange membranes of fuel cells or proton batteries. Additionally, ILs are also used in analytical chemistry as absorptive sensors and chromatography materials43−45 and in organic synthesis as media or catalysts for chemical reactions such as Diels−Alder reactions, nucleophilic displacement reactions, Friedel−Crafts alkylation reactions, etc.14

1. INTRODUCTION Bis(trifluoromethanesulfonyl)imide (HTFSI) is a typical strong Brønsted monoprotic acid in aqueous solutions. For example, a 0.1 M aqueous solution of HTFSI has a pH close to 1.0.1 Its aqueous solutions can be titrated by both strong and weak bases, yielding typical titration curves and TFSI− salts. In ionic liquid (IL) solutions, HTFSI has also been reported to be a monoprotic Brønsted acid and, therefore, a possible proton source for imidazole- and pyrazole-based proton-conducting materials.2−4 In the gas phase, HTFSI is an acid that is stronger than triflic acid (CF3SO3H). In nonpolar solutions, however, it cannot even protonate water.5 HTFSI can be added to TFSI−containing ILs to create an acidic ionic liquid (AIL) solution. The acidic strength (Ka) of HTFSI in ILs is unknown, but it is likely a stronger acid than imidazolium, pyridinium, pyrrolidinium, and trialkylammonium, as facile proton transfer from HTFSI to corresponding conjugate bases has been observed.3,6 Once HTFSI dissolves in ILs, the HTFSI molecule might dissociate to produce hydrogen ion (H+) and TFSI− anion. The H+ cation might be further solvated by multiple anions in the solution. In a previous publication, we have reported the solution properties of HTFSI acid in an IL 1-butyl-3methylimidazolium bis(trifluoromethanesulfonyl)imide (BMITFSI).1 At a lower concentration, HTFSI completely dissociates in BMITFSI solution. The H+ ion may be solvated by multiple, most likely three, TFSI− anions. The H+ solvation clusters are analogues of Li+ and Na+ ions in TFSI− ILs.7−13 In order to form a solvation cluster, the trans-TFSI− in the bulk BMITFSI converts to cis-TFSI−. This ionization-solvation effect increases the overall solution conductivity. When the HTFSI concentration in BMITFSI is higher than 0.1 M, the “extra” H+ © 2015 American Chemical Society

Received: March 20, 2015 Revised: April 23, 2015 Published: May 1, 2015 6304

DOI: 10.1021/acs.jpcb.5b02715 J. Phys. Chem. B 2015, 119, 6304−6310

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The Journal of Physical Chemistry B

Density functional theory (DFT) calculations of IL solvation structures were performed with the GAMESS code incorporated with the ChemBio3D program (PerkinElmer). This software package was chosen because, unlike many other commercial computational chemistry packages, ChemBio3D can compute Raman intensities. Geometries of IL molecular cation−anion complexes with and without acid solutes were computed at the B3LYP/6-31+G* level. IR and Raman spectra (frequencies and IR/Raman intensities) were computed at the same level of theory and simulated by scaling the harmonic frequencies by 0.97 and then convolving the frequencies and intensities with 10 cm−1 fwhm Lorentzian line shapes.

Because of their versatile applications, a greater understanding of the physicochemical properties in AILs is imperative. Furthermore, although the applications of AILs are well-known, microscopic evidence about the ionic structures and solvation mechanisms is not available.46 The proton solvation structure in an IL is a key unknown in understanding the solution’s acidity and proton-transport properties. In this current work, we investigated AIL solutions of HTFSI dissolved in the IL 1-butyl-3-methylpyrrolidinium bis(trifloromethanesulfonyl)imide (PyrrTFSI) with HTFSI concentration up to 5.0 M. We used conductivity measurements, UV−vis, fluorescence, Raman, and infrared spectroscopy (FTIR) and a “mini-clusters” computational simulation together to characterize the proton-solvation structures. The PyrrTFSI IL was selected for two reasons. First, pyrrolidinium-based ILs have intrinsically higher ionic conductivities and excellent thermostabilities along with imidazolium-based ILs compared to tetraalkylamonium and tertraalkylphosphonium ILs. Therefore, these ILs and their solutions are excellent candidates for use as electrolytes in electrochemical devices at elevated temperatures. Second, in the literature, the imidazolium cation is the most popular one to make ILs and electrolytes of electrochemistry. Instead of simply being a spectator, it may be involved in proton solvation and transportation through a Grotthuss mechanism or a vehicular mechanism.3,47 We have found that HTFSI/BMITFSI AIL solutions have pink color at high HTFSI concentrations. To eliminate the imidazolium effects and isolate the H+ solvation mechanism, we therefore chose the HTFSI/PyrrTFSI AIL system to create a TFSI−-rich environment to investigate the proton solvation and to rule out the effects of imidazolium in proton solvation and transportation.

3. RESULTS AND DISCUSSION 3.1. Conductivity of HTFSI/PyrrTFSI Solutions. Results of the conductivity measurements are shown in Figure 1. The

Figure 1. Conductivity (mS/cm) of HTFSI in PyrrTFSI solution shown as a function of increasing HTFSI concentration. Error bars are included in the data summarized in Table S1.

2. MATERIALS AND EXPERIMENTAL METHODS The PyrrTFSI (>98%) and HTFSI were purchased from EMD/Millipore and Sigma-Aldrich, respectively. HTFSI solutions in PyrrTFSI solvent were prepared under a hood with quick operation in order to prevent the absorption of moisture from air. Pure HTFSI absorbs water rapidly in air; however, no significant absorption of water was observed once solutions were prepared. Therefore, the solutions were stored in capped containers without special moisture control. Conductivities of pure ILs and solutions were measured by an AC mode Traceable conductivity meter at a constant frequency of 3 kHz with a pair of parallel Pt plate electrodes at room temperature, approximately 23 °C in our laboratory. The cell constant is designed to be unity. The cell was calibrated by standard solutions with conductivity of 1 and 10 mS/cm prior to measurement. UV/vis absorbance spectra were acquired with a PerkinElmer Lambda 35 UV/vis spectrometer, using an empty quartz cuvette as background. Fluorescence spectra were acquired with a PerkinElmer LS55 fluorescence spectrometer. Fourier transform infrared (FTIR) spectra of all samples were measured with a Varian FTS 7000 FTIR spectrometer at 1 cm−1 resolution. Spectra of liquid samples were acquired with a ZnSe attenuated total reflectance (ATR) kit and a thin layer of liquid sample on the ZnSe crystal. Raman spectra were measured using a Horiba Jobin-Yvon LabRam Evolution Raman microscope (Horiba, Edison, NJ) fitted with a liquid sample acquisition accessory. Samples were measuring using either 532 or 633 nm excitation wavelengths. Fluorescence backgrounds were subtracted using the Labspec 6 software.

values with statistical errors (standard deviations) are included in Table S1 of the Supporting Information. The standard deviations are less than 0.04 mS/cm, and therefore the trend shown in Figure 1 is significant. At room temperature, pure PyrrTFSI has conductivity of about 1.25 mS/cm, which is about one-third of the conductivity BMITFSI at the same temperature. The conductivity of the acidic PyrrTFSI solutions with HTFSI is higher than the pure PyrrTFSI IL. Conductivity of the solutions increased with HTFSI present at concentrations up to 1.0 M. However, at concentrations higher than 1.5 M, the conductivity decreased gradually until 4.0 M and then leveled off. A maximum conductivity of ∼2.80 mS/cm (∼2.2-fold higher) was observed at HTFSI concentration of 1.0 M. In our previous work,1 we have shown a similar conductivity vs acid concentration pattern in HTFSI/BMITFSI AILs, in which the maximum conductivity exists at 0.1 M HTFSI. The increase and then decrease of conductivity has been attributed to the ionization/solvation and then protonation of the HTFSI imide group in the solution at different concentrations. 3.2. Electronic Spectroscopy. The PyrrTFSI is normally colorless or faintly yellow in color due to impurities. HTFSI is also colorless. However, visual inspection of the AILs shows that the addition of more than 1.0 M HTFSI results a deep yellow/orange color. We quantified this color change with absorption and fluorescence spectroscopies as presented in Figure 2. The UV absorbance spectra (Figure 2A) show that with 2.0 M HTFSI the AILs has a higher UV absorbance profile relative to the pure IL and exhibits two bands at about 355 and 500 nm (not observed in the pure IL) that can explain the 6305

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the pure IL spectrum. There are five IR bands, at 1025, 1057, 1127, 1183, and 1345 cm−1, that are not observed in the pure IL spectrum. Following the IR spectra from the pure IL to the high-concentration AILs, there seems to be a somewhat smooth evolution from the bottom spectrum to the top spectrum, which would indicate a structural change in the ILs upon HTFSI addition. It is also notable that the spectrum of the 2.0 M HTFSI AIL exhibits a band at 1300 cm−1 that is not present in the pure IL spectrum and is significantly reduced in the 6.0 M solution spectrum. This would suggest that the 2.0 M HTFSI AIL solution structure is different from the 6.0 M solution and would be consistent with the conductivity differences between the 2.0 and 6.0 M solutions (Figure 1). Previous structural investigations of TFSI-based IL systems have used Raman spectroscopic measurements of the TFSI CF3 umbrella mode to infer structural information.13 We measured the Raman spectra of the pure IL and the 2.0 M HTFSI AIL (the same solution shown in Figure 3). The Raman spectra results are shown in Figure 4. For the most part, the Raman

Figure 2. (A) UV−vis spectra of pure PyrrTFSI and 2.0 M HTFSI in PyrrTFSI. (B) Fluorescence spectra of pure PyrrTFSI and 2.0 M HTFSI in PyrrTFSI excited at 355 nm.

yellow/orange solution color. In the HTFSI/BMITFSI solutions, similar color and peaks at very close positions (∼380 and 500 nm) have also been observed.1 At a concentration lower than 1.0 M, the color is too light to be observed and the absorption peaks are very weak. This result in PyrrTFSI solution, in which the imidazolium ion is absent, clearly rules out the aromatic imidazolium cation as contributing to this color change in high-concentration HTFSI AIL solutions. The absorption peaks at ∼355 and ∼500 nm may correspond to solvated species in solution. The fluorescence spectra (Figure 2B) show that after excitation of the 355 nm band the 2.0 M HTFSI/PyrrTFSI AILs exhibits significantly higher fluorescence intensity at ∼450 nm relative to the pure IL. It is known that pyrrolidinium ion does not fluoresce around this wavelength. It is also well-known that molecular species with lone-pair electrons and π bonds usually fluoresces in the visible light range. Therefore, this solvated species that absorbs at ∼355 and 500 nm and fluoresces at ∼450 nm should constitute the TFSI− ion that has lone-pair electrons and π bonds should also has different structure compared with that in pure PyrrTFSI or BMITFSI. 3.3. Vibrational Spectroscopy. The results presented to this point suggest that the HTFSI AILs have different molecular species than the pure PyrrTFSI IL. To characterize the AILs structures, we performed a series of vibrational (FTIR and Raman) spectroscopy measurements on the AILs of increasing HTFSI concentrations. The FTIR spectra from 700 to 1800 cm−1 are shown in Figure 3. In the spectrum of the pure IL PyrrTFSI (bottom trace of Figure 3), two very sharp bands are observed at 1028 and 1220 cm−1 and two prominent broad bands are observed at 1148 and 1255 cm−1. The spectrum of the AILs with 6.0 M HTFSI (top trace) is clearly different from

Figure 4. Raman spectra of PyrrTFSI with and without 2.0 M HTFSI in solution. (A) Full spectra. (B) Expanded region showing the doublet observed for the −CF3 umbrella vibration.

spectrum of the AIL is identical to that of the pure IL. The noise-level increase in the AIL spectrum is due to the increased fluorescence background (subtracted; see section 2). However, there are significant differences regarding the Raman band at 755 cm−1, as illustrated in Figure 4B. This band corresponds to the −CF3 umbrella-bending vibration. In the pure IL spectrum (lower trace), the band is a single peak centered at 755 cm−1, while in the AIL spectrum (upper trace), the band is split into two peaks: one at 743 cm−1 and the other one at 765 cm−1. These results suggest that the TFSI− conformational structure is significantly affected by the presence of HTFSI at high concentrations. 3.4. DFT Calculations. To assist in interpreting the vibrational spectroscopic results, we simulated the IR and Raman spectra for different IL conformational structures using DFT (B3LYP/6-31+G*) calculations. The structures used for the calculations are shown in Figure 5. While we used the butylmethylpyrrolidinium molecular cation for the experiments, for simplicity we used the dimethylpyrrolidinium as cation for the DFT calculations. For the pure IL (Figure 5A), we used a cluster of two pyrrolidinium cations and two TFSI− anions. The molecular ions interact via ionic and other electrostatic forces and do not form intermolecular hydrogen bonds. For the AIL species, we considered two different structures, each with one pyrrolidinium cation and one TFSI− anion with one neutral HTFSI molecule to simulate a high-concentration

Figure 3. IR spectra of HTFSI/PyrrTFSI solutions at different HTFSI concentrations. 6306

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Figure 5. DFT structures of IL clusters. (A) PyrrTFSI (2 cations and 2 anions). (B) PyrrTFSI with HTFSI in N−H−N structure. (C) PyrrTFSI with HTFSI in N−H−O structure.

AIL structure. In Figure 5B, the NH group from the HTFSI acid strongly interacts with the TFSI− imide nitrogen in an N− H−N “bridged proton” configuration. In Figure 5C, the NH from the HTFSI binds to the SO2 group of a TFSI− anion in an N−H−O configuration. The DFT calculations predict that the N−H−O configuration is slightly lower in energy, by ∼2 kJ/ mol. Within the accuracy level of the DFT calculations, the two structures can be considered isoenergetic. To compare the DFT calculations to the experiments, we simulated the IR and Raman spectra using the computed frequencies (scaled by 0.97), relative IR intensities, and relative Raman intensities. The scaled frequencies and intensities were used to generate simulated spectra with Lorentzian profiles. The spectral line widths for the simulations were chosen to match experimental line widths. The experiment−theory comparison for the pure IL system is shown in Figure 6. For the most part, the agreement is very good. In the IR spectra (Figure 6A), the only major discrepancy is that the theory

predicts two bands around 1000 cm−1 (one at 960 cm−1 and one at 1040 cm−1) while only a single sharp band is observed at 1000 cm−1. In the Raman spectra (Figure 6B), the main discrepancy is that the theory predicts a band at 650 cm−1, while experimentally this band appears at ∼740 cm−1. Based on previous literature reports,1 and as discussed above, this band corresponds to the TFSI− −CF3 umbrella mode and may be difficult to predict with harmonic frequency calculations. For both IR and Raman spectra, we performed simulations with only one cation and one anion present. The simulated spectra with two cations and two anions (Figure S1 in Supporting Information) are identical to the spectra presented in Figure 6, which indicates that inaccuracies in the simulations are not due to cluster size effects. We compare the experimental and theoretical spectra for the 2.0 M HTFSI AIL in Figure 7. Simulated IR and Raman spectra for both structures (Figure 5B,C) are considered. From the IR spectra (Figure 7A), the predicted spectra from the two

Figure 6. Comparison between experimental and simulated vibrational spectra for the pure PyrrTFSI, from DFT calculations from Figure 5A. (A) IR spectra. (B) Raman spectra.

Figure 7. Comparison between experimental and simulated vibrational spectra for the bridged-proton in HTFSI/PyrrTFSI, from DFT calculations from Figure 5B,C. (A) IR spectra. (B) Raman spectra. 6307

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increasing HTFSI concentration up to about 1.0 M and then decreases with additional added HTFSI. Electronic absorbance measurements show two bands in the spectrum of the 2.0 M HTFSI AIL solution, and these electronic bands strongly increase the fluorescence of the solution. These results suggest the formation of a new species in solution. The IR spectra show that the structures of the IL molecular ions change as HTFSI concentration increases and evolve into a new structural motif at 2.0 M and then evolve into a different structural motif at even higher HTFSI concentration. The Raman spectra show that with 2.0 M HFTSI in solution the −CF3 umbrella band splits into two bands. DFT calculations show that this spectroscopic observation is consistent with a “bridged-proton” structure in which the NH from the HTFSI molecule bonds directly to the imide nitrogen of the TFSI− anion in an N−H− N configuration. Taken together, the results provide strong evidence for the formation of the N−H−N configuration in AIL solution (although a N−H−O configuration is also likely present). Such a structural configuration is a key component in the Grotthuss-like proton-conduction mechanism hypothesized for AIL solutions involving the TFSI molecular anion. At moderately high Brønsted acid concentration (up to 2.0 M), the formation of this bridged structure contributes positively to high proton/charge conductivity and points to optimum performance for such solutions in technical applications. However, our results suggest that at very high concentration the charge-transport mechanism is diminished due to further structural changes in the AIL solvation structure, which would be consistent with literature reports indicating permanent covalent protonation of the TFSI− anion.

structures are very similar and both match the experiment with essentially the same (fair) degree of agreement. The comparison for the simulated Raman spectra (Figure 7B) provides a more convincing conclusion. It is clear that the position, intensity, and splitting in the −CF3 umbrella mode band at ∼750 cm−1 are reproduced very well with the computed Raman spectrum for the N−H−N configuration. The simulated Raman spectrum for the N−H−O configuration appears to match many of the experimental bands, but the −CF3 umbrella mode band is not as well reproduced for this structure. The experimental and theoretical Raman spectra show that in the pure IL the −CF3 umbrella mode exhibits only a single band. With HTFSI present at 2.0 M in the AIL solution, this mode exhibits two bands split by ∼20 cm−1. The appearance of two bands could correspond to splittings in the degeneracies of the −CF3 groups due to structural changes, or the two bands could arise from the TFSI− anion and HFTSI molecule independently. In either case, the agreement between the experiment and the theory suggest that the structures presented in Figure 5B,C, in which the NH from the HTFSI molecule bonds to the TFSI− (either through the imide nitrogen or sulfonyl oxygen), are consistent with the experiments. Between the two structural possibilities, the N−H−N structure exhibits better agreement with the appearance (position, splitting, and relative intensities) of the CF3 band, which provides evidence to support the presence of the N−H−N configuration. However, both configurations could contribute to the experimental results. The absorption and fluorescence results further support the identification of the “bridged-proton” structure predicted in Figure 5B. Both pure PyrrTFSI and HFTSI are mostly colorless and only weakly fluorescent. The strong visible-light absorbance and fluorescence of the 2.0 M HTFSI AIL (Figure 2) must arise from a structure different from the pure IL molecular anion and cation and from the neutral HTFSI molecule. It is reasonable to suggest that a bridged-proton structure could give rise to a lowenergy excited state involving the imide bonds and SO groups on the TFSI− ions. The observation of the “bridged-proton” structure is consistent with the conductivity and electronic spectroscopic results presented in Figures 1 and 2. The conductivity increase up to 1.0 M HTFSI concentration followed by decrease after 2.0 M HTFSI is consistent with previous results that suggest a solvation structure at low concentration and covalent protonated structure at high concentration. Previous reports for BMITFSI show decrease in conductivity after 0.25 M acid concentration, while the PyrrTFSI reported here appears to exhibit increased conductivity past 1.0 M.1 From the results presented here, the increased conductivity of the HTFSI/ PyrrTFSI AIL solution may be due to the bridged-proton structure providing a faster proton transport at the intermediate concentrations between ∼1 and 2 M acid concentrations. This explanation is consistent with the IR, Raman, and conductivity results in Figures 1, 3, and 4. With increase concentration from 0 to 2 M, the solution structure evolves into the bridged-proton motif, increasing conductivity. However, beyond 2 M the structure further evolves into a covalent protonated motif or other new structures that exhibits low conductivity.

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ASSOCIATED CONTENT

S Supporting Information *

Data, including errors, for Figure 1 and comparison between the simulated spectra for the 1:1 and 2:2 Pyrr:TFSI clusters. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcb.5b02715.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected] (L.Y.). *E-mail [email protected] (T.D.V.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by NSF grants CHE-1362493 and MRI1338014. REFERENCES

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4. CONCLUSIONS HFTSI was added as a Brønsted acid in PyrrTFSI IL to make series of AIL solutions. The AIL conductivity increases with 6308

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DOI: 10.1021/acs.jpcb.5b02715 J. Phys. Chem. B 2015, 119, 6304−6310

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DOI: 10.1021/acs.jpcb.5b02715 J. Phys. Chem. B 2015, 119, 6304−6310