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Correlation Between Soft X-ray Absorption and Emission Spectra of the Nitrogen Atoms Within Imidazolium-Based Ionic Liquids Yuka Horikawa, Takashi Tokushima, Osamu Takahashi, Hiroshi Hoke, and Toshiyuki Takamuku J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.6b04132 • Publication Date (Web): 07 Jul 2016 Downloaded from http://pubs.acs.org on July 19, 2016

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The Journal of Physical Chemistry

Correlation between Soft X-ray Absorption and Emission Spectra of the Nitrogen Atoms within Imidazolium-based Ionic Liquids Yuka Horikawa,a,b Takashi Tokushima,b Osamu Takahashi,c,b Hiroshi Hoke,d and Toshiyuki Takamuku*,d,b

a

Department of Physics and Information Science, Faculty of Science, Yamaguchi University, 1677-1 Yoshida, Yamaguchi 753-8512, Japan b

RIKEN SPring-8 Center, Soft X-ray Spectroscopy Instrumentation Unit, Sayo-cho, Sayo, Hyogo 679-5148, Japan c

Institute for Sustainable Sciences and Development, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima 739-8526, Japan d

Department of Chemistry and Applied Chemistry, Graduate School of Science and Engineering, Saga University, Honjo-machi, Saga 840-8502, Japan

Corresponding Author Tel: +81-952-28-8554 *E-mail: [email protected]

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ABSTRACT

Soft X-ray absorption spectroscopy (XAS) has been performed on the N K-edge of two imidazolium-based

ionic

liquids

bis(trifluoromethylsulfonyl)amide

(ILs)

([C2mim][TFSA])

of

1-ethyl-3-methylimidazolium

and

1-ethyl-3-methylimidazolium

bromide ([C2mim][Br]) to clarify the electronic structure of the ILs. Soft X-ray emission spectroscopy (XES) has also been applied on the ILs by excitation at various X-ray energies according to the XAS spectra. The XAS peaks were able to be fully associated with the XES peaks. Additionally, both XAS and XES spectra of the ILs were well reproduced by the theoretical spectra for a single molecule model on [C2mim]+ and [TFSA]− using the density functional theory (DFT). The assignments for the XAS and XES peaks of the ILs were accomplished from both experimental and theoretical approaches. The theoretical XAS and XES spectra of [C2mim]+ and [TFSA]− did not significantly depend on the conformations of the ions. The reproducibility of the theoretical spectra for the single molecule model suggested that the interactions between the cations and the anions are very weak in the ILs to scarcely influence the electronic structure of the nitrogen atoms at least.


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1. INTRODUCTION

A large number of reports on room temperature ionic liquids (ILs) have been published in various journals in the fields of fundamental and applied chemistries. Most of them aim at application of ILs as a solvent1-7 due to their negligible volatility, nonflammablity, and electroconductivity. Thus, their physicochemical properties, such as thermal stability and wide potential window, should be well understood. Such properties should arise from the electronic structure of cation and anion of ILs. However, the direct observation of the electronic structure for ILs is methodologically limited. Ultraviolet and X-ray photoelectron spectroscopic (UPS and XPS) techniques are one of the methods to observe the electronic states of atoms within ILs.8-13 Unfortunately, the atomic selectivity of UPS and XPS is not excellent for ILs because of the complex spectra due to the overlap of the valence states of atoms within the cation and anion. Furthermore, both techniques give us information mainly on the surface of liquids due to the short mean free path of photoelectrons. Nevertheless, several UPS and XPS investigations have been made on ILs. The previous UPS and XPS investigations on [C2mim][TFSA], together with the metastable impact electron spectroscopy (MIES), showed that the top of the valence band is dominated by the cation-anion combination.8-10 In such investigations, the UPS and XPS spectra have been explained by the DFT calculations.8,13 However, information on the electronic structure of bulk ILs using UPS and XPS techniques has not yet been obtained. 3 ACS Paragon Plus Environment


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Recently, synchrotron facilities enable us to use bright and stable low energy X-rays (soft X-rays) with a wide energy range (ca. 0.1−2 keV). The absorption edges of light elements, such as nitrogen and oxygen, within a sample fall into the soft X-ray energy range. However, soft X-rays may be easily absorbed by air. Hence, sample liquids should be put under a high vacuum to avoid the decay of soft X-rays. In this case, the negligible vapor pressure of ILs is an advantage, i.e., ILs can be placed in a vacuum. However, XAS and XES investigations have been scarcely conducted on ILs. XAS spectra of the K-edge of nitrogen, oxygen, fluorine, chlorine, and sulfur atoms were measured for several imidazolium-based ILs with various anions and alkyl chain lengths.14 The assignments of the observed XAS peaks were qualitatively made by a comparison of the spectra. On the other hand, the XES measurements were

conducted

on

1-butyl-3-methylimidazolium

([C4mim]+)

and

1-methyl-3-octylimidazolium-based ([C8mim]+) ILs with different anions.13,15 The band gap theory was adopted to interpret the results of XES, UPS, XPS, and inverse photoemission spectroscopy (IPES), i.e., the ILs were treated as a resemblance of solid materials. XAS experiments have been made only on [C4mim][PF6]. The local structures around bromide ion in bromide-based ILs have been determined from their XANES spectra for Br K-edge.16-18 However, the electronic structure of such ionic liquids has not been discussed. Thus, the correlation between XAS and XES peaks of ILs has not been reported.


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In the present study, we have conducted both XAS and XES measurements on the N K-edge of [C2mim][TFSA] using the synchrotron facility, SPring-8, Japan, to elucidate the electronic structure of the nitrogen atoms within [C2mim]+ and [TFSA]−. XAS and XES spectra of [C2mim][Br] have also been measured for a comparison. DFT calculations were performed on [C2mim]+ and [TFSA]− with each pair of two conformations; planar and nonplanar of the ethyl group against the imidazolium ring plane19 and the cis- and trans-forms of the two trifluoromethyl groups against the S−N−S bond of [TFSA]− (Figure 1).20 We have succeeded in establishing the correlation between XAS and XES peaks of the ILs. Moreover, the experimental XAS and XES spectra have been reproduced by the theoretical ones calculated within the framework of DFT. Based on the present results from XAS and XES measurements for the N K-edge of the ILs, together with the DFT calculations, the states of the cations and the anions in the ILs have been discussed.

2. EXPERIMENTAL SECTION

2.1. Materials.

Distilled 1-methylimidazole (Wako Pure Chemicals, primary grade) and bromoethane (Tokyo Chemical Industry, extra grade) were reacted with each other in acetonitrile to synthesize [C2mim][Br].21 To obtain [C2mim][TFSA], bromide ions in aqueous solution of 5 ACS Paragon Plus Environment


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[C2mim][Br] were then replaced with TFSA− by slowly dropping aqueous solution of bis(trifluoromethylsulfonyl)amide ([H][TFSA]) (Morita Chemical Industries). The lower phase of [C2mim][TFSA] separated from the upper aqueous phase was dried under vacuum for a few days. [C2mim][Br] is in the crystalline state at 25 °C, while [C2mim][TFSA] is in the liquid state. The purity of the ILs was evaluated by an elemental analysis, density and 1H and 13

C NMR measurements. The water contents of the ILs were less than 50 ppm determined by a

Karl Fischer titration.

2.2 XAS and XES measurements.

XAS and XES spectra for the ILs at a room temperature in a vacuum were recorded at the beamline BL17SU in the synchrotron radiation facility, SPring-8, Japan using circularly polarized soft X-rays.22,23 Each IL was spread on a gold foil pasted on a cupper substrate. The lower edge of the gold foil was bended to keep enough amount of IL on the substrate. The IL on the gold foil was directly set into the sample vacuum chamber. During measurements the state of the IL in the vacuum chamber was monitored using an optical camera (WAT-250D, WATEC Co., Japan) with a zoom lens (MLH-10X, CBC Co., Japan). XAS spectra were recorded as total fluorescence yields (FY) using a 100 mm2 standard Si photodiode (IRD AXUV-100G, Opto Diode Corp., USA). XES spectra were recorded using HEPA2.5 emission spectrometer.24 The energy resolutions of the XAS and XES spectra were 0.1 and 0.42 eV, respectively. 6 ACS Paragon Plus Environment

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Soft X-rays may often damage a sample. In addition, charge-build up occurred for the ILs. Hence, the soft X-ray beam with the spot size of 5−10µm scanned on the IL surface spread on the gold foil to avoid damage of the IL. The scan rate was 0.5 µm/s in the vertical direction during the XAS and XES measurements. Thus, a spot on the IL surface was exposed by X-rays for about 10−20 s. The total irradiation times were 30 and 15 min per sample for the XAS and XES measurements, respectively. Signals due to the charge-build up were not detected by scanning the X-ray beam on the IL surface during measurements. 2.3 Computational methods.

The geometries of [C2mim]+ and [TFSA]− as a model of a single molecule were optimized at B3LYP/6-31G(d,p) level of approximation using Gaussian09 suite of programs.25 The geometries obtained for the cation and anion and the ionization and excitation energies calculated for the ions were listed in the Supporting Information. The XAS spectrum calculations were performed using the StoBe-deMon program;26 details of the computational method for XAS spectroscopy have been described in the previous reports.27-29 The theoretical XAS spectra were generated by the transition potential (DFT-TP) method.27 The gradient-corrected exchange (PD86)30 and correlation functional (PD91)31 established by Perdew and Wang were applied. The orbitals and densities for the species in the excited states were determined using a half-occupied core orbital at the core-ionized site. The orbitals for the excited electron were then obtained by diagonalizing the Kohn–Sham matrix built from this 7 ACS Paragon Plus Environment


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density with the basis set extended by a large set of diffuse basis functions centered on the excited atom. The obtained orbital energies and computed transition moments provided the excitation energy and associated intensities in the theoretical absorption spectrum. For a more accurate estimation of the absolute excitation energy, relativistic and functional corrections were added to the excitation energy.32 The relativistic correction added to the ionization potential was 0.18 eV for the N K-edge. The non-core-excited atoms were described by effective core potentials.33 Finally, the spectra were generated by a Gaussian convolution of the discrete lines by varying the broadenings.28,34

Theoretical calculations of XES spectra35 were performed at the same level for the XAS calculations. Relative intensity of calculated XES spectra was evaluated by dipole transition probabilities between core and valence orbitals. In order to determine the energy position of the excited states, normal ∆Kohn–Sham calculations were performed to compute the ionization energies of the highest occupied molecular orbital (HOMO) and a core orbital.

3. RESULTS AND DISCUSSION

3.1 XAS Spectra of the ILs.

Figure 2 shows XAS spectra of [C2mim][TFSA] and [C2mim][Br] at the N K-edge. The former IL involves the two and one nitrogen atoms in the cation and anion, respectively, while 8 ACS Paragon Plus Environment

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the latter possesses the atoms only in the cation. A prominent peak at 401.9 eV is observed in the spectra for both ILs. Thus, this peak is mainly attributed to the core-excited states of the nitrogen atoms of [C2mim]+, although the nitrogen atom of [TFSA]− may slightly contribute to the peak. This is shown in the +1.0 eV shifted theoretical spectra of [TFSA]− in Figure 3. A small peak at ∼403.5 eV in each spectrum is also thought to arise from the core-excited states of the nitrogen atoms in the cation. This peak for [C2mim][TFSA] is more distinctly stronger than that for [C2mim][Br]. Moreover, the valley at 403 eV for the former is shallower compared to the latter. Two humps at ∼405.5 and ∼408 eV and a wide band centered at ∼408 eV in the spectra of [C2mim][TFSA] and [C2mim][Br], respectively, cannot be easily assigned only by the comparison between the spectra.

To distinguish the peaks observed in both spectra, DFT calculations were performed on [C2mim]+ and [TFSA]− as models of a single molecule. The previous Raman and DFT studies on [C2mim][TFSA]19,20 have shown that the ethyl group of [C2mim]+ has two conformations of planar and nonplanar against the imidazolium ring (Figure 1).19 Additionally, the trifluoromethyl groups of [TFSA]− may exist in the cis- and trans-forms against the S−N−S bond.20 All of the four conformations of the cation and anion were assumed in the DFT calculations. In Figure 3, the theoretical XAS spectra calculated for each conformation of the cation are compared with the experimental spectrum of [C2mim][TFSA]. The theoretical oscillator strengths of absorption for the N1 and N3 atoms of the cation with the two 9 ACS Paragon Plus Environment


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conformations of [C2mim]+ are shown by the red and black bars, respectively (Figure 1 shows the notation of the nitrogen atoms). The conformation dependence is not significant in the theoretical spectra of the cation. Hence, the change in the conformation does not remarkably influence the spectrum shape. The theoretical spectra suggest that the prominent peak at 401.9 eV in the experimental XAS spectra for [C2mim][TFSA] and [C2mim][Br] can be ascribed to the N 1s → π* excitation of the cation. The wide band centered at ∼408 eV is mainly assigned to the absorption of the core-excited states from the nitrogen atoms in the cation. Particularly, this is reasonable with the wide band observed for [C2mim][Br] due to no nitrogen atoms in the anion. The theoretical absorptions for the N1 and N3 atoms of [C2mim]+ at 403.8 eV as shown by the small bars well explain the small peak at ∼403.5 eV in the experimental spectrum.

As shown by the solid lines in Figure 3, the theoretical spectra for the cis- and trans-forms of [TFSA]− are not remarkably different from each other as found for the cation. Main and satellite peaks at 403 and 401.5 eV in the theoretical spectra correspond to the valley and the prominent peak in the experimental spectrum, respectively. Hence, the photon energy of the theoretical spectra of [TFSA]− seems to be underestimated by the DFT calculations. The energy shift may arise from the use of the simple model for the single molecule and underestimated work functions of the IL. As shown by the dashed lines, the shift of +1.0 eV brings the agreement of the positions of the main and satellite peaks in the theoretical spectra 10 ACS Paragon Plus Environment

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with those of the small peak at ∼403.5 eV and the valley at 403 eV in the experimental spectrum. This is reasonable to the features of the experimental spectrum of [C2mim][TFSA]; the stronger peak at ∼403.5 eV and the shallower valley at 403 eV in the spectrum of [C2mim][TFSA] than those of [C2mim][Br] (Figure 2). A tail above ∼406 eV in the shifted theoretical spectra of [TFSA]− contributes to the broad band centered at 408 eV in the experimental spectrum of [C2mim][TFSA]. However, the assignments of the small peaks at ∼403.5 and ∼405.5 eV in the XAS spectra still include ambiguity. Thus, XES measurements were conducted on the N K-edge of [C2mim][TFSA] and [C2mim][Br] with tuning the excitation energy according to the XAS spectra.

3.2 XES Spectra of the ILs.

Figure 4 indicates the N K-edge XES spectra obtained by excitation at various energies as depicted in the XAS spectra inset. When the nitrogen atoms within [C2mim][TFSA] and [C2mim][Br] are excited at high energies of 415.6 and 412.3 eV, respectively, non-resonant XES spectra for the ILs are obtained as shown in the top layer of Figure 4. A peak observed at the excitation energy in the XES spectrum results from the elastic scattering of the incident X-rays. In the XES spectrum for [C2mim][TFSA], three XES peaks appear at 392.0, 394.0, and 396.4 eV. Tiny humps at 383 and 387 eV are also found. In the spectrum of [C2mim][Br], the strongest peak at 394.0 eV observed for [C2mim][TFSA] cannot be found, whereas the



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other peaks appear at the same energies as those for [C2mim][TFSA]. Hence, the peak at 394.0 eV may be assigned to the HOMO of [TFSA]−.

According to the XAS spectra, excitation energies were chosen to selectively observe each component. Resonant XES spectra measured at the excitation energies of 408.3, 405.1, 403.5, and 401.9 eV are shown in the second, third, fourth, and bottom layers of Figure 4, respectively. At the excitation energy of 408.3 eV, where the wide band is observed in the XAS spectra for both ILs, the resonant XES spectra of the ILs do not markedly differ from the non-resonant spectra. Therefore, the wide band of the XAS spectra for both ILs consists of various absorptions of the cation, especially, both cation and anion of [C2mim][TFSA] because the peak at 394.0 eV is still observed in the XES spectrum. This is consistent with the theoretical XAS spectra of the cation and anion, as shown in Figure 3. When the nitrogen atoms of the ILs are excited at 405.1 eV corresponding to the small XAS peak of the [TFSA]−−IL, the peak at 394.0 eV remarkably strengthens in the XES spectrum of the [TFSA]−−IL (the third layer of Figure 4). In contrast, the peaks at 392.0 and 396.4 eV for the [TFSA]−−IL significantly weaken. Both peaks in the spectrum of the [Br]−−IL also become weaker than those excited at 408.3 eV. Thus, the small XAS peak at 405.5 eV for the [TFSA]−−IL mainly arises from the absorption of the anion. However, the nitrogen atoms of the cation still contribute to the peak at 405.5 eV because of the partial overlap of the wide band centered at 408 eV. In fact, the energy of 405.5 eV corresponds to the low energy side of 12 ACS Paragon Plus Environment

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the wide band of the [Br]−−IL. At the excitation energy of 403.5 eV, where the small XAS peak appears for both ILs, the peak at 394.0 eV is still strong in the XES spectrum of the [TFSA]−−IL (the fourth layer of Figure 4). However, the peaks at 392.0 and 396.4 eV become weaker than those excited at 405.1 eV. The corresponding XES peaks for the [Br]−−IL are observed as very weak humps. These results agree with the discussion on the XAS spectra above. Thus, the small XAS peak at 403.5 eV for both ILs is attributed to the absorption for the nitrogen atoms of the cation, however, the contribution of the nitrogen atom of the anion is the main for [C2mim][TFSA]. This shows the validity of the shift of the theoretical XAS spectra for [TFSA]− with +1.0 eV.

Furthermore, the validity of the shift of the theoretical XAS spectra for [TFSA]− is proved by the relation between the experimental XES spectrum for [C2mim][TFSA] and the excitation energies. In the present study, XAS spectra were measured as FY by detecting total intensity of the X-ray emission using photodiode. Thus, in the case of a multi-component system including the same elements, such as the nitrogen atoms of [C2mim]+ and [TFSA]− in the IL, the experimental XAS spectrum consists of both spectra for cation and anion. The spectrum cannot be easily separated into each component of the cation and anion. If X-ray emission peaks of the ions are independent of each other in XES spectra, XAS spectrum for each cation and anion may be reproduced from XES spectra measured at various excitation energies. Figure 5 shows the change of the N K-edge XES spectra for [C2mim][TFSA] with varying the 13 ACS Paragon Plus Environment


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excitation energy from 401.1 to 415.7 eV. As discussed on the non-resonant XES spectrum of [C2mim][TFSA], the strongest peak at 394.0 eV in Region A is assigned to the X-ray emissions from [TFSA]−. On the other hand, the large peak at 392.0 eV in Region B is attributable to the emissions from [C2mim]+ as discussed below. Fortunately, the XES peaks for both cation and anion individually appear in the XES spectra. Thus, XAS spectrum of each cation and anion of the IL can be reproduced from the limited integration windows of Regions B and A in the XES spectra, respectively. The X-ray emission intensities of Regions A and B were firstly estimated by integration of the intensities of each region in the XES spectra. In Figure 6, the intensities estimated for Regions A and B are plotted against the excitation energies. The points are connected by a solid line for clarity of the change in the intensities against the photon energy. The solid lines for Regions A and B correspond to the reproduced absorption spectra for [TFSA]− and [C2mim]+, respectively. The experimental XAS spectrum of [C2mim][TFSA] is also depicted for a comparison in Figure 6. The reproduced spectrum for [C2mim]+ well explains the prominent peak at 401.9 eV in the experimental XAS spectrum. Small peaks at 403.5 and 405.5 eV in the reproduced spectrum agree with those in the experimental spectrum. Furthermore, the reproduced spectrum for [C2mim]+ bears a resemblance to the theoretical spectra calculated by the DFT (Figure 3).

Figure 6 shows that the shape of the reproduced spectrum for [TFSA]− is also comparable with that of the theoretical spectra for the anion depicted by the solid lines in Figure 3. The 14 ACS Paragon Plus Environment

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main peak in the reproduced spectrum is observed at 404.06 eV. This value is higher by +1.0 eV compared to the original position of the main peak in the theoretical XAS spectra from the DFT calculations. This agrees with the above discussion; the photon energy of the theoretical XAS spectra for [TFSA]− was underestimated by the DFT. When the reproduced spectra for [C2mim]+ and [TFSA]− are summed up to estimate the total XAS spectrum, the valley of the reproduced spectrum for [C2mim]+ in the energy range from 403 to 405 eV will be raised by the broad peak of the reproduced spectrum for [TFSA]−. This is consistent with the stronger peak at ∼403.5 eV and the shallower valley at 403 eV in the experimental XAS spectrum of [C2mim][TFSA] compared to those of [C2mim][Br] (Figure 2). These facts show the reasonable position of the main peak at 404.06 eV for [TFSA]−. Hence, the theoretical XAS spectra for [TFSA]− with the cis- and trans-forms calculated by the DFT should be shifted to +1.0 eV, as shown by the dashed lines in Figure 3.

In Figure 4, as the nitrogen atoms of the ILs are excited at 401.9 eV, where the prominent XAS peak is observed for both ILs, the peak at 394.0 eV is scarcely observed in the XES spectrum of [C2mim][TFSA], whereas the two peaks at both sides of the 394.0 eV peak re-strengthen for both ILs. These results are consistent with the assignment of the prominent XAS peak to the X-ray absorption by the nitrogen atoms of the cation. The two XES peaks at 391.5 and 395.0 eV shift toward the lower energy about 1 eV compared to those in the non-resonant XES spectra. This may arise from a spectator shift due to the localized excited 15 ACS Paragon Plus Environment


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electron.36,37 The present results show that the two approaches for the comparison of spectra of the ILs with and without the nitrogen atom of the anion and the DFT calculations are compatible with each other.

To surely establish the assignments of the XAS and XES peaks through their correlation, the experimental XES spectra of [C2mim][TFSA] and [C2mim][Br] are compared with the theoretical ones from the DFT calculations. The theoretical XES spectra of [C2mim]+ and [TFSA]− are depicted in Figure 7, accompanied by the non-resonant experimental spectra of the ILs. The theoretical spectra of the cation and anion are indicated by a shift of −0.5 and +3.8 eV, respectively, to adjust the position of the main peak in the theoretical spectra to that in the experimental ones. It is more difficult to determine the absolute X-ray emission energies of XES spectra by the DFT calculations compared to the photon energies of XAS spectra. The conformation dependence of the theoretical XES spectra for both cation and anion is not significantly observed as well as the XAS spectra. The theoretical XES spectra of the cation with the planar and nonplanar conformations well reproduce the experimental one for [C2mim][Br]. The theoretical spectra of the cation also explain the experimental one for [C2mim][TFSA], except for the strongest peak at 394.0 eV. The two strong peaks at 392.0 and 396.4 eV in the XES spectra of both ILs are surely attributed to the emissions from the cation. The tiny humps at 383 and 387 eV observed in the experimental XES spectra of both ILs are satisfactorily reproduced by the theoretical spectra for the cation calculated by the DFT, 16 ACS Paragon Plus Environment

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although the theoretical peaks appear at the slightly higher energy compared to the experimental ones. The strongest peak at 394.0 eV for the [TFSA]−−IL can be explained by the dominant peak in the theoretical spectra of [TFSA]− with both cis- and trans-forms. The theoretical XES spectra of [TFSA]− suggest the contribution of the other small peaks at 380.3, 382.8, 387.0, and 390.0 eV to the XES spectrum of [C2mim][TFSA]. Indeed, the large peak at 392.0 eV for the nitrogen atoms of the cation in the non-resonant XES spectrum of the [TFSA]−−IL has a shoulder at the lower energy, while the shoulder is not observed in the non-resonant spectrum of the [Br]−−IL. The shoulder may be due to the small contribution of the peak at 390.0 eV in the theoretical spectra of [TFSA]−. Despite the remarkable intensities, the theoretical peak at 380.3 eV is not found in the experimental XES spectrum. The X-ray emission at the energy may be suppressed in the liquid state by the interactions between the cation and anion.

3.3 States of the cation and the anion in the ILs.

The previous Raman studies with the DFT calculations19,20 suggest that the energetic barriers for the conformational changes of the planar and nonplanar [C2mim]+ and the cis- and trans-[TFSA]− are very small. Thus, both conformers of the cation and anion coexist in [C2mim][TFSA]. In the present study, the theoretical XAS and XES spectra for the cation and anion do not significantly depend on the conformations of each ion. Then, the theoretical spectra of the four conformations of the ions can well explain the experimental XAS and XES 17 ACS Paragon Plus Environment


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spectra. There are few unreproducible peaks in the experimental spectra. Hence, the XAS and XES spectra suggest that the electronic structures for the nitrogen atoms in the cation and anion are not significantly affected by their conformational change, although the nitrogen atoms are located in the central part of the cation and anion and are directory bound to the functional groups.

One can expect that the electronic structures of the cation and anion might be remarkably affected by each other in the ILs due to the electrostatic forces of both ions. However, the present results suggest that the weak interactions between the cations and the anions. In the present DFT calculations, any interactions between [C2mim]+ and [TFSA]− were not assumed, i.e., the single molecular model was adopted for the calculations of the cation and anion. Nevertheless, the experimental XAS and XES spectra of [C2mim][TFSA] are well reproduced by the theoretical spectra of the isolated ions based on the DFT calculations. This suggests that the cations and the anions weakly interact with each other in the IL. This consideration is consistent with the previous X-ray diffraction study for [C2mim][TFSA]; [TFSA]− widely and loosely distributes around the imidazolium ring of [C2mim]+ in the liquid structure.38 Such floppy distribution of the anion around the cation in the liquid state may result from very weak interaction between the cations and the anions. The present XAS and XES experiments with the DFT calculations prove the weak interactions between [C2mim]+ and [TFSA]−.


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On the other hand, the experimental XAS and XES spectra of [C2mim][Br] can also be explained by the DFT calculation for the cation isolated, although the three hydrogen atoms of the imidazolium ring are hydrogen-bonded with bromide ions in the crystalline state as previously reported.16,39 Thus, the interionic interactions in the [Br]−−IL is not strong to significantly affect the electronic structure of the nitrogen atoms in the cation at least. As shown in Figure 2, the dominant peak at 401.9 eV in the XAS spectrum for the [TFSA]−−IL hardly shifts by the replacement of the anion by [Br]−. This is in agreement with the previous XAS investigation on the N K-edge of the imidazolium-based IL with several anions, such as [Cl]−, [Br]−, [PF6]−, and [TFSA]−, and the different alkyl chain lengths.14 The photon energies of the first prominent peak in the XAS spectra of the ILs are independent of the anions and the alkyl chain lengths. In contrast, the previous XPS investigation showed that the binding energies of the N 1s peak for the nitrogen atoms within [C8mim]+−IL depend on the anions, e.g. the energy for [C8mim][TFSA] is higher of 0.36 eV compared to that for [C8mim][Br].40 It has been concluded that the shift of the N 1s peak is caused by the weak charge transfer from [Br]− to [C8mim]+. The reasons for such difference between the XAS and XPS results have not been established yet. Further investigations of both XAS and XPS measurements on ILs are needed to solve this question. In the present stage, a plausible reason may be considered by the following. The charge transfer from the anion to the cation may induce the change in the energies of both core level and valence band of the nitrogen atoms. XAS observes the electron 19 ACS Paragon Plus Environment


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transition between the core level and the valence band. Thus, the photon energy of the XAS peak of the imidazolium ring nitrogen atoms might not significantly change, if the energy gap between the core level and the valence band would scarcely change with the charge transition, in other word, both core and valence band levels would shift with the same extent.

4. CONCLUSIONS

XAS and XES spectra for [C2mim][TFSA] and [C2mim][Br] at the N K-edge were measured using synchrotron light source, SPring-8. Both XAS and XES spectra could be well explained by the theoretical ones calculated by the DFT using the single molecular model. Additionally, the XAS peaks for the nitrogen atoms of the ILs could be well correlated with the XES peaks. The two conformations of [C2mim]+ and [TFSA]− did not markedly affect the XAS and XES spectra of the N K-edge. The present experimental and theoretical results suggested that the cations and the anions very weakly interact with each other in the ILs to scarcely influence the electronic structure of the nitrogen atoms of the ILs.

ASSOCIATED CONTENT


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Supporting Information. The geometries of [C2mim]+ and [TFSA]−, their atomic coordinates, the ionization and excitation energies calculated by DFT. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION

Corresponding Author Tel: +81-952-28-8554 Email: [email protected]

ACKNOWLEDGMENT

This work was supported partly by Grants-in-Aid (No. 26410018 (T.Ta.) and No. 15K04755 (O.T.)) from the Japan Society for the Promotion of Science and Dean’s Grant for Progressive Research Projects from Saga University. XAS and XES experiments at SPring-8 BL17SU were carried out with the approval of the RIKEN SPring-8 Center (Proposal Nos. 20140050 and 20150064). In addition, this work was partially supported by the RIKEN SPDR program. We would like to thank Dr. M. Oura for his valuable help with experiments at BL17SU. The density and NMR measurements of the ILs for evaluation of their purity were made at the Analytical Research Center for Experimental Sciences of Saga University. The DFT 21 ACS Paragon Plus Environment


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computations were performed using a Fujitsu PRIMEQUEST installed at the Research Center for Computational Science, Okazaki, Japan.

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Figure Captions

Figure 1. Conformations of [C2mim]+ and [TFSA]−.

Figure 2. Experimental N K-edge XAS spectra for [C2mim][TFSA] (black line) and [C2mim][Br] (green line).

Figure 3. Comparison of N K-edge XAS spectra for [C2mim][TFSA] with theoretical spectra for the planar and nonplanar conformations of [C2mim]+ and the cis- and trans-forms of [TFSA]− calculated by DFT. The dashed lines represent the theoretical spectra for [TFSA]− shifted with +1.0 eV from the original spectra (solid lines) obtained from DFT calculations. The theoretical oscillator strengths are indicated by bars, especially, those for the N1 and N3 atoms of [C2mim]+ are distinguished by red and black bars, respectively.

Figure 4. N K-edge XES spectra for [C2mim][TFSA] (black lines) and [C2mim][Br] (green lines) measured by excitation at various energies depicted in XAS spectra in the inset.

Figure 5. XES spectra for [C2mim][TFSA] with varying excitation X-ray energy from 401.1 to 415.7 eV.

Figure 6. Comparison of experimental XAS spectrum of [C2mim][TFSA] with reproduced spectra of [TFSA]− and [C2mim]+ obtained from the intensities for Regions A and B in XES spectra (Figure 5), respectively

Figure 7. Comparison of non-resonant N K-edge XES spectra for [C2mim][TFSA] and [C2mim][Br] with theoretical spectra for the planar and nonplanar conformations of [C2mim]+ 26 ACS Paragon Plus Environment

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and the cis- and trans-forms of [TFSA]− calculated by DFT. The theoretical emissions of the nitrogen atoms are indicated by bars, especially, those for the N1 and N3 atoms of [C2mim]+ are distinguished by red and black bars, respectively.



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Figure 1. Conformations of [C2mim]+ and [TFSA]−.


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Figure 2. Experimental N K-edge XAS spectra for [C2mim][TFSA] (black line) and [C2mim][Br] (green line).



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Figure 3. Comparison of N K-edge XAS spectra for [C2mim][TFSA] with theoretical spectra for the planar and nonplanar conformations of [C2mim]+ and the cis- and trans-forms of [TFSA]− calculated by DFT. The dashed lines represent the theoretical spectra for [TFSA]− shifted with +1.0 eV from the original spectra (solid lines) obtained from DFT calculations. The theoretical oscillator strengths are indicated by bars, especially, those for the N1 and N3 atoms of [C2mim]+ are distinguished by red and black bars, respectively.


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Figure 4. N K-edge XES spectra for [C2mim][TFSA] (black lines) and [C2mim][Br] (green lines) measured by excitation at various energies depicted in XAS spectra in the inset.



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Figure 5. XES spectra for [C2mim][TFSA] with varying excitation X-ray energy from 401.1 to 415.7 eV.


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Figure 6. Comparison of experimental XAS spectrum of [C2mim][TFSA] with reproduced spectra of [TFSA]− and [C2mim]+ obtained from the intensities for Regions A and B in XES spectra (Figure 5), respectively.



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Figure 7. Comparison of non-resonant N K-edge XES spectra for [C2mim][TFSA] and [C2mim][Br] with theoretical spectra for the planar and nonplanar conformations of [C2mim]+ and the cis- and trans-forms of [TFSA]− calculated by DFT. The theoretical emissions of the nitrogen atoms are indicated by bars, especially, those for the N1 and N3 atoms of [C2mim]+ are distinguished by red and black bars, respectively.


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