The structure and interaction properties of two task-specific ionic liquids and acetonitrile mixtures: A combined FTIR and DFT study

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 226 (2020) 117641

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The structure and interaction properties of two task-specific ionic liquids and acetonitrile mixtures: A combined FTIR and DFT study Yan-Zhen Zheng a, Yu Zhou b, Geng Deng c, Rui Guo a, Da-Fu Chen a, * a

College of Animal Sciences (College of Bee Science), Fujian Agriculture and Forestry University, Fuzhou, 350002, PR China School of Chemistry and Chemical Engineering, Qingdao University, Qingdao, 266071, PR China c Key Laboratory of Bioorganic Phosphorous Chemistry and Chemical Biology (Ministry of Education), Department of Chemistry, Tsinghua University, Beijing, 100084, PR China b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 7 January 2019 Received in revised form 8 October 2019 Accepted 8 October 2019 Available online xxx

The mixtures of ionic liquid (IL) and acetonitrile (CH3CN) can be used as reaction media, supercapacitors and thermally stable electrolytes. The macroscopic properties of ILs-CH3CN mixtures have been extensively studied. However, some fundamental questions regarding the microscopic properties of ILs-CH3CN mixtures still remain to be answered. In this work, the structure properties and hydrogen-bond interactions of two task-specific ILs, i.e., 1-propylnitrile-3-methylimidazolium bis(trifluoromethylsulfonyl) imide ([PCNMIM][Tf2N]) and 1-(20 -hydroxylethyl)-3-methylimidazolium bis(trifluoromethylsulfonyl) imide ([C2OHMIM][Tf2N]), and CH3CN were studied using the combination of Fourier transform infrared spectroscopy (FTIR) and density functional theory (DFT) calculations. The aromatic C‒H stretching vibration region of the cation was an area of special focus. Excess infrared spectroscopy with enhanced resolution was applied to analyse the original infrared spectra. It is found that: (1) The two ILs form stable hydrogen-bonds with CH3CN. (2) Ion cluster, ion clusteracetonitrile, and ion pairacetonitrile are identified in the mixture. Acetonitrile cannot break apart the strong electronic interaction between the cation and anion in the examined concentration range. (3) The hydrogen-bonds are weak strength, closed shell and electrostatic dominant interactions. (4) The preferred interaction site of [PCNMIM]þ cation is the hydrogen atom at the C2 site, while that of [C2OHMIM]þ cation is the hydrogen atom in the hydroxyl group. © 2019 Elsevier B.V. All rights reserved.

Keywords: Ionic liquids Hydrogen-bond Excess infrared spectra Acetonitrile Density functional theory Fourier transform infrared spectroscopy

1. Introduction Ionic liquids (ILs) are a type of charged fluid chemical entirely composed of organic cations and inorganic or organic anions. ILs remain liquid near or at approximately room temperature [1e4]. ILs are a subject of rapidly growing interest due to their unique physicochemical properties such as extremely low vaporization pressure, wide electrochemical window, excellent chemical and thermal stability, remarkable ion conductivity and favourable solvation behaviour [5e8]. Another amazing property of ILs is that ILs can be tailored towards specific processes and acquire desired properties by the combinations of different cations and anions. These attractive properties allow ILs to be used or potentially applied in synthesis [9,10], electrochemistry [11,12], organic catalysts [13,14], lubricant additives [15,16], gas absorbents [17,18], and many other fields.

* Corresponding author. E-mail address: [email protected] (D.-F. Chen). https://doi.org/10.1016/j.saa.2019.117641 1386-1425/© 2019 Elsevier B.V. All rights reserved.

Similar to anything else in nature, ILs are not perfect. Their practical uses are greatly hindered by their high viscosities [19e21], which are normally 102e103 times higher than those of traditional molecular solvents [19]. It is difficult to design a novel class of ILs that shows low viscosity and demonstrates the attractive properties of ILs as their high viscosities result from the strong Coulombic electrostatic interactions between the charged species of ILs [22,23]. The high viscosity would consequently lead to the decrease in mass transfer rates and increase in pumping costs [20,21]. A simple strategy to improve the fluidity of ILs is the use of cosolvents to reduce the viscosity of ILs. IL-cosolvent mixtures have greatly expanded the usage of ILs. It is found that mixing ILs and molecular solvents can enhance the efficiency of auto-partitioning extracted lipids as well as oil from lipid-bearing biomass [24,25]. An increased extraction efficiency in liquidliquid extractions was also observed in mixtures of molecular cosolvents and ILs [26,27]. Furthermore, the mixing of ILs and some cosolvents can enhance cellulose dissolution [28,29]. The mixing with a molecular cosolvent not only increases the fluidity but also modifies the related properties of the ILs system

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provide more information than the original IR spectra [46,53,54,59,60]. This method can illustrate the transformation of association complexes in the binary systems [46,53,54,60]. In this work, excess infrared spectroscopy was applied to analyse the original IR spectra.

such as electrical conductivity, diffusivity and polarity [30e32]. The macroscopic physicochemical properties are directly influenced by the microstructure of the system. In terms of the development and efficient practical use of an ILecosolvent mixture, the key fundamental issue is to understand the structure and interaction properties of the mixture and to establish a comprehensive microscopic understanding of the system. In a rough approximation, the ILcosolvent solvents can be regarded as electrolyte solutions, in which the mutual effect of three forces, i.e., electrostatic interactions, dispersive forces and hydrogen-bonding interactions, controls the microstructure [33e36]. There is no doubt that the electrostatic interaction plays the most vital role. The importance of the hydrogen-bonding interaction is also worthy of attention. It is believed that the hydrogen-bonding interaction can introduce a random factor into the ordered structure constructed by the electrostatic interaction and lead to a reduced melting point and increased fluidity of the ILs [37,38]. The dissolution of cellulose by ILs is mainly determined by the hydrogen-bonding interactions between the anion and the hydroxyl group of the cellulose [29,39]. The mixing with aprotic solvents such as dimethyl formamide and dimethylsulfoxide can partially break down the ionic association of ILs, and the dissociated anion can readily interact with cellulose resulting in an increase in cellulose solubility. In contrast, protic solvents such as H2O and CH3OH can compete with cellulose for the hydrogen-bonding interactions resulting in a decrease in the cellulose solubility [29,39]. Acetonitrile (CH3CN) is an important polar liquid that is able to solvate hydrophilic, hydrophobic, inorganic, and organic substances [40]. Mixtures of CH3CN and ILs show better ionic con-

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2. Experimental sections 2.1. Materials and sample preparation In this work, the properties of the samples are described in Table 1. Deuterated acetonitrile (CD3CN) was used to avoid the overlap of the absorption bands with the CeH stretching bands of the ILs. A series of different concentration mixtures was prepared by weighing the two components. The mole fraction of CD3CN in the mixtures ranged from 0.1 to 0.9 increasing by increments of 0.1. 2.2. FTIR spectroscopy A Nicolet 5700 FTIR spectrometer with an MCT detector was used to perform the IR measurements. This spectrometer is equipped with trapezoidal ZnSe crystal in a horizontal ATR cell. The ATR cell was carefully dried before each measurement. The spectral range of this work was from 4000 cm1 to 650 cm1. Each spectrum was obtained using an accumulation of 32 parallel scans, a resolution of 2 cm1 and a zero-filling factor. To obtain the penetration depth (de), ATR correction was conducted by the following equation [61]:

1



B C 2n21 cos q 2 sin2 q  n221 B C Nl 2n21 cos q C* de ¼ B þ 1 1       B
C 4pn  2 2 1 2 2 2 @ 1  n2 2 2 4 2 2 sin q  n21 n21 cos q þ sin q  n21 A sin q  n21 21

ductivity and mobility than pure ILs [41]. Mixtures of ILs-CH3CN can be used as reaction media for organic synthesis, supercapacitors with low toxicity and high efficiency, and thermally stable electrolytes of solar cells and Li-ion batteries [42,43]. It is also reported that the nitrile group of CH3CN can be used as a molecular infrared probe to detect the Lewis acidity and microenvironments of ILs [44e46]. The macroscopic properties of ILsCH3CN mixtures have been extensively studied [30,47]. However, some fundamental questions regarding the microscopic properties of ILs-CH3CN mixtures still remain to be answered. In this work, we pay special attention to the structural properties and hydrogen-bonding interactions of mixtures of CD3CN and two task-specific ILs: 1-propylnitrile-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([PCNMIM][Tf2N]) and 1-(20 -hydroxylethyl)-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C2OHMIM][Tf2N]). The structure of [PCNMIM][Tf2N] is shown in Fig. 1A and that of [C2OHMIM][Tf2N] is shown in Fig. 1B. Among the different experimental and theoretical techniques, Fourier transform infrared spectroscopy (FTIR) and density functional theory (DFT) calculations are convenient and effective approaches to study the microscopic properties of ILs at the molecular level [37,38,46,48e58]. In this work, a combination of FTIR and DFT calculations was applied to study the structure and hydrogenbonding interaction properties of [PCNMIM][Tf2N]/[C2OHMIM] [Tf2N] and CH3CN binary mixtures. Excess infrared absorption spectroscopy can be used to analyse the original IR spectra and can

(1)

In equation (1), n21 is the refractive index ratio of the sample to the crystal; n1 is the refractive index of the crystal. For ZnSe, the refractive index is 2.43. q is the angle of incidence, and in this work, it is 45 . N is the number of reflections, and in this work, it is 12. l is the wavelength of incident light. We used a Shenguang WYA-2S Abbe refractometer to measure the refractive index. Excess spectroscopy can enhance the spectral resolution [46,53,54,59,60]. Excess spectroscopy is suitable for analysing the original IR spectra of the IL-cosolvent system. In this work, excess spectroscopy was applied to analyse the original IR data. The concept of excess spectroscopy is devised from excess thermodynamic functions. Briefly, an excess spectrum is defined as a difference spectrum between the spectra of real and ideal solutions [59,60]. The calculated equation for the excess spectra of a binary mixture system is as follows [59,60]:

εE ¼


 A  x1 ε*1 þ x2 ε*2 dðC1 þ C2 Þ

(2)

In equation (2), the two components are numbered as 1 and 2. A is the IR absorbance of the mixture; d is the light path length of the mixture and is solved using equation (1); C is the molarity in the mixture; x is the mole fraction in the mixture; and ε* is the molar absorption coefficient of the pure component.

Y.-Z. Zheng et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 226 (2020) 117641

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Fig. 1. ATR-FTIR (A and B) and excess infrared (C and D) spectra of [PCNMIM][Tf2N]eCD3CN (A and C) and [C2OHMIM][Tf2N]eCD3CN (B and D) systems in 3250 cm1e2850 cm1 region. The mole fractions of CD3CN are labelled in the excess spectra (C and D). The dashed-dotted and dashed lines in A and B depict the spectra of pure ILs and CD3CN, respectively.

Table 1 Sample table. Chemical name a

[PCNMIM][Tf2N] [C2OHMIM][Tf2N]b CD3CNc a b c

Source

Initial purity specified by supplier

Purification method

Final mass fraction purity

Analysis method

Cheng Jie Chemical Cheng Jie Chemical Cambridge Isotopes Laboratories

0.99 0.99 0.998

Evacuation Evacuation

0.998 0.997

Karl Fischer titration Karl Fischer titration

1-propylnitrile-3-methylimidazolium bis(trifluoromethylsulfonyl)imide. 1-(20 -hydroxylethyl)-3-methylimidazolium bis(trifluoromethylsulfonyl)imide. Deuteration acetonitrile.

2.3. Quantum chemical calculations The electronic structure, optimized geometry and frequency property of the individual cation and anion ([PCNMIM]þ, [C2OHMIM]þ and [Tf2N]), CD3CN monomer, [PCNMIM]þCD3CD complex, [C2OHMIM]þCD3CD complex, [PCNMIM]þ[Tf2N] complex, [C2OHMIM]þ[Tf2N] complex, 2[PCNMIM]þ2[Tf2N] complex, 2[C2OHMIM]þ2[Tf2N] complex, [PCNMIM][Tf2N] CD3CN complex, [C2OHMIM][Tf2N]CD3CN complex, 2[PCNMIM]2 [Tf2N]CD3CN complex and 2[C2OHMIM]2[Tf2N]CD3CN complex were computed by the DFT calculations using the Gaussian 09 program package [62]. The Grimme’s dispersion correction D3 is essential in systems with noncovalent interactions [63,64]. In addition, the correction of Becke’s three parameter hybrid exchange functionals with Lee-Yang-Parr correlation functional (B3LYP) and D3 could well describe weak interactions [65]. In this work, the B3LYP-D3(BJ) method combined with the 6-311G** basis set was employed to conduct the structure optimization and frequency analysis. First, the structures of [PCNMIM]þ, [C2OHMIM]þ,

[Tf2N] and CD3CN were optimized. Then, the structures of different complexes were optimized using the optimized structures of [PCNMIM]þ, [C2OHMIM]þ, [Tf2N] and CD3CN. The harmonic vibrational frequencies were computed to verify that each geometry was a true minimum and were found to have no negative frequencies for the most stable structures. Single point energy calculations were calculated for the optimized structures using the B3LYP-D3(BJ)/6-311 þ G(2d,p) method. The interaction energies were corrected using the counterpoise (CP) method to avoid the influence of the basis set superposition error (BSSE) [66]. In the frequency analysis, the frequency correction factor was used to consider the amount of thermodynamic correction. The frequency correction factor was 0.967 for the method used [67]. The atoms in molecules (AIM) theory devised by Bader was used to study the topological properties of the hydrogen-bond [68]. This theory was performed on the final optimized structures obtained by the B3LYP-D3(BJ)/6-311G** method. Five topological descriptors at the bond critical point (BCP), namely, the electron density (rBCP), the Laplacian of the electron density (V2rBCP), the Lagrangian

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kinetic energy (GBCP), the potential electron density (VBCP) and the energy density (HBCP), were used to characterize the hydrogenbond. The search for the BCPs at the hydrogen-bond and the detailed topological analysis were performed using the Multiwfn 3.5 suite [69]. 3. Results and discussion 3.1. The original infrared spectra in the v(CH) region The CH stretching vibrational regions, especially those in the aromatic ring of the cation, contain rich information about the structure and interaction properties of ILs. In this work, the ATRFTIR spectra in the 32502850 cm1 wavenumber region that belongs to v(CH) are shown in Fig. 1A and B. The IR spectra of pure ILs are denoted by dashed lines and that of pure CD3CN is present in a dashed-dotted line. For [PCNMIM][Tf2N], the bands at approximately 3158.8 cm1, 3121.6 cm1 and 2967.7 cm1 are from the absorbance of v(C4,5H), v(C2H) and v(alkyl CH), respectively [46,53]. For [C2OHMIM][Tf2N], the bands at approximately 3160.6 cm1 and 3123.3 cm1 are from the absorbance of v(C4,5H) and v(C2H), respectively, and those at approximately 2966.9 cm1 and 2895.4 cm1 are from the absorbance of v(alkyl CH) [46,53]. In pure ILs, shoulder peaks can be found at approximately 3104 cm1 and 3106 cm1 for [PCNMIM][Tf2N] and [C2OHMIM][Tf2N], respectively. In the 32502850 cm1 wavenumber region, CD3CN has a weak absorption band approximately 3087.5 cm1, which is attributed to the combination tone of v(C≡N) and v(CC) [46,53]. The ATR-FTIR spectra of [PCNMIM][Tf2N]CD3CN and [C2OHMIM][Tf2N]CD3CN systems in different concentration ranges are shown in Fig. 1A and B. With the increase in x(CD3CN), all of the intensities of v(CH) show a gradual decrease; the band widths of v(C4,5eH) become broader. With the addition of CD3CN, the relative intensities of the shoulder peak gradually increase. However, as the peak position of the shoulder peak of v(C2H) is close to the combination tone of v(C≡N) and v(CC) of CD3CN, the origin of the increasing intensities of the shoulder peak cannot be distinguished directly from the original infrared spectra. From the analysis of the peak position changes of v(C‒H), it is found that the peaks all gradually redshift with the addition of CD3CN. For the [PCNMIM][Tf2N]‒CD3CN system, when x(CD3CN) reaches 0.9, the peaks centred at 3158.8 cm1, 3121.6 cm1 and 2967.7 cm1 shift to 3158.1 cm1, 3120.0 cm1 and 2966.0 cm1, respectively. For the [C2OHMIM][Tf2N]‒CD3CN system, when x(CD3CN) reaches 0.9, the peaks centred at 3160.6 cm1, 3123.3 cm1, 2966.9 cm1 and 2895.4 cm1 shift to 3158.4 cm1, 3121.6 cm1, 2965.9 cm1 and 2892.2 cm1, respectively. 3.2. The excess infrared spectra in the v(CH) region The corresponding excess infrared spectra of [PCNMIM][Tf2N] CD3CN and [C2OHMIM][Tf2N]CD3CN systems in the 32502850 cm1 wavenumber region are shown in Fig. 1C and D. Positive and negative peaks are observed in the excess spectra, indicating that the mixing progresses of the two ILs and CD3CN are non-ideal and the microscopic interactions between [PCNMIM] [Tf2N]/[C2OHMIM][Tf2N] and CD3CN also occur. The peak of the excess spectrum is dependent on the wavelength. As shown in Fig. 1C and D, the v(alkyl CH) region (3050 cm1 to 2850 cm1) has positive excess peaks at the lower wavenumber and negative excess peaks at the higher wavenumber in the concentration range. In the v(C2eH) region, each excess spectrum has three positive peaks at the lower wavenumber of 3121.5 cm1 (the peak position of v(C2H) of pure [PCNMIM][Tf2N]) and 3123.3 cm1 (the peak position of v(C2H) of pure [C2OHMIM][Tf2N]) throughout the

concentration range. For v(C4,5eH), each excess spectrum has one positive band at the lower wavenumber and a negative band at the higher wavenumber of 3158.8 cm1 (the peak position of v(C4,5H) of pure [PCNMIM][Tf2N]) and 3160.6 cm1 (the peak position of v(C4,5H) of pure [C2OHMIM][Tf2N]) throughout the concentration range. The positions of the excess bands in the v(CH) region are fixed. After careful analysis, the positions of the excess bands in the v(aromatic CH) region of [PCNMIM][Tf2N] are fixed at 3182.7 cm1, 3153.7 cm1, 3116.5 cm1, 3098.3 cm1 and 3087.8 cm1 from higher wavenumber to lower wavenumber. The positions of the excess bands in the v(aromatic CH) region of [C2OHMIM][Tf2N] are fixed at 3183.4 cm1, 3153.0 cm1, 3119.3 cm1, 3100.7 cm1 and 3087.2 cm1 from higher wavenumber to lower wavenumber. The wavenumber of the excess peaks at approximately 3087.8 cm1 for the [PCNMIM][Tf2N] CD3CN system and 3087.2 cm1 for the [C2OHMIM][Tf2N]CD3CN system is close to the wavenumber of the combination tone of v(C≡N) and v(CC) of CD3CN. Thus, this excess peak is not related to v(C2‒H) and is from the combination tone of v(C≡N) and v(CC) of CD3CN. In addition to the negative and positive bands, a fixed number of position peak valleys were also observed in the v(aromatic CH) region. For the [PCNMIM][Tf2N]CD3CN system, the wavenumbers of the peak valleys are 3138.7 cm1, 3129.7 cm1, 3109.4 cm1 and 3090.7 cm1 from higher wavenumber to lower wavenumber. For the [C2OHMIM][Tf2N]CD3CN system, the wavenumbers of the peak valleys are 3137.3 cm1, 3125.0 cm1, 3110.1 cm1 and 3089.2 cm1 from higher wavenumber to lower wavenumber. After careful analysis, it is found that the peak valleys at approximately 3138.7 cm1 for the [PCNMIM][Tf2N]‒CD3CN system and 3137.3 cm1 for the [C2OHMIM][Tf2N]‒CD3CN system are raised from the peak valleys between v(C4,5‒H) and v(C2‒H) in the original infrared spectra. The peak valleys at approximately 3109.4 cm1 for the [PCNMIM][Tf2N]‒CD3CN system and 3110.1 cm1 for the [C2OHMIM][Tf2N]‒CD3CN system are raised from the peak valleys between v(C2‒H) and its shoulder peak in the original infrared spectra. The peak valleys at approximately 3090.7 cm1 for the [PCNMIM][Tf2N]‒CD3CN system and 3089.2 cm1 for the [C2OHMIM][Tf2N]‒CD3CN system are raised from the peak valleys between the shoulder peak of v(C2‒H) and the combination tone of v(C≡N) and v(CC) of CD3CN in the original infrared spectra. For a compound in different association forms, increasing the amount of one or some species is also accompanied by a decreased amount of other species. Therefore, in the excess spectra, there should be both positive and negative bands reflecting the increase and decrease in the related species [46,53,54,59,60]. In Fig. 1C and D, it is apparent that the v(C2eH) region has two positive peaks and the v(C4,5eH) region has a negative peak at the higher wavenumber and a positive peak at the lower wavenumber throughout the studied concentrations. Both the C2 hydrogen atom and the C4,5 hydrogen atoms are on the cation. Thus, decreasing the amounts of the respective species of the C4,5 hydrogen atoms is also accompanied by that of the C2 hydrogen atom. There also should be a negative excess peak at the higher wavenumber side of the v(C2H) region similar to the v(C4,5H) region. The peak valley at approximately 3129.7 cm1 for [PCNMIM][Tf2N]‒CD3CN and 3125.0 cm1 for [C2OHMIM][Tf2N] may be the negative peaks for the v(C2‒H) region. The overlap with the positive excess peaks of v(C2eH) may make their intensities larger than zero. There should be two positive peaks in the v(C4,5‒H) region similar to the v(C2H) region. Due to the resolution factor limit of the equipment, only one broad positive peak was observed in the v(C4,5‒H) region. Overall, in the excess spectra, the excess peaks related with v(C2‒H) are at approximately 3129.7 cm1, 3116.5 cm1 and 3098.3 cm1 for the [PCNMIM][Tf2N] system and 3125.0 cm1,

Y.-Z. Zheng et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 226 (2020) 117641

3119.3 cm1 and 3100.7 cm1 for the [C2OHMIM][Tf2N] system. Those related to v(C4,5‒H) are at approximately 3182.7 cm1 and 3153.7 cm1 for the [PCNMIM][Tf2N] system and 3183.4 cm1 and 3153.0 cm1 for the [C2OHMIM][Tf2N] system. In Fig. 1C and D, the excess peaks related to v(C4,5‒H) and v(C2‒H) are marked by dashed lines and numbered as 1, 2, 3, 4, and 5 from higher wavenumber to lower wavenumber. The other peaks and peak valleys are marked by full lines. The above section illustrated that all of the peaks in the v(C‒H) region are gradually redshifted with the increasing amount of CD3CN. The continuous change in the solvent effect, the combination of a few individual peaks from the absorption of multiple hydrogen-bonded complexes or both phenomena can induce the gradual change in peak position [37,38,50e57]. The peak positions in the excess spectra would be fixed throughout the studied concentration range if the hydrogen-bonding interaction played the dominant role [46,53,54]. A contrary result occurs when the continuous change of the solvent effect is the dominant factor [46,53,54]. The excess peaks in the 3250 cm1e3050 cm1 region related to v(C4,5‒H) and v(C2‒H) are marked by dashed lines. These peaks are fixed in the concentration range. Thus, in this work, hydrogen-bond is the main cause that induces the gradual peak position change in the v(C‒H) region. In the studied concentration, CD3CN forms a hydrogen-bonded complex with [PCNMIM][Tf2N] and [C2OHMIM][Tf2N]. 3.3. The origin of the excess peaks in the v(C2H) region The v(CeH) bands are often the focus of research. The 28503050 cm1 region is complex as all of the alkyl CH stretching vibrations of the cation are present in this region, and it is difficult to distinguish the absorption information for each alkyl CH. The peak position of v(C2H) is distinguished from that of v(C4,5H) in the 30503250 cm1 region of the original IR spectra. However, due to the resolution factor limit of the equipment, only one broad positive peak was observed in the v(C4,5‒H) region. The excess spectra of the v(C2‒H) region show more information than the excess spectra of the v(C4,5‒H) region. The v(C2‒H) region were especially focused on in the following analysis. The v(C2H) region has three excess peaks throughout the concentration range. On the other hand, this result illustrates that there are three distinct associations of [PCNMIM][Tf2N]/ [C2OHMIM][Tf2N] present in the binary mixtures. Quantum chemical calculations were applied to reveal what the types of associations are. In pure ILs, there are ion clusters and ion pairs of different sizes [51]. Based on the previous studies [46,51,53,54], we took two-cationsetwo-anions (2[PCNMIM]þe2[Tf2N] and 2 [C2OHMIM]þe2[Tf2N]) to represent an ion cluster and cationeanion ([PCNMIM]þe[Tf2N] and [C2OHMIM]þe[Tf2N]) to represent an ion pair. When the cosolvent is gradually added into the ILs, the ion clusters may be separated into ion pairs and even individual cations and anions or remain as clusters. The ion pairs

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may be separated into individual cations and anions or maintain their original state. The ion cluster, ion pair and individual cation and anion would interact with the cosolvent. According to the previous work [46,53,54], we took two-cationsetwoanionseCD3CN (2[PCNMIM]þe2[Tf2N]dCD3CN/2[C2OHMIM]þe2 [Tf2N]dCD3CN) to represent an ion clustereCD3CN interaction complex, cationeanioneCD3CN ([PCNMIM]þe[Tf2N]eCD3CN/ [C2OHMIM]þe[Tf2N]eCD3CN) to represent an ion paireCD3CN interaction complex and cationeCD3CN ([PCNMIM]þeCD3CN/ [C2OHMIM]þeCD3CN) to represent a cationeCD3CN interaction complex. Only the most stable structures of the above complexes were used in the frequency analysis. The frequencies of v(C2H) of [PCNMIM]þ cation and [C2OHMIM]þ cation in the most stable structures of the ion cluster, ion pair, cation, ion clusterCD3CN complex, ion pairCD3CN complex and cationCD3CN complex are summarized in Table 2. As seen in Table 2, all of the calculated frequencies are higher than the observed frequencies. That finding is mainly ascribed to the basis set deficiencies or electron correlation effects or both [70,71]. Thus, it is practically impossible to determine the origin of the observed excess peak positions via completely matching the calculated frequencies. Even so, the observed peak positions still have a relationship with the calculated frequencies. The linear relationship method is often applied to determine the origin of the observed excess peak position [46,53,54]. The larger the correlation coefficient (R2), the greater the correlation between the observed frequencies and the calculated frequencies. We also exploited this method to make the assignments of the excess peak in the v(C2H) region. After all of the possible combinations, the most relevant set between the calculated frequencies and the experimentally observed excess peak positions was found. The results are shown in Fig. 2. Both of the R2 of the two lines are larger than 0.99, which shows a high correlation of the calculated frequencies and the experimentally observed excess peak positions. Thus, the observed peaks at 3129.7 cm1 for the [PCNMIM][Tf2N]CD3CN system and 3125.0 cm1 for the [C2OHMIM][Tf2N]CD3CN system are attributed to v(C2H) of the ion clusters. The peaks at 3116.5 cm1 for the [PCNMIM][Tf2N]CD3CN system and 3119.3 cm1 for the [C2OHMIM][Tf2N]CD3CN system are related with the ion clusterCD3CN complex. The peaks at 3098.3 cm1 for the [PCNMIM] [Tf2N]CD3CN system and 3100.7 cm1 for the [C2OHMIM][Tf2N] CD3CN system are from the ion pairCD3CN complex. The bands centred at approximately 3129.7 cm1 and 3125.0 cm1 are negative. Thus, the ion clusters of the ILs are lower in the mixture compared with those in the pure ILs. The bands centred at 3116.5 cm1 and 3098.3 cm1 for the [PCNMIM][Tf2N] CD3CN system and 3119.3 cm1 and 3100.7 cm1 for the [C2OHMIM][Tf2N]CD3CN system are positive. Thus, the amounts of the ion clusterCD3CN and ion pairCD3CN complexes in the mixtures are larger than in the pure ILs. And the ion clusterCD3CN and ion pairCD3CN complexes are the main species in the binary mixtures. To better understand the mixing process, a schematic

Table 2 Assignments of the absorption bands in the C2-H stretching vibrational region of the cation based on our density functional theory (DFT) calculations. Component

[PCNMIM][Tf2N]aCD3CNb system

Ion cluster Ion pair Ion clusterCD3CN complex Ion pairCD3CN complex Cation CationCD3CN complex

Calculated frequency (cm1) 3270.4 3267.8 3225.4 3173.0 3279.4 3179.7

a b c

[C2OHMIM][Tf2N]cCD3CN system Observed frequency (cm1) 3129.7 3116.5 3098.3

1-Propylnitrile-3-methylimidazolium bis(trifluoromethylsulfonyl)imide. Deuteration acetonitrile. 1-(20 -Hydroxylethyl)-3-methylimidazolium bis(trifluoromethylsulfonyl)imide.

Calculated frequency (cm1) 3292.1 3246.2 3269.7 3176.3 3278.8 3181.0

Observed frequency (cm1) 3125.0 3119.3 3100.7

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Fig. 2. Linear relationship between the observed peak positions in the v(C2H) region and the calculated v(C2H) wavenumbers of different [PCNMIM][Tf2N]CD3CN and [C2OHMIM][Tf2N]CD3CN complexes.

diagram which takes the [C2OHMIM][Tf2N]CD3CN system as the example is shown in Fig. 3. After all of the possible combinations, it is found that the set having the calculated frequencies of v(C2H) in the individual [PCNMIM]þ/ [C2OHMIM]þ cation or [PCNMIM]þ/[C2OHMIM]þCD3CN complex is weakly correlated and cannot be fitted well into the linear relationship. This result implies that the [PCNMIM]þ/[C2OHMIM]þ cation and [PCNMIM]þ/[C2OHMIM]þCD3CN complex are not the significant species contributing to v(C2H). Thus, in the concentration range, CD3CN cannot break the strong interaction between the cation and anion. 3.4. The hydrogen-bond properties in the two binary mixtures In the above discussions, from pure ILs to the ILs‒CD3CN binary mixture, the association complex changes from ion cluster and ion pair to ion clusterCD3CN and ion pairCD3CN hydrogen-bonded complexes. In addition to the frequencies, the quantum chemical calculations can also illustrate other information about the complex including the interaction energies and hydrogen-bond properties. The most stable optimized structures of the ion cluster (2 [PCNMIM]þ2[Tf2N] and 2[C2OHMIM]þ2[Tf2N]), ion pair ([PCNMIM]þe[Tf2N] and [C2OHMIM]þe[Tf2N]), ion clusterCD3CN complex (2[PCNMIM]2[Tf2N]CD3CN and 2[PCNMIM]2 [Tf2N]CD3CN) and ion pairCD3CN complex ([PCNMIM][Tf2N] CD3CN and [PCNMIM][Tf2N]CD3CN) are shown in Fig. 4. For the ion cluster and ion pair, the BSSE correlated interaction energies

between the cation and anion are shown below the complex. For the ion clusterCD3CN and ion pairCD3CN complexes, the interaction energies between CD3CN and the ion cluster/ion pair are shown below the corresponding complexes. The data in Fig. 4 clearly illustrated that the interaction energies between the cation and anion are larger than those between CD3CN and the ion pair/ion cluster. Thus, it is difficult for CD3CN to break the strong interaction between the cation and anion. In the concentration range, the excess peak related to individual cation or cationCD3CN complexes is not found in the excess spectra. It is also clear that in the optimized 2[PCNMIM]2[Tf2N]CD3CN and 2[C2OHMIM]2 [Tf2N]CD3CN complexes, CD3CN interacts with the cluster and ion pair at the periphery instead of inserting into the ion pair or ion cluster. The AIM theory can provide a significant amount of information about the properties of chemical bonds, including hydrogen-bonds [68]. It is a very useful tool in analysing hydrogen-bonds [68]. In the AIM theory, bond critical point (BCP) is used to identify the formation of a chemical bond between two adjacent atoms. In this work, the denoted hydrogen-bonds of the complexes in Fig. 4 all have BCPs. The nature of the chemical bond is well described by the following five topological descriptors at the BCPs: rBCP, V2rBCP, GBCP, VBCP and HBCP. The five topological descriptors at the BCPs of the denoted hydrogen-bonds in Fig. 4 are shown in Table S1 (Supplementary Material). The above sections clearly demonstrate that ILs and CD3CN form hydrogen-bonded complexes in the binary mixtures. Both the geometrical and topological parameters are useful tools to characterize the existence and nature of the hydrogen-bond. The geometrical criteria for identifying the existence of the hydrogenbond are as follows: (1) the distance between proton acceptor and hydrogen atom is smaller than the sum of the corresponding van der Waals atomic radii of the two atoms. (2) The angle of the donor proton/acceptor is larger than 90 [72]. The sum of the van der Waals atomic radii between hydrogen and oxygen, hydrogen and fluorine, hydrogen and nitrogen is 2.5 Å, 2.45 Å and 2.6 Å respectively [72]. The distance between the proton acceptor and hydrogen atom and the angle of the donor proton/acceptor are carefully examined in the most stable structures of ion cluster, ion pair, ion clusterCD3CN and ion pairCD3CN complexes. The identified hydrogen-bonds in Fig. 4 are marked by dashed lines, and the corresponding hydrogen-bond distances are marked. The angles of the donor proton /acceptor are all larger than 90 . As seen in Fig. 4, all of the ion cluster, ion pair, ion clusterCD3CN and ion pairCD3CN complexes exhibit hydrogen-bonds. Both the hydrogen atoms in the aromatic ring and the alkyl chains in the [PCNMIM]þ and [C2OHMIM]þ cations participate in the hydrogen-

Fig. 3. Schematic models for the transformation of the species in the mixing process of [C2OHMIM][Tf2N]CD3CN system.

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Fig. 4. The optimized geometries and the corresponding interaction energies for 2[PCNMIM]2[Tf2N], [PCNMIM][Tf2N], 2[PCNMIM]2[Tf2N]CD3CN, [C2OHMIM][Tf2N]CD3CN, 2 [C2OHMIM]2[Tf2N], [C2OHMIM][Tf2N], 2[C2OHMIM]2[Tf2N]CD3CN and [C2OHMIM][Tf2N]CD3CN complexes. Hydrogen-bonds are denoted by dashed lines and the distances are labelled by the side of the corresponding hydrogen-bond.

bonds. In 2[PCNMIM]2[Tf2N]CD3CN and [PCNMIM][Tf2N]CD3CN complexes, both the methyl group and the C≡N group of CD3CN interact with the IL. However, in the 2[C2OHMIM]2[Tf2N]CD3CN and [C2OHMIM][Tf2N]CD3CN complexes, only the hydrogen atoms of CD3CN form hydrogen-bonds with the [Tf2N] anion. As the geometrical criteria are often considered as insufficient, the existence of hydrogen-bonds could be further supported by the topological criteria based on the AIM theory: (1) the existence of the BCP at the hydrogen-bond; and (2) the value of rBCP should be within the range of 0.002e0.040 au, and V2rBCP should be in the range from 0.02 au to 0.15 au [73]. The topological criteria provide a basis to distinguish the hydrogen-bonding interactions from the van der Waals interactions. These criteria have been proved to be valid for identifying standard and non-conventional hydrogenbonds [73]. All of the hydrogen-bonds denoted in Fig. 4 exhibit BCPs. In Table S1, the values of rBCP of the [PCNMIM][Tf2N]‒CD3CN system are from 0.008 au to 0.025 au. The values of rBCP of the [C2OHMIM][Tf2N]‒CD3CN system range from 0.008 au to 0.027 au. These values are within the range of 0.002e0.040 au. The values of V2rBCP of the [PCNMIM][Tf2N]‒CD3CN system range from 0.034 au to 0.094 au, and those of the [C2OHMIM][Tf2N]‒CD3CN system are from 0.025 au to 0.104 au. These values are within 0.02 au to 0.15 au. Thus, the topological data also confirm the existence of hydrogenbonds in the optimized complexes in Fig. 4. According to the theory proposed by Espinosa [74], the energy of a hydrogen-bond (E) can be calculated using the equation: E ¼ (1/ 2)(VBCP). An increase in the absolute value of VBCP correlates with the increase in the hydrogen-bond energy. In addition to VBCP, an increase in the value of V2rBCP also corresponds to an increased strength of the hydrogen-bond [68]. In Table S1, the absolute values of VBCP and V2rBCP at the BCP of H2/O or H2/N hydrogen-bond are the highest in each complex of the [PCNMIM][Tf2N]‒CD3CN system. The absolute values of VBCP and V2rBCP at the BCP of OH/O hydrogen-bond are the highest in each complex of the [C2OHMIM] [Tf2N]‒CD3CN system. Thus, the hydrogen atom at the C2 site is the strongest hydrogen-bonding interaction site in the [PCNMIM]þ cation. The hydrogen atom at the hydroxyl group of the [C2OHMIM]þ cation is the strongest interaction site.

Apart from the strength, the sign of V2rBCP is also relates to the type of hydrogen-bond [75]. A negative value of V2rBCP indicates the concentration of charge towards the hydrogen-bonding interaction line, and the corresponding hydrogen-bond is a shared type interaction [75]. A positive value of V2rBCP at the BCP implies that the hydrogen-bond interaction is dominated by the contraction of electron density towards each nucleus, and the corresponding hydrogen-bond is a closed shell interaction [75]. All of the V2rBCP in Table S1 are positive. Thus, the hydrogen-bonding interactions in the ion cluster, ion pair, ion cluster‒CD3CN and ion pair‒CD3CN complexes are closed shell interactions. According to the theory proposed by Rozas et al. [76], the hydrogen-bonding interaction can be classified into three types according to the following criteria: (1) V2rBCP > 0 and HBCP > 0, weak hydrogen-bond and electrostatic in nature; (2) V2rBCP > 0 and HBCP < 0, medium hydrogen-bond and partially covalent in nature; and (3) V2rBCP < 0 and HBCP < 0, strong hydrogen-bond and covalent in nature. All of the V2rBCP and HBCP are positive in Table S1. Thus, the hydrogen-bonding interactions in the ion cluster, ion pair, ion cluster‒CD3CN and ion pair‒CD3CN complexes are weak-strength and electrostatic in nature. 4. Conclusions The mixing of ILs with a molecular cosolvent can largely reduce the high viscosities of ILs, which can consequently lead to the decrease in mass transfer rates and increase in pumping costs. Mixtures of ILs and CH3CN can be used as reaction media, supercapacitors and thermally stable electrolyte. The macroscopic properties of the ILs-CH3CN mixtures have been extensively studied. However, some fundamental questions regarding the microscopic properties of ILs-CH3CN mixtures still remain to be answered. In this work, the structure properties and hydrogenbonding interactions between two ILs ([PCNMIM][Tf2N] and [C2OHMIM][Tf2N]) and CD3CN were studied using the combination of FTIR and DFT calculations. Excess infrared absorption spectroscopy with enhanced spectral resolution property was used to analyse the original IR spectra. The fixed position positive and

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negative excess peaks were found in the excess spectra. Thus, acetonitrile forms stable hydrogen-bonded complexes with the ILs in the mixtures. The observed peaks in the excess spectra were assigned with the help of quantum chemical calculations. They are related to the ion cluster, ion clusterCD3CN and ion paireCD3CN. Throughout the concentration range investigated, CD3CN cannot break the ion cluster and ion pair into individual cations and anions. Both the hydrogen atoms in the aromatic ring and the alkyl chain of the [PCNMIM]þ and [C2OHMIM]þ cations participate in the hydrogen-bonds. The properties of the hydrogen-bond were carefully analysed by the AIM theory. All of the hydrogen-bonds in the ion cluster, ion pair, ion clusterCD3CN and ion paireCD3CN are weak strength, closed shell and electrostatic dominant interactions. The preferred interaction site of the [PCNMIM]þ cation is the hydrogen atom at the C2 site, while that of the [C2OHMIM]þ cation is the hydrogen atom in the hydroxyl group. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgment This work was supported by the National Natural Science Foundation of China (21703035 and 21703115), Earmarked Fund for China Agriculture Research System (CARS-44-KXJ7) and the Fujian Agriculture and Forestry University Foundation for excellent youth teachers (xjq201715). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.saa.2019.117641. References [1] P. Wassercheid, T. Welton, Ionic Liquids in Synthesis, Wiley-VCH Verlag, Germany, 2002. [2] H. Weingartner, Understanding ionic liquids at the molecular level: facts, problems, and controversies, Angew. Chem. Int. Ed. 47 (2008) 654e670. [3] A. Corma, S. Iborra, A. Velty, Chemical routes for the transformation of biomass into chemicals, Chem. Rev. 107 (2007) 2411e2502. [4] R.G. Azevedo, J.S.S. Esperanca, V.N. Visak, Z.P. Visak, H.J.R. Guedes, M.N. Ponte, L.P.N. Rebelo, Thermophysical and thermodynamic properties of 1-butyl-3methylimidazolium tetrafluoroborate and 1-butyl-3-methylimidazolium hexafluorophosphate over an extended pressure range, J. Chem. Eng. Data 50 (2005) 997e1008. [5] M.J. Earle, J. Esperanca, M.A. Gilea, J.N.C. Lopes, L.P.N. Rebelo, J.W. Magee, K.R. Seddon, J.A. Widegren, The distillation and volatility of ionic liquids, Nature 439 (2006) 831e834. [6] R.D. Rogers, K.R. Seddon, Ionic liquids-solvents of the future? Science 302 (2003) 792e793. [7] Y. Chen, Y. Cao, Y. Shi, Z. Xue, T. Mu, Quantitative research on the vaporization and decomposition of [EMIM][Tf2N] by thermogravimetric analysisemass spectrometry, Ind. Eng. Chem. Res. 51 (2012) 7418e7427. [8] C. Maton, N. De Vos, C.V. Stevens, Ionic liquid thermal stabilities: decomposition mechanisms and analysis tools, Chem. Soc. Rev. 42 (2013) 5963e5977. [9] T. Welton, Room-temperature ionic liquids. Solvents for synthesis and catalysis, Chem. Rev. 99 (1999) 2071e2083. [10] J.I. Kadokawa, Ionic liquids as components in fluorescent functional materials, in: Ionic Liquids-New Aspects for the Future, InTech, 2013. [11] D. Kuang, P. Wang, S. Ito, S.M. Zakeeruddin, M. Gr€ atzel, Stable mesoscopic dye-sensitized solar cells based on tetracyanoborate ionic liquid electrolyte, J. Am. Chem. Soc. 128 (2006) 7732e7733. [12] Y. Cao, J. Zhang, Y. Bai, R. Li, S.M. Zakeeruddin, M. Gratzel, P. Wang, Dyesensitized solar cells with solvent-free ionic liquid electrolytes, J. Phys. Chem. C 112 (2008) 13775e13781. [13] P. Wasserscheid, T. Welton (Eds.), Ionic Liquids in Synthesis, second ed., Wiley-VCH, Weinheim, Germany, 2007, 174287. [14] M. Freemantle, Ionic liquids in organic synthesis, Chem. Eng. News 82 (2004) 44e49.

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