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Hydrogen-bonding interactions between [BMIM][BF4] and dimethyl sulfoxide
Journal of Molecular Structure xxx (2014) xxx–xxx
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Hydrogen-bonding interactions between [BMIM][BF4] and dimethyl sulfoxide Yan-Zhen Zheng a, Hong-Yan He b, Yu Zhou a, Zhi-Wu Yu a,⇑ a b
Key Laboratory of Bioorganic Phosphorous Chemistry and Chemical Biology (Ministry of Education), Department of Chemistry, Tsinghua University, Beijing 100084, PR China Beijing Key Laboratory of Ionic Liquids Clean Process, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, PR China
h i g h l i g h t s It is first used excess spectra to investigate the interactions between [BMIM][BF4] and DMSO. C2–H is the main interaction site in investigated complexes. The two turning points of the wavenumber shift changes of C2–H may indicate the dilution process. Hydrogen-bond interactions are the main cause influencing the wavenumber shift changes.
a r t i c l e
i n f o
Article history: Available online xxxx Keywords: ATR-FTIR Excess infrared spectra Ion cluster Ion pair Hydrogen-bond Ionic liquid
a b s t r a c t Mixtures of Ionic liquids and small polar organic solvent are potential green solvents for cellulose dissolution under mild conditions. In this work, the interactions between a representative imidazolium-based ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF4]) and dimethyl sulfoxide (DMSO) were investigated in detail by attenuated total reflection infrared spectroscopy (ATR-IR) and density functional theory calculations (DFT). The main conclusions are: (1) C2–H is the main interaction site in forming cation–anion, cation–DMSO, and [BMIM][BF4]–DMSO complexes. (2) The two turning points of the wavenumber shift changes of C2–H may indicate that the dilution process can be divided into several stages: from larger ion clusters to smaller ion clusters, then to ion pairs, and finally to individual ions. The solvent molecules cannot break apart the strong Coulombic interaction between [BMIM]+ and [BF4] but can break apart the ion clusters into ion pairs when the mole fraction of DMSO is less than 0.9. When the mole fraction of DMSO is greater than 0.9, ion pairs can be broke into ions. (3) The hydrogen-bonds of the aromatic C–Hs in [BMIM]+ are strengthened in the dilution process while those of the alkyl C–Hs of [BMIM]+ are weakened. (4) The aromatic C–Hs of the [BMIM]+ cation strength before the weakening of the alkyl C–Hs. These in-depth studies on the properties of the ionic liquid-DMSO mixed solvents may shed light on exploring their applications as mixed solvents in cellulose dissolution and other practices. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Concerns about the gradual depletion of conventional fossil resources and the pressure from global climate changes have led to an intensified need for new alternative resources [1]. A potential solution to the problems is the utilization of cellulose as an alternative and sustainable energy source. Other than this, as the most abundant and bio-renewable resource found on the earth, cellulose has been widely used because of its many attractive properties such as biocompatibility, biodegradability, and thermal and chemical stability [2]. Generally, processing cellulose into desired products first requires the dissolution of cellulose. Due to its stiff and close packing via numerous inter-molecular and intra-molecular ⇑ Corresponding author. Tel.: +86 10 6279 2492; fax: +86 10 6277 1149. E-mail address: [email protected] (Z.-W. Yu).
hydrogen-bonds, cellulose cannot be melted and also extremely difficult to be dissolved in common solvents, which greatly limits its processing and applications [3]. Although some of the developed solvents have achieved great successes in cellulose dissolution, the inherent disadvantages such as toxicity, volatility, and instability are still critical problems in their practical applications [4]. Therefore, it is in urgent need of developing greener solvents for cellulose dissolution under mild conditions. Over the last decade, ionic liquids (ILs) have become popular solvents for the pretreatment of cellulose. ILs are organic salts with ultra-low vapor pressure, high thermal and chemical stability, and high solvent capacity and thermal stability [5]. In 2002, Swatloski et al. first reported that a certain class of ionic liquids (ILs) can be used as solvents for cellulose under relatively mild conditions, which inspired great interest in understanding the dissolution mechanism for the design of effective solvent systems [6]. Since
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then, many kinds of ILs have been reported to use as cellulose solvents. Although ILs can be used in cellulose dissolution, sometimes their large-scale use is hindered due to their high viscosity and cost [7,8]. For example, most currently used ILs, especially those with halide anions, have high viscosities, typically 102–103 higher than traditional neutral organic solvents [9]. These problems greatly hinder large-scale application and further development of ILs to dissolve cellulose. Designing novel ILs with specific structures may bring on some improvements, but can hardly solve all the problems. Using mixtures of ILs and polar molecular solvents instead of pure ILs could be an effective approach to solving these problems in certain cases. For example, Rinaldi et al. found that some organic solutions, which contain just a small amount of IL, instantaneously dissolved large amounts of cellulose [10]. Xu et al. has also reported several highly effective cellulose solvents consisting of imidazolium ILs and polar solvents [11]. Noncovalent interactions greatly influence the physical properties of IL-cosolvent mixtures, and the key to understand the modulation effect of cosolvent on ILs is to investigate the molecular interactions between them [12–20]. Among various molecular interactions, electrostatic interaction is the most important one, however, other interactions especially hydrogen-bonding interactions are considered to be also very important [14–20]. DMSO is an extraordinary dipolar solvent, completely miscible with water and with very wide range of organic and inorganic substances [21]. Recent investigations showed that adding dimethyl sulfoxide (DMSO) into ILs can accelerate the cellulose dissolution dramatically [22]. The physical properties such as conductivity, viscosity, density, and volumetric property of the mixtures of IL and DMSO have been reported [23,24]. The interactions between DMSO and some ILs such as 1-butylpyridinium tetrafluoroborate ([BuPy][BF4]) [17] and 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF4]) [20] have been studied in the concentration range (the mole fraction of DMSO between 0.1 and 0.9). But the molecular pictures of the binary systems in the full concentration range (the mole fraction of DMSO between 0.1 and 0.99) are not clear yet. In this work, we chose DMSO and a representative imidazolium salt [BMIM][BF4] to investigate in detail their interactions, particularly the hydrogen-bonding interactions in the full concentration range. Attenuated total reflection infrared spectroscopy (ATR-FTIR) and density functional theory (DFT) calculation have been employed in the present study. In particular, excess infrared spectroscopy [25–29] and two-dimensional correlation spectroscopy (2D-COS) have been employed to analyze the infrared spectra [30,31]. 2. Experimental section 2.1. Materials and sample preparation [BMIM][BF4] (>98%) was purchased from Sigma–Aldrich. DMSO-d6 (>99.8 of deuterium) was purchased from Cambridge Isotopes Laboratories (CIL). DMSO (>99.5%) was from Beijing Chemical Plant (Beijing, China). The IL was dried for 72 h under vacuum, and then was stored in a desiccator. A series of [BMIM][BF4]–DMSO-d6 mixtures were prepared by weighing for the FTIR experiments. The mole fractions of DMSO-d6 in the mixtures are displayed in the respective excess spectra.
filling factor of 2. For each sample, three parallel measurements were carried out. The refractive indexes of different solutions were measured with an Abbe refractometer at 25 °C. The formula suggested by Hansen was used to do the ATR corrections [32]. 2.3. Excess infrared spectroscopy The theory of excess infrared spectroscopy has been described in detail elsewhere [25,26]. Briefly, an excess infrared spectrum is defined as the difference between the spectrum of a real solution and that of the respective ideal solution under identical conditions. The working equation in calculating the excess infrared spectrum is
eE ¼
A ðx1 e1 þ x2 e2 Þ dðC 1 þ C 2 Þ
ð1Þ
where A is the absorbance of the mixture, d is the light path length, C1 and C2 are the molarities of the two components, x1 and x2 are the mole fractions of components 1 and 2, e1 and e2 are the molar absorption coefficients of the two components in their pure states, respectively. If our attention is the absorbance of a particular absorption band, the excess molar absorbance of the band can be calculated by its integrated intensity. Matlab 7.0 (Math Works Inc., Natick, MA) was used to manipulate spectral data pretreatment, i.e., the subtraction, truncation, and baseline correction, to calculate excess infrared spectra and to do the integration of the bands. 2.4. Two-dimensional correlation spectroscopy Standard 2D correlation spectral analysis was performed using Matlab 7.0, based on the algorithm developed by Noda [30]. In order to remove the linear contribution to absorbance by concentration variation and thus to obtain the information of specific interactions, the selected bands were normalized with the modified component-normalization method [28,33]. Specifically, the absorption bands of m(aromatic C–Hs) and v(alkyl C–Hs) in excess infrared spectra were divided by the corresponding molarity of [BMIM][BF4]. The average of all the spectra over the full concentration range was used as the reference spectrum. In the 2D correlation contour map, solid (red) and dashed (blue) lines represent positive and negative correlation intensities, respectively. 2.5. Quantum chemical calculations All computations were performed with the Gaussian 03 program [34]. The B3LYP method with the 6-31++G** basis set was used to optimize molecular energies and geometries of the isolated DMSO and [BMIM]+–[BF4], [BMIM]+–DMSO, and [BMIM] [BF4]–DMSO complexes. The optimized geometries at local energy minimum were ensured by the absence of imaginary vibrational frequency. Meanwhile, the basis set superposition error (BSSE) correction computed via the counterpoise method of Boys and Bernardi was estimated for obtaining accurate interaction energies of the complexes [35]. 3. Results and discussion
2.2. FTIR spectroscopy FTIR spectra over the range from 4000 to 650 cm1 were collected at room temperature (25 °C) using a Nicolet 5700 FTIR spectrometer, equipped with a MCT detector. The attenuated total reflection (ATR) cell, made of trapezoidal ZnSe crystal with incident angle of 45° and 12 reflections, was employed. Spectra were recorded with a resolution of 2 cm1, 32 parallel scans, and a zero
Infrared spectroscopy and quantum chemical calculations have been employed to study hydrogen-bonding interactions between [BMIM][BF4]–DMSO. Due to the fact that information from the former can only provide necessary conditions of hydrogen-bond interactions, whilst information from the latter provides sufficient conditions, we present the quantum chemical calculation results first.
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3.1. Theoretical Investigation of the Interaction between [BMIM][BF4] and DMSO A set of possible conformations of the complexes consisting of one DMSO molecule and the [BMIM]+ cation, with or without the anion, have been examined by DFT calculations to characterize the interactions between DMSO and [BMIM][BF4]. The optimized geometries of [BMIM]+–[BF4], [BMIM]+–DMSO, [BF4]–DMSO, and [BMIM][BF4]–DMSO complexes were studied at the B3LYP/631++G** level. The sum of van der Waals atomic radii of hydrogen and oxygen (2.5 Å) and that of hydrogen and fluorine (2.45 Å) are used as critical values for judging the existence of hydrogen-bonds between hydrogen and oxygen/fluorine [36]. Hydrogen-bonds are denoted by dashed lines, and the corresponding H O and H F distances are labeled (Figs. 1–3). For the cation [BMIM]+, there are many possible sites to interact with DMSO. To find out the preferred interaction sites, a number of possible mutual orientations of the complexes consisting of one DMSO molecule and one [BMIM]+ cation were examined. The optimized geometries and the respective interaction energies are shown in Fig. 1A–C. As shown in the figure, both of the aromatic and alkyl CHs in [BMIM]+ can form hydrogen-bonds with the oxygen atom. The absolute value of the interaction energy of the first conformer (Fig. 1A) is much larger than the other two, indicating that conformer A is the most stable one. Besides, the sequential order of the hydrogen-bond indicates that C2–H is the most preferred interaction site, followed by C4,5H. As to the interactions between DMSO and the anion, two hydrogen-bonds are identified. Before examining the cation–anion-cosolvent ‘‘three-body’’ complex, interactions between the cation and anion was first investigated. The optimized geometries and respective interaction energies of [BMIM]+–[BF4] complex are shown in Fig. 2. The absolute value of the interaction energy of the first conformer is much
Fig. 2. Four optimized geometries and corresponding interaction energies for [BMIM]+–[BF4] complex. Hydrogen-bonds are denoted by dashed lines and the corresponding H F distances are labeled. The corresponding interaction energies are shown below the complexes.
Fig. 3. Four optimized geometries and corresponding interaction energies for [BMIM][BF4]–DMSO complex. Hydrogen-bonds are denoted by dashed lines and the corresponding H O and H F distances are labeled. The corresponding interaction energies are shown below the complexes. Fig. 1. Optimized geometries and corresponding interaction energies for an DMSO molecule interacting with [BMIM]+ and [BF4]. Hydrogen-bonds are denoted by dashed lines and the corresponding H O and H F distances are labeled. The corresponding interaction energies are shown below the complexes.
larger than the other three conformers, meaning that it is the most stable one and the anion is more easily captured by C2–H. Comparison of the most stable interaction pair in Fig. 1 and that
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in Fig. 2, we can see that C2–H is the most active hydrogen atom in the cation. Charge distribution calculation has pointed out that, the positive charge is distributed on the peripheral hydrogen atoms of aromatic ring of the cation, while the negative charge is distributed at N1 and N3 atoms. C4 and C5 atoms appear to be neutral while C2 atom exclusively possesses a positive charge owing to the electron deficit nature of the C2@N bond. The large positive charge at the C2–H unit accounts for the observed higher acidity of C2–H [37]. Shown in Fig. 3 are four optimized conformers of the [BMIM][BF4]–DMSO complex, giving the interaction modes in the presence of both the anion and the cation. It can be seen that, in the three-body complex, both of the methyl group and the oxygen atom of DMSO can take part in the hydrogen-bond interactions with the anion and cation of IL simultaneously. Comparing the most stable conformers in Figs. 1–3, we see that the absolute values of the interaction energies of [BMIM]+–[BF4] (345.10 kJ/mol) and [BMIM][BF4]–DMSO (395.54 kJ/mol) complexes are much larger than those of [BMIM]+–DMSO (77.36 kJ/ mol) and [BF4]–DMSO (57.50 kJ/mol). The large interaction energies of the former are mainly due to the strong Coulombic interaction between the cation and the anion. From the optimized stable geometries shown in Fig. 3, we can see that DMSO molecule cannot insert into the [BMIM]+–[BF4] complexes, or in other words, stay between the cation and the anion. It illuminates that it is not easy for the neutral molecule to break apart the strong Coulombic interaction between [BMIM]+ and [BF4] by simple insertion between them when the concentration of the neutral molecule is low [38].
influence from overlap, [BMIM][BF4]–DMSO-d6 system was chosen to analysis the v(C–H) in the cations and v(C–D) in DMSO-d6. 3.2.2. ATR-FTIR analysis of v(C–Hs) ATR-FTIR spectra and excess infrared spectra of [BMIM][BF4]– DMSO-d6 system within the concentration range in the v(C–Hs) region are shown in Fig. 5. A few features can be seen readily in the original infrared spectra with increasing concentration of DMSOd6. In the ATR-FTIR spectra (Fig. 5A), the imidazolium ring C–H stretching bands, centered around 3162 cm1 and 3122 cm1 in pure [BMIM][BF4], exhibit significant changes in the spectral profiles in the presence of DMSO-d6. The absorption peak positions of v(C2–H) and v(C4,5–H) gradually move to lower wavenumbers with increasing DMSO-d6. When the mole fraction of DMSO-d6 is higher than 0.6, the changes are evident, particularly a shoulder peak appears for v(C2–H). The bandwidth of the v(C2–H) and v(C4,5–H) are broader and broader with increasing DMSO-d6, which are more pronounced for v(C2–H). For the alkyl C–Hs, apparently, they do not exhibit obvious changes in the spectral
3.2. ATR and excess infrared spectra analysis 3.2.1. IR spectra of pure [BMIM][BF4], DMSO, and DMSO-d6 Stretching vibrations are effective bands to characterize the weak interactions. The partial ATR-FTIR spectra of pure [BMIM][BF4], DMSO and DMSO-d6 are shown in Fig. 4. For pure [BMIM][BF4], the bands at around 3120 cm1 and 3160 cm1 are attributed to the v(C2–H) and v(C4,5–H) of the imidazolium ring, respectively, while the three bands in 3000–2800 cm1 range are from the v(C–H) in the alkyl chains of imidazolium ring [39]. It should be noted that there have been debates on the assignments of the aromatic C–Hs in imidazolium ILs in literatures [39,40]. Our assignments are in line with the majority of the community. For pure DMSO-d6 and DMSO, two bands around 2250 and 2125 cm1 are due to the vas(C–D) and vs(C–D) of DMSO-d6, and the bands around 2994 and 2912 cm1 are due to the vas(C–H) and vs(C–H) of DMSO, respectively [41]. In order to get rid of the
Fig. 4. ATR-FTIR spectra of pure [BMIM][BF4], DMSO, and DMSO-d6 in the range of 3300–2000 cm1.
Fig. 5. ATR-FTIR (A) and excess infrared (B) spectra of [BMIM][BF4]–DMSO-d6 system in the range of the C–H stretching vibrations. From top to bottom, the mole fraction of DMSO-d6 increases from 0 to 1 in (A) and 0.1 to 0.99 in (B) with the corresponding mole fractions of DMSO-d6 shown in (B). The dashed and dashdotted lines depict spectra of pure [BMIM][BF4] and DMSO-d6.
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Fig. 6. Wavenumber shifts (A) and excess molar absorbance (B) of the v(C–Hs) in [BMIM]+ at different mole fractions of DMSO-d6. Solid and open symbols represent the alkyl and aromatic C–H, respectively.
profiles upon dilution, implying that they may not be the interaction sites. The wavenumber shifts of the v(C–Hs) are plotted as functions of the mole fraction of DMSO-d6 in Fig. 6A. Starting in the neat RTIL, the v(C2–H) shows a slight but monotonic red-shift until the DMSO-d6 mole fraction of 0.6. With further addition of the solvent the slope of the curve substantially decreases, showing a more pronounced change. When the mole fraction of DMSO-d6 reaches 0.9, the red-shift becomes more pronounced. In contrast, the v(C4,5H) and v(alkyl C–Hs) show a monotonic red-shifting behavior. Careful analysis shows that the peak positions of the bands at 3162 cm1
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and 3122 cm1 red-shift by about 15 cm1 and 50.3 cm1, respectively, while the v(alkyl C–Hs), centered around 2964 cm1, 2938 cm1, and 2877 cm1, are all red-shifted by only about 3 cm1, suggesting that the C–H groups on the imidazolium cation experience different environment in the mixture as compared to that in pure IL. Based on our previous work and other literatures, ion clusters are the dominating constituents in pure ILs [19,42]. Mixing with molecular solvents can break apart the ion clusters into ion pairs, ion pairs into individual ions. However, when the concentration of molecular solvents is not high enough, it can only break larger ion clusters into smaller ion clusters. C2–H is the main interaction site of the cation and is thus the most sensitive compared to other C–Hs in the cation as can be seen in the quantum chemical calculations discussed in the above section. From the wavenumber changes of C2–H, we might get the following dilution process: When the mole fraction of DMSO is lower than 0.6, larger ion clusters are broken into smaller ion clusters. When the mole fraction of DMSO is among 0.6–0.9, ion clusters are broken into ion pairs. When the mole fraction of DMSO is greater than 0.9, all the ion clusters are broken into ion pairs and ion pairs begin to change to individual ions. Compared to the original IR spectra, excess infrared spectra can better reveal the changes of the infrared spectra than the original IR spectra [16–19,25–29]. It can reveal the positions of new complexes and the changes in molar absorptivity directly, reflecting details of the molecular interactions in the mixtures. For a compound existing in different distinct forms, if the resolution of an excess spectrum is good enough, there should be both positive and negative peaks for a particular vibrational mode, representing the increasing and decreasing amounts of the respective species including the interacting complexes in the mixtures. As shown in Fig. 5B, both m(C2–H) and m(C4,5–H) regions have positive peaks at lower wavenumber and negative peaks at the higher wavenumber, attributed to the red shift of the bands. For v(C–H) of the alkyl chains, there are four positive peaks at the lower wavenumber side (corresponding to 2964, 2938, and 2877 cm1 of the pure ionic liquid v(alkyl C–H)), attributed to red shifts of the bands. Interestingly, the positions of the negative bands and positive bands in the excess spectra are nearly fixed in the concentration range investigated, indicating they are from disappearing and appearing species in the mixtures.
Fig. 7. Synchronous (A) and asynchronous (B) 2D correlation spectra contour maps of v(C–Hs) of [BMIM][BF4] in the dilution process with DMSO-d6. Solid (red) and dashed (blue) lines represent positive and negative correlation intensities, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.).
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Integral values of the bands in excess infrared spectra can be used to represent the molar absorptivity variation of a certain vibrational mode. The integral values of the excess infrared bands (eE) for the aromatic and alkyl C–H stretches in the cation are given in Fig. 6B. For the C–Hs in the cation, they are all positive over the entire concentration range. The integral value of the aromatic C–Hs is larger than the alkyl C–Hs and the maximum appears before the alkyl C–Hs. In the IL solutions, both of the solvent effects (including the effect of charged particles in the solution) and hydrogen-bonding interactions can influence the IR spectra [14–20,40]. If the solvent effect dominated, the positions of the positive bands and negative bands in the excess spectra would have shifted in a gradual manner with increasing cosolvent. On the contrary, the positions of the negative bands and positive bands in the excess spectra are nearly fixed as can be seen in Fig. 5B. This is attributed to the relative stable hydrogen-bonding interaction pairs/complex. Hydrogen-bond from the imidazolium ring C–Hs is classified as ‘‘proper red-shift hydrogen-bond’’ [14–20], while that related to alkyl C– Hs or C–Ds is classified as ‘‘improper blue-shift hydrogen-bond’’
[43]. During the dilution process by DMSO-d6, m(C2–H), v(C4,5–H), and m(alkyl C–Hs) show red-shifts and increase in IR absorbance. This implies that, with the addition of DMSO-d6, the hydrogenbonding interactions involving the C–H groups in imidazolium ring are strengthened, while, that in the alkyl C–H groups are weakened. 3.2.3. Sequential order of the interactions of C–Hs with DMSO-d6 The preference of DMSO-d6 in the interactions with imidazolium ring C–Hs and alkyl C–Hs can be evaluated by (2D-COS). Absorption bands of m(Aromatic C–Hs) and m(alkyl C–Hs) in [BMIM][BF4]–DMSO-d6 system were selected to do the 2D-COS analysis. The synchronous and asynchronous 2D correlation contour maps of m(aromatic C–Hs) and m(alkyl C–Hs) in [BMIM][BF4]– DMSO-d6 system are shown in Fig. 7. The two central bands in imidazolium ring and alkyl C–H stretching regions of [BMIM][BF4], at around 3162 and 2964 cm1 (see Fig. 4), are taken as the representative absorptions for 2D correlation analysis. A negative cross peak at (3162 cm1, 2964 cm1) is seen in the synchronous spectrum shown in Fig. 7A. While in the asynchronous spectrum given in Fig. 7B, a negative cross peak at the same position is seen. According to Noda’s rule [30,31], the change of the absorption coefficient of the imidazolium ring is prior to that of the alkyl C–H. It means that DMSO prefers to interact with the aromatic C–Hs compared to alkyl C–Hs in the dilution process. The integral value of the aromatic C–Hs is larger than the alkyl C–Hs and the maximum appears before the alkyl C–Hs. These may also indicate the DMSO prefers to interact with the aromatic C–Hs compared to alkyl C–Hs in the dilution process. 3.2.4. ATR and excess infrared spectra analysis of v(C–D) ATR-FTIR spectra and excess infrared spectra of [BMIM][BF4]– DMSO-d6 system over the entire mole fraction rang in the v(C–D) region are shown in Fig. 8. A few features can be seen readily in
Fig. 8. ATR-FTIR (A) and excess infrared (B) spectra of vas(C–D) and vs(C–D). From top to bottom, the mole fraction of [BMIM][BF4] increases from 0 to 1 in (A) and 0.01 to 0.9 in (B) with the corresponding mole fractions of IL shown in (B). The dashed and dash-dotted lines depict spectra of pure DMSO-d6 and [BMIM][BF4].
Fig. 9. Wavenumber shifts (A) and excess molar absorbance (B) of vs(C–D) in DMSO-d6 at different mole fractions of [BMIM][BF4].
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vas(C–D)
and
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the original infrared spectra with increasing concentration of ILs. For the original IR spectra, the vas(C–D) and vs(C–D) gradually move to higher wavenumber with increasing IL. The band width of mas(C–D) becomes broader. When the mole fraction of IL is greater than 0.3, it can be seen that the vas(C–D) band is consisting of two components: the amount of the species in the lower wavenumber gradually decreases, while the amount of the species in the higher wavenumber gradually increases. The wavenumber shifts are presented in Fig. 9A. They are blue-shifted by 13.4 and 5.8 cm1 for vas(C–D) and vs(C–D), respectively. As can be seen in the figure, the wavenumber shift change is more pronounced for vas(C–D). Compared to the original IR spectra, excess infrared spectra can better reveal the changes of the infrared spectra than the original IR spectra [16–19,25–29]. It can reveal the positions of new complexes and the changes in molar absorptivity directly, reflecting details of the molecular interactions in the mixtures. In Fig. 8B, excess infrared spectra show positive bands at higher wavenumbers and negative bands at lower wavenumbers in vas(C–D) and vs(C–D) region. The positions of the positive bands and negative bands are nearly fixed, indicating that there are distinct complexes in the mixture. The positive bands in the higher wavenumber are related to the DMSO-IL complex, while the negative bands in the lower wavenumbers are attributed to the DMSO self-association complex. The increasing of the complex in the higher wavenumber and the reducing of the complex in the lower wavenumber are the origin of the blue-shift of vas(C–D) and vs(C–D). The integral values of the excess infrared bands (eE) for both ms(C–D) and vas(C–D) are given in Fig. 9B. The values are both negative in the concentration range. As ‘‘unconventional hydrogenbond’’ [43], the blue-shifts and the negative integral values of eE indicate that the hydrogen-bond interactions of the methyl group in DMSO-d6 are strengthened upon mixing with the ionic liquid. 4. Conclusions In this work, ATR-FTIR and DFT calculation were used to study the hydrogen-bonding interactions in [BMIM][BF4]–DMSO system. Excess infrared spectroscopy and 2D-COS have been employed to analyze IR spectra in detail. From the fixed positions of positive and negative peaks in excess infrared spectra, it can be drawn that hydrogen-bonding is the main cause that influences the wavenumber shift of C–Hs in [BMIM]+ and DMSO, and the hydrogen-bonds involving aromatic C–Hs in the [BMIM]+ and DMSO are strengthened with the addition of DMSO. 2D-COS reveals that the O atom of DMSO prefers to interact with the imidazolium ring C–Hs than with the alkyl C–Hs in [BMIM][BF4]. The quantum chemical calculations illustrate that v(C2–H) is the main interaction site in the three complexes: [BMIM]+– [BF4], [BMIM]+–DMSO, and [BMIM][BF4]–DMSO. The wavenumber shifts of the v(C2–H) can be used to indicate roughly the state change of the cation in the mixing process. The turning point appearing in the wavenumber shift curve of v(C2–H) might be used to identify the dilution process. When the mole fraction of DMSO is lower than 0.6, only larger ion clusters can be broken into smaller ion clusters; when the mole fraction is between 0.6 and 0.9, ion clusters can be broken into ion pairs; when the mole fraction exceeds 0.9, ion pairs begin to be broken into individual cations and anions. These studies on the hydrogen-bonding interactions between [BMIM][BF4] and DMSO can provide in-depth information for understanding the physical properties of the mixtures of [BMIM][BF4] and DMSO. This is of importance for cellulose dissolution. It may also shed light on the studies of other ionic liquids with cosolvent in the future.
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Acknowledgment This work was supported by the Natural Science Foundation of China (Grant Nos. 21133009 and 21273130). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34]
[35] [36] [37] [38] [39] [40] [41] [42] [43]
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Hydrogen-bonding interactions between [BMIM][BF4] and dimethyl sulfoxide Yan-Zhen Zheng a, Hong-Yan He b, Yu Zhou a, Zhi-Wu Yu a,⇑ a b
Key Laboratory of Bioorganic Phosphorous Chemistry and Chemical Biology (Ministry of Education), Department of Chemistry, Tsinghua University, Beijing 100084, PR China Beijing Key Laboratory of Ionic Liquids Clean Process, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, PR China
h i g h l i g h t s It is first used excess spectra to investigate the interactions between [BMIM][BF4] and DMSO. C2–H is the main interaction site in investigated complexes. The two turning points of the wavenumber shift changes of C2–H may indicate the dilution process. Hydrogen-bond interactions are the main cause influencing the wavenumber shift changes.
a r t i c l e
i n f o
Article history: Available online xxxx Keywords: ATR-FTIR Excess infrared spectra Ion cluster Ion pair Hydrogen-bond Ionic liquid
a b s t r a c t Mixtures of Ionic liquids and small polar organic solvent are potential green solvents for cellulose dissolution under mild conditions. In this work, the interactions between a representative imidazolium-based ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF4]) and dimethyl sulfoxide (DMSO) were investigated in detail by attenuated total reflection infrared spectroscopy (ATR-IR) and density functional theory calculations (DFT). The main conclusions are: (1) C2–H is the main interaction site in forming cation–anion, cation–DMSO, and [BMIM][BF4]–DMSO complexes. (2) The two turning points of the wavenumber shift changes of C2–H may indicate that the dilution process can be divided into several stages: from larger ion clusters to smaller ion clusters, then to ion pairs, and finally to individual ions. The solvent molecules cannot break apart the strong Coulombic interaction between [BMIM]+ and [BF4] but can break apart the ion clusters into ion pairs when the mole fraction of DMSO is less than 0.9. When the mole fraction of DMSO is greater than 0.9, ion pairs can be broke into ions. (3) The hydrogen-bonds of the aromatic C–Hs in [BMIM]+ are strengthened in the dilution process while those of the alkyl C–Hs of [BMIM]+ are weakened. (4) The aromatic C–Hs of the [BMIM]+ cation strength before the weakening of the alkyl C–Hs. These in-depth studies on the properties of the ionic liquid-DMSO mixed solvents may shed light on exploring their applications as mixed solvents in cellulose dissolution and other practices. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Concerns about the gradual depletion of conventional fossil resources and the pressure from global climate changes have led to an intensified need for new alternative resources [1]. A potential solution to the problems is the utilization of cellulose as an alternative and sustainable energy source. Other than this, as the most abundant and bio-renewable resource found on the earth, cellulose has been widely used because of its many attractive properties such as biocompatibility, biodegradability, and thermal and chemical stability [2]. Generally, processing cellulose into desired products first requires the dissolution of cellulose. Due to its stiff and close packing via numerous inter-molecular and intra-molecular ⇑ Corresponding author. Tel.: +86 10 6279 2492; fax: +86 10 6277 1149. E-mail address: [email protected] (Z.-W. Yu).
hydrogen-bonds, cellulose cannot be melted and also extremely difficult to be dissolved in common solvents, which greatly limits its processing and applications [3]. Although some of the developed solvents have achieved great successes in cellulose dissolution, the inherent disadvantages such as toxicity, volatility, and instability are still critical problems in their practical applications [4]. Therefore, it is in urgent need of developing greener solvents for cellulose dissolution under mild conditions. Over the last decade, ionic liquids (ILs) have become popular solvents for the pretreatment of cellulose. ILs are organic salts with ultra-low vapor pressure, high thermal and chemical stability, and high solvent capacity and thermal stability [5]. In 2002, Swatloski et al. first reported that a certain class of ionic liquids (ILs) can be used as solvents for cellulose under relatively mild conditions, which inspired great interest in understanding the dissolution mechanism for the design of effective solvent systems [6]. Since
0022-2860/$ - see front matter Ó 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.molstruc.2014.01.013
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then, many kinds of ILs have been reported to use as cellulose solvents. Although ILs can be used in cellulose dissolution, sometimes their large-scale use is hindered due to their high viscosity and cost [7,8]. For example, most currently used ILs, especially those with halide anions, have high viscosities, typically 102–103 higher than traditional neutral organic solvents [9]. These problems greatly hinder large-scale application and further development of ILs to dissolve cellulose. Designing novel ILs with specific structures may bring on some improvements, but can hardly solve all the problems. Using mixtures of ILs and polar molecular solvents instead of pure ILs could be an effective approach to solving these problems in certain cases. For example, Rinaldi et al. found that some organic solutions, which contain just a small amount of IL, instantaneously dissolved large amounts of cellulose [10]. Xu et al. has also reported several highly effective cellulose solvents consisting of imidazolium ILs and polar solvents [11]. Noncovalent interactions greatly influence the physical properties of IL-cosolvent mixtures, and the key to understand the modulation effect of cosolvent on ILs is to investigate the molecular interactions between them [12–20]. Among various molecular interactions, electrostatic interaction is the most important one, however, other interactions especially hydrogen-bonding interactions are considered to be also very important [14–20]. DMSO is an extraordinary dipolar solvent, completely miscible with water and with very wide range of organic and inorganic substances [21]. Recent investigations showed that adding dimethyl sulfoxide (DMSO) into ILs can accelerate the cellulose dissolution dramatically [22]. The physical properties such as conductivity, viscosity, density, and volumetric property of the mixtures of IL and DMSO have been reported [23,24]. The interactions between DMSO and some ILs such as 1-butylpyridinium tetrafluoroborate ([BuPy][BF4]) [17] and 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF4]) [20] have been studied in the concentration range (the mole fraction of DMSO between 0.1 and 0.9). But the molecular pictures of the binary systems in the full concentration range (the mole fraction of DMSO between 0.1 and 0.99) are not clear yet. In this work, we chose DMSO and a representative imidazolium salt [BMIM][BF4] to investigate in detail their interactions, particularly the hydrogen-bonding interactions in the full concentration range. Attenuated total reflection infrared spectroscopy (ATR-FTIR) and density functional theory (DFT) calculation have been employed in the present study. In particular, excess infrared spectroscopy [25–29] and two-dimensional correlation spectroscopy (2D-COS) have been employed to analyze the infrared spectra [30,31]. 2. Experimental section 2.1. Materials and sample preparation [BMIM][BF4] (>98%) was purchased from Sigma–Aldrich. DMSO-d6 (>99.8 of deuterium) was purchased from Cambridge Isotopes Laboratories (CIL). DMSO (>99.5%) was from Beijing Chemical Plant (Beijing, China). The IL was dried for 72 h under vacuum, and then was stored in a desiccator. A series of [BMIM][BF4]–DMSO-d6 mixtures were prepared by weighing for the FTIR experiments. The mole fractions of DMSO-d6 in the mixtures are displayed in the respective excess spectra.
filling factor of 2. For each sample, three parallel measurements were carried out. The refractive indexes of different solutions were measured with an Abbe refractometer at 25 °C. The formula suggested by Hansen was used to do the ATR corrections [32]. 2.3. Excess infrared spectroscopy The theory of excess infrared spectroscopy has been described in detail elsewhere [25,26]. Briefly, an excess infrared spectrum is defined as the difference between the spectrum of a real solution and that of the respective ideal solution under identical conditions. The working equation in calculating the excess infrared spectrum is
eE ¼
A ðx1 e1 þ x2 e2 Þ dðC 1 þ C 2 Þ
ð1Þ
where A is the absorbance of the mixture, d is the light path length, C1 and C2 are the molarities of the two components, x1 and x2 are the mole fractions of components 1 and 2, e1 and e2 are the molar absorption coefficients of the two components in their pure states, respectively. If our attention is the absorbance of a particular absorption band, the excess molar absorbance of the band can be calculated by its integrated intensity. Matlab 7.0 (Math Works Inc., Natick, MA) was used to manipulate spectral data pretreatment, i.e., the subtraction, truncation, and baseline correction, to calculate excess infrared spectra and to do the integration of the bands. 2.4. Two-dimensional correlation spectroscopy Standard 2D correlation spectral analysis was performed using Matlab 7.0, based on the algorithm developed by Noda [30]. In order to remove the linear contribution to absorbance by concentration variation and thus to obtain the information of specific interactions, the selected bands were normalized with the modified component-normalization method [28,33]. Specifically, the absorption bands of m(aromatic C–Hs) and v(alkyl C–Hs) in excess infrared spectra were divided by the corresponding molarity of [BMIM][BF4]. The average of all the spectra over the full concentration range was used as the reference spectrum. In the 2D correlation contour map, solid (red) and dashed (blue) lines represent positive and negative correlation intensities, respectively. 2.5. Quantum chemical calculations All computations were performed with the Gaussian 03 program [34]. The B3LYP method with the 6-31++G** basis set was used to optimize molecular energies and geometries of the isolated DMSO and [BMIM]+–[BF4], [BMIM]+–DMSO, and [BMIM] [BF4]–DMSO complexes. The optimized geometries at local energy minimum were ensured by the absence of imaginary vibrational frequency. Meanwhile, the basis set superposition error (BSSE) correction computed via the counterpoise method of Boys and Bernardi was estimated for obtaining accurate interaction energies of the complexes [35]. 3. Results and discussion
2.2. FTIR spectroscopy FTIR spectra over the range from 4000 to 650 cm1 were collected at room temperature (25 °C) using a Nicolet 5700 FTIR spectrometer, equipped with a MCT detector. The attenuated total reflection (ATR) cell, made of trapezoidal ZnSe crystal with incident angle of 45° and 12 reflections, was employed. Spectra were recorded with a resolution of 2 cm1, 32 parallel scans, and a zero
Infrared spectroscopy and quantum chemical calculations have been employed to study hydrogen-bonding interactions between [BMIM][BF4]–DMSO. Due to the fact that information from the former can only provide necessary conditions of hydrogen-bond interactions, whilst information from the latter provides sufficient conditions, we present the quantum chemical calculation results first.
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3.1. Theoretical Investigation of the Interaction between [BMIM][BF4] and DMSO A set of possible conformations of the complexes consisting of one DMSO molecule and the [BMIM]+ cation, with or without the anion, have been examined by DFT calculations to characterize the interactions between DMSO and [BMIM][BF4]. The optimized geometries of [BMIM]+–[BF4], [BMIM]+–DMSO, [BF4]–DMSO, and [BMIM][BF4]–DMSO complexes were studied at the B3LYP/631++G** level. The sum of van der Waals atomic radii of hydrogen and oxygen (2.5 Å) and that of hydrogen and fluorine (2.45 Å) are used as critical values for judging the existence of hydrogen-bonds between hydrogen and oxygen/fluorine [36]. Hydrogen-bonds are denoted by dashed lines, and the corresponding H O and H F distances are labeled (Figs. 1–3). For the cation [BMIM]+, there are many possible sites to interact with DMSO. To find out the preferred interaction sites, a number of possible mutual orientations of the complexes consisting of one DMSO molecule and one [BMIM]+ cation were examined. The optimized geometries and the respective interaction energies are shown in Fig. 1A–C. As shown in the figure, both of the aromatic and alkyl CHs in [BMIM]+ can form hydrogen-bonds with the oxygen atom. The absolute value of the interaction energy of the first conformer (Fig. 1A) is much larger than the other two, indicating that conformer A is the most stable one. Besides, the sequential order of the hydrogen-bond indicates that C2–H is the most preferred interaction site, followed by C4,5H. As to the interactions between DMSO and the anion, two hydrogen-bonds are identified. Before examining the cation–anion-cosolvent ‘‘three-body’’ complex, interactions between the cation and anion was first investigated. The optimized geometries and respective interaction energies of [BMIM]+–[BF4] complex are shown in Fig. 2. The absolute value of the interaction energy of the first conformer is much
Fig. 2. Four optimized geometries and corresponding interaction energies for [BMIM]+–[BF4] complex. Hydrogen-bonds are denoted by dashed lines and the corresponding H F distances are labeled. The corresponding interaction energies are shown below the complexes.
Fig. 3. Four optimized geometries and corresponding interaction energies for [BMIM][BF4]–DMSO complex. Hydrogen-bonds are denoted by dashed lines and the corresponding H O and H F distances are labeled. The corresponding interaction energies are shown below the complexes. Fig. 1. Optimized geometries and corresponding interaction energies for an DMSO molecule interacting with [BMIM]+ and [BF4]. Hydrogen-bonds are denoted by dashed lines and the corresponding H O and H F distances are labeled. The corresponding interaction energies are shown below the complexes.
larger than the other three conformers, meaning that it is the most stable one and the anion is more easily captured by C2–H. Comparison of the most stable interaction pair in Fig. 1 and that
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in Fig. 2, we can see that C2–H is the most active hydrogen atom in the cation. Charge distribution calculation has pointed out that, the positive charge is distributed on the peripheral hydrogen atoms of aromatic ring of the cation, while the negative charge is distributed at N1 and N3 atoms. C4 and C5 atoms appear to be neutral while C2 atom exclusively possesses a positive charge owing to the electron deficit nature of the C2@N bond. The large positive charge at the C2–H unit accounts for the observed higher acidity of C2–H [37]. Shown in Fig. 3 are four optimized conformers of the [BMIM][BF4]–DMSO complex, giving the interaction modes in the presence of both the anion and the cation. It can be seen that, in the three-body complex, both of the methyl group and the oxygen atom of DMSO can take part in the hydrogen-bond interactions with the anion and cation of IL simultaneously. Comparing the most stable conformers in Figs. 1–3, we see that the absolute values of the interaction energies of [BMIM]+–[BF4] (345.10 kJ/mol) and [BMIM][BF4]–DMSO (395.54 kJ/mol) complexes are much larger than those of [BMIM]+–DMSO (77.36 kJ/ mol) and [BF4]–DMSO (57.50 kJ/mol). The large interaction energies of the former are mainly due to the strong Coulombic interaction between the cation and the anion. From the optimized stable geometries shown in Fig. 3, we can see that DMSO molecule cannot insert into the [BMIM]+–[BF4] complexes, or in other words, stay between the cation and the anion. It illuminates that it is not easy for the neutral molecule to break apart the strong Coulombic interaction between [BMIM]+ and [BF4] by simple insertion between them when the concentration of the neutral molecule is low [38].
influence from overlap, [BMIM][BF4]–DMSO-d6 system was chosen to analysis the v(C–H) in the cations and v(C–D) in DMSO-d6. 3.2.2. ATR-FTIR analysis of v(C–Hs) ATR-FTIR spectra and excess infrared spectra of [BMIM][BF4]– DMSO-d6 system within the concentration range in the v(C–Hs) region are shown in Fig. 5. A few features can be seen readily in the original infrared spectra with increasing concentration of DMSOd6. In the ATR-FTIR spectra (Fig. 5A), the imidazolium ring C–H stretching bands, centered around 3162 cm1 and 3122 cm1 in pure [BMIM][BF4], exhibit significant changes in the spectral profiles in the presence of DMSO-d6. The absorption peak positions of v(C2–H) and v(C4,5–H) gradually move to lower wavenumbers with increasing DMSO-d6. When the mole fraction of DMSO-d6 is higher than 0.6, the changes are evident, particularly a shoulder peak appears for v(C2–H). The bandwidth of the v(C2–H) and v(C4,5–H) are broader and broader with increasing DMSO-d6, which are more pronounced for v(C2–H). For the alkyl C–Hs, apparently, they do not exhibit obvious changes in the spectral
3.2. ATR and excess infrared spectra analysis 3.2.1. IR spectra of pure [BMIM][BF4], DMSO, and DMSO-d6 Stretching vibrations are effective bands to characterize the weak interactions. The partial ATR-FTIR spectra of pure [BMIM][BF4], DMSO and DMSO-d6 are shown in Fig. 4. For pure [BMIM][BF4], the bands at around 3120 cm1 and 3160 cm1 are attributed to the v(C2–H) and v(C4,5–H) of the imidazolium ring, respectively, while the three bands in 3000–2800 cm1 range are from the v(C–H) in the alkyl chains of imidazolium ring [39]. It should be noted that there have been debates on the assignments of the aromatic C–Hs in imidazolium ILs in literatures [39,40]. Our assignments are in line with the majority of the community. For pure DMSO-d6 and DMSO, two bands around 2250 and 2125 cm1 are due to the vas(C–D) and vs(C–D) of DMSO-d6, and the bands around 2994 and 2912 cm1 are due to the vas(C–H) and vs(C–H) of DMSO, respectively [41]. In order to get rid of the
Fig. 4. ATR-FTIR spectra of pure [BMIM][BF4], DMSO, and DMSO-d6 in the range of 3300–2000 cm1.
Fig. 5. ATR-FTIR (A) and excess infrared (B) spectra of [BMIM][BF4]–DMSO-d6 system in the range of the C–H stretching vibrations. From top to bottom, the mole fraction of DMSO-d6 increases from 0 to 1 in (A) and 0.1 to 0.99 in (B) with the corresponding mole fractions of DMSO-d6 shown in (B). The dashed and dashdotted lines depict spectra of pure [BMIM][BF4] and DMSO-d6.
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Fig. 6. Wavenumber shifts (A) and excess molar absorbance (B) of the v(C–Hs) in [BMIM]+ at different mole fractions of DMSO-d6. Solid and open symbols represent the alkyl and aromatic C–H, respectively.
profiles upon dilution, implying that they may not be the interaction sites. The wavenumber shifts of the v(C–Hs) are plotted as functions of the mole fraction of DMSO-d6 in Fig. 6A. Starting in the neat RTIL, the v(C2–H) shows a slight but monotonic red-shift until the DMSO-d6 mole fraction of 0.6. With further addition of the solvent the slope of the curve substantially decreases, showing a more pronounced change. When the mole fraction of DMSO-d6 reaches 0.9, the red-shift becomes more pronounced. In contrast, the v(C4,5H) and v(alkyl C–Hs) show a monotonic red-shifting behavior. Careful analysis shows that the peak positions of the bands at 3162 cm1
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and 3122 cm1 red-shift by about 15 cm1 and 50.3 cm1, respectively, while the v(alkyl C–Hs), centered around 2964 cm1, 2938 cm1, and 2877 cm1, are all red-shifted by only about 3 cm1, suggesting that the C–H groups on the imidazolium cation experience different environment in the mixture as compared to that in pure IL. Based on our previous work and other literatures, ion clusters are the dominating constituents in pure ILs [19,42]. Mixing with molecular solvents can break apart the ion clusters into ion pairs, ion pairs into individual ions. However, when the concentration of molecular solvents is not high enough, it can only break larger ion clusters into smaller ion clusters. C2–H is the main interaction site of the cation and is thus the most sensitive compared to other C–Hs in the cation as can be seen in the quantum chemical calculations discussed in the above section. From the wavenumber changes of C2–H, we might get the following dilution process: When the mole fraction of DMSO is lower than 0.6, larger ion clusters are broken into smaller ion clusters. When the mole fraction of DMSO is among 0.6–0.9, ion clusters are broken into ion pairs. When the mole fraction of DMSO is greater than 0.9, all the ion clusters are broken into ion pairs and ion pairs begin to change to individual ions. Compared to the original IR spectra, excess infrared spectra can better reveal the changes of the infrared spectra than the original IR spectra [16–19,25–29]. It can reveal the positions of new complexes and the changes in molar absorptivity directly, reflecting details of the molecular interactions in the mixtures. For a compound existing in different distinct forms, if the resolution of an excess spectrum is good enough, there should be both positive and negative peaks for a particular vibrational mode, representing the increasing and decreasing amounts of the respective species including the interacting complexes in the mixtures. As shown in Fig. 5B, both m(C2–H) and m(C4,5–H) regions have positive peaks at lower wavenumber and negative peaks at the higher wavenumber, attributed to the red shift of the bands. For v(C–H) of the alkyl chains, there are four positive peaks at the lower wavenumber side (corresponding to 2964, 2938, and 2877 cm1 of the pure ionic liquid v(alkyl C–H)), attributed to red shifts of the bands. Interestingly, the positions of the negative bands and positive bands in the excess spectra are nearly fixed in the concentration range investigated, indicating they are from disappearing and appearing species in the mixtures.
Fig. 7. Synchronous (A) and asynchronous (B) 2D correlation spectra contour maps of v(C–Hs) of [BMIM][BF4] in the dilution process with DMSO-d6. Solid (red) and dashed (blue) lines represent positive and negative correlation intensities, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.).
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Integral values of the bands in excess infrared spectra can be used to represent the molar absorptivity variation of a certain vibrational mode. The integral values of the excess infrared bands (eE) for the aromatic and alkyl C–H stretches in the cation are given in Fig. 6B. For the C–Hs in the cation, they are all positive over the entire concentration range. The integral value of the aromatic C–Hs is larger than the alkyl C–Hs and the maximum appears before the alkyl C–Hs. In the IL solutions, both of the solvent effects (including the effect of charged particles in the solution) and hydrogen-bonding interactions can influence the IR spectra [14–20,40]. If the solvent effect dominated, the positions of the positive bands and negative bands in the excess spectra would have shifted in a gradual manner with increasing cosolvent. On the contrary, the positions of the negative bands and positive bands in the excess spectra are nearly fixed as can be seen in Fig. 5B. This is attributed to the relative stable hydrogen-bonding interaction pairs/complex. Hydrogen-bond from the imidazolium ring C–Hs is classified as ‘‘proper red-shift hydrogen-bond’’ [14–20], while that related to alkyl C– Hs or C–Ds is classified as ‘‘improper blue-shift hydrogen-bond’’
[43]. During the dilution process by DMSO-d6, m(C2–H), v(C4,5–H), and m(alkyl C–Hs) show red-shifts and increase in IR absorbance. This implies that, with the addition of DMSO-d6, the hydrogenbonding interactions involving the C–H groups in imidazolium ring are strengthened, while, that in the alkyl C–H groups are weakened. 3.2.3. Sequential order of the interactions of C–Hs with DMSO-d6 The preference of DMSO-d6 in the interactions with imidazolium ring C–Hs and alkyl C–Hs can be evaluated by (2D-COS). Absorption bands of m(Aromatic C–Hs) and m(alkyl C–Hs) in [BMIM][BF4]–DMSO-d6 system were selected to do the 2D-COS analysis. The synchronous and asynchronous 2D correlation contour maps of m(aromatic C–Hs) and m(alkyl C–Hs) in [BMIM][BF4]– DMSO-d6 system are shown in Fig. 7. The two central bands in imidazolium ring and alkyl C–H stretching regions of [BMIM][BF4], at around 3162 and 2964 cm1 (see Fig. 4), are taken as the representative absorptions for 2D correlation analysis. A negative cross peak at (3162 cm1, 2964 cm1) is seen in the synchronous spectrum shown in Fig. 7A. While in the asynchronous spectrum given in Fig. 7B, a negative cross peak at the same position is seen. According to Noda’s rule [30,31], the change of the absorption coefficient of the imidazolium ring is prior to that of the alkyl C–H. It means that DMSO prefers to interact with the aromatic C–Hs compared to alkyl C–Hs in the dilution process. The integral value of the aromatic C–Hs is larger than the alkyl C–Hs and the maximum appears before the alkyl C–Hs. These may also indicate the DMSO prefers to interact with the aromatic C–Hs compared to alkyl C–Hs in the dilution process. 3.2.4. ATR and excess infrared spectra analysis of v(C–D) ATR-FTIR spectra and excess infrared spectra of [BMIM][BF4]– DMSO-d6 system over the entire mole fraction rang in the v(C–D) region are shown in Fig. 8. A few features can be seen readily in
Fig. 8. ATR-FTIR (A) and excess infrared (B) spectra of vas(C–D) and vs(C–D). From top to bottom, the mole fraction of [BMIM][BF4] increases from 0 to 1 in (A) and 0.01 to 0.9 in (B) with the corresponding mole fractions of IL shown in (B). The dashed and dash-dotted lines depict spectra of pure DMSO-d6 and [BMIM][BF4].
Fig. 9. Wavenumber shifts (A) and excess molar absorbance (B) of vs(C–D) in DMSO-d6 at different mole fractions of [BMIM][BF4].
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vas(C–D)
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the original infrared spectra with increasing concentration of ILs. For the original IR spectra, the vas(C–D) and vs(C–D) gradually move to higher wavenumber with increasing IL. The band width of mas(C–D) becomes broader. When the mole fraction of IL is greater than 0.3, it can be seen that the vas(C–D) band is consisting of two components: the amount of the species in the lower wavenumber gradually decreases, while the amount of the species in the higher wavenumber gradually increases. The wavenumber shifts are presented in Fig. 9A. They are blue-shifted by 13.4 and 5.8 cm1 for vas(C–D) and vs(C–D), respectively. As can be seen in the figure, the wavenumber shift change is more pronounced for vas(C–D). Compared to the original IR spectra, excess infrared spectra can better reveal the changes of the infrared spectra than the original IR spectra [16–19,25–29]. It can reveal the positions of new complexes and the changes in molar absorptivity directly, reflecting details of the molecular interactions in the mixtures. In Fig. 8B, excess infrared spectra show positive bands at higher wavenumbers and negative bands at lower wavenumbers in vas(C–D) and vs(C–D) region. The positions of the positive bands and negative bands are nearly fixed, indicating that there are distinct complexes in the mixture. The positive bands in the higher wavenumber are related to the DMSO-IL complex, while the negative bands in the lower wavenumbers are attributed to the DMSO self-association complex. The increasing of the complex in the higher wavenumber and the reducing of the complex in the lower wavenumber are the origin of the blue-shift of vas(C–D) and vs(C–D). The integral values of the excess infrared bands (eE) for both ms(C–D) and vas(C–D) are given in Fig. 9B. The values are both negative in the concentration range. As ‘‘unconventional hydrogenbond’’ [43], the blue-shifts and the negative integral values of eE indicate that the hydrogen-bond interactions of the methyl group in DMSO-d6 are strengthened upon mixing with the ionic liquid. 4. Conclusions In this work, ATR-FTIR and DFT calculation were used to study the hydrogen-bonding interactions in [BMIM][BF4]–DMSO system. Excess infrared spectroscopy and 2D-COS have been employed to analyze IR spectra in detail. From the fixed positions of positive and negative peaks in excess infrared spectra, it can be drawn that hydrogen-bonding is the main cause that influences the wavenumber shift of C–Hs in [BMIM]+ and DMSO, and the hydrogen-bonds involving aromatic C–Hs in the [BMIM]+ and DMSO are strengthened with the addition of DMSO. 2D-COS reveals that the O atom of DMSO prefers to interact with the imidazolium ring C–Hs than with the alkyl C–Hs in [BMIM][BF4]. The quantum chemical calculations illustrate that v(C2–H) is the main interaction site in the three complexes: [BMIM]+– [BF4], [BMIM]+–DMSO, and [BMIM][BF4]–DMSO. The wavenumber shifts of the v(C2–H) can be used to indicate roughly the state change of the cation in the mixing process. The turning point appearing in the wavenumber shift curve of v(C2–H) might be used to identify the dilution process. When the mole fraction of DMSO is lower than 0.6, only larger ion clusters can be broken into smaller ion clusters; when the mole fraction is between 0.6 and 0.9, ion clusters can be broken into ion pairs; when the mole fraction exceeds 0.9, ion pairs begin to be broken into individual cations and anions. These studies on the hydrogen-bonding interactions between [BMIM][BF4] and DMSO can provide in-depth information for understanding the physical properties of the mixtures of [BMIM][BF4] and DMSO. This is of importance for cellulose dissolution. It may also shed light on the studies of other ionic liquids with cosolvent in the future.
7
Acknowledgment This work was supported by the Natural Science Foundation of China (Grant Nos. 21133009 and 21273130). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34]
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