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Hydrogen-bonding interactions between a pyridinium-based ionic liquid [C4Py][SCN] and dimethyl sulfoxide
Chemical Engineering Science ∎ (∎∎∎∎) ∎∎∎–∎∎∎
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Hydrogen-bonding interactions between a pyridinium-based ionic liquid [C4Py][SCN] and dimethyl sulfoxide Hongyan He a, Hui Chen a, Yanzhen Zheng b, Suojiang Zhang a,n, Zhiwu Yu b a Beijing Key Laboratory of Ionic Liquids Clean Process, Key Laboratory of Green Process and Engineering, State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China b Department of Chemistry, Tsinghua University, Beijing 100084, China
H I G H L I G H T S
We present novel insight into the molecular interactions between co-solvent and IL. Universal H-bonds play important role for the miscibility of DMSO with IL. H-bonding interactions are the main cause influencing the wavenumber shift changes. We analyze the essential difference of the interactions between DMSO and ion of IL.
art ic l e i nf o
a b s t r a c t
Article history: Received 26 April 2014 Received in revised form 7 July 2014 Accepted 12 July 2014
In this work, the interactions between a representative pyridinium-based ionic liquid (IL) with cyanofunctionalized anion ([C4Py][SCN]) and dimethyl sulfoxide (DMSO) were investigated in detail using attenuated total reflection infrared spectroscopy (ATR-FTIR), hydrogen nuclear magnetic resonance (1H NMR), and density functional theory (DFT) calculations. It was found that H-bonds are universally involved and play important role for the miscibility of DMSO with [C4Py][SCN] IL and maintain the stability of this system. ATR-FTIR and excess spectroscopy analysis indicated that the H-bonds involving the alkyl C–Hs and C N are strengthened with the addition of DMSO, while the H-bonds involving pyridinium ring C–Hs as well as the H-bonds formed between the [C4Py] þ and [SCN] are weakened. The addition of [C4Py][SCN] IL led to the H-bonds involving the C–Hs in DMSO weakened comparing with the associated H-bonds in the pure DMSO system. The results of DFT calculations indicated that the DMSO molecules cannot disrupt the strong Coulombic interaction between the [C4Py] þ and [SCN] . Natural bond orbital (NBO) analysis further confirmed that the interaction mechanisms of DMSO molecule with the anion and cation are different in nature. These studies will shed light on exploring the applications of ILs as reaction or separation media. & 2014 Elsevier Ltd. All rights reserved.
Keywords: Quantum chemical calculation ATR-FTIR Ionic liquids Hydrogen bond Excess infrared spectroscopy
1. Introduction As novel and green class of chemical components, room temperature ionic liquids (ILs) are becoming popular for a variety of applications due to their unique physiochemical properties, such as ultra-low vapor pressure, high thermal and chemical stability, high solvent capacity, etc. Particularly, their properties could be tuned by varying either anion, cation or other substituent (Forsyth et al., 2004; Wilkes, 2002). However, in many industrial applications, the use of pure ILs is practically hindered due to their
n
Corresponding author. E-mail address: [email protected] (S. Zhang).
high polarity (Reichardt, 2005), high viscosity (Weingartner, 2008) and high cost (Clark and Tavener, 2006). Thus, the design of novel ILs with specific structures will bring new hope for dealing with these challenges, but could hardly overcome all these problems at the present stage. Recently, the using mixtures of ILs and other cosolvents such as water, alcohol, and DMSO is proved to be an effective approach to solve these problems (He et al., 2013a; Long et al., 2011). These co-solvents have significant effect on the physiochemical properties of the IL systems such as density, viscosity, polarity, surface tension and reactivity (Cappelli et al., 2000; Chang et al., 2008b; Navas et al., 2009; Umebayashi et al., 2009; Vreekamp et al., 2011; Zhang et al., 2011). Therefore, it is of great importance to investigate the interactions between ILs and co-solvents at the molecular level intensively and to obtain
http://dx.doi.org/10.1016/j.ces.2014.07.024 0009-2509/& 2014 Elsevier Ltd. All rights reserved.
Please cite this article as: He, H., et al., Hydrogen-bonding interactions between a pyridinium-based ionic liquid [C4Py][SCN] and dimethyl sulfoxide. Chem. Eng. Sci. (2014), http://dx.doi.org/10.1016/j.ces.2014.07.024i
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desirable physicochemical properties of such systems with specific functions. It has been reported that non-covalent interactions influence the properties of ILs and co-solvent mixtures greatly, especially H-bonding interactions, which are considered as the most important interaction in ILs system except for the electrostatic interaction (Dong and Zhang, 2012; Fumino et al., 2008; Peppel et al., 2011; Wang et al., 2010). Numerous efforts have been devoted to the experimental and theoretical investigations involving H-bonding interactions of pure ILs and ILs-cosolvent system. These researches demonstrated that H-bonds affect both the structures and properties in such systems. For example, the single, local and directional interactions such as H-bonds can decrease the melting point of imidazolium-based ILs by 100 K, which greatly extends the working temperature range of the ILs (Fumino et al., 2008). Water molecules absorbed from air (0.2–1.0 mol/dm3) interact mainly with various anions such as [PF6] , [BF4] , and [CF3CO2] in the form of symmetric anion–HOH–anion H-bonded complexes in imidazolium-based ILs (Cammarata et al., 2001). In a representative mixture system including imidazolium IL and acetone, the peculiarities of volumetric property data have been revealed by H-bonding interactions (Kiefer et al., 2012). In our previous reports, the H-bond networks in a number of ILs and the mechanism accounting for hydrogen bond-promoted fixation of CO2 have been investigated (Dong et al., 2012; Dong et al., 2006; He et al., 2013b; Wang et al., 2012). Infrared spectroscopy showed that the H-bonds and structure organizations in ILs with the presence of co-solvents revealed continuously decreasing of free O–H band intensity with the increasing of the pressure of 1-butyl3-methylimidazolium tetrafluoroborate/water mixtures (Chang et al., 2008a; Chang et al., 2008b, c; Chang et al., 2006). The above-mentioned studies have greatly enhanced our perspective on the interactions between ILs and co-solvents. However, most of the studies are limited to the imidazolium-based ILs. In fact, alkylated pyridinium-based ILs is the second major category of cations in the family of ILs in addition to imidazolium-based cations (Navas et al., 2009; Yunus et al., 2012; Zhu et al., 2011). Compared with most reported imidazolium-based ILs, pyridinium-based ILs have many advantages, for example, high thermal stability, excellent SO2 absorption performance (Anderson et al., 2006; Zeng et al., 2014), lower cost and higher biodegradability (Docherty et al., 2006). On the other hand, DMSO is an extraordinary dipolar aprotic solvent, completely miscible with wide range of organic and inorganic chemicals (Brayton, 1986), and ILs-DMSO binary systems are frequently used in the experimental fields (Rinaldi, 2011), such as cellulose dissolving (Andanson et al., 2014), which are of particular interest to us to study the ILs–DMSO interactions, especially ILs contain pyridinium-based cations. In this work, N-butylpyridiniumthiocyanate IL containing pyridinium-based cation, [C4Py][SCN], is chosen as the representative IL for the studying of the H-bonding behaviors with DMSO as the co-solvent. To the best of our knowledge, there have been few publications concerning about the H-bonding interactions of N-butylpyridiniumthiocyanate ILs. Attenuated total reflection infrared spectroscopy (ATR-FTIR), hydrogen nuclear magnetic resonance (1H NMR), and density functional theory (DFT) calculations were employed in the present study. In order to get rid of the overlap in the 5 4
6 1
8
10 9
7
O S C N
N
3
S H 3C
CH 3
2
[C4Py][SCN]
DM S O
Fig. 1. Structure and atom numbering of [C4Py][SCN] IL and DMSO.
C–H stretching vibration region of the [C4Py] þ cation and DMSO, deuterated DMSO (DMSO-d6) and DMSO were used in the FTIR experiment. In particular, excess infrared absorption spectroscopy (Li et al., 2006; Wang et al., 2009; Wang et al., 2010; Zhang et al., 2010) was used for detailed investigation of the interaction, which has been demonstrated to enhance the spectral resolution and can help reveal the details of the molecular interactions in liquid solution.
2. Experimental details 2.1. Chemicals [C4Py][SCN] IL was synthesized following the reported methods (Yunus et al., 2012). The product, a transparent yellow viscous liquid, was dried in vacuum for 48 h at 323 K. Water content was lower than 550 ppm measured by a Karl Fischer coulometric titration. The 1H NMR chemical shifts of [C4Py][SCN] were determined as follows: δH (600 MHz, DMSO, TMS), 9.10 (2H, d, C2,6–H), 8.62 (1H, t, C4–H), 8.18 (2H, t, C3,5–H), 4.62 (2H, t, C7–H), 1.91 (2H, m, C8–H), 1.31 (2H, m, C9–H), 0.93 (3H, t, C10–H). The chemical structure and atom numbering for [C4Py][SCN] IL are presented in Fig. 1. DMSO (4 99%) and DMSO-d6 (D, 99.8%) were purchased from Cambridge Isotope Laboratories, without further purification before use.
2.2. Sample preparation A series of [C4Py][SCN]–DMSO and [C4Py][SCN]–DMSO-d6 binary mixtures were prepared by weighing with a METTER TOLEDO ML 204/02 balance ( 71 10 4 g), thus leading to 71 10 4 accuracy for mole fraction. The mole fractions of DMSO in [C4Py][SCN]–DMSO mixtures are of 0.1001, 0.1992, 0.3001, 0.3997, 0.4999, 0.6000, 0.7001, 0.8000 and 0.9000, and DMSO-d6 in [C4Py][SCN]–DMSO-d6 mixtures are of 0.1076, 0.1999, 0.3181, 0.3996, 0.5002, 0.5999, 0.7004, 0.7995 and 0.9000.
2.3. FTIR spectroscopy FTIR spectra over the range from 4000 to 650 cm 1 were collected at temperature of 20.3 1C using a Nicolet 5700 FTIR spectrometer, equipped with a MTC detector. Two attenuated total reflection cells, ZnSe and Ge crystals with incident angles of 451 and 601 and 12 reflections, were employed. The Ge crystal, with shorter effective light path was used to examine the strong stretching bands of C N. Spectra were recorded with a resolution of 2 cm 1, a zero filling factor of 2, and 16 parallel scans. Three parallel measurements for each sample were carried out. The refractive indexes of solutions were measured with a refractometer at 20.3 1C. The formulas suggested by Hansen (1965) were used to do the ATR corrections.
2.4. Excess absorption spectroscopy The calculation of excess infrared spectra was performed on Matlab software, based on the theory developed by Yu's group (Li et al., 2006; Wang et al., 2009). The theory of excess infrared spectroscopy and its applications have been described in detail in their papers (Wang et al., 2010; Zhang et al., 2011; Zhang et al., 2010; Zheng et al., 2013). Briefly speaking, 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 formula for calculating the excess
Please cite this article as: He, H., et al., Hydrogen-bonding interactions between a pyridinium-based ionic liquid [C4Py][SCN] and dimethyl sulfoxide. Chem. Eng. Sci. (2014), http://dx.doi.org/10.1016/j.ces.2014.07.024i
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infrared spectrum is described as follows:
εE ¼
A ðx1 εn1 þ x2 εn2 Þ dðC 1 þ C 2 Þ
ð1Þ
where A is the absorbance of the mixture, d is the light path length, C 1 and C 2 are molarities of the two components, x1 and x2 are mole fractions of components 1 and 2, and εn1 and εn2 are molar absorption coefficients of the two components in their pure states, respectively. 2.5.
1
H NMR measurements
The 1H NMR measurements were carried out on a JEOL JNMECA 300 NMR spectrometer (300 MHz) at 298 K. To avoid the influence of the possible interactions between the standard chemical TMS and DMSO–[C4Py][SCN], TMS was dissolved in CCl4 and used as external standard. Fig. 2. ATR-FTIR spectra of [C4Py][SCN], DMSO and DMSO-d6 in the range of 3200– 2000 cm 1.
3. Computational details DFT calculations were carried out using Gaussian 09 program (Frisch et al., 2010). All geometric optimizations were performed at B3LYP/6–31þ þg (d, p) level. In the optimization process, all complexes were relaxed under symmetric constraints. The final obtained geometries were recognized as local minima without any negative vibrational frequency. Second-order perturbation delocalization energies (E(2)) were obtained by natural bond orbital (NBO) analysis at B3LYP/6–31þ þg (d, p) level. The interaction energies calculated at B2PLYPD/6–31 þ þg (d,p) theoretical level were compared with the results obtained at B3LYP/6–31þ þg (d, p) level. B2PLYPD is a new empirical hybrid function considering both the correction from perturbation theory and dispersion correlation, which has been successfully used to obtain the thermodynamic data with the smallest mean absolute deviation (Goerigk et al., 2009; Sancho-Garcia and Adamo, 2013; Schwabe and Grimme, 2007). The interaction energies for a dimer AB complex and trimer ABC complex were obtained as below (Young, 2001)
ΔE ¼ EAB EA EB
ð2Þ
ΔE ¼ EABC EA EB EC
ð3Þ
The structure of the [C4Py] þ –[SCN] complex was initially optimized in the gas phase as a tentative interaction model. Then the solvent effect was introduced by using the polarized continuum model (PCM) (Barone and Cossi, 1998). The method creates a solute cavity by a set of overlapping spheres. The solvent DMSO was used, ε ¼46.7. 4. Results and discussion 4.1. IR spectra of pure [C4Py][SCN] IL, DMSO and DMSO-d6 The partial ATR-FTIR spectra of pure [C4Py][SCN] IL, DMSO and DMSO-d6 are shown in Fig. 2 and the concerned spectra of IL–DMSO interactions are restricted to these regions. For pure [C4Py][SCN] IL, three bands above 3020 cm 1 are attributed to the coupled v(Cring–H) of the pyridinium ring, while those below 3020 cm 1 are from the v(Calkyl–H) in the alkyl chain (Wang et al., 2010), and the strong band around 2052 cm 1 is attributed to the v(C N) in the anion (Zheng et al., 2013). For pure DMSO-d6 and DMSO, two bands around 2249 and 2124 cm 1 are due to the vas(C–D) and vs(C–D) of DMSO-d6, and the bands around 2994 and 2911 cm 1 are due to the vas(C–H) and vs(C–H) of DMSO, respectively (Zhang et al., 2009). Thus, the [C4Py][SCN]–DMSO system was
chosen for the analysis of the v(C N) to avoid the influence of band overlap, while the [C4Py][SCN]–DMSO-d6 system was selected to analyze the coupled v(Cring–H) of the pyridinium ring, v(Calkyl–H) in the alkyl chain and vas(C–D) of DMSO-d6. 4.2. ATR and excess infrared spectra analysis of v(C–H) It is well known that the v(C–H) is a key band to characterize the H-bonds such as C–H O/N, and it can be used as a probe to reflect the interactions between DMSO and ILs (Chang et al., 2006). The partial ATR-FTIR and excess infrared spectra of [C4Py][SCN]– DMSO-d6 system in the v(C–H) region over the entire mole fraction range are shown in Fig. 3. A few features can be found readily in the original infrared spectra with the increment of DMSO content (Fig. 3a). The pyridinium ring C–Hs stretching bands, centered around 3126 and 3052 cm 1 in pure [C4Py] [SCN] IL, exhibit obvious changes in the spectral profiles in the presence of DMSO-d6. It is interesting to see that both shoulder peaks on the left and right sides of v(Cring–H) around 3052 cm 1 gradually disappear while the mole fraction of DMSO-d6 is higher than 0.7. The peak of v(Cring–H) around 3126 cm 1 gradually moves to higher wavenumber with the increasing of the mole fraction of DMSO-d6 by blue-shifted 4.6 cm 1, while the v(Cring–H) around 3052 cm 1 is nearly fixed, with less than70.3 cm 1 change in wavenumber (Fig. 3b). As for the alkyl C–Hs stretching bands, centered around 2960, 2935 and 2873 cm 1 in pure [C4Py] [SCN] IL, they exhibit no obvious changes in the spectral profiles in the presence of DMSO-d6. The peak positions of these three bands show less obvious shifts than that of v(Cring–H) around 3126 cm 1 with merely blue-shifted by 1.8, 1.9 and 2.0 cm 1, respectively (Fig. 3d), which implies that pyridinium ring C–Hs are the preferred sites for interactions between the cation and DMSO-d6. Excess infrared spectra can reveal the positions of new complexes and the changes in molar absorptivity more clearly than the original IR spectra (Li et al., 2006). In the excess infrared spectra of C–Hs stretching vibrations (Fig. 3c), the most obvious feature is the mainly positive bands from both the pyridinium ring C–Hs and alkyl C–Hs stretches, which represents that the IR activity of C–Hs stretches is enhanced (Kiefer et al., 2012), and also means that the newly arisen H-bonded complexes of pyridinium ring C–Hs/alkyl C–Hs and DMSO, i.e., C–H OQS H-bonded complexes. By comparing the excess bands of v(Cring–H) and v(Calkyl–H) stretches, it is clear to see that the positive excess bands of v (Cring–H) around 3052 cm 1 are extremely stronger than that of the v(Calkyl–H), indicating that the pyridinium ring C–Hs participate in stronger H-bonding interactions. Furthermore, the
Please cite this article as: He, H., et al., Hydrogen-bonding interactions between a pyridinium-based ionic liquid [C4Py][SCN] and dimethyl sulfoxide. Chem. Eng. Sci. (2014), http://dx.doi.org/10.1016/j.ces.2014.07.024i
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Fig. 3. ATR-FTIR (a) and excess infrared (c) spectra of [C4Py][SCN]–DMSO-d6 binary systems in the range of the C–H stretching vibrations; wavenumber shifts of the pyridinium C–H (b) and alkyl C–H (d) stretching vibrations at different mole fractions of DMSO-d6.
shoulder peaks on v(Cring–H) around 3052 cm 1 in the original IR spectra become sharp, located at about 3094 cm 1 denoted by arrows in Fig. 3c (taking xDMSO d6 ¼ 0:3996 and 0.5002 as examples), can be attributed to the C–Hs stretching vibrations from C–H OQS H-bonded complexes. According to the literatures (Fumino et al., 2008; Wang et al., 2010; Zhang et al., 2008; Zhang et al., 2010), the H-bond involving aromatic C–H on the pyridinium ring is classified as a “proper redshift H-bond”, while that related to alkyl C–H is classified as an “improper red-shift H-bond”. Thus, the blue-shifts of v(Cring–H) around 3126 cm 1 observed in this work imply that DMSO weakens the Cring–H anion H-bonding interactions between the cation and anion during the dilution process, which also represents the formation of H-bonding interactions between DMSO-d6 and [C4Py] þ cation. For alkyl C–Hs in [C4Py][SCN] IL, the generation of the blue-shifts indicates that the H-bonding interactions involving the Calkyl–H groups in the pyridinium cation are strengthened. It should be pointed out emphatically that the electrostatic interactions in the mixtures containing cations and anions still play important role and result in electrostatic-enhanced H-bonds. 4.3. ATR and excess infrared spectra analysis of v(C–D) and v(C N) The partial ATR-FTIR and excess infrared spectra of [C4Py] [SCN]–DMSO-d6 and [C4Py][SCN]–DMSO systems in the vas(C–D) and v(C N) regions over the entire mole fraction range are
shown in Fig. 4. For v(C–D), only the band of vas(C–D) at around 2249 cm 1 is analyzed as the vs(C–D) around 2124 cm 1 overlapped with that of v(C N). In the original spectra (Fig. 4a and d), vas(C–D) moves to lower wavenumber gradually, while the v (C N) moves to higher wavenumber inchmeal in the mixing process. The peak position of the former is red-shifted by 4.2 cm 1 in the presence of [C4Py][SCN] IL, while that of the latter is blueshifted by 1.9 cm 1 in the presence of DMSO. The concentration dependences of the wavenumber shift are summarized in Fig. 4c and f. As can be seen in Fig. 4b, the most obvious feature of the excess infrared spectra in vas(C–D) region is the negative bands at higher wavenumber located at about 2254 cm 1 and positive bands at lower wavenumber located at about 2241 cm 1. The facts show distinctly that the existence of complexes in the mixture for the nearly fixed positions of the negative and positive bands. The positive ones at the higher wavenumber can be attributed to the new H-bonding complexes formed by the C–D of DMSO-d6 and the anion of [C4Py][SCN] IL, whereas the negative ones at the lower wavenumber can be attributed to the decrease of self-associated H-bonds in DMSO-d6. For the infrared spectra in v (C N) region (Fig. 4e), there are a tiny negative band at around 2068 cm 1, a very strong positive band at around 2054 cm 1 and a negative band at around 2039 cm 1, which can be explained as the result of the electron-donating effect of C N in the formation of the C N DMSO H-bond. The result will be further confirmed by the following theoretical calculation. In brief, the
Please cite this article as: He, H., et al., Hydrogen-bonding interactions between a pyridinium-based ionic liquid [C4Py][SCN] and dimethyl sulfoxide. Chem. Eng. Sci. (2014), http://dx.doi.org/10.1016/j.ces.2014.07.024i
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Fig. 4. ATR-FTIR spectra (a and d), excess infrared spectra (b and e), wavenumber shifts (c and f) of the vas(C–D) and v(C N) at different mole fractions of DMSO-d6/DMSO of [C4Py][SCN]–DMSO-d6/DMSO binary systems.
H-bonding behaviors in the [C4Py][SCN]–DMSO system discussed above suggest the possible existence of selective interactions among DMSO and different functional groups of the cation and anion. 4.4.
1
H NMR analysis
1 H NMR measurements of [C4Py][SCN] IL, DMSO and a series of [C4Py][SCN]–DMSO mixtures were carried out, and the chemical shift changes of individual H atoms during the dilution process
were evaluated. Assignments of the 1H NMR signals to the H atoms on [C4Py] þ are displayed in Fig. 5a. The chemical shifts variations (Δδ) are expressed by the differences between the pure chemicals and their mixtures. The dependences of Δδ of different H atoms on the mole fraction of DMSO are displayed in Fig. 5b–d. The Δδ values of the H atoms on [C4Py] þ are all positive (40.34 ppm), which means clearly that the presence of DMSO resulted in downfield shift of the H atoms on [C4Py] þ . It is well known that the formation of H-bonds cause downfield chemical shift of the
Please cite this article as: He, H., et al., Hydrogen-bonding interactions between a pyridinium-based ionic liquid [C4Py][SCN] and dimethyl sulfoxide. Chem. Eng. Sci. (2014), http://dx.doi.org/10.1016/j.ces.2014.07.024i
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Fig. 5. 1H NMR assignment of pure [C4Py][SCN] IL (a) and chemical shift changes of the H atoms in an pyridinium ring (b), H atoms in alkyl groups of the pyridinium ring (c), and H atoms in the methyl group of DMSO (d) in the [C4Py][SCN]–DMSO binary systems.
related H atoms (Wulf et al., 2007). Thus the observation of this downfield change of H atoms on [C4Py] þ can support above results about H-bonding interactions formed between the cation of IL and solvent molecules. In addition, the negative value of Δδ for the H atoms on DMSO states that the H atoms show upfield shift in the presence of [C4Py][SCN] IL. This result indicates that in the presence of IL, the self-associated H-bonds of pure DMSO are weakened, which are in good agreement with the conclusion drawn from FT-IR. Moreover, in real the [C4Py][SCN]–DMSO binary system, the DMSO molecules can form H-bonding interactions with pyridinium ring C–Hs and alkyl C–Hs simultaneously as both of the Δδ values are changed significantly.
4.5. Theoretical investigation of the interaction between [C4Py][SCN] IL and DMSO In order to investigate the H-bonding interactions of [C4Py] [SCN] IL and DMSO and further determine the preferred interaction sites contributing to the formation of H-bonds, complexes including [C4Py] þ [SCN] , [C4Py] þ –DMSO, [SCN] –DMSO, [C4Py][SCN]–DMSO, [C4Py][SCN]–2DMSO, 2[C4Py] þ 2[SCN] and 2[C4Py][SCN] DMSO were selected to perform DFT calculations. The sum of the van der Waals atomic radii of H and O (2.72 Å) and that of H and N (2.75 Å), H and S (3.00 Å) (Bondi, 1964) are used as the geometric criteria values for judging the formation of H-bonds between H and O/N/S atoms. H-bonds are
denoted by dashed lines, and the corresponding H O, H N and H S bond distances are labeled in Figs. 6–9.
4.5.1. Interactions between [C4Py] þ /[SCN] and DMSO Interactions between the cation and anion were investigated before the examination of the cation/anion-cosolvent “two-body” complex (cation/anion-cosolvent). The optimized geometries of [C4Py] þ –[SCN] complex at B3LYP/6–31 þ þ g (d,p) level are shown in Fig. 6, and the corresponding interaction energies both at B3LYP/6–31þ þG(d,p) and B2PLYPD/6–31 þ þG(d,p) levels are listed in Table 1. The absolute value of the interaction energy of the first complex is larger than that of the other six complexes, meaning which is the most stable one. It is noted that the interactions between the cation and anion are characterized by multiple H-bonds formed mainly between the electronegative N and/or S atom of the [SCN] anion and the pyridinium ring C–H and /or alkyl C–H of the [C4Py] þ cation. However, there are many possible sites of [C4Py] þ cation and [SCN] anion to interact with DMSO, therefore, a number of possible mutual orientations of the complexes consisting of one DMSO molecule and one [C4Py] þ cation or [SCN] anion were examined. The optimized geometries are shown in Fig. 7, and respective interaction energies are shown in Table 2. For [C4Py] þ – DMSO complexes (Fig. 7A, B), there are two most stable complexes with minor interaction energy difference ( 0.8 kJ/mol) and very similar structures. C–H O H-bonding interactions can exist
Please cite this article as: He, H., et al., Hydrogen-bonding interactions between a pyridinium-based ionic liquid [C4Py][SCN] and dimethyl sulfoxide. Chem. Eng. Sci. (2014), http://dx.doi.org/10.1016/j.ces.2014.07.024i
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Fig. 6. Optimized geometries of [C4Py][SCN] complexes (A–G). Dashed lines denote H-bonds and corresponding distances are labeled. (A0 ) is the capture of the front view of complex A in respect to pyridinium ring, which is presented in tube form to show clearly.
Fig. 7. Optimized geometries of [C4Py] þ /[SCN] –DMSO complexes (A–C). Dashed lines denote H-bonds and the corresponding distances are labeled.
between aromatic C–H even alkyl C–H of [C4Py] þ cation and the O atom in DMSO. For the [SCN] –DMSO complex (Fig. 7C), it can be seen that H-bond only exist between the C–H group of DMSO and the N atom in [SCN] , which indicates that N atom rather than S atom in [SCN] is the preferred site for the formation of H-bond with DMSO molecule. It can be found that DMSO molecule can form stable H-bonds with [C4Py] þ and [SCN] through different atoms, i. e. H and O atom, respectively, by the examination of the interaction pairs of [C4Py] þ –DMSO and [SCN] –DMSO (Fig. 7A–C). The result means that there is no obvious competitive interaction between DMSO molecule and ions even though the interaction energies difference between [C4Py] þ –DMSO and [SCN] –DMSO complexes are
considerable. Namely, DMSO molecule can interact with the C–Hs of [C4Py] þ and N atom of [SCN] simultaneously when the DMSO concentration is lower. By comparison of the interaction energies in Tables 1 and 2, it can be determined that the interaction energy of [C4Py] þ –[SCN] complexes is much higher in absolute value than that of the [C4Py] þ –DMSO and [SCN] –DMSO complexes. The larger interaction energy of [C4Py] þ –[SCN] is due to the strong electrostatic interaction of the cation and anion. The interaction energy of [C4Py] þ –DMSO is larger than that of [SCN] –DMSO indicates that the interaction between DMSO and the [C4Py] þ cation is stronger than that of DMSO and the [SCN] anion. In fact, the interaction energy of [C4Py] þ –[SCN] is too strong because the DFT
Please cite this article as: He, H., et al., Hydrogen-bonding interactions between a pyridinium-based ionic liquid [C4Py][SCN] and dimethyl sulfoxide. Chem. Eng. Sci. (2014), http://dx.doi.org/10.1016/j.ces.2014.07.024i
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Fig. 8. Optimized geometries of [C4Py][SCN]–DMSO complexes (A–I). Dashed lines denote H-bonds and the corresponding distances are labeled.
Fig. 9. Optimized geometries of 2[C4Py] þ –2[SCN] , 2[C4Py] þ –2[SCN] –DMSO and [C4Py] þ –[SCN] –2DMSO complexes (A–C). Dashed lines denote H-bonds and the corresponding distances are labeled.
Please cite this article as: He, H., et al., Hydrogen-bonding interactions between a pyridinium-based ionic liquid [C4Py][SCN] and dimethyl sulfoxide. Chem. Eng. Sci. (2014), http://dx.doi.org/10.1016/j.ces.2014.07.024i
H. He et al. / Chemical Engineering Science ∎ (∎∎∎∎) ∎∎∎–∎∎∎ Table 1 Interaction energies (ΔE) of [C4Py] þ –[SCN] complexes as shown in Fig. 6 at B3LYP/6–31 þ þG(d,p) and B2PLYPD/6–31 þ þG(d,p) levels.a ΔE (kJ/mol)
Complex
Fig. Fig. Fig. Fig. Fig. Fig. Fig.
6(A) 6(B) 6(C) 6(D) 6(E) 6(F) 6(G)
B3LYP/6–31þ þ G(d,p)
B2PLYPD/6–31þ þG(d,p)
336.63 333.39 332.28 330.08 329.84 328.76 328.11
337.04 335.71 335.49 336.18 333.89 327.87 330.68
a Interaction energies are corrected with the basis set superposition error (BSSE).
Table 2 Interaction energies (ΔE), NBO analysis, delocalization energies (E(2)) of selected anti-bonding orbital of [C4Py] þ /[SCN] –DMSO complexes as shown in Fig. 7: [C4Py] þ –DMSO (A–B); [SCN] –DMSO (C). Complex ΔEa (kJ/mol)
NBOb
E(2) (kJ/mol)
B3LYP/6– 31 þ þG(d,p)
B2PLYPD/6– 31þ þG(d,p)
Proton acceptordonor
Fig. 7(A)
72.57
81.93
Fig. 7(B)
71.77
81.05
Fig. 7(C)
58.94
62.88
LP(O)-σn(C2–H)ring 50.94 LP(O)-σn(C7–H)alkyl 8.12 n LP(O)-σ (C2–H)ring 32.02 LP(O)-σn(C7–H)alkyl 16.07 LP(N)-σn(C1–H)DMSO 13.52 LP(N)-σn(C2–H)DMSO 14.32
a Interaction energies are corrected with the basis set superposition error (BSSE). b NBO analysis was performed at the B3LYP/6–31þ þ G(d,p) level.
calculations were applied in a gas phase. The interaction energy decreases drastically to 13.99 kJ/mol when [C4Py] þ –[SCN] was optimized in DMSO solvent. Correspondingly, the geometric structures of [C4Py] þ –[SCN] also change obviously with the solvent (taking Fig. 6A as an example, see Fig. S5 in the Supplementary Material).
4.5.2. NBO analysis of the interactions between [C4Py] þ /[SCN] and DMSO To further understand the interactions between DMSO and [C4Py] þ /[SCN] , NBO analysis was carried out. For the [C4Py] þ – DMSO complexes (see Fig. 7A, B), the NBO analysis suggests strong orbital interactions between the anti-bonding orbital of the proton donor in [C4Py] þ , σn(C–H), and the lone pairs (LP) of the proton acceptor in DMSO, LP(O), which is supported by the second-order delocalization energies (E(2)) in the related localized anti-bonding orbital (see Table 2). Here, the E(2) values are 50.94 and 8.12 kJ/mol for the LP(O)-σn(C2–H) and LP(O)-σn(C7–H) interactions for complex A, 32.02 and 16.07 kJ/mol for the LP(O)-σn(C2–H) and LP(O)-σn(C7–H) interactions for complex B. It is obvious that the H-bonds formed by the O atom in DMSO and the aromatic H atoms on the pyridinium ring are generally stronger than those with the H atoms on the alkyl chain, indicating that the Cring–H O H-bonds play an important role in stabilizing the [C4Py] þ –DMSO complexes. For the [SCN] –DMSO complex, the calculated result shows that the interaction between the lone pair of the N atom in the [SCN] , LP(N), and the anti-bonding orbital of the C–H bond in DMSO, σn(C–H), dominates over the interaction of DMSO and [SCN] . Such a LP(N)-σn(C–H) interaction results in electron transfer from the proton acceptor LP(N) to the donor σn(C–H) and
9
C–H N H-bond formation, and hence stabilizes the [SCN] – DMSO complex. The E(2) values are 13.52 and 14.32 kJ/mol for the LP(N)-σn(C–H) interactions for complex C. It can be found from the above analysis that the interaction mechanisms of the DMSO molecule with the cation and anion are very different in nature: the former relates to the decisive orbital overlap of the type of LP (O)-σn(C–H)cation, while the latter mainly involves LP(N)σn(C–H)DMSO interactions. By comparing the second-order delocalization energies of DMSO/ions complexes, the larger E(2) value of cation–DMSO complexes indicates that the DMSO molecule preferentially interact with [C4Py] þ , which accords well with the above structural analysis.
4.5.3. Interactions between [C4Py][SCN] ion pair and DMSO [C4Py][SCN]–DMSO complexes were evaluated in order to understand how DMSO interacts with both the cation and anion at the same time. The results including optimized geometries and H-bonding interactions are depicted in Fig. 8, and the corresponding interaction energies are shown in Table 3. As can be seen in the figures, these nine structures can be divided into two categories: structures A and C, structures B, D–I. In the former, the Cring– H S H-bonds can be formed between the [C4Py] þ cation and [SCN] anion with S H bond distances of 2.842 and 2.839 Å, respectively. In addition, Cring–H O and Calkyl–H O H-bonds can be formed between DMSO and the cation and anion synchronously, as well as CDMSO–H N H-bonds also exist between DMSO and the anion, which shows that the DMSO molecule acts as both an H-bond donor and acceptor. In the latter, N atom of the anion can form H-bonds with Cring–H and/or Calkyl–H on the cation, and can form CDMSO–H N H-bonds with DMSO at the same time in some complexes. As to the S atom of the anion, it can form H-bonds with the O atom of DMSO or Calkyl–H on the cation in some complexes. All structures in Fig. 8 demonstrate that DMSO molecule cannot insert into the [C4Py] þ cation and [SCN] anion, which illuminate that it is not easy for a neutral molecule like DMSO to disrupt the strong Coulombic interaction between the cation and anion by simple insertion when the concentration of DMSO is low. In addition, the above results also indicate that in [C4Py][SCN]–DMSO complexes, DMSO molecule can form H-bonds with aromatic C–H and alkyl C–H simultaneously, but the complex of DMSO interacting with aromatic C–H is more dominant than that of with alkyl C–H. This is in general agreement with above 1 H NMR result. The fact that all the interaction energies of the [C4Py][SCN]–DMSO complexes are larger than those of [C4Py] [SCN] implies that the mixing process of the [C4Py][SCN] and DMSO is enthalpically favorable and consistent with the prompt solubility of [C4Py][SCN] in DMSO. Table 3 Interaction energies (ΔE) of [C4Py][SCN]–DMSO complexes (A–I) as shown in Fig. 8 at B3LYP/6–31þ þ G(d,p) and B2PLYPD/6–31þ þ G(d,p) levels.a Complex
Fig. Fig. Fig. Fig. Fig. Fig. Fig. Fig. Fig.
8(A) 8(B) 8(C) 8(D) 8(E) 8(F) 8(G) 8(H) 8(I)
ΔE (kJ/mol) B3LYP/6–31þ þ G(d,p)
B2PLYPD/6–31 þ þ G(d,p)
392.96 387.15 387.00 382.36 382.07 378.28 374.77 369.19 360.93
412.33 405.51 403.55 399.61 396.82 388.25 389.69 384.32 370.62
a Interaction energies are corrected with the basis set superposition error (BSSE).
Please cite this article as: He, H., et al., Hydrogen-bonding interactions between a pyridinium-based ionic liquid [C4Py][SCN] and dimethyl sulfoxide. Chem. Eng. Sci. (2014), http://dx.doi.org/10.1016/j.ces.2014.07.024i
H. He et al. / Chemical Engineering Science ∎ (∎∎∎∎) ∎∎∎–∎∎∎
10
Table 4 Interaction energies (ΔE) of 2[C4Py] þ –2[SCN] , 2[C4Py] þ –2[SCN] –DMSO and [C4Py] þ –[SCN] –2DMSO complexes (A-C) as shown in Fig. 9 at B3LYP/6–31þ þ G (d,p) and B2PLYPD/6–31 þ þG(d,p) levels.a Complex
Fig. 9 (A) Fig. 9 (B) Fig. 9 (C)
ΔE, kJ/mol B3LYP/6–31þ þG(d,p)
B2PLYPD/6–31þ þG(d,p)
767.91 804.64 431.68
788.42 833.42 464.26
a
Interaction energies are corrected with the basis set superposition error (BSSE).
Ionic cluster consisting of a certain number of ions can partly explain the complicated network structure of IL system. The optimized geometries and H-bonding interactions of 2[C4Py] þ –2 [SCN] , 2[C4Py] þ –2[SCN] –DMSO and [C4Py] þ –[SCN] –2DMSO are presented in Fig. 9, and the corresponding interaction energies are shown in Table 4. The [SCN] anions are situated between the two [C4Py] þ cations and interact with them at the same time (Fig. 9A). For 2[C4Py] þ –2[SCN] –DMSO complex (Fig. 9B), the DMSO molecule stays on the outer skirt of the cluster with its O and H atom interacting with the Cring–H of the cation and N atom in the anion simultaneously. The [C4Py] þ –[SCN] –2DMSO complex shown in Fig. 9C was selected to record the representative interaction modes at higher DMSO concentration range. There exist strong H-bonding interactions between the cation and anion, while the DMSO molecules still are unable to insert into the cation and anion, meaning that the strong Coulombic interaction between [C4Py] þ cation and [SCN] anion is difficult to disrupt. At the same time, it can be found that most of the H-bond distances between the cation and anion in structures A–C are lengthened with the addition of the DMSO. This phenomenon implies that this addition weakens the H-bond interactions between the cation and anion, and may lead to a decrease of the viscosity of ILs by comparing with the optimized geometries in Fig. 9.
5. Conclusions In this work, ATR-FTIR, 1H NMR and DFT calculations were used to study the H-bonding interactions in [C4Py][SCN]–DMSO system. It was found that the H-bonding interactions exist widely in this system, which plays a key role towards the stability and miscibility of the systems. Detailed investigation about the positions of positive and negative peaks in excess infrared spectroscopy showed that H-bonds involving the alkyl C–Hs and C N of IL are strengthened with the addition of DMSO, while the H-bonds involving pyridinium ring C–Hs as well as the H-bonds formed between the [C4Py] þ cation and [SCN] anion are weakened. The addition of [C4Py][SCN] led to the H-bonds involving the C–Hs in DMSO molecule weakened comparing with the associated H-bonds in pure DMSO system. The results of DFT calculations indicated that the solvent molecules cannot disrupt the strong Coulumbic interaction between the [C4Py] þ and [SCN] . NBO analysis further confirmed that the interaction mechanism of DMSO molecule with the anion and cation are different in nature: the former relates to the decisive orbital overlap of the type of LP(O)-σn(C–H)ring/alkyl, while the latter mainly involves LP(N)σn(C–H)DMSO interactions, which mean that the DMSO molecule can acts as both an H-bond donor and acceptor in [C4Py][SCN]– DMSO system. Through comparison of the mutual interactions among the species [C4Py] þ , [SCN] , and DMSO with the help of DFT calculations, the following sequential order of interaction
strength was established: [C4Py] þ –[SCN] –DMSO 4[C4Py] þ – [SCN] 4 [C4Py] þ –DMSO 4[SCN] –DMSO. These studies on the properties of the IL–DMSO system may deepen our understanding of the system at the molecular level and shed light on exploring the applications of ILs.
Acknowledgments This work was supported by the National Natural Science Foundation of China (21127011, 21336002), and National High Technology Research and Development Program of China (2012AA063001). The authors sincerely thank Professor Xuehui Li of South China University of Technology for his meaningful help in discussions and writing.
Appendix A. Supplementary Material The Supplementary Material contains the geometries and absolute energies of all calculated structures, which can be found in the online version at http://www.sciencedirect.com/. Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.ces.2014.07.024.
References Andanson, J.M., Bordes, E., Devémy, J., Leroux, F., Pádua, A.A.H., Gomes, M.F.C., 2014. Understanding the role of co-solvents in the dissolution of cellulose in ionic liquids. Green Chem. 16, 2528–2538. Anderson, J.L., Dixon, J.K., Maginn, E.J., Brennecke, J.F., 2006. Measurement of SO2 solubility in ionic liquids. J. Phys. Chem. B 110, 15059–15062. Barone, V., Cossi, M., 1998. Quantum calculation of molecular energies and energy gradients in solution by a conductor solvent model. J. Phys. Chem. A 102, 1995–2001. Bondi, A., 1964. Van der Waals volumes and radii. J. Phys. Chem. 68, 441–451. Brayton, C.F., 1986. Dimethyl sulfoxide (DMSO): a review. Cornel. Vet. 76, 61–90. Cammarata, L., Kazarian, S.G., Salter, P.A., Welton, T., 2001. Molecular states of water in room temperature ionic liquids. Phys. Chem. Chem. Phys. 3, 5192–5200. Cappelli, C., Mennucci, B., da Silva, C.O., Tomasi, J., 2000. Refinements on solvation continuum models: hydrogen-bond effects on the OH stretch in liquid water and methanol. J. Chem. Phys. 112, 5382–5392. Chang, H.C., Jiang, J.C., Chang, C.Y., Su, J.C., Hung, C.H., Liou, Y.C., Lin, S.H., 2008a. Structural organization in aqueous solutions of 1-butyl-3-methylimidazolium halides: a high-pressure infrared spectroscopic study on ionic liquids. J. Phys. Chem. B 112, 4351–4356. Chang, H.C., Jiang, J.C., Liou, Y.C., Hung, C.H., Lai, T.Y., Lin, S.H., 2008b. Effects of water and methanol on the molecular organization of 1-butyl-3methylimidazolium tetrafluoroborate as functions of pressure and concentration. J. Chem. Phys. 129, 044506. Chang, H.C., Jiang, J.C., Liou, Y.C., Hung, C.H., Lai, T.Y., Lin, S.H., 2008c. Local structures of water in 1-butyl-3-methylimidazolium tetrafluoroborate probed by high-pressure infrared spectroscopy. Anal. Sci. 24, 1305–1309. Chang, H.C., Jiang, J.C., Tsai, W.C., Chen, G.C., Lin, S.H., 2006. Hydrogen bond stabilization in 1,3-dimethylimidazolium methyl sulfate and 1-butyl-3methylimidazolium hexafluorophosphate probed by high pressure: The role of charge-enhanced C–H O interactions in the room-temperature ionic liquid. J. Phys. Chem. B 110, 3302–3307. Clark, J.H., Tavener, S.J., 2006. Alternative solvents: shades of green. Org. Process Res. Dev. 11, 149–155. Docherty, K.M., Dixon, J.K., Kulpa Jr, C.F., 2006. Biodegradability of imidazolium and pyridinium ionic liquids by an activated sludge microbial community. Biodegradation 18, 481–493. Dong, K., Song, Y.T., Liu, X.M., Cheng, W.G., Yao, X.Q., Zhang, S.J., 2012. Understanding structures and hydrogen bonds of ionic liquids at the electronic level. J. Phys. Chem. B 116, 1007–1017. Dong, K., Zhang, S.J., 2012. Hydrogen bonds: a structural insight into ionic liquids. Chem.–A Eur. J. 18, 2748–2761. Dong, K., Zhang, S.J., Wang, D.X., Yao, X.Q., 2006. Hydrogen bonds in imidazolium ionic liquids. J. Phys. Chem. A 110, 9775–9782. Forsyth, S.A., Pringle, J.M., MacFarlane, D.R., 2004. Ionic liquids – an overview. Aust. J. Chem. 57, 113–119. Frisch, M.J., Trucks, G.W., Schlegel, H.B., Scuseria, G.E., Robb, M.A., Cheeseman, J.R., et al., 2010. Gaussian 09, Revision B.01. Gaussian, Inc., Wallingford CT. Fumino, K., Wulf, A., Ludwig, R., 2008. Strong, localized, and directional hydrogen bonds fluidize ionic liquids. Angew. Chem. Int. Ed., 8731–8734.
Please cite this article as: He, H., et al., Hydrogen-bonding interactions between a pyridinium-based ionic liquid [C4Py][SCN] and dimethyl sulfoxide. Chem. Eng. Sci. (2014), http://dx.doi.org/10.1016/j.ces.2014.07.024i
H. He et al. / Chemical Engineering Science ∎ (∎∎∎∎) ∎∎∎–∎∎∎ Goerigk, L., Moellmann, J., Grimme, S., 2009. Computation of accurate excitation energies for large organic molecules with double-hybrid density functionals. Phys. Chem. Chem. Phys. 11, 4611–4620. Hansen, W.N., 1965. Expanded formulas for attenuated total reflection (A.T.R.) and the derivation of absorption rules for single and multiple A.T.R. spectrometer cells. Spectrochim. Acta 21, 815–833. He, H.Y., Chen, H., Zheng, Y.Z., Zhang, X.C., Yao, X.Q., Yu, Z.W., Zhang, S.J., 2013a. The hydrogen-bonding interactions between 1-ethyl-3-methylimidazolium lactate ionic liquid and methanol. Aust. J. Chem. 66, 50–59. He, H.Y., Zhang, S.J., Liu, X.M., Wang, J.Q., Yao, X.Q., Zhang, X.P., 2013b. Structures and hydrogen bonds of biodegradable naphthenate ionic liquids. Fluid Phase Equilib. 360, 169–179. Kiefer, J., Martinez Molina, M., Noack, K., 2012. The peculiar nature of molecular interactions between an imidazolium ionic liquid and acetone. ChemPhysChem 13, 1213–1220. Li, Q.Z., Wu, G.S., Yu, Z.W., 2006. The role of methyl groups in the formation of hydrogen bond in DMSO–methanol mixtures. J. Am. Chem. Soc. 128, 1438–1439. Long, J., Guo, B., Li, X., Jiang, Y., Wang, F., Tsang, S.C., Wang, L., Yu, K.M.K., 2011. One step catalytic conversion of cellulose to sustainable chemicals utilizing cooperative ionic liquid pairs. Green Chem. 13, 2334–2338. Navas, A., Ortega, J., Vreekamp, R., Marrero, E., Palomar, J., 2009. Experimental thermodynamic properties of 1-butyl-2-methylpyridinium tetrafluoroborate b2mpyBF4 with water and with alkan-1-ol and their interpretation with the COSMO-RS methodology. Ind. Eng. Chem. Res. 48, 2678–2690. Peppel, T., Roth, C., Fumino, K., Paschek, D., Köckerling, M., Ludwig, R., 2011. The influence of hydrogen-bond defects on the properties of ionic liquids. Angew. Chem. Int. Ed. 50, 6661–6665. Reichardt, C., 2005. Polarity of ionic liquids determined empirically by means of solvatochromic pyridinium N-phenolate betaine dyes. Green Chem. 7, 339–351. Rinaldi, R., 2011. Instantaneous dissolution of cellulose in organic electrolyte solutions. Chem. Commun. 47, 511–513. Sancho-Garcia, J.C., Adamo, C., 2013. Double-hybrid density functionals: merging wavefunction and density approaches to get the best of both worlds. Phys. Chem. Chem. Phys. 15, 14581–14594. Schwabe, T., Grimme, S., 2007. Double-hybrid density functionals with long-range dispersion corrections: higher accuracy and extended applicability. Phys. Chem. Chem. Phys. 9, 3397–3406. Umebayashi, Y., Jiang, J.C., Lin, K.H., Shan, Y.L., Fujii, K., Seki, S., Ishiguro, S.I., Lin, S.H., Chang, H.C., 2009. Solvation and microscopic properties of ionic liquid/ acetonitrile mixtures probed by high-pressure infrared spectroscopy. J. Chem. Phys. 131, 234502–234507. Vreekamp, R., Castellano, D., Palomar, J., Ortega, J., Espiau, F., Fernandez, L., Penco, E., 2011. Thermodynamic behavior of the binaries 1-butylpyridinium tetrafluoroborate
11
with water and alkanols: their interpretation using 1H NMR spectroscopy and quantum-chemistry calculations. J. Phys. Chem. B 115, 8763–8774. Wang, J.Q., Sun, J., Cheng, W.G., Dong, K., Zhang, X.P., Zhang, S.J., 2012. Experimental and theoretical studies on hydrogen bond-promoted fixation of carbon dioxide and epoxides in cyclic carbonates. Phys. Chem. Chem. Phys. 14, 11021–11026. Wang, N., Li, Q., Yu, Z., 2009. Hydrogen bonding interactions in three 2-mercaptoethanol systems: an excess infrared spectroscopic study. Appl. Spectrosc. 63, 1356–1362. Wang, N., Zhang, Q., Wu, F., Li, Q., Yu, Z., 2010. Hydrogen bonding interactions between a representative pyridinium-based ionic liquid [BuPy][BF4] and water/ dimethyl sulfoxide. J. Phys. Chem. B 114, 8689–8700. Weingartner, H., 2008. Understanding ionic liquids at the molecular level: facts, problems, and controversies. Angew. Chem. Int. Ed. 47, 654–670. Wilkes, J.S., 2002. A short history of ionic liquids-from molten salts to neoteric solvents. Green Chem. 4, 73–80. Wulf, A., Fumino, K., Michalik, D., Ludwig, R., 2007. IR and NMR properties of ionic liquids: do they tell us the same thing? ChemPhysChem 8, 2265–2269. Young, D.C., 2001. Computational chemistry: A Practical Guide for Applying Techniques to Real-World Problems. John Wiley & Sons, Inc, New York. Yunus, N.M., Mutalib, M.I.A., Man, Z., Bustam, M.A., Murugesan, T., 2012. Solubility of CO2 in pyridinium based ionic liquids. Chem. Eng. J. 189–190, 94–100. Zeng, S., Gao, H., Zhang, X., Dong, H., Zhang, X., Zhang, S., 2014. Efficient and reversible capture of SO2 by pyridinium-based ionic liquids. Chem. Eng. J. 251, 248–256. Zhang, L.Q., Wang, Y., Xu, Z., Li, H.R., 2009. Comparison of the blue-shifted C–D stretching vibrations for DMSO-d6 in imidazolium-based room temperature ionic liquids and in water. J. Phys. Chem. B 113, 5978–5984. Zhang, L.Q., Xu, Z., Wang, Y., Li, H.R., 2008. Prediction of the solvation and structural properties of ionic liquids in water by two-dimensional correlation spectroscopy. J. Phys. Chem. B 112, 6411–6419. Zhang, Q.G., Wang, N.N., Wang, S.L., Yu, Z.W., 2011. Hydrogen bonding behaviors of binary systems containing the ionic liquid 1-butyl-3-methylimidazolium trifluoroacetate and water/methanol. J. Phys. Chem. B 115, 11127–11136. Zhang, Q.G., Wang, N.N., Yu, Z.W., 2010. The hydrogen bonding interactions between the ionic liquid 1-ethyl-3-methylimidazolium ethyl sulfate and water. J. Phys. Chem. B 114, 4747–4754. Zheng, Y.Z., Wang, N.N., Luo, J.J., Zhou, Y., Yu, Z.W., 2013. Hydrogen-bonding interactions between [BMIM][BF4] and acetonitrile. Phys. Chem. Chem. Phys. 15, 18055–18064. Zhu, X., Cui, P., Zhang, D., Liu, C., 2011. Theoretical Study for pyridinium-based ionic liquid 1-ethylpyridinium trifluoroacetate: synthesis mechanism, electronic structure, and catalytic reactivity. J. Phys. Chem. A 115, 8255–8263.
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Hydrogen-bonding interactions between a pyridinium-based ionic liquid [C4Py][SCN] and dimethyl sulfoxide Hongyan He a, Hui Chen a, Yanzhen Zheng b, Suojiang Zhang a,n, Zhiwu Yu b a Beijing Key Laboratory of Ionic Liquids Clean Process, Key Laboratory of Green Process and Engineering, State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China b Department of Chemistry, Tsinghua University, Beijing 100084, China
H I G H L I G H T S
We present novel insight into the molecular interactions between co-solvent and IL. Universal H-bonds play important role for the miscibility of DMSO with IL. H-bonding interactions are the main cause influencing the wavenumber shift changes. We analyze the essential difference of the interactions between DMSO and ion of IL.
art ic l e i nf o
a b s t r a c t
Article history: Received 26 April 2014 Received in revised form 7 July 2014 Accepted 12 July 2014
In this work, the interactions between a representative pyridinium-based ionic liquid (IL) with cyanofunctionalized anion ([C4Py][SCN]) and dimethyl sulfoxide (DMSO) were investigated in detail using attenuated total reflection infrared spectroscopy (ATR-FTIR), hydrogen nuclear magnetic resonance (1H NMR), and density functional theory (DFT) calculations. It was found that H-bonds are universally involved and play important role for the miscibility of DMSO with [C4Py][SCN] IL and maintain the stability of this system. ATR-FTIR and excess spectroscopy analysis indicated that the H-bonds involving the alkyl C–Hs and C N are strengthened with the addition of DMSO, while the H-bonds involving pyridinium ring C–Hs as well as the H-bonds formed between the [C4Py] þ and [SCN] are weakened. The addition of [C4Py][SCN] IL led to the H-bonds involving the C–Hs in DMSO weakened comparing with the associated H-bonds in the pure DMSO system. The results of DFT calculations indicated that the DMSO molecules cannot disrupt the strong Coulombic interaction between the [C4Py] þ and [SCN] . Natural bond orbital (NBO) analysis further confirmed that the interaction mechanisms of DMSO molecule with the anion and cation are different in nature. These studies will shed light on exploring the applications of ILs as reaction or separation media. & 2014 Elsevier Ltd. All rights reserved.
Keywords: Quantum chemical calculation ATR-FTIR Ionic liquids Hydrogen bond Excess infrared spectroscopy
1. Introduction As novel and green class of chemical components, room temperature ionic liquids (ILs) are becoming popular for a variety of applications due to their unique physiochemical properties, such as ultra-low vapor pressure, high thermal and chemical stability, high solvent capacity, etc. Particularly, their properties could be tuned by varying either anion, cation or other substituent (Forsyth et al., 2004; Wilkes, 2002). However, in many industrial applications, the use of pure ILs is practically hindered due to their
n
Corresponding author. E-mail address: [email protected] (S. Zhang).
high polarity (Reichardt, 2005), high viscosity (Weingartner, 2008) and high cost (Clark and Tavener, 2006). Thus, the design of novel ILs with specific structures will bring new hope for dealing with these challenges, but could hardly overcome all these problems at the present stage. Recently, the using mixtures of ILs and other cosolvents such as water, alcohol, and DMSO is proved to be an effective approach to solve these problems (He et al., 2013a; Long et al., 2011). These co-solvents have significant effect on the physiochemical properties of the IL systems such as density, viscosity, polarity, surface tension and reactivity (Cappelli et al., 2000; Chang et al., 2008b; Navas et al., 2009; Umebayashi et al., 2009; Vreekamp et al., 2011; Zhang et al., 2011). Therefore, it is of great importance to investigate the interactions between ILs and co-solvents at the molecular level intensively and to obtain
http://dx.doi.org/10.1016/j.ces.2014.07.024 0009-2509/& 2014 Elsevier Ltd. All rights reserved.
Please cite this article as: He, H., et al., Hydrogen-bonding interactions between a pyridinium-based ionic liquid [C4Py][SCN] and dimethyl sulfoxide. Chem. Eng. Sci. (2014), http://dx.doi.org/10.1016/j.ces.2014.07.024i
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desirable physicochemical properties of such systems with specific functions. It has been reported that non-covalent interactions influence the properties of ILs and co-solvent mixtures greatly, especially H-bonding interactions, which are considered as the most important interaction in ILs system except for the electrostatic interaction (Dong and Zhang, 2012; Fumino et al., 2008; Peppel et al., 2011; Wang et al., 2010). Numerous efforts have been devoted to the experimental and theoretical investigations involving H-bonding interactions of pure ILs and ILs-cosolvent system. These researches demonstrated that H-bonds affect both the structures and properties in such systems. For example, the single, local and directional interactions such as H-bonds can decrease the melting point of imidazolium-based ILs by 100 K, which greatly extends the working temperature range of the ILs (Fumino et al., 2008). Water molecules absorbed from air (0.2–1.0 mol/dm3) interact mainly with various anions such as [PF6] , [BF4] , and [CF3CO2] in the form of symmetric anion–HOH–anion H-bonded complexes in imidazolium-based ILs (Cammarata et al., 2001). In a representative mixture system including imidazolium IL and acetone, the peculiarities of volumetric property data have been revealed by H-bonding interactions (Kiefer et al., 2012). In our previous reports, the H-bond networks in a number of ILs and the mechanism accounting for hydrogen bond-promoted fixation of CO2 have been investigated (Dong et al., 2012; Dong et al., 2006; He et al., 2013b; Wang et al., 2012). Infrared spectroscopy showed that the H-bonds and structure organizations in ILs with the presence of co-solvents revealed continuously decreasing of free O–H band intensity with the increasing of the pressure of 1-butyl3-methylimidazolium tetrafluoroborate/water mixtures (Chang et al., 2008a; Chang et al., 2008b, c; Chang et al., 2006). The above-mentioned studies have greatly enhanced our perspective on the interactions between ILs and co-solvents. However, most of the studies are limited to the imidazolium-based ILs. In fact, alkylated pyridinium-based ILs is the second major category of cations in the family of ILs in addition to imidazolium-based cations (Navas et al., 2009; Yunus et al., 2012; Zhu et al., 2011). Compared with most reported imidazolium-based ILs, pyridinium-based ILs have many advantages, for example, high thermal stability, excellent SO2 absorption performance (Anderson et al., 2006; Zeng et al., 2014), lower cost and higher biodegradability (Docherty et al., 2006). On the other hand, DMSO is an extraordinary dipolar aprotic solvent, completely miscible with wide range of organic and inorganic chemicals (Brayton, 1986), and ILs-DMSO binary systems are frequently used in the experimental fields (Rinaldi, 2011), such as cellulose dissolving (Andanson et al., 2014), which are of particular interest to us to study the ILs–DMSO interactions, especially ILs contain pyridinium-based cations. In this work, N-butylpyridiniumthiocyanate IL containing pyridinium-based cation, [C4Py][SCN], is chosen as the representative IL for the studying of the H-bonding behaviors with DMSO as the co-solvent. To the best of our knowledge, there have been few publications concerning about the H-bonding interactions of N-butylpyridiniumthiocyanate ILs. Attenuated total reflection infrared spectroscopy (ATR-FTIR), hydrogen nuclear magnetic resonance (1H NMR), and density functional theory (DFT) calculations were employed in the present study. In order to get rid of the overlap in the 5 4
6 1
8
10 9
7
O S C N
N
3
S H 3C
CH 3
2
[C4Py][SCN]
DM S O
Fig. 1. Structure and atom numbering of [C4Py][SCN] IL and DMSO.
C–H stretching vibration region of the [C4Py] þ cation and DMSO, deuterated DMSO (DMSO-d6) and DMSO were used in the FTIR experiment. In particular, excess infrared absorption spectroscopy (Li et al., 2006; Wang et al., 2009; Wang et al., 2010; Zhang et al., 2010) was used for detailed investigation of the interaction, which has been demonstrated to enhance the spectral resolution and can help reveal the details of the molecular interactions in liquid solution.
2. Experimental details 2.1. Chemicals [C4Py][SCN] IL was synthesized following the reported methods (Yunus et al., 2012). The product, a transparent yellow viscous liquid, was dried in vacuum for 48 h at 323 K. Water content was lower than 550 ppm measured by a Karl Fischer coulometric titration. The 1H NMR chemical shifts of [C4Py][SCN] were determined as follows: δH (600 MHz, DMSO, TMS), 9.10 (2H, d, C2,6–H), 8.62 (1H, t, C4–H), 8.18 (2H, t, C3,5–H), 4.62 (2H, t, C7–H), 1.91 (2H, m, C8–H), 1.31 (2H, m, C9–H), 0.93 (3H, t, C10–H). The chemical structure and atom numbering for [C4Py][SCN] IL are presented in Fig. 1. DMSO (4 99%) and DMSO-d6 (D, 99.8%) were purchased from Cambridge Isotope Laboratories, without further purification before use.
2.2. Sample preparation A series of [C4Py][SCN]–DMSO and [C4Py][SCN]–DMSO-d6 binary mixtures were prepared by weighing with a METTER TOLEDO ML 204/02 balance ( 71 10 4 g), thus leading to 71 10 4 accuracy for mole fraction. The mole fractions of DMSO in [C4Py][SCN]–DMSO mixtures are of 0.1001, 0.1992, 0.3001, 0.3997, 0.4999, 0.6000, 0.7001, 0.8000 and 0.9000, and DMSO-d6 in [C4Py][SCN]–DMSO-d6 mixtures are of 0.1076, 0.1999, 0.3181, 0.3996, 0.5002, 0.5999, 0.7004, 0.7995 and 0.9000.
2.3. FTIR spectroscopy FTIR spectra over the range from 4000 to 650 cm 1 were collected at temperature of 20.3 1C using a Nicolet 5700 FTIR spectrometer, equipped with a MTC detector. Two attenuated total reflection cells, ZnSe and Ge crystals with incident angles of 451 and 601 and 12 reflections, were employed. The Ge crystal, with shorter effective light path was used to examine the strong stretching bands of C N. Spectra were recorded with a resolution of 2 cm 1, a zero filling factor of 2, and 16 parallel scans. Three parallel measurements for each sample were carried out. The refractive indexes of solutions were measured with a refractometer at 20.3 1C. The formulas suggested by Hansen (1965) were used to do the ATR corrections.
2.4. Excess absorption spectroscopy The calculation of excess infrared spectra was performed on Matlab software, based on the theory developed by Yu's group (Li et al., 2006; Wang et al., 2009). The theory of excess infrared spectroscopy and its applications have been described in detail in their papers (Wang et al., 2010; Zhang et al., 2011; Zhang et al., 2010; Zheng et al., 2013). Briefly speaking, 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 formula for calculating the excess
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infrared spectrum is described as follows:
εE ¼
A ðx1 εn1 þ x2 εn2 Þ dðC 1 þ C 2 Þ
ð1Þ
where A is the absorbance of the mixture, d is the light path length, C 1 and C 2 are molarities of the two components, x1 and x2 are mole fractions of components 1 and 2, and εn1 and εn2 are molar absorption coefficients of the two components in their pure states, respectively. 2.5.
1
H NMR measurements
The 1H NMR measurements were carried out on a JEOL JNMECA 300 NMR spectrometer (300 MHz) at 298 K. To avoid the influence of the possible interactions between the standard chemical TMS and DMSO–[C4Py][SCN], TMS was dissolved in CCl4 and used as external standard. Fig. 2. ATR-FTIR spectra of [C4Py][SCN], DMSO and DMSO-d6 in the range of 3200– 2000 cm 1.
3. Computational details DFT calculations were carried out using Gaussian 09 program (Frisch et al., 2010). All geometric optimizations were performed at B3LYP/6–31þ þg (d, p) level. In the optimization process, all complexes were relaxed under symmetric constraints. The final obtained geometries were recognized as local minima without any negative vibrational frequency. Second-order perturbation delocalization energies (E(2)) were obtained by natural bond orbital (NBO) analysis at B3LYP/6–31þ þg (d, p) level. The interaction energies calculated at B2PLYPD/6–31 þ þg (d,p) theoretical level were compared with the results obtained at B3LYP/6–31þ þg (d, p) level. B2PLYPD is a new empirical hybrid function considering both the correction from perturbation theory and dispersion correlation, which has been successfully used to obtain the thermodynamic data with the smallest mean absolute deviation (Goerigk et al., 2009; Sancho-Garcia and Adamo, 2013; Schwabe and Grimme, 2007). The interaction energies for a dimer AB complex and trimer ABC complex were obtained as below (Young, 2001)
ΔE ¼ EAB EA EB
ð2Þ
ΔE ¼ EABC EA EB EC
ð3Þ
The structure of the [C4Py] þ –[SCN] complex was initially optimized in the gas phase as a tentative interaction model. Then the solvent effect was introduced by using the polarized continuum model (PCM) (Barone and Cossi, 1998). The method creates a solute cavity by a set of overlapping spheres. The solvent DMSO was used, ε ¼46.7. 4. Results and discussion 4.1. IR spectra of pure [C4Py][SCN] IL, DMSO and DMSO-d6 The partial ATR-FTIR spectra of pure [C4Py][SCN] IL, DMSO and DMSO-d6 are shown in Fig. 2 and the concerned spectra of IL–DMSO interactions are restricted to these regions. For pure [C4Py][SCN] IL, three bands above 3020 cm 1 are attributed to the coupled v(Cring–H) of the pyridinium ring, while those below 3020 cm 1 are from the v(Calkyl–H) in the alkyl chain (Wang et al., 2010), and the strong band around 2052 cm 1 is attributed to the v(C N) in the anion (Zheng et al., 2013). For pure DMSO-d6 and DMSO, two bands around 2249 and 2124 cm 1 are due to the vas(C–D) and vs(C–D) of DMSO-d6, and the bands around 2994 and 2911 cm 1 are due to the vas(C–H) and vs(C–H) of DMSO, respectively (Zhang et al., 2009). Thus, the [C4Py][SCN]–DMSO system was
chosen for the analysis of the v(C N) to avoid the influence of band overlap, while the [C4Py][SCN]–DMSO-d6 system was selected to analyze the coupled v(Cring–H) of the pyridinium ring, v(Calkyl–H) in the alkyl chain and vas(C–D) of DMSO-d6. 4.2. ATR and excess infrared spectra analysis of v(C–H) It is well known that the v(C–H) is a key band to characterize the H-bonds such as C–H O/N, and it can be used as a probe to reflect the interactions between DMSO and ILs (Chang et al., 2006). The partial ATR-FTIR and excess infrared spectra of [C4Py][SCN]– DMSO-d6 system in the v(C–H) region over the entire mole fraction range are shown in Fig. 3. A few features can be found readily in the original infrared spectra with the increment of DMSO content (Fig. 3a). The pyridinium ring C–Hs stretching bands, centered around 3126 and 3052 cm 1 in pure [C4Py] [SCN] IL, exhibit obvious changes in the spectral profiles in the presence of DMSO-d6. It is interesting to see that both shoulder peaks on the left and right sides of v(Cring–H) around 3052 cm 1 gradually disappear while the mole fraction of DMSO-d6 is higher than 0.7. The peak of v(Cring–H) around 3126 cm 1 gradually moves to higher wavenumber with the increasing of the mole fraction of DMSO-d6 by blue-shifted 4.6 cm 1, while the v(Cring–H) around 3052 cm 1 is nearly fixed, with less than70.3 cm 1 change in wavenumber (Fig. 3b). As for the alkyl C–Hs stretching bands, centered around 2960, 2935 and 2873 cm 1 in pure [C4Py] [SCN] IL, they exhibit no obvious changes in the spectral profiles in the presence of DMSO-d6. The peak positions of these three bands show less obvious shifts than that of v(Cring–H) around 3126 cm 1 with merely blue-shifted by 1.8, 1.9 and 2.0 cm 1, respectively (Fig. 3d), which implies that pyridinium ring C–Hs are the preferred sites for interactions between the cation and DMSO-d6. Excess infrared spectra can reveal the positions of new complexes and the changes in molar absorptivity more clearly than the original IR spectra (Li et al., 2006). In the excess infrared spectra of C–Hs stretching vibrations (Fig. 3c), the most obvious feature is the mainly positive bands from both the pyridinium ring C–Hs and alkyl C–Hs stretches, which represents that the IR activity of C–Hs stretches is enhanced (Kiefer et al., 2012), and also means that the newly arisen H-bonded complexes of pyridinium ring C–Hs/alkyl C–Hs and DMSO, i.e., C–H OQS H-bonded complexes. By comparing the excess bands of v(Cring–H) and v(Calkyl–H) stretches, it is clear to see that the positive excess bands of v (Cring–H) around 3052 cm 1 are extremely stronger than that of the v(Calkyl–H), indicating that the pyridinium ring C–Hs participate in stronger H-bonding interactions. Furthermore, the
Please cite this article as: He, H., et al., Hydrogen-bonding interactions between a pyridinium-based ionic liquid [C4Py][SCN] and dimethyl sulfoxide. Chem. Eng. Sci. (2014), http://dx.doi.org/10.1016/j.ces.2014.07.024i
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Fig. 3. ATR-FTIR (a) and excess infrared (c) spectra of [C4Py][SCN]–DMSO-d6 binary systems in the range of the C–H stretching vibrations; wavenumber shifts of the pyridinium C–H (b) and alkyl C–H (d) stretching vibrations at different mole fractions of DMSO-d6.
shoulder peaks on v(Cring–H) around 3052 cm 1 in the original IR spectra become sharp, located at about 3094 cm 1 denoted by arrows in Fig. 3c (taking xDMSO d6 ¼ 0:3996 and 0.5002 as examples), can be attributed to the C–Hs stretching vibrations from C–H OQS H-bonded complexes. According to the literatures (Fumino et al., 2008; Wang et al., 2010; Zhang et al., 2008; Zhang et al., 2010), the H-bond involving aromatic C–H on the pyridinium ring is classified as a “proper redshift H-bond”, while that related to alkyl C–H is classified as an “improper red-shift H-bond”. Thus, the blue-shifts of v(Cring–H) around 3126 cm 1 observed in this work imply that DMSO weakens the Cring–H anion H-bonding interactions between the cation and anion during the dilution process, which also represents the formation of H-bonding interactions between DMSO-d6 and [C4Py] þ cation. For alkyl C–Hs in [C4Py][SCN] IL, the generation of the blue-shifts indicates that the H-bonding interactions involving the Calkyl–H groups in the pyridinium cation are strengthened. It should be pointed out emphatically that the electrostatic interactions in the mixtures containing cations and anions still play important role and result in electrostatic-enhanced H-bonds. 4.3. ATR and excess infrared spectra analysis of v(C–D) and v(C N) The partial ATR-FTIR and excess infrared spectra of [C4Py] [SCN]–DMSO-d6 and [C4Py][SCN]–DMSO systems in the vas(C–D) and v(C N) regions over the entire mole fraction range are
shown in Fig. 4. For v(C–D), only the band of vas(C–D) at around 2249 cm 1 is analyzed as the vs(C–D) around 2124 cm 1 overlapped with that of v(C N). In the original spectra (Fig. 4a and d), vas(C–D) moves to lower wavenumber gradually, while the v (C N) moves to higher wavenumber inchmeal in the mixing process. The peak position of the former is red-shifted by 4.2 cm 1 in the presence of [C4Py][SCN] IL, while that of the latter is blueshifted by 1.9 cm 1 in the presence of DMSO. The concentration dependences of the wavenumber shift are summarized in Fig. 4c and f. As can be seen in Fig. 4b, the most obvious feature of the excess infrared spectra in vas(C–D) region is the negative bands at higher wavenumber located at about 2254 cm 1 and positive bands at lower wavenumber located at about 2241 cm 1. The facts show distinctly that the existence of complexes in the mixture for the nearly fixed positions of the negative and positive bands. The positive ones at the higher wavenumber can be attributed to the new H-bonding complexes formed by the C–D of DMSO-d6 and the anion of [C4Py][SCN] IL, whereas the negative ones at the lower wavenumber can be attributed to the decrease of self-associated H-bonds in DMSO-d6. For the infrared spectra in v (C N) region (Fig. 4e), there are a tiny negative band at around 2068 cm 1, a very strong positive band at around 2054 cm 1 and a negative band at around 2039 cm 1, which can be explained as the result of the electron-donating effect of C N in the formation of the C N DMSO H-bond. The result will be further confirmed by the following theoretical calculation. In brief, the
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Fig. 4. ATR-FTIR spectra (a and d), excess infrared spectra (b and e), wavenumber shifts (c and f) of the vas(C–D) and v(C N) at different mole fractions of DMSO-d6/DMSO of [C4Py][SCN]–DMSO-d6/DMSO binary systems.
H-bonding behaviors in the [C4Py][SCN]–DMSO system discussed above suggest the possible existence of selective interactions among DMSO and different functional groups of the cation and anion. 4.4.
1
H NMR analysis
1 H NMR measurements of [C4Py][SCN] IL, DMSO and a series of [C4Py][SCN]–DMSO mixtures were carried out, and the chemical shift changes of individual H atoms during the dilution process
were evaluated. Assignments of the 1H NMR signals to the H atoms on [C4Py] þ are displayed in Fig. 5a. The chemical shifts variations (Δδ) are expressed by the differences between the pure chemicals and their mixtures. The dependences of Δδ of different H atoms on the mole fraction of DMSO are displayed in Fig. 5b–d. The Δδ values of the H atoms on [C4Py] þ are all positive (40.34 ppm), which means clearly that the presence of DMSO resulted in downfield shift of the H atoms on [C4Py] þ . It is well known that the formation of H-bonds cause downfield chemical shift of the
Please cite this article as: He, H., et al., Hydrogen-bonding interactions between a pyridinium-based ionic liquid [C4Py][SCN] and dimethyl sulfoxide. Chem. Eng. Sci. (2014), http://dx.doi.org/10.1016/j.ces.2014.07.024i
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Fig. 5. 1H NMR assignment of pure [C4Py][SCN] IL (a) and chemical shift changes of the H atoms in an pyridinium ring (b), H atoms in alkyl groups of the pyridinium ring (c), and H atoms in the methyl group of DMSO (d) in the [C4Py][SCN]–DMSO binary systems.
related H atoms (Wulf et al., 2007). Thus the observation of this downfield change of H atoms on [C4Py] þ can support above results about H-bonding interactions formed between the cation of IL and solvent molecules. In addition, the negative value of Δδ for the H atoms on DMSO states that the H atoms show upfield shift in the presence of [C4Py][SCN] IL. This result indicates that in the presence of IL, the self-associated H-bonds of pure DMSO are weakened, which are in good agreement with the conclusion drawn from FT-IR. Moreover, in real the [C4Py][SCN]–DMSO binary system, the DMSO molecules can form H-bonding interactions with pyridinium ring C–Hs and alkyl C–Hs simultaneously as both of the Δδ values are changed significantly.
4.5. Theoretical investigation of the interaction between [C4Py][SCN] IL and DMSO In order to investigate the H-bonding interactions of [C4Py] [SCN] IL and DMSO and further determine the preferred interaction sites contributing to the formation of H-bonds, complexes including [C4Py] þ [SCN] , [C4Py] þ –DMSO, [SCN] –DMSO, [C4Py][SCN]–DMSO, [C4Py][SCN]–2DMSO, 2[C4Py] þ 2[SCN] and 2[C4Py][SCN] DMSO were selected to perform DFT calculations. The sum of the van der Waals atomic radii of H and O (2.72 Å) and that of H and N (2.75 Å), H and S (3.00 Å) (Bondi, 1964) are used as the geometric criteria values for judging the formation of H-bonds between H and O/N/S atoms. H-bonds are
denoted by dashed lines, and the corresponding H O, H N and H S bond distances are labeled in Figs. 6–9.
4.5.1. Interactions between [C4Py] þ /[SCN] and DMSO Interactions between the cation and anion were investigated before the examination of the cation/anion-cosolvent “two-body” complex (cation/anion-cosolvent). The optimized geometries of [C4Py] þ –[SCN] complex at B3LYP/6–31 þ þ g (d,p) level are shown in Fig. 6, and the corresponding interaction energies both at B3LYP/6–31þ þG(d,p) and B2PLYPD/6–31 þ þG(d,p) levels are listed in Table 1. The absolute value of the interaction energy of the first complex is larger than that of the other six complexes, meaning which is the most stable one. It is noted that the interactions between the cation and anion are characterized by multiple H-bonds formed mainly between the electronegative N and/or S atom of the [SCN] anion and the pyridinium ring C–H and /or alkyl C–H of the [C4Py] þ cation. However, there are many possible sites of [C4Py] þ cation and [SCN] anion to interact with DMSO, therefore, a number of possible mutual orientations of the complexes consisting of one DMSO molecule and one [C4Py] þ cation or [SCN] anion were examined. The optimized geometries are shown in Fig. 7, and respective interaction energies are shown in Table 2. For [C4Py] þ – DMSO complexes (Fig. 7A, B), there are two most stable complexes with minor interaction energy difference ( 0.8 kJ/mol) and very similar structures. C–H O H-bonding interactions can exist
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Fig. 6. Optimized geometries of [C4Py][SCN] complexes (A–G). Dashed lines denote H-bonds and corresponding distances are labeled. (A0 ) is the capture of the front view of complex A in respect to pyridinium ring, which is presented in tube form to show clearly.
Fig. 7. Optimized geometries of [C4Py] þ /[SCN] –DMSO complexes (A–C). Dashed lines denote H-bonds and the corresponding distances are labeled.
between aromatic C–H even alkyl C–H of [C4Py] þ cation and the O atom in DMSO. For the [SCN] –DMSO complex (Fig. 7C), it can be seen that H-bond only exist between the C–H group of DMSO and the N atom in [SCN] , which indicates that N atom rather than S atom in [SCN] is the preferred site for the formation of H-bond with DMSO molecule. It can be found that DMSO molecule can form stable H-bonds with [C4Py] þ and [SCN] through different atoms, i. e. H and O atom, respectively, by the examination of the interaction pairs of [C4Py] þ –DMSO and [SCN] –DMSO (Fig. 7A–C). The result means that there is no obvious competitive interaction between DMSO molecule and ions even though the interaction energies difference between [C4Py] þ –DMSO and [SCN] –DMSO complexes are
considerable. Namely, DMSO molecule can interact with the C–Hs of [C4Py] þ and N atom of [SCN] simultaneously when the DMSO concentration is lower. By comparison of the interaction energies in Tables 1 and 2, it can be determined that the interaction energy of [C4Py] þ –[SCN] complexes is much higher in absolute value than that of the [C4Py] þ –DMSO and [SCN] –DMSO complexes. The larger interaction energy of [C4Py] þ –[SCN] is due to the strong electrostatic interaction of the cation and anion. The interaction energy of [C4Py] þ –DMSO is larger than that of [SCN] –DMSO indicates that the interaction between DMSO and the [C4Py] þ cation is stronger than that of DMSO and the [SCN] anion. In fact, the interaction energy of [C4Py] þ –[SCN] is too strong because the DFT
Please cite this article as: He, H., et al., Hydrogen-bonding interactions between a pyridinium-based ionic liquid [C4Py][SCN] and dimethyl sulfoxide. Chem. Eng. Sci. (2014), http://dx.doi.org/10.1016/j.ces.2014.07.024i
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Fig. 8. Optimized geometries of [C4Py][SCN]–DMSO complexes (A–I). Dashed lines denote H-bonds and the corresponding distances are labeled.
Fig. 9. Optimized geometries of 2[C4Py] þ –2[SCN] , 2[C4Py] þ –2[SCN] –DMSO and [C4Py] þ –[SCN] –2DMSO complexes (A–C). Dashed lines denote H-bonds and the corresponding distances are labeled.
Please cite this article as: He, H., et al., Hydrogen-bonding interactions between a pyridinium-based ionic liquid [C4Py][SCN] and dimethyl sulfoxide. Chem. Eng. Sci. (2014), http://dx.doi.org/10.1016/j.ces.2014.07.024i
H. He et al. / Chemical Engineering Science ∎ (∎∎∎∎) ∎∎∎–∎∎∎ Table 1 Interaction energies (ΔE) of [C4Py] þ –[SCN] complexes as shown in Fig. 6 at B3LYP/6–31 þ þG(d,p) and B2PLYPD/6–31 þ þG(d,p) levels.a ΔE (kJ/mol)
Complex
Fig. Fig. Fig. Fig. Fig. Fig. Fig.
6(A) 6(B) 6(C) 6(D) 6(E) 6(F) 6(G)
B3LYP/6–31þ þ G(d,p)
B2PLYPD/6–31þ þG(d,p)
336.63 333.39 332.28 330.08 329.84 328.76 328.11
337.04 335.71 335.49 336.18 333.89 327.87 330.68
a Interaction energies are corrected with the basis set superposition error (BSSE).
Table 2 Interaction energies (ΔE), NBO analysis, delocalization energies (E(2)) of selected anti-bonding orbital of [C4Py] þ /[SCN] –DMSO complexes as shown in Fig. 7: [C4Py] þ –DMSO (A–B); [SCN] –DMSO (C). Complex ΔEa (kJ/mol)
NBOb
E(2) (kJ/mol)
B3LYP/6– 31 þ þG(d,p)
B2PLYPD/6– 31þ þG(d,p)
Proton acceptordonor
Fig. 7(A)
72.57
81.93
Fig. 7(B)
71.77
81.05
Fig. 7(C)
58.94
62.88
LP(O)-σn(C2–H)ring 50.94 LP(O)-σn(C7–H)alkyl 8.12 n LP(O)-σ (C2–H)ring 32.02 LP(O)-σn(C7–H)alkyl 16.07 LP(N)-σn(C1–H)DMSO 13.52 LP(N)-σn(C2–H)DMSO 14.32
a Interaction energies are corrected with the basis set superposition error (BSSE). b NBO analysis was performed at the B3LYP/6–31þ þ G(d,p) level.
calculations were applied in a gas phase. The interaction energy decreases drastically to 13.99 kJ/mol when [C4Py] þ –[SCN] was optimized in DMSO solvent. Correspondingly, the geometric structures of [C4Py] þ –[SCN] also change obviously with the solvent (taking Fig. 6A as an example, see Fig. S5 in the Supplementary Material).
4.5.2. NBO analysis of the interactions between [C4Py] þ /[SCN] and DMSO To further understand the interactions between DMSO and [C4Py] þ /[SCN] , NBO analysis was carried out. For the [C4Py] þ – DMSO complexes (see Fig. 7A, B), the NBO analysis suggests strong orbital interactions between the anti-bonding orbital of the proton donor in [C4Py] þ , σn(C–H), and the lone pairs (LP) of the proton acceptor in DMSO, LP(O), which is supported by the second-order delocalization energies (E(2)) in the related localized anti-bonding orbital (see Table 2). Here, the E(2) values are 50.94 and 8.12 kJ/mol for the LP(O)-σn(C2–H) and LP(O)-σn(C7–H) interactions for complex A, 32.02 and 16.07 kJ/mol for the LP(O)-σn(C2–H) and LP(O)-σn(C7–H) interactions for complex B. It is obvious that the H-bonds formed by the O atom in DMSO and the aromatic H atoms on the pyridinium ring are generally stronger than those with the H atoms on the alkyl chain, indicating that the Cring–H O H-bonds play an important role in stabilizing the [C4Py] þ –DMSO complexes. For the [SCN] –DMSO complex, the calculated result shows that the interaction between the lone pair of the N atom in the [SCN] , LP(N), and the anti-bonding orbital of the C–H bond in DMSO, σn(C–H), dominates over the interaction of DMSO and [SCN] . Such a LP(N)-σn(C–H) interaction results in electron transfer from the proton acceptor LP(N) to the donor σn(C–H) and
9
C–H N H-bond formation, and hence stabilizes the [SCN] – DMSO complex. The E(2) values are 13.52 and 14.32 kJ/mol for the LP(N)-σn(C–H) interactions for complex C. It can be found from the above analysis that the interaction mechanisms of the DMSO molecule with the cation and anion are very different in nature: the former relates to the decisive orbital overlap of the type of LP (O)-σn(C–H)cation, while the latter mainly involves LP(N)σn(C–H)DMSO interactions. By comparing the second-order delocalization energies of DMSO/ions complexes, the larger E(2) value of cation–DMSO complexes indicates that the DMSO molecule preferentially interact with [C4Py] þ , which accords well with the above structural analysis.
4.5.3. Interactions between [C4Py][SCN] ion pair and DMSO [C4Py][SCN]–DMSO complexes were evaluated in order to understand how DMSO interacts with both the cation and anion at the same time. The results including optimized geometries and H-bonding interactions are depicted in Fig. 8, and the corresponding interaction energies are shown in Table 3. As can be seen in the figures, these nine structures can be divided into two categories: structures A and C, structures B, D–I. In the former, the Cring– H S H-bonds can be formed between the [C4Py] þ cation and [SCN] anion with S H bond distances of 2.842 and 2.839 Å, respectively. In addition, Cring–H O and Calkyl–H O H-bonds can be formed between DMSO and the cation and anion synchronously, as well as CDMSO–H N H-bonds also exist between DMSO and the anion, which shows that the DMSO molecule acts as both an H-bond donor and acceptor. In the latter, N atom of the anion can form H-bonds with Cring–H and/or Calkyl–H on the cation, and can form CDMSO–H N H-bonds with DMSO at the same time in some complexes. As to the S atom of the anion, it can form H-bonds with the O atom of DMSO or Calkyl–H on the cation in some complexes. All structures in Fig. 8 demonstrate that DMSO molecule cannot insert into the [C4Py] þ cation and [SCN] anion, which illuminate that it is not easy for a neutral molecule like DMSO to disrupt the strong Coulombic interaction between the cation and anion by simple insertion when the concentration of DMSO is low. In addition, the above results also indicate that in [C4Py][SCN]–DMSO complexes, DMSO molecule can form H-bonds with aromatic C–H and alkyl C–H simultaneously, but the complex of DMSO interacting with aromatic C–H is more dominant than that of with alkyl C–H. This is in general agreement with above 1 H NMR result. The fact that all the interaction energies of the [C4Py][SCN]–DMSO complexes are larger than those of [C4Py] [SCN] implies that the mixing process of the [C4Py][SCN] and DMSO is enthalpically favorable and consistent with the prompt solubility of [C4Py][SCN] in DMSO. Table 3 Interaction energies (ΔE) of [C4Py][SCN]–DMSO complexes (A–I) as shown in Fig. 8 at B3LYP/6–31þ þ G(d,p) and B2PLYPD/6–31þ þ G(d,p) levels.a Complex
Fig. Fig. Fig. Fig. Fig. Fig. Fig. Fig. Fig.
8(A) 8(B) 8(C) 8(D) 8(E) 8(F) 8(G) 8(H) 8(I)
ΔE (kJ/mol) B3LYP/6–31þ þ G(d,p)
B2PLYPD/6–31 þ þ G(d,p)
392.96 387.15 387.00 382.36 382.07 378.28 374.77 369.19 360.93
412.33 405.51 403.55 399.61 396.82 388.25 389.69 384.32 370.62
a Interaction energies are corrected with the basis set superposition error (BSSE).
Please cite this article as: He, H., et al., Hydrogen-bonding interactions between a pyridinium-based ionic liquid [C4Py][SCN] and dimethyl sulfoxide. Chem. Eng. Sci. (2014), http://dx.doi.org/10.1016/j.ces.2014.07.024i
H. He et al. / Chemical Engineering Science ∎ (∎∎∎∎) ∎∎∎–∎∎∎
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Table 4 Interaction energies (ΔE) of 2[C4Py] þ –2[SCN] , 2[C4Py] þ –2[SCN] –DMSO and [C4Py] þ –[SCN] –2DMSO complexes (A-C) as shown in Fig. 9 at B3LYP/6–31þ þ G (d,p) and B2PLYPD/6–31 þ þG(d,p) levels.a Complex
Fig. 9 (A) Fig. 9 (B) Fig. 9 (C)
ΔE, kJ/mol B3LYP/6–31þ þG(d,p)
B2PLYPD/6–31þ þG(d,p)
767.91 804.64 431.68
788.42 833.42 464.26
a
Interaction energies are corrected with the basis set superposition error (BSSE).
Ionic cluster consisting of a certain number of ions can partly explain the complicated network structure of IL system. The optimized geometries and H-bonding interactions of 2[C4Py] þ –2 [SCN] , 2[C4Py] þ –2[SCN] –DMSO and [C4Py] þ –[SCN] –2DMSO are presented in Fig. 9, and the corresponding interaction energies are shown in Table 4. The [SCN] anions are situated between the two [C4Py] þ cations and interact with them at the same time (Fig. 9A). For 2[C4Py] þ –2[SCN] –DMSO complex (Fig. 9B), the DMSO molecule stays on the outer skirt of the cluster with its O and H atom interacting with the Cring–H of the cation and N atom in the anion simultaneously. The [C4Py] þ –[SCN] –2DMSO complex shown in Fig. 9C was selected to record the representative interaction modes at higher DMSO concentration range. There exist strong H-bonding interactions between the cation and anion, while the DMSO molecules still are unable to insert into the cation and anion, meaning that the strong Coulombic interaction between [C4Py] þ cation and [SCN] anion is difficult to disrupt. At the same time, it can be found that most of the H-bond distances between the cation and anion in structures A–C are lengthened with the addition of the DMSO. This phenomenon implies that this addition weakens the H-bond interactions between the cation and anion, and may lead to a decrease of the viscosity of ILs by comparing with the optimized geometries in Fig. 9.
5. Conclusions In this work, ATR-FTIR, 1H NMR and DFT calculations were used to study the H-bonding interactions in [C4Py][SCN]–DMSO system. It was found that the H-bonding interactions exist widely in this system, which plays a key role towards the stability and miscibility of the systems. Detailed investigation about the positions of positive and negative peaks in excess infrared spectroscopy showed that H-bonds involving the alkyl C–Hs and C N of IL are strengthened with the addition of DMSO, while the H-bonds involving pyridinium ring C–Hs as well as the H-bonds formed between the [C4Py] þ cation and [SCN] anion are weakened. The addition of [C4Py][SCN] led to the H-bonds involving the C–Hs in DMSO molecule weakened comparing with the associated H-bonds in pure DMSO system. The results of DFT calculations indicated that the solvent molecules cannot disrupt the strong Coulumbic interaction between the [C4Py] þ and [SCN] . NBO analysis further confirmed that the interaction mechanism of DMSO molecule with the anion and cation are different in nature: the former relates to the decisive orbital overlap of the type of LP(O)-σn(C–H)ring/alkyl, while the latter mainly involves LP(N)σn(C–H)DMSO interactions, which mean that the DMSO molecule can acts as both an H-bond donor and acceptor in [C4Py][SCN]– DMSO system. Through comparison of the mutual interactions among the species [C4Py] þ , [SCN] , and DMSO with the help of DFT calculations, the following sequential order of interaction
strength was established: [C4Py] þ –[SCN] –DMSO 4[C4Py] þ – [SCN] 4 [C4Py] þ –DMSO 4[SCN] –DMSO. These studies on the properties of the IL–DMSO system may deepen our understanding of the system at the molecular level and shed light on exploring the applications of ILs.
Acknowledgments This work was supported by the National Natural Science Foundation of China (21127011, 21336002), and National High Technology Research and Development Program of China (2012AA063001). The authors sincerely thank Professor Xuehui Li of South China University of Technology for his meaningful help in discussions and writing.
Appendix A. Supplementary Material The Supplementary Material contains the geometries and absolute energies of all calculated structures, which can be found in the online version at http://www.sciencedirect.com/. Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.ces.2014.07.024.
References Andanson, J.M., Bordes, E., Devémy, J., Leroux, F., Pádua, A.A.H., Gomes, M.F.C., 2014. Understanding the role of co-solvents in the dissolution of cellulose in ionic liquids. Green Chem. 16, 2528–2538. Anderson, J.L., Dixon, J.K., Maginn, E.J., Brennecke, J.F., 2006. Measurement of SO2 solubility in ionic liquids. J. Phys. Chem. B 110, 15059–15062. Barone, V., Cossi, M., 1998. Quantum calculation of molecular energies and energy gradients in solution by a conductor solvent model. J. Phys. Chem. A 102, 1995–2001. Bondi, A., 1964. Van der Waals volumes and radii. J. Phys. Chem. 68, 441–451. Brayton, C.F., 1986. Dimethyl sulfoxide (DMSO): a review. Cornel. Vet. 76, 61–90. Cammarata, L., Kazarian, S.G., Salter, P.A., Welton, T., 2001. Molecular states of water in room temperature ionic liquids. Phys. Chem. Chem. Phys. 3, 5192–5200. Cappelli, C., Mennucci, B., da Silva, C.O., Tomasi, J., 2000. Refinements on solvation continuum models: hydrogen-bond effects on the OH stretch in liquid water and methanol. J. Chem. Phys. 112, 5382–5392. Chang, H.C., Jiang, J.C., Chang, C.Y., Su, J.C., Hung, C.H., Liou, Y.C., Lin, S.H., 2008a. Structural organization in aqueous solutions of 1-butyl-3-methylimidazolium halides: a high-pressure infrared spectroscopic study on ionic liquids. J. Phys. Chem. B 112, 4351–4356. Chang, H.C., Jiang, J.C., Liou, Y.C., Hung, C.H., Lai, T.Y., Lin, S.H., 2008b. Effects of water and methanol on the molecular organization of 1-butyl-3methylimidazolium tetrafluoroborate as functions of pressure and concentration. J. Chem. Phys. 129, 044506. Chang, H.C., Jiang, J.C., Liou, Y.C., Hung, C.H., Lai, T.Y., Lin, S.H., 2008c. Local structures of water in 1-butyl-3-methylimidazolium tetrafluoroborate probed by high-pressure infrared spectroscopy. Anal. Sci. 24, 1305–1309. Chang, H.C., Jiang, J.C., Tsai, W.C., Chen, G.C., Lin, S.H., 2006. Hydrogen bond stabilization in 1,3-dimethylimidazolium methyl sulfate and 1-butyl-3methylimidazolium hexafluorophosphate probed by high pressure: The role of charge-enhanced C–H O interactions in the room-temperature ionic liquid. J. Phys. Chem. B 110, 3302–3307. Clark, J.H., Tavener, S.J., 2006. Alternative solvents: shades of green. Org. Process Res. Dev. 11, 149–155. Docherty, K.M., Dixon, J.K., Kulpa Jr, C.F., 2006. Biodegradability of imidazolium and pyridinium ionic liquids by an activated sludge microbial community. Biodegradation 18, 481–493. Dong, K., Song, Y.T., Liu, X.M., Cheng, W.G., Yao, X.Q., Zhang, S.J., 2012. Understanding structures and hydrogen bonds of ionic liquids at the electronic level. J. Phys. Chem. B 116, 1007–1017. Dong, K., Zhang, S.J., 2012. Hydrogen bonds: a structural insight into ionic liquids. Chem.–A Eur. J. 18, 2748–2761. Dong, K., Zhang, S.J., Wang, D.X., Yao, X.Q., 2006. Hydrogen bonds in imidazolium ionic liquids. J. Phys. Chem. A 110, 9775–9782. Forsyth, S.A., Pringle, J.M., MacFarlane, D.R., 2004. Ionic liquids – an overview. Aust. J. Chem. 57, 113–119. Frisch, M.J., Trucks, G.W., Schlegel, H.B., Scuseria, G.E., Robb, M.A., Cheeseman, J.R., et al., 2010. Gaussian 09, Revision B.01. Gaussian, Inc., Wallingford CT. Fumino, K., Wulf, A., Ludwig, R., 2008. Strong, localized, and directional hydrogen bonds fluidize ionic liquids. Angew. Chem. Int. Ed., 8731–8734.
Please cite this article as: He, H., et al., Hydrogen-bonding interactions between a pyridinium-based ionic liquid [C4Py][SCN] and dimethyl sulfoxide. Chem. Eng. Sci. (2014), http://dx.doi.org/10.1016/j.ces.2014.07.024i
H. He et al. / Chemical Engineering Science ∎ (∎∎∎∎) ∎∎∎–∎∎∎ Goerigk, L., Moellmann, J., Grimme, S., 2009. Computation of accurate excitation energies for large organic molecules with double-hybrid density functionals. Phys. Chem. Chem. Phys. 11, 4611–4620. Hansen, W.N., 1965. Expanded formulas for attenuated total reflection (A.T.R.) and the derivation of absorption rules for single and multiple A.T.R. spectrometer cells. Spectrochim. Acta 21, 815–833. He, H.Y., Chen, H., Zheng, Y.Z., Zhang, X.C., Yao, X.Q., Yu, Z.W., Zhang, S.J., 2013a. The hydrogen-bonding interactions between 1-ethyl-3-methylimidazolium lactate ionic liquid and methanol. Aust. J. Chem. 66, 50–59. He, H.Y., Zhang, S.J., Liu, X.M., Wang, J.Q., Yao, X.Q., Zhang, X.P., 2013b. Structures and hydrogen bonds of biodegradable naphthenate ionic liquids. Fluid Phase Equilib. 360, 169–179. Kiefer, J., Martinez Molina, M., Noack, K., 2012. The peculiar nature of molecular interactions between an imidazolium ionic liquid and acetone. ChemPhysChem 13, 1213–1220. Li, Q.Z., Wu, G.S., Yu, Z.W., 2006. The role of methyl groups in the formation of hydrogen bond in DMSO–methanol mixtures. J. Am. Chem. Soc. 128, 1438–1439. Long, J., Guo, B., Li, X., Jiang, Y., Wang, F., Tsang, S.C., Wang, L., Yu, K.M.K., 2011. One step catalytic conversion of cellulose to sustainable chemicals utilizing cooperative ionic liquid pairs. Green Chem. 13, 2334–2338. Navas, A., Ortega, J., Vreekamp, R., Marrero, E., Palomar, J., 2009. Experimental thermodynamic properties of 1-butyl-2-methylpyridinium tetrafluoroborate b2mpyBF4 with water and with alkan-1-ol and their interpretation with the COSMO-RS methodology. Ind. Eng. Chem. Res. 48, 2678–2690. Peppel, T., Roth, C., Fumino, K., Paschek, D., Köckerling, M., Ludwig, R., 2011. The influence of hydrogen-bond defects on the properties of ionic liquids. Angew. Chem. Int. Ed. 50, 6661–6665. Reichardt, C., 2005. Polarity of ionic liquids determined empirically by means of solvatochromic pyridinium N-phenolate betaine dyes. Green Chem. 7, 339–351. Rinaldi, R., 2011. Instantaneous dissolution of cellulose in organic electrolyte solutions. Chem. Commun. 47, 511–513. Sancho-Garcia, J.C., Adamo, C., 2013. Double-hybrid density functionals: merging wavefunction and density approaches to get the best of both worlds. Phys. Chem. Chem. Phys. 15, 14581–14594. Schwabe, T., Grimme, S., 2007. Double-hybrid density functionals with long-range dispersion corrections: higher accuracy and extended applicability. Phys. Chem. Chem. Phys. 9, 3397–3406. Umebayashi, Y., Jiang, J.C., Lin, K.H., Shan, Y.L., Fujii, K., Seki, S., Ishiguro, S.I., Lin, S.H., Chang, H.C., 2009. Solvation and microscopic properties of ionic liquid/ acetonitrile mixtures probed by high-pressure infrared spectroscopy. J. Chem. Phys. 131, 234502–234507. Vreekamp, R., Castellano, D., Palomar, J., Ortega, J., Espiau, F., Fernandez, L., Penco, E., 2011. Thermodynamic behavior of the binaries 1-butylpyridinium tetrafluoroborate
11
with water and alkanols: their interpretation using 1H NMR spectroscopy and quantum-chemistry calculations. J. Phys. Chem. B 115, 8763–8774. Wang, J.Q., Sun, J., Cheng, W.G., Dong, K., Zhang, X.P., Zhang, S.J., 2012. Experimental and theoretical studies on hydrogen bond-promoted fixation of carbon dioxide and epoxides in cyclic carbonates. Phys. Chem. Chem. Phys. 14, 11021–11026. Wang, N., Li, Q., Yu, Z., 2009. Hydrogen bonding interactions in three 2-mercaptoethanol systems: an excess infrared spectroscopic study. Appl. Spectrosc. 63, 1356–1362. Wang, N., Zhang, Q., Wu, F., Li, Q., Yu, Z., 2010. Hydrogen bonding interactions between a representative pyridinium-based ionic liquid [BuPy][BF4] and water/ dimethyl sulfoxide. J. Phys. Chem. B 114, 8689–8700. Weingartner, H., 2008. Understanding ionic liquids at the molecular level: facts, problems, and controversies. Angew. Chem. Int. Ed. 47, 654–670. Wilkes, J.S., 2002. A short history of ionic liquids-from molten salts to neoteric solvents. Green Chem. 4, 73–80. Wulf, A., Fumino, K., Michalik, D., Ludwig, R., 2007. IR and NMR properties of ionic liquids: do they tell us the same thing? ChemPhysChem 8, 2265–2269. Young, D.C., 2001. Computational chemistry: A Practical Guide for Applying Techniques to Real-World Problems. John Wiley & Sons, Inc, New York. Yunus, N.M., Mutalib, M.I.A., Man, Z., Bustam, M.A., Murugesan, T., 2012. Solubility of CO2 in pyridinium based ionic liquids. Chem. Eng. J. 189–190, 94–100. Zeng, S., Gao, H., Zhang, X., Dong, H., Zhang, X., Zhang, S., 2014. Efficient and reversible capture of SO2 by pyridinium-based ionic liquids. Chem. Eng. J. 251, 248–256. Zhang, L.Q., Wang, Y., Xu, Z., Li, H.R., 2009. Comparison of the blue-shifted C–D stretching vibrations for DMSO-d6 in imidazolium-based room temperature ionic liquids and in water. J. Phys. Chem. B 113, 5978–5984. Zhang, L.Q., Xu, Z., Wang, Y., Li, H.R., 2008. Prediction of the solvation and structural properties of ionic liquids in water by two-dimensional correlation spectroscopy. J. Phys. Chem. B 112, 6411–6419. Zhang, Q.G., Wang, N.N., Wang, S.L., Yu, Z.W., 2011. Hydrogen bonding behaviors of binary systems containing the ionic liquid 1-butyl-3-methylimidazolium trifluoroacetate and water/methanol. J. Phys. Chem. B 115, 11127–11136. Zhang, Q.G., Wang, N.N., Yu, Z.W., 2010. The hydrogen bonding interactions between the ionic liquid 1-ethyl-3-methylimidazolium ethyl sulfate and water. J. Phys. Chem. B 114, 4747–4754. Zheng, Y.Z., Wang, N.N., Luo, J.J., Zhou, Y., Yu, Z.W., 2013. Hydrogen-bonding interactions between [BMIM][BF4] and acetonitrile. Phys. Chem. Chem. Phys. 15, 18055–18064. Zhu, X., Cui, P., Zhang, D., Liu, C., 2011. Theoretical Study for pyridinium-based ionic liquid 1-ethylpyridinium trifluoroacetate: synthesis mechanism, electronic structure, and catalytic reactivity. J. Phys. Chem. A 115, 8255–8263.
Please cite this article as: He, H., et al., Hydrogen-bonding interactions between a pyridinium-based ionic liquid [C4Py][SCN] and dimethyl sulfoxide. Chem. Eng. Sci. (2014), http://dx.doi.org/10.1016/j.ces.2014.07.024i