Insights into the Hydrogen Bond Interactions in Deep Eutectic Solvents

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Insights into the Hydrogen Bond Interactions in Deep Eutectic Solvents Composed of Choline Chloride and Polyols Huiyong Wang, Shuyan Liu, Yuling Zhao, Jianji Wang, and Zhiwu Yu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b06676 • Publication Date (Web): 19 Mar 2019 Downloaded from http://pubs.acs.org on March 20, 2019

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ACS Sustainable Chemistry & Engineering

Insights into the Hydrogen Bond Interactions in Deep Eutectic Solvents Composed of Choline Chloride and Polyols Huiyong Wang a,b, Shuyan Liu b, Yuling Zhao b, Jianji Wang b,*, Zhiwu Yu a,* aKey

Laboratory of Bioorganic Phosphorous Chemistry and Chemical Biology (Ministry

of Education), Department of Chemistry, Tsinghua University, Beijing 100084, P. R. China.

bSchool

of Chemistry and Chemical Engineering, Key Laboratory of Green Chemical

Media and Reactions, Ministry of Education, Henan Normal University, 46 Jianshe Road E., Xinxiang, Henan 453007, P. R. China.

*Corresponding

author: E-mail: [email protected] (Jianji Wang) E-mail: [email protected] (Zhiwu Yu)

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Abstract Deep eutectic solvents (DESs) consisting of cholinium chloride (ChCl) and alcohols have been widely applied in the purification of bioactive compounds, biodiesel and flavonoids. However, an explicit and complete knowledge of the interactions between ChCl and alcohols are still lacking. In this work, the interactions between ChCl and polyols (1,2-butanediol, 1,3-butanediol, 1,4-butanediol, 2,3-butanediol, 1,3-propanediol, glycerol, 1,5-pentanediol, 1,2,5-pentanetriol and xylitol) have been investigated at different molar ratios of ChCl to polyols by Fourier transform middle infrared, far infrared, 1H and 35Cl NMR spectroscopy as well as quantum chemistry calculation. It is shown that hydrogen bond interaction between Cl atom of ChCl and H atom of O-H group in the polyols is predominant and its strength decreases with both the increase of carbon number between two hydroxyl groups in butanediol and the decrease of hydroxyl number in polyols. As butanediol content is increased in the mixture, the hydrogen bond interaction of choline cations with Cl- is weakened, while that among butanediol molecules is enhanced. A molar ratio of ChCl to butanediol at 1:2 is indispensable for the formation of DES in a reasonable strength of hydrogen bond. The present results offer possible explanation for viscosities of the DESs and may afford useful knowledge for the design and development of new DESs.

Keywords Deep eutectic solvents, hydrogen bonds, Fourier transform infrared spectroscopy, 1H and 35Cl NMR spectroscopy, quantum chemistry calculation

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Introduction Deep eutectic solvents (DESs), reported by Abbott and co-workers in 2003,1 are generally comprised of two or three components which can form eutectic mixture with melting point far below that of the single component on account of the association of components. These solvents show a low vapor pressure, relatively wide liquid range, and structural designability, which are analogous to ionic liquids (ILs).2 However, compared with traditional ILs, DESs have some advantages such as lower price, easy preparation, strong biocompatibility, and without any purification steps.3 Therefore, these characters render DESs potential alternatives to replace conventional organic solvents as well as some ILs in many applications such as inorganic and organic synthesis, electrochemistry, extraction, separation, bio-transformations and biomedicine.4-7 In recent years, many kinds of DESs with different hydrogen bond acceptors (HBAs) and hydrogen bond donors (HBDs) have been reported, but the most popular DESs investigated so far are those based on cholinium chloride (ChCl, used as HBA) due to its low cost, low toxicity, biocompatibility, and biodegradability.8 In this context, ChCl has been combined with several kinds of HBDs such as carbohydrates, renewable polyols, amides, alcohols, amines, or carboxylic acids to form DESs.9 Recently, it is found that the DESs formed by cholinium chloride and alcohols (such as glycerol, 1,2-butanediol, 1,3-butanediol, 1,4-butanediol) can be used for the purification of biodiesel10 and flavonoids.11 These DESs are also useful in the separation of bioactive compounds. As one expected, these applications depend on the structure and properties of DESs, and the interactions between the components of DES usually control its structure and physicochemical properties. Therefore, a fundamental understanding of the interactions between alcohol and ChCl is very important. In recent years, a certain number of experimental and computational researches on the interactions of ChCl and urea as well as the microstructures of their DES have been completed.1,

12-18

It is shown that the

formation of hydrogen bond network between urea and choline salt is the main reason for the deep freezing-point depression of the mixture.1 Nevertheless, the studies on the interactions between cholinium chloride and alcohols are very scarce, which may hinder our understanding of the properties and further application of the ChCl- alcohol DESs. In this work, we focused our attention on the DESs composed of cholinium chloride 3



and aliphatic polyols (1,2-butanediol, 1,3-butanediol, 1,4-butanediol , 2,3-butanediol, 1, 3-propanediol, glycerol, 1,5-pentanediol, 1,2,5-pentanetriol and xylitol), and studied the hydrogen bond interactions between ChCl and polyols by Fourier transform mid-infrared (FT-MIR), far infrared (FIR), 1H NMR, and 35Cl NMR spectroscopy as well as quantum chemistry calculation. The effect of the hydroxyl position in butanediol, molar ratio of ChCl to butanediol and the number of hydroxyl group in polyols on the interactions was examined, and the relationship of hydrogen bond interactions with viscosity of the DESs was discussed, thus providing a deeper understanding on structure and properties of these useful DESs. Experimental Section Materials Choline chloride (>98.0%), 1,3-butanediol (>99.0%), 1,4-butanediol (>99.0%), 1,3propanediol (>98.0%), glycerol (>99.5%), 1,5-pentanediol (>97.0%), 1,2,5-pentanetriol (>97.0%) and xylitol were purchased from Aladdin Co., Ltd (Shanghai, China). 1,2-Butanediol (>98.0%) and 2,3-butanediol (>97.0%) were purchased from Saen Chemical Technology Co., Ltd (Shanghai, China). Deuterium dimethylsulfoxide (DMSO-d6) was brought from J&K Scientific Ltd (Beijing, China). These chemicals were used as received. The DESs were prepared through mixing the two components under stirring until a homogeneous clear liquid was observed.1 The water contents in DESs were less than 0.08% mass fraction, which was ascertained by a Karl Fischer titration. The abbreviation of the produced DESs was listed in Table 1. FT-MIR and FIR experiments FT-MIR spectrum experiments were completed with a FIR spectrometer (PerkinElmer Spectrum 400, USA) consisting of a room temperature DTGS detector (15000-370 cm-1) and KBr windows. The wavenumber was in the range from 4000 to 600 cm − 1 and the resolution of FT-MIR spectrometer was 4 cm −1. The system temperature was maintained within ± 1 oC by A PE-94 temperature controller (Perkin-Elmer, USA). During the measurement process, a small amount of DES was put on the upper part of KBr windows, and 16 scans were performed. For FIR experiment, the scanning window was changed into polyethylene (PE) window and the detector was DTGS (720-30cm-1). The scanning range was 600-30 cm-1, the resolution was 2 cm − 1, and the scanning number was 250. 4


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During the experiments, the measuring cell was blown with dehumidified air produced by an accessory of the instrument to avoid water absorption from air in the samples. 1H

NMR and 35Cl NMR measurements

All the 1H NMR and 35Cl NMR measurements were finished on a Bruker AVANCE 400 and AV III HD 600 spectrometer at 20 oC, respectively. For 1H NMR spectrum measurement, the signal of TMS in DMSO-d6 in capillary insert was used as the reference and the DMSO-d6 in the same capillary insert was used for locking. In addition, the signal of HCl in DMSO-d6 in capillary insert was used as the reference in 35Cl NMR spectrum measurements. The chemical shifts were expressed in parts per million downfield from TMS and HCl, respectively. Computation method The geometry optimization of the DES mixtures was conducted by the Gaussian 09 D.01 program applying different starting directions of the HBD molecules (out-of-plane and in-plane) round choline chloride, with the HBDs positioned at the end of ammonium and/or hydroxyl group.19 The structural optimizations were performed at the B3LYP/6-31+G* theoretical level. All optimized geometries were identified as local minima without any negative vibrational frequency. The binding energies were adjusted on the basis of set superposition error (BSSE).20 For all the DESs, the vibrational frequencies were rectified by the standard factor of 0.96. The interaction energy, ΔEDES, was computed according to the following equation: ΔEDES=EDES-(EChCl+2EHBD)

(1)

where EDES is the single point energy of DES, and EHBD and EChCl are the single point energies of HBD and ChCl in the geometry of the corresponding DES pair, respectively. Table 1.

The abbreviation of DESs

Abbreviation DES-1 DES-2 DES-3 DES-4 DES-5 DES-6 DES-7 DES-8 DES-9

Salt ChCl ChCl ChCl ChCl ChCl ChCl ChCl ChCl ChCl

HBD 1,2-butanediol 1,3-butanediol 1,4-butanediol 2,3-butanediol 1,3-propanediol glycerol 1,5-pentanediol 1,2,5-pentanetriol xylitol 5


Salt/HBD ratio 1:2 1:2 1:2 1:2 1:2 1:2 1:2 1:2 1:2


Results and discussion The effect of hydroxyl position on the hydrogen bond interactions from FT-MIR, FIR, 1H and 35Cl NMR studies It is known that the intermolecular interactions, such as hydrogen bond, in the systems have a vital effect on the mid infrared spectrum.21 Figures 1 and 2 show the FT-MIR spectra of neat ChCl, neat butanediol and the corresponding DES at 20 oC, respectively. It can be seen from Figure 1 that the peaks at about ∼3220 and 3327 cm−1 can be belonging to O-H stretching vibration of ChCl and butanediol. And the O-H stretching frequency in DES-1, DES-2, DES-3 and DES-4 is 3323, 3308, 3305 and 3325 cm-1, respectively. Compared with O-H stretching frequency of ChCl, that of DES is increased, suggesting that the O−H stretching vibration in DESs is blue shift. In theory, the frequency of a normal vibration is closely depends on the force constants of the particular bond. Intermolecular interactions (such as a hydrogen bond) can give rise to the length and strength perturbation of a bond and cause spectral signature variations. It is adopted that the formation of X-H···Y will make the energy of X-H bond decrease and bond length increase, thus leading to red shifts. Based on the analysis of the molecular structures of ChCl and butanediol, intermolecular hydrogen bonds may exist in ChCl and butanediol molecules. So, we can conclude from the O-H stretching vibration in all the DESs that with the addition of butanediol, intermolecular hydrogen bond of ChCl itself is gradually broken, leading to the blue-shift of the O-H stretching vibration. In addition, compared with neat 1, 2-butanediol or 2, 3-butanediol, O-H stretching in DES-1 and DES-4 exhibits red-shift, suggesting that the hydrogen bond interaction between ChCl and 1, 2-butanediol (2,3-butanediol) is stronger than that between diol molecules. However, for DES-2 and DES-3, O-H stretching is blue-shift, indicating that the hydrogen bond formed by two alcohol molecules is weaken due to the formation of DES. Furthermore, as the carbon number between two hydroxyl groups in butanediol is increased, the degree of the blue-shift of O-H stretching in ChCl become small, which indicates that as the carbon number between two hydroxyl groups of butanediol increases, the ability of butanediol to break the hydrogen bond among ChCl molecules is reduced. In other word, the hydrogen bond interaction of ChCl with butanediol is weakened with the increase of carbon number between two hydroxyl groups in butanediol. 6


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Figure 1. FT-MIR spectra of neat ChCl and butanediol in the range of 4000−1000 cm−1.

Figure 2. FT-MIR spectra for different DESs. In addition, broad bands were observed in the O-H stretching vibration region (3100-3700 cm-1) for the DESs investigated, which are attributed to the summation of several contributions of different types of hydroxyl groups in the DESs. In order to determine the relative contribution from different hydroxyl groups, the broad O-H stretching vibration peaks in DESs-1, DESs-2, DESs-3 and DESs-4 were resolved using a

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curve-fitting procedure, which resulted in three different types of O-H peaks in each of these DESs (Figure 3). The deconvoluted peaks at 3423, 3364, and 3267 cm-1 could be assigned to the O-H stretching vibration peaks for free, inter-/intramolecular O-H···O-H, and O-H···Cl hydroxyl groups, respectively. It was found that the majority of the hydroxyl groups in the DESs were H-bonded rather than free. The strength of hydrogen bond in O-H···Cl follows the sequence DES-1 ≈ DES-4 > DES-2 > DES-3, as judged from their relative peak area. This result conforms again that the hydrogen bond interaction between ChCl and butanediol become weaken with the increase of carbon number between two hydroxyl groups in butanediol.

Figure 3. FT-IR O-H stretching vibration peak deconvolution in different DESs: (a) DES-1, (b) DES-2, (c) DES-3, (d) DES-4. In view of the fact that FIR technology is a powerful means to study the intermolecular and intramolecular hydrogen bonds,22-24 this technology was used to study the hydrogen

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bond interactions among constitutes of the DESs. The FIR spectra of neat ChCl, neat butanediol and corresponding DESs were shown in Figures 4 and S1-S3 (Supporting Information). To better understand these peaks, density functional theory calculation was carried out. First of all, stable configuration was optimized for DES, and FIR spectra were collected and analyzed. The calculated and experimental values of the vibrational frequencies in the range from 50 to 450 cm−1 were listed in Table 2 together with the corresponding vibrational assignments for these DESs. It is noted that the experimental values of these peaks were consistent with the calculated ones, and the hydrogen bond peaks were mainly shaped by the interaction between Cl atom of ChCl and H atom in O-H group of butanediol. The vibrational frequencies of hydrogen bonds in the DESs decreased as the number of methylene between two hydroxyl groups in butanediol was increased. This consequence indicates that the hydrogen bond interaction gradually weakens with the number of methylene between two hydroxyl groups in butanediol increasing, which is in line with the conclusion from FT-MIR.

Figure 4. FIR spectra of neat ChCl, neat 1, 2-butanediol and the corresponding DES at molar ratio of 1:2 (ChCl to 1, 2-butanediol)

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Table 2. Wavenumber of different DESs at 20 oC and the related assignments DES

Experimental value /cm-1

Calculated value/cm-1

Assignment

DES-1

141

146

stretching vibration of hydrogen bond between Cl atom of ChCl and H atom of O-H group in 1,2-butanediol

DES-2

121

126

DES-3

114

115

DES-4

131

165

DES-5

119

121

stretching vibration of hydrogen bond between Cl atom of ChCl and H atom of O-H group in 1,3-propanediol

DES-6

122

125

stretching vibration of hydrogen bond between Cl atom of ChCl and H atom of O-H group in glycerol

DES-7

96

101

stretching vibration of hydrogen bond between Cl atom of ChCl and H atom of O-H group in 1,5-pentanediol

DES-8

119

118

stretching vibration of hydrogen bond between Cl atom of ChCl and H atom of O-H group in 1,2,5-pentanetriol

DES-9

125

123

stretching vibration of hydrogen bond between Cl atom of ChCl and H atom of O-H group in xylitol

stretching vibration of hydrogen bond between Cl atom of ChCl and H atom of O-H group in 1,3-butanediol stretching vibration of hydrogen bond between Cl atom of ChCl and H atom of O-H group in 1,4-butanediol stretching vibration of hydrogen bond between Cl atom of ChCl and H atom of O-H group in 2,3-butanediol

To further investigate the hydrogen bond interaction among components of the DESs, 1H

NMR technology was also applied. To expediently discuss the hydrogen bond

interaction, the difference between the chemical shift of each proton in the DES and that in the neat components (ChCl or butanediol) was used to describe the chemical shift of each proton. Namely, chemical shift variation (Δδ) rather than chemical shift (δ) was used to discuss these spectroscopic variations. Tables 3 and S1-S3 listed the chemical shifts of H proton of O-H group in ChCl, butanediol and the corresponding DESs as well as the related Δδ values. Some factors (hydrogen bond, anisotropy effect of aromatic ring, and structural change) may affect Δδ values for these protons in the mixtures.25 Without doubt, hydrogen bond is a vital factor to impact the chemical shift of protons. 10


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Theoretically, with respect to a X-H···Y hydrogen bond, a decrease in electron density of the proton in the X-H bond results in a downfield shift, while an increase in electron density of the proton in the X-H bond causes an upfield shift.26 It was shown that when DES was formed by ChCl and butanediol, the chemical shift value of H proton of O-H group in ChCl was decreased, manifesting that hydrogen bond formed by Cl atom of ChCl and H atom of O-H in butanediol weakened the hydrogen bond between ChCl molecules itself, thus leading to an increase in electron density of the proton of O-H in ChCl and then up-shift. However, for H proton of O-H group in butanediol, the observed downfield shifts implied that the strength of hydrogen bond between Cl atom of ChCl and H atom of O-H in butanediol was stronger than that between butanediol molecules, which made the electron density of the proton decrease. These results supported the conclusion from FT-MIR analysis of DESs. Table 3. The chemical shift and Δδ values of H proton of hydroxyl group in ChCl, 1, 2-butanediol and the corresponding DESs O-H

ChCl

1,2-butanediol

DES-1

Δδ

δ(-OH)/ppm

5.53

—

5.51

-0.02

δ(2-OH)/ppm

—

4.43

4.48

0.05

δ(1-OH)/ppm

—

4.37

4.42

0.05

According to the above analysis, the hydrogen bond may be formed between Cl atom of ChCl and H proton of O-H group in butanediol. The presence of this hydrogen bond will make the chemical shift of Cl atom to change, which should be reflected in NMR spectrum. Therefore,

35Cl

35Cl

NMR spectroscopy was used to probe the hydrogen

bonds mentioned. Figure 5 shows the chemical shift of Cl atom in neat ChCl and DES. It is noted that as DES was formed, the chemical shift values of Cl atom in DES-1, DES-2, and DES-3 all became smaller. These results suggest that the intensity of hydrogen bonds between Cl atom of ChCl and H atom of O-H group in butanediol were weaker than those formed by ChCl itself, resulting in the increase in electron density of Cl atom and upfield shift in chemical shift. In addition, chemical shift change of Cl atom in the three DESs gradually diminished with the increase of methylene number between two hydroxyl 11



groups in butanediol, which indicates that the hydrogen bond interaction between Cl atom of ChCl and H atom of O-H group in butanediol was reduced with the carbon number increase. These results also supported the conclusion obtained from FT-MIR and FIR. However, for DES-4, the chemical shift of Cl atom was larger than that of neat ChCl. This finding implies that the hydrogen bond between ChCl and 2, 3-butanediol is stronger than ChCl oneself, which leads to the decrease of electron density of Cl atom and then downshift in the chemical shift.

Figure 5. 35Cl NMR spectra of ChCl and the related DESs. The effect of molar ratios of ChCl to butanediol on the hydrogen bond interactions It is known that different ratios of the components of DES influence the properties of DESs (such as viscosity, conductivity and stability), which may be ascribed to the change in hydrogen bond interactions.27 Therefore, we studied the effect of molar ratios on the hydrogen bond interactions in the DESs at 20 oC. The FIR spectra for the mixtures of ChCl and butanediol at different molar ratios (1:3, 1:2.5, 1:2, 1:1) were shown in Figures 6 and S9-S11. Taking the mixture of ChCl and 1, 2-butanediol as an example, it can be seen from Figure 6 that the vibration frequency of hydrogen bond in the mixture increased from 115 to 141 cm-1 as the molar ratio of ChCl to 1, 2-butanediol was increased from 1:1 to 1:3. Based on the relationship of vibration frequency and force constant of one bond, we can infer that as the molar ratio of ChCl to butanediol was 1:2,

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the hydrogen bond interaction was strongest, implying that at this molar ratio, the two compounds had the strongest tendency to form DESs. Similar results were observed in other kinds of DESs investigated in this work.

Figure 6. FIR spectra of the mixture of ChCl and 1, 2-butanediol at different molar ratios of ChCl to 1, 2-butanediol. In order to have a deeper understanding for the hydrogen bond interaction between ChCl and butanediol at different molar ratios, 1H NMR spectra of the mixture of ChCl with butanediol in DMSO-d6 at different molar ratios were determined at 20 oC. Figures 7 and S12-S14 describe the Δδ change in H proton of O-H group in the DES as a function of molar ratio. It is clear that with increasing molar ratio, Δδ value of H proton in O-H group of ChCl increased and changed from negative to positive. This result manifests that as the content of ChCl in the mixture was increased, the hydrogen bond strength of ChCl itself was enhanced gradually, causing the electron density in H proton of O-H group decreased and then chemical shift increased. At the same time, Δδ value for H proton in O-H group of butanediol also increased with increasing content of ChCl in the mixture, indicating the reduced electron density of H proton. According to the analysis of the variation of chemical shift for H proton of O-H group in the mixture, the addition of butanediol in ChCl impaired the hydrogen bond interaction among ChCl itself. With the

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decrease of butanediol content in the mixture, the ability of butanediol to break the hydrogen bond of ChCl oneself was reduced, while the hydrogen bond interaction of butanediol itself was enhanced. The similar results were also found in the DES of ChCl and urea.13 Therefore, we speculated that at the molar ratio of 1:2 (ChCl to butanediol), a reasonable strength of hydrogen bond was formed between H atom of O-H group in butanediol and Cl atom in ChCl, which was responsible for the formation of ChCl butanediol DES.

Figure 7. The change of Δδ values for H proton of O-H group in the mixture of ChCl and 1, 2-butanediol at different molar ratios of ChCl to 1, 2-butanediol. Based on the above discussion, the chemical shift of Cl atom in the mixture of ChCl and butanediol will change with the variety of its composition. To test this, we carried out 35Cl

NMR experiments. The variety in Δδ value of Cl atom in the mixtures with molar

ratio was shown in Figure 8. It is clear that except the mixture of ChCl and 2, 3-butanediol, the Δδ values of Cl atom in the mixture were negative. This result indicates that the presence of butanediol impaired the hydrogen bond interaction between ChCl itself, resulting in the increase of electron density in Cl atom and upshift in chemical shift. Moreover, |Δδ| value of Cl atom became smaller with increasing molar ratio, which suggests that the ability of butanediol to break the hydrogen bond of ChCl itself was reduced. This trend was similar to that of H proton of O-H group in ChCl. We also found that at a given molar ratio, |Δδ| values of Cl atom became smaller with the increase of carbon number between two hydroxyl groups in butanediol, which indicates that the 14


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strength of hydrogen bond interaction between Cl atom and H atom in O-H group of butanediol was reduced. This conclusion was in agreement with that extracted from FIR results.

Figure 8. Variety of Δδ values for Cl atom in the mixture of ChCl and butanediol at different molar ratios. The effect of hydroxyl number in polyols on the hydrogen bond interactions The hydrogen bond interactions of ChCl with 1, 3-propanediol, glycerol, 1,5- pentanediol, 1,2,5-pentanetriol or xylitol were also evaluated by FT-MIR, FIR, and

35Cl

NMR

techniques at the molar ratio of 1:2 (ChCl to polyols). It can be seen from Figures 1 and 2 that compared with O-H stretching frequency of polyols, O-H stretching frequency in DESs was increased, which suggests that the O−H stretching vibration in DESs was blue-shift. From this result, we can conclude that with the addition of ChCl, intermolecular hydrogen bond in polyols itself was gradually broken. In addition, taking the polyol with five carbons as an example, the difference between the wavenumber of O−H stretching vibration in the DES and that in the neat polyol increased in the order: DES-9 (14 cm-1) >DES-8 (9 cm-1) >DES-7 (7 cm-1), indicating that with the increase of hydroxyl number, the hydrogen bond interaction became stronger. To understand the nature of hydrogen bond, the curve fitting of the hydroxyl vibration peaks in DES-7, DES-8 and DES-9 was also performed, and the results were shown in Figure 9. It was 15



shown that the majority of the hydroxyl groups in DESs were still H-bonded rather than free. The strength of hydrogen bond of O-H···Cl increased in the sequence DES-9 > DES-8 >DES-7. In addition, the vibration frequencies of hydrogen bond in DES-5, DES-6, DES-7, DES-8 and DES-9 were also listed in Table 2. It was found that the vibration frequencies of hydrogen bond in the DES-7, DES-8, and Des-9 also increased in the sequence DES-9 >DES-8 >DES-7, which suggests that the hydrogen bond interaction between ChCl and polyols became stronger with the increase of hydroxyl number in polyols. These results supported the conclusion obtained from FT-MIR analysis of DESs.

Figure 9. FT-IR O-H stretching vibration peak deconvolution in different DESs: (a) DES-7, (b) DES-8, (c) DES-9. On the other hand, 35Cl NMR chemical shift of Cl atom in the DES-5, DES-6, DES-7, DES-8 and DES-9 was found to be 60.2, 53.6, 61.6, 56.3 and 54.6 ppm, respectively. The difference (Δδ(Cl)) between the chemical shift of Cl in the DESs and that in ChCl (62.0 ppm) was -1.8, -8.4, -0.4, -5.7 and -7.4 ppm, respectively. It is clear that the Δδ value of 16


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Cl atom in the DESs was negative, which indicates that the presence of polyols weakened the hydrogen bond interaction between ChCl itself, resulting in the increase of electron density in Cl atom and upshift in chemical shift. In addition, |Δδ| value of Cl atom in DES-7, DES-8 or DES-9 increased in the order DES-9 >DES-8 >DES-7, indicating that with the increase of hydroxyl number in polyols, the ability of polyols to break the hydrogen bonds among ChCl itself was enhanced and the hydrogen bond interaction between Cl atom in ChCl and H atom in O-H group of polyols was strengthened. This conclusion was in agreement with that from FIR measurements. The Link of viscosity with hydrogen bond interaction in the DESs It is known that the macroscopic properties of DESs are closely related with the interactions between components of DES. Thus, the hydrogen bond interactions discussed above would have a vital impact on the physical properties of the DESs. Considering the fact that viscosity is an indication of intrinsic resistance to flow and reflects the strength of intramolecular interactions existed in the liquid, we try to have a correlation of the strength of hydrogen bond in DESs and viscosities of the DESs. It was reported that at molar ratio of 1:2 of ChCl to butanediol, the viscosities of the DESs were decreased in the sequence DES-4 > DES-2 > DES-3 > DES-1 at room temperature.11 It can be derived from the foregoing discussion that the strength of hydrogen bond interaction in the DESs decreased in the sequence DES-1 > DES-4 > DES-2 > DES-3. Except for DES-1, this sequence is in line with that of the strength of hydrogen bond interaction in the DESs. These results suggest that it is the strong hydrogen bond interaction that dominants the viscosity of the DESs. The similar correlation of the viscosities of some cholinium chloride based DESs with the strength of hydrogen bond interaction in the DESs was found.8,

28

Furthermore, the viscosities of the mixtures of

ChCl and 1,4-butanediol at different molar ratios were reported in the literature.29 It was found that the viscosity increased with increasing molar ratio. This result could be explained from the trend in the change of the strength of hydrogen bond interaction between ChCl and butanediol with molar ratio. As the molar ratio of ChCl to butanediol was increased, the hydrogen bond interaction of butanediol itself was weakened and more number of hydrogen bond between ChCl and butanediol was formed. Therefore, viscosity 17



of the mixture of ChCl and butanediol was enhanced with increasing molar ratio.

Conclusion In summary, the interactions between choline chloride and each of nine polyols were investigated at different molar ratios of choline chloride to aliphatic polyols by FT-MIR, FIR, 1H NMR, 35Cl NMR and quantum chemistry calculation. It was found that the main interaction between components of the DESs was the hydrogen bond interaction between Cl atom of ChCl and H atom in O-H group of polyols, and its interaction strength decreased with increasing carbon number between two hydroxyl groups in butanediol and decreasing hydroxyl number in polyols. Furthermore, the molar ratio of ChCl to butanediol was found to have an important influence on hydrogen bond interaction in the DESs. With the increase of butanediol content, the hydrogen bond interaction of choline cations with Cl- decreased, whereas that among butanediol molecules increased. A molar ratio of ChCl to butanediol at 1:2 is obligatory for the formation of DES with a reasonable strength of hydrogen bond. In addition, the high viscosity of the DESs had been reasonably explained by the strong hydrogen bond interactions between Cl atom of ChCl and H atom in O-H group of butanediol in the DESs. The present results may offer valuable information on the understanding of the microstructure and physicochemical properties of the eutectic mixture of choline chloride with polyols, and are useful for the design and development of new DESs.

Supplementary Information FT-MIR spectra and the variation of chemical shift in protons of hydroxyl group in the mixtures of ChCl and butanediol at different molar ratios of ChCl to butanediol.

Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 21573060, U1704251 and 21733011), the National Key Research and Development Program of China (No.2017YFA0403101), the Program for Backbone Teacher in University of Henan Province (No. 2016GGJS-049) and the 111 project (No. D17007).

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TOC

The hydrogen bond interaction between Cl atom of ChCl and H atom of O-H group in polyols is predominant in the DESs, and may offer possible explanation for viscosity of the systems.

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Insights into the Hydrogen Bond Interactions in Deep Eutectic Solvents

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