Rhenium-Catalyzed Decarboxylative Tri-/Difluoromethylation of Styrenes with Fluorinated Carboxylic Acid-Derived Hypervalent Iodine Reagents

Article abstract of DOI:10.1002/cjoc.201900296

Herein, unprecedented rhenium-catalyzed decarboxylative oxytri-/difluoromethylation and Heck-type trifluoromethylation of styrenes have been developed by using hypervalent iodine(III) reagents derived from cheap, stable, and easy-handling fluorinated carboxylic acids. Mechanistic studies revealed a radical decarboxylative trifluoromethylation pathway occurring in these reactions.

Full text of DOI:10.1002/cjoc.201900296

Research Article
pubs.acs.org/journal/ascecg
Cite This: ACS Sustainable Chem. Eng. 2019, 7, 7760−7767
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*,†
†Key Laboratory of Bioorganic Phosphorous Chemistry and Chemical Biology (Ministry of Education), Department of Chemistry,
Tsinghua University, Beijing 100084, P. R. China
‡School 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
S
* Supporting Information
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 is still lacking. In this work, the interactions between ChCl
and polyols (1,2-butanediol, 1,3-butanediol, 1,4-butanediol, 2,3-butane-
diol, 1,3-propanediol, glycerol, 1,5-pentanediol, 1,2,5-pentanetriol, and
xylitol) have been investigated at different molar ratios of ChCl to
1
polyols by Fourier transform middle infrared, far-infrared, H 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
the 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,
whereas 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 a possible explanation for viscosities of
the DESs and may afford useful knowledge for the design and development of new DESs.
1
KEYWORDS: Deep eutectic solvents, Hydrogen bonds, Fourier transform infrared spectroscopy, H and 35Cl NMR spectroscopy,
Quantum chemistry calculation
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 under-
standing 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 a 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
INTRODUCTION
■
Deep eutectic solvents (DESs), reported by Abbott and co-
workers in 2003,1 are generally composed of two or three
components which can form a eutectic mixture with a 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 desig-
nability, 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, biotransformations, 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
Received: December 20, 2018
Revised: February 13, 2019
Published: March 19, 2019
© 2019 American Chemical Society
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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 the 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) around 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
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 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-
1
MIR), far-infrared (FIR), H NMR, and 35Cl NMR spectros-
copy 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 groups 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.
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.
EXPERIMENTAL SECTION
■
Materials. Choline chloride (>98.0%), 1,3-butanediol (>99.0%),
1,4-butanediol (>99.0%), 1,3-propanediol (>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 pur-
chased 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
abbreviations of the produced DESs are listed in Table 1.
RESULTS AND DISCUSSION
■
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 a
hydrogen bond, in the systems have a vital effect on the mid-
infrared spectrum.21 Figures 1 and 2 show the FT-MIR spectra
Table 1. Abbreviations of DESs
abbreviation
salt
HBD
salt/HBD ratio
DES-1
DES-2
DES-3
DES-4
DES-5
DES-6
DES-7
DES-8
DES-9
ChCl
ChCl
ChCl
ChCl
ChCl
ChCl
ChCl
ChCl
ChCl
1,2-butanediol
1,3-butanediol
1,4-butanediol
2,3-butanediol
1,3-propanediol
glycerol
1,5-pentanediol
1,2,5-pentanetriol
xylitol
1:2
1:2
1:2
1:2
1:2
1:2
1:2
1:2
1:2
Figure 1. FT-MIR spectra of neat ChCl and butanediol in the range
of 4000−1000 cm−1.
FT-MIR and FIR Experiments. FT-MIR spectrum experiments
were completed with an 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 the FT-MIR
spectrometer was 4 cm−1. The system temperature was maintained
within 1 °C by a PE-94 temperature controller (PerkinElmer, 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 a
polyethylene (PE) window and the detector was DTGS (720−30
cm−1). The scanning range was 600−30 cm−1, the resolution was 2
cm−1, and the scanning number was 250. 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.
of neat ChCl, neat butanediol, and the corresponding DES at
20 °C, respectively. It can be seen from Figure 1 that the peaks
at about ∼3220 and 3327 cm−1 can be belonging to the O-H
stretching vibration of ChCl and butanediol, respectively. 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 the O-H stretching frequency of ChCl, that of
DES is increased, suggesting that the O-H stretching vibration
in DESs is blue-shifted. In theory, the frequency of a normal
vibration 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 the X−H bond decrease and bond length increase,
1
1H NMR and 35Cl NMR Measurements. All the H NMR and
35Cl NMR measurements were finished on a Bruker AVANCE 400
and AV III HD 600 spectrometer at 20 °C, respectively. For 1H NMR
spectrum measurement, the signal of TMS in DMSO-d6 in a capillary
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DES-3, O-H stretching is blue-shifted, indicating that the
hydrogen bond formed by two alcohol molecules is weakened
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 becomes 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 words, the hydrogen bond
interaction of ChCl with butanediol is weakened with the
increase of carbon number between two hydroxyl groups in
butanediol.
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 DES-1, DES-2, DES-3, and DES-4 were resolved using
a 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 the 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 confirms again that the
Figure 2. FT-MIR spectra for different DESs.
thus leading to red shifts. On the basis of 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, the
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 a
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
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.
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hydrogen bond interaction between ChCl and butanediol
becomes weakened with the increase of carbon number
between two hydroxyl groups in butanediol.
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
bond interactions among constitutes of the DESs. The FIR
spectra of neat ChCl, neat butanediol, and corresponding
DESs are shown in Figures 4 and S1−S3 (Supporting
number of methylene between two hydroxyl groups in
butanediol increasing, which is in line with the conclusion
from FT-MIR.
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 list the chemical shifts of H proton of the
Table 3. Chemical Shift and Δδ Values of H Proton of
Hydroxyl Group in ChCl, 1,2-Butanediol, and the
Corresponding DESs
O-H
ChCl
5.53
1,2-butanediol
DES-1
Δδ
δ(-OH)/ppm
δ(2-OH)/ppm
δ(1-OH)/ppm
5.51
4.48
4.42
−0.02
0.05
0.05
4.43
4.37
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, a hydrogen bond is a vital factor to impact the chemical
shift of protons. Theoretically, with respect to an 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
the O-H group in ChCl was decreased, manifesting that the
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 upshift. However, for
H proton of the O-H group in butanediol, the observed
downfield shifts implied that the strength of the 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
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).
Information). To better understand these peaks, density
functional theory calculation was carried out. First of all, a
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 are 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 the 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
Table 2. Wavenumber of Different DESs at 20 °C and the Related Assignments
experimental
value/cm−1
calculated
DES
value/cm−1
assignment
DES-1
141
121
114
131
119
146
126
115
165
121
stretching vibration of hydrogen bond between Cl atom of ChCl and H atom of O-H group in 1,2-
butanediol
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
DES-2
DES-3
DES-4
DES-5
stretching vibration of hydrogen bond between Cl atom of ChCl and H atom of O-H group in 1,3-
propanediol
DES-6
DES-7
122
96
125
101
stretching vibration of hydrogen bond between Cl atom of ChCl and H atom of O-H group in glycerol
stretching vibration of hydrogen bond between Cl atom of ChCl and H atom of O-H group in 1,5-
pentanediol
DES-8
DES-9
119
125
118
123
stretching vibration of hydrogen bond between Cl atom of ChCl and H atom of O-H group in 1,2,5-
pentanetriol
stretching vibration of hydrogen bond between Cl atom of ChCl and H atom of O-H group in xylitol
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decrease. These results supported the conclusion from FT-
MIR analysis of DESs.
According to the above analysis, the hydrogen bond may be
formed between Cl atom of ChCl and H proton of the O-H
group in butanediol. The presence of this hydrogen bond will
cause the chemical shift of the Cl atom to change, which
should be reflected in the 35Cl NMR spectrum. Therefore, 35Cl
NMR spectroscopy was used to probe the hydrogen bonds
mentioned. Figure 5 shows the chemical shift of the Cl atom in
Figure 6. FIR spectra of the mixture of ChCl and 1,2-butanediol at
different molar ratios of ChCl to 1,2-butanediol.
hydrogen bond interaction was the 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.
In order to have a deeper understanding of the hydrogen
bond interaction between ChCl and butanediol at different
molar ratios, H NMR spectra of the mixture of ChCl with
Figure 5. 35Cl NMR spectra of ChCl and the related DESs.
1
butanediol in DMSO-d6 at different molar ratios were
determined at 20 °C. Figures 7 and S12−S14 describe the
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 the
O-H group in butanediol was weaker than that formed by
ChCl itself, resulting in the increase in electron density of the
Cl atom and upfield shift in chemical shift. In addition,
chemical shift change of the Cl atom in the three DESs
gradually diminished with the increase of methylene number
between two hydroxyl groups in butanediol, which indicates
that the hydrogen bond interaction between Cl atom of ChCl
and H atom of the 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 the 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 itself,
which leads to the decrease of electron density of Cl atom and
then downshift in the chemical shift.
Effect of Molar Ratios of ChCl to Butanediol on the
Hydrogen Bond Interactions. It is known that different
ratios of the components of a 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 °C. The
FIR spectra for the mixtures of ChCl and butanediol at
different molar ratios (1:3, 1:2.5, 1:2, 1:1) are 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 the 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. On the basis of 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, the
Figure 7. 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.
Δδ change in H proton of the O-H group in the DES as a
function of molar ratio. It is clear that, with increasing molar
ratio, the Δδ value of H proton in the 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 the O-H group to decrease and then the chemical shift
increased. At the same time, the Δδ value for H proton in the
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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 the O-H group in
the mixture, the addition of butanediol in ChCl impaired the
hydrogen bond interaction among ChCl itself. With the
decrease of butanediol content in the mixture, the ability of
butanediol to break the hydrogen bond of ChCl itself 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 the O-H
group in butanediol and Cl atom in ChCl, which was
responsible for the formation of ChCl-butanediol DES.
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-shifted.
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 the 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 are shown in
Figure 9. It was shown that the majority of the hydroxyl groups
in DESs were still H-bonded rather than free. The strength of
the hydrogen bond of O-H···Cl increased in the sequence
DES-9 > DES-8 > DES-7. In addition, the vibration
frequencies of the hydrogen bond in DES-5, DES-6, DES-7,
DES-8, and DES-9 are also listed in Table 2. It was found that
the vibration frequencies of the 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.
On the basis of 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 is shown in Figure 8. It is clear that,
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 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,
the |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 the O-H group of polyols was strengthened.
This conclusion was in agreement with that from FIR
measurements.
The Link of Viscosity with Hydrogen Bond Inter-
action in the DESs. It is known that the macroscopic
properties of DESs are closely related to 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 existing in
the liquid, we try to have a correlation of the strength of
hydrogen bonds in DESs and viscosities of the DESs. It was
reported that, at a 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
Figure 8. Variety of Δδ values for Cl atom in the mixture of ChCl and
butanediol at different molar ratios.
except for 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, the |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 the
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 strength of hydrogen bond interaction
between Cl atom and H atom in the O-H group of butanediol
was reduced. This conclusion was in agreement with that
extracted from FIR results.
Effect of Hydroxyl Number in Polyols on the
Hydrogen Bond Interactions. The hydrogen bond
interactions of ChCl with 1,3-propanediol, glycerol, 1,5-
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Research Article
Figure 9. FT-IR O-H stretching vibration peak deconvolution in different DESs: (a) DES-7, (b) DES-8, (c) DES-9.
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 dominates 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 a larger number of hydrogen bonds between ChCl and
butanediol were formed. Therefore, viscosity of the mixture of
ChCl and butanediol was enhanced with increasing molar
ratio.
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 the 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.
ASSOCIATED CONTENT
CONCLUSION
In summary, the interactions between choline chloride and
each of nine polyols were investigated at different molar ratios
■
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acssusche-
meng.8b06676.
1
of choline chloride to aliphatic polyols by FT-MIR, FIR, H
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 the O-H group of polyols, and its
FT-MIR spectra and the variation of chemical shift in
protons of the hydroxyl group in the mixtures of ChCl
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Research Article
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and butanediol at different molar ratios of ChCl to
butanediol (PDF)
AUTHOR INFORMATION
Corresponding Authors
*E-mail: jwang@htu.cn (J.W.).
*E-mail: yuzhw@tsinghua.edu.cn (Z.Y.).
■
ORCID
Jianji Wang: 0000-0003-2417-4630
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the National Natural Science
Foundation of China (Nos. 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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enhanced anion−cation interaction from hydrogen bonding in pure
imidazolium ionic liquids. Angew. Chem., Int. Ed. 2010, 49, 449−453.
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FIR and Raman spectroscopy studies of imidazolium-based ionic
liquids covering the frequency range 2−300 cm−1. ChemPhysChem
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