Influence of methyl and propyl groups on the vibrational spectra of two imidazolium ionic liquids and their non-ionic precursors

Article abstract of DOI:10.1016/j.molstruc.2017.01.008

Imidazolium-based ionic liquids (ILs) are usually synthesized using non-ionic imidazole compounds as precursors. While the ILs have been extensively studied in the past, the precursors was not paid much attention to. The structural analysis of the precursors, however, may offer an opportunity to better understand the behavior of the ionic compounds of interest. In this paper, a comparative study of two ionic liquids and their imidazole precursors is presented. The precursors 1-methylimidazole [1-MIM] and 1,2-dimethylimidazole [1,2-DMIM] are compared in order to explain the influences of the methyl group at the C(2) position (methylation). Since the imidazole compounds are non-ionic, the spectroscopic properties of [1-MIM] and [1,2-DMIM] are not affected by cation-anion interactions. In addition, the products obtained by alkylation using propyl iodide leading to the corresponding IL compounds 1-methyl-3-propylimidazolium iodide [1-MPrIM⁺][I^?] and 1,2-dimethyl-3-propylimidazolium iodide [1,2-DMPrIM⁺][I^?] were studied. For this purpose, vibrational spectroscopy in terms of FT-Raman and FTIR in the wavenumber range from [45 to 3500?cm^?1] and from [600 to 4000?cm^?1], respectively, was performed. Moreover, to aid the spectral assignment, density functional theory (DFT) calculations were carried out. The aim was to investigate the vibrational structure, to understand the effects of the propyl group at the N(3) and of the methyl group at the C(2) position, and to analyze the resulting cation-anion interactions. The data indicate that the iodide ion predominantly interacts with the C(2)[sbnd]H group via hydrogen bonding. Upon methylation, the C(4/5)[sbnd]H moiety becomes the main interaction site. However, an interaction takes place only with one of the two hydrogen atoms resulting in a split of the initially degenerate CH stretching modes.

Full text of DOI:10.1016/j.molstruc.2017.01.008

Accepted Manuscript
Influence of methyl and propyl groups on the vibrational spectra of two imidazolium
ionic liquids and their non-ionic precursors
Boumediene Haddad, Drai Mokhtar, Mimanne Goussem, El-habib Belarbi, Didier
Villemin, Serge Bresson, Mustapha Rahmouni, Nilesh R. Dhumal, Hyung J. Kim,
Johannes Kiefer
PII:
S0022-2860(17)30008-X
10.1016/j.molstruc.2017.01.008
MOLSTR 23313
DOI:
Reference:
To appear in: Journal of Molecular Structure
Received Date: 5 September 2016
Revised Date: 29 December 2016
Accepted Date: 2 January 2017
Please cite this article as: B. Haddad, D. Mokhtar, M. Goussem, E.-h. Belarbi, D. Villemin, S. Bresson,
M. Rahmouni, N.R. Dhumal, H.J. Kim, J. Kiefer, Influence of methyl and propyl groups on the vibrational
spectra of two imidazolium ionic liquids and their non-ionic precursors, Journal of Molecular Structure
(2017), doi: 10.1016/j.molstruc.2017.01.008.
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ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT
Influence of methyl and propyl groups on the vibrational spectra of two
imidazolium ionic liquids and their non-ionic precursors
a,b,d
b,c
c
b
Boumediene Haddad , Drai Mokhtar , Mimanne Goussem , El-habib Belarbi , Didier
c
e
b
f
f
Villemin , Serge Bresson , Mustapha Rahmouni , Nilesh R. Dhumal, Hyung J. Kim,
g,h,i,*
Johannes Kiefer
a
b
c
Department of Chemistry, Dr Moulay Tahar University of Saida , Algeria.
Synthesis and Catalysis Laboratory LSCT, University of Tiaret, Algeria.
Laboratoire de Matériaux & Catalyse, Université Djillali Liabès, BP 89 Sidi-Bel-Abbès, 22000 Algérie.
d
LCMT, ENSICAEN, UMR 6507 CNRS, University of Caen, 6 bd Ml Juin, 14050 Caen, France.
e
Laboratoire de Physique des Systèmes Complexes, Université Picardie Jules Verne, 33 rue St Leu 80039
Amiens cedex, France.
f
Department of Chemistry, Carnegie Mellon University, Pittsburgh, USA
g
Technische Thermodynamik, Universität Bremen, Badgasteiner Str. 1, 28359 Bremen, Germany.
h
MAPEX Center for Materials and Processes, Universität Bremen, 28359 Bremen, Germany.
i
School of Engineering, University of Aberdeen, Fraser Noble Building, Aberdeen AB24 3UE, UK
*
Corresponding author.
E-mail :jkiefer@uni-bremen.de
1

ACCEPTED MANUSCRIPT
Abstract
Imidazolium-based ionic liquids (ILs) are usually synthesized using non-ionic imidazole
compounds as precursors. While the ILs have been extensively studied in the past, the
precursors was not paid much attention to. The structural analysis of the precursors, however,
may offer an opportunity to better understand the behavior of the ionic compounds of interest.
In this paper, a comparative study of two ionic liquids and their imidazole precursors is
presented. The precursors 1-methylimidazole [1-MIM] and 1,2-dimethylimidazole [1,2-
DMIM] are compared in order to explain the influences of the methyl group at the C(2)
position (methylation). Since the imidazole compounds are non-ionic, the spectroscopic
properties of [1-MIM] and [1,2-DMIM] are not affected by cation-anion interactions. In
addition, the products obtained by alkylation using propyl iodide leading to the corresponding
+
-
IL compounds 1-methyl-3-propylimidazolium iodide [1-MPrIM ][I ] and 1,2-dimethyl-3-
+
-
propylimidazolium iodide [1,2-DMPrIM ][I ] were studied. For this purpose, vibrational
-
spectroscopy in terms of FT-Raman and FTIR in the wavenumber range from [45 to 3500cm
1
-1
]
and from [600 to 4000 cm ], respectively, was performed. Moreover, to aid the spectral
assignment, density functional theory (DFT) calculations were carried out. The aim was to
investigate the vibrational structure, to understand the effects of the propyl group at the N(3)
and of the methyl group at the C(2) position, and to analyze the resulting cation-anion
interactions. The data indicate that the iodide ion predominantly interacts with the C(2)-H
group via hydrogen bonding. Upon methylation, the C(4/5)-H moiety becomes the main
interaction site. However, an interaction takes place only with one of the two hydrogen atoms
resulting in a split of the initially degenerate CH stretching modes.
Keywords: Ionic liquids ILs; FT-Raman, FTIR/ATR, imidazolium, methylation, propylation.
2

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1
. Introduction
-Methylimidazole (or N-methylimidazole) and 1,2-dimethylimidazole are heterocyclic
1
aromatic compounds having the chemical formula CH C H N and CH CH C H N ,
3
3
3
2
3
3
3
2
2
respectively. They are colourless liquids that are used as specialty solvents [1], bases, and as
precursors in the synthesis of ionic liquids (ILs) [2]. Furthermore, the properties of these
important compounds make them a very promising class of materials for a wide range of
applications such as in pharmaceuticals [3] and pesticides [4]. In addition, they are widely
utilized in corrosion inhibitors [5] and as catalytic deprotonation reagents [6].
The chemical reaction of these two precursors with alkyl halides yields transfer of the alkyl
group to the imidazolium ring and results in an imidazolium halide ionic liquid [7]. The large
variety of possible combinations of cations and anions leads to ionic compounds with a wide
range of physicochemical properties [8]. For example, ILs based on dialkylimidazolium
cations like 1-methyl-3-propylimidazolium combined with the iodide anion provide excellent
efficiency and good stability in dye-sensitized solar cells [9]. They have also been extensively
studied as electrolytes because of their outstanding thermal and electrochemical stability [10].
Besides the cation-anion combination, simple changes in the length of the spacer or the
aliphatic chains of the cation allows for tailoring the physical properties of these ILs for
specific applications. With this objective, several studies have been done to control the
vibrational spectroscopic properties of ILs through the design of their cations and anions for
practical use [11-12].
Recent studies have been performed to elucidate the relation between the structure of ILs and
their physicochemical properties, especially, the thermophysical properties including the
viscosity, density, and melting point. These properties determine whether or not an IL is
suitable for a given application [14]. Previous reports indicate that the methylation of the C2
position [N-C-N site in the aromatic ring] of 1,3-dialkylimidazolium-based ILs disrupts the
predominant hydrogen-bonding interaction between cation and anion leading to unexpected
changes of the physicochemical properties [15-16]. The intermolecular cation-anion
interactions have been studied by several groups [17-18]. These interionic interactions
determine the macroscopic (chemical and thermophysical) properties [19].
The present work focuses on the synthesis and vibrational properties of two ionic liquids and
their precursors. The alkylation reaction of 1-methylimidazole and 1,2-dimethylimidazole
+
-
with propyl iodide using a microwave process yields the ionic liquids [1-MPrIM ][I ] and
+
-
1
13
[
1,2-DMPrIM ][I ]. The obtained ILs are confirmed by H-NMR, C-NMR, and FTIR
3

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spectroscopy. In the first step of our work, the precursors, 1-methylimidazole and 1,2-
dimethylimidazole, are studied. To date, only few experimental studies have been dedicated to
illustrate the vibrational mode assignments of methylimidazoles. For example, Carter and
Pemberton have reviewed and discussed the Raman and SERS (Surface-enhanced Raman
scattering) spectra as well as the vibrational assignments of 1- and 2-methylimidazole [13]. A
vibrational analysis of 1,2-dimethylimidazole has not been published, to the best of our
knowledge. The understanding of the vibrational structures is important to explain the
influences of the methyl group at the C2 position (effect of methylation) in the two resulting
ILs. For this purpose, the Raman and attenuated total reflection Fourier transform infrared
(ATR/FTIR) spectra are analysed. In the second stage, the 1-methyl-3-propylimidazolium
+
-
+
-
iodide [1-MPrIM ][I ] and 1,2-dimethyl-3-propylimidazolium iodide [1,2-DMPrIM ][I ] ionic
liquids are studied to unravel the impact of propylating the N3 position and converting the
precursors into ionic compounds. Their inter- and intramolecular interactions are analyzed
utilizing the Raman and IR spectra.
2
. Experimental
2
.1 Materials, synthesis, and characterization
The reagents used in this study are 1-methylimidazole (>99%), 1,2-dimethylimidazole (98%),
and propyl iodide (98%). They were purchased from Fluka and used as received.
1
13
H NMR and C NMR spectra were recorded on a DRX 400 MHz NMR instrument. The
chemical shifts (δ) are given in ppm and referenced to the internal solvent signal from TMS
and CFCl , respectively. The IR spectra were recorded on a FT-IR Perkin-Elmer Spectrum
3
-1
BX spectrophotometer (using KBr pellets) with a resolution of 4 cm in the range 4000-650
-1
cm .
The synthesis of 1-methyl-3-propylimidazolium iodide [1-MPrIM ][I ] is depicted in scheme
. Under microwave exposure, a mixture of 1-methylimidazole (7.78 mL, 100 mmol) and
+
-
1
propyl iodide (9.74 mL, 50 mmol) was heated for 3 min at 120°C. The yield of this reaction
was 87%. The reaction mixture was evaporated at reduced pressure and the product was
washed repeatedly with diethyl ether (5 × 20 mL) to remove any excess propyl iodide. Then
the solvent was removed and the product was dried under vacuum for 8 h to obtain a product
with high purity. The resulting 1-methyl-3-propylimidazolium iodide was obtained as a
+
-
yellowish viscous liquid. 1,2-Dimethyl-3-propylimidazolium iodide [1,2-DMPrIM ][I ] was
4

ACCEPTED MANUSCRIPT
+
-
synthesized in a similar way as the [1-MPrIM ][I ] ionic liquid. The yield of the synthesis was
2%. The compound was obtained as a slightly yellow liquid at room temperature.
9
R
R
7
2
7
2
CH
10
3
8
H C
H C
3
3
6
6
N
N
I
N
N
1
1
9
3
3
I-
MW, 120°C
5
4
5
4
H
H
H
H
R= H, Methyl
+
-
+
-
Scheme 1. General synthesis of [1-MPrIM ][I ] and [1,2-DMPrIM ][I ] and numbering of
atoms. MW = microwave.
1
13
The structures of the obtained products are confirmed using H-, C-NMR, and FTIR
spectroscopy, which also confirms the absence of any major impurities. The spectroscopic
1
13
data are given below and the H- and C-NMR spectra are presented in Fig. S1 and S2 of the
supplementary material:
+
-
1
+
+
[
1-MPrIM ][I ]: H-NMR (CDCl ) δ (ppm): 9.08 (1H, s, N C HN), 7.72 (1H, s, N C H), 7.67
3
H
3
2
3
4
+
(
1H, s, N C H), 4.11-4.14 (2H, t, J=4 Hz, N C H2), 3.86 (3H, s, N CH ), 1.79-1.84 (2H, m, J=8 Hz,
1 5 3 8 1 3
+
+
13
N C H C H ), 0.85-0.88 (3H, t, J=8 Hz, N C H C H C H ); C-NMR (CDCl ) δ (ppm): 10.4,
3
8
2
9
2
3
8
2
9
2
10
3
3
C
~
–1
2
3.3, 36.8, 51.1, 122.2, 123.5, 136.0; IR (
ν
/cm ): 3071 [ν(=C-H)], 2962,2875 [ν (C–H)], 1567
[
ν(C=C)], 1455 [δ(C-H)], 1189 [ν(C-N)], 747 [ν(C-H)].
+
-
1
+
[
1,2-DMPrIM ][I ]: H-NMR (CDCl ) δ (ppm): 7.63(1H, s, N C H), 7.60 (1H, s, N C H), 4.06-
3
H
3
4
1
5
+
+
4
.09 (2H, t, J= 4 Hz, N C H ), 3.75 (3H, s, N CH ), 2.58 (3H, s, N C CH N), 1.71-1.77 (2H, m, J=4
3
8
2
5
3
3
2
3
+
+
13
Hz, N C H C H ), 0.86-0.89 (3H, t, J=4 Hz, N C H C H C H ); C-NMR (CDCl ) δ (ppm): 10.9,
3
8
2
9
2
3
8
2
9
2
10
3
3
C
~
–1
1
1.7, 23.2, 36.7, 50.4, 121.3, 122.8, 143.7; IR (
ν /cm ): 3074 [ν(=C-H)], 2963,2876 [ν(C–H)], 1586
[
ν(C=C)], 1456 [δ(C-H)], 1135 [ν(C-N)], 751 [ν(C-H)].
2
.2. FTIR/ATR and FT-RAMAN measurements
The measurements for the structural analysis were performed in the Walloon Agricultural
Research Center (Craw) Belgium. The FTIR/ATR measurements were acquired on a Bruker
Vertex II-70RAM Spectrometer (Bruker Analytical, Madison, WI) operating with a Golden
Gate diamond ATR accessory (Specac Ltd, Slough, United Kingdom). The FTIR/ATR
-1
-1
spectra [600-4000 cm ] were collected with 1 cm nominal resolution by co-adding 64 scans
for each spectrum. The OPUS 6.0 Software for Windows was used for the management of the
instrument.
5

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FT-Raman spectra were acquired on a Vertex 70-RAM II Bruker FT-Raman spectrometer.
This instrument is equipped with a Nd:YAG laser (yttrium aluminium garnet crystal doped
with triply ionized neodymium) with a wavelength of 1064 nm and a maximum power of 1.5
W. The measurement accessory is pre-aligned, only the Z-axis of the scattered light is
adjusted to set the sample in the appropriate position regarding the local measurement point.
The RAM II spectrometer is equipped with a liquid-nitrogen-cooled Ge detector. FT-Raman
-1
-1
spectra [45-4000 cm ] were collected with 1 cm resolution by co-adding 128 scans for each
spectrum at room temperature. The OPUS 6.0 software was used for the spectral acquisition,
manipulation, and transformation.
2
.3. DFT calculations
In order to aid the spectral assignments, the state-of-the-art density functional theory (DFT)
calculations of the four compounds were carried out. For this purpose, isolated molecules or
ion pairs were considered. Hybrid density functional theory, which incorporates Becke’s
three-parameter exchange with the Lee, Yang, and Parr correlation functional [20-21] method
(B3LYP) in the GAUSSIAN-09 program [22] were performed on the four compounds by
employing the 6-31(d, p) basis set for C, N H and LANL2DZ for I. Vibrational frequencies
were calculated for confirmation of local minima on the potential energy surface, which were
found to have no imaginary components.
3
. Results and discussion
In general, the vibrational structure of ILs is determined by the characteristics of the
counterions and their molecular interactions. Therefore, the spectra of ILs are frequently
studied. However, only few studies have been reported on the neutral (non-ionic) precursors
[
23-24]. In the present case, the precursors are [1-MIM] and [1,2-DMIM]. These species carry
no net charges. Hence, the molecular complexity is significantly reduced compared to an IL
as only a single species is present in the sample. The vibrational analysis may therefore
provide interesting insights into the local structure and intramolecular interactions in these
substances. Moreover, it can aid the interpretation of the IL compounds.
Figure 1 shows the IR spectra of 1-methylimidazole [1-MIM], 1,2-dimethylimidazole [1,2-
+
-
DMIM], 1-methyl-3-propylimidazolium iodide [1-MPrIM ][I ], and 1,2-dimethyl-3-
+
-
propylimidazolium iodide [1,2-DMPrIM ][I ]. The individual panels show the enlarged
-1
-1
-1
spectra in the ranges 600-1000 cm , 1000-1700 cm , and 2800-3200 cm . The observed IR
6

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bands and their assignments are listed in Table 1. The Raman spectra of [1-MIM], [1,2-
+
-
+
-
DMIM], [1-MPrIM ][I ], and [1,2-DMPrIM ][I ] are illustrated in Fig. 2, and the observed
Raman bands and their assignments are summarized in Table 2. For an overview, the full IR
and Raman spectra are shown in Figs. S3 and S4 of the supplementary material, respectively.
+
-
Figure 1. FTIR/ATR spectra of [1-MIM], [1,2-DMIM] (a, b and c), [1-MPrIM ][I ], [1,2-
+
-
-1
-1
DMPrIM ][I ] (a', b' and c') in the spectral range: 600-1000 cm , 1000-1700 cm and 2800-
-1
3
200 cm .
7

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+
-
Figure 2. FT-Raman Spectra of [1-MIM], [1,2-DMIM] (a, b, c and d), [1-MPrIM ][I ], and
+
-
-1
-1
[
1,2-DMPrIM ][I ] (a', b' c' and d') in the spectral ranges 0-600 cm (a), 600-1000 cm (b),
-1
-1
1
000-1700 cm (c), and 2800-3200 cm (d).
3
.1 Non-ionic Precursors
8

ACCEPTED MANUSCRIPT
The observed peak wavenumbers in the computational and experimental data of the
precursors are summarized in Table 1. The assignments are made on the basis of the DFT
results and the literature [23-39].
-1
Region 0-600 cm
Unlike FTIR/ATR, the Raman instrument allows recording the vibrational spectrum in the
-1
low wavenumber region down to 45 cm (the spectra of [1-MIM] and [1,2-DMIM] are shown
-1
in Fig. 2a). Below 200 cm , however, the spectra are dominated by a strong interference from
elastically scattered light and thus the features observed should not be over-interpreted.
-1
Beyond 200 cm , the spectra show predominantly peaks that can be assigned to N-CH
3
vibrations.
-1
Region 600-1000 cm
-1
The bands between 600 and 1000 cm can be mainly ascribed to contributions from ring
-1
deformation modes of the imidazole (Fig 1a). The very strong band at 615 cm in [1-MIM] is
assigned to the CN bond oscillation. The DFT suggests a very weak peak for this mode and
no intensity at all for methylated species. The latter is confirmed by the absence of this peak
-1
-1
in the spectrum of [1,2-DMIM]. The IR bands at 734 cm for [1-MIM] and 723/725 cm
-1
(
[
IR/Raman) for [1,2-DMIM] are related to N-C-H wagging modes. The peak at 815 cm for
1-MIM] is assigned to the wagging contribution of the N-C2-H moiety. The rather strong [1-
-1
MIM] peak at 906/907 cm is attributed to the bending vibration of the N-C2-N moiety. The
corresponding band in the methylated species [1,2-DMIM] is weak and observed at 925/920
-
1
cm .
-1
Region 1000-1700 cm
In this spectral region, the methylation has further characteristic effects. For instance, the IR
-1
band at 1438 cm can be assigned to (CH )C2 scissoring in [1,2-DMIM] does not have a
3
corresponding mode in the spectrum of [1-MIM]. On the other hand, [1-MIM] exhibits an N-
-1
C2-H rocking mode at 1230/1231 cm , which is absent in the methylated precursor.
-1
Region 2800-3200 cm
-1
The spectral range 2800-3200 cm contains the CH-stretching vibrations of the alkyl groups
as well as those of the imidazole ring. The IR and Raman spectra in this range are shown in
Fig.1c and 2d, respectively. The signals are relatively weak in the IR owing to the selection
9

ACCEPTED MANUSCRIPT
rules for absorption. The bands at low wavenumber are assigned to CH stretching in the
-1
aliphatic groups. The two weak bands at 2950/2953 and 2952/2953 cm in the spectra of the
precursors, [1-MIM] and [1,2-DMIM], are likely due to the N1-methyl groups. The bands at
-1
higher wavenumber, i.e. around 3130 and 3178 cm , are a result of CH vibrations of the
aromatic ring.
Figure 1(c) shows that the IR spectra of the two precursors [1-MIM] and [1-DMIM] are very
-1
similar in the CH stretching region between 2800 and 3200 cm . On the other hand, the bands
-1
-1
at the highest frequency, 3132 cm for [1,2-DMIM] and 3134 cm for [1-MIM], are assigned
to the symmetric stretching modes of C-H at the 4 and 5 positions of the imidazole ring. The
C(2)-H stretching mode in [1-MIM] is not observed as an individual peak as the frequency is
rather similar to that of the C4,5-H modes.
-1
The weak Raman peaks at 2816 and 2803 cm for [1-MIM] and [1-DMIM], respectively, are
-1
likely due to overtone vibrations methyl rocking modes at 1420 and 1412 cm . Their
appearance at a wavenumber that is slightly below the doubled values of the fundamental
modes can be explained by anharmonicity effects.
-1
Table 1. Computational and experimentally observed wavenumbers (in cm ) of the imidazole
precursors and their vibrational assignments.
1
-methylimidazole
1,2-dimethylimidazole
Assignment
DFT
FTIR Raman DFT
FTIR Raman
3
3
3
184 (3) 3178
155 (3) 3134 3135
153 (3)
3182 (5)
3154 (7)
3179
H-C4-C5-H symmetric stretch
H-C4-C5-H asymmetric stretch
C2-H stretch
3132 3135
3065 (4)
CH (C2) asymmetric stretch
3
3
056
3105 3109
3014
3056 (11) 3108 3109
CH asymmetric stretch
3
(
10)
022
20)
3
3023 (18)
CH asymmetric stretch
3
(
2
2
989 (23) 2999
942 (28) 2952 2953
CH (C2) asymmetric stretch
3
CH (C2) symmetric stretch
3
2
959
2950 2953
CH symmetric stretch
3
(
47)
2
2
917
816
2928
2803
overtone
1535 (21) 1527
CN stretch
1
1
511 (7) 1515 1516
1480 (6)
CH scissor
N3-C4-H rock + CH3 rock
3
1
1
502 (24) 1504 1504
460 (13) 1469 1465
CH (C2) rocking
3
451 (7)
1467
1452 (1)
451 (3)
1
CH (C2) scissor
3
1
0

ACCEPTED MANUSCRIPT
1438 (22) 1438
CH (C2) scissor + CH rocking
3
3
1
418
1419 1420
1408 (43) 1411 1412
CH rock
3
(
19)
1
375 (1)
1348 (6)
1280 (35) 1280 1284
CH (C2) rock
N1-C2-N3 asymmetric stretch
CN stretch + CH rocking
3
1
1
366 (1)
349 (3) 1350 1378
1350 1345
3
1
276
1284 1285
1230 1231
H-C4-C5-H rock
N1-C2-H rock
(
27)
236
20)
1
(
1
186
1189 (2)
1186 1202
1
116
1135
1148 (10) 1132 1143
N1-C5-H rock
(
15)
1
107 1109
1
(
1
(
070
11)
047
11)
1076 1077
1075 (9)
1083 1084
1054
H-C4-C5-H scissor
1054
CH scissor
3
1
014 (9) 1027 1029
N1-C2-N3 symmetric stretch
CH3 wag
971 (14)
985
8
8
7
6
6
6
6
88 (11) 906
31 (2)
82 (25) 815
99 (18) 734
907
900 (2)
822 (5)
925
920
725
642
N1-C2-N3 bend
H-C4-C5-H twist
N1-C2-H wag
702 (21)
723
N3-C4-H wag
60 (4)
54 (13)
04 (4)
663
668
663 (15)
608 (0)
675
640
CN bond oscillation
CN bond oscillation
615
3
.2 Ionic liquids
The observed peak wavenumbers in the computational and experimental data of the ionic
liquids are summarized in Table 2. The assignments are made on the basis of the DFT results
and the literature [23-39].
-1
Table 2. Computational and experimentally observed wavenumbers (in cm ) of the imidazole
precursors and their vibrational assignments.
1
-methyl-3-
propylimidazolium iodide
DFT FTIR Raman DFT
1,2-dimethyl-3-
propylimidazolium iodide
Assignment
FTIR
3174
3121
Raman
3
3
3
3
3
316 (2)
198 (6)
086 (1)
060 (6)
043 (11)
3222 (2)
3204 (5)
3094 (0)
3079 (7)
3056 (7)
3171
H-C-C-H symmetric stretch
H-C-C-H asymmetric stretch
3138 3145
3071 3076
3122
CH asymmetric stretch
3
CH asymmetric stretch
3
3074
3074
CH (propyl chain) asymmetric
3
stretch
3
035 (10)
3040 (21)
CH (propyl chain) asymmetric
2
1
1

ACCEPTED MANUSCRIPT
stretch
3
3
024 (25)
000 (4)
3038 (7)
CH (propyl chain) asymmetric
stretch
3
3037 (13)
CH (propyl chain) asymmetric
2
stretch
3
2
015 (19)
993 (10)
CH (propyl chain) asymmetric
stretch
2
CH (propyl chain) asymmetric
2
stretch
2
2
983 (22)
974 (47)
2975 (42)
2957 (13)
2972
2944
CH symmetric stretch
3
2962 2973
2945
2962
CH (propyl chain) symmetric
2
stretch
2
2
954 (27)
945 (38)
2948 (39)
CH (propyl chain) symmetric
3
stretch
2933 2927
CH (propyl chain) symmetric
2
stretch
2
2
939 (41)
925 (98)
2933
2875
CH (C ) symmetric stretch
3
2
CH (C ) symmetric stretch
3
2
2
875
CH
2
(propyl chain) symmetric
stretch
2
826
2826
overtone
2
1
1
758 (1140) 2849
C2-H stretch
C -C + C-N stretch
566 (16)
552 (80)
1585 1564
1535
1580 (26)
522 (76)
1567
1515
4
5
N -C -H rock
1
2
1
N1-C2-N3 stretch + CH (C )
3 2
rock
1
1
1
1
1
476 (13)
474 (15)
471 (7)
458 (3)
455 (13)
CH (propyl chain) scissor
3
1504 (24)
1481 (8)
CH scissor
3
CH (propyl chain) scissor
2
CH (propyl chain) scissor
2
1456
1446
1472 (8)
CH scissor
3
1470 (10)
CH (propyl chain) scissor
3
1
449 (6)
1467 (4)
CH (propyl chain) scissor
2
1
1
1
1
462 (6)
454 (4)
433 (8)
424 (27)
CH (propyl chain) scissor
2
1456
1419
CH scissor
3
1446
1378
CH (C ) scissor
3
2
CH scissor
3
1
1
425 (8)
413 (21)
1429
1416
1401 (8)
1388 (1)
CH rocking
3
CH (propyl chain) rocking + C-N
2
stretch
1
387 (7)
1386
CH (propyl chain) rock
3
1
385 (6)
CH (C ) rock
3
2
1
1
380 (6)
364 (5)
1382
1351
1379 (10)
1368 (4)
1332 (9)
1304 (2)
1299 (8)
1382
1351
1328
CH (propyl chain) rock
2
CH (propyl chain) rock
2
1
328 1337
1242
1303
N1-C2-N3 bend
1
1
1
1
324 (1)
305 (2)
292 (5)
276 (25)
CH (propyl chain) rock
2
CH (propyl chain) rock
2
CH (propyl chain) rock
2
1267 1271
235
1253 (6),
1243
H-C -C -H rock
4
5
1227 (17)
1
1
2

ACCEPTED MANUSCRIPT
1
1
1
231 (4)
163 (102)
124 (4)
1189
1164
1187 (12)
1186
CH (propyl chain) twist
2
N -C -H rock
1
2
CH rock
3
1123 (17)
1135
CH (C ) rock
3
2
1
1
1
1
1
120 (7)
099 (4)
084 (1)
074 (29)
020 (1)
1113
1117 (4)
1095 (7)
1091 (5)
1114
1058
CH (propyl chain) twist
2
1087
1056
1043
H-C -C -H scissor
4
5
CH rock
3
C-C (propyl chain) stretch
CH (C ) wag
1020 (48)
1056
752
3
2
1
9
9
8
7
7
008 (3)
99 (24)
20 (39)
84 (6)
35 (12)
23 (15)
1021
904
Ring breathing
N -C -H wag
1
2
880 (6)
737 (2)
747 (12)
880
760
CH (propyl chain) wag
3
CH (propyl chain) wag
2
748
CN stretch
7
24
7
12 (38)
696 696
708 (16),
703
704
667
H-C -C -H wag
4
5
703 (10)
6
6
52 (12)
09 (1)
646 659
615 619
634 (53)
598 (103)
663
CN bond oscillation
CN bond oscillation
-1
Region 0-600 cm
-1
The Raman spectra in the low wavenumber region down to 45 cm are shown in Fig. 2a'.
-1
Below 200 cm , the IL spectra are dominated by a strong interference from elastically
-1
scattered light like for the non-ionic precursors. Beyond 200 cm , the spectra show
predominantly peaks that can be assigned to N-CH vibrations.
3
-1
Region 600-1000 cm
-1
The bands between 600 and 1000 cm can be mainly ascribed to contributions imidazolium
-1
cation (Fig 1.a'). Like in [1-MIM], we noticed a very strong band at 615/619 cm in [1-
+
-
MPrIM ][I ]. This band is assigned to CN bond oscillations. The corresponding mode in the
species with a methylated C2 position is absent. However, according to the DFT simulations,
it is likely located outside the range accessible by the IR instrument. The IR bands of the
-1
-1
precursors at 734 cm for [1-MIM] and 723/725 cm for [1,2-DMIM] are shifted after
-1
+
-
-1
+
-
propylation to 748 cm for [1-MPrIM ][I ] and at 752 cm for [1,2-DMPrIM ][I ],
respectively. These bands are assigned to vibrations of the N-C-N moiety and therefore are
strongly affected by methylation at C2 and propylation at C3.
-1
Region 1000-1700 cm
1
3

ACCEPTED MANUSCRIPT
As for the precursors, the methylation has some characteristic effects in this spectral range.
-1
+
-
For instance, the strong band at 1164 cm for [1-MPrIM ][I ] can be assigned to (N)-C2-H
-1
rocking. The IR modes at 1351 cm are assigned to CH rocking in the propyl chain. In this
2
-1
range, the Raman spectrum exhibits also bands at 1350/1348 cm for [1-MIM], at 1350/1345
-1
cm for [1,2-DMIM], which are due to CN stretching and CH rocking. The corresponding
3
-1
+
-
-1
bands in the ionic compounds are observed at 1337 cm for [1-MPrIM ][I ], and at 1342 cm
+
-
-1
for [1,2-DMPrIM ][I ]. The medium intensity bands centered around 1456 cm in the IR
+
-
+
-
spectra of both propylated compounds [1-MPrIM ][I ] and [1,2-DMPrIM ][I ] can be assigned
-1
to CH scissoring vibrations. Moreover, the strong band centered around 1515 cm in the
3
+
-
Raman spectrum of [1,2-DMPrIM ][I ] is assigned to a NC(2)N stretching band.
-1
Region 2800-3200 cm
-1
Again, the spectral range 2800-3200 cm contains the CH-stretching vibrations of the alkyl
groups as well as those of the imidazolium ring. The latter are likely carrying information
about possible interionic hydrogen bonding interactions if present. The IR and Raman spectra
in this range are shown in Fig. 1c' and 2d', respectively. Both IL compounds exhibit bands at
-1
2
875 and 2933 cm , which can be assigned to CH vibrations in the propyl chain. Moreover,
-1
the ILs exhibit a strong band at 2962 cm , which is again a contribution of the propyl chain.
-1
The bands at higher wavenumber, i.e. beyond 3100 cm , is a result of CH vibrations of the
-1
aromatic ring. Only the C2-H stretching mode is significantly shifted to 2849 cm as a result
of interionic interactions. A detailed discussion is provided in the next section.
3
.3 Effects of interionic interactions
-1
After propylation, the spectra in the region 2800-3200 cm become more distinctly divided
into two regions (see Figure 1c'). The presence of the propyl side chains results in three joint
-1
peaks in the region below 3000 cm . These bands can be attributed to antisymmetric and
symmetric vibrations of the methyl and methylene groups. The bands at wavenumbers above
-1
3
100 cm can be assigned to the aromatic CH stretching modes, which may be influenced by
-
-1
anion-cation interactions via C(2,4,5)-H⋯I . For example, the band at 3138 cm in the
+
-
spectrum [1-MPrIM ][I ] was assigned to C(4,5)-H stretching. In the methylated IL [1,2-
+
-
-1
DMPrIM ][I ], this band virtually disappears, while a very weak mode at 3174 cm and a
-1
shoulder band at 3121 cm appear. These bands are marked by arrows in Fig. 1c'. A similar
effect can be observed in the Raman spectra, see Fig. 2d'. This observation can be explained
1
4

ACCEPTED MANUSCRIPT
+
-
in terms of the interionic interactions. In [1-MPrIM ][I ], the C2-H is the predominant
interaction site due to the polarization of this bond and the resulting slightly acidic character.
-1
+
-
Consequently, it is significantly red-shifted to 2849 cm in [1-MPrIM ][I ] while it has
virtually the same frequency as the C(4,5)-H modes in the non-ionic precursor [1-MIM],
where no hydrogen bond acceptor is present and the C2-H bond is not acidic. This interaction
site is removed, when the IL is methylated (see Fig. 3). As a consequence, interionic
-
interactions at C(4,5)-H become favorable. When the iodide ion forms a C-H⋯I hydrogen
bond at either the C4 or the C5 position, the interactions will result in two distinguishable C-
H bonds [30]. One will interact with the iodide and hence the corresponding C-H stretching
-1
mode will be red-shifted (3121 cm ). The other hydrogen atom will not (or at least less) be
involved in the interactions. This, combined with the induced re-distribution of charge in the
-1
imidazolium ring leads to a blue-shift that manifests as the peak at 3174 cm . Endo et al. [40]
+
-
observed a similar behavior in the Raman spectra of the ILs [BMIM ][I ] and [1,2-
+
-
DMBuIM ][I ]. The red-shift was also attributed to the interionic interactions at the C4 and
C5 position, but the weak blue-shifted band was not assigned. The peak-split mechanism
proposed above could be a possible explanation for their data.
The experimental results presented in this study reveal that propylating the precursors [1-
MIM] and [1,2-DMIM] and converting them into ionic liquids has a significant impact on the
vibrational structure. This is particularly true for the modes in the fingerprint region 600-1700
-1
cm . On the other hand, replacing the C(2) proton with a methyl group shows the most
significant effects in the high frequency region, where the vibrational bands of the C(2)-H and
C(4/5)-H groups of the imidazolium ring have their stretching modes. The methylation at the
C(2) position removes the predominant site for interionic hydrogen bonding. As a
consequence, the imidazolium CH groups at C(4) and C(5) become more involved in the
-
interactions. It was found that when the I anion interacts with one of the two initially very
similar and vibrationally degenerate CH groups, a split of the vibrational band can be
observed. This results in a red-shifted contribution for the hydrogen-bonded group and a blue-
shifted one for the non-hydrogen-bonded group. Other studies also reported similar effects
upon methylation of the C2 position in imidazolium ILs. Noack et al. studied imidazolium
NTf -ILs with and without a methyl group at the C2 position using vibrational and NMR
2
spectroscopy [25]. They found that the methylation removes the strong positive partial charge
of the hydrogen atom leading to a decrease of the electron density at the C2 atom, which is
not compensated by the +I-effect of the methyl group or stronger hydrogen bonding at C4/C5-
1
5

ACCEPTED MANUSCRIPT
H. Namboodiri et al. studied the same set of ILs by femtosecond coherent anti-Stokes Raman
scattering (CARS) spectroscopy [41]. They observed that the ILs exhibiting a proton at the C2
position can transfer vibrational energy between the cation and the anion via hydrogen bonds.
This ultrafast energy transfer was disabled when the C2 position was methylated. In a follow-
up study, however, they showed for 1-ethyl-3-methylimidazolium ethylsulfate that the
presence of a C2-H group is not the only pre-requisite for this energy transfer and that the
underlying mechanisms are yet to be fully understood [42]. Furthermore, Lehmann and
coworkers also observed the importance of the C(2) position in two important co-conformers
+
-
in IL [1-MEIM ][Cl ] (on-top and in-plane) [26]. They found an increase in intensity and a
large red-shift of the C2-H stretching frequency for the in-plane compared to the on-top
conformation. Our experimental IR data suggest that a significant fraction of ion pairs interact
via hydrogen bonds, hence arrange themselves in-plane. The existence of on-top conformers
however cannot be excluded.
2
CH3
2
H C
H C
3
3
N
1
N
N
N
1
3
3
5
4
5
4
Figure 3. C2,4,5-H positions of H-bond abilities in two investigated ILs.
4
. Conclusion
+
-
Two ionic liquids, 1-methyl-3-propylimidazolium iodide [1-MPrIM ][I ] and 1,2-dimethyl-3-
+
-
propylimidazolium iodide [1,2-DMPrIM ][I ], were successfully synthesized from the
precursors 1-methylimidazole [1-MIM] and 1,2-dimethylimidazole [1-DMIM], respectively.
1
13
The structures of the synthesized ILs were confirmed by H, C-NMR, and IR spectroscopy.
A detailed structural study of the ILs and their precursors was carried out using Raman and IR
-1
-1
spectroscopy in the wavenumber ranges [45 to 3500 cm ] and [600 to 4000 cm ],
respectively. The spectra allowed unraveling the effects of (1) methylation of the C2 position
1
6

ACCEPTED MANUSCRIPT
in the imidazolium ring, and of (2) converting the neutral compound into an ionic species with
a propyl side chain. The chemical conversion via propylation adds significant complexity to
the vibrational spectra, in particular, to the Raman spectrum. This can be attributed to the high
Raman activity of the additional CH bending and stretching modes in the propyl side chain.
Moreover, it was found that blocking the predominant interaction site at C2 with a methyl
group removed the possibility for hydrogen bonding interactions at this position. The iodide
ion is then interacting with the hydrogen atoms at C4 and C5. The spectra suggest that this
interaction is basically the formation of a hydrogen bond with one of the two C-H groups.
This was evidenced by a splitting of an initially degenerate C4,5-H stretching vibration into a
red- and a blue-shifted peak. The red-shift indicates the formation of a hydrogen bond, while
the blue-shift is a result of the charge re-distribution along the non-hydrogen-bonded C-H
bond.
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42] J. Kiefer, M. Namboodiri, M.M. Kazemi, A. Materny, Time-resolved femtosecond
CARS of the ionic liquid 1-ethyl-3-methylimidazolium ethylsulfate, J. Raman Spectrosc. 46
2015) 722–726. doi:10.1002/jrs.4692
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ACCEPTED MANUSCRIPT
Highlights:
•
•
The vibrational analysis of nonionic precursors can aid the understanding of ionic liquids.
Methylation of the C2 position in imidazolium ILs triggers interionic hydrogen bonding at
C4/5.
•
Iodide interactions with either C4-H or C5-H are evident as a split of the initially degenerate
stretching modes.