Structural Arrangement and Computational Exploration of Guanidinium-Based Ionic Liquids with Benzoic Acid Derivatives as Anions

Article abstract of DOI:10.1021/acs.cgd.8b01754

Crystallization of ionic liquids (ILs) is of great interest for determining the crystal structure and molecular interactions. Hydrogen bonding is an important noncovalent interaction, which is related to the properties of ILs. In this context, we have prepared 1,1,3,3-tetramethylguanidine (TMG)-based ILs with different benzoic acid derivatives as anions and the crystal structures of these ILs were determined at low temperature (173(2) K) to understand the interactions and their network formation. Using Hirshfeld surface analysis, the intermolecular interactions in the crystal packing of ILs were analyzed and quantified. Density functional theory (DFT) calculations were carried out to ascribe the thermodynamic properties of these ILs. The molecular geometry of the guanidinium ionic liquids was optimized, and the electronic properties and HOMO and LUMO energies were calculated using DFT at the B3LYP/6-311G++(d,p) level. The characterization of the ILs was done by TGA, DSC, NMR, and IR.

Full text of DOI:10.1021/acs.cgd.8b01754

Subscriber access provided by University of South Dakota
Article
Structural Arrangement and Computational Exploration of Guanidinium
Based Ionic Liquids with Benzoic Acid Derivatives as Anion
V. Ramkumar, Indrajit Das, and Ramesh L. Gardas
Cryst. Growth Des., Just Accepted Manuscript • Publication Date (Web): 26 Mar 2019
Downloaded from http://pubs.acs.org on March 27, 2019
Just Accepted
“Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted
online prior to technical editing, formatting for publication and author proofing. The American Chemical
Society provides “Just Accepted” as a service to the research community to expedite the dissemination
of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in
full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully
peer reviewed, but should not be considered the official version of record. They are citable by the
Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore,
the “Just Accepted” Web site may not include all articles that will be published in the journal. After
a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web
site and published as an ASAP article. Note that technical editing may introduce minor changes
to the manuscript text and/or graphics which could affect content, and all legal disclaimers and
ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or
consequences arising from the use of information contained in these “Just Accepted” manuscripts.
is published by the American Chemical Society. 1155 Sixteenth Street N.W.,
Washington, DC 20036
Published by American Chemical Society. Copyright © American Chemical Society.
However, no copyright claim is made to original U.S. Government works, or works
produced by employees of any Commonwealth realm Crown government in the course
of their duties.

Page 1 of 37
Crystal Growth & Design
1
2
3
4
5
6
Structural Arrangement and Computational Exploration of Guanidinium
Based Ionic Liquids with Benzoic Acid Derivatives as Anion
7
8
9
V. Ramkumar, Indrajit Das and Ramesh L. Gardas*
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Department of Chemistry, Indian Institute of Technology Madras, Chennai – 600036, India
*Corresponding author: e-mail address: gardas@iitm.ac.in
URL: http://www.iitm.ac.in/info/fac/gardas
Tel.: +91 44 2257 4248; Fax: +91 44 2257 4202
1
ACS Paragon Plus Environment

Crystal Growth & Design
Page 2 of 37
1
2
3
4
Abstract
5
6
7
8
Crystallisation of ionic liquids [ILs], is of great interest for determining the crystal structure and
molecular interactions. Hydrogen bonding is an important non-covalent interaction, which is
related to the properties of ILs. In this context, we have prepared 1,1,3,3-tetramethylguanidine
[TMG] based ionic liquids (ILs), with different benzoic acid derivatives as anions and the crystal
structure of these ILs were determined at low temperature (173(2) K) to understand the
interactions and their network formation. Using Hirshfeld surface analysis the intermolecular
interactions in crystal packing of ILs were analyzed and quantified. Density functional theory
(DFT) calculation was done to ascribe the thermodynamic properties of these ILs. Molecular
geometry of the guanidinium ionic liquids was optimized and the electronic properties and HOMO
and LUMO energies were calculated using the DFT at B3LYP/6-311G++(d,p) level. The
characterization of the ILs was done by TGA, DSC, NMR and IR.
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
1. Introduction
Ionic liquids (ILs) are green solvents possess fascinating physical properties and provide good
opportunity to industrialists furnishing alternative solvents, which are environmental friendly1-2.
They generally exist as anions and cations in the liquid-state. The importance of hydrogen bonding
in ILs is a subject of debate for several decades and there is a general view that ILs are dominated
by Coulombic interactions. Determination of the type of interaction, be it Coulombic interaction,
dispersive forces, hydrogen bonding, is the need of the hour3. Seddon et al. first reported the
presence of hydrogen bonding in imidazolium salts4. Later, the presence of hydrogen bonded ion-
ion interaction in molten salt was studied by Dymek et. al.5 and several studies were done using
techniques like NMR, X-Ray diffraction, IR6 and theoretical studies7 to substantiate the presence
of hydrogen bonding in ILs .
Hydrogen bonding interaction in ILs is considered crucial for the evaluation of ILs structure and
it is the most important interaction in molecular crystals. This also distinguishes protic ILs from
other ILs, which form the proton-donor-acceptor sites and the network structure. The attractive
interaction between a hydrogen atom from a molecule or a molecular fragment and an atom or a
group of atoms in the same or different molecule, which provide evidence for the bond formation
is referred to as hydrogen bonding8. Hydrogen bonding is formed during the synthesis as an ion
transfer and this is a trade mark of ILs9. For understanding the modes of interaction and
2
ACS Paragon Plus Environment

Page 3 of 37
Crystal Growth & Design
1
2
3
4
5
6
conformational influence of cations and anions on the lattice energies, melting points and general
10
behavior of ILs, the determination of hydrogen-bonding environments is critical . A careful
selection of anion and cation is essential as they influence the molecular structure and packing
resulting in several distinct conformations11. Anions of different size, symmetry and hydrogen
bonding capability are chosen to study the effect of the anion on crystalline behavior12. Ralf
Ludwig et al.13,6 have used X-ray diffraction tool to determine the existence of H-bonding between
anion and cation, which is a strong evidence in both solid and liquid states.
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
1,1,3,3-tetramethylguanidinium [TMG] based ILs have wide applications such as SO2
capture14,15,16,17, production of biofuels18, synthesis of biolubricants19, as catalysts for aldol
reaction20 and as a medium for Mannich reactions21. The gas solubility22 with efficient and
reversible CO2 capture through [TMG] based ILs was studied by Lei et al.23.
The X-ray crystal structure of ILs is difficult to study, as they are in the liquid state at room
temperature and it is a challenging task to crystallize them24. A modern day technique is used now-
a-days for structure determination of compounds with low melting points by in situ crystallisation
using optical heating and crystallization device (OHCD)25. Winterton et al.26 have studied
imidazolium based ILs by in situ cryo-crystallisation method using OHCD. Deepak et al.
demonstrated the fact, that nature of intra and intermolecular interactions, which evolve on
crystallizing liquids are similar to those observed in the solid-state27. TMG based ILs are rare and
very few studies on crystal structures of [TMG]Cl28, [TMG]Br29, [TMG][Ac]30, [TMG][IM]23,
[TMG][NTf2]31, [TMG][H2PO4]32 and [TMG][NO3]33 have been reported. Unlike other ILs, only
less work on the study of cation-anion interaction has been done for TMG based ILs but recently
a detailed comparison of thermophysical properties of [TMG][BEN] and [TMG][SAL] in the
presence of water has been reported by Akhil et al.34. Carboxylic acid molecules, which usually
form cyclic hydrogen-bonded dimers, are a commonly observed motif in the solid-state and
derivatives of benzoic acid were used for this purpose as anions to study the intermolecular
interactions and network formation.
Theoretical calculations are one way of accessing the fundamental nature of H-bonding in ILs,
providing information through electronic structure on the geometry and vibrational spectra7.
Studies to investigate H-bonding interactions in imidazolium based ILs by theoretical method
confirmed that C–H… H-bonds existed between cations and anions35. H-bonds in imidazolium-
based ILs perturb the Coulombic forces to deviate from charge symmetry and significantly
3
ACS Paragon Plus Environment

Crystal Growth & Design
Page 4 of 37
1
2
3
4
5
6
influence the physicochemical properties due to cation.10 The H-bonding through C-H and N-H
bonds of the cations has significant influence on melting points, viscosities and enthalphy of
vaporization in some ILs35.
7
8
9
Thermal studies also reveal the stability and interaction of H-bonding in ILs, which govern the
important properties like glass transition temperature (Tg), melting point (Tm), boiling-point (Tb),
viscosity and iconicity of the ILs. By modifying the interaction between the cation and anion, the
Tg values can be controlled36. It is also observed that the thermal behavior of salts strongly depends
on the anion and its shape12.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Hirshfeld surface analysis for the ILs was performed to get an insight and quantify the
intermolecular interactions in the crystal packing.
Hence, four TMG based ILs with different anions were synthesized and their hydrogen bonding
and network arrangement were studied by single crystal XRD. Thermal studies and theoretical
calculations with temperature dependence of the thermodynamic functions, HOMO-LUMO
analysis to predict the charge transfer between the anion and cation, vibrational energy, and
minimum energy studies using B3LYP calculations with 6-311++G(d,p) basis set were carried
out.
2. Experimental Section
2.1 Synthesis and Characterization Data
The TMG unit is highly basic (pKa=13.6)37 and so it can be easily protonated in presence of any
weak acid. We have synthesized four ILs by neutralization method using equimolar mixture of
TMG (base) with different acids. TMG was added to equimolar amount of acids namely, benzoic
acid, salicylic acid, cinnamic acid and para toluenic acid in the presence of methanol (scheme 1).
The reaction was continuously stirred for 24 h under N2 atmosphere at room temperature. The
excess solvent was removed by rotary evaporator and the product was dried under high vacuum
for nearly 24 h under N2 atmosphere at room temperature. The ILs formed are
tetramethylguanidinium
benzoate
[TMG][BEN],
tetramethylguanidinium
salicylate
[TMG][SAL], (semi-solid in nature), tetramethylguanidinium cinnamate [TMG][CNA] and
tetramethylguanidinium tolunate [TMG][PTA] (semi viscous in nature).
1
13
The obtained ILs were characterized by H NMR, C NMR, differential scanning calorimetry,
thermogravimetric analysis and IR spectrophotometry.
4
ACS Paragon Plus Environment

Page 5 of 37
Crystal Growth & Design
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Scheme 1. Synthesis of TMG based ILs with benzoic acid derivatives
1H NMR and 13C NMR spectra were recorded on a Bruker Avance-III spectrometer operating at
500 MHz with tetramethylsilane as the internal standard. The following abbreviations are used for
the multiplicity of signals in 1H NMR: (s) singlet, (d) doublet, (t) triplet, (q) quartet, (m) multiplet.
NMR spectra (CDCl3, 25 oC) were recorded using Bruker Avance 500 MHz NMR spectrometer
giving peak positions for the four ILs (Figures S1-S4).
1
[TMG][BEN]: H NMR (500 MHz) δ (ppm): 2.87 (s, 12H, CH3), 4.85 (broad singlet, 2H, NH2),
7.29 (m, 3H, aromatic), 8.02 (d, J = 6.9 Hz, 2H, aromatic).
13C NMR (125 MHz) δ (ppm): 39.6 (N-CH3), 127.6 (aromatic), 129.4 (aromatic), 129.6
(aromatic), 138.7 (aromatic), 162.3 (COO-), 172.6 (N=C).
[TMG][SAL]: 1H NMR (500 MHz) δ (ppm): 2.93 (s, 12H, CH3), 6.75 (t, J = 7.15 Hz, 1H,
aromatic), 6.86 (d, J = 7.15 Hz, 1H, aromatic), 7.26 (t, J = 7.47 Hz, 1H, aromatic), 7.88 (d, 1H, J
= 7.47 Hz, aromatic), 8.75 (broad s, 2H, NH) 14.45 (s,1H, OH)
13C NMR (125 MHz) δ (ppm): 39.8 (N-CH3), 116.4 (aromatic), 117.6 (aromatic), 119.7(C-C=O,
aromatic), 130.7 (aromatic), 132.5 (aromatic), 162.1 (C-OH), 162.4 (COO-), 174.5 (N=C).
5
ACS Paragon Plus Environment

Crystal Growth & Design
Page 6 of 37
1
2
3
4
5
6
[TMG][CNA] : 1H NMR (500 MHz) δ (ppm): 2.95 (s, 12H, CH3), 3.47 (broad s, 2H, NH) 6.57
(d, J = 15.6 Hz, 1H, olefin), 7.25 (t, J = 7.3 Hz, 1H, aromatic), 7.30 (t, J = 7.94 Hz, 2H, aromatic)
7.4 (d, J = 15.6 Hz, 1H, olefin), 7.48(d, J = 7.94 Hz, 2H, aromatic).
7
8
9
13C NMR (125 MHz) δ (ppm): 39.8 (N-CH3), 116.5 (aromatic), 127.6 (aromatic), 128.4(C-C=O,
aromatic), 128.6 (aromatic), 136.7 (aromatic), 138.5 (aromatic), 163.2 (C=O), 173.6 (N=C).
[TMG][PTA]: 1H NMR (500 MHz) δ (ppm): 2.33 (s, 3H, CH3), 2.90 (s, 12H, N-CH3), 4.90 (broad
s 2H, NH), 7.10 (d, J = 7.90 Hz, 2H, aromatic), 7.91(d, J = 7.90 Hz, 2H, aromatic).
13C NMR (125 MHz) δ (ppm): 21.5 (CH3), 39.70 (N-CH3), 128.3 (aromatic), 129.6 (aromatic),
135.9 (aromatic), 139.70 (aromatic), 162.6 (C=O), 172.9 (N=C).
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
The water content of each ionic liquid after drying was determined by Karl Fisher titration method
before measuring their thermophysical properties. The measurement was made with the help of
Metrohm KF 870 Titrino in triplicates for each IL and the average values are presented in Table
S1.
2.2 X-ray Diffraction Structure Determination
The x-ray crystal structure was determined for all the TMG based ILs. Single-crystal X-ray
structure data collection was performed on a Bruker AXS Kappa Apex II CCD diffractometer with
graphite monochromated Mo Kα radiation (λ = 0.71073 Å). The ILs were taken in a vial and heated
and cooled suddenly using liquid nitrogen and this process was repeated for several times till small
crystals appeared. Crystal was picked carefully from the sides of the vial with a cryo-loop with a
constant flow of liquid nitrogen. Cryo-loop was mounted onto the goniometer and the data were
collected at 173 K. The unit cell determination was done with 32 frames at three different
orientations of the detector to collect reflections and the program APEX2-SAINT38 was used for
finding the unit cell parameters. The frames were integrated with the Bruker SAINT software
package using a narrow-frame algorithm. The integration of the data was done to a maximum θ
angle of 25.0°.
The program SADABS was used for absorption correction using multi-scan procedure. The
structures were solved by direct methods using SHELXTL software39 and refined by full-matrix
least squares techniques using SHELXT computer program39.
All the hydrogens, except the nitrogen hydrogen were fixed at chemically meaningful positions
and riding model refinement was applied. The hydrogen attached to the nitrogen was located
6
ACS Paragon Plus Environment

Page 7 of 37
Crystal Growth & Design
1
2
3
4
5
6
7
8
9
through the difference Fourier map and refined with isotropic thermal parameters. The hydrogen
atom bound to carbon atoms was constrained to a distance of C–H = 0.93–0.97 Å and Uiso(H) =
1.2Ueq(C) and 1.5Ueq(C). The hydrogen atom associated with the oxygen atom was allowed to ride
on the parent atom with a distance of 0.82 Å and Uiso(H) = 1.5Ueq(C). Molecular graphics were
drawn using ORTEP3240 and Mercury programs41. The crystal data with refinement details are
summarized in Table 1. The refinement of all the crystals was done using the data collected at low
temperature with the help of kryoflex cooling system.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
3. Result and Discussion
3.1 Crystal Studies
Figure 1. (a) ORTEP of [TMG][BEN] with 30% ellipsoid probability (b) N-H…O hydrogen
bonding connected through 22 (6) chain motif (c) N-H…O interactions forming
퐶
chains and wavelike pattern.
7
ACS Paragon Plus Environment

Crystal Growth & Design
Page 8 of 37
1
2
3
4
5
6
7
8
9
The structure of [TMG][BEN] was solved in the orthorhombic crystal system, space group
P212121. The other three crystal structures were solved in the monoclinic crystal system, the space
groups were C2/c, P21/c and P21/c for [TMG][SAL], [TMG][CNA] and [TMG][PTA],
respectively. The ORTEP40 diagrams for the four ILs are drawn with 30% probability in Figure
1a, 2a, 3a and 4a, respectively. In all the four ILs, the TMG core (N=C(N)2) shows a nearly ideal
trigonal-planar geometry, as expected for sp2 hybridization and the anions benzoate, salicylate,
cinnamate and para-tolunate show a planar structure due to the presence of sp2 carbon atoms42. A
similar [TMG] cation structure has already been reported by Berg et. al.31 with [NTf2] as anion.
An intramolecular hydrogen bonding C-H…O is seen particularly in [TMG][SAL] and N-H…O
intermolecular hydrogen bonding is seen in all the four ILs. Generally the H…A ( A = acceptor)
distance of < 2.2 Å is described as strong hydrogen bonds.43 The C-H…O and the N-H…O
interaction distance and angles of the ILs show that the interactions are strong and they are
hydrogen bonds in nature and are observed in all the four ILs.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 2. (a) ORTEP of [TMG][SAL] with 30% ellipsoid probability (b) N-H…O hydrogen
bond chains forming a “butterfly like pattern (c) N-H…O interactions along c-axis.
8
ACS Paragon Plus Environment

Page 9 of 37
Crystal Growth & Design
1
2
3
4
5
6
7
8
The two [TMG] cations are hydrogen–bonded to two anions related to each other by an inversion
center in the case of [TMG][SAL], [TMG][PTA] and [TMG][CNA] giving rise to a network of
molecules connected by hydrogen bonds in the form of a tetramer. The tetramer seems to be planar
in the case of [TMG][PTA] and [TMG][CNA] but it is slightly folded like an ‘envelope’ in the
case of [TMG][SAL] giving rise to a ‘butterfly like’ network pattern (Figure 2b).
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 3. (a) ORTEP of [TMG][CNA] with 30% ellipsoid probability (b) N-H…O hydrogen
bonding connected through 4(12) ring motif (c) N-H…O interactions along c-axis.
푅4
The single crystal data of [TMG][BEN] clearly revealed that the cation was involved in hydrogen
퐶2
bonding interactions with two benzoate anions forming a chain 2 (6) graph set44 interaction
(Figure 2b). The NH2 group in [TMG] cation facilitates formation of hydrogen bond with the
oxygens atoms of two anions. This forms hydrogen bonds with a distance of 1.91(2) Å and 1.93(2)
Å (Table 2). N-H…O intermolecular hydrogen bond result in the formation of a “wavelike” pattern
along the b-axis (Figure 1c). It is interesting to note that benzoic acid without TMG crystallises to
9
ACS Paragon Plus Environment

Crystal Growth & Design
Page 10 of 37
1
2
3
4
5
6
form a herringbone packing pattern by C-H…π interactions forming a dimer45. The single crystal
data of [TMG][SAL] showed that the cation was involved in hydrogen bonding interactions with
two salicylate anions forming a ring type 4(12) graph set interaction. The NH group in [TMG]
푅4
2
7
8
9
cation facilitates the formation of hydrogen bond with the oxygen atoms of two anions. This forms
a hydrogen bond with a distance of 1.87(2) Å and 2.08(2) Å (Table 2). The network arrangements
of the cation and the anion of the molecules along the c-axis is shown in Figure 2c.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 4. (a) ORTEP of [TMG][PTA] with 30% ellipsoid probability (b) N-H…O hydrogen
bonding connected through 4(12) ring motif (c) N-H…O interactions forming
푅4
parallel shown along b-axis.
A similar ideal trigonal-planar arrangement of the cation core N=C(N)2 was also seen in
[TMG][CNA]. The cinnamate anion phenyl ring shows a planar structure and the -COOH group
linked to the -CH=CH- carbon atoms shows a slight deviation from the planarity. The torsion angle
for the carbon atom attached to the –COOH group C12-C13-C14-O1 is -0.2 (3)o and for C12-C13-
10
ACS Paragon Plus Environment

Page 11 of 37
Crystal Growth & Design
1
2
3
4
5
6
C14-O2 is -175.8 (2)o. This deviation in the mean plane was due to the presence of -CH=CH-
group in the anion.
The hydrogen bond interaction between the [TMG] cation and the two cinnamate anions is in the
7
8
9
form of 4(12) graph set interaction (Figure 3b). The NH group in [TMG] cation facilitates the
푅4
2
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
formation of hydrogen bonds (N-H… O) with two oxygen atoms of cinnamate anion and the
distances are 1.91 (3) Å and 1.85 (3) Å (Table 2). The packing diagram of the molecules along the
b-axis is shown in Figure 3c.
Similar to [TMG][BEN], [TMG][SAL] and [TMG][CNA], trigonal-planar arrangement was
observed in the N=C(N)2 core of the [TMG][PTA]. The [TMG] cation in [TMG][PTA] has N-
H…O hydrogen bond interactions with two tolunate anions forming a 4(12) graph set interaction
푅4
(Figure 4b). The NH2 group in [TMG] cation facilitates the formation of hydrogen bonds with two
oxygen atoms of the different anion and the distances are 1.90 (2) Å and 1.93 (2) Å. The network
arrangement of cation and anion of the molecules along the b-axis is shown in Figure 4c.
3. 2 Hirshfeld Surface analysis
The Hirshfeld surface is becoming a valuable tool for analysing intermolecular interactions while
maintaining a whole-of-molecule approach. They can also be used in conjunction with a more
direct atom…atom based approach to gain a fuller appreciation of the important interactions in a
molecular crystal46. The molecular Hirshfeld surfaces reflect intermolecular interactions and the
picture of molecular shape in a crystalline environment visually. The surface distances to the
nearest atoms outside are represented as de, and inside as di , which are well defined and from these
properties it is easy to explore the type as well as the proximity of intermolecular contacts in a
molecular crystal. The function dnorm, which is normalized contact distance, takes into account the
van der Waals radius of the appropriate atom internal or external to the surface.46
The red colour visualised in the Hirshfeld surface analysis represents a close contact and the value
of the dnorm would be negative indicating the sum of di and de is shorter than the sum of the van der
Waals radii of the atoms. The blue colour denotes a contact longer than the van der Waals radii
and the dnorm would be positive, whereas the white colour denotes intermolecular distance close to
van der Waals contacts with dnorm equal to zero (Figure 5a-5d).
11
ACS Paragon Plus Environment

Crystal Growth & Design
Page 12 of 37
1
2
3
4
5
6
7
8
9
Hirshfeld surfaces and 2D fingerprint plots were created from Crystal Explorer 1747 program based
on the CIF generated from single crystal X-ray diffraction studies.48 The existence of different
types of intermolecular interactions is represented by 2D fingerprint plots showing di versus de.
The Hirshfeld surfaces for the prepared ILs were analysed to quantify the various intermolecular
interactions, and their associated fingerprint plots were also calculated. The quantitative measures
like area, volume, globularity and asphericity were also computed using the Hirshfeld surfaces.
The calculated value of globularity49 for all the ILs was < 1.0, indicating that the molecular surface
is not a sphere, but structured. The order of globularity for the ILs is [TMG][BEN > [TMG][PTA]
> [TMG][SAL] > [TMG][CNA] (Table 3).
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
The asphericity, which is a measure of anisotropy was also calculated and the order for the ILs is
[TMG][CNA] > [TMG][PTA] > [TMG][SAL] > [TMG][BEN] (Table 3).
The fingerprint plots of the ILs showed spike of various thickness and lengths. Most importantly,
in the two-dimensional fingerprint plots for the carboxylic acids, the hydrogen bond dominates
and the appearance of sharp spikes pointing towards the bottom left of each plot is observed, in
which case the upper spike (where de >di) corresponds to the hydrogen-bond donor and the lower
spike (where de < di) corresponds to the hydrogen-bond acceptor48.
The overall fingerprint plot for [TMG][BEN] (Figure 5a) and the corresponding plots delineated
into H-H, O-H/H-O and C-H/H-C contacts are illustrated in Figure 5b–5d. Similarly the overall
fingerprint plot of [TMG][SAL], [TMG][CNA] and [TMG][PTA] is illustrated in Figure 6a,
Figure 7a and Figure 8a respectively. In all the ILs the non-polar H-H interaction is dominant and
a considerable percentage of O-H/ H-O, C-H/H-C interactions is also seen (Table 4).
12
ACS Paragon Plus Environment

Page 13 of 37
Crystal Growth & Design
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 5. Two-dimensional fingerprint plots calculated for [TMG][BEN] (a) overall plot , (b) H-H,
(c) O-H/H-O, (d) C-H/H-C contacts and (e) Hirshfeld surface mapped over dnorm for
[TMG][BEN] with Isovalue 0.5.
Figure 6. Two-dimensional fingerprint plots calculated for [TMG][SAL] (a) overall plot , (b) H-H,
(c) O-H/H-O, (d) C-H/H-C contacts and (e) Hirshfeld surface mapped over dnorm for
[TMG][SAL] with Isovalue 0.5.
13
ACS Paragon Plus Environment

Crystal Growth & Design
Page 14 of 37
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 7. Two-dimensional fingerprint plots calculated for [TMG][CNA] (a) overall plot , (b) H-H,
(c) O-H/H-O, (d) C-H/H-C contacts and (e) Hirshfeld surface mapped over dnorm for
[TMG][CNA] with Isovalue 0.5.
Figure 8. Two-dimensional fingerprint plots calculated for [TMG][PTA] (a) overall plot , (b) H-H,
(c) O-H/H-O, (d) C-H/H-C contacts and (e) Hirshfeld surface mapped over dnorm for
[TMG][PTA] with Isovalue 0.5.
14
ACS Paragon Plus Environment

Page 15 of 37
Crystal Growth & Design
1
2
3
4
4. Computational details
5
6
7
8
The density functional theory (DFT) calculation has been a very useful tool to understand the
fundamental vibrations and the electronic structure of a compound. The computational calculations
were performed using Gaussian 09 Program50 and the output files are visualized using Gauss
View software51. The optimized structure, vibrational spectra, structural characteristics, energy,
stability and the thermodynamic properties of the four ILs were determined with B3LYP (Becke’s
three parameter hybrid model using Lee-Yang-Parr correlation functional)52,53 method and by
using basis set 6-311G ++(d,p). Furthermore, HOMO and LUMO energy values and the energy
gap were calculated for the four ILs by the same method.
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
4.1 Molecular geometry
The optimized molecular structure of the four ILs was obtained from Gaussian 0950 and Gauss
view programs51 using B3LYP/6311-G++(d,p) method. The molecular structures of the optimized
four ILs are shown in Figure 9. It is found that most of the optimized bond lengths obtained from
theoretical values are slightly larger than the experimental values as theoretical calculations belong
to isolated molecules in gaseous phase while the experimental results belong to molecules in solid
state and liquid state. The DFT results show a fairly good agreement with the experimental data
obtained from crystallographic studies.
4.2 Thermodynamic properties
Quantum mechanics, spectroscopy and thermodynamics are linked to partition function, which is
an important parameter in thermodynamics. Partition functions such as, transitional partition
function, rotational partition function, vibrational partition function, and electronic partition
function are used to calculate the heat capacity, entropy, equilibrium and rate constant. The total
energy of a molecule contributes to sum of translational, rotational, vibrational and electronic
energies i.e., E=Et+Er+Ev+Ee. The thermodynamic parameters such as zero point vibrational
energies, specific heat, rotational constants, the entropy and dipole moment values of four ILs were
calculated using B3LYP/6-311++G(d,p) basis set at room temperature (298.15 K) and one
atmospheric pressure. According to the obtained data, several interesting results are deduced
(Table 5).
15
ACS Paragon Plus Environment

Crystal Growth & Design
Page 16 of 37
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure. 9. Optimized geometries of the four ILs (a) [TMG][BEN], (b) [TMG][SAL], (c) [TMG][CNA]
and (d) [TMG][PTA] as obtained from DFT/B3LYP calculations with Gaussian 6-311++(d,p)
basis set.
First, there is a reasonable correlation between the dipole moment of these ILs and their
corresponding energy values. In other words, [TMG][SAL] has a higher dipole moment (9.152
Debye) than those of the other ILs. Strikingly, a significant declining trend of the HOMO and
LUMO energy levels was evident. These are vital in predicting the reactive properties of a
chemical reaction.
The thermodynamic functions such as heat capacity, entropy and enthalpy changes at different
temperatures (100 K -1000 K) were also obtained from the harmonic frequency calculations based
on the vibrational analysis using B3LYP method and 6-311++G(d, p) basis set for the four ILs and
are listed in Table 6.
The molecular vibrational intensities of the molecule increases with the increase in temperature
and this is observed in all the cases of the compound studied with temperature ranging from 100
to 1000 K. The correlation equations between heat capacity, entropy, enthalpy, Gibb’s free energy
and temperature for each ILs were calculated using a second order polynomial function. The
16
ACS Paragon Plus Environment

Page 17 of 37
Crystal Growth & Design
1
2
3
4
5
6
7
8
9
corresponding regression factor (R2) for the thermodynamic properties was found to be close to
0.999 for all ILs. The corresponding fitting equations for all the ILs are given in supporting
information (S1), values are listed in Table 6 and the correlation graphs are shown in Figure 10a-
10d. The thermodynamic properties calculated from the above equation will be useful to estimate
the spontaneity of the reaction, if these ILs were used in a reaction. Further, these are important in
evaluating ILs in thermal storage, heat transfer applications and heat capacity measurements.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
(a)
(b)
(c)
(d)
Figure 10. Variation in thermodynamic parameters with temperature for (a) [TMG][BEN],
(b) [TMG][SAL], (c) [TMG][CNA] and (d) [TMG][PTA].
17
ACS Paragon Plus Environment

Crystal Growth & Design
Page 18 of 37
1
2
3
4
4.3. HOMO-LUMO analysis
5
6
7
8
The chemical reactivity can be understood by the electronic absorption transition from the ground
state to the excited state and is mainly described by one electron excitation from the highest
occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO). The
HOMO-LUMO energy gap calculated by computational study reflects on the chemical reactivity
of the molecule, where LUMO represents the electron acceptor and HOMO represents the ability
to donate an electron. The HOMO-LUMO energy gap reveals whether charge transfer interactions
are taking place between the cation and anion of the molecule and to understand the ionic
interaction between the cation and anions of the ILs. Generally, LUMO energies correspond to
cations (electrophile), which receive the electrons from donors and HOMO energies correspond to
the anions (nucleophile), which donate the electrons to the acceptors. The energy of the HOMO is
related directly to the ionization potential and that of the LUMO to electron affinity. From the
HOMO-LUMO energy gap, the molecular properties like chemical hardness, reactivity and
stability could be predicted. The greater the energy gap, harder the molecule will be, less reactive
and more stable.
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
The other important parameters such as Chemical hardness (η), electronegativity (χ), chemical
potential (µ) and electrophilicity index (ψ) were also calculated from the ionization potential (-
EHOMO) and electron affinity (- ELUMO) values for the four ILs.
Chemical hardness (η) = (ELUMO - EHOMO) / 2
Electronic chemical potential (µ) = (EHOMO + ELUMO) / 2
Electronegativity (χ) = - (EHOMO + ELUMO) / 2
Electrophilicity index (ψ) = µ2/2η
(1)
(2)
(3)
(4)
From the optimized geometry, the HOMO and LUMO region levels were found to spread over the
entire anion molecule and the charge transfer interface between the cation and anion is calculated
by the energy gap between the HOMO-LUMO. The energy levels of the HOMO, LUMO, LUMO
+ 1 and HOMO - 1 orbitals were computed at B3LYP/6-311G++(d,p) level for the four ILs and
are represented in Figure 11a-11d. The calculated values are given in Table 7. The surface of the
frontier orbitals was drawn to understand the bonding scheme of the four ILs. The positive is
represented by red and the negative by green colours.
18
ACS Paragon Plus Environment

Page 19 of 37
Crystal Growth & Design
1
2
3
4
5
6
7
8
9
Table 7 shows the HOMO and LUMO energy gap of the studied ILs. The observed HOMO-LUMO
energy gap for [TMG][SAL] is the least value compared to the energy levels of other ILs. Hence,
the salicylate anion has more ionic interactions with cation when compared to the other cation
anion interactions.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
(a)
(b)
(c)
(d)
Figure 11. Frontier molecular orbitals of (a) [TMG][BEN], (b) [TMG][SAL], (c) [TMG][CNA]
and (d) [TMG][PTA]
19
ACS Paragon Plus Environment

Crystal Growth & Design
Page 20 of 37
1
2
3
4
5. Thermal analysis
5
6
7
8
9
The thermal stability of ILs is found to depend on the anion type, chain length and allyl-
functionalization54. Anion type in ILs plays a major role on the thermal stability than the cation
type. The thermal stability of ILs increases with shorter chain length and the allyl-functionalization
decreased the thermal stability54. The dynamic decomposition experiment of four ILs was
performed under nitrogen atmosphere using TA Instruments (Q 500 Hi-Res TGA). The important
information on thermal stability is the decomposition onset temperature (Td-onset), which was
arrived at from the thermogravimetric data. The Td-onset temperatures of [TMG][BEN],
[TMG][SAL], [TMG][CNA] and [TMG][PTA] are 210.2 oC, 131.3 oC, 102.4 oC and 115.6 oC,
respectively (Figure 12). The Tonset temperatures for 5% decomposition of [TMG][BEN],
[TMG][SAL], [TMG][CNA] and [TMG][PTA] are 121.6 oC, 160.9 oC, 87.1 oC and 144.4 oC,
respectively. The DSC analysis, performed on TA instruments Q200 MSDC showed that only
[TMG][BEN] exhibited crystallization and melting transitions but the other three ILs did not
exhibit crystallization or melting transitions, instead glass transition (Tg) was observed. The Tg
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
o
o
temperature of [TMG][SAL], [TMG][CNA] and [TMG][PTA] are -7.90 C, -22.13 C and -
o
o
o
18.74 C, respectively at 5 C/min heating. [TMG][BEN] showed melting at 95.26 C and
crystallization below room temperature (Figures S5-S9). ILs usually tend to form glasses easily
at low temperature and so, Tg values determine the lower limit of operating range, where the IL
is liquid, and Td onset can be regarded as the upper operating range, where ILs do not evaporate.
Figure 12. TGA analysis of [TMG][BEN], [TMG][SAL,[TMG][CNA] and [TMG][PTA]
20
ACS Paragon Plus Environment

Page 21 of 37
Crystal Growth & Design
1
2
3
4
6. Vibrational spectral analysis
5
6
7
8
Theoretical calculations were performed using density functional theory B3LYP with 6-
31G++(d,p) basis set. Since the DFT potentials overestimate the vibrational wavenumbers, the
discrepancies caused are corrected by scaling the calculated wavenumbers with the proper factor
directly55. Here, we have adopted the scaling factor of 0.978 for B3LYP method. The optimized
structural parameter was calculated with the above said basis sets and the values are compared
with the experimental values of all ILs ornately. The FTIR spectra of the ILs were recorded in the
region 4000– 400 cm-1 using JASCO 4100 model FTIR spectrometer equipped with MCT detector,
a KBr beam splitter and High-Intensity ceramic source.
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
The infrared stretching vibration frequencies of N-H and C=N in TMG as reported by Anderson
and Hammer 56 are 3311 and 1594 cm-1, respectively and these are observed in all the three ILs.
+
During the protonation of the =NH group in TMG to –NH2 a prominent band was observed from
2700 to 2250 cm-1, which may be due to the formation of triaminocarbonium ion. Three types of
vibrations (stretching, in-plane bending and out-of-plane bending vibrations) of O–H group
(anion) were seen. The synthesized ILs were moisture sensitive and so the O–H group vibrations
are likely to be present as a broad peak, which are generally observed in the region around 3500
cm−1. The recorded spectrum of [TMG] [BEN] shows the vibrations of C=O (COO-) asymmetric
and symmetric stretching at 1666 cm-1 and 1381 cm-1, respectively (Figure 13a). Further, C=C
(phenolic) multiple peaks appear at 1551 cm-1. The C-C stretching peaks were observed at 1551-
1407 cm-1. The COO-(C-O) stretching peak appeared at 1235 cm-1. The vibrational peaks
appearing in the region 760-670 cm-1 were attributed to =C-H bending. The calculated vibrational
frequencies by B3LYP/6-311G(d,p) method for [TMG][BEN] shows a dominant O-H vibration
at 2813cm-1. The asymmetric and symmetric stretching frequencies were observed at 1641 cm-1
and 1346 cm-1, respectively.
The recorded spectrum of [TMG][SAL] showed vibrations of C=O (COO-) asymmetric and
symmetric stretching at 1652-1670 cm-1 and 1386 cm-1, respectively (Figure 13b). Further, C=C
(phenolic) multiple peaks appeared at 1558-1610 cm-1. The C-C stretching peaks were observed
at 1444-1503 cm-1. The O-H (phenolic) bending peak appeared at 1324 cm-1 and the O-H stretching
was observed around 3100 cm-1. The COO- (C-O) stretching and C-OH (phenolic) stretching
peaks appeared at 1296 cm-1 and 1156-1248 cm-1, respectively. The vibrational peaks appearing
in the region 759-669 cm-1 were attributed to =C-H bending. The calculated vibrational
21
ACS Paragon Plus Environment

Crystal Growth & Design
Page 22 of 37
1
2
3
4
5
6
frequencies by B3LYP/6-311G(d,p) method for [TMG][SAL] showed a dominant N-H vibration
at 2386cm-1 and O-H stretching at 3052 cm-1. The asymmetric and symmetric stretching frequency
was observed at 1617 cm-1 and 1383 cm-1 respectively.
7
8
9
The IR spectrum of [TMG][CNA] has its own characteristic peaks corresponding to the functional
groups –COOH and C=C that are present in the cinnamic acid. C=O double bond stretching
frequency was observed at 1642 cm-1 due to the conjugation effect (Figure 13c). Further, the -
COOH functional group has a broad band O-H stretching from 3100 cm-1 to 2800 cm-1 and a C-
O stretching at 1240 cm-1 ~ 1380 cm-1. The two bands at 1412 cm-1 and 1642 cm-1 are assigned to
the aromatic C=C double bond in cinnamic acid. The aromatic C=C double bond was actually
observed between 1700 cm-1 and 1500 cm-1, but they shifted to lower wavenumber due to the
conjugation effect of benzene ring. In addition, the band of aliphatic C=C double bond is
overlapped by the aromatic C=C double bond stretch, which is actually present between 1642
cm-1 to 1616 cm-1 with a single peak (aromatic has two peaks). The calculated vibrational
frequencies by B3LYP/6-311G(d,p) method for [TMG][CNA] showed a dominant N-H vibration
at 2879 cm-1 . The asymmetric and symmetric stretching frequencies were observed at around 1617
cm-1 and 1383 cm-1, respectively. The aromatic double bond vibration frequency in the cinnamic
acid was found at 1639 cm-1 and the stretching frequency of aliphatic C=C double bond was
observed at 1683 cm-1.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
The IR spectrum of [TMG] [PTA] showed an aromatic C-H stretching peak at 3033 cm-1. The
C=O stretching (carboxylic acid) peak appeared at 1655 cm-1. The peak due to aromatic ring
stretching appeared at 1547 cm-1. The CH3 bending peak appeared at 1453 cm-1 and C-C stretching
peak was found at 1402 cm-1 (Figure 13d). Similarly C-O stretching (carboxylic acid) peak
appeared at 1381 cm-1 and C-OH stretching peak appeared at 1287 cm-1. The O-H bending peak
was found at 916 cm-1.
22
ACS Paragon Plus Environment

Page 23 of 37
Crystal Growth & Design
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
(a)
(b)
(c)
(d)
Figure 13. Calculated IR spectra using B3LYP/6-31G++(d,p) basis set (---- dashed line) and Experimental
FTIR absorption spectra ( continuous line) of compounds (a) [TMG][BEN], (b) [TMG][SAL], (c)
[TMG][CNA] and (d) [TMG][PTA].
23
ACS Paragon Plus Environment

Crystal Growth & Design
Page 24 of 37
1
2
3
4
7. Conclusions
5
6
7
8
Four ILs, [TMG][BEN], [TMG][SAL], [TMG][CNA] and [TMG][PTA] were synthesized and
their structures were determined by crystallographic technique to understand the intermolecular
interaction among the cations and anions. It is interesting to note that N-H…O hydrogen bonding
plays an important role in crystal packing of molecule and in crystal structures of the ILs.
The structural insights of the ionic liquids, to quantify the intermolecular interactions in the
molecular crystals are explored and visualized by Hirsfled surface analysis and by associated
fingerprint plots generated from the dnorm surface. Hirshfeld surface analysis showed a clear
dominance of H…H, O…H and C…H interactions.
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
The thermal properties of the ILs were evaluated by TGA and DSC techniques to establish the
thermal stability, weight loss and glass transition temperature giving a close accordance with the
structure.
Density functional calculation provided valuable information on the geometry and the electronic
structure of the ILs. The FT-IR spectra were recorded and the vibrational frequencies were
explained by DFT- B3LYP method with 6-311G++(d,p) basis set for the four ILs. The difference
between the observed and calculated wavenumbers was very small for most of the fundamentals.
Furthermore, the thermodynamic properties, such as heat capacity, entropy, enthalpy and Gibb’s
free energy for the ILs were calculated theoretically. It can be observed that the molecular
vibrational intensity increases with temperature and thereby the thermodynamic function also
increases with temperature ranging from 100 to 1000 K. The frontier molecular orbital of ILs and
excitation energies of surfaces of molecular orbitals from HOMO-1 and HOMO to LUMO,
LUMO+1 were calculated using DFT–B3LYP 6-311G++(d,p).
Supporting Information
The Supporting Information is available free of charge on the ACS Publications website.
1H and 13C NMR spectra, DSC heating and cooling curves, water content, CCDC numbers for X-
ray crystallographic files, and the second order polynomial correlation equations between
thermodynamic parameters and temperature for studied ionic liquids are provided in supporting
information.
24
ACS Paragon Plus Environment

Page 25 of 37
Crystal Growth & Design
1
2
3
4
Acknowledgements
5
6
7
8
Authors are thankful to IIT Madras for financial support through Institute Research and
Development Award (IRDA): CHY/15-16/833/RFIR/RAME and Dr. A. Narayanan, IIT Madras
for thermal analysis measurements of few samples.
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
25
ACS Paragon Plus Environment

Crystal Growth & Design
Page 26 of 37
1
2
3
4
5
Table 1.
Crystal data and details of the structure determination of [TMG][BEN],
[TMG][SAL], [TMG][CNA] and [TMG][PTA]
6
7
8
9
Identification code
[TMG][BEN]
C12 H19 N3 O2
237.30
[TMG][SAL]
C24 H38 N6 O6
506.60
173(2)
0.71073
[TMG][CNA]
C14 H21 N3 O2
263.34
173(2)
0.71073
[TMG][PTA]
C13H21N3O2
251.33
173(2)
0.71073
Empirical formula
Formula weight/gmol-1
Temperature/K
Wavelength/ Å
Crystal system
Space group
a/ Å
173(2)
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
0.71073
Orthorhombic
P212121
7.1764(3)
10.1869(5)
17.4148(8)
-
1273.11(10)
4
1.238
Monoclinic
C2/c
Monoclinic
P2(1)/c
Monoclinic
P2(1)/c
20.2575(12)
9.9330(6)
13.5455(8)
103.779(3)
2647.2(3)
4
12.1053(16)
7.0086(9)
17.352(3)
92.464(6)
1470.8(3)
4
11.0111(4)
7.0841(2)
17.7940(5)
92.806(15)
1386.33(7)
4
b/ Å
c/ Å
β/0
Volume/ Å3
Z
Density/ g/cm3
Absorption coefficient/
mm-1
1.271
1.189
1.204
0.086
0.093
0.081
0.083
Crystal size/mm
Theta range for data
collection/0
0.12 x 0.16 x 0.25 0.20 x 0.13 x 0.12 0.22 x 0.15 x 0.12 0.10 x 0.22 x 0.25
2.32 to 25.00
2.070 to 24.999
2.350 to 24.997
2.29 to 25.00
-8<=h<=6,
-9<=k<=12,
-20<=l<=20
-24<=h<=24,
-11<=k<=11,
-14<=l<=16
-10<=h<=14,
-8<=k<=6,
-20<=l<=20
-13<=h<=12,
-8<=k<=8,
-20<=l<=21
Limiting indices
Reflections collected /
unique
R (int)
Completeness to theta =
24.999/%
Absorption correction
Data / restraints /
parameters
Goodness-of-fit
on F^2
Final R indices
[I>2sigma(I)]
5521 / 223
0.0164
7410 / 2289
0.0261
8529 / 2587
0.0568
9378/2449
0.0246
99.6
98.3
100
100.0
Multi-scan
2236 / 0 / 166
Multi-scan
2289 / 0 / 179
Multi-scan
2587 / 0 / 184
Multi-scan
2449/0/177
1.061
1.040
1.002
1.055
R1 = 0.0265,
wR2 = 0.0679
R1 = 0.0280,
wR2 = 0.0689
R1 = 0.0432,
wR2 = 0.1104
R1 = 0.0580,
wR2 = 0.1226
R1 = 0.0506,
wR2 = 0.1022
R1 = 0.1082,
wR2 = 0.1292
R1 = 0.0398,
wR2 = 0.1045
R1 = 0.0570,
wR2 = 0.1169
R indices (all data)
26
ACS Paragon Plus Environment

Page 27 of 37
Crystal Growth & Design
1
2
3
4
Table 2. Hydrogen-bonding geometry (Å, o)
5
6
7
8
9
[TMG][BEN]
D-H…A
N2-H1N…O1i
N2-H1N…O2ii
D-H
0.93(2)
0.83(2)
H…A
1.91(2)
1.93(2)
D…A
2.823(2)
2.767(2)
D-H…A
170.0(19)
179.0(2)
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Symmetry codes : (i) 1+x, y, z; (ii) 1–x, ½+y, ½-z;
[TMG][SAL]
H…A
1.87(2)
2.08(2)
2.53
D-H…A
N2-H1N…O1i
N2-H2N…O2ii
C5-H5C…O3iii
D-H
0.90(2)
0.88(2)
0.98
D…A
D-H…A
165.7(19)
160.5(18)
133.0
2.747(2)
2.921(2)
3.276(3)
Symmetry codes : (i) x, 1+y, z; (ii) –x,1+y, ½-z; (iii) x, 1-y, ½+z.
[TMG][CNA]
D-H…A
N2-H1N…O1i
N2-H2N…O2ii
D-H
0.94(3)
0.95(2)
H…A
1.91(3)
1.85(3)
D…A
2.837(3)
2.794(3)
D-H…A
170(2)
171(2)
Symmetry codes : (i) 1-x, 1-y, -z; (ii) 1–x,-y, -z.
[TMG][PTA]
H…A
1.90(2)
1.93(2)
D-H…A
N2-H1N…O1i
N2-H2N…O2ii
D-H
0.93(2)
0.89(2)
D…A
2.818(2)
2.821(3)
D-H…A
172(2)
174.9(18)
Symmetry codes : (i) 1-x, -1/2+y, 1/2-z; (ii) x,3/2-y,-1/2+z.
Table 3. Quantitative measure computed for studied ILs using Hirshfeld surface analysis
Anion
[TMG][BEN]
312.22
292.92
0.760
[TMG][SAL]
323.85
318.09
0.717
[TMG][CNA]
360.23
347.05
0.705
[TMG][PTA]
339.64
325.47
0.723
Volume (Å3)
Area (Å2)
Globurality
Asphericity
dnorm min
dnorm max
0.105
0.308
0.424
0.353
-0.657
-0.509
-0.596
-0.615
1.226
1.490
1.561
1.218
27
ACS Paragon Plus Environment

Crystal Growth & Design
Page 28 of 37
1
2
3
4
5
6
7
Table 4. Relative Percentage Contributions of Interactions for Cation- Anion
to overall Hirshfeld Surface Area.
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Anion
[TMG][BEN]
62.9 %
15.3 %
18.6 %
0.1 %
[TMG][SAL]
57.9 %
20.6 %
17.8 %
0.1 %
[TMG][CAN]
[TMG][PTA]
H-H
63.9%
15.4 %
17.0%
-
65.6 %
14.4 %
17.7 %
-
O-H / H-O
C-H / H-C
C-O/ O-C
O-O
0.1 %
0.1 %
-
-
C-C
1.1 %
0.5 %
1.9 %
0.5 %
Table 5. Thermodynamic parameters of studied ILs with DFT/B3LYP method using
6-311++G(d,p) basis set.
Parameter
[TMG][BEN] [TMG][SAL] [TMG][CNA] [TMG][PTA]
SCF energy E(a.u)
-783.6679
192.2755
1.1014
-858.9244
193.5246
0.8361
-861.0903
211.2945
1.1044
-822.7141
207.6263
1.0875
Zero-point Vibrational energy (kcal/mol)
Rotational constant (GHz) A
Rotational constant (GHz) B
Rotational constant (GHz) C
Thermal energy (kcal/mol)
0.1800
0.2130
0.0941
0.1447
0.1725
0.1977
0.0880
0.1400
204.065
67.432
205.385
70.551
224.745
76.000
220.203
73.309
Specific heat at constant volume Cv
(cal mol-1K-1)
Entropy S (cal mol-1K-1)
145.664
3.361
141.323
9.1520
164.243
3.7796
147.814
3.1038
Dipole moment (Debye)
28
ACS Paragon Plus Environment

Page 29 of 37
Crystal Growth & Design
1
2
3
4
Table 6. Thermodynamic functions of studied ILs.
5
6
7
8
[TMG][BEN]
퐜0
푝,푚 (cal mol-1 K-1)
퐒푚0
퐒푚0
퐒푚0
퐇푚0
퐇푚0
퐇푚0
퐆0
푚 (Kcal mol-1 )
(cal mol-1 K-1)
(Kcal mol-1 )
T (K)
100
200
298.15
400
500
600
700
800
900
1000
30.39
49.20
67.76
87.90
116.32
150.52
163.13
184.67
205.10
224.38
242.52
259.61
275.70
192.47
196.68
203.03
210.61
220.30
230.33
244.04
257.64
272.15
287.44
183.69
173.42
158.16
145.36
127.97
108.47
86.98
63.63
38.51
11.74
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
86.27
103.00
117.14
128.93
138.82
147.19
154.32
[TMG][SAL]
(cal mol-1 K-1)
퐜0
퐆0
푚 (Kcal mol-1 )
푝,푚 (cal mol-1 K-1)
(Kcal mol-1 )
T (K)
100
200
298.15
400
500
600
700
800
900
1000
30.47
51.32
70.55
87.33
116.47
141.32
165.41
187.86
209.10
229.11
247.92
265.60
195.48
199.80
205.97
214.37
224.46
236.13
249.13
263.23
278.25
294.07
186.75
176.51
163.84
148.21
130.54
110.68
88.76
64.90
39.21
11.81
90.14
107.28
121.74
133.80
143.90
152.42
159.68
282.26
[TMG][CNA]
(cal mol-1 K-1)
퐜0
퐆0
푚 (Kcal mol-1 )
푝,푚 (cal mol-1 K-1)
(Kcal mol-1 )
T (K)
100
200
298.15
400
500
600
700
800
900
1000
36.69
56.14
76.00
104.47
137.39
164.24
190.14
214.29
237.16
258.70
278.96
298.01
213.81
218.67
225.33
234.36
245.22
257.79
271.78
286.96
303.15
320.18
203.37
191.19
176.37
158.31
138.08
115.50
90.69
63.80
34.94
4.23
97.10
115.64
131.25
144.24
155.13
164.34
172.19
315.95
[TMG][PTA]
(cal mol-1 K-1)
퐜0
퐒푚0
퐇푚0
퐆0
푚 (Kcal mol-1 )
푝,푚 (cal mol-1 K-1)
(Kcal mol-1 )
T (K)
100
200
298.15
400
500
600
700
800
900
1000
35.38
54.94
73.31
98.13
130.23
147.81
181.57
204.93
227.05
247.89
267.51
285.97
303.37
209.95
214.69
220.79
229.96
240.46
252.61
266.15
280.85
296.54
313.06
200.14
188.65
176.73
157.33
138.00
116.39
92.63
66.85
39.17
9.69
93.94
111.75
126.88
139.55
150.22
159.28
167.02
29
ACS Paragon Plus Environment