Diethylamine-based ionic liquids: quantum chemical calculations and experiment

Article abstract of DOI:10.1007/s11172-019-2660-7

The structural and energetic characteristics of the compounds formed by the reaction of diethylamine (DEA) with protic acids (sulfuric (H₂SO₄), methanesulfonic (MsOH), trifluoromethanesulfonic (TfOH), and para-toluenesulfonic (TsOH)) were examined using quantum chemical computations (B3LYP-GD3/6-31++G(d,p)). The strength of the hydrogen bonds in the ion pairs formed was quantitatively estimated by the QTAIM theory and NBO analysis. The results of the quantum chemical computations and the obtained thermal (phase transition and decomposition temperatures) and physicochemical (viscosity and conductivity) characteristics indicate that the reactions of DEA with the acids afford salts. The salts with the melting points higher than 100 °C are formed in the case of DEA/OTf(OTs), while protic ionic liquids are produced in the case of DEA/OMs(HSO₄).

Full text of DOI:10.1007/s11172-019-2660-7

Russian Chemical Bulletin, International Edition, Vol. 68, No. 11, pp. 2009—2019, November, 2019
2009
Diethylamine-based ionic liquids:
quantum chemical calculations and experiment*
L. E. Shmukler, I. V. Fedorova, М. S. Gruzdev, Yu. A. Fadeeva, and L. P. Safonova
G. A. Krestov Institute of Solution Chemistry, Russian Academy of Sciences,
1 ul. Akademicheskaya, 153045 Ivanovo, Russian Federation.
Fax: +7 (493 2) 33 6259. E-mail: les@isc-ras.ru
The structural and energetic characteristics of the compounds formed by the reaction of
diethylamine (DEA) with protic acids (sulfuric (H2SO4), methanesulfonic (MsOH), trifluoro-
methanesulfonic (TfOH), and para-toluenesulfonic (TsOH)) were examined using quantum
chemical computations (B3LYP-GD3/6-31++G(d,p)). The strength of the hydrogen bonds in
the ion pairs formed was quantitatively estimated by the QTAIM theory and NBO analysis. The
results of the quantum chemical computations and the obtained thermal (phase transition and
decomposition temperatures) and physicochemical (viscosity and conductivity) characteristics
indicate that the reactions of DEA with the acids afford salts. The salts with the melting points
higher than 100 C are formed in the case of DEA/OTf(OTs), while protic ionic liquids are
produced in the case of DEA/OMs(HSO4).
Key words: diethylamine, protic acids, hydrogen bond, quantum chemistry, thermal
characteristics, electrical conductivity.
Protic ionic liquid (PILs) capable of proton transport-
ing under anhydrous conditions form one of the most
important subclasses of ionic liquids (ILs). According to
the data available from the ILThermo database on ionic
liquids, the number of studies devoted to protic ionic
liquids is much lower than that on aprotic liquids. Aprotic
ILs are fairly well studied in both the individual state and
solutions using a wide range of experimental methods,
as well as various numerical methods, including quan-
tum chemical computations and molecular dynamic
modeling.1—5
points, due to which they are attractive for use as electro-
lytes for fuel cells.10—17
The problem about the proton transfer during PILs
formation is widely discussed.18—21 The interaction of the
base and acid via the neutralization reaction does not
always result in the complete transfer of a proton from the
acid to base and in the formation of the PIL but can be
more complicated. Depending on the strength of the acid
and base, the processes presented in Scheme 1 can occur.
Scheme 1
Protic ionic liquids are salts containing a large organic
cation and organic or inorganic anion. The key feature of
PILs is the presence of an accessible proton on a cation
that allows the cation to form hydrogen bonds with the
anion. Alkylammonium salts discussed in the present study
have a number of advantages over the whole series of PILs,
since they are lowly toxic and relatively cheap, which
provides a substantial extension of areas of their applica-
tion. These ILs are used in the dissolution of polymers and
separation processes and in organic synthesis, including
the production of new drugs.6—9 Many of the salts are
classified as room-temperature ILs or exist in the liquid
metastable state at temperatures lower than their melting
When choosing the initial acids and bases for the
evaluation of the degree of proton transfer in the synthe-
sized PILs, the ΔpKa value (difference between pKa of the
acid and base in an aqueous solution)22,23 is often used.
The dependence of ΔpKa on the probability of proton
transfer between acid—base pairs was examined for 6465
crystalline complexes.18 The results of the analysis showed
the predominant formation of ion pairs at pKa > 4
and non-ionized acid-base complexes at pKa < –1, while
* Based on the materials of the Russian National Conference
"Interplay between Ionic and Covalent Interactions in Design
of Molecular and Nano Chemical Systems" (ChemSci-2019)
(May 13—17, 2019, Moscow, Russia).
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 2009—2019, November, 2019.
1066-5285/19/6811-2009 © 2019 Springer Science+Business Media, Inc.

2010
Russ. Chem. Bull., Int. Ed., Vol. 68, No. 11, November, 2019
Shmukler et al.
a linear dependence between the probability of salt forma-
tion and pKa was obtained at –1 < pKa < 4. However,
it is impossible to predict the degree of proton transfer in
PILs based only on the difference in dissociation constants
of the initial acids and bases,19 because some reactions of
proton transfer do not occur to the extent that can be
expected from the data on pKa. It was also mentioned16
that the pKa values determined for aqueous solutions are
not always correct for analysis of nonaqueous PILs. The
methods of IR and NMR spectroscopy24—27 and quantum
chemistry28—31 are used to determine the degree of proton
transfer. In addition, a complex approach combining
theoretical and experimental investigation methods are
used more frequently for this purpose.20,21 It was found20
using this complex approach that the reactions of acetic
acid with primary and tertiary amines having similar pKa
values occurred with different degrees of proton transfer.
Based on an analysis of the ionicities of n-butyl(heptyl)-
ammonium, triethylammonium, n-methylpyrrolidinium,
and dimethylbutylammonium acetates according to
Walden´s rule and using a series of observations of the
change in the physical properties of these compounds, the
authors concluded20 that simple primary amines formed
highly ionized ILs, while tertiary amines tended to form
mixtures with a low degree of proton transfer. A similar
conclusion about different degrees of proton transfer (χ)
in the primary and tertiary ammonium salts was made21
on the basis of the molecular dynamic modeling results
using experimental data on the physicochemical properties.
In addition, it was found that the properties of the PILs
changed depending in χ. For all salts, an increase in χ
was accompanied by an increase in the density and
viscosity and a decrease in the diffusion coefficients,
whereas the dependence of the ion conductivity on χ
was extremal.21 A similar analysis31—33 of the data ob-
tained by the quantum chemical method and experimen-
tal results showed that the reactions of di- and trisubsti-
tuted alkylamines with strong protic acids resulted in the
proton transfer from the acid to amine and in the forma-
tion of ion pairs with the hydrogen bond. It was ob-
served31 that both ion pair and molecular complex can be
formed upon the transfer of a proton from phosphorous
(pKa = 1.3)34 and trifluoroacetic acids (pKa = 0.5234) to
diethylamine.
publications devoted to the synthesis and properties of the
compounds formed by diethylamine and some acids used
in this work10,12,16,17,26,35 and also to the properties of
their solutions in various polar solvents.36—39
Experimental
Diethylamine (≥99.5%, Sigma-Aldrich), para-toluenesul-
fonic acid monohydrate (99%, Tokyo Chemical Industries,
Japan), trifluoromethanesulfonic acid (≥99%, Sigma-Aldrich),
methanesulfonic acid (≥99.5%, Sigma-Aldrich), and sulfonic
acid (reagent grade) were used as received.
Proton ionic liquids were prepared by the neutralization of
diethylamine directly with the protic sulfo acids taken in equi-
molar amounts via Scheme 2.
Scheme 2
R = OH (HSO4),
(OTs), Me (OMs), CF3 (OTf).
The obtained product was dried on a vacuum line at 80 C
with continuous stirring. The synthesized compounds were
identified by NMR and IR spectroscopy.
Diethylammonium hydrosulfate (DEA/HSO4). 1H NMR
(DMSO-d6), δ: 1.2—1.117 (t, 6 H, CH3, J = 7.34 Hz); 2.97—2.9
(m, 4 H, CH2); 7.71 (s, 1 H, HSO4); 8.31 (br.s, 2 H, NH+).
13C NMR (DMSO-d6), δ: 10.83 (CH3); 42.53 (CH2). 15N NMR:
δN –332.74. IR (KBr), νmax/cm–1: 3385 (NH), 2977—2742 (CH2,
СН3), 2604 (R2NH+), 2472 (NH+...–O—S), 1180 s (νas(S=O)),
1071 s (νs(S=O)), 578 s (СН2).
Diethylammonium tosylate (DEA/OTs). 1H NMR (CDCl3),
δ: 1.28—1.25 (t, 6 H, CH3, J = 7.39 Hz); 2.31 (s, 3 H, CH3);
2.95—2.91 (q, 6 H, CH2); 7.15—7.13 (d, 2 Н, Ph—H, J = 8 Hz),
7.69—7.68 (d, 2 Н, Ph—H, J = 8.04 Hz). 13C NMR (CDCl3),
δ: 11.19 (CH3); 21.43 (CH3); 42.55 (CH2); 125.83 (C—Ph);
129.08 (C—Ph); 140.57 (C—Ph); 141.99 (C—Ph). 15N NMR:
δN –332.5. IR (KBr), νmax/cm–1: 3437 (NH), 3005—2874 (СН3),
2804—2736 (CH2), 2677—2527 (R2NH+), 2492 (NH+…–O—S),
1204 (νas(S=O)), 1057 (νs(S=O)), 561—538 (СН2).
Diethylammonium triflate (DEA/OTf). 1H NMR (DMSO-d6),
δ: 1.18—1.15 (t, 6 H, CH3, J = 7.34 Hz); 2.97—2.9 (m, 4 H,
CH2); 8.17 (s, 2 H, NH+). 13C NMR (DMSO-d6), δ: 10.95
(CH3); 41.41 (CH2); 124.6—116.2 (CF3). 15N NMR: δN –332.44.
IR (KBr), νmax/cm–1: 3194 (NH), 3031—2982 (СН3), 2921—
2854 (CH2), 1289—1244 (νas(S=O)), 1032 (νs(S=O)), 578 (СН2).
Diethylammonium mesylate (DEA/OMs). 1H NMR (DMSO-d6),
δ: 1.19—1.16 (t, 6 H, CH3, J = 7.38 Hz); 2.42 (s, 3 H, CH3);
2.95—2.88 (m, 4 H, CH2); 8.38 (s, 2 H, NH+). 13C NMR
(DMSO-d6), δ: 10.96 (CH3); 41.48 (CH2). 15N NMR: δN
–332.89. IR (KBr), νmax/cm–1: 2966—2746 (CH2, СН3),
2525—2463 (R2NH+), 2411 (NH+…–O—S), 1215 (νas (S=O)),
1056 (νs(S=O)), 568 s (СН2).
Thus, it is difficult to predict a priori the result of interac-
tion between base and acid (formation of PIL, molecular
complex, or acid—amine mixture). In this work, we pres-
ent the results of studying the structures and thermal and
physicochemical characteristics of the compounds based
on diethylamine with various sulfonic acids (sulfonic,
methanesulfonic, trifluoromethanesulfonic, and para-
toluenesulfonic) using a complex approach combining
chemical calculations and experimental methods. Note
that literature data on the properties of the compounds
presented in this work are restricted. There are only several

Diethylamine-based ionic liquids
Russ. Chem. Bull., Int. Ed., Vol. 68, No. 11, November, 2019
2011
The IR spectra of the synthesized compounds are presented
in Fig. 1. The protonation of diethylamine by sulfo acids results
in the disappearance of the frequency of the N—H group
(3280 cm–1), and the group of frequencies appears in a range of
3000—3250 cm–1. We assign this group to stretching vibrations
of the (NH2+) fragment of the DEAH+ cation.
lyzed acids, in all cases, the proton of the acid is transferred to
the nitrogen atom of diethylamine to form the cation and anion,
which are bound to each other into ion pairs due to hydrogen
bonds. The optimized geometries of the ion pairs, being struc-
tural units of the studied diethylammonium PILs, were obtained
by the density functional theory41 using the B3LYP hybrid ex-
change correlation potential42,43 taking into account the GD3
dispersion correction44 in the 6-31++G(d,p) basis set45 using
the Gaussian09 software package of quantum chemical pro-
grams.46 The equilibrium geometries of randomly arranged
molecules were used when searching for minima on the potential
energy hypersurface in order to determine stable configurations
formed upon the interaction of the acid and amine. The normal
vibration frequencies were calculated for all optimized structures.
The absence of negative values indicates that the energy minimum
was achieved.
The content of water in the synthesized ionic liquids was
determined by coulonometric titration using a Karl Fischer
Titrator V30 titrator (Mettler Toledo). The IR spectra of the
compounds (in KBr pellets) were recorded on a Vertex 80V
spectrometer (Bruker) in a frequency range of 7500—370 cm–1
with the resolution 2 cm–1. 1Н, 13С, and 15N NMR spectra were
obtained on an Avance-500 (Bruker) spectrometer in deuterated
solvents DMSO-d6 and CDCl3. The 1H and 13C chemical shifts
were determined relative to the signals from TMS, whose position
was accepted to be 0 ppm. The phase transition temperatures
were determined on a DSC 204 F1 Phoenix differential scanning
The charges on the atoms in the cation and anion were esti-
mated using the approach based on the natural population
analysis (NPA).47
The interionic interaction energy (ΔE) was calculated as
a difference between the total energy of the ion pair and energies
of the cation and anion in the ion pair. The BSSE correction to
the superposition of the basis sets was applied in the calculation
of the interaction energy.48
calorimeter with a μ sensor (NETZCH) at a rate of 10 K min–1
.
The temperatures of decomposition onset were determined on
a TG 209 F1 Iris microbalance (NETZCH) in an argon flow
(20 mL min–1) at a heating rate of 10 K min–1
.
The volume resistance of the samples was determined by
electrochemical impedance spectroscopy on a Solartron SI 1260
analyzer of impedance and amplitude-phase characteristics
complete with a Solartron SI1287A electrochemical interface in
a frequency range of 0.1—1 MHz. The resistance was measured
in a cell with platinum electrodes. The cell was calibrated by
a standard solution of KCl (12.88 mS cm–1, Mettler Toledo) at
298 K. The constants of the cell at higher temperatures were
calculated using the equation proposed previously.40 A VT4
liquid themostat (Thermex, Russia) filled with the PMS-20 sili-
cone oil was applied to maintain a constant temperature. The
accuracy of temperature maintaining was 0.1 C. The dy-
namic viscosity of the samples was measured on a Brookfield
LVDV-II+ Pro rotation viscosimeter (Brookfield, USA) in a cell
for sample measuring (SSA, SC4-18 spindle).
Two independent approaches were applied for the identifica-
tion of hydrogen-bonded interactions in the ion pairs: natural
orbital analysis (NBO49 using the Gaussian NBO Version 3.146
)
and quantum topological analysis of electron density in terms of
the theory "Atoms in Molecules" (QTAIM)50 using the AIMAll
program package (Version 10.05.04)51 based on the wave function
calculated by the B3LYP-GD3/6-31++G(d,p) method.
In the QTAIM theory, the interaction between a pair of atoms
is identified with the presence of a bonding path between them
and critical point (CP) with the signature (3, –1). The critical
points in the analyzed structures of ion pairs are characterized
by the values of electron density ρ(r), Laplacian of electron
density 2ρ(r), and densities of kinetic G(r), potential V(r), and
total Н(r) energies. The electron energy density Н(r) was deter-
Calculation methods. According to the results of the quantum
chemical computation of the interaction of amine with the ana-
I
(N—H)
1
6
2
3
4
5
2800
3000
3200
3400
3600 /cm–1
Fig. 1. IR spectra of the ILs (1—4), diethylamine (5), and TsОН (6). The spectra for DEA (5), DEA/OMs (1), and DEA/HSO4 (3)
were obtained by the ATR method, and the spectra of the crystalline samples (TsОН (6), DEA/OTs (2), DEA/OTf (4)) were recorded
for pellets with KBr.

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Russ. Chem. Bull., Int. Ed., Vol. 68, No. 11, November, 2019
Shmukler et al.
mined as the sum of the densities G(r) and V(r) at the correspond-
ing CP. For the observed hydrogen-bonded interactions (ЕHB
distribution in the cation in the following order: qН(CH ) 
3
)
 qН(CH )  qН(N) (0.248, 0.261, and 0.465 e, respectively).
The cha3racter of the electrostatic potential for the studied
anions is of the same type and has a broad range of nega-
tive potential (red region) around three oxygen atoms of
the sulfo group, thus indicating the equal probabilities of
proton addition to each atom. The charges on the oxygen
atoms increase in the following series of the anions:
ОTf  HSO4  ОТs  OMs (–1.000, –1.020, –1.029, and
–1.047 e, respectively), which results, in turn, in the
observed decrease in the length and strengthening of
the hydrogen bond between the cation and anion in
the ion pair.
in the ion pairs, their energy was estimated according to the
Lecomte—Espinosa equation52
ЕHB = 0.5V(r).
(1)
In the framework of the NBO analysis, we considered the
donor-acceptor mechanism of formation of hydrogen bonds in
the ion pairs and determined the stabilization energy that reflects
the energy effect from the interaction of the orbitals involved in
hydrogen bond formation Е(2) and the charge transfer value q
according to the equations
Е
(2) = nFij 2/(εi – εj),
(2)
Note that the calculated enthalpy of formation for the
triflate anion upon proton detachment from the acid (see
Fig. 2) is much lower than those for other anions of the
studied acids and corresponds to stronger proton-donating
properties of the acid (рКа –12).53
The most energetically stable structures of ion pairs
are shown in Fig. 3. The ion pairs with OTs and OTf can
have both one (see Fig. 3, compounds 1 and 2) and two
(see Fig. 3, compounds 3 and 4) hydrogen bonds. In all
DEA/A ion pairs, the position of the proton between the
nitrogen and oxygen atoms in the N—Н…O fragment is
nonsymmetric, and the N—Н bond (~1.0 Å) is shorter
than the Н…О bond. All bonding Н…О interactions in the
studied pairs are situated at distances shorter than the sum
of the van der Waals radii of the corresponding atoms
(vdW(Н—О) = 2.72 Å).54
q = n[Fij /(εi – εj)]2,
(3)
where n is the population of orbitals, εi and εj are the energies of
orbitals, and Fij is the non-diagonal element of the effective
orbital Hamiltonian that characterizes the interaction (over-
lapping) of these orbitals.
Results and Discussion
Quantum chemical calculations. The maps of the elec-
trostatic potential for the optimized geometries of the ions
are presented in Fig. 2 for the visual qualitative evaluation
of the reactivities of the cation and anion formed due to
the reaction of diethylamine and acid and of the possibil-
ity of hydrogen bond formation. The violet region cor-
responding to the most positive potentials is located on
two hydrogen atoms of the amino group of the cation and
indicates the most probable site of attack by the oxygen
atom of the anion, which is well consistent with the charge
The bond in the DEA/OTs(OTf) pairs with one
hydrogen bond (see Fig. 3, compounds 1 and 2) is char-
acterized by the very short distance and linearity of the
N—H…O fragment, which makes it possible to classify it
+
–
OTs (1325.11)
OMs (1330.34)
HSO4 (1290.34)
OTf (1247.94)
DEA
Fig. 2. Map of the electrostatic potential for the studied anions and diethylammonium cation. The proton affinities (kJ mol–1) of the
anions are given in parentheses.
Note. Figure 2 is available in full color on the web page of the journal (https://link.springer.com/journal/volumesAndIssues/11172).

Diethylamine-based ionic liquids
Russ. Chem. Bull., Int. Ed., Vol. 68, No. 11, November, 2019
2013
1.470
173.7
1.548
174.9
1
2
1.631
153.9
1.763
147.1
1.799
144.0
1.816
140.2
3
4
1.724
146.9
1.719
149.3
1.724
146.9
1.805
145.8
5
6
Fig. 3. Optimized structures of the DЕА/А ion pairs, where А = OTs (1, 3),31 OTf (2, 4), OMs (5), and HSO4 (6). Dashed lines show
hydrogen bonds. Bond lengths and angles are given in Å and deg, respectively.
as a strong bond according to the geometric criterion of
hydrogen bonding.55 In these cases, the topological
analysis of the electron density unambiguously indicates
the presence of the CP (3, –1) between the hydrogen-
bonded atoms, the characteristics of this CP are given in
Table 1. The low values of electron density (~10–2),
2ρ(r) > 0, and H(r) < 0 at the CP of the Н…О bond cor-
respond to an intermediate type of interatomic distances,
among which are both polar bonds and strong hydrogen
bonds.56,57 The values of ρ(r) and 2ρ(r) at the CP of
the bonds exceed the range of values characteristic of
"usual" hydrogen bonds according to Koch—Popelier
(0.002—0.035 and 0.024—0.139 a.u., respectively).58,59
The |V(r)|/G(r) ratio is higher than 1, indicating a partial
contribution of the covalent component to the hydrogen
bond.60 The estimation of the energy of hydrogen bonds
by the Lecomte—Espinosa approach52 gives the values
higher than 60 kJ mol–1, which is characteristic of very
strong (ionic) hydrogen bonds. The calculated energies of
interionic interaction for the ion pairs exceed the energies
of hydrogen bonds by many times, which is primarily
caused by electrostatic interaction forces.
The DEA/OTs(OMs, OTf, HSO4) ion pairs with two
hydrogen bonds (see Fig. 3, compounds 3—6) are analo-
gous in geometry to many respects. Hydrogen bonds in
ion pairs of this structure have different lengths (except
for the DEA/OMs pair in which both bonds are identical)
and the N—H—O bond angles are strongly distorted in all

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Shmukler et al.
Table 1. Energies of the interaction between the cation and anion in the ion pairs, energies of hydrogen
bonds (kJ mol–1), and topological parameters at the CP (3, –1) of the Н…О bonds (ρ(r), 2ρ(r), and
H(r) in a.u.)
Ion pair
ρ(r)
r
H(r)
|V(r)|/G(r)
ЕHB
ΔE
DEA/OTs (1)
DEA/OTf (2)
DEA/OTs (3)
0.0849
0.0683
0.0569;
0.0376
0.0415;
0.0383
0.0459;
0.0459
0.0463;
0.0384
0.1523
0.1624
0.1504;
0.1064
0.1177;
0.1093
0.1271;
0.1271
0.1277;
0.1076
–0.0232
–0.0099
–0.0045;
–0.0014
–0.0014;
–0.0012
–0.0021;
–0.0021
–0.0021;
–0.0014
1.3793
1.1964
1.1078;
1.0488
1.0450;
1.0425
1.0610;
1.0610
1.0617;
1.0501
–111.08
–83.36
–60.27;
–38.49
–42.28;
–39.06
–47.12;
–47.12
–47.42;
–39.04
–490.62
–445.54
–486.81
DEA/OTf (4)
DEA/OMs (5)
DEA/HSO4 (6)
–443.42
–493.51
–462.51
cases. The both bonds in the ion pairs have a character of
intermediate interactions (ρ(r) > 0 and H(r) < 0). The
topological characteristics of the CP of the Н…О bonds
are well discernible: the shorter bonds are characterized
by higher values of ρ(r) and 2ρ(r) and more negative
values of Н(r). It can be seen that the potential energy at
the CP takes more negative values with an increase in the
covalent component of the hydrogen bond. The calculated
energy of each hydrogen bond in the discussed structures
of DEA/А does not exceed 60 kJ mol–1, indicating the
formation of medium-strength hydrogen bonds. Both
hydrogen bonds in the ion pairs make a noticeable con-
tribution to the energy of interionic interaction (18—20%),
and the total energy of bonds increases for the ion pairs
in the order OTf < HSO4 < OMs < OTs (–81.34, –86.46,
–94.24, –98.76 kJ mol–1, respectively). It follows from
Table 1 that the formation of two hydrogen bonds in the
DEA/OTs(OTf) pairs does not result in an energy advan-
tage compared to their ion pairs with one hydrogen bond.
The ion pairs with both one and two hydrogen bonds
have similar energies of the ion-ionic interaction. It is
difficult to conclude on the basis of an analysis of the
energetic characteristics which of the forms is preferable,
and both forms can take place with equal probabilities
during the synthesis. On the whole, the DEA/A ion pairs are
characterized by a tendency to enhancing the interionic
interaction in the series of anions OTf < HSO4 < OTs < OMs.
The same tendency is observed in the series of ion
pairs with the triethylammonium cation and analyzed
anions.3 The data of the NBO analysis presented in Table 2
indicate the donor-acceptor mechanism of the formation
of a hydrogen bond between the cation and anion in the
ion pair. According to the NBO analysis, the hydrogen
bonds in the studied ion pairs represent the redistribution
of the electron density from the donor orbitals of lone
electron pairs of the oxygen atom of the acid anion to the
antibonding orbital of N—H of the cation: LPO  BD*N—H
.
This phenomenon is fairly characteristic of all "usual"
hydrogen bonds and is a reason for the observed elongation
of the covalent bond in the hydrogen-bonded fragment.
In all ion pairs, hydrogen bonding results in an increase
in the population of the BD*N—Н orbital compared to its
initial value for the cation and correlates well with the
elongation of the covalent N—H bond (ΔrNH, see Table 2).
The orbital interaction LPO  BD*N—H during the forma-
tion of the hydrogen bond in all ion pairs is accompanied
by a fairly high stabilization energy and the charge trans-
fer value is considerably higher than the Weinhold criterion
established for hydrogen bonding (q ≥ 0.01 e).61
Thus, the performed quantum chemical computations
showed that the reactions of diethylamine with the studied
acids leads to the proton transfer from the acid to amine
Table 2. Elongation of the covalent N—Н bond in the N—H…O fragment (Å), stabilization
energy (kJ mol–1), charge transfer (e), and population of the BD*N—H orbital
Ion pair
ΔrNH
Population of BD*N—H
E(2)
q
DEA/OTs (1)
DEA/OTf (2)
DEA/OTs (3)
DEA/OTf (4)
DEA/OMs (5)
DEA/HSO4 (6)
0.080
0.1189 (0.0152)*
0.1178
0.0974; 0.0613
0.0686; 0.0627
0.0776; 0.0776
0.0777; 0.0625
312.05
0.179
0.054
265.52
0.140
0.041; 0.019
0.023; 0.019
0.029; 0.029
0.028; 0.020
195.68; 96.06
118.91; 102.63
138.41; 138.41
144.01; 102.84
0.109; 0.053
0.065; 0.056
0.076; 0.076
0.082; 0.058
* The value for the isolated cation is given in parentheses.

Diethylamine-based ionic liquids
Russ. Chem. Bull., Int. Ed., Vol. 68, No. 11, November, 2019
2015
DSC/mW mg–1
and formation of ion pairs with various numbers of
hydrogen bonds. The main properties of the synthesized
compounds, whose energetic and geometric characteristics
are given above, are presented further.
1
0
Phase behavior and electrical conductivity of the syn-
thesized salts. Since the character of interactions in the
system affects the physicochemical properties of the com-
21
pounds, we analyzed the phased behavior, thermal
–1
–2
–3
stability, electrical conductivity, and dynamic viscosity
of the synthesized compounds. The temperatures of
phase transitions and decomposition onsets of the salts
measured in this work and available literature data are
presented in Table 3.
As can be seen from the data in Table 3, the salts formed
by trifluoromethane sulfonic and toluenesulfonic acids
have the melting points higher than 100 C and, hence,
they cannot be assigned to ILs but are referred to so-called
molten salts. According to the quantum chemical com-
putation results, the formation of ion pairs with both one
and two hydrogen bonds is possible with equal probabili-
ties in the case of DEA/OTs(OTf). Evidently, the differ-
ence in the number and strength of hydrogen bonds can
change the physical properties of the studied PILs. It is
found64 that in the ILs based on primary alkylammonium
cations strong hydrogen bonds lead to the formation of a
"solid-like" structure, whereas weaker and distorted bonds
favor weakening of the interaction between the ions and
the ILs manifest the "liquid-like" properties.
It is most likely that the structures with one strong
hydrogen bond leading to the formation of "solid-like"
compounds (see Fig. 3, compounds 1 and 2) are formed
in the case of DEA/OTs(OTf). Note that the reactions of
triethylamine with OTf and OTs afford PILs with the
melting points 26—36 (see Refs 10, 13, 16, 35, and 65)
and 80.2 C,66 respectively. It is found by the study of the
thermal characteristics of the PILs based on primary,
secondary, and tertiary ammonium cations with fluori-
nated (heptafluorobutyrate, pentadecafluorooctanoate)
and non-fluorinated (octanoate) anions that the salts with
fluorinated anions had higher melting temperatures com-
pared to similar non-fluorinated salts.67
–20
0
20
40
60
80 100 120 t/C
Fig. 4. DSC thermograms of DEA/OTs for the second heating
and cooling cycles.
For the compounds synthesized in this work, the DSC
curves contain melting points on heating the samples and
transitions to the crystalline state on cooling the melts,
but no transitions to the glassy state were observed. The
DSC thermograms of diethylammonium tosylates are
presented in Fig. 4 as an example.
At the same time, the transitions of the DEA/OMs(HSO4)
samples to the glassy state were observed10 under similar con-
ditions of the DSC experiment, and no transition to the crys-
talline state was detected. When studying16 the thermal beha-
vior of DEA/HSO4, the temperatures of three phase transi-
tions were detected on the DSC curve: at first the transition
of the sample to the glassy state was observed, an overcooled
liquid was formed on heating the glass, and the liquid then crys-
tallized (cold crystallization) and melted on further heating.
The values obtained for the melting points of the
DEA/OMs(OTf) samples agree satisfactorily with the
published data.10,17,26,35 Some difference in the tm value
for DEA/HSO4 from the published data10,12 can be caused
by different contents of water.
The thermal stability of the ionic liquids is character-
ized most frequently by the temperature of the thermal
decomposition onset tonset (Fig. 5).10,68,69
Table 3. Temperatures of glass transitions and decomposition onset for the synthesized compounds (published data
are given in brackets)
Salt
tcr
tm
tg
tonset
ΔpKa
Н2О (wt.%)
C
DEA/OMs
DEA/OTf
125.1
45.5 [31.410, 45.126
]
[–67.810
—
]
,
254.5 [18510
]
12.76 1.0 [<1•10–6 10
22.84 0.4 [<1•10–6 10
18.85 1.4 [<1•10–6 10
]
107.3 118.5 [12510, 12717,35
]
376.5 [36210, 390 (5%)17,35
]
]
DEA/HSO4 110.4
42.2 [77.312, 60.510
5315
101.8
,
[–67.110
289.2 [301.512, 26210
323.1
]
]
[1015
121.1
]
]
–54.415
—
]
DEA/OTs
17.4
0.2
Note. tcr is the crystallization temperature, tm is the melting point, tg is the glass transition temperature, and tonset is
the temperature of decomposition onset; pKa: MsOH –1.9262; TfOH –1253; H2SO4 –1.9934; and TsOH –6.5663
.

2016
Russ. Chem. Bull., Int. Ed., Vol. 68, No. 11, November, 2019
Shmukler et al.
TG (%)
100
logκ
–1.0
–1.5
–2.0
2
1
80
60
40
20
tonset
2
3
4
3
1
2.6
2.8
3.0
3.2
T–1•103/K–1
Fig. 6. Temperature dependences of the electrical conductivity
(κ/Ohm–1 cm–1) for the salts 1, HSO4; 2, OMs; and 3, OTs.
100
200
300
400 t/C
Fig. 5. TG curves of the salts OTs (1), OMs (2), OTf (3), and
HSO4 (4) demonstrating their thermal stability.
at 25 C. The restricted temperature range of measure-
ments of the conductivity of diethylammonium tosylate
is caused by its melting temperature. The highest values
of electrical conductivity are characteristic of DEA/HSO4.
The value of 19.7 mS cm–1 at 120 С, which is fivefold
lower than the conductivity value obtained in the present
work, is presented10 for this IL. On going from DEA/HSO4
to DEA/OMs, the replacement of the hydroxyl group in
the hydrosulfate anion by the methyl group results in the
conductivity increase, which can be induced by the en-
hancement of interionic interactions (see Table 1). On the
whole, the electrical conductivity values correlate with the
total energy of hydrogen bonds in the ion pairs, namely,
the electrical conductivity decreases with an increase in
ЕHB. The electrical conductivity value of PIL is determined
by various factors: viscosity, size of ions, ion aggregation,
and mechanism of proton transfer (hopping and ion-
migration/vehicle). It is known that for the ion-migration
mechanism of proton transfer the electrical conductivity
mainly depends on the viscosity. We measured the visco-
sity of the salts with the melting points lower than 100 C.
The viscosity value for DEA/HSO4 measured in the pres-
ent work approximately three times exceeds the published
value,37,38 where it is 34.1 mPa s at 25 С. It is difficult to
unambiguously explain this difference in viscosity. A com-
parison of the results of the NMR study with that presented
in Ref. 37, which was performed for the identification of
the synthesized substances, showed that in the decoding
of the spectrum37 the number of protons does not coincide
with that declared in the empirical formula. It should be
mentioned that, according to the published results,21 an
increase in the degree of proton transfer results in an in-
crease in the density and viscosity of the IL. The density
value presented in Ref. 37 is 1.28395 g cm–3 at 25 С,
which is also lower than the value obtained by us earlier74:
1.37953 g cm–3 at the same temperature.
However, according to the earlier works,70,71 the up-
per limit of the working temperature at which the ILs
retain their structural integrity is overestimated. The
isothermal studies71 showed that 1-butyl(hexyl)-3-me-
thylimidazolium chlorides demonstrated noticeable de-
composition at the temperatures significantly lower than
tonset, which is determined by fast scanning. The first
measured mass loss (tstart) and the temperature corre-
sponding to the peak on the differential curve (tpeak) are
presented69 as a decomposition temperature for a large
series of ionic liquids based on pyridinium. In addition,
the temperature corresponding to the mass loss of the
sample by 5%,7,16,72 which can coincide with tonset, is
often accepted to be the decomposition temperature. The
values of tonset are given as a decomposition temperature
in Table 3. The obtained decomposition temperatures are
mainly well consistent with similar temperatures presented
in the literature (see Table 3).
It is mentioned in several studies that the thermal
stability of the salts increases with an increase in pKa.68,73
Strong acids should form stable bonds with the cation, and
a large amount of energy is needed to decompose them.
Therefore, salts of strong acids should have higher decom-
position temperatures. As a whole, a tendency to increas-
ing decomposition temperatures of the samples with
an increase in pKa is observed for the compounds syn-
thesized in this work.
The temperature dependences of the electrical con-
ductivity of the synthesized compounds, except for diethyl-
ammonium triflate, are shown in Fig. 6.
The values obtained for the electrical conductivity of
the salts range from 10–3 to 10–1 Ohm–1 cm–1 depending
on the temperature and anion. Diethylammonium hydro-
sulfate exists in the liquid metastable state at the tem-
peratures lower than its melting point, which made it
possible to measure the electrical conductivity of this salt
Although DEA/HSO4 and DEA/OMs have close values
of viscosity (26.7 and 23.5 mPa s, respectively, at 65 С)

Diethylamine-based ionic liquids
Russ. Chem. Bull., Int. Ed., Vol. 68, No. 11, November, 2019
2017
log[/[Ohm–1 cm–2 mol–1
]
and energy of hydrogen bonds (see Table 1), whereas their
electrical conductivities differ as said above (see Fig. 6).
It was concluded26,75 when studying the properties of the
PILs based on a large number of amines (di(tri)ethylamine,
di(tri)-n-butylamine, di-n-propylamine, di(tri)ethanol-
amine, pyrrolidine, ethylenediamine, and others) with
formic, acetic, methanesulfonic, and sulfaminic acids,
saccharin, and di-n-butyl phosphate that the electrical
conductivity of the salts is determined by the type of anion
rather than by viscosity. For example, ethylenediammo-
nium and di-n-butylammonium formates are characterized
by similar viscosity values, but their electrical conduc-
tivities differ significantly. In addition, o-sulfobenzimide
and pyrrolidium acetate had close conductivity values,
while their viscosities differed by nearly two orders of
magnitude. It was also found76 that the molar electrical
conductivities of triethylammonium (TEA) and dimethyl-
ethylammonium (DMEA) trifluoroacetate (TFA) at 65 С
differed insignificantly: from 1.1 Ohm–1 cm2 mol–1 (for
TEA/TFA) to 2.1 Ohm–1 cm2 mol–1 (for DMEA/TFA).
At the same time, the dynamic viscosity of TEA/TFA
(34.9 mPa s) is higher by 4.4 times than that of DMEA/
TFA (7.9 mPa s).
3
2
1
DEA/HSO4
DEA/OMs
0
1
–1
–2
–2
–1
0
1
2
log[–1/P–1
]
Fig. 7. Walden diagram for DEA/HSO4 and DEA/OMs (1,
"ideal" line for KCl). The values of equivalent electrical conduc-
tivity and viscosity for a 0.01 M solution of KCl (designated by
crosses) are taken from Refs 79—81.
tinguished as "bad," "good," or "superionic" depending on
the position of the dependences of the studied compounds:
above or below this line.23 The Walden diagram for
DEA/HSO4 and DEA/OMs is presented in Fig. 7.
Thus, the high electrical conductivity of diethyl-
ammonium hydrosulfate can also be related to the fact
that the jump mechanism during proton transfer can
contribute to the conductivity value. A similar conclusion
was drawn in Ref. 26.
The temperature dependences of the electrical con-
ductivity of the synthesized samples with the melting points
lower than 100 C are well described by the Vogel—
Fulcher—Tammann nonlinear equation (VFT)77,78
The position of the points of the dependence of
DEA/HSO4 above the "ideal" line of KCl can confirm that
the proton transfer in this ionic liquid can proceed via the
jump mechanism. The points of the dependence of
DEA/OMs are close to the line of KCl but somewhat lower
than this line. This deviation can be due to the formation
of both ionic associates and molecular particles. In our
opinion, a comparison of the electrical conductivities and
viscosities of a dilute aqueous solution of KCl containing
almost no ion-ionic interactions and molten salts cannot
adequately characterize the ionicity of the latter. Since
there are no theoretical foundations for choosing an aque-
ous solution of KCl as an "ideal" one, the authors82 also
make doubt concerning the interpretation of the Walden
diagram for the evaluation of the ionicity of ILs.
κ = κ0•exp[–B/(T – To)],
where κ0 is the conductivity at the infinitely high T, B is
the coefficient related to the activation energy of the elec-
trical conductivity Ea, and To is an ideal glass transition
temperature. The calculated parameters of the VFT equa-
tion are presented in Table 4.
To conclude, the structural and energetic characteris-
tics of the ion pairs DEA/А (А = OTs, OTf, OMs, HSO4)
were determined by the density functional theory (B3LYP-
GD3/6-31++G(d,p)). It is shown that the cation and
anion are retained in the ion pair due to electrostatic forces,
hydrogen bonds, and dispersion interactions. Various
relationships between the geometric characteristics of
the hydrogen bond and parameters calculated in the
framework of the NBO and QTAIM methods were ob-
tained. It is shown that the differences in the number and
strength of the hydrogen bonds affected the thermal
characteristics of the synthesized compounds. The phase
behavior and thermal stability of the synthesized salts were
examined. It was found that DEA/OTf(OTs) formed
structures with a very strong hydrogen bond with the
melting points higher than 100 C. The formation of two
The ionicities of the compounds (for which the visco-
sity was measured and the density values are known74) can
be discussed using Walden´s rule. If the liquid consists of
independent ions and their diffusion is determined by
viscosity, we obtain an "ideal" line with the slope close to
1 on the Walden diagram (dilute aqueous solutions of KCl
were chosen as a reference system). Ionic liquids are dis-
Table 4. Coefficients of the VFT equation
PIL
κo/Ohm–1 cm–1
B
To
R2
K
DEA/HSO4 0.027 0.005
DEA/MsO 1.87 0.79
623 73
179 10 0.999
0.999
1206 228 97 25

2018
Russ. Chem. Bull., Int. Ed., Vol. 68, No. 11, November, 2019
Shmukler et al.
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distorted hydrogen bonds of medium strength in the
DEA/OMs(HSO4) ion pairs produces PILs. On the whole,
a tendency to increasing the temperature of decomposition
onset of the samples with an increase in pKa is observed.
The obtained electrical conductivity values of the synthe-
sized compounds, except for DEA/OTf, range from 10–3
to 10–1 Ohm–1 cm–1 depending on the temperature and
anion. The experimental results are consistent with the
data of quantum chemical computations.
The DSC, TG, and NMR studies and impedance
measurements were carried out using the equipment of
the centre for joint use of scientific equipment "The Upper
Volga Region Centre for Physicochemical Research."
This work was financially supported in part by the
Russian Foundation for Basic Research (Project No. 19-
03-00505).
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Received July 9, 2019;
in revised form September 19, 2019;
accepted September 26, 2019