Cetylpyridinium picrate: Spectroscopy, conductivity and DFT investigation of the structure of a new ionic liquid

Article abstract of DOI:10.1016/j.molstruc.2020.129803

A new cetylpyridinium picrate ionic liquid has been synthesized and characterized by differential thermal analysis, XRD, FT-IR, and NMR (¹H and ¹³C) spectroscopy. Differential thermal analysis indicates the congruent melting of cetyl

Full text of DOI:10.1016/j.molstruc.2020.129803

Journal of Molecular Structure 1229 (2021) 129803
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstr
Cetylpyridinium picrate: Spectroscopy, conductivity and DFT
investigation of the structure of a new ionic liquid
a,∗
a,b
a
c
d
Maksym Fizer , Michael Filep , Oksana Fizer , Oľga Fri cˇ ová , Ruslan Mariychuk
a
Faculty of Chemistry, Uzhhorod National University, Pidhirna 46, Uzhhorod 88000, Ukraine
b
Ferenc Rákóczi II Transcarpathian Hungarian Institute, Kossuth Sq. 6, Beregszász 90200, Ukraine
c
Department of Physics, Faculty of Electrical Engineering and Informatics, Technical University of Košice, Letná 9, Košice 04200, Slovak Republic
Faculty of Humanity and Natural Sciences, University of Prešov in Prešov, Prešov 08116, Slovak Republic
d
a r t i c l e i n f o
a b s t r a c t
Article history:
A new cetylpyridinium picrate ionic liquid has been synthesized and characterized by differential thermal
analysis, XRD, FT-IR, and NMR ( H and 13 C) spectroscopy. Differential thermal analysis indicates the con-
gruent melting of cetylpyridinium picrate and a temperature range of liquid state is 47–267 °C. A solid
form of the sample characterized with a polymorphic transformation at 37 °C, which was confirmed via
the XRD analysis.
Received 19 October 2020
Revised 12 November 2020
Accepted 19 December 2020
Available online 24 December 2020
1
Keywords:
Cetylpyridinium
Charge transfer
Conductometry
DFT
The interionic charge-transfer interactions, namely the charge transfer from the picrate anion to the
cetylpyridinium cation, were detected by comparison of NMR spectra of DMSO solutions of the ionic liq-
uid and cetylpyridinium chloride. The H–NMR chemical shifts’ differences of about 0.2 ppm and 0.1 ppm
were observed in the case of the o-protons and the α-methylene group protons, respectively. Additionally,
the charge-transfering between the attracted ions was confirmed by DFT calculations.
NMR
Based on the solid state NMR technique, the higher ions mobility was suggested for the solid sample
of the ionic liquid in comparison to the solid cetylpyridinium chloride monohydrate. Moreover, the as-
sociation constants KA at 22, 30, 40, and 50 °C, obtained from the electrical conductivity measurements,
clearly testifies much lower dissociation in the case of picrate. The KA values of cetylpyridinium picrate
and chloride in DMSO medium at 22 °C equal 748.7 L/mol and 525.0 L/mol, respectively.
The performed DFT computations of the reduced density gradient function in four proposed struc-
tures of the new ionic liquid cation-anion pair identifies the presence of weak non-covalent C–H•••O=N
XRD
interactions between cetyl chain hydrogen atoms and nitro groups of picrate was explored through DFT
calculations and analysis of the RDG function.
©
2020 Elsevier B.V. All rights reserved.
1
. Introduction
The CP cation is also very popular in the field of analytical
chemistry, primarily for its cationic surfactant nature and ability to
act as a counter-ion in ion-pair based ionophores. CP cations have
been used for the construction of ion-selective electrodes’ mem-
branes sensitive to iodide [10], periodate [11], and alkyl sulfates
[12], and can be used for the detection of cholesterol [13]. More-
over, CP can be used to modify silica to improve the analysis of
food dyes [14].
Cetylpyridinium (CP) cation, also called 1-hexadecylpyridinium,
has been a steady focus of scientific interest. According to the
Google Scholar database, “hexadecylpyridinium” was mentioned in
2
560 and “cetylpyridinium” in 15,300 publications and patents in
the period 2010–2019.
The scientific interest in the CP cation is motivated by the ex-
tensive usage of its salts in industrial and commercial cleaning
formulations, cosmetics, and pharmaceuticals. The use of CP salts
mainly due to its antiseptic [1–3] and surfactant [4–8] properties,
while some are used as catalysts in organic synthesis [9]. The most
common commercially available salts of CP are chloride (CPC) and
bromide, in monohydrate or anhydrous forms.
Taking into account the wide usage of CP-containing com-
pounds and the longstanding interest in CP-selective membranes,
we investigated the CP dipicrylamide ionic pair as an ion-
exchanger for a plasticized CP-selective electrode in a previous
work [15]. Continuing our investigations in this field, we have
synthesized cetylpyridinium picrate (CPPA). The CPPA ionic pair
has a low melting point (47 °C), and can be considered a low-
temperature ionic liquid [16]. It must be noted that the formation
of the CPPA ion pair has been studied previously [17] and this as-
∗
Corresponding author.
E-mail address: max.fizer@uzhnu.edu.ua (M. Fizer).
https://doi.org/10.1016/j.molstruc.2020.129803
0
022-2860/© 2020 Elsevier B.V. All rights reserved.

M. Fizer, M. Filep, O. Fizer et al.
Journal of Molecular Structure 1229 (2021) 129803
sociation process in water can be used for the detection of CPC
DMSO–d6 solutions of CPPA and CPC were obtained by dissolving
of 49 mg and 33 mg of salts in 0.6 mL of the solvent, respectively.
Solid-state NMR spectra of CPPA and CPC were recorded on a
Varian 400 MHz NMR spectrometer using adamantane as an ex-
[
18].
Taking a step aside from analytical measurements and applica-
tions, we decided to explore the molecular and electronic structure
of this compound with the assumption that such knowledge will
support the explanation and interpretation of future studies of its
role as an ion-exchange agent. Thus, in the present work, we dis-
cuss the structure of the ionic liquid that consists of the CP cation
and the picrate anion (PA) – CPPA.
ternal standard. The MAS frequency used for the measuring of 1
H
NMR spectra was 12 kHz. 13C NMR spectra were acquired using
cross-polarization, dipolar decoupling (DD), and magic angle spin-
ning (MAS) with a spinning rate of 10 kHz.
The XRD powder patterns of CPPA samples were recorded using
an AXRD Benchtop powder diffractometer (Proto Manufacturing
Limited) equipped with a hybrid photon counting detector. The ex-
perimental data were collected in the θ/2θ mode (Bragg–Brentano
geometry) using the CuKα radiation (Ni filter) in 5–70 2θ angle
range with 0.02° step, counting time of 1 s per step and a dy-
namic region of interest. Lattice constants were determined using
Expo2014 software [34]. PowderCell 2.4 [35] was used for phase
analysis.
Ionic liquids (ILs) form a wide class of organic salts that have
relatively low melting temperatures (below 100 °C) and remain liq-
uid across a wide temperature range [19,20]. ILs’ crystal structures
are responsible for their low melting points; they are generally
built from large, asymmetric organic cations and organic/inorganic
anions bonded together by weak van der Waals or hydrogen
bonds to form a dimensional framework [19,21,22]. These struc-
tures determine their interesting physico-chemical properties, like
high electrical conductivity and thermal stability [23], good sol-
vating characteristics, negligible vapor pressure, and high viscosity
The CPPA samples were characterized using differential thermal
analysis (DTA). The DTA curves of CPPA were recorded in the air
atmosphere in the temperature range 25–400 °C with a heating
rate of 12 °C/min. The temperature of the effects on the DTA curves
was determined as proposed in [36].
[
19,21,24,25].
The large number of available bulky organic ions and their
many possible combinations enable wide variability in the com-
position of ILs and their adjustment to meet the requirements of
a particular process (as “designer solvents”). Due to the wide vari-
ability of ILs and the possibility of tuning their composition and
properties, ILs have found application as novel solvents in organic
synthesis [26], for separation in analytical chemistry [27], and as
solvents for nanoparticles stabilization [28] in addition to serving
as catalysts [29], corrosion inhibitors [30], lubricants [31], and elec-
trochemical sensors and electrolytes [32]. ILs also exhibit biological
activity and can be used in pharmaceuticals [33].
Conductometric studies were carried out with a WTW inoLab
Multi 9620 IDS digital conductivity meter. The specific conductance
(κ) of CPPA and CPC monohydrate in DMSO was measured for the
temperature range from 22 to 50 °C. The temperature of solutions
was maintained with an accuracy of ± 0.1 °C using the LabExpert
1021 water thermostat.
2.3. Computational software and methods
The starting geometries were created and pre-optimized in Avo-
gadro [37]. Refinements of the geometries of considered CPPA
structures were performed with PBE GGA functional [38] and 6–
2. Experimental
3
11G(d,p) basis sets [39]. The selection of this method was moti-
2
.1. Synthesis of cetylpyridinium picrate (CPPA)
Caution! Synthesis must be executed with specific safety pre-
vated by the good performance of GGA functionals with triple-zeta
basis sets for modeling of geometric parameters of ionic liquids
[
40,41] and by our previous good experience in using the PBE/6–
11G(d,p) combination [42–44]. All computations related to PBE
cautions. Picric acid is explosive and corrosive. CPC is irritating to
skin and eyes and toxic if swallowed or inhaled. Distillated water
was used in all experiments.
3
functional were done using PRIRODA 19 code [45].
The computations related to PBE0 [46], M06–2X [47] and
ωB97X [48] functionals were performed in the ORCA 4.2.1 pack-
age [49]. The investigation of non-covalent interactions (NCI)
with reduced-density-gradient (RDG) method [50] and related
atoms-in-molecules (AIM) analysis of critical points were per-
formed with Multiwfn 3.6 [51]. In the present study, differ-
ent atomic population analyses were performed to obtain partial
atomic charges: Mulliken, atomic dipole moment corrected Hirsh-
feld (ADCH) [52] scheme, and natural population analysis (NPA).
NPA was performed with JANPA 2.01 [53]. Gabedit [54] was used
for input file preparation, and VMD [55] for visualization.
Picric acid (2.30 g, 10 mmol) was dissolved in 50 mL of wa-
ter with NaOH (0.41 g, 10 mmol) to obtain clear yellow solution
A. CPC monohydrate (3.58 g, 10 mmol) was dissolved in 100 mL
of warm water (solution B). Solutions A and B were mixed with
the immediate formation of bright yellow oil. The yellow oil solid-
ified upon standing at room temperature for 2 days. The precipi-
tate was filtered, washed with water, and air-dried. The result was
a yellow solid in 92% yield (4.90 g), mp 46 °C. 1H NMR (DMSO–
d6, 400 MHz), δ (ppm): 9.08 (2H, d, o-CHpyridinium), 8.60 (1H, t, p-
CHpyridinium), 8.58 (2H, s, CHpicrate), 8.15 (1H, t, m-CHpyridinium), 4.58
(
2H, t, CH N), 1.90 (2H, m, β-CH ), 1.22–1.26 (26H, m, 13[CH ]),
2 2 2
0
.84 (3H, t, CH ). 13C NMR (DMSO, 100 MHz), δ (ppm): 160.80,
3
3. Results and discussion
1
45.43, 144.71, 141.83, 128.06, 125.15, 124.10, 60.78, 31.28, 30.71,
9.03, 29.00, 28.90, 28.76, 28.69, 28.36, 25.39, 22.07, 13.89. Ele-
mental analysis, found (%): C, 61.1; H, 7.8; N, 10.3; molecular for-
mula C33H42N O requires (%): C, 60.9; H, 7.6; N, 10.5.
2
3
.1. Structure of the cetylpyridinium picrate ionic liquid
The CPPA ion pair was obtained as a product of an ion-exchange
8
12
reaction between CPC and PA in water (Scheme 1). An equimolar
amount of NaOH was used for the neutralization of picric acid. This
step also prevents possible hydrogen chloride formation from the
reaction of CPC with picric acid and shifts the reaction equilibrium
to the formation of the desired CPPA. The association of the PA
and CP cation is driven by the electrostatic attraction between the
negatively charged phenolic oxygen of the PA and the positively
charged nitrogen of the pyridinium ring. However, literature data
testify that interactions of PA with organic cations in ionic liquids
2
.2. Instruments and measurements
Fourier-transform infrared spectra (FT-IR) were recorded on a
Shimadzu IRPrestige-21 spectrometer in attenuated total reflection
(
ATR) mode with a zinc selenide crystal.
NMR spectra of CPPA and CPC solutions in deuterated dimethyl
sulfoxide (DMSO–d6) were recorded on a Varian Mercury-400
instrument with tetramethylsilane used as an internal standard.
2

M. Fizer, M. Filep, O. Fizer et al.
Journal of Molecular Structure 1229 (2021) 129803
Scheme 1. Synthesis of CPPA ionic liquid.
Table 1
Relative total energies (E), enthalpies (H) and Gibbs free energies (G) (in kcal/mol) of forms A–D computed with three
different functionals; the number of non-covalent BCPs between anion and cation in CPPA (NBCPs); the maximal value
of BPC’s electron density (MAXBCP, e× A˚ 3); the sum of electron density values of all interionic BPCs (ꢀρ(r)BCP, e× A˚ –3).
–
PBE-D3BJ
PBE0-D3BJ
M06–2X
NBPCs
ꢀρ(r)BCP
MAXBCP
E
H
G
E
H
G
E
H
G
PBE0-D3BJ
A
B
C
D
2.12
2.09
0.00
2.41
2.12
2.00
0.00
2.39
1.43
2.05
0.00
1.35
3.27
3.59
0.00
3.06
3.28
3.50
0.00
3.05
2.58
3.54
0.00
2.01
2.53
3.45
0.00
1.89
2.54
3.36
0.00
1.87
1.84
3.41
0.00
0.83
6
7
7
4
0.410
0.444
0.621
0.295
0.136
0.150
0.226
0.123
have a complex character, and numerous N=O•••H–C interactions
are present [56].
BCPs is 4 in D, 6 in A, and 7 in B and C (Table 1). In the case
of structures A, B, and D, N=O•••C and N=O•••H–C interactions
are present, whereas in structure C, only N=O•••H–C interionic in-
To explore the anion–cation interactions in the synthesized
ionic liquid, we analyzed the electronic structure of the CPPA
molecule using DFT. Numerous works on organic ionic pairs
teractions were detected. Moreover, in geometry C, four BCPs cor-
respond to the anion–alkyl chain interactions, unlike A, B, and D,
where the anion–cetyl chain interactions are characterized by only
two BCPs. The correlation between the A–D forms’ total enthalpies
and maximal electron density of interionic BCP, or the sum of elec-
tron densities of interionic BCPs, is presented in supplementary
materials. The sum of characteristic values of non-covalent BCPs
was used as for the explanation of ion association in our previous
study [42].
[
15,40,57] have shown that different optimized relative positions
of ions cause only marginal energy differences among the corre-
sponding structures; consequently, numerous different geometries
can exist in equilibrium. In the present work, we have adopted
an established known methodology for modeling pyridinium-based
ionic associates [40]. We have considered four cation–anion dispo-
sitions, where the anion is placed in the inside and outside cav-
ities formed by two rotamers of CP, one with the perpendicular
arrangement of the cetyl chain with the pyridinium ring, and the
other almost linear cation (see Fig. 1). PA can interact with the
CP cation in different ways. That is why, in all structures consid-
ered, we have imposed anion such that the phenolic oxygen was
as close as possible to the o-position of the pyridinium and the
α-methylene group of the cetyl chain, as these are the most posi-
tively charged areas of the CP cation [15]. Optimization of molec-
ular geometries and calculations of corresponding Hessians, zero-
point vibrational energies and entropies were performed at the
PBE/6–311G(d,p) level.
Visual analysis of the nature of non-covalent interactions (NCI)
can be done using the reduced density gradient (RDG) method
[50]. A visualization of this analysis for structures A–D is shown
in Fig. 1. Red localized areas inside the rings correspond to the
steric repulsive interactions, whereas the large green areas repre-
sents van der Waals interactions in the CPPA associate. Blue areas
correspond to H-bonds (strong interaction). This analysis explains
the stability of structure C; a strong interaction is present between
the phenolic oxygen of PA and the o-hydrogen of the pyridinium
ring. The scatter graphs of the RDG analysis presented in supple-
mentary materials.
During the geometry optimization of structure C, the anion
shifted from the cavity significantly and coordinated near the side
of the pyridinium cycle. In the resulting geometry, both the anion
and the cation rings were nearly in one plane. Cartesian atomic
coordinates of optimized geometries are presented in the supple-
mentary materials.
AIM analysis of BCPs and NCI explored via the RDG function
indicate the presence of numerous weak interactions between the
CP cation and the PA. Many weak van der Waals interactions are
due to the N=O•••H–C attraction between anion and cation. The
analysis of the anion’s electron density generated at the PBE/6–
311G(d,p) level, identifies the partial negative charge of the ni-
tro groups. Hence, Mulliken, ADCH, and NPA partial charges and
the Mayer bond order analysis indicate high charge delocalization,
which can be explained by the presence of numerous resonance
forms (see Scheme 2). The choice of ADCH partial charges was
dictated by the known excellent performance of these charges for
the reproduction of molecular dipole moments and chemical re-
activity [58], in contrast to Mulliken and natural partial atomic
charges, which are useful in the prediction of physical properties
(e.g. logPo/w) [59].
Table 1 shows the relative total energies, enthalpies, and Gibbs
free energies of all four possible structures A–D. Single point cal-
culations of total energies with PBE, PBE0, M06–2X functionals in
combination with the 6–311G(d,p) basis set were performed on the
geometries optimized at the PBE/6–311G(d,p) level. According to
the data in Table 1, the arrangement C is for 0.83–3.59 kcal/mol
more stable than the other considered structures. This stabilization
can be explained by the formation of multiple non-covalent inter-
actions between the anion and the alkyl chain and the ring of the
cation. It is possible that multiple forms exist in the real bulk ionic
liquid sample, and some kind of agglomerates may even be prefer-
able. However, at this point, consideration of the simplified forms
A–D gives a general idea of the different anion–cation interactions.
Analysis of structures A–D in terms of atoms-in-molecules the-
ory shows the presence of several interionic bond critical points
In Mulliken population analysis, the partial charge of pheno-
lic oxygen equals –0.28e, and the charges of oxygen atoms of the
nitro groups are in the range–0.25e to –0.29e. Similar electron
density redistribution is observed for the ADCH (NPA) population
scheme: the partial charge of the phenolic oxygen is –0.37e (–
0.54e) and the partial charges of the nitro group oxygen atoms are
in the range–0.29 to –0.32e (from –0.35 to –0.42). The bond order
(BCP) (purple dots in Fig. 1). The number of these non-covalent
3

M. Fizer, M. Filep, O. Fizer et al.
Journal of Molecular Structure 1229 (2021) 129803
Fig. 1. Four structures represent the different reciprocal arrangement of CP and PA ions in the CPPA ionic associate: (A) CP rotamer is linear and PA coordinated outside the
cavity; (B) CP cation is linear and PA located inside the cavity; (C) the cetyl chain is perpendicular to the pyridinium ring, and the anion is inside the cavity; (D) the same
structure as C, but with the anion outside the cavity. The non-covalent bond critical points are shown as purple dots and the corresponding paths are shown as yellow lines.
Reduced density gradient isosurface plot for four different anion–cation arrangements. The strong H-bond in structure C is shown.
Scheme 2. Resonance structures of PA.
4

M. Fizer, M. Filep, O. Fizer et al.
Journal of Molecular Structure 1229 (2021) 129803
Fig. 2. DTA curve of CPC monohydrate (A), as-synthesized CPPA after first and second heating (B) and DTA curves of once-melted CPPA (C).
analysis indicates the presence of a double bond between the phe-
nolic oxygen and the ring, which is in agreement with the lit-
erature [60]. The nitrogen–oxygen bond orders are close to 1.5,
whereas the C–NO2 bonds are single, with orders of about 0.8–
0
.9. The four analyses indicate that the formal negative charge was
nearly equally delocalized over all oxygen atoms of the PA. The full
list of the partial charges and bond orders is provided in the sup-
plementary material.
3
.2. Differential thermal analysis
DTA analysis was used to establish the melting point and the
decomposition temperature of CPPA. The DTA heating curve of
CPPA was compared with the heating curves of CPC and PA to con-
firm the absence of CPC and picric acid impurities. PA melts at 117
°C and begins to decompose at 163 °C [61]. No data on thermal
studies of CPC were found in the literature. The experimental DTA
curve of CPC monohydrate (see Fig. 2A) contains four endothermic
effects, at 82, 177, 212, and 326 °C.
Fig. 3. Powder XRD diffraction patterns of as-synthesized CPPA (A), CPPA after
heating (B) and CPC monohydrate (C).
Defined amounts of as-synthesized CPPA (0.202 g) were placed
in specialized quartz containers and heated to 400 °C. The DTA
heating curve (see Fig. 2B) contains one endothermic effect, at 47
°
C, and one exothermic effect, at 267 °C. The peaks in the heating
Analysis of the phase composition of CPPA was carried out by
comparing the experimental diffraction pattern with the simulated
diffraction patterns of CPC monohydrate and picric acid [63]. The
absence of the initial phase’s peaks indicates the identity of the
obtained compound. The powder pattern of CPPA reveals sharp
diffraction peaks, and the crystallinity of the investigated sample
is 75%. Indexing of obtained powder pattern indicates that the as-
grown CPPA crystals are a monoclinic crystal system with the fol-
curve of the CPPA do not match the thermal effects of the initial
compounds. The exothermic effect corresponds to the decomposi-
tion of the CPPA sample, confirmed by the change in appearance
(
(
from yellow crystals to black powder and film) and weight loss
ꢁm = 74.5%). The sharpness of the effect confirms the rapid de-
composition of CPPA.
The second heating was performed to only 100 °C to prevent
decomposition. Due to the absence of weight loss, the endothermic
effect at 47 °C corresponds to the melting of CPPA.
The once-melted sample of CPPA was repeatedly investigated
by DTA in a temperature range of 25–100 °C (see Fig. 2C). In the
heating curves, one additional endothermic effect appears at 33 °C.
Taking into account the absence of weight loss, the appearance of
the additional endothermic peak can be explained by a polymor-
phic transformation. The endothermic peaks at 33 and 47 °C in the
heating curve also agree with the exothermic effects in the cooling
curve.
˚
˚
˚
lowing cell parameters: a = 12.88 A, b = 19.47 A, c = 11.29 A,
˚ 3
β = 103.58°, with cell volume V = 2753 A . Based on the ob-
3
tained cell volume and density measurements (d = 1.2075 g/cm ),
the number of formula units Z was determined to be 4.
Analysis of the melted CPPA sample powder pattern (Fig. 3B)
indicates differences between its diffraction pattern and that of
as-synthesized CPPA (Fig. 3A). Considering both heating curves
(
Fig. 2B and C), it can be noted that a high-temperature poly-
morphic modification of CPPA occurs during synthesis from solu-
tion; this indicated by the presence of only one endothermic ef-
fect in the heating curve. After melting, an additional peak ap-
pears (Fig. 2C), which corresponds to phase transformation. Thus,
the CPPA recrystallizes in a low-temperature modification.
However, the diffraction peaks of high-temperature modifica-
tion can be detected in low-temperature modification diffraction
patterns due to the low melting point, slow crystallization rate,
and high viscosity of the CPPA melt. Determination of the lat-
tice parameters of the low-temperature modification CPPA sam-
ple was not possible due to the complexity of the separation of
the reflex systems belonging to the high- and low-temperature
modifications.
3
.3. XRD analysis
The phase content of CPPA and CPC monohydrate was investi-
gated by XRD. XRD powder patterns of as-synthesized CPPA, once-
melted CPPA, and CPC monohydrate are shown in Fig. 3. Analysis
of the phase composition of CPC monohydrate was carried out by
comparing the experimental diffraction pattern with data from the
literature [62]. The CPC monohydrate diffraction pattern is charac-
terized by the presence of only one system of reflexes which cor-
responds to triclinic crystal system or symmetry.
5

M. Fizer, M. Filep, O. Fizer et al.
Journal of Molecular Structure 1229 (2021) 129803
appear as a medium peak at 1471 cm 1. A sharp medium peak
−
at 1433 cm 1 corresponds to C–C stretching in PA. Two bands, at
−
364 cm 1 and 1334 cm , could be attributed to O–N–O sym-
−
−1
1
metric stretching vibrations. Calculated modes 141 and 148, with
frequencies of 1288 cm 1 and 1316 cm , respectively, correspond
−
−1
to these vibrations. The most intense experimental peak at 1274
cm 1, according to the DFT-computed spectrum, can be assigned to
−
the rocking vibration of the C–C(O)–C group, with the correspond-
ing calculated mode 137 characterized by frequency of 1281 cm 1.
−
The peaks at 1157, 1182, and 1073 cm 1 are related to in-plane
aromatic C–H deformation of the PA benzene ring. The C–NO2
stretching vibrations are seen as two low-intensity peaks at 908
−
and 928 cm 1. Two sharp medium peaks at 787 cm
−
−1
and 778
cm 1 correspond to the out-of-plane vibrations of aromatic C–H
−
groups.
The calculated IR spectra of structures A–D are presented in the
supplementary material.
3
.5. NMR study
NMR spectra of CPPA were recorded in (CD ) SO and compared
3
2
with the NMR spectra of CPC. Figs. 5 and 6 show 1H NMR and 13
C
Fig. 4. FT-IR ATR spectra of solid-state CPPA, and CPPA in solutions of DMSO, chlo-
NMR spectra of CPPA overlapped with the corresponding spectra
of CPC.
roform.
The two protons of PA appear in the spectrum as a sharp sin-
glet at 8.58 ppm (marked with a star in Fig. 5). The signal of
the o-protons of the pyridinium ring appears at 9.08 ppm (dou-
blet), which is upfield by about 0.20 ppm in comparison to CPC. At
8.60 ppm, the triplet of the p-proton is present, and only 0.03 ppm
upfield relative to CPC. The chemical shift of the triplet of the pyri-
dinium m-protons is 8.15 ppm, whereas in the CPC spectrum, the
corresponding signal is located at 8.18 ppm.
3
.4. FT-IR spectroscopy
FT-IR ATR spectra of CPPA were recorded for the solid-state
sample and three solutions with solvents of different polarities:
DMSO, chloroform, and p-xylene (Fig. 4). The signals of the pure
solvents were subtracted from the recorded solutions’ data. The
characters of all spectra are similar, with only a marginal difference
of some peaks’ intensities, which testifies the same CPPA structure
is present in different phases. m and p-xylene. The computed FT-IR
spectrum of structure C is marked as DFT.
Chemical shifts of cetyl chain protons in CPPA and CPC
molecules are well-matched, except the signal of the α-methylene
group. In the case of CPPA, the α-CH2 group signal shows as a
triplet at 4.58 ppm, whereas, in the case of CPC, the signal of
the same group appears at 4.68 ppm. The chemical shift of the
β-CH2 group protons shifts upfield and appears as a multiplet at
1.90 ppm. The signal of the preterminal methylene group is present
at 1.26 ppm, overlapping with the intense signal at 1.22 ppm that
corresponds to protons attached to C –C carbons of the cetyl
To assign the experimental peaks, we compared the experimen-
tal solid-state CPPA spectrum with the DFT-computed wavenum-
bers for structure C (Fig. 4). Weak peaks in the region 3069–
3
126 cm 1 (computed modes 225, 226, and 227 with correspond-
−
ing frequencies of 3153, 3158, and 3159 cm 1, respectively) were
−
3
14
assigned to C–H stretching of PA and pyridinium ring. Two ex-
chain. The peak of the terminal methyl group of the cetyl chain
appears as a triplet at 0.84 ppm.
−
1
perimental medium bands at 2915 and 2849 cm
correspond to
asymmetric and symmetric C–CH2 stretching vibration of the cetyl
chain, respectively. The corresponding calculated modes 190–203
The signal at 160.80 ppm corresponds to the C–O carbon of
PA (see Fig. 6). Peaks at 141.83, 125.15, and 124.10 ppm corre-
spond to the o-, m- and p-carbons of PA, respectively. Signals of
p-, m- and o-carbons of pyridinium ring appear at 145.43, 128.06,
and 144.71 ppm, respectively. A peak of the α-methylene group
(
symmetric) and 204–222 (asymmetric) are characterized by fre-
−
1
quency ranges from 2937 to 2956 cm
and from 2958 to 3057
cm 1, respectively. Most intense symmetric and asymmetric cetyl
chain C–H stretching vibrations in the calculated IR spectrum are
represented by modes 203 and 217, with respective frequencies of
−
appears at 60.78 ppm. Signals of the cetyl chain C –C carbons
2
16
are present in the upfield region. Two close peaks at 31.28 and
30.71 ppm correspond to the C2 (β-methylene group) and C14 car-
bons, respectively. The six peaks in the range 28.36–29.03 ppm
correspond to the C –C carbons. A signal of the γ -methylene
2
956 and 3007 cm 1
A medium-broad peak at 1642 cm
stretching of PA. Computed mode 187 corresponds to this C–O vi-
−
.
−
1
corresponds to the C–O
4
13
bration with frequency of 1625 cm 1. Medium sharp signals at
−
group appears at 25.39 ppm. In the most upfield region, the sig-
nals of preterminal and terminal carbons of the cetyl chain appear
at 22.07 and 13.89 ppm, respectively.
−
−1
−1
1
642 cm 1, 1637 cm , 1624 cm , and 1610 cm
−1
are caused by
the C–C stretching in both aromatic rings. The corresponding com-
puted modes 188 (symmetric, pyridinium), 187 (symmetric, PA),
We must spotlight the perfect match of chemical shifts of CPPA
and CPC, except in the cases of pyridinium o-carbons and α-
methylene group. In contrast to CPC, the CPPA signals of the o-
carbons and α-CH2 group are shifted upfield by 0.14 and 0.28 ppm,
respectively. The differences are marginal but rather high in com-
parison to other signals; similar upfield shifts of CP NMR peaks
were observed in our previous work [15].
1
86 (asymmetric, pyridinium) and 185 (asymmetric, PA) have fre-
quencies of 1631, 1625, 1573 and 1570 cm 1, respectively. Visual
analysis of mode 187 indicates that C–C stretching coincides with
the intense C–O stretching.
−
−
1
A medium sharp peak at 1547 cm
caused by asymmetric O–
N–O bending. Related computed modes 231 and 232 are charac-
−1
−1
terized by frequencies of 1552 cm
and 1562 cm , respectively.
These upfield shifts can be caused by electron shielding of
the pyridinium ring via charge transfer (CT) interaction with
the electron-donating PA. Numerous works have indicated the
The rocking vibrations of the pyridinium ring are observed as a
strong peak at 1483 cm 1, whereas the scissoring C–C–H vibrations
−
6

M. Fizer, M. Filep, O. Fizer et al.
Journal of Molecular Structure 1229 (2021) 129803
Fig. 5. Overlapped 1H NMR spectra of CPPA and CPC.
Fig. 6. Overlapped 13 C NMR spectra of CPPA and CPC.
Table 2
tor, rather than donating, properties of CP. The difference between
d(PA) and b(CP) can be considered as the number of net electrons
transferred from PA to CP due to the formation of the correspond-
ing complex orbital. The ECDA method was employed to calculate
net electron transfer values without the contribution of the elec-
tron polarization effect. These values (from 0.1388 to 0.1995) indi-
cate the transferring of electron density from PA to CP, and explain
the experimentally observed electron shielding of the α-methylene
group and o-hydrogens of the CP ion.
The number of donated electrons from PA to CP – d(PA); elec-
trons back donated from CP to PA – b(CP); net electron transfer
values calculated via ECDA. Net charge difference (NCD) calcu-
lated with Hirshfeld partial charges.
d(PA)
b(CP)
d(PA)-b(CP)
ECDA
NCD
A
B
C
D
0.1066
0.1180
0.1642
0.1026
−0.0095
−0.0112
−0.0018
−0.0084
0.1161
0.1292
0.1660
0.1110
0.1469
0.1670
0.1995
0.1388
0.204
0.218
0.299
0.195
Moreover, the quantity of electrons transferred between two
fragments was calculated by the net charge difference (NCD) ap-
proach. The partial charge of fragments (CP and PA) were calcu-
lated by summing all atomic charges in the ion and then sub-
tracted from the charge of an isolated ion. The Hirshfeld charges
were used for the investigation of CT effects via NCD calculation.
The net charge difference values in Table 2 represent the num-
ber of electrons donated from the PA to CP. The observed values in
the range 0.195–0.299 testify to the significant CT effect. We found
a good correlation between all approaches (CDA, ECDA, and NCD)
used for the quantitative analysis of CT effects.
presence of CT interactions in organic picrates [64,65]. A complex
DFT study was performed to explore CT interactions in CPPA. The
superior performance of range-separated density functionals ver-
sus single hybrid functionals in the description of CT processes and
interactions is well-known. That is why we have used the range-
separated wB97X functional in combination with the 6–311G(d,p)
basis set for the generation of electron densities of structures A–D
and the corresponding separate ions.
The NMR spectra described above give a general picture of the
CPPA structure and interactions between the ions. However, to ex-
clude possible solvent effects on the CPPA structure, the samples of
the ionic liquid were explored using solid-state 1H and 13C cross-
polarization/MAS/DD NMR techniques. Overlapped 1H MAS NMR
spectra of CPC and CPPA are presented in Fig. 7. In the case of CPC,
the spectrum contains a very broad area with only two identifiable
small peaks at 1.35 ppm and 3.55 ppm. These correspond to the
hydrogen atoms of the cetyl chain and the hydrogen atoms of the
co-crystallized water molecule, as the monohydrated form of CPC
was used in this study. The broad area at about 7 ppm relates to
the pyridinium hydrogen atoms. In the case of CPPA, the spectrum
Analysis of the electron density differences between the CPPA-
associated and separate CP and PA ions indicate the electron den-
sity transfer from PA to the o-hydrogens of the pyridinium ring
and α-methylene group (see supplementary materials). For quanti-
tative analysis of CT, charge decomposition analysis (CDA) [66] and
its extended form (ECDA) [67] were performed in Multiwfn (see
Table 2). The extent of electron donation (denoted as d(PA)) from
PA to CP via the corresponding complex orbital is in the range of
0
.1026–0.1642.
In contrast, the number of electrons back-donated (denoted
b(CP)) from CP to the anion has negligible values, from –0.0112
to –0.0018, and the negative values indicate the higher accep-
7

M. Fizer, M. Filep, O. Fizer et al.
Journal of Molecular Structure 1229 (2021) 129803
Table 3
Molar conductivities at infinite dilution (ꢂ∞) and associa-
tion constants (KA) for solutions of CPPA and CPC in DMSO
at different temperatures.
2
ꢂ
∞, S×cm /mol
KA, L/mol
Temperature, °C
CPPA
CPC
CPPA
CPC
2
3
4
5
2
0
0
0
38.3
36.6
34.3
33.6
37.3
35.6
35.1
33.7
748.7
695.0
508.3
467.1
525.0
436.2
439.8
351.3
The specific conductance of CPC and CPPA (see supplemen-
tary materials) solutions in the concentration range 2.510–4
.610–1 mol/L increases linearly with increasing solute content. The
–
1
Fig. 7. Overlapped (unnormalized) solid-state 1H MAS NMR spectra of CPPA and
specific conductance of the investigated solutions decreases with
increasing temperature. In all cases, the conductance of CPC so-
lutions is slightly higher than that of the corresponding CPPA so-
lutions, and this phenomenon can be related to the higher molar
ionic conductivity of the chloride anion in comparison to the PA
CPC.
contains relatively sharp peaks at 1.16 ppm, 5.08 ppm, 8.62 ppm,
and 11.56 ppm, which correspond to the cetyl chain protons, the
two protons of the N–CH2 group, pyridinium and the PA rings hy-
drogen atoms, respectively. In MAS NMR studies, broader peaks in-
dicate lower internal mobility of atomic groups in the molecule,
whereas sharp signals indicate higher mobility. Consequently, the
CPC spectra indicate a relatively fixed internal structure, while the
spectra of CPPA ionic liquid with flexible and rotatable structure
contains relatively sharp peaks.
[
72].
The slopes of the plots of the concentration dependence of spe-
cific conductivity against the total concentration of CPC and CPPA
do not contain sharp differences. The molar conductivity (ꢂm) for
CPC and CPPA in DMSO was calculated to determine the CMC (mo-
lar conductivity can be calculated as ꢂm = κ / CM). Fig. 9 presents
the values of molar conductivity obtained at different temperatures
as a function of the molar concentration square root (CM1/2) of CPC
Similarly, in the case of 13C cross-polarization/MAS/DD NMR
and CPPA. The molar conductivity in both cases decreases with the
concentration increase.
(
Fig. 8), the relatively narrow signal at 16.46 ppm indicates the
high mobility of the corresponding methyl group. The broad area
from 24.40 to 34.41 ppm is caused by the overlapping of signals
The DMSO solutions of CPC are characterized by two values of
CMC: The first occurs in a dilute region and refers to micelle for-
mation. The value of CMC (0.01 mol/L) does not depend on tem-
perature. The second CMC is related to a change of micelle type,
and it is sensitive to temperature change. Thus, at 22 and 30 °C,
the CMC is 0.14 mol/L, whereas at 40 and 50 °C, it is 0.12 mol/L.
The above relationships were used to calculate the limiting mo-
lar conductivity (ꢂ∞) and related standard-state association con-
of similar carbons of the cetyl chain. The broadness of the N–CH2
group of peaks at 61.87–63.17 ppm is caused by cetyl chain disor-
der due to its flexibility, as shown in the discussion of DFT calcula-
tions above. Peaks in the range of 124.80–148.94 ppm correspond
to the carbons of the pyridinium and PA rings. The most downfield
signal, at 164.13 ppm, is related to the C–O group of the PA carbon
atoms.
stants (K ). Ostwald’s dilution law combined with the Arrhenius
A
relation [73] enables the determination of the limiting value of the
3
.6. Conductometric study
molar conductivity (ꢂ∞) (Eq. (1)).
Many compounds with long or bulky ions, such as ILs or surfac-
1
1
KA
=
+
× ꢂm × CM
(1)
2
tants, can aggregate and form micelles [68]. In order to investigate
the possible influence of ion association and micelle formation on
the NMR chemical shifts, conductometric studies were performed.
Electrical conductivity measurements can be used for the direct
determination of critical micelle concentrations (CMC) [69]. Most
of the conductometric studies related to CPC micelle formation
ꢂ
m
ꢂ
∞
ꢂ
∞
For these purposes, the linear form of the Ostwald’s dilution
law was plotted in the form of 1/ꢂm = f (c × ꢂm). Extrapolation
of the line to the zero concentration point gives the required value
of ꢂ∞.
(
CMC, energetic parameters) are deal with dilute (10–3–10–4 mol/L)
The ion-pair formation is characterized by the association con-
water–polar organic binary mixtures [70,71].
stant K . Table 3 summarizes derived molar conductivities at infi-
A
Fig. 8. 13 C solid-state cross-polarization/MAS/DD NMR spectra of CPPA.
8

M. Fizer, M. Filep, O. Fizer et al.
Journal of Molecular Structure 1229 (2021) 129803
Fig. 9. Molar conductivity dependence of CPC (A) and CPPA (B) concentration in DMSO.
nite dilution (ꢂ∞) and related standard-state association constants
KA). The KA values are relatively large, in comparison to simi-
Declaration of Competing Interest
(
lar ionic liquids [74], which testifies the higher association in the
case of CP salts. Moreover, at all tested temperatures, ions in CPPA
seems to be more associated than in CPC.
The authors declare that they have no known competing finan-
cial interests or personal relationships that could have appeared to
influence the work reported in this paper.
In the case of CPPA DMSO solution, the first CMC value remains
constant (0.02 mol/L) at all examined temperatures. The second
CMC of CPPA solutions equals 0.08 mol/L and 0.1 mol/L at 22 °C
and 30 °C, respectively. The similar behavior of micelle formation
of CPC and CPPA solutions is probably determined by the surfac-
tant properties of the CP cation.
CRediT authorship contribution statement
Maksym Fizer: Conceptualization, Supervision, Investigation,
Writing - original draft, Writing - review & editing. Michael Filep:
Conceptualization, Investigation, Writing - original draft, Writing
-
review & editing. Oksana Fizer: Conceptualization, Investiga-
4
. Conclusions
tion, Writing - original draft. Oľga Fri cˇ ová: Investigation, Writing -
original draft. Ruslan Mariychuk: Conceptualization, Investigation,
Writing - original draft, Writing - review & editing.
The ion pair of cetylpyridinium cation with picrate anion is
an above room temperature ionic liquid was synthesized and the
structure was determined using XRD, FT-IR, and NMR methods.
DTA results established that the investigated compound congru-
ently melts at 47 °C, and remains a liquid up to 267 °C. Vigor-
ous decomposition was observed at higher temperatures. CPPA un-
dergoes a polymorphic transformation at 33 °C. The two different
modifications were obtained when CPPA was crystallized from a
solution or during recrystallization from a melt.
Acknowledgments
This study was partially supported by the Ministry of Educa-
tion and Science of Ukraine [Projects GR-0119U100232 and GR-
0
120U100431], and by the Slovak Academic Information Agency
National Scholarship Programme of the Slovak Republic, Grants ID
5718 and 30750].
(
2
The differences of about 0.2 ppm and 0.1 ppm in H–NMR chem-
ical shifts’ of the o-protons and the α-methylene group protons
of CPPA and CPC can be explained by: (a) high interionic charge-
transfer effects caused by the electron-donating picrate anion in
the case of CPPA, which were established via the DFT-based inves-
tigation of CDA, ECDA, and the net charge differences; (b) higher
ion association in the case of CPPA DMSO solution in comparison
to CPC, which can be concluded from the KA values of 748.7 L/mol
and 525.0 L/mol, respectively.
Supplementary materials
Supplementary material associated with this article can be
found, in the online version, at doi:10.1016/j.molstruc.2020.129803.
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