Crystal structure, phase transition, and disorder in pyridinium methanesulfonate

Article abstract of DOI:10.1016/j.jpcs.2017.01.001

Thermal analysis, X-ray diffraction and vibrational spectroscopy have been used to investigate pyridinium methanesulfonate in order to verify its crystal structure and to have insight into its anomalous dynamical properties. This compound undergoes a solid-solid, reversible phase transition at 255.8/266.8 K (cooling/heating). It crystallizes in non-centrosymmetric Pna2₁ space group at the high-temperature phase (I) and in centrosymmetric Pbca one at the low-temperature phase (II). The entropy gain occurred at the II→I transition has shown that two independent orientations of ions in the disorder phase (I) are allowed. Infrared spectra have confirmed that the pyridinium cation as well as the –CH₃ group of the anion exhibit changes in their dynamics when entering the phase I. These spectra have also suggested residual disorder of both, the pyridinium cation and the –CH₃ group, in the low temperature phase (II).

Full text of DOI:10.1016/j.jpcs.2017.01.001

Author’s Accepted Manuscript
Crystal structure, phase transition, and disorder in
pyridinium methanesulfonate
Dominik Jesariew, Maria M. Ilczyszyn
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http://dx.doi.org/10.1016/j.jpcs.2017.01.001
PCS7948
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Reference:
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Journal of Physical and Chemistry of Solids
Received date: 13 October 2016
Revised date: 7 December 2016
Accepted date: 3 January 2017
Cite this article as: Dominik Jesariew and Maria M. Ilczyszyn, Crystal structure
phase transition, and disorder in pyridinium methanesulfonate, Journal o
Physical and Chemistry of Solids, http://dx.doi.org/10.1016/j.jpcs.2017.01.001
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Crystal structure, phase transition, and disorder in
pyridinium methanesulfonate.
Dominik Jesariew, Maria M. Ilczyszyn
Faculty of Chemistry, Wrocław University, Joliot Curie 14, 50 383 Wrocław, Poland
Email: maria.ilczyszyn@chem.uni.wroc.pl
Abstract
Thermal analysis, X-ray diffraction and vibrational spectroscopy have been used to investigate
pyridinium methanesulfonate in order to verify its crystal structure and to have insight into its anomalous
dynamical properties. This compound undergoes a solid-solid, reversible phase transition at 255.8/266.8 K
(cooling/heating). It crystallizes in non-centrosymmetric Pna21 space group at the high-temperature phase (I)
and in centrosymmetric Pbca one at the low-temperature phase (II). The entropy gain occurred at the II→I
transition has shown that two independent orientations of ions in the disorder phase (I) are allowed. Infrared
spectra have confirmed that the pyridinium cation as well as the –CH3 group of the anion exhibit changes in their
dynamics when entering the phase I. These spectra have also suggested residual disorder of both, the pyridinium
cation and the –CH3 group, in the low temperature phase (II).
Keywords: pyridinium, methanesulfonate, hydrogen bond, DSC, temperature-dependent IR spectra, phase
transition, disorder
1. Introduction
Although plastic crystals have been known for decades [1] they have still attracted tremendous
attention and have been studied from different points of view. It is because of their very interesting
physical properties and practical applications. Most of them exhibit, among other properties, high
conductivities at ambient temperature [2] what makes them promising materials for devices such as
fuel cell, batteries and solar cell. Such physical properties have been found, among other things, for
the compounds comprising cations such as pyrrolidinium, imidazolium and pyridinium and anions
-
-
-
-
such as BF4 , PF6 , CF3SO3 , CH3SO3 etc [2-7]. Pyridinium methanesulfonate appears to belong to this
family.
The crystal structure of the pyridinium methanesulfonate (abbreviated as PyHMS) was
determined and two reports of these studies are available in the scientific literature [8, 9]. Although the
both structures were determined at the same temperature (173 K) the data reported in refs 8 and 9
differ. According to Bolte et al. [8] the crystal in question crystallizes in non-centosymmetric Pna21
space group. In turn, according to Lehtonen et al. [9] it crystallizes in centrosymmetric Pbca space
group. This difference may arise from different ways of the PyHMS crystal preparation. It is also
possible that each of these research groups has obtained different polymorphic forms of the PyHMS
crystal.
To explain this uncertainty, we have used various methods to investigate properties of the
PyHMS compound. First of all, the thermal properties of the crystal in question have been measured.
Further, the structural properties have been reexamined at various temperatures. The temperature
1

dependent infrared spectra of the PyHMS crystal in the wide temperature range (298 – 11 K) have
been also measured. Infrared spectroscopy can provide information on dynamical properties of crystals
near structural phase transitions or on structural changes near structural phase transitions. Those data
can be easily obtained from evolutions of infrared band shapes as well as from infrared band positions
versus temperature. The utility of infrared spectroscopy in structural phase transition investigations
and in better insight into structural phase transition mechanism has been described in many
publications (see for example [10]). Here we report on the thermal characteristics, structural and
spectroscopic properties of PyHMS with particular attention to the changes of these properties near
structural phase transition temperature. This allows us to get some knowledge on molecular
mechanism of structural phase transition in the PyHMS crystal.
2. Experimental
2.1. Synthesis and crystal growth
PyHMS was synthesized by mixing pyridine and methanesulfonic acid in the 1:1 molar ration
in water at room temperature. The single crystal suitable for the X-ray diffraction experiment was
grown from aqueous solution by slow evaporation at room temperature. Our preparation method is
very close to that reported by Lehtonen et al. [9] and differs from that reported by Bolte et al. [8].
2.2. DSC studies
The DSC curves were measured using a Perkin-Elmer DSC-8500 instrument at 20 K∙min-1
scanning rate in the 100 – 400 K temperature range (the melting point temperature is equal to 434.6
K). Aluminum capsules were used for all experiments. The measurements were carried out for: (1) a
fresh sample without its drying capsulated in hermetically sealed capsule; (2) a sample dried at 300 K
for 1 h and after that capsulated in hermetically sealed capsule; (3) a sample capsulated in pricked
capsule, placed in instrument and heated above the melting point for half an hour. The results of all
these experiments are the same. The DSC curves are presented in Fig. 1.
2

phase II
2 mW
phase I
100
150
200
250
300
350
400
Temperature [K]
Figure 1. The DSC curves for the PyHMS crystal.
2.3. Single crystal X-ray diffraction
XRD experiment was carried out by means of KUMA KM-4 CCD -axis diffractometer with
graphite–monochromator and Mo K radiation. Single crystal XRD data were collected for four
different samples at different conditions:
(1) a single crystal of 0.34 x 0.20 x 0.15 mm size was cooled down to 280 K and after the temperature
stabilization the XRD data were collected. Then the crystal was cooled down to 100 K and after the
temperature stabilization the XRD data were collected;
(2) a single crystal of good quality (of 0.42 x 0.34 x 0.37 mm size) was quickly cooled down directly
to 100 K and after the temperature stabilization the XRD data were collected. It was noticed that the
sample cracked while cooling and reflections collected at 100 K became more blurred;
(3) the same experiment as described in point (2) was carried out for a smaller single crystal of good
quality (of 0.30 x 0.20 x 0.15 mm). The obtained results were the same as in the case (2);
(4) a single crystal of good quality (of 0.62 x 0.39 x 0.30 mm size) was cooled down slowly to 230 K
and after the temperature stabilization the XRD data were collected. Next the sample was cooled down
slowly to 100 K and after the temperature stabilization the XRD data were collected once more.
Data reduction was carried out using the CrysAlis ‘RED’ program. The crystal structures were solved
and refined using SHELXTL-97 software package [11]. Non-hydrogen atoms were refined with
anisotropic thermal parameters. The positions of the H atoms were refined using a riding model, with
the fixed N-H and C-H distances: 0.88 Å at 100 K and 0.86 Å at 280 K for N-H and 0.95 Å and 0.98 Å
at 100 K and 0.93 Å and 0.96 Å at 280 K for C-H.
3

2.4. Vibrational studies
Infrared spectra were recorded using a Bruker IFS-88 FT-IR spectrometer at a resolution of 2
cm-1 with no background correction or smoothing. The room temperature infrared spectra were
recorded in the 4000 - 400 cm-1, the powder samples were prepared as Nujol and hexachlorobutadiene
mulls on KBr window. The low temperature spectra were recorded using a closed helium cycle
cryostat (Displex closed cycle cryogenics refrigeration system, model CSW-202) to which a dual
sensor temperature controller of a Lake-shore model 300-11 was joined in the 300 - 11 K temperature
range and in the 4000-500 cm-1 wavenumbers region. The powder samples for temperature dependent
infrared spectra measurement were prepared as Nujol mull on NaCl windows.
The room temperature Raman spectrum for polycrystalline sample was recorded using a
Nicolet Magna 860 spectrometer (laser Nd: YAG, λ = 1064 nm, power 150 mW) at a resolution of 2
cm-1 in the 4000-150 cm-1 wavenumbers region.
3. Results and discussion
3.1. Thermal properties
The DSC curves recorded upon the heating and cooling are displayed in Fig. 1. The
calorimetric measurements show that the crystal undergoes fully reversible solid-solid, III, phase
transition at 255.8/266.8 K (on cooling-heating, respectively) of the first order type and with
noticeable thermal hysteresis. The entropy gain (ΔSonset) at the III transition, equals to 6.4/5.9 J mol-
1 K-1 (on cooling-heating, respectively, (Table 1)) and shows that the phase transition may be classified
as the ‘order-disorder’ one with two independent orientations of the ions in the disorder phase. This
value is significantly lower than corresponding values found in crystals exhibiting solid-solid phase
transitions from an order phase to a rotationally disorder phase (organic ionic plastic crystal) [5, 7, 12,
13].
Table 1. The II→I phase transition parameters (Tmax, Tonset, ΔSonset and ΔHonset) estimated from DSC
experiment.
Phase transition
Tmax
[K]
250.3/268.7*
Tonset
[K]
255.8/266.8
ΔHonset
[J*g-1]
-9.3/9.00
ΔSonset
[J*mol-1*K-1]
-6.4/5.9
II → I
* the values given before and after “/” are related to the cooling and heating processes, respectively.
3.2. Crystal structure description
Details of the crystal data and structural refinements in phases I and II are given in Table 2.
Table 3 collects the selected bond distances and angles of the PyHMS crystal in these phases. The
geometrical parameters of the N-H∙∙∙O hydrogen bonds and C-H∙∙∙O short contacts detected in both
phases are given in Table 4.
4

Table 2. Crystal data and structural refinement for the PyHMS crystal in phases I and II.
-
-
C5H6N+ CH3SO3 (phase I)
C5H5N+ CH3SO3 (phase II)
Empirical formula
Formula weight
Temperature
Wavelenght [Å]
Crystal system
175.20
280(2) K
0.71073
orthorombic
Pna21
175.20
100(2) K
0.71073
orthorombic
Pbca
Space group
a (Å)
b (Å)
11.805(2)
7.688(2)
8.692(2)
90o
7.604(2)
11.594(2)
17.212(3)
90o
c (Å)
 (o)
90o
90o
 (o)
90o
788.9(3)
4
90o
1517.4(5)
8

Volume [Å3]
Z
Reflections
13548/3723
12625/2464
collected/independent reflections
Rint
0.0448
0.0397
Data/parameters
S
3723/101
1.015
2464/102
1.000
R1 = 0.0471
wR2 = 0.1116
R1 = 0.0453
wR2 = 0.1195
Final R indices
The PyHMS crystal in the high temperature phase (the room temperature phase, phase I)
crystallizes in the non-centrosymmetric space group Pna21 of orthorhombic system, Z=4. In turn, in
the low temperature phase (below 266.8 K, phase II) PyHMS belongs to the centrosymmetric space
group Pbca of orthorhombic system, Z=8. The asymmetric unit of PyHMS in both phases consists of
the pyridinium cation (PyH+) and the methanesulfonate anion (MS-) linked by the N-H∙∙∙O hydrogen
bonds (Figs 2a and 2b). In both phases the PyH+ and MS- ions are arranged in the same way into the
hetero-dimers but in each phase the adjacent dimers are related by different symmetry elements. In the
phase I the adjacent hetero-dimers are related by the diagonal glide plane (n) but in the phase II the
adjacent dimers are related by the center of inversion (Figs 3a and 3b).
Figure 2a. Asymmetric unit of the PyHMS crystal in phase I.
5

Figure 2b. Asymmetric unit of the PyHMS crystal in phase II.
In both phases, II and I, two types of the hydrogen bonds, N-H∙∙∙O and C-H∙∙∙O, are present.
The N-H∙∙∙O hydrogen bonds join the PyH+ and MS- ions into the hetero-dimers, the C-H∙∙∙O short
contacts were found between adjacent pyridinium-methanesulfonate dimers. The N-H∙∙∙O hydrogen
bonds in phase II appear to be much stronger than that ones apparent in the phase I. In the former
phase the N∙∙∙O distance is equal to 2.726(2) Å and the NHO angle is equal to 171o, in the latter one
the corresponding values are equal to 3.025(6) Å and 159o, respectively (Table 4). Contrary to that the
C-H···O short contacts were found to be distinctly weaker in the phase II than in the phase I (Table 4).
From Table 3 it is seen that the -SO3 group of the MS- anion is more distorted in the phase II
than in the phase I. This probably arises from the N-H···O hydrogen bond strength in which the S-O
bond is involved. The N-H∙∙∙O hydrogen bond in the phase II is stronger than in the phase I.
Table 3. Selected bond lengths (Å), angles and torsion angles (o) determined by X-ray diffraction
studies at 100 and 280 K for the PyHMS crystal.
Parameter
Phase II
100 K
Phase I
280 K
Bond lengths
S1-O1
1.4572(10)
1.4530(12)
1.4767(11)
1.7609(18)
1.3411(19)
1.346(2)
1.439(5)
S1-O2
1.4442(14)
1.453(4)
S1-O3
S1-C1
1.7583(17)
1.392(11)
1.292(5)
N1-C6
N1-C2
C3-C2
C3-C4
C4-C5
C6-C5
1.374(2)
1.309(11)
1.376(8)
1.398(2)
1.389(2)
1.377(2)
1.359(4)
1.284(10)
Bond angles
O2-S1-O1
113.58(7)
6
113.5(3)

O2-S1-O3
O1-S1-O3
O2-S1-C1
O1-S1-C1
O3-S1-C1
C6-N1-C2
C2-C3-C4
C3-C4-C5
C4-C5-C6
N1-C2-C3
N1-C6-C5
111.83(6)
111.85(7)
107.17(8)
106.16(7)
105.65(7)
122.46(14)
119.10(14)
119.86(14)
118.69(13)
119.64(13)
120.24(14)
111.8(3)
112.49(9)
106.34(9)
106.9(3)
105.2(3)
120.5(6)
118.8(6)
118.5(6)
121.9(6)
122.1(6)
118.2(6)
Torsion angles
C2-C3-C4-C5 -1.1(2)
C2-N1-C6-C5 -0.7(2)
C6-N1-C2-C3 0.4(2)
C4-C3-C2-N1 0.6(2)
N1-C6-C5-C4 0.1(2)
C3-C4-C5-C6 0.8(2)
-2.9(11)
-1.3(10)
2.2(14)
0.02
1.5(11)
0.6(12)
Contrary to –SO3 the pyridinium cation ring is more distorted in the phase I than in the phase
II. In the phase II, the pyridinium ring is more flat and the opposite bonds in the ring are of the same
length (Table 3). The less distortion of the pyridinium cation ring in the phase II is probably due to
noticeable weakness of the C-H···O short contacts in this phase (Table 4).
Our XRD experiments showed that the crystal structure of PyHMS is strongly dependent on
the way of the sample cooling. When the sample is rapidly cooled down to 100 K, the structure of the
high temperature phase (Pna21) is kept at the low temperature phase. Otherwise, as the sample is
slowly cooled down to 100 K, the crystal at this temperature exhibits symmetry of the low temperature
phase (Pbca).
7

O3
O3
C2
C2
O2
C3
C4
C3
C4
O2
C1
N1
C6
N1
C6
C1
C5
C5
O1
C5
O1
C5
O1
O1
C6
C6
N1
C4
C3
C1
C4
C3
N1
C1
O2
O2
C2
C2
O3
O3
Figure 3a. The arrangement of the molecules in the PyHMS crystal at 100 K (the low temperature
phase II). The hetero-dimers are related by centre of inversion.
C6
O1
O3
O1
C6
C5
C4
C5
C4
O2
O2
N1
C2
N1
C2
C1
C1
C3
O1
C3
O1
O3
C6
C6
C5
C4 C1
C5
C4
O2
N1
C2
O2
N1
C2
C1
C3
C3
O3
O3
Figure 3b. The arrangement of the molecules in the PyHMS crystal at 280 K (the high temperature
phase I). The hetero-dimers are related by diagonal glide plane (n).
Table 4. The geometrical parameters of the N-H∙∙∙O hydrogen bonds and the C-H∙∙∙O short contacts
detected in the PyHMS crystal in the phase I (280 K) and the phase II (100 K).
Temperature
280 K
D-H∙∙∙A
D-H [Å]
0.86
0.93
0.93
0.93
H∙∙∙A [Å]
2.21
2.26
2.35
2.59
D∙∙∙A [Å]
3.025(6)
3.065(8)
3.192(9)
3.428(6)
3.498(7)
3.209(9)
<(DHA) [o]
159
145
151
150
N1-H1∙∙∙O1
C2-H2∙∙∙O3
C3-H3∙∙∙O2a
C4-H4∙∙∙O3b
C5-H5∙∙∙O1b
C6-H6∙∙∙O2c
0.93
0.93
2.70
2.35
144
154
8

100 K
N1-H1∙∙∙O3
C6-H6∙∙∙O1
C5-H5∙∙∙O2d
C4-H4∙∙∙O1e
C3-H3∙∙∙O3e
C2-H2∙∙∙O2f
0.88
0.95
0.95
0.95
0.95
0.95
1.85
2.39
2.51
2.43
2.81
2.47
2.726(2)
3.133(2)
3.323(2)
3.335(2)
3.557(2)
3.172 (2)
171
135
144
158
131
131
Kody operacji symetrii: a –x+1/2, y+1/2, z-1/2; b x+1/2, -y+5/2, z; c –x+1/2, y+1/2, z+1/2 d x-1/2, -y+1/2, -z+1;
e –x-1/2, y+1/2, z; f –x, y+1/2, -z+1/2
3.3. The selection rules for the PyHMS crystal at the low and high temperature phases.
In both phases, I and II, the PyH+ cations and MS- anions occupy sites of C1 symmetry.
Because of this the site-group selection rules in these both phases are the same, e.g. thirty internal
vibrations of the PyH+ cations and eighteen internal vibrations of the MS- anions of species A are
infrared and Raman active. The presence of four ions per primitive unit cell, at the phase I, can result
in splitting of each internal vibration into four components (Table 5a). Three of them (A1+B1+B2) are
predicted in infrared spectrum and four of them ( A1+A2+ B1+B2) are expected in Raman spectrum. In
turn, the presence of eight molecules per primitive unit cell at the phase II can result in splitting of
each internal vibration into eight components (Table 5b). Three of them (B1u+B2u+B3u) should be
infrared active and four of them (Ag+B1g+B2g+B3g) should be Raman active. Note that although the
selection rules differ in both phases, the number of bands due to the internal vibrations of both ions,
expected in the infrared and Raman spectra of both phases, are the same. The main difference
concerns the value of the wavenumbers of the corresponding bands. In the phase I the bands arising
from a given internal vibration should be observed at the same wavenumber in the infrared and Raman
spectra. Contrary to this, in the phase II, the band arising from a given internal vibration should
exhibit various wavenumbers in the infrared and Raman spectra.
Table 5a. Analysis of the fundamental modes (k=0) for the PyHMS crystal in the phase I.
Lattice modes
Internal vibrations
Selection rules
-
C5H5NH+ CH3SO3
IR
Raman
C2v
N
A
T
L
A1
A2
B1
B2
60
60
60
60
1
0
1
1
5
6
5
5
6
6
6
6
30
30
30
30
18
18
18
18
z
x2,y2,z2
xy
xz
x
y
yz
Abbreviations: N-total number of modes; A-acoustic modes; T-translation modes; L-libration modes.
Table 5b. Analysis of the fundamental modes (k=0) for the PyHMS crystal in the phase II.
Lattice modes
Internal vibrations
Selection rules
-
C5H5NH+ CH3SO3
IR
Raman
x2,y2,z2
xy
D2h
Ag
B1g
N
60
60
A
0
0
T
6
6
L
6
6
30
30
18
18
9

B2g
B3g
Au
B1u
B2u
B3u
60
60
60
60
60
60
0
0
0
1
1
1
6
6
6
5
5
5
6
6
6
6
6
6
30
30
30
30
30
30
18
18
18
18
18
18
xz
yz
z
y
x
Abbreviations: N-total number of modes; A-acoustic modes; T-translation modes; L-libration modes.
3.4. Vibrational spectra
Fig. 4 shows the room temperature vibrational spectra of PyHMS in the region of 4000-400
cm-1. Figs 5a-5d show the infrared spectra of PyHMS at selected temperatures in the 1680-1520 cm-1,
1540-1300 cm-1, 1125-900 cm-1 and 880-500 cm-1 wavenumber regions. The wavenumbers of the
bands observed in the room temperature spectra, at a few degrees above (270 K) and below (260 K)
the phase transition temperature and at 11 K are reported in Table 6. The wavenumbers of the bands
observed in the all temperature dependent infrared spectra of the PyHMS crystal are collected in Table
S1. The temperature dependent infrared spectra of the PyHMS crystal are shown in Fig. S1.
a
b
N
N
N
c
3000
2500
2000
1500
1000
500
wavenumbers/cm-1
Figure 4. Infrared (a and b) and Raman (c) powder spectra of the PyHMS crystal recorded at room
temperature. Infrared spectra are recorded as hexachlorobutadien mull (a) and nujol mull (b). Bands
due to nujol absorption are marked by “N”.
-
The studied crystal is the inter-ionic compound (PyH+···MS-; MS- = CH3SO3 ). For that reason
the bands observed in the 4000-400 cm-1 wavenumber region are due to the internal vibrations of the
MS- anion and PyH+ cation. Next, the internal vibrations of PyH+ may be divided into the internal
vibrations of the pyridine ring and vibrations of the proton transferred to the pyridine nitrogen. The
10

latter vibrations are tantamount to the vibrations of the N-H···O hydrogen bond in which the PyH+ and
MS- ions are involved.
298 K
280 K
270 K
260 K
250 K
200 K
1639
1650
1600
1550
wavenumbers/cm-1
Figure 5a. Infrared spectra of the PyHMS crystal recorded at six different temperatures in the 1680-
1520 cm-1 region (ν8a (1639 cm-1), ν8b (1622 cm-1) and ν19b (1549 cm-1)).
298 K
N
N
280 K
N
N
270 K
N
N
260 K
250 K
N
N
N
N
N
200 K
N
1500
1450
1400
1350
1300
wavenumbers/cm-1
Figure 5b. Infrared spectra of the PyHMS crystal recorded at six different temperatures in the 1530-
1300 cm-1 region (ν19a (1492 cm-1), δaCH3 (1433 cm-1), δaCH3 (1395 cm-1) and ν3 (1342 cm-1)).
11

298 K
280 K
270 K
260 K
250 K
200 K
1100
1050
1000
950
900
wavenumbers/cm-1
Figure 5c. Infrared spectra of the PyHMS crystal recorded at six different temperatures in the 1125-
900 cm-1 region (ν18a (1059 cm-1), CH3 (971 cm-1) and ν5 (937 cm-1)).
N
298 K
N
280 K
N
270 K
N
260 K
N
250 K
N
200 K
850 800 750 700 650 600 550 500
wavenumbers/cm-1
Figure 5d. Infrared spectra of the PyHMS crystal recorded at six different temperatures in the 880-500
cm-1 region (ν11 (683 cm-1)).
The tentative assignment of the bands observed in the vibrational spectra of the studied crystal
given in Table 6 has been based on the theoretical predictions of the internal vibrations of pyridine and
d5-pyridine [14-16] and on the comparison of the assignments proposed for: (i) pyridinium, d5-
pyridinium and N-deuterated of h5- and d5-pyridinium cations [17-20]; (ii) some simple salts of
methanesulfonic acid [21-23]. Assignment of the internal vibrations of pyridinium
12

trifluoromethanesulfonate proposed in [20] has also been helpful. The denotation of the phenyl
internal vibrations [24, 25] was adopted for the labeling of the PyH+ cation internal vibrations.
3.4.1 Hydrogen bond vibration
According to the crystal structure the PyH+ cation and MS- anion are linked by an intermediate
N-H∙∙∙O hydrogen bond. In the infrared spectra at room temperature this bond is manifested by νNH
mode which gives rise to a broad, medium intense band in the high wavenumber region of the
spectrum. The NH band is observed in the 3500-2200 cm-1 region and its maximum is about 2770
cm-1 in the room temperature infrared spectrum (Fig. 4, phase I). There are several bands appearing on
the high wavenumbers side of the broad band (Fig. 4) in the infrared spectra of the high as well as low
temperature phase. These bands are due to the fundamental and combination modes of the internal
vibrations of the pyridine ring (Table 6) [18]. When the temperature is reduced the NH broad band
becomes narrower and shifts to the lower wavenumbers (Fig. 6). At 11 K its maximum is observed at
about 2605 cm-1. This result shows that the N-H∙∙∙O hydrogen bond at the low temperature phase
(phase II) is much stronger than in the high temperature one. This finding is consistent with the XRD
data. As it is seen from Fig. 6, Tables 6 and S1, the most abrupt changes in the wavenumber of the
broad band due to NH of the N-H∙∙∙O hydrogen bond are observed in the vicinity of the phase
transition (266.8 K). The NH mode shifts towards lower frequency from about 2769 cm-1 at 280 K to
about 2672 cm-1 at 260 K. This indicates that the structural phase transition in the crystal studied
provokes changes in the strength of the N-HO bond.
2800
2780
NH
phase II
2760
2740
2720
2700
2680
2660
phase I
2640
2620
2600
0
50
100
150
200
250
300
temperature/K
Figure 6. NH stretching wavenumbers as a function of temperature.
13

The in-plane deformation mode of the N-HO hydrogen bond, NH, in the infrared spectra
of simple pyridinium salts is expected near 1300 cm-1 and usually it is strongly coupled to the B1
symmetry modes of the pyridinium ring: the C=C and C=N stretches appearing in 1650-1200 cm-1
[17]. In turn, the band due to the out-of-plane deformation mode of N-HO hydrogen bond, NH, in
the infrared spectra of simple pyridinium salts is expected in the 1080-903 cm-1 frequency region,
however its position and its behavior strongly depends on a H-bond strength [17]. In the stronger H-
bonded salts, the NH band overlaps with the 12, 18a and 18b modes or vibrationaly interacts with the
5 mode. In the weaker H-bonded salts the NH mode is only loosely coupled to the 10b and 11 modes
[17, 26]. In the studied crystal the NH mode probably gives rise to the medium intense band observed
at 1340 cm-1 or to the weak intense band at 1322 cm-1. In turn, the band due to the γNH mode probably
overlaps with the 12, 18a, 18b and sSO3 bands apparent in the 1060-1000 cm-1 region. On cooling, it
firmly shifts to higher wavenumbers and it is observed as very intensive and separate band at 1073 cm-
1
(11 K) (see Fig S1 and Table S1). Such assignment is consistent with our expectation: usually the
γNH mode is highly temperature sensitive what results in a great change in the wavenumber and in the
intensity of the band due to this mode. Moreover, our assignment of the γNH mode is consistent with
the assignment given by Glazunov et al. [18] for pyridinium chloride, pyridinium bromide and
pyridinium iodide involving the N-H∙∙∙X (X = Cl-, Br- and I-) hydrogen bonds of very similar strength
as the N-H∙∙∙O hydrogen bond in PyHMS. The γNH bands, in the infrared spectra of those salts, are
observed at 1080, 1045 and 1020 cm-1, respectively. In the former salt, PyHCl, the broad absorption
due to the NH mode has been identified at about 2510 cm-1, the band due to the γNH vibration has
been found at 1080 cm-1 [17, 18].
The bands due to the hydrogen bond vibrations (NH, NH and γNH) are not observed in the
Raman spectrum.
3.4.2 Internal vibrations of PyH+ cation
The wavenumbers of the bands due to the internal vibrations of the pyridinium cation and their
assignments are summarized in Table 6. Generally, they resemble assignments reported for simple h5-
pyridinium salts [17] and for pyridinium trifluoromethanesulfonate [20], though some differences are
observed in the infrared spectra of the studied crystal and of separate pyridinium salts. First of all these
differences concern the out–of–plane internal modes of the PyH+ cation which mix with the γNH
mode [17]. In such cases these modes remain hydrogen-bond sensitive and their band shape and
position strongly depend on the strength of the H-bonds. E.g. because of the vibrational interaction
between the 5(CH) and γNH modes the band due to the former vibration (5(CH)) is observed at
996 cm-1 in the infrared spectra of pyridinium trifluoromethanesulfonate [20], while the corresponding
band in the infrared spectrum of the studied crystal is localized at 936 cm-1. The N-H∙∙∙O hydrogen
14

bond in the pyridinium trifluoromethanesulfonate crystal is weaker (R(N∙∙∙O) = 3.214 and 3.159 Å)
than that in the studied crystal (R(N∙∙∙O) = 3.025 Å).
On cooling the PyHMS crystal below 266.8 K (III phase transition) several changes in the
internal vibrations of the pyridinium cation, are noted. The most prominent are:
(1) the slow disappearance of the bands at 1622 (8b(CC)) and 1492 cm-1 (19a(CC)) parallel to a
sudden appearance of new bands at 1631 and 1486 cm-1, respectively (Figs 5a and 5b)
(2) the splitting of the bands at 1059 cm-1 (18a (CH)), 937 cm-1 (5 (CH)) and 683 cm-1 (11 (CH),)
(Figs 5c and 5d, Table S1).
On further cooling the sample to 11 K, the bands become considerably sharpened and well defined.
The splitting of the bands (phase II) cannot be explained by the vibrational coupling between
the eight molecules in the unit cell (correlation splitting). According to the selection rules discussed
above (3.3 chapter) the same number of bands due to the internal vibrations of the pyridinium cation
should be expected in the infrared spectra measured for both phases, I and II (Table 5). The observed
changes are not due to the sharpening of bands at the lower temperature either. The described above
changes are noticed in the spectra measured at a few degrees above and below the phase transition
temperature. This indicates that they are due to III transition. Moreover, they reveal that this
transition relates to the PyH+ ion dynamics. What is more, the observed splitting may suggest that
III transition from completely disordered to completely ordered phase should be excluded. Twofold
disorder of the ions in the phase I has been estimated from our DCS measurement (see Thermal
properties chapter). PyH+ cation reorientations in the phase I are probably fast and only average bands
due to its internal vibrations are observed in the infrared spectra. In the phase II, reorientations of
PyH+ cations are probably slower than in the phase I (some residual disorder of the pyridinium cation
is frozen in the low temperature phase) what results in the band splitting and slow disappearance of
some bands parallel to the new ones appearing in the infrared spectra recorded below the III phase
transition temperature. Some residual disorder of the PyH+ cation in salts of pyridinium halides at the
low temperature (order) phase has been detected by NMR and quasi-elastic neutron scattering [27, 28].
Table 6. Wavenumbers (cm-1), relative intensity and assignment of bands observed in the vibrational
spectra of the PyHMS crystal recorded at different temperatures.
IR
Ramana
IR
Tentative assignmentsb
298K
298K
270K
260K
11K
3284vw
3274vw
3266vw
3252vw
3230vw
3200vw
3185vw
3143sh
3136w





3249vw
3250vw
3258vw
8a + 8b
3177w
3141w
3180vw
3141 w
3187vw

8a + 19b
3145sh
3136 w
3128vw
3127*
3116w



8a + 19a
8b + 19a /2(CH)
3117sh(9)
3119vw
3116vw
3106m
3120w
3108m
15

3090m
3097(19)
3096m
3095sh
3102w
3093w
3072m
3057w
3042w
3034w
3007m
3002w
2 x 19b /20b(CH)
3070s
3050m
3033m
3066(7) sh
3054(10)
3069m
3054w
3035w
3073m
3051w
3034w
13(CH)
7(CH)
aCH320a/7b(CH)

20a(CH)
3002shm
3003(19)
2973 (3)
2925(27)
3003w
3007w
sCH3
sCH3
aCH3
aCH3
sCH3
2971 m
2962m
2894s
2884m
2848
2841(5)
aCH3
2814
2800s

2776 vb m
2714mb
2789*
2672sb
2790*
2605sb
2796*
νNH∙∙∙O
2769m
2770m
2765m
2778m
2762*
2736m
2728*
2737*
2724*
2710m
2697s
2712m
2701m
2708m
2697s
2688m
2634m
2607m
2686s
2652sh
2643s
2675m
2674m
2610m
2630s
2624*
2619s
2614s
2614sh
2604s
2465sh
2397vw
2352vw
2306vw
2261vw
2186vw
2466vw
2397vw
2346vw
2315vw
2280vw
2397vw
2349vw
2311vw
2261vw
2182vw
2310vw
2258vw
2183vw
2161vw
2122m
2122vw
2122vw
2115w
γNH+18a
γNH+5
2107vw
2092vw
2072vw
2061vw
2001m
2059vw
1996w
2002w
2002w
1967vwsh
1936vwsh
1876vw
1822vw
1970vw
1934vw
1886vw
1831vw
1756vw
1726vw
1705vw
1664vw
1646sh
1642vs
1635s
1879vw
1881vw
Overtones and combination
modes of 5, 10, 11, 17
1695vw
1665vw
1699vw
1663vw
1664vw




8a (νCC)
1639m
1621vs
1640(14)
1624(10)
1639m
1624vs
1638vs
1631vs
1623sh
8a (νCC)
8b (νCC)
8b (νCC)
1624w
1607vw
1594vw
1555vs
1549m
1491vs
1550m
1492sh
1551m
1493sh
1486vs
1436w
1429w
1403vw
1399sh
1339s
1320vw
1282vw
1259m
1224vsb
1197vs
1176vs



19b (νCC)
19a (νCC)
19a (νCC)
1485vs
1438m
1430m
1406m
1399m
1342s
1324w
1290vw
1262s
1433w
1395w
1432(12)
1432
aCH3
aCH3
aCH3
aCH3
3 (δCH) / δNH???
3 (δCH)
1396
1399sh
1342s
1322vw
1342m
1322w
1279sh
1257m
1225vs
1199vs
1172vssh
1259(7)
1221(6)
1206(10)
1258m
1225vsb
1198vs
sCH3
aSO3
9a(δCH)
15(δCH)
1222vsb
1195vs
1178vs
1173vssh
16

1158vs
1158(5)
1158vsb
1121w
1156vsb
1118m
1092w
1154vsb
1142sh
1121m
1096w
1090wb
1073s
aSO3
1119shm

18b (δCH)
γNH
1061m
1056m
1062s



18a (δCH), 15(CH) (γNH?)
18a (δCH), 15(CH) (γNH?)
18a (δCH), 15(CH) (γNH?)
1058m
1061(8.5)
1059m
1055m
1037vs
1033vsb
1020vs
1037vs
1023vs
1032(100)
1021(20.5)
1037vs
1023vs
1035vs
1021vs
sSO3/12(CC)
12(δCC)
Transmission window arises from
12(CC) and sSO3 coupling
1009*
1010*
1002s
1006(68)
972(4.6)
1002s
1003s
1003vs
999vs
993m
974w
1(νCC)
1(νCC)
1(νCC)
CH3
995 sh
971vw
995sh
972vw
994sh
971vw
967vw
967w
CH3
962vw
949m
940m
880vw
841vw
779vs
772vw
762vs
757vs
689vs
681vs
641vw
629vw
609msh
606m
591vw
573m
549vs
538vs
528vs
CH3 (?)
C
C
943sh
938w
878vw
937vwb
884shvw
938wb
776vs
776vs
776(35)
776vs
C-S
755vs
751shvs
683s
762sh(5)
756vs
751vssh
686vs
682sh
639vw
758vs
754vssh
686vs
679vs
640vw
627sh

10b(δCH)






11 (δCH)
11 (δCH)
6b (δCC)
6b (δCC)
6a (δCC)
 (δCC)
639vw
608w
640(20)
609(12)
608m
607m
552s
537vs
530vs
552vs
537vs
529vs
550vs
538vs
528vs
aSO3
aSO3
aSO3
536(9)
530(11)
398(3)

16b(δCC)
344(19)
128(111)
C-S
a values of intensities of the Raman bands given in parenthesis are determined in comparison with the intensity
of the band at 1032 cm-1. Its intensity was assumed to be 100.
b denotation of the internal vibrations of pyridinium cation was adopted from denotation of the phenyl internal
vibrations [24, 25].
3.4.3 Internal vibrations of MS- anion
-
Isolated MS- (CH3SO3 ) anion exhibits C3v symmetry and in this case its eighteen internal
vibrations belong to the 5A1+A2+6E irreducible representations (Tables 7a and 7b). All of them expect
the mode of A2 symmetry, are infrared and Raman active. In the studied crystal in both, I and II
phases, MS- occupies position of C1 symmetry. The effect of the site symmetry results in the lowering
of the effective molecular symmetry of the MS- anion from C3v to C1. As a result, the doubly
degenerate species (E) split into two components and bands due to eighteen internal vibrations of the
-
CH3SO3 anion of species A should be observed in the infrared and Raman spectra. The presence of
four anions per primitive unit cell in the phase I and eight anions per primitive unit cell in the phase II
results in the correlation splitting of each A-species internal mode of the MS- anion into four
(A1+A2+B1+B2) and eight (Ag+B1g+B2g+B3g+Au+B1u+B2u+B3u) components, respectively. Note, that in
both, I and II phases only three components of each mode are infrared active: A1+B1+B2 and
17

B1u+B2u+B3u, respectively. Correlation diagrams for the MS- internal vibrations and their wavenumbers
in the studied crystal in phases I and II are given in Tables 7a and 7b, respectively.
-
Table 7a. Correlation diagram for the internal vibrations of the CH3SO3 anion in the PyHMS crystal
and their wavenumbers (cm-1) observed in the infrared and Raman spectra at room temperature (298
K, phase I).
Internal vibrations of free
CH3SO3
Symmetry of
free anion
C3v
Site symmetry
Factor group symmetry
Proposed assignment
-
infrared
Raman
a
a
a
C1
C2v
1 (sCH3 = 2943 cm-1)b
2 (sCH3 = 1268 cm-1)
3 (sSO3 = 1050 cm-1)
4 (C-S = 780 cm-1)
A1
A1
A1
A1
A1
E
A
A
A
A
A
2A
A1+A2+B1+B2
A1+A2+B1+B2
A1+A2+B1+B2
A1+A2+B1+B2
A1+A2+B1+B2
2(A1+A2+B1+B2)
3004 m
1258 m
1037 vs
776 vs
552 vs
3003 (19), 2925 (27)
1259 (7)
1032 (100)
776 (35)
5 (sSO3 = 560-530 cm-1)
6 (aCH3 = 3020 cm-1)
551 (15)
3033 m, 2971 m 2973 (3)
2962 m, 2848
1433 w, 1395 w
1225 vs
1158 vs
971 vw
2841 (5)
1432 (12)
1221(6), 1206 (10)
1158 (5)
972 (5)
7 (aCH3 = 1435 cm-1)
E
E
2A
2A
2(A1+A2+B1+B2)
2(A1+A2+B1+B2 )
8 (aSO3 = 1177-1200 cm-1)
9 (CH3 = 960 cm-1)
E
E
E
A2
2A
2A
2A
A
2(A1+A2+B1+B2 )
2(A1+A2+B1+B2 )
2(A1+A2+B1+B2 )
A1+A2+B1+B2
10 (aSO3 = 560-550 cm-1)
11 (SO3 = 360-350 cm-1)
12 (C-S = 346 cm-1)
537 vs, 530 vs
536 (6), 530 (11)



344 (19)
a selection rules for C3v: A1(IR, R), A2(i.a.), E(IR, R); selection rules for C1: A(IR, R); selection rules for C2v:
A1(IR, R), A2(R), B1(IR,R), B2(IR,R);.
b wavenumbers taken from [21-23].
-
Table 7b. Correlation diagram for the internal vibrations of the CH3SO3 anion in the PyHMS crystal
and their wavenumbers (cm-1) observed in the infrared spectra at low temperature (11 K, phase II).
Internal vibrations of free
CH3SO3
Symmetry of
free anion
C3v
Site
Factor group symmetry
Proposed assignment
infrared
-
symmetry
a
a
a
C1
D2h
1 (sCH3 = 2943cm-1)b
2 (sCH3 = 1268cm-1)
3 (sSO3 = 1050 cm-1)
4 (C-S = 780 cm-1)
A1
A1
A1
A1
A1
E
A
A
A
A
A
2A
2A
Ag+B1g+B2g+B3g+Au+B1u+B2u+B3u
Ag+B1g+B2g+B3g+Au+B1u+B2u+B3u
Ag+B1g+B2g+B3g+Au+B1u+B2u+B3u
Ag+B1g+B2g+B3g+Au+B1u+B2u+B3u
Ag+B1g+B2g+B3g+Au+B1u+B2u+B3u
2(Ag+B1g+B2g+B3g+Au+B1u+B2u+B3u)
2(Ag+B1g+B2g+B3g+Au+B1u+B2u+B3u)
3007 m, 3002 w
1262 s
1037 vs (?), 1033 vsb
779 vs
5 (sSO3 = 560-530 cm-1)
6 (aCH3 = 3020 cm-1)
7 (aCH3 = 1435 cm-1)
549 vs
3042 wc
E
1437 m, 1430 m,
1406 m, 1398 m
1223 vsb, 1154 vsb
974 w, 967 w,
962 vw
8 (aSO3 = 1177-1200 cm-1)
9 (CH3 = 960 cm-1)
E
E
2A
2A
2(Ag+B1g+B2g+B3g+Au+B1u+B2u+B3u)
2(Ag+B1g+B2g+B3g+Au+B1u+B2u+B3u)
10 (aSO3 = 560-550 cm-1)
11 (SO3 = 360-350 cm-1)
12 (C-S = 346 cm-1)
E
E
A2
2A
2A
A
2(Ag+B1g+B2g+B3g+Au+B1u+B2u+B3u)
2(Ag+B1g+B2g+B3g+Au+B1u+B2u+B3u)
Ag+B1g+B2g+B3g+Au+B1u+B2u+B3u
538 vs, 528 vs



a selection rules for C3v: A1(IR, R), A2(i.a.), E(IR, R); selection rules for D2h: Ag(R), B1g (R), B2g(R), B3g(R),
Au(i.a.), B1u(IR), B2u(IR), B3u(IR).
b wavenumbers taken from [21-23]
c the remaining bands due to 6(aCH3) vibration in the low temperature infrared spectra are overlapped by
absorption due to nujol .
As it is easily seen from Table 7a the wavenumbers of the bands due to the internal vibrations
of the MS- anion in studied crystal are rather close to those observed in the infrared spectra of other
simple salts of methanesulfonic acid [21-23]. From Table 7a it is also seen that the number of bands
observed in the vibrational spectra of the studied crystal at the phase I is less than the number of bands
18

predicted by the factor group selection rules. This observation indicates that vibrational couplings
between four MS- anions in the unit cell in the phase I are not present.
As the sample goes through the III transition several changes in the position, shape and
number of bands due to the internal modes of the MS- anion have been observed. The observed
changes bear a close resemblance to those observed for the bands due to the internal vibrations of the
PyH+ cation. The most prominent changes are as follows:
(1) splitting of the bands due to the aCH3 (1433 cm-1) and CH3 (971 cm-1) modes into two
components at 1436 and 1429 cm-1 and at 971 cm-1 and 967 cm-1, respectively (Figs 5b and 5c).
(2) a slow vanishing of the bands at 1395 cm-1 (aCH3) accompanied by a sudden appearance of the
new band at 1406 cm-1 (Fig. 5b).
All these changes are observed in the spectra recorded at a few degrees above and below the phase
transition temperature (266.8 K) what indicates that they are due to this transition and not just due to
the bands sharpening at the lower temperature. The splitting of the bands, similarly as in the case of
the PyH+ cation, does not result from the vibrational coupling between the eight molecules in the unit
cell (correlation splitting). Although the factor group selection rules for the MS- anion in both phases,
I and II, differ the number of bands due to the MS- anion vibrations expected in the infrared spectrum
of both phases are the same (Tables 7a and 7b). Similarly as in the case of the PyH+ cation, the
observed splitting probably relates to the MS- anion dynamics. Note, that only bands due to the
internal vibrations of the methyl (-CH3) group ending of the MS- anion split and vanish. The bands due
to the internal modes of the sulfonyl (-SO3) ending only slightly shift to the lower wavenumbers.
These observations may point at the reorientational motion of the –CH3 ending in the phase I (with
two possible positions) and hindered of this reorientation in the phase II resulting in the band splitting
in this phase. Taking this into account one may state that the –CH3 group of the MS- anion exhibits
dynamical disorder in the phase I which is partly “frozen” the in phase II.
Detailed examination of the temperature dependent infrared spectra in the region of the
internal vibrations of MS- reveals some bands due to these vibrations which only slightly (by 2 or 1
cm-1) shift to the lower or higher wavenumbers on cooling the sample below 266.8 K (III phase
transition). These observations apply to bands at 3003 (sCH3), 1258 (sCH3) cm-1, 1225 (aSO3), 1158
(aSO3), 530 (aSO3) and 552 (sSO3) cm-1. The relatively small shifts of the bands due to vibrations of
MS- as well as PyH+ (see Table 6) suggest a predominance of electrostatic interactions in the
intermolecular interactions between the anions and cations in the studied crystal.
4. Summary and concluding remarks
The pyridinium methanesulfonate crystal has been investigated in order to:
(i) verify its crystal structure;
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(ii) have insight into anomalous dynamical properties of the crystal at the low and high temperature
phases and its structural changes accompanied by its structural phase transition .
It was shown that pyridinium methanesulfonate undergoes reversible solid-solid structural
phase transition at 255.8/266.8 K. This transition has the order-disorder character with two
distinguishable allowed orientations in the high temperature phase. The XRD experiments have
revealed that studied crystal crystallizes in the non-centrosymmetric Pna21 space group and in the
centrosymmetric Pbca space group in the high and low temperature phase, respectively. Those
experiments have also revealed that the crystal structure of PyHMS strongly depends on the way of the
sample cooling. When the sample is cooled down rapidly to the low temperature, the structure of the
high temperature phase is kept at the low temperature phase. We suppose that this may explain why
two different structures of the crystal at the same temperature are available in the scientific literature.
The temperature dependent infrared spectra revealed that the order-disorder transition is related to the
changes in the pyridinium cation as well as the –CH3 group dynamics. Moreover they have shown that
some residual disorder of both, the pyridinium cation and the –CH3 group, is present in the low
temperature phase (II).
Appendix A. Supplementary data
1. CCDC 1447533 and 1447534 contains the supplementary crystallographic data for this paper. These
data can be obtained free of charge via http://www.ccdc.cam.ac.uk/deposit (or from the Cambridge
Crystallographic Data Centre, e-mail: deposit@ccdc.cam.ac.uk).
2. Temperature dependent infrared spectra are shown in Fig. S1. The wavenumbers of the bands
observed in temperature dependent infrared spectra are collected in Table S1.
References
[1] J.Timmermans, Plastic crystals: A historical review, J. Phys. Chem. Solids 18 (1961) 1-8.
[2] J.M.Pringle, P.C.Howlett, D.R.MacFarlane, M.Forsyth, Organic ionic plastic crystals: recent
advances, J. Mater. Chem. 20 (2010) 2056-2062.
[3] D.R.MacFarlane, P.Meakin, N.Amini, M.Forsyth, Structural studies of ambient temperature plastic
crystal ion conductors, J. Phys.: Condens. Matter 13 (2001) 8257-8267.
[4] S.J.Pas, J.M.Pringle, M.Forsyth, D.R.MacFarlane, Thermal and physical properties of an
archetypal organic ionic plastic crystal electrolyte, Phys. Chem. Chem. Phys. 6 (2004) 3721-3725.
[5] J.Adebahr, M.Grimsley, N.M.Rocher, D.R.MacFarlane, M.Forsyth, Rotational and translational
mobility of a highly plastic salt: Dimethylpyrrolidinium thiocyanate, Solid State Ionics 178 (2008)
1798-1803.
20

[6] M.Lee, U.H.Choi, S.Wi, C.Slebodnick, R.H.Colby, H.W.Gibson, 1,2-Bis[N-(N′-
alkylimidazolium)]ethane salts: a new class of organic ionic plastic crystals, J. Mater. Chem. 21
(2011) 12280-12287.
[7] J.Luo, O.Conrad, I.F.J.Vankelecom, Imidazolium methanesulfonate as a high
temperature proton conductor, J. Mater. Chem. A 1 (2013) 2238-2247.
[8] M.Bolte, Ch.Griesinger, P.Sakhaii, Pyridinium methylsulfonate: a pseudosymmetric structure,
Acta Crystallogr. E57 (2001) o458-o460.
[9] O.Lehtonen, J.Hartikainen, K.Rissanen, O.Ikkala, L.O.Pietilä, Hydrogen bonding and protonation
in acid–base complexes: Methanesulfonic acid-pyridine, J. Chem. Phys. 116 (2002) 2417-2424.
[10] J.Petzelt, V.Dvorak, Z.Iqbal, F.J.Owens (Eds.), Vibrational Spectroscopy of Phase Transitions, ed
Z.Iqbal, F.J.Owens, Academic Press, Orlando, 1984, p. 55.
[11] G.M.Sheldrick, A short history of SHELX, Acta Cryst. A64 (2008) 112-122.
[12] R.Asayama, J.Kawamura, T.Hattori, Phase transition and ionic transport mechanism of
(C4H9)4NI, Chem. Phys. Lett. 414 (2005) 87-91.
[13] D.Jesariew, M.M.Ilczyszyn, A.Pietraszko, The crystal structure and the phase transitions of
pyridinium trifluoromethanesulfonate, Material Research Express 1 (2014) 015705.
[14] G.Zerbi, B.Crawford Jr., J.Overend, Normal Coordinates of the Planar Vibrations of Pyridine and
Its Deuteroisomers with a Modified Urey—Bradley Force Field, J. Chem. Phys. 38 (1963) 127-133.
[15] K.B.Wiberg, V.A.Walters, K.N.Wong, S.D.Colson, Azines: vibrational force field and intensities
for pyridine, J. Phys. Chem. 88 (1984) 6067-6075.
[16] F.Partal Ureña, M.Fernandez Gomez, J.J.López González, E.Martinez Torres, A new insight into
the vibrational analysis of pyridine, Spectrochim.Acta 59A (2003) 2815-2839.
[17] V.P.Glazunov, S.E.Odinokov, Infrared spectra of pyridinium salts in solution—I. The region of
middle frequencies, Spectrochim. Acta 38A (1982) 399-408.
[18] V.P.Glazunov, S.E.Odinokov, Infrared spectra of pyridinium salts in solution—II. Fermi
resonance and structure of νNH bands, Spectrochim. Acta 38A (1982) 409-415.
[19] S.E.Odinokov, A.A.Moskovsky, A.A.Nabullin, Infrared spectra of pyridinium salts in solution—
III. Associations of pyridinium salts with bases, Spectrochim. Acta 39A (1983) 1065-1071.
[20] D.Jesariew, M.M.Ilczyszyn, Phase transitions in non-centrosymmetric pyridinium
trifluoromethanesulfonate crystal: Vibrational studies, Spectrochim. Acta 148A (2015) 203-214.
[21] H.Gerding J.W.Maarsen, The Raman and infra-red spectra of methane-sulphonic acid and the
methanesulphonate ion, Recl. Trav. Chim. 77 (1958) 374-382.
[22] H.Siebert, Schwingungsspektren einiger Derivate der Schwefelsäure, Z. Anorg. Allg. Chem. 289
(1957) 15-28.
[23] A.Simon, H.Kriegsmann, Berg 89 (1965) 1718.
[24] B.Wojtkowiak, M.Chabanel, Spectrochimie moleculaire, Technique at Documentation, Paris,
1977.
21

[25] G.Varsányi, Vibrational Spectra of Benzene Derivatives, Akadémiai Kiadó, Budapest, 1969
[26] A.Novak, Hydrogen bonding in solids correlation of spectroscopic and crystallographic data,
Struct. Bonding 18 (1974) 177-216.
[27] J.A.Ripmeester, Molecular motion, phase transitions, and disorder in the pyridinium halides, Can.
J. Chem. 54 (1976) 3453-3457.
[28] R.Mukhopadhyay, S.Mitra, I.Tsukushi, S.Ikeda, Order–disorder transition in pyridinium iodide:
quasi-elastic neutron scattering study, Chem. Phys. Lett. 341 (2001) 45-50.
Caption of supplementary Figure:
Figure S1. Infrared spectra of the PyHMS crystal recorded at various temperatures.
Highlights
 Pyridinium methanesulfonate undergoes structural phase transition.
 Pyridinium methanesulfonate crystallizes in non-centrosymmetric group above Tc.
 Phase transition is accompanied by the changes in the pyridinium cation and –CH3
group dynamics.
 Residual disorder of the pyridinium cation in order phase was detected.
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