Preparation of quaternary pyridinium salts as possible proton conductors

Article abstract of DOI:10.1515/chempap-2015-0037

On basis of earlier experimental experience, the transfer of protons in salts of the organic cation-inorganic anion type occurs primarily through directional arrangement of the anion-anion type short hydrogen bonds. The submitted work presents the prepara

Full text of DOI:10.1515/chempap-2015-0037

Chemical Papers 69 (3) 448–455 (2015)
DOI: 10.1515/chempap-2015-0037
ORIGINAL PAPER
Preparation of quaternary pyridinium salts
as possible proton conductors
b
b
aJiří Urban, David Havlíček*, Josef Krajbich
aJ. Heyrovský Institute of Physical Chemistry of the ASCR, v.v.i., Dolejškova 3, 182 23 Praha 8, Czech Republic
bDepartment of Inorganic Chemistry, Faculty of Science, Charles University in Prague,
Hlavova 2030, 128 40, Prague 2, Czech Republic
Received 11 April 2014; Revised 29 July 2014; Accepted 5 August 2014
On basis of earlier experimental experience, the transfer of protons in salts of the organic
cation–inorganic anion type occurs primarily through directional arrangement of the anion–anion
type short hydrogen bonds. The submitted work presents the preparation of quaternary pyri-
dinium salts of inorganic hydrogen anions in the absence of solvent molecules in their crys-
tal structure. These substances can form only the above-described anion–anion type hydrogen
bonds; in addition, the absence of solvate anions increases the stability of the prepared com-
pounds. A total of six substituted pyridinium salts were prepared, four of which have not
been described yet: 1,2,4,6-tetraphenylpyridinium perchlorate, 1-benzyl-2,4,6-trimethylpyridinium
perchlorate, 1,4-dimethylpyridinium hydrogen sulphate, 1,4-dimethylpyridinium dihydrogen phos-
phate, 1,4-dimethylpyridinium hydrogen sulphate, and 1,2-dimethyl-5-ethylpyridinium dihydrogen
phosphate. Three of these substances were characterised by X-ray structural analysis: 1,2,4,6-
tetraphenylpyridinium perchlorate crystallises in the orthorhombic system, space group Pbca; 1-
benzyl-2,4,6-trimethylpyridinium perchlorate crystallises in the monoclinic system, space group
P21/c; and 1,4-dimethylpyridinium dihydrogen phosphate crystallises in the monoclinic system,
space group C2/c. This structure contains an oriented anion network bond by short anion–anion
˚
˚
type hydrogen bonds with the donor acceptor lengths of 2.567(3) A and 2.557(3) A and thus fulfils
the requirements of a good proton conductor.
c
ꢀ 2014 Institute of Chemistry, Slovak Academy of Sciences
Keywords: quaternary pyridinium salts, synthesis, structural analysis, hydrogen bonding and pro-
ton conductivity
Introduction
Sanui, 2000) are most frequently used on an indus-
trial scale, the study of compounds exhibiting these
properties has a fixed place in basic research as
these substances can become a part of hybrid ma-
terials, where they will find practical use in the fu-
ture. Here, we would like to mention crystal struc-
tures with proton conductivity, synthesised and stud-
ied at the Department of Inorganic Chemistry, Fac-
ulty of Science, Charles University in Prague. Němec
et al. (2004) studied three phases of aminoguani-
dinium dihydrogen phosphate, whose structure con-
tains chains of dihydrogen phosphate anions con-
Recently, proton conductors have been greatly in
demand as materials with a wide range of techni-
cal applications. In addition to being used as materi-
als for proton-exchange membranes (Li et al., 2008)
in low-temperature fuel cells or in solid oxide fuel
cells (Mehta & Cooper, 2003), they are also used
in other branches as potentiometric sensors for hy-
drogen, for gaseous mixtures analysis, or as high-
temperature sensors for alkanes and alcohols. Al-
though materials of the “nafion” type (Rikukawa &
*Corresponding author, e-mail: havlicek@natur.cuni.cz
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J. Urban et al./Chemical Papers 69 (3) 448–455 (2015)
449
the preparation of four so-far not described hydrogen
salts of cations, where R1 = R3 = R5 = H, R2
=
R4 = CH3 (1,4-dimethylpyridinium), and R1 = R4 =
CH3, R2 = R3 = H, and R5 = C2H5 (1,2-dimethyl-5-
ethylpyridinium).
Experimental
Chemical synthesis and analysis: quaternary pyri-
dinium salts were prepared by alkylation or benzyla-
tion of commercially available pyridines: 4-methylpyri-
dine (Aldrich), 5-ethyl-2-methylpyridine (Aldrich),
and 2,4,6-trimethylpyridine (Fluka). Alkylation or
benzylation was performed using methyl iodide or
benzyl bromide (Fluka). Pyridinium iodides and bro-
mides were transferred to hydroxides by freshly pre-
pared silver oxide (from AgNO3, Penta) and hy-
droxides solutions were neutralised using the relevant
acids.
Fig. 1. General formula of the quaternary bases of interest.
nected by short hydrogen bonds with lengths be-
˚
tween 2.496–2.521 A. The proton exhibits proton
conductivity in the direction of these chains. In
the work of Smrčok et al. (2009), a crystal of 1,4-
diazabicyclo[2.2.2]octane-1,4-diium dihydrogen phos-
phate monohydrate was studied in which, while it
forms a spatial network of various types of hydro-
gen bonds, the proton conductivity in the direction
of channels formed of dihydrogen phosphate anions
Elemental analyses were performed on a Perkin–
Elmer CHNS/O 2400 instrument. 1H NMR spectra
were obtained with a Varian Mercury Plus 300 instru-
1
ment (299.939 MHz for H) in D2O. Chemical shifts
˚
connected by hydrogen bonds with lengths of 2.424 A
are given in the δ-scale. t-Butanol was used as the in-
ternal standard. FTIR spectra were measured in KBr
on a Magna 760 Nicolet instrument. MAS spectra were
measured on an Esquire 3000 (Bruker) instrument us-
ing the ESI technique.
1,4-Dimethylpyridinium iodide: methyl iodide
(29.64 g, 0.21 mol) and 4-methylpyridine (18.38 g, 0.15
mol) were dissolved in 20 mL of ether. The solution
was heated to 45◦C for 3 h. After cooling, the product
was filtered off, washed with ether, and dried (48 g,
96 %). The product was crystallised from methanol.
1,2-Dimethyl-5-ethylpyridinium iodide was pre-
pared from 36.48 g (0.26 mol) of methyl iodide and
29.41 g (0.24 mol) of 5-ethyl-2-methylpyridine. The
reaction afforded 57 g (81 %) of the product, which
was crystallised from methanol.
1-Benzyl-4-methylpyridinium bromide was pre-
pared from 4 g (0.043 mol) of 4-methylpyridine and
6.5 g (0.038 mol) of benzyl bromide. The reaction af-
forded 7.3 g (73 %) of the product, which was crys-
tallised from methanol.
1-Benzyl-5-ethyl-2-methylpyridinium bromide was
prepared from 12 g (0.099 mol) of 5-ethyl-2-methyl-
pyridine and 20 g (0.117 mol) of benzyl bromide. The
reaction afforded 16.2 g (54 %) of the product, which
was crystallised from methanol.
˚
and 2.425 A can be observed. In another study (Ka-
man et al., 2011), the base (diazabicyclooctane) was
quaternized on the nitrogen atoms, excluding thus the
incorporation of cations in the hydrogen bond sys-
tem. In addition, the compound formed, 1,4-dimethyl-
1,4-diazabicyclo[2.2.2]octane-1,4-diium hydrogen sul-
phate, did not contain water originating from crystalli-
sation. The crystal structure obtained in this way con-
tains chains of hydrogen sulphate anions connected by
˚
˚
hydrogen bonds with lengths of 2.575 A and 2.594 A.
However, proton conductivity of the crystal has not
yet been measured.
It is apparent from these results that the hydrogen
anions mutually connected in fibres, chains, or chan-
nels are the most promising structural motifs provid-
ing for proton conductivity in these structures. Thus,
an acidic salt of an inorganic acid and base with qua-
ternary nitrogen that does not contain water from
crystallisation is desired. Urban and Volke (1994) were
concerned with the preparation of the salts of these
quaternary bases, synthesising mainly perchlorates of
cations with the general formula shown in Fig. 1, and
studying their electrochemical properties (Volke et al.,
1992).
Although the perchlorates of these cations can-
not exhibit proton conductivity because of the ab-
sence of hydrogen bonds, this work describes two so-
far unpublished crystal structures of such cation salts,
where R1 = R2 = R3 = R4 = C6H5 and R5 = H
(1,2,4,6-tetraphenylpyridinium perchlorate), and R1
= R2 = R3 = CH3, R4 = CH2C6H5, and R5 = H (1-
benzyl-2,4,6-trimethylpyridinium perchlorate). How-
ever, hydrogen salts of these cations have not been
synthesised. The submitted work is concerned with
General preparation procedure for pyridinium hy-
droxides: halogenides were dissolved in water and ex-
cess Ag2O was added. The mixture was stirred for 3 h.
Silver halogenides and unreacted silver oxide were fil-
tered out and the solution was titrated using H2SO4
over phenolphthalein to determine the hydroxide con-
centration. The presence and purity of the desired
cations in the solutions were controlled by mass spec-
troscopy.
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J. Urban et al./Chemical Papers 69 (3) 448–455 (2015)
Table 1. Characterisation data of newly prepared compounds
wi(calc.)/mass %
wi(found)/mass %
Compound
Formula
Mr
Remarks
C
H
N
I
C7H11NO4S 204.15
C7H12NO4P 205.15
C9H15NO4S 233.28
C9H16NO4P 233.20
41.18
38.18
40.99
37.73
46.33
42.39
46.36
40.35
5.43
6.31
5.89
6.11
6.48
7.21
6.92
7.73
6.86
6.40
6.82
6.29
6.00
5.54
6.01
5.15
contains moisture; molar ratio C : N = 6.96 (7.00 calc.)
contains moisture; molar ratio C : N = 7.00 (7.00 calc.)
contains moisture; molar ratio C : N = 8.93 (9.00 calc.)
contains moisture; molar ratio C : N = 9.14 (9.00 calc.)
II
III
IV
Table 2. Spectral data of newly prepared compounds
Compound
Spectral data
I
IR (KBr), ν˜/cm−1: 3381 (s), 3041 (m), 1712 (w), 1627 (s), 1589 (w), 1573 (m), 1486 (m), 1187 (s), 1063 (m)
1H NMR (D2O), δ: 2.64 (s, 3H, CH3—Ar), 4.31 (s, 3H, CH3—N), 7.85 (d, 2H, J = 6.3 Hz, H-3,5), 8.58 (d, 2H, J
= 6.3 Hz, H-2,6)
II
IR (KBr), ν˜/cm−1: 3427 (s), 3039 (s), 1644 (s), 1577 (w), 1521 (m), 1479 (m), 1293 (m), 1187 (s), 1107 (w), 1042
(w), 920 (w), 825 (m), 701 (m), 545 (m), 488 (s)
1H NMR (D2O), δ: 2.64 (s, 3H, CH3—Ar), 4.30 (s, 3H, CH3—N), 7.84 (d, 2H, J = 6.3 Hz, H-3,5), 8.57 (d, 2H, J
= 6.3 Hz, H-2,6)
III
IV
IR (KBr), ν˜/cm−1: 3478 (s), 1643 (m), 1583 (w), 1532 (s), 1462 (m), 1385 (w), 1238 (s), 1050 (m), 1009 (w), 896
(m), 857 (m), 754 (w)
1H NMR (D2O) δ: 1.25 (t, 3H, J = 7.8 Hz, CH3CH2—Ar), 2.73 (s, 3H, CH3—Ar), 2.79 (q, 2H, J = 7.8 Hz,
CH3CH2—Ar), 4.19 (s, 3H, CH3—Ar), 7.78 (d, 1H, J = 8.39 Hz, H-3), 8.22 (d, 1H, J = 8.39 Hz, H-4), 8.53 (s, 1H,
H-6)
IR (KBr), ν˜/cm−1: 3427 (s), 2973 (w), 1637 (m), 1578 (m), 1530 (s), 1289 (m), 1089 (w), 1062 (w), 1041 (w), 910
(w), 857 (w), 548 (m), 439 (m)
1H NMR (D2O), δ: 1.25 (t, 3H, J = 7.81 Hz, CH3CH2—Ar), 2.73 (s, 3H, CH3—Ar), 2.79 (q, 2H, J = 7.81 Hz,
CH3CH2—Ar), 4.18 (s, 3H, CH3—N), 7.77 (d, 1H, J = 8.39 Hz, H-3), 8.22 (d, 1H, J = 8.39 Hz, H-4), 8.52 (s, 1H,
H-6)
1,4-Dimethylpyridinium hydrogen sulphate (I ):
the solution of relevant hydroxides of known concen-
tration was neutralised by 10 mL of H2SO4 (0.3 M),
molar ratio 1 : 1. The solution was concentrated to
5 mL and left to crystallise over H2SO4. Hygroscopic
crystalline powder was washed with ether.
1,4-Dimethylpyridinium dihydrogen phosphate
(II ), 1,2-dimethyl-5-ethylpyridinium hydrogen sul-
phate (III ), and 1,2-dimethyl-5-ethylpyridinium dihy-
drogen phosphate (IV ) were prepared by the neutrali-
sation of relevant hydroxides by sulphuric or phospho-
ric acid in the molar ratio 1 : 1.
X-ray analysis of single crystals: primary data were
measured on a Nonius Kappa CCD four-circle diffrac-
tometer. Radiation from the molybdenum lamp was
modified using a graphite monochromator. The mea-
surement was controlled by the Collect (Hooft, 1998)
and Denzo (Otwinowski & Minor, 1997) sets of pro-
grams and was performed at the temperature of 150 K.
The phase problem was resolved using direct meth-
ods by the SIR 97 program (Altomare et al., 1997).
The data obtained were treated and refined by the
least squares method using the SHELX 97 program
(Sheldrick, 1997) with function F2 minimisation and
the Platon program (Spek, 1999). Hydrogen atoms in
1,4-dimethylpyridinium dihydrogen phosphate on the
oxygen atoms were found, hydrogen atoms bond to
carbon atoms were located in the calculated positions.
In case of 1,2,4,6-tetraphenylpyridinium perchlorate
and 1-benzyl-2,4,6-trimethylpyridinium perchlorate,
all hydrogen atoms (bond to carbons) were located in
the calculated positions. Basic crystallographic data,
data collection, and refinement parameters for all
three compounds are given in Table 3. Crystallo-
graphic data for 1,2,4,6-tetraphenylpyridinium per-
chlorate, 1-benzyl-2,4,6-trimethylpyridinium perchlo-
rate, and 1,4-dimethylpyridinium dihydrogen phos-
phate have been deposited with the Cambridge Crys-
tallographic Data Centre as supplementary publi-
cations CCDC 898066, CCDC 898065, and CCDC
898067, respectively. Copies of the data can be ob-
tained free of charge on application to CCDC, 12
Union Road, Cambridge CG21EZ, UK (fax: +441223
336 031; e-mail: deposit@ccdc.cam.ac.uk).
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J. Urban et al./Chemical Papers 69 (3) 448–455 (2015)
451
Table 3. Experimental data for single-crystal X-ray diffraction studies of the compounds studied
Compound
1,2,4,6-Tetraphenylpyridinium
perchlorate
1-Benzyl-2,4,6-trimethylpyridinium
perchlorate
IV
Formula
C29H22NClO4
C15H18NClO4
311.75
Monoclinic
P21/c
C7H12NO4P
205.15
Monoclinic
C2/c
Formula mass
Crystal system
Space group
483.93
Orthorhombic
Pbca
˚
a/A
16.3580(3)
16.5530(3)
17.5080(3)
90
9.7753(4)
12.0300(4)
14.4020(6)
90
116.528(2)
90
15.8940(4)
16.1480(3)
7.5850(1)
90
108.429(10)
90
1846.90(6)
8
1.476
˚
b/A
˚
c/A
α/◦
β/◦
γ/◦
90
90
3
˚
V/A
4710.71(15)
8
1515.32(11)
4
Z
D(calc)/(g mL−1
µ(MoKα)/mm−1
F(000)
)
1.356
0.198
2016
1.367
0.267
656
0.281
864
Crystal size/mm
0.30 × 0.30 × 0.10
0.20 × 0.27 × 0.40
0.12 × 0.22 × 0.40
˚
Radiation/A
0.71073
0.71073
0.71073
θ/◦ (min–max)
2.1–27.47
–21,21; –21,21; –22,22
10282/5424/4076
3.42–27.47
–12,12; –15,15; –18,18
6377/3458/2359
1.85–27.52
–20,20; 20,20; 9,9
4164/2119/1891
Index ranges
Number of reflections
measured/unique/observed
Data/restrains/parameters
Goodness-of-fit on F 2
5424/0/338
1.114
3458/0/221
1.027
2119/0/128
1.070
Final R, R(alldata)
Largest difference of peak and hole
0.0450, 0.0686
0.340, –0.668
0.0446, 0.0786
0.291, –0.331
0.0290, 0.0370
0.412, –0.474
Fig. 2. Molecular structure and atom numbering of 1,2,4,6-tetraphenylpyridinium perchlorate.
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J. Urban et al./Chemical Papers 69 (3) 448–455 (2015)
Fig. 3. Molecular structure and atom numbering of 1-benzyl-2,4,6-trimethylpyridinium perchlorate.
Results and discussion
depicted in Fig. 3 for clarity and they are “hidden” in
thermal ellipsoids.
In this work, structural data of two perchlo-
rates that have already been described in litera-
ture, 1,2,4,6-tetraphenylpyridinium perchlorate and 1-
benzyl-2,4,6-trimethylpyridinium perchlorate are pub-
lished for the first time. Basic crystallographic data
are given in Table 3.
Both structures (Figs. 2 and 3) are formed by the
relevant cation and a perchlorate anion. Oxygen atoms
of perchlorates are usually disordered in such struc-
tures. Disorder of perchlorates in the structure de-
picted in Fig. 2 (1,2,4,6-tetraphenylpyridinium per-
chlorate) was not solved using an exactly calculated
model. The structure in Fig. 2 is depicted as “average
structure” which causes somewhat “ugly” thermal el-
lipsoids especially on oxygen atoms.
The following model was used to describe the dis-
order of the perchlorate anion in the second structure
depicted in Fig. 3 (1-benzyl-2,4,6-trimethylpyridinium
perchlorate): O1 atom is fixed in one position and
it is not disordered, while atoms O2, O3, and O4
were found in two positions (O2A, O3A, and O4A).
The same occupation factors were refined for all three
remaining oxygen atoms and fixed on the value of
0.56/0.44. Oxygen atoms O2A, O3A, and O4A are not
However, both cations contain quaternary nitrogen
and thus cannot participate in the network of short hy-
drogen bonds. In addition, they crystallise quite read-
ily. However, to induce proton conductivity in the pre-
pared substances, the perchlorate cation has to be re-
placed by a hydrogen anion, which was not achieved
in spite of multiple attempts.
Thus, the pyridinium salt preparation employing
cations with the general formula given in the introduc-
tion which correspond to the commercially available
amines 4-methylpyridine, 5-ethyl-2-methylpyridine,
and 2,4,6-trimethylpyridine was attempted. Methy-
lation of the first two amines on the nitrogen atom
yielded the corresponding iodides, while benzylation
yielded the corresponding bromides. N-substituted
halogenides derived from 2,4,6-trimethylpyridine
could not be isolated in sufficient amounts. Halo-
genides prepared in this way were converted to aque-
ous solutions of the corresponding hydroxides using
silver oxide. Successful progress of the conversion was
monitored by mass spectroscopy. Concentrations of
the hydroxide solutions were determined by titration
with a standard solution of sulphuric acid; these solu-
tions were then used to prepare the acidic salts. How-
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J. Urban et al./Chemical Papers 69 (3) 448–455 (2015)
453
Fig. 4. Molecular structure and atom numbering of 1,4-dimethylpyridinium dihydrogen phosphate (IV ).
ever, for the N-benzyl derivatives, the concentrations
of the hydroxide solutions obtained were too low (the
reaction with Ag2O progressed with a very low yield)
and thus the attention was further concentrated on the
N-methyl derivatives. Based on experience from pre-
vious works, preparation of the corresponding hydro-
gen sulphates, dihydrogen phosphates, hydrogen sele-
nates, and hydrogen selenites through a reaction with
the corresponding mineral acids was attempted. How-
ever, the reactions with selenic acid did not lead to any
crystalline products, while selenious acid oxidised the
methyl group in the para position of the pyridinium
ring to a carboxyl group and thus, only hydrogen sul-
phates and dihydrogen phosphates were considered in
the following work.
The described procedure enabled the isolation of
four salts: 1,4-dimethylpyridinium hydrogen sulphate,
1,4-dimethylpyridinium dihydrogen phosphate, 1,2-
dimethyl-5-ethylpyridinium hydrogen sulphate, and
1,2-dimethyl-5-ethylpyridinium dihydrogen phospha-
te. All the synthesised substances (see the Experi-
mental section) form highly hygroscopic crystalline
materials very difficult to free of water. Elemental
analysis indicates lower carbon and nitrogen con-
tents while their corresponding molar ratios is main-
tained, which results from the presence of water. The
NMR and FTIR spectra correspond to the formula-
tion of the prepared salts. However, only in one case,
1,4-dimethylpyridinium dihydrogenphosphate, the re-
moval of a single crystal suitable for X-ray structural
analysis from the crystalline material was achieved.
Basic crystallographic data for this substance are
given in Table 3, and the independent part of the
structure is presented in Fig. 4.
The structure of this substance contains three ba-
sic motifs: one dihydrogen phosphate and two 1,4-
dimethylpyridinium cations, although the stoichiome-
try of the salt is 1 : 1. Each of the cations contributes
only by one half to the independent part, while the
second half is generated by symmetry operation. For
cations with carbon C7 and nitrogen N7, occupation
of 0.5 was found for the studied structural data on
these atoms, while occupation of 1 was found for the
other atoms. Occupations of 0.5 on carbon C7 and
nitrogen N7 is caused by disorder as carbon and ni-
trogen alternate in their positions with a fifty percent
probability. For the second unit, occupation of 0.25
was found on atoms C11 and N6 due to the similar
alternation of carbon and nitrogen and, in this case,
also due to the special position of atoms C11 and N6.
Dihydrogen phosphate anions in Fig. 5 are connected
˚
by hydrogen bonds with the length of 2.5666(3) A
˚
(O5—H1· · ·O4) and 2.5572(3) A (O2—H2· · ·O3),
and angles (O—H· · ·O) of 175◦ and 178◦, respec-
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J. Urban et al./Chemical Papers 69 (3) 448–455 (2015)
Fig. 5. Dihydrogenphosphate chains in the structure of 1,4-dimethylpyridinium dihydrogen phosphate. Organic part omitted for
clarity.
tively. These anion chains in the direction of crys-
tallographic axis c are not mutually connected by
other hydrogen bonds and can thus be considered to
be “one-dimensional”. This structure contains there-
fore motifs that determine the ability of the sub-
stance to carry protons, like in the case of amino
guanidinium(1+) dihydrogen phosphate (Němec et
al., 2004), 1,4-diazabicyclo[2.2.2]octane-1,4-diium di-
hydrogen phosphate monohydrate (Smrčok et al.,
2009; Kaman & Havlíček 2005), and other similar
compounds (Taraba, 2012). Consequently, the anion
network is connected by sufficiently short hydrogen
bonds that pass through the structure in a single,
given direction. Proton conductivity can thus be an-
ticipated. However, in order to verify this expectation,
it is necessary to grow a sufficiently large single crys-
tal of this substance and to measure its anisotropic
properties.
Conclusions
Several salts of phosphoric, sulphuric, and perchlo-
ric acid with 1,4-dimethylpyridinium, 1,2-dimethyl-5-
ethylpyridinium, 1,2,4,6-tetraphenylpyridinium, and
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455
1-benzyl-2,4,6-trimethylpyridinium were prepared and
studied. Crystal structures of 1,2,4,6-tetraphenyl-
pyridinium perchlorate, 1-benzyl-2,4,6-trimethylpyri-
dinium, and 1,4-dimethylpyridinium dihydrogen phos-
phate were solved. In the last mentioned structure, the
network of dihydrogenphosphate anions contains short
Li, M. Q., Shao, Z. G., & Scott, K. (2008). A high conductivity
Cs2.5H0.5PMo12O40/polybenzimidazole (PBI)/H3PO4 com-
posite membrane for proton-exchange membrane fuel cells
operating at high temperature. Journal of Power Sources,
183, 69–75. DOI: 10.1016/j.jpowsour.2008.04.093.
Němec, I., Macháčková, Z., Teubner, K., Císařová, I., Vaněk,
P., & Mička, Z. (2004). The structural phase transitions
of aminoguanidinium(1+) dihydrogen phosphate—study of
crystal structures, vibrational spectra and thermal be-
haviour. Journal of Solid State Chemistry, 177, 4655–4664.
DOI: 10.1016/j.jssc.2004.08.010.
Otwinowski, Z., & Minor, W. (1997). Processing of X-ray
diffraction data collected in oscillation mode. In C. W.
Carter, Jr., & R. M. Sweet (Eds.), Methods in enzymol-
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Rikukawa, M., & Sanui, K. (2000). Proton-conducting poly-
mer electrolyte membranes based on hydrocarbon poly-
mers. Progress in Polymer Science, 25, 1463–1502. DOI:
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˚
hydrogen bonds (approximately 2.55 A) which form
infinite chains throughout the structure (see Fig. 5).
This structure motif can lead to long-distance transfer
of protons through the crystal and the compound can
act as a proton conductor.
Acknowledgements. The authors wish to thank Dr. Ivana
Císařová for single crystal X-ray measurements. The study is a
part of a long term Research Plan of the Ministry of Education
of the Czech Republic No. MSM0021620857.
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