β-Alaninium picrate: A new salt with di-β-alaninium dimeric cation

Article abstract of DOI:10.1016/j.molstruc.2012.03.066

β-Alaninium picrate crystallizes in the form of yellow columns, in the triclinic system (space group P1?, Z = 2). The asymmetric unit contains one β-alaninium cation and one picrate anion. Nevertheless, the unit cell comprises a dimeric centrosymmetric β-Ala⁺?β- Ala⁺ cation and two picrate anions. The O?O distance (2.636(2) A?) in the dimeric cation is among the shortest distances of known salts with A⁺?A⁺ type dimeric cations. The infrared and Raman spectra are shown and discussed. A comparison of known crystals with β-Ala⁺?β-Ala⁺ cations is presented, which shows that four different types can be distinguished.

Full text of DOI:10.1016/j.molstruc.2012.03.066

Journal of Molecular Structure 1019 (2012) 91–96
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
b-Alaninium picrate: A new salt with di-b-alaninium dimeric cation
M. Fleck a, V.V. Ghazaryan b, A.M. Petrosyan b,
⇑
a Institute of Mineralogy and Crystallography, University of Vienna, Althanstr. 14, A-1090 Vienna, Austria
b Institute of Applied Problems of Physics, NAS of Armenia, 25 Nersessyan Str., 0014 Yerevan, Armenia
a r t i c l e i n f o
a b s t r a c t
ꢀ
Article history:
b-Alaninium picrate crystallizes in the form of yellow columns, in the triclinic system (space group P1,
Z = 2). The asymmetric unit contains one b-alaninium cation and one picrate anion. Nevertheless, the unit
cell comprises a dimeric centrosymmetric b-Ala+ꢀ ꢀ ꢀb-Ala+ cation and two picrate anions. The Oꢀ ꢀ ꢀO dis-
tance (2.636(2) Å) in the dimeric cation is among the shortest distances of known salts with A+ꢀ ꢀ ꢀA+ type
dimeric cations. The infrared and Raman spectra are shown and discussed. A comparison of known crys-
tals with b-Ala+ꢀ ꢀ ꢀb-Ala+ cations is presented, which shows that four different types can be distinguished.
Ó 2012 Elsevier B.V. All rights reserved.
Received 14 February 2012
Received in revised form 5 March 2012
Accepted 26 March 2012
Available online 5 April 2012
Keywords:
b-Alaninium picrate
Dimeric cation
Structure
Vibrational spectra
1. Introduction
of ‘‘bis b-alanine picrate’’ corresponds well to the IR spectrum of
b-alanine, although the unit cell parameters are not in good agree-
One of the topics of our current research is the family of salts of
amino acids with Aꢀ ꢀ ꢀA+ dimeric cation with short hydrogen bonds
[1–5], where A and A+ are amino acids in zwitter-ionic and singly
charged state respectively. There are two types of such salts,
depending on the valency of the anion, i.e., (Aꢀ ꢀ ꢀA+)ꢀXꢁ (this
encompasses the vast majority of such salts known to date, see
[4] and references therein) and 2(Aꢀ ꢀ ꢀA+)ꢀY2ꢁ [6,7]. In addition to
said (Aꢀ ꢀ ꢀA+) type dimeric cations, there are two types of salts with
dimeric (A+ꢀ ꢀ ꢀA+) type cation: (A+ꢀ ꢀ ꢀA+)ꢀ2Xꢁ [8–11] and
(A+ꢀ ꢀ ꢀA+)ꢀY2ꢁ [12–16]. In all these salts, the amino acid is b-alanine,
thus A+ is the b-alaninium cation (an overview of these compounds
with some data is given in Table 1).
It is interesting to note that b-alanine b-alaninium picrate ob-
tained and investigated in Ref. [17] is perhaps the first example of
the 2(Aꢀ ꢀ ꢀA+)ꢀ2Xꢁ type salts. Vibrational spectra of this crystal have
been investigated in Ref. [18]. According to the authors of Refs.
[17,18] crystals of b-alanine b-alaninium picrate were obtained by
evaporation from aqueous solution containing equimolar quantities
of b-alanine and picric acid. Recently another paper was published
[19], which is devoted to the growth and characterization of ‘‘novel
bis b-alanine picrate’’. It does not contain quotations of previous
works on b-alanine b-alaninium picrate [17,18]. The grown crystal
of ‘‘bis b-alanine picrate’’ (see Fig. 1 in Ref. [19]) contains colorless
parts, which shows that it cannot be picrate or picric acid, because
in this case it must be homogeneously yellow. Infrared spectrum
ment with those of b-alanine [20]. In this context it should be men-
tioned another paper on an alleged L-alanine compound, which in
fact is L-alanine, namely, ‘‘Synthesis, growth and material charac-
terization of bis L-alanine triethanol amine (BLATEA) single crystals
grown by slow evaporation technique’’ [21]. No crystal and molec-
ular structure is reported, while unit cell parameters, powder XRD
pattern and IR spectrum correspond well to respective characteris-
tics of L-alanine with the exception of that the IR spectrum contains
additional absorption band at 3408.18 cmꢁ1 caused probably by ab-
sorbed water due to hygroscopicity of KBr.
For further study of the b-alanine b-alaninium picrate crystal
(we expected to find a phase transition at low temperatures) we
have tried to reproduce this crystal. However, the infrared spec-
trum of crystals obtained from aqueous solution containing equi-
molar quantities differed from the spectrum shown in [18].
Subsequent crystal structure determination showed that the actual
composition is 1:1, that is, the obtained crystal is b-alaninium pic-
rate. Moreover, it turned out that b-alaninium picrate is a new
member of the salts with (A+ꢀ ꢀ ꢀA+) type dimeric cation. In this pa-
per, we describe the structure and vibrational spectra of b-alanini-
um picrate and compare it with other similar crystals.
2. Experimental
2.1. Synthesis and crystal growth
⇑
As initial reagents we used b-alanine purchased from Reanal
(Hungary), and picric acid ‘‘pure’’ made in Poland. Elongated,
Corresponding author. Tel.: +374 10 241106; fax: +374 10 281861.
E-mail address: apetros@iapp.sci.am (A.M. Petrosyan).
0022-2860/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molstruc.2012.03.066

92
M. Fleck et al. / Journal of Molecular Structure 1019 (2012) 91–96
Fig. 1. Spontaneously formed crystal of b-alaninium picrate.
Fig. 4. Crystal structure of b-alaninium picrate, viewed along [010]. Notice the
ꢀ
orientation of the picrate anions along ð205Þ.
Fig. 2. Molecular structure of b-alaninium picrate, with ellipsoids at the 50%
probability level.
Fig. 3. Detail of the dimeric centrosymmetric b-Ala+ꢀ ꢀ ꢀb-Ala+ cation. Hydrogen
bonds shown as dashed lines. Notice that both molecules are symmetry related via
the inversion center, which is in the middle of the dimeric unit.
transparent yellow crystals of b-alaninium picrate (Fig. 1) have
been obtained by slow evaporation at room temperature from
aqueous solution containing equimolar quantities of respective
components. The same crystal was obtained also at 1.5:1 M ratio,
while from stoichiometric 2:1 M ratio we were barely able to obtain
the b-alanine b-alaninium picrate crystal described in [17,18].
2.2. Crystal structure determination
Fig. 5. Dimeric b-Ala+ꢀ ꢀ ꢀb-Ala+ cations in b-alaninium compounds b-Ala+ NOꢁ3 (type
For X-ray diffraction studies, a suitable single crystal sample of
b-alaninium picrate with the size of approximately 0.09 ꢂ 0.05 ꢂ
0.05 mm3 was prepared by manually cutting a larger crystal and
mounted on a thin glass needle with laboratory grease. The crystal
was measured on a Bruker-Nonius Kappa diffractometer, equipped
with a CCD area detector and employing graphite-monochromated
B), (b-Ala+)2 BPDS2ꢁ (type C), (b-Ala+)2 SnI2ꢁ (type D). For references see Table 1.
4
with anisotropic displacement parameter, the hydrogen atoms
were located form the difference Fourier maps and refined without
any restraints. Relatively high displacement parameters of the ni-
tro groups’ oxygen atoms suggested possible disorder of these
groups. However, attempts to refine these atoms as disordered
did not give better values, so we decided to use the model as pre-
sented here, although the relatively high residual electron density
in the vicinity of the nitro groups resulted in rather high R-values.
Crystallographic data and refinement details are given in Table 2.
Mo K
a radiation (k = 0.71073 Å). A full Ewald sphere was mea-
sured. The reflection data were collected and processed using the
Bruker-Nonius program suites COLLECT, DENZO-SMN and related
analysis software [22,23]. The structures were solved by direct
methods and subsequent Fourier and difference Fourier syntheses,
followed by full-matrix least-square refinements on F2, using the
program SHELX [24]. All non-hydrogen atoms have been refined

M. Fleck et al. / Journal of Molecular Structure 1019 (2012) 91–96
93
Table 1
Space groups, Oꢀ ꢀ ꢀO distances (Å) and CACACAN torsion angles (°) of b-alaninium (b-Ala+) compounds containing A+ꢀ ꢀ ꢀA+ cations.
#
Crystal
Space group
(Oꢀ ꢀ ꢀO)
2.636(2)
2.690(2)
2.726(4)
2.640(5)
2.629(7); 2.646(7)
2.750(2)
2.64(5)
2.682(1)
CACACAN
Refs.
a
b-Ala+ picrateꢁ
66.2(2)
174.3(2)
64.9(3)
62.3(1)
ꢀ
1
2
3
4
5
6
7
8
9
P1
(b-Ala+)2 (BPDS)2ꢁb
[12]
[10]
[8]
[15]
[11]
[16]
[13]
[14]
[9]
ꢀ
P1
b-Ala+ ClOꢁ4
P21/n
P21/c
P21/c
C2/c
C2/c
I2/c
b-Ala+ (FeCl4)ꢁ
(b-Ala+)2(SnI4)2ꢁ
b-Ala+ oxalateꢁ.0.5H2O
(b-Ala+)2(Re2Cl8)2ꢁ
(b-Ala+)2(CuCl4)2ꢁ
(b-Ala+)2(CuBr4)2ꢁc
b-Ala+ NOꢁ3
55.0(7); ꢁ61.7(7)
77.0(2)
69.8(12)
67.5(1)
66.8(1)
67.6(3); ꢁ70.0(4)
I2/c
Pca21
2.704(1)
2.638(3); 2.643(3)
10
a
b
c
This work.
BPDS = biphenyl-4,40-disulfonate.
Note: The atomic position parameters x and y of C3 given in the CIF (CSD Refcode CAYPOH) differ from that in the publication and are erroneous.
Table 2
Crystal data and details of the refinement for b-alaninium picrate.
Table 3
Formula
Mr
C9 H10 N4 O9
318.21
Hydrogen bond parameters for b-alaninium picrate (in Å and °).
Crystal system
Space group
a (Å)
b (Å)
c (Å)
Triclinic
DAHꢀ ꢀ ꢀA
DAH
Hꢀ ꢀ ꢀA
Dꢀ ꢀ ꢀA
<DHA
ꢀ
P1
N1AAH11Aꢀ ꢀ ꢀO1B
N1AAH12Aꢀ ꢀ ꢀO1B
N1AAH13Aꢀ ꢀ ꢀO2A
O1AAH1Aꢀ ꢀ ꢀO2A
0.87(4)
0.91(3)
0.90(3)
0.89(2)
2.01(4)
1.96(3)
2.23(3)
1.76(2)
2.821(3)
2.850(2)
2.888(2)
2.636(2)
155(3)
165(2)
129(2)
168(5)
4.5764(9)
12.060(2)
12.291(3)
104.72(3)
97.69(3)
95.83(3)
643.6(2)
2
a
(°)
b (°)
c
(°)
V (Å3)
Table 4
Selected interatomic distances and angles in b-alaninium picrate (in Å and °).
Z
Dcalcd (g cmꢁ3
)
1.642
0.149
) (cmꢁ1
)
Distance
Angle
l
(Mo K
a
F(000)
T (K)
hkl range
Reflections measured
Reflections unique
328
296(2)
O1AAC1A
O2AAC1A
C1AAC2A
C2AAC3A
C3AAN1A
O1BAC1B
C1BAC2B
C1BAC6B
C2BAC3B
C2BAN1B
C3BAC4B
C4BAC5B
C4BAN2B
C5BAC6B
C6BAN3B
N1BAO3B
N1BAO2B
N2BAO5B
N2BAO4B
N3BAO6B
N3BAO7B
1.309(2)
1.221(2)
1.499(3)
1.513(3)
1.492(2)
1.255(2)
1.449(2)
1.452(3)
1.376(3)
1.458(2)
1.383(3)
1.384(3)
1.448(3)
1.370(3)
1.459(2)
1.212(2)
1.226(2)
1.218(3)
1.219(2)
1.158(3)
1.228(3)
O2AAC1AAO1A
O2AAC1AAC2A
O1AAC1AAC2A
C1AAC2AAC3A
N1AAC3AAC2A
O1BAC1BAC2B
O1BAC1BAC6B
C2BAC1BAC6B
C3BAC2BAC1B
C3BAC2BAN1B
C1BAC2BAN1B
C2BAC3BAC4B
C3BAC4BAC5B
C3BAC4BAN2B
C5BAC4BAN2B
C6BAC5BAC4B
C5BAC6BAC1B
C5BAC6BAN3B
C1BAC6BAN3B
123.5(2)
122.3(2)
114.2(2)
114.0(2)
112.9(2)
124.9(2)
123.9(2)
111.3(2)
124.7(2)
116.1(2)
119.2(2)
118.9(2)
121.4(2)
119.6(2)
119.0(2)
119.1(2)
124.7(2)
115.7(2)
119.6(2)
6, ꢁ17/16, 18
5959
4344
2953
0.017
240
0.069
0.0995
0.177
0.048/0.379
0.880/ꢁ0.637
Data with Fo > 4
Rint
r
(Fo)
Parameters refined
R(F)a (for Fo > 4
r(Fo)
R(F)a (all reflections)
wR(F2)a (all reflections)
Weighting parameters a/b
D
qfin (max/min) (e Åꢁ3
)
The CCDC data set 864240 contains supplementary crystallographic
data for b-alaninium picrate. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
wR2 = [R R
w(Fo2–Fc2)2/ wF4o]1/2,w = 1/
a
R1 =
R
||Fo| ꢁ |Fc||
R
|Fo|,
[
r
2(F2o) + (a ꢂ P)2 + b ꢂ P], P = (Fo2 þ 2F2c )/3.
2.3. Vibrational spectroscopy
Fourier-transform Raman spectra were registered by a NXP FT-
Raman Module of a Nicolet 5700 spectrometer (number of scans:
512, laser power at the sample: 0.16 W, resolution 4 cmꢁ1) at room
temperature. The same spectrometer was used for measuring
attenuated total reflection Fourier-transform infrared spectra (FTIR
ATR) (ZnSe prism, 4000–500 cmꢁ1, Happ-Genzel apodization, ATR
distortion is corrected, number of scans 32, resolution 4 cmꢁ1). Part
of the IR spectrum in the region 500–400 cmꢁ1 were taken from
FTIR spectra registered with Nujol mull (4000–400 cmꢁ1, number
of scans 32, resolution 2 cmꢁ1).
the expected 2:1 ratio. All atoms are located on general positions
in the triclinic unit cell (Fig. 2). Although the name of the species
somehow suggests that the b-alaninium cation is connected to
the picrate anion, this is not the case. In contrast, two crystallo-
graphically related b-alaninium molecules are connected to each
other via two symmetry-equivalent OAHꢀ ꢀ ꢀO type hydrogen
bonds. Thus, although no atom of the crystal is located on a special
ꢀ
position, the dimeric cation does display point symmetry 1 (Fig. 3).
Nevertheless, there is some connection between the dimeric units
(which carry two positive charges) and the picrate units, namely
via NAHꢀ ꢀ ꢀO type hydrogen bonds (Table 3). It is interesting to no-
tice that these bonds only connect to one of the seven oxygen
atoms of the picrate anion, viz. to the deprotonated O1B atom. This
fact also is expressed in the rather large values of the displacement
parameters of the other oxygen atoms of the picrate anion – a fact
that is responsible for the high residual electron density found
around the nitro groups of the picrate anion (see Section 2).
3. Results and discussion
3.1. Molecular and crystal structure of b-alaninium picrate
The molecular structure of b-alaninium picrate comprises b-
alaninium cations and picrate anions in a 1:1 ratio, in contrast to

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M. Fleck et al. / Journal of Molecular Structure 1019 (2012) 91–96
Table 5
Wavenumbers (in cmꢁ1) and assignment of peaks in IR and Raman spectra of b-
alaninium picrate.
IR
Raman
Assignment
3373; 3358
3261
3261
m(NH)
3176
3079; 3039sh
2998; 2917; 2880
2726; 2679
2590
2104; 2040
1687
3084; 3041
3008; 2951
m
m
(NH);
(CH);
m
(CH)
m
(NH); m(OH)
Sum tones
Sum tone
Sum tones
COOH
1633; 1619
1588
1652sh; 1633sh; 1620
1590
m(C@C) ring
+
das(NH3
)
1564; 1557
1523; 1487
1470; 1451
1428
1572; 1549
1535sh; 1494
1481
m
as(NO2)
+
ds(NH3
d(CH2)
d(CH2)
)
1433
1404
1407
x(CH2)
1391
1392
1362
1368
x(CH2)
1331; 1320
1262
1241
1338; 1323
1279
1249
m
m
m
s(NO2)
(CAO) phenolic
(CAOH)
1165
1143
1101; 1087
1054
960
1170
1155; 1146
1090
1057
963; 949
934
d(CH) in-plane
q(NH3 )
+
m(CAN)
m(CAN)
m(CAC)
928
913
918
862
864
m(CAC)
828
827
d(NO2)
790
745
813; 793
753
d(CH) out-of-plane
710
703sh
582; 549; 522
507; 437; 408
716
706
551
q(CH2)
Fig. 6. Schematic depiction of structure types arranged by geometries and
symmetries in dimeric b-Ala+ꢀ ꢀ ꢀb-Ala+ cations in b-alaninium compounds. Symme-
try centers are depicted as circles. Most b-alaninium compounds belong to type A.
The structure type with trans-oriented molecules without inversion symmetry
(drawn in gray) is theoretical, i.e. has not been found in any crystal.
498; 408
370; 356; 332; 318
298; 202; 168
The geometry of the organic molecules themselves is well with-
in the range of the expected values (selected distances and angles
are given in Table 4), with the exception of the hydrogen bond in
the dimeric cation. The Oꢀ ꢀ ꢀO distances may be divided into two
groups. In one group (## 1, 4, 5, 7, 10) the Oꢀ ꢀ ꢀO distances are rel-
atively short (the mean value is 2.639 Å), while in second group
(## 2, 3, 6, 8, 9) the Oꢀ ꢀ ꢀO distances are longer (the mean value
is 2.710 Å). Thus the value of the Oꢀ ꢀ ꢀO distance in b-alaninium pic-
rate (2.636(2) Å) is among the shortest distances in such A+ꢀ ꢀ ꢀA+
cations (see Table 1).
The third type is realized in crystals of (b-Ala+)2 biphenyl-4,40-
disulfonate, which has centrosymmetrical dimers of trans-oriented
b-alaninium cations (type C, Fig. 5). Finally, we found a fourth type,
which is represented by (b-Ala+)2 SnI4, where the two amino ends
of both gauche-oriented cations are facing in the same direction
(type D, Fig. 5). As in type B, the oxygen atoms are not coplanar,
in (b-Ala+)2 SnI4 the torsion angle O3AO1AO2AO4 equals ꢁ8.2°.
Theoretically, a fifth type is conceivable, namely with b-alaninium
cations in trans-conformation, but without an inversion center.
However, none of the crystals reported assumes this geometry.
From all b-alaninium compounds containing b-Ala+ꢀ ꢀ ꢀb-Ala+ cat-
ions, the majority (## 1, 3, 4, 6, 7, 8, 9 from Table 1) belongs to type
A, i.e. has centrosymmetrical dimeric pairs in gauche-conforma-
tion. All other types have only one representative, viz. the one
named above. A schematic overview of the geometries of these
types is shown in Fig. 6.
As far as the packing of the units is considered, the structure can
be described as layers of picrate anions, with the amino acid moi-
eties in the interstices (Fig. 4). The aromatic ring of the picrate an-
ꢀ
ions lies in the plane ð205Þ, although the nitro groups of N-atoms
N1b and N3b are twisted out of this plane. The b-alaninium cations
in between assume gauche conformation (torsion angle
C1AAC2AAC3AAN1A equals 66.2(2)°) – the most common confor-
mation when compared with related compounds (see Table 1 for
values calculated from the CIFs of the listed species).
3.2. Vibrational spectra of b-alaninium picrate
When comparing the structural properties of all known b-
Ala+ꢀ ꢀ ꢀb-Ala+ dimers, we found four different types. The first type
is that of b-alaninium picrate is characterized by a symmetric
arrangement of two b-alaninium cations in gauche conformation
(type A, Fig. 3). Another type is represented by b-Ala+ NO3, in which
there is no inversion center as in the above type, although the b-
alaninium cations also assume gauche conformation and adopt a
similar geometry (type B, Fig. 5). Since the inversion center is miss-
ing the four oxygen atoms are not coplanar, in the case of b-Ala+
NO3 the torsion angle O21AO22AO11AO12 equals ꢁ4.0°.
The IR and Raman spectra of b-alaninium picrate are shown in
Fig. 7, wavenumbers and assignment of peaks are provided in Ta-
ble 5. In the high-frequency region one expect to find bands caused
by stretching vibrations of NAH, CAH and OAH bonds of NHþ, CH2,
3
OH groups of b-alaninium cation and CH groups of picrate anion.
The absorption band at 3261 cmꢁ1 and respective Raman-line we
assign to vibration of N1AAH13A bond, which forms weak hydro-
gen bond (see Table 3). The broad absorption band centered at
2917 cmꢁ1 and respective Raman lines in this region are caused

M. Fleck et al. / Journal of Molecular Structure 1019 (2012) 91–96
95
Fig. 7. Infrared and Raman spectra of b-alaninium picrate.
by vibrations of N1AAH11A and N1AAH12A bonds forming stron-
ger hydrogen bonds as well as OH, CH2 and CH groups. The ob-
served positions of m(NH) and m(OH) correspond well to expected
the spectra of salts with (Aꢀ ꢀ ꢀA+) type dimeric cation with very
short (OAHꢀ ꢀ ꢀO) hydrogen bonds [4].
positions on the basis of respective correlations [25,26]. The picrate
anion has some characteristic bands [3,5,18]. In the Raman spec-
trum these are the most intensive lines caused by vibrations of ni-
tro group of picrate anion. The line at 1549 cmꢁ1 we assign to
asymmetric stretching, the most intensive band with peaks at
1338 and 1323 cmꢁ1 to symmetric stretching and the line at
827 cmꢁ1 to deformation vibration of NO2 groups. Respective
4. Conclusion
With this contribution we report a new crystal species, b-alan-
inium picrate, unexpectedly found in the system b-alanine + picric
acid + H2O (in addition to the already known compound b-alanine
b-alaninium picrate). We have characterized this novel species
structurally and by infrared and Raman spectra. It belongs to the
group of crystals that contain dimeric amino acid (A+ꢀ ꢀ ꢀA+) cations.
Comparison with other known similar salts showed that there are
two groups of salts, i.e., with short and relatively long (Oꢀ ꢀ ꢀO) dis-
tances. Furthermore, it was found that all these salts can be ar-
ranged into four structural types, according to conformation and
symmetry of the dimers of b-alaninium cations. b-Alaninium pic-
rate belongs to the group with short (Oꢀ ꢀ ꢀO) distances and to the
bands in the IR spectrum are at 1564, 1557 cmꢁ1
1320 cmꢁ1 s(NO2)) and 828 cmꢁ1 (d(NO2)). In addition, the IR
(mas(NO2)), 1331,
(m
spectrum comprises a characteristic band at 1262 cmꢁ1 caused
by stretching vibration of the phenolic CAO bond and a band with
peaks at 1633 and 1619 cmꢁ1 caused by stretching vibration of
short C@C bonds in the ring. The IR spectrum also comprises a
band at 1687 cmꢁ1, which is not active in the Raman spectrum.
We assign it to the COOH group. Similar bands were observed in

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M. Fleck et al. / Journal of Molecular Structure 1019 (2012) 91–96
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