Synthesis and spectral investigations of pyridinium picrate

Article abstract of DOI:10.1016/j.molstruc.2016.07.032

This article describes the vibrational spectra of a crystalline compound called “Pyridinium Picrate. The vibrational spectra measurements were recorded as a function of temperature by both FT-IR and Raman spectroscopy. The presence of various functional g

Full text of DOI:10.1016/j.molstruc.2016.07.032

Accepted Manuscript
Synthesis and spectral investigations of pyridinium picrate
Meddour Ahmane, Azizi Abdelkader
PII:
S0022-2860(16)30703-7
10.1016/j.molstruc.2016.07.032
MOLSTR 22735
DOI:
Reference:
To appear in: Journal of Molecular Structure
Received Date: 7 August 2014
Revised Date: 6 July 2016
Accepted Date: 8 July 2016
Please cite this article as: M. Ahmane, A. Abdelkader, Synthesis and spectral investigations of
pyridinium picrate, Journal of Molecular Structure (2016), doi: 10.1016/j.molstruc.2016.07.032.
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ACCEPTED MANUSCRIPT
Synthesis and Spectral investigations of Pyridinium picrate
*
MEDDOUR Ahmane, AZIZI Abdelkader
Materials Physics Laboratory
8
May 1945 Universite, BP 401 Guelma 24000, Algeria
ABSTRACT
This article describes the vibrational spectra of a crystalline compound called "Pyridinium
Picrate. The vibrational spectra measurements were recorded as a function of temperature
by both FT-IR and Raman spectroscopy. The presence of various functional groups and
modes of their vibrations were assigned and followed evolutions. The stability of the
crystalline compound arising from has been analyzed using DSC analysis. This crystal
composed of two symmetry groups C2v and P-1 respectively named phase I and II, this
transition was followed by a function of temperature and XRD, is agreement with crystal
structure data. In the present investigation, IR and Raman spectroscopy were employed for
the identification of the transitions phase of this dimorphic. Essentially studied by Raman
low frequency, the shift, disappearance and appearance of the peaks are used for the
determination of the phase transition by the characteristic bands NO , C–H deformation and
2
C–O stretching, these results reveal a reversibility of this structural transition, depending on
the operating conditions.
Keywords: Pyridinium Picrate, phase transition, FT-Raman, FT-IR, XRD, DSC.
1
. INTRODUCTION
Many active ingredients have several polymorphic forms, with one of them only being the desired
form to be formulated in the drug. It is thus imperative to control within the mixture which form
is present. Being non-invasive, and very chemically selective to discriminate polymorphs, Raman
spectroscopy is well adapted to determine within tablets or capsules which form(s) are included.
The structural phase transitions alter the crystal’s vibrational behavior, so that: Raman spectra
show changes across those transitions; Raman spectroscopy provides a convenient proxy for
structural phase changes. Doubling of features, frequency shifts, line width changes, are the
typical signatures of structural transitions (Soft modes signal softening of restoring forces typical
of many structural transitions. The pyridinium Picrate properties has allowed to intervene in
various fields such as biology, optics, catalysis and as purification in the pharmaceutical industry,
hence the importance of knowing the polymorphic behavior of the active substance to optimize
operation and storage conditions so that only the desired shape will be present in these processes,
or the importance of the thermal stability of this dimorphic compound (two phase).
The work presented in this article is an contribution to the study of phenomena related to
-
+
dimorphic of an organic salt called [(C H N O ) .(C H N) ] '' Picrate Pyridine '', by comparing the
6
2
3
7
5
6
behavior of its two forms, respectively named phase I and II, to provide additional structural and
spectral data with previous results of other authors, by monitoring the behavior and activity of any
group spectroscopy essential to the understanding of its mechanism of structural changes to tackle
the question of reversibility in this transition. To do this, a particular technique was preferred;
Raman spectroscopy has the ability to provide guests with information on the molecule structure,
its binding mode and the presence or absence of an interaction, which makes particularly
interesting target for certain influenced by phase transition phenomena particular groupings. To
complete the data structural studies of this dimorphic, because of our knowledge, the vibration
spectra of crystals PicPy has never been a comprehensive spectral study. The unusual work is
essentially based on the diffraction XRD, on which, we refer in this study.
*
Corresponding Author. Email: azizikader@yahoo.fr

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2
2
. EXPERIMENTAL DETAILS
.1. Synthesis
The title’s compound is a yellowish crystalline powder which has a molecular formula C H N O
11
8
4
7
with molecular weight 308.206ꢀgꢀ/mol. The saturated solution of the title’s compound was
prepared at 1:1 stoichiometric ratio the pyridine solution and picric acid with methanol as the
solvent. The solubility for 298K was determined by dissolving the picric acid salt in 10 mL of
alcohol (The solubility of the crystal was analyzed by three different solvents). A constant weight
of 10 mg of the picric acid powdered was used throughout the experiment. The acid picric and
pyridinium solutions were mixed together and stirred well using mechanical stirrer for about
30 min to get a homogeneous solution and the resulting solution was filtered into a clean dry
beaker using what man filter paper. After filtration at primary vacuum, the filtrate was kept in
3
dust-free and humidity, average dimensions of the grown crystals were 0,9 x 0,8 x 0,04 mm , the
grown crystal is optically transparent as shown in Fig. 1.
Fig. 1. Photograph of as-grown PicPy crystal (yellow).
2
.2. DSC Studies
For structural characterization, it is very important to start with the study of the thermal behavior
in order to identify areas of stability and transition, highlight thermal anomalies revealing any
change in these properties under the influence of the thermal environment. For this purpose, we
used the thermal analysis by differential scanning calorimetry.
Fig. 2. DSC Thermograms.
The curves were obtained on a sail Labsys Evo TG- DSC , with a mass of 35 mg of sample in an
aluminum crucible and in a measuring cell with a capsule called a reference, which is subjected to
a temperature program ranging from 293 to 424K, as ramps 285, 283 and 281K/min under
nitrogen atmosphere (35ml/min). These curves in Fig. 2, shows that at the beginning of a 358K
thermal activity gives rise to a first endothermic peak center to 368K, which is the melting start
point of the phase I, followed by an exothermic peak intermediate between the two bands
endothermic what combines a recrystallization solution phase II with a second endothermic peak

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at 383K. The melting peak does not appear on the curve, as in a limited temperature 423K to
prevent product degradation in the device and thus damaging its smooth operation (the melting
point is confirmed by Kotler banc method).
Both endothermic point of the hypothesis of the formation of phase II, following the merger of
Phase I at the same temperature (358K) are find by Koflor Mark Botoshansky [1,3], who gave us
an idea of areas in which the temperatures must focus for XRD, IR and Raman records. It is noted
that the recordings done by cooling the samples at these speeds, shows no sign of activity.
2
.3. X-ray Diffraction analysis
In order to follow the structure, that is to say to ensure that the compounds prepared in the
expected crystallize well system (in our conditions), two samples were prepared in one stored at
ambient temperature) (293K) and another treated in an oven at a temperature of 368K for 30 min
(
in order to cause the transition phase), were characterized by X-ray diffraction using a diffract
meter operating with copper Kα radiation (λ=1.5406 Å), the records obtained are shown in Fig. 3.
Fig. 3. DRX pattern at different temperatures
A):298K, and (B): 368K.
(
Indexing experimental diffractograms peaks is calculated and performed using the program and
Mercury.CCDC [4,5]. The refinement of the cell parameters of phase I (at ambient temperature),
shows that it crystallizes in a monoclinic system, space group P2(1)/c, with lattice parameters.
a = 12.122(2) Å; b = 3.7830(10) Å; c = 26.621(3) Å ;
α = 90 °;β = 92.56 ° (5); γ = 90 °, Z=4 and Z’=4;
And at the processing temperature of 368K for 30 min (the transition stage II) in a triclinic system,
space group P-1 with cell parameters:
a = 10.156(2) Å; b = 8.984(2) Å; c = 7.2300(10) Å ;
α = 86.38(5) °; β = 80.10(5)°; γ = 89.97(5) °, Z=2 and Z’=0;

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3
3
. RESULTS AND DISCUSSION
.1. SPECTROSCOPIC STUDIES (IR and RAMAN)
The IR spectrum of the grown crystals was recorded using the infrared spectrometer, in the region
−
1
of 4000 to 400ꢀcm . The absorption of IR radiation causes various bands in a spectrum which
can be treated as the finger print of different modes of vibration of a molecule, and it
clearly exhibits the presence of various functional groups present in a compound. The infrared
absorption arises when the vibrational motion produces a change in the permanent dipole moment
of the molecule [6]. The recorded IR spectrum is shown in Fig. 4.
The FT-Raman spectral measurements were made with laser beam of wavelength 1.06 µm (IR),
the output power being 60ꢀmw, was used. The spectrum was recorded for a range of 3500 to
-1
-1
2
00ꢀcm with a spectral resolution of 2ꢀcm . Raman spectroscopy is an inelastic photon scattering
between a laser beam and a molecule. Raman scattering occurs when the vibrational motion
produces an induced dipole moment or change in the polarizability of the molecule [7]. The
comparison of FT-Raman and infrared spectral analyses is shown in Fig. 4.
-
1
Frequency (cm )
Fig. 4. FT-IR and FT-Raman spectra of PicPy at ambient temperature.

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As with any complex molecules, and for simplicity, vibrational interactions occur and these labels
only indicate the predominant vibration, the bands were compared with FTIR spectra of picric
acid spectrum. The Pyridinium picrate crystals have functional groups and skeletal groups such
as C-H, NO and C-O ring structure. The absence of O-H stretching indicates the presence of
2
-
picrate ion instead of picric acid. The picrate (C H N O ) forms the nitro group and are twisted
6
2
3
7
out of plane of the ring. Picrate being the trinitrophenolate has the characteristic bands of phenol,
the nitro group and the phenoxy group.
Tableau 1: Vibrational assignments [7-22]
-1
-1
Infrared /(cm )
Raman /(cm )
Assignment
Phase I
204 w
Phase II
Phase I
Phase II
3
3204w
-
-
-
-
3
2
080ms
920ms
3080ms
C-H stretch
C-C stretch
2921ms
2853ms
1620w
1510w
-
-
-
2
1
1
850 ms
631 ms
641 ms
-
-
1634vw
1598w
-
-
-
1
592w
548w
-
1
1553w
1522s
1460s
1367w
-
1555s
-
1553s
-
2
C-NO stretch
1
522s
460s
364w
-
NO asy. stretch
2
1
1496w
1365vw
1365w
1376vs
1346w
1365ms
1324vw
1283ms
1176w
1496w
-
C-C stretch
C=N stretch
1
1365w
1374vs
1336w
1365ms
-
C-C stretch
-
-
1
342ms
1345 ms
-
2
NO sym. stretch
-
1
330w
266vs
173ms
1330w
1266vs
1172ms
C-C stretch
C-N stretch
1
1298ms
1198w
1
-
1168ms
1167ms
-
C-H ipb
C-H opb
1
084vs
1085vs
995w
1086 s
1000w
943w
1082s
1020w
941w
9
95w
9
42ms
942ms
9
25w
38vs
20ms
86ms
-
925 w
837vs
819ms
786ms
-
926ms
-
929ms
8
-
NO
2
in plane deform
C-H opb
8
7
822w
783w
763w
742w
821w
783w
-
-
C-C ipb
C-C opb
7
41ms
745ms
6
79vs
06vs
678vs
695vs
-
6
606vs
550w
472s
397w
-
607ms
548 w
550ms
390w
605ms
541w
5
50w
NO
2
rocking, C–C–C ipb
C-C opb
4
72s
541ms
397w
-
-
-
297ms
207ms
295w
C- NO
C- NO
NO
2
ipb
-
205ms
2
opb
-
-
120ms
120ms
2
torsion
Note: ipb - in-plane deformation; opb - out-of-plane deformation;
s- strong; b-broad; v-very; w-weak; m-medium.

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3
.2. Analysis in Temperature dependence
Fig. 5. Photos the powder compound at different temperatures:
(A): 333K, (B): 388K, (C): 398, (D): 418K, (X): With Chalumeau (closed room).
Samples have been heated until they have reached the temperature shown by the thermal analysis
was kept constant for a duration of treatment of 30 min, and ambient temperature Fig. 5,
illustrates the aspects of powder at different temperature, although we note the color of change
with the temperature. He observed Raman and IR bands are assigned in terms of fundamentals,
overtones and combination bands. The observed wave numbers along with their relative
intensities and probable assignments are given in Table 1. The wave numbers were assigned on
the basis of earlier assignments and the literature values.
3
.3. Temperature dependence of IR Spectral
-
1
Frequency (cm )
Fig. 6. Evolution of the FT-IR spectra of PicPy at different temperatures.

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Although the IR spectra (Fig.6) of the two phases are the same at different temperatures, the
appearance of the XRD diffractogram of the two different powders: one is of monoclinic structure,
the yellowish color and another under a system triclinic, brown color.
In general, we observe two sets of parallel spectra with the same rhythm and the same bands, the
spectra recorded at 293K and 333K are very similar and they fit perfectly, so we think that there is
no change. However, at 358K, the spectra are slightly modified compared to those recorded at
lower temperatures, and beyond the 358K sound recordings also almost identical with that
recorded at 388, 398 and 418K. However, there are differences between the massive 600 and
-1
-1
-1
-1
8
00 Cm , around 1450 Cm and 1350 Cm and between 1400 and 1750 Cm . At 388K, there is
a remarkable evolution of the spectra, there is a contrast reversal of intensities between 600 and
-
1
4
8
00 Cm , and between, we note that the number of bands decreased for regions between
-1
-1
50 - 750 Cm and 1350 - 1750 Cm [7-15].
3
.4. Temperature dependence of Raman Spectral
-
1
Frequency (cm )
Fig. 7. Evolution of the Raman spectra of PicPy at different temperatures.
The nitro group has characteristic vibrations active in both IR and Raman spectra. Inspection of
Raman spectra (Fig. 7) reveals that the NO asymmetric and symmetric stretching vibrations are
2
coupled with CC stretching vibrations and in-plane CH bending, and the associated IR intensities
are distributed over a large number of vibrations. It was shown that nitro aromatic compounds can
-
1
be identified in the FT-IR spectra by the characteristic bands that appear in 1550-1475 cm
-1
-1
(
νasym (NO)) and 1360-1290 cm (ν (NO)) regions [15-22]. The bands at 1620 and 1598 cm are
sym
assigned to similar complex vibrations consisting of CC stretch, in plane CH bend and NO2
asymmetric stretch. The CN stretching vibrations is coupled to δ (CH) vibration giving rise to the
-1
bands in 1140-1067 cm region that is overlapped by the absorption of (νC-O) constituents. The
-1
band at 1548 cm has a major contribution from the asymmetric stretch of the three nitro groups.
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1
The NO out-of plane wag, (scissors) appear at around 780, 853 and 872 cm , the peak at
8
2
-1
-1
20ꢀcm is assigned to C–H out-of-plane bending. The medium band at 935ꢀcm is attributed to

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-1
the presence of C–C stretching. The strong peak at 1266 cm illustrates the presence of C–N
-1
stretching and at 1167ꢀcm is assigned to C–H in-plane deformation. C–C–C in-plane
-1
deformation is located at the weak peak of 550ꢀcm [10-22].
3
.5. Vibrational spectral analysis
Fig. 8. Observed thermally induced variations peak of the IR and Raman modes.
The fig. 8, illustrates the changing frequencies of the lines of the Raman spectra as a function of
temperature. It is found the all modes have frequency substantially stable (is insensitive with to
temperature). However, the intensity of these bands varies inversely with temperature
(
attenuation of intensity); these anomalies are indicative of the existence of the structural changes
at a temperature of about 345K (phase transition). Other hand, the latter (with Chalumeau in
closed room), present signs of Return to the initial state; under the effect of a change in
operational terms
4
. CONCLUSIONS :
The Raman spectra depend on the vibrations, and structure, they can be used very effectively as a
witness for structural changes. The crystals of pyridinium picrate were synthesized by slow
evaporation method, and the structure of the compound was confirmed with the help of powder
X-ray diffraction pattern. The thermal analysis (differential scanning calorimetry), was carried
out to find the thermal stability of the synthesized crystal. The thermal anomalies observed in this
curves indicate the title compound shows a first order phase transition. As a result, the non-
destructive techniques FTIR and FT-Raman spectra (reveal the mode of vibrations) have been
attempted to identify the numbers of the different functional groups in the compound (monitoring
of change in with temperature). The presence of picrate ions in the crystal structures is reflected in
the highly mixed bands of the C-O stretch, in-plane deformation and together out-of-plane

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-1
bending vibrations. The medium to high peak at 820 cm is attributed to CH out-of-plane bending,
and also because of the presence of NO out-of-plane deformation, was followed their indicative
2
of a phase transformation in the compound, with signs of realignment of the vibrations, this is
purely due to the effect of the reversibility. These results information the favorable conditions for
the preservation of phase I (reversibility), when applied in the pharmaceutical industry and
provides some insight into the capabilities of Raman spectroscopy for pharmaceutical applications.
Stability of products in their real state over time might be evaluated using this technique, without
having to transfer a sub sample into any vial – with the risk of transferring a non-representative
fraction of the crystals phase or add contamination.
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