Characterization of new cocrystals by raman spectroscopy, powder X-ray diffraction, differential scanning calorimetry, and transmission raman spectroscopy

Article abstract of DOI:10.1021/cg100156a

Cocrystals have been increasingly recognized as an attractive alternative delivery form for solids of drug products. In this work, salicylic acid was employed as a cocrystal former with the nicotinic acid, dl-phenylalanine, and 6-hydroxynicotinic acid (6HNA). Also, 3,4-dihydroxybenzoic acid with oxalic acid was studied. The cocrystals in all cases were prepared by slow evaporation from ethanol followed by characterization using Raman spectroscopy, powder X-ray diffraction, transmission Raman spectroscopy (TRS), and differential scanning calorimetry. Full understanding of the effects of formation on the vibrational modes of motion was obtained by the complete assignment of the spectra of the starting materials and of the cocrystal components. The results show that all the cocrystals, prepared in a 1:1 molar ratio, possess unique thermal, spectroscopic, and X-ray diffraction properties. Raman and TRS spectra showed that the vibrational modes of the cocrystal were different from those of the starting materials, suggesting that Raman spectroscopy and TRS are effective tools to evaluate cocrystal formation through interaction of their components. In addition, we have used a synthetic standard containing a 1:1:1 mixture of KNO₃ and raw material for which each sample was analyzed at seven random positions, with each point sampled twice. We have done the same with all cocrystals (1:1 KNO₃ and cocrystal), the ratios confirming that the cocrystal components (were in a 1:1 molar ratio).

Full text of DOI:10.1021/cg100156a

DOI: 10.1021/cg100156a
Characterization of New Cocrystals by Raman Spectroscopy,
Powder X-ray Diffraction, Differential Scanning Calorimetry,
and Transmission Raman Spectroscopy
2010, Vol. 10
2360–2371
M. A. Elbagerma,*,† H. G. M. Edwards,† T. Munshi,† M. D. Hargreaves,‡
Pavel Matousek,‡,# and I. J. Scowen†
†Raman Spectroscopy Group, University Analytical Centre, Division of Chemical and Forensic Sciences,
University of Bradford, West Yorkshire, BD7 1DP, U.K., ‡Cobalt Light Systems Ltd, Start Electron,
Fermi Avenue, Harwell Science & Innovation Campus, Didcot, Oxfordshire, OX11 0QR, UKU.K., and
#Central Laser Facility, Science & Technology Facilities Council, Rutherford Appleton Laboratory,
Oxfordshire, OX11 0QX, U.K.
Received February 1, 2010; Revised Manuscript Received March 29, 2010
ABSTRACT: Cocrystals have been increasingly recognized as an attractive alternative delivery form for solids of drug pro-
ducts. In this work, salicylic acid was employed as a cocrystal former with the nicotinic acid, DL-phenylalanine, and 6-hydroxy-
nicotinic acid (6HNA). Also, 3,4-dihydroxybenzoic acid with oxalic acid was studied. The cocrystals in all cases were prepared
by slow evaporation from ethanol followed by characterization using Raman spectroscopy, powder X-ray diffraction,
transmission Raman spectroscopy (TRS), and differential scanning calorimetry. Full understanding of the effects of formation
on the vibrational modes of motion was obtained by the complete assignment of the spectra of the starting materials and of the
cocrystal components. The results show that all the cocrystals, prepared in a 1:1 molar ratio, possess unique thermal,
spectroscopic, and X-ray diffraction properties. Raman and TRS spectra showed that the vibrational modes of the cocrystal
were different from those of the starting materials, suggesting that Raman spectroscopy and TRS are effective tools to evaluate
cocrystal formation through interaction of their components. In addition, we have used a synthetic standard containing a 1:1:1
mixture of KNO3 and raw material for which each sample was analyzed at seven random positions, with each point sampled
twice. We have done the same with all cocrystals (1:1 KNO3 and cocrystal), the ratios confirming that the cocrystal components
(were in a 1:1 molar ratio).
1. Introduction
high-throughput screening technologies.4,5 However, drawbacks
of this technique are the long measurement times, insensitivity
toward isostructural crystals, and, for some materials, inter-
ference from preferred orientation effects; consequently, Raman
spectroscopy is now being advocated as an analytical alter-
native to PXRD for cocrystal characterization.6-8 Raman
spectroscopy probes the effect of crystal structure on bond
vibrational energies and is potentially able to selectively
distinguish between the polymorphs of a given API. Further-
more, measurements are noninvasive, nondestructive, and
rapid (data acquisition is achievable within seconds rather
than minutes), which makes Raman spectroscopy ideal for
automated high-throughput systems. Since Raman spectros-
copy and PXRD are complementary techniques at the mole-
cular level, in combination they can provide an increased
understanding of solid-state phenomena. Also, the crystalliza-
tion process and other physicochemical characteristics can be
determined by differential scanning calorimetry (DSC) and
infrared (IR).9 On the other hand, Brittain has concluded that
that the Raman spectroscopy is not useful in characterization
of benzamide-benzoic acid cocrystal, though, Raman spectros-
copy was found to be a useful vibrational spectroscopic
technique for characterization of the benzylamine-benzoic
acid system and benzenecarboxylic acids and their sodium
salts systems.10-12 Also, Elbagerma et al. have found that
Raman spectroscopic is a very useful tool for characterization
of benzamide-salicylic acid cocrystal.13
Cocrystallization is a crystallization method that allows the
binding of two or more building blocks within one periodic
crystal line lattice without making or breaking covalent
bonds.1 These building blocks are generally neutral molecular
species linked via hydrogen bonds.2 Cocrystals represent a
class of unexplored compounds and offer huge opportunities
for the pharmaceutical industry. Indeed, cocrystallization can
be a new way to crystallize molecules that crystallize with
difficulty in their pure forms or as salts. Pharmaceutical
cocrystallization is a relatively recent technology which offers
an alternative platform in improving the physicochemical
properties of active pharmaceutical ingredients (APIs), such
as the melting point, solubility, stability, and dissolution rate.3
APIs are complex chemical structures with functional
groups that can take part in molecular recognition events. It
is the presence of these functional groups, such as amides and
carboxylic acids, that provides an ability to engage in supra-
molecular events with cocrystal formers having complemen-
taryhydrogen bond donor and acceptor sites, therebyforming
pharmaceutical cocrystals.
The solid product of each reaction is commonly analyzed
using X-ray powder diffractometry (PXRD) as the primary
technique. PXRD detects changes in the crystal lattice and
is therefore a powerful tool for studying polymorphism,
pharmaceutical salts, and cocrystalline phases. PXRD can
also be used as a component of automated robotic systems in
The principal benefit of transmission Raman spectroscopy
*To whom correspondence should be addressed.
(TRS) lies in its ability to provide rapid volumetric information
r
pubs.acs.org/crystal
Published on Web 04/15/2010
2010 American Chemical Society

Article
Crystal Growth & Design, Vol. 10, No. 5, 2010 2361
on the content of pharmaceutical formulations including
intact tablets and capsules eliminating the subsampling issues
of conventional Raman spectroscopy and suppressing surface
Raman and fluorescence components (e.g., from capsule shell
or tablet coating). These features are important in quality
control within a production environment.14 From this point
of view, the cocrystals were also analyzed with this method.
Table 1. Assignments of Major Bands of Raman Spectra of Salicylic Acid
and Nicotinic Acid and Their 1:1 Cocrystal Products
salicylic
acid
nicotinic
acid
cocrystal
3070
assignment22-24
3086
3075
7b (C-H) stretching
2 (C-H) stretching
(C-H) stretching
v(CdO)
3065
1694m
1675
1672m
1633m
1603m
1603
2. Experimental Section
1632s
CdO str
2.1. Materials. Salicylic acid, nicotinic acid, DL-phenylalanine,
oxalic acid, 6-hydroxynicotinic acid (6HNA), and 3,4-dihydroxy-
benzoic acid were purchased from Sigma-Aldrich at >98%
purity. These materials were used as received. The solvent
€
(ethanol) was HPLC grade and purchased from Reidel de Haen
or Fisher Scientific.
Cocrystal formation was identified initially using Raman spectros-
copy and the difference in melting points from the pure components
and the structures were confirmed by X-ray diffraction.
2.2. Raman Spectroscopy. Raman spectra of the samples derived
from the general preparative method described below as well as the
1599s
1588m
v4(CdC)N
v2(CdC)N
8a
1586m
1580m
1483w
1471m
1324 doublet
broad
shoulder
1471m
1321s
19a
(C-O)c str
1318m
1306m
1242w
1306m
βOH þ βCH(9a) þ
vC-O)
(C-O)h str
1245s
1187m
1247m
1169m
1159s
1163 m
1066w
3
13
Scheme 1. The Starting Components of the
Seven 1:1 Stoichiometry
1113w
1042s
βCH(18b) þ v(CC) þ
v(CN)
σ ring (12) þ βCH(18a)
1036s
1031m
σ ring (12) þ v ring (1)
1027s
805w
18b
850m
818m
810s
σ ring (6a) þ v(C-X) þ
σ (COO)
774s
768s
652w
652w
639w
385 m
σ ring (6b) þ σ (COO)
564m
531m
449m
564m
531m
449w
439w
385m
v(C-X) þ σ ring (6a)
σ (COO)
378w broad
255s
204w
255m
177s
255w
204s
182m
Figure 1. Raman spectra obtained for (a) salicylic acid, (b) nicotinic acid, and (c) the cocrystal.

2362 Crystal Growth & Design, Vol. 10, No. 5, 2010
Elbagerma et al.
Figure 2. Powder X-ray diffraction patterns for (a) salicylic acid, (b) nicotinic acid, and (c) the cocrystal for full range.
Figure 3. Raman spectra obtained for (a) salicylic acid, (b) the cocrystal, and (c) DL-phenylalanine.
Figure 4. Raman spectra obtained for (a) salicylic acid, (b) the cocrystal, and (c) DL-phenylalanine.
single components were obtained using a Raman microscope
(Renishaw plc.) with 785 nm stabilized diode excitation. The laser
power at the sample was approximately 25 mW. A 50ꢀ objective
lens was used, giving a laser spot diameter (footprint) of 2 μm at the
sample. Spectra were obtained for a 10 s exposure of the CCD
detector in the region 3600-50 cm-1 using the extended scanning
mode of the instrument.
2.3. Transmission Raman Spectroscopy. Transmission Raman
spectra were collected using a Cobalt Light Systems TRS100
dispersive Raman spectrometer. The vial was held in place (at an
angle ∼45°) and the excitation laser was directed onto the underside
of the sample. Light was collected at ∼180° from the direction of the
excitation laser light, on the side of the vial. A NIR laser at 830 nm
was used, with a power at the sample of approximately 1 W and a
beam diameter of 3-4 mm. The imaging zone on the vial, deter-
mined by the collection optics, was 5 mm. All samples were mea-
sured for a 1 s exposure time, with 10 accumulations, giving a total
scan time of 10 s. Transmission data were collected at a resolution
of ∼5-6 cm-1, over the full dispersive range of the instrument
∼45-2500 cm-1
.
2.4. Powder X-ray Diffraction. Data were collected on a Bruker
D8 diffractometer in Bragg-Brentano θ-θ geometry with Cu
˚
KR1,2 radiation (1.5418 A) using a secondary curved graphite
monochromator. The X-ray tube was operated at 40 kV, 30 mA.
Samples were scanned in a vertical Bragg-Brentano (θ/2θ) geo-
metry (reflection mode) from 5° to 40° (2θ) using a 0.005° step width

Article
Crystal Growth & Design, Vol. 10, No. 5, 2010 2363
and a 1.5 s count time at each step. The receiving slit was 1° and the
scatter slit 0.2°.
Table 2. Assignments of Major Bands of Raman Spectra of Salicylic Acid,
DL-Phenylalanine, and Their 1:1 Cocrystal Products
2.5. Differential Scanning Calorimetry. DSC profiles were gene-
rated in the range of 50 -160 °C using a TA Q2000 DSC instrument
with an RGS90 cooling unit. Temperature calibration was perfor-
med using an indium metal standard supplied with the instrument at
the appropriate heating rate of 10 °C min-1. Accurately weighed
samples (1-2 mg) were placed in Tzero aluminum pans using a
similar empty pan as reference. The data were collected in triplicate
for each sample and were analyzed using a TA Instruments Univer-
sal Analysis 2000 version 4.3A software.
2.6. Sample Preparation. Molecular structures of the cocrystal
components used to prepare cocrystals 1-5 are illustrated in Scheme 1;
experimental details of their preparation are presented below.
The standard method of preparation is exemplified as follows:
salicylic acid (28.2 mg) and nicotinic acid (25.14 mg) were dissolved
in 3 mL of ethanol. The mixture was warmed on a hot plate and the
solution was allowed to cool to room temperature.
Slow evaporation of the solution resulted in the formation of
a solid product which was isolated by filtration and air-dried.
Similarly, for the other cocrystals: salicylic acid (28.2 mg) and
DL-phenylalanine (33.73 mg) were dissolved in 3 mL of ethanol;
salicylic acid (100 mg) and 6HNA (102.20 mg) were dissolved in
4 mL of ethanol; oxalic acid (100 mg) and 3,4 dihydroxybenzoic acid
(171.19 mg) were dissolved in 5 mL of ethanol, and caffeine (100 mg)
and 3,4 dihydroxybenzoic acid (79.36 mg) were dissolved in 5 mL of
ethanol; they were also treated similarly to prepare their respective
cocrystalline products.
salicylic
acid
DL-
phenylalanine
cocrystal
assignment20-28
3086
3075
7b (C-H) stretching
2 (C-H) stretching
(C-H) stretching
1666m
1655m
1632s
CdO str
1609s
1607m broad
[NH3]þ asym def;
j quad. ring str (ν8a
)
1602s
1585s
1582m
j quad. ring str (ν8b);
[NH3]þ sym def
8a
1580m
1471m
1561w
1510w
COO- asym. str.
C-N asym str
19a
1463m broad
1458w
disappear under j s.c. ring str þ CH
1463 def (ν19b)
1442s
1411s
1441w
1410vw abroad COO- sym. str.
1363m
double broad
peaks 1325m,
1317m
1383m
1321s
(O-H)h i.p. bend
(O-H) i.p. def
1346m
1337s
1322m
j sextant ring str. (v14
(C-O)c str
CH2 wagging
14
(C-O)h str
j CH o.p. def (chain);
CH2 twist
j i.p. CH def (ν9a);
C -O (H) str
3
j i.p. CH def (ν15)
15
C-C str
18b
j i.p. ring. def (ν12)
)
1309s
1306m
1245s
1301w
1240s
1207m
3. Results and Discussion
1213vs
1185m
The seven starting components have the following stoichio-
metry (Scheme 1).
broad 1192,
1183,1175 vw
1162m
1155m
1139m
1030s
3.1. Characterization of the Cocrystals. 3.1.1. Salicylic
Acid and Nicotinic Acid (Vitamin B3) Cocrystal. The Raman
spectra of salicylic acid, nicotinic acid, and cocrystal are
presented in Figure 1. The assignments for the most charac-
teristic vibrational bands are listed in Table 1. Raman spectro-
scopic data were utilized first to evaluate whether the com-
plex is a cocrystal or salicylic acid contains a carboxylic group
and a hydroxyl group whereas nicotinic acid is a derivative
of pyridine, with a carboxylic acid group (COOH) at the
3-position.
1163 m
1151s
1027s
1156s
1031s
1002vs
1000vs
965m
941w
903w broad
847w
948m
909w
847m
C-C str
C-C str
C-H out-of plane
deformation
C-C str
827s
822w
811w
doublet 773,
768
737w
805w
768s
A comparison of the spectra reveals that there are several
band shifts occurring between the individual components
and the cocrystal; as shown in Figure 1, Table 1, the Raman
spectra for pure nicotinic acid in the starting material have
bands at 1694, 1599, and 1588 cm-1, corresponding to v(CdO),
v(CdC)N, and v2(CdC)N, respectively. During the cocrys-
tallization of nicotinic acid with salicylic acid, these bands in
777vw
C-C skeletal str
742 m
726w
618s
CH2 rock
CH2 rock
618w
597vw
560m
527w broad
j i.p. ring. def (ν6b
)
602m
O CdO i.p. def
564m
531m
the cocrystal were shifted to 1675, 1603, and 1586 cm-1
,
518m
468m
C CdO i.p. def; j in.
p. ring. def (ν6a
)
respectively. The decrease in the v(CdO) frequency of nico-
tinic acid from 1694 to 1675 cm-1 and 1536 to 1533 cm-1 in
the SA component indicates that the carbonyl group is parti-
cipating in hydrogen bonding. The Raman spectra of pure
salicylic acid and nicotinic acid have bands at 1321 cm-1
((C-O) str) of the carboxylic group and 1318 cm-1, respec-
tively. During the cocrystallization, these bands appeared as
a broad band at 1324 cm-1 as shown in Figure 1. Also, pure
nicotinic acid spectrum has a band at 1306 cm-1, corres-
ponding to (βOH þ βCH (9a) þ vC-O)); through the co-
crystal this band appears as a shoulder.
490w
478w
COO- rocking
449m
385m
448w
438w
365m
313m
σ(C-C), σ(C-C-C)
C-C-C-C o. ph. def
313w
252w
198w
255m
208m
lattice modes
177s
166,160m broad
Upon cocrystallization, this band shifted to 1247 cm-1
with a decrease in the intensity; this observation indicates
that the carboxylic group participates in hydrogen bonding.
The peak at 1163 cm-1 in the spectrum of salicylic acid is
shifted to 1159 cm-1 in the spectrum of the cocrystal and
becomes stronger. Moreover, new bands at 1169 and 850 cm-1
,
which do not occur in either salicylic acid or nicotinic acid,
appear in the salicylic acid-nicotinic acid cocrystal.
Using a synthetic standard containing a 1:1:1 mixture of
KNO3, salicylic acid, and nicotinic acid for which each sample

2364 Crystal Growth & Design, Vol. 10, No. 5, 2010
Elbagerma et al.
Figure 5. Transmission Raman spectra obtained for (a) salicylic acid, (b) the cocrystal, and (c) DL-phenylalanine.
fraction patterns of the raw material and the cocrystal
product.
Table 3. Major Bands of Transmission Raman Spectroscopy (TRS) of
Salicylic Acid, DL-Phenylalanine, and their 1:1 Cocrystal Products
The DSC thermograms of nicotinic acid, salicylic acid,
and their 1:1 cocrystal product are presented. The thermo-
gram of salicylic acid shows a melting endotherm maximum
at 159 °C and the thermogram of nicotinic acid shows a
melting endotherm maximum at 236 °C. The DSC thermo-
gram of the 1:1 stoichiometric product proved that the
nicotinic acid-salicylic acid cocrystal had been formed;
this substance exhibits a melting endotherm maximum at
128 °C.
salicylic
acid
DL-
phenylalanine
cocrystal
1670m
1659m
assignment24-28
1632s
CdO str
1611w
1606w broad
[NH3]þ asym def;
j quad. ring str (ν8a
)
1604w
1580w
1582m
j quad. ring str (ν8b);
[NH3]þsym def
8a
1583m
1473m
1385w
3.1.2. Salicylic Acid and DL-Phenylalanine Cocrystal. The
Raman spectra in the regions of 1800 to 1100, 1100 to 650,
and 600 to 200 cm-1 for salicylic acid, DL-phenylalanine, and
the salicylic acid-DL-phenylalanine crystal are presented in
Figures 3 and 4. The assignments for the most characteristic
vibrational bands are listed in Table 2. During the formation
of the salicylic acid-DL-phenylalanine crystal, the OH and
CdO bands of the salicylic acid are shifted to lower or higher
wavenumber by 5-21 cm-1, which suggests that the mole-
cular complex of salicylic acid-DL-phenylalanine is a cocrys-
tal.15 The Raman spectrum for pure salicylic acid in the
starting material has strong bands at 1632 cm-1, 1245 cm-1
and a medium intensity peak at 1383 cm-1 corresponding to
CdO stretching, ((C-O)h stretching (vibrational of hydro-
xylic group)) and ((O-H)h in-plane bending, respectively.
During the cocrystallization, the band at 1632 cm-1 dis-
appears and the bands at 1383 cm-1 and 1245 cm-1 shift to
1363 cm-1, respectively. The disappearance of the CdO
stretching mode and decrease in the ((C-O)h str, (O-H)h)
wavenumber of salicylic acid indicate that the carboxylic
group is participating in strong hydrogen bonding. The
peaks in the spectrum of DL-phenylalanine occurring at
1346 cm-1 ((O-H) in-plane deformation), 1337 cm-1, 1322
cm-1, and 1309 cm-1 (CH2 wagging) and the peak of sali-
cylic acid at 1321 cm-1 ((C-O)c stretching (vibration of the
carboxylic group)) appear as a bond doublet at 1325 and
1561w
1510w
COO- asym. str.
C-N asym str
19a
1465 m broad
1441w
1410 vw abroad
1365vw
double broad peaks (O-H) i.p. def
1325m, 1317m
1443w
1411w
COO- sym. str.
(O-H)h i.p. bend
1346m
1337s
1322m
j sextant ring str. (ν14
(C-O)c str
j CH o.p. def (chain);
CH2 twist
j i.p. CH def (ν9a);
C-O (H) str
)
1321s
1214m
1193w
1208w
1185w
1163m
1162w
1156m
1142m
1031s
1158m
j i.p. CH def (ν15)
15
1152s
1028vs
18b
C-C str
j i.p. ring. def (ν12
1028s
1003vs
827w
1000s
770w
)
C-C str
lattice modes
770w
188w
217w
149m
114w
177w broad
127m
132m
114w
95m
83m
73s
71m
66s
1317 cm-1
.
54m
57w
48vwsh
49w sh
The spectra of pure DL-phenylalanine have strong bands at
1609 corresponding to ([NH3]þ asym deformation phenyl
ring quadrant. ring str (v8a)) and 1602 cm-1 appear as broad
bands centered at 1582 cm-1 with decreased intensity. Also,
the bands at 1411, 1213, and 1185 cm-1 correspond to
(COO- sym. str.), phenyl ring CH out of plane deformation
(chain); CH2 twist and phenyl ring in plane CH def (ν9a);
C-O (H) str, respectively. During cocrystal formation, the
strong band at 1411 cm-1 appears at 1410 cm-1 as a broad
was analyzed at seven random positions, with each point
sampled twice, the ratio of the 1599/768 cm-1 bands is 0.188;
in the cocrystal the ratio of these bands is 0.195 confirming
that it is a 1:1 cocrystal of salicylic acid and nicotinic acid.
The powder XRD diffractograms of the products of the
specimen cocrystallized via slow evaporation from ethanol
(Figure 2) show significant differences between the dif-

Article
Crystal Growth & Design, Vol. 10, No. 5, 2010 2365
Figure 6. Powder X-ray diffraction patterns for (a) salicylic acid, (b) DL-phenylalanine, and (c) the cocrystal.
Figure 7. DSC melting curves of DL-phenylalanine (blue line), cocrystal (red line), and salicylic acid (green line).
very weak intensity band, the peak at 1213 cm-1 was shif-
ted to 1207 cm-1 with decreased intensity, and the peak at
1185 cm-1 appeared as a weak broad peak at 1192, 1183, and
1175 cm-1. These observations indicate that the amide and
carboxylic groups are participating in strong hydrogen
bonding.
On the other hand, new bands at 1666, 1655, 965, 940, and
478 cm-1 are not present in the pure reference spectra. Using
a synthetic standard containing a 1:1:1 mixture of KNO3,
salicylic acid, and DL-phenylalanine for which each sam-
ple was analyzed at seven random positions, with each
point sampled twice, the ratio of the 1213/1322 cm-1 bands
is 0.381; in the cocrystal, the ratio of these bands is 0.375
confirming that it is a 1:1 cocrystal of salicylic acid and
DL-phenylalanine.
Transmission Raman Spectroscopy (TRS). The transmis-
sion Raman spectroscopy (TRS) spectra of salicylic acid, the
DL-phenylalanine, and the cocrystal are shown in Figure 5
and the vibrational wavenumbers and assignments are listed
in Table 3.
The Raman spectrum of DL-phenylalanine in the starting
material has bands at 1611, 1604, 1561, 1510, 1214, and 1193
cm-1. In the cocrystal the bands at 1611 and 1604 cm-1
appear as a broad band at 1606 cm-1; at the same time the
other bands at 1561 and 1510 cm-1 have disappeared in the
cocrystal and the bands at 1214 and 1193 cm-1 have shifted
to lower wavenumbers at 1208 and 1185 cm-1, respectively.
The bands at 827, 217, and 149 cm-1 have disappeared in the
cocrystal, while the peaks at 114 and 66 cm-1 have shifted to
127 and 71 cm-1, respectively. On the other hand, the Raman

2366 Crystal Growth & Design, Vol. 10, No. 5, 2010
Elbagerma et al.
Table 4. Assignments of Major Bands of Raman Spectra of
3,4-Dihydroxybenzoic Acid, Oxalic Acid and Their 1:1
Cocrystal Products
Table 5. Assignments of Major Bands of Raman Spectra of Salicylic Acid,
and 6-Hydroxynicotinic Acid and Their 1:1 Cocrystal Products
salicylic
acid
6-hydroxynicotinic
acid
oxalic 3,4-dihydroxybenzoic
acid
cocrystal
1715w
1686w
1642s
assignment24,34-37
acid
cocrystal
assignment29-33
1717sh
1699s
1647m
3077w
2584vw
3094vw broad ν(CH)
2584vw 1792m vOH
vOH
CdO str
ant(COOH)str
CdO str
2584vw
1792m
1778m
1724s
1632s
1622m
1554s
1467s
1622s
1583w
1555m
1468
i.p. β(NH)
1583m
1473m
1383w
8a (C-C)str
vas (CC/CN)
19a (c-C)str
vas (CC/CN)
(O-H)h i.p. bend
1704 broad
1616w
1604s
va (CdO)
ν(CC)ar
1604s
1599s
1514w
1455sh
ν(CC)ar
1481m
1454vw
1480w broad
1454vw
δ(COH) þ v(C-O)
vs (C-O) þ v (C-C)
CH3 bend C ring st
CH3 bend C ring st.
1355m
1343w
1337w
1444w
1379w
1364w
1338w
1291m
1335m
1373m
1322s
1321vw broad
1265s
1257s
1231sh
1243s
(C-O)c str
v(C-O) str
1275vs
1254m
1237m
v(OH) phenolic
β(OH)carboxylic,
(CC)ar
1313m
1249m
1245s
1163m
1152s
(C-O)h str
3
15(CC) i.p. bend
1242m
CH bending
1173m
1156m
1140m
1132w
1033s
doublet 859vs,
845s
846s
1097m
944m
CO stretch (COOH)
β(CH)
944w
1128w
859vs
852w
837m
824s
vC-C)
vC-C)
1026vs
18b(CC) i.p. bend
12(CC) i.p. bend
837s
824m
794w broad
794m
771w
754vs
636m
593m
ring breathing
CH wagging
ring deformation
scissors (COOH)
806vw
772s
795sh
769s
761w
768w
727w
642w
722w
541w
535w
478m
doublet 638s,
635s
559m
541w broad
527 broad
502w
477w
450w
435m
395m
σ ring (6b) þ
531w
ring deformation
ring wagging
σ (COO)
564s
(O-H)c o.p. bend
o.p. bend π (NH)
(C-O)h torsion
463m
455m
449m
542m
512w
458w
402w
530m
449m
438m
382s
343s
230m
429vw
382vw
ring deformation
OH wagging
lattice mode
448m
255w
9b
343w broad
225w broad
174s
332 broad
249 broad
181s
10a
256w
spectrum for salicylic acid has bands at 1632, 1473, 1385,
1152, 1028, and 188 cm-1. For the cocrystal, the peak at 1632
cm-1 has disappeared and the other peaks in the cocrystal
were shifted to 1365, 1142, 1031, and 177 cm-1, respectively.
Moreover, new peaks at 1670, 1659, and 950 cm-1 are
observed which do not occur in either salicylic acid or DL-
phenylalanine.
PXRD patterns obtained for salicylic acid, DL-phenylala-
nine, and the stoichiometric 1:1 salicylic acid, DL-phenylala-
nine component are shown in Figure 6. The diffraction
patterns of the three materials were found to be very differ-
ent, with the product exhibiting characteristic peaks at 5.5,
15.7, 11.17, 19.34, and 24.23 deg 2θ, and salicylic acid
exhibits characteristic peaks at 11.00 and 17.1 deg 2θ,
whereas DL-phenylalanine exhibits characteristic peaks at
5.6 and 22 deg 2θ.
The DSC thermograms of benzamide, salicylic acid, and
their 1:1 cocrystal product are presented in Figure 7 showing
the thermogram of salicylic acid (melting endotherm maxi-
mum at 159 °C) and the thermogram of DL-phenylalanine
(melting endotherm maximum at 276 °C). The DSC thermo-
gram of the 1:1 stoichiometric product proved that the
benzamide-salicylic acid cocrystal formed exhibits a melt-
ing endotherm maximum at 181 °C.
3.1.3. 3,4-Dihydroxybenzoic Acid and Oxalic Acid Cocrys-
tal. The molecular interaction between 3,4-dihydroxyben-
zoic acid and oxalic acid was examined by Raman spectros-
copy, and the vibrational wavenumbers and assignments
are listed in Table 4. The spectrum of oxalic acid showed a
peak corresponding to the carbonyl va(CdO) of carboxylic
acid at 1724 cm-1, The decrease in the CdO stretching
wavenumber from 1724 cm-1 to 1704 cm-1 indicates that
the carbonyl carboxylic group is participating in strong
hydrogen bonding. This band was shifted in the Raman spec-
trum of the cocrystal to a lower wavenumber at 1704 cm-1
and appears as a broad peak as shown in Figure 8. The
Raman spectra for pure oxalic acid in the starting material
has bands at 1481 cm-1, corresponding to the δ(COH) þ
v(C-O), and 1173 cm-1. During the cocrystallization of
the oxalic acid with 3,4-dihydroxybenzoic acid, the band at
1481 cm-1 appears at 1480 cm-1 as a broad peak and the
peak at 1173 cm-1 has disappeared in the cocrystal.
The Raman spectrum for pure 3,4-dihydroxybenzoic acid
in the starting material has bands at 1604 cm-1 ν(CC)ar,
1599 and 1514 cm-1 ν(CC)ar. Upon cocrystallization, these
bands in the cocrystal were shifted to 1616 cm-1, 1604 cm-1
and disappear, respectively. Also, the bands at 1291 cm-1
,

Article
Crystal Growth & Design, Vol. 10, No. 5, 2010 2367
Table 6. Major Bands of Transmission Raman Spectroscopy (TRS) of
Salicylic Acid, 6-Hydroxynicotinic Acid and Their 1:1 Cocrystal Products
1242 cm-1, 1097 cm-1 and 754 cm-1, corresponding to the
β(OH)carboxylic, (CC)ar, CH bending, CO stretch (COOH),
and CH wagging, respectively, and upon cocrystal for-
mation, these bands were shift to higher wavenumbers
with the exception of the band at 1097 cm-1, which has
disappeared.
salicylic 6-hydroxynicotinic
acid
acid
cocrystal
1686w
assignment24,34-37
1702m
1651w
ant(COOH)str
CdO str
8a
1634s
1583m
1635 broad
1583w
1557w
Furthermore, the O-H stretching mode of the 3,4-dihy-
droxybenzoic acid shifted from 1291 to 1313 cm-1 in the
cocrystal indicating that the intermolecular hydrogen bonds
between 3,4-dihydroxybenzoic acid molecules were broken.
Moreover, a new peak at 478 cm-1, which does not occur in
oxalic acid or 3,4-dihydroxybenzoic acid, is observed.
The Raman spectral data also confirmed that only oxalic
acid is present in the complex, as no bands can be assigned to
the oxalate16 or hydrogen oxalate17,18 ions. Furthermore, the
v(C-C)stretching wavenumber, 837 cm-1, is in agreement
with that reported for H2C2O4 molecule,19 but this value is
too low for HC2O4- or C2O4-2 ions, where v(C-C) appears
1557m
1510w
va,s (CC/CN)
1473m
1386w
1469m broad 19a
1468m broad va,s (CC/CN)
1386w broad (O-H)h i.p.bend
1342w broad
1469m
1337m
1322m
1308m
1323w broad (C-O)c str
14
1277s
1258w
1268m
1258m
1246m
v( C-O) str
1246s
(C-O)h str
1235m
1152s
1028s
1152m broad 15 (C-C) i.p. bend
at about 880 cm-1 16-18
.
1028m
18b(C-C) i.p. bend
938w broad
doublet 859m,
848m
769s
637m
Using a synthetic standard containing a 1:1:1 mixture of
KNO3, oxalic acid, and 3,4-dihydroxybenzoic acid for which
each sample was analyzed at seven random positions, with
each point sampled twice, and the ratio of the 834/754 cm-1
bands is 0.327; in the cocrystal the ratio of these bands is
0.329 confirming that it is a 1:1 cocrystal of oxalic acid and
3,4-dihydroxybenzoic acid.
The cocrystal formed between oxalic acid and 3,4-dihy-
droxybenzoic acid at a molar ratio of 1:1 was investigated by
other solid-state analysis methods. The PXRD data for the
oxalic acid-3,4-dihydroxybenzoic acid system are depicted
in Figure 9. The PXRD pattern of the (oxalic acid-3,4-
dihydroxybenzoic acid) differed from those of the constitu-
ents, confirming the formation of a new complex phase.
Furthermore, the DSC thermogram of the product exhi-
bits a melting endotherm maximum at 96 and 159 °C pro-
viding further support for a new crystal phase. The ther-
mal behavior of the new phase is significantly different from
oxalic acid (melting endotherm maximum at 100.1 °C)
3,4-dihydroxybenzoic acid (melting endotherm maximum
at 203 °C).
859s
769s
643m
σ ring (6b) þ σ (COO)
591w
188w broad
lattice modes
188s
155m
120m
96s
132m
115m
128m
72s
53m
72s broad
53w shoulder
46m
Transmission Raman Spectroscopy (TRS). The transmis-
sion Raman (TRS) spectra of oxalic acid, 3,4-dihydroxyben-
zoic acid, and the cocrystal are shown in Figure 10 and the
vibrational wavenumbers are listed in Table 7.
The Raman spectrum for 3,4-dihydroxybenzoic acid in
the starting material has bands at 1611 and 1299 cm-1
.
Figure 8. Raman spectra obtained for (a) 3,4-dihydroxybenzoic
acid, (b) oxalic acid, and (c) the cocrystal.
Figure 9. Powder X-ray diffraction patterns for (a) 3,4-dihydroxybenzoic acid, (b) oxalic acid, (c) and cocrystal.

2368 Crystal Growth & Design, Vol. 10, No. 5, 2010
Elbagerma et al.
Figure 10. Transmission Raman spectra from 180 -40 cm-1 region obtained for (a) 3,4-dihydroxybenzoic acid, (b) oxalic acid, and (c) the
cocrystal.
Raman spectra of the 6HNA, the in-plane β(C-OH) band
Table 7. Major Bands of Transmission Raman Spectroscopy (TRS)
of Oxalic Acid, 3,4-Dihydroxybenzoic Acid and Their 1:1
is now missing at ca. 1280 cm-1. Also, the strong band
Cocrystal Productsa
around 1023 cm-1, attributed to the pyridine ring,20 is not
oxalic acid
3,4-dihydroxybenzoic acid
cocrystal29-33
seen. These features suggest that the 6HNA could possibly
have adopted the ketonic rather than the hydroxylated form.
Furthermore, the presence of the v(CdO)stretching vibra-
tion at 1699 cm-1 and the in-plane β(NH) bend at 1622 cm-1
in the Raman spectra suggests that the ketonic form pre-
dominates in the solid state.21
1796m
1781m
1725s
1631w
1621sh
1611s
1299s
1620m
1605s
1315s
The Raman spectrum for pure 6HNA in the starting
material has bands at 1699 and 1467 cm-1 corresponding
to CdO stretching and va,s (CC/CN), respectively; in the
cocrystal formed between salicylic acid and 6HNA, these
bands were shifted to 1686 and 1455 cm-1 and became
shoulders. The decrease in the CdO stretching wave-
number of 6HNA from 1699 to 1686 cm-1 indicates that
the carbonyl group is participating in hydrogen bonding.
The Raman spectrum for pure salicylic acid in the start-
ing material has strong bands at 1632, 1322 cm-1 and a
medium intensity peak at 1383 cm-1 corresponding to
CdO stretching, (C-O)c stretching and (O-H)h in plane
bend, respectively. In the cocrystal, the band at 1632 cm-1
was shifted to 1642 cm-1, the band at 1383 cm-1 disap-
pears and the peak at 1322 cm-1 appears as a very weak
broad band; these observations indicate that the carboxy-
lic group is participating in hydrogen bonding as shown in
Figure 11. The peaks in the spectrum of 6HNA occurr-
1173m
1156m
807m
1155m
791s
1130w
1105s
797s
543s
448 broad
542 broad
435m
466s, 456s doublet
362w
363w
348 broad
238s
354s
243s
456m
156m
117s
142m
73m
94s
77s
88vs
a vs, very strong; s, strong; m, medium; w, weak; vw, very weak; sh,
shoulder; v, stretching; σ, scissoring; β in-plane deformation; a, anti-
symmetric; s, symmetric; as, asymmetric; i.p, in plane; o.p, out of plane;
j, phenyl ring; def, deformation; str, stretch.
ing at 1275 cm-1 v( C-O) str, 1237 cm-1 and 642 cm-1
σ
ring (6b) þ σ (COO) in the cocrystal were shifted to 1265
cm-1, 1231 cm-1 as a shoulder and a doublet at 638 and
635 cm-1, respectively. Also, the peak at 859 cm-1 now
appears as a doublet at 858 and 845 cm-1. Furthermore,
the Raman spectra for salicylic acid in the starting material
has bands at 1163, 564, and 530 cm-1, corresponding to
(3)mode, (O-H)c o.p. bend and (C-O)h torsion, respec-
tively. In forming the cocrystal, the band at 1163 cm-1
disappears and the other two bands were shifted to 559 cm-1
with (decreased intensity) and 527 cm-1 are broading,
respectively.
These bands in the cocrystal were shifted to 1605 and 1315
cm-1, respectively, while the peaks at 1173, 1130, and 1105
cm-1 disappear from the cocrystal spectrum. The Raman
spectrum for pure oxalic acid has bands at 1796, 1725, 1156,
and 543 cm-1, whereas in the cocrystal the bands at 1796,
1725, and 1156 cm-1 have disappeared, and that band at 543
cm-1 is broadened but is still centered on 542 cm-1. On the
other hand, the peak at 156 cm-1 in the spectrum of pure
salicylic acid has shifted to a lower wavenumber at 142 cm-1
.
3.1.4. Salicylic Acid and 6-Hydroxynicotinic Acid (6HNA)
Cocrystal. The Raman spectra obtained in the fingerprint
region for salicylic acid, 6HNA, and their cocrystal contained
additional information regarding the intermolecular inter-
actions associated with the cocrystal formation. Assignments
for the observed Raman bands are collected in Table 6.
Using a synthetic standard containing a 1:1:1 mixture of
KNO3, salicylic acid, and 6HNA for which each sample was
analyzed at seven random positions, with each point sampled
twice, the ratio of the 858:768 cm-1 bands is 0.479; in the
cocrystal the ratio of these bands is 0.452 confirming that it is
a 1:1 cocrystal of salicylic acid and 6HNA.

Article
Crystal Growth & Design, Vol. 10, No. 5, 2010 2369
Figure 11. Raman spectra obtained for (a) 6-dihydroxynicotinic acid, (b) salicylic acid, and (c) the cocrystal.
Figure 12. Powder X-ray diffraction patterns for (a) 6HNA, (b) salicylic acid, and (c) the cocrystal.
Scheme 2. Ketonic Form and Hydroxyl Tautomeric Form of 6HNA
The powder XRD diffractograms of 6HNA, salicylic acid,
and the product from their cocrystallization via slow eva-
poration from ethanol are compared in Figure 12. The
intensity of the characteristic peaks at 27.9° and 11° of
6HNA and salicylic acid, respectively, were decreased during
the cocrystal. Furthermore, the cocrystal has a strong peak at
4.42°, which indicated that a new phase was formed during
the cocrystallization of 6HNA and salicylic acid.
Transmission Raman Spectroscopy (TRS). The TRS spec-
tra of salicylic acid, 6HNA, and the cocrystal are shown in
Figures 13 and 14 and the vibrational wavenumbers and
assignments are listed in Table 6.
The Raman spectrum for pure salicylic acid has bands at
1634, 1473, 1386, and 1322 cm-1, corresponding to the CdO
str, 19a, (O-H)h i.p. bend, and (C-O)c str, modes respec-
tively. In the cocrystal formed between of salicylic acid with
6HNA, these bands appeared as broad bands at 1635, 1469,
1386, and 1323 cm-1, respectively. On the other hand, the
Raman spectrum for 6HNA has bands at 1235, 155, 120, 96,
and 46 cm-1 which do not occur in the cocrystal. Also, the
peak of medium intensity at 1308 cm-1 in pure salicylic acid has
disappeared during the cocrystal formation. Moreover, new
bands at 938 and 591 cm-1 which do not occur in either salicylic
acid or 6HNA appear in the salicylic acid-6HNA cocrystal.
The DSC of 6HNA, salicylic acid, and cocrystallization
product from slow evaporation are presented in Figure 15.
The new molecular complex has melting points at 153
and 202 °C, while salicylic acid has a melting endotherm
The Raman spectrum for 6HNA in the starting material
has bands at 1702, 1651, and 1510 cm-1. In the cocrystal the
bands at 1651 and 1510 cm-1 have disappeared while that at
1702 cm-1 was shifted to lower wavenumber at 1686 cm-1
.

2370 Crystal Growth & Design, Vol. 10, No. 5, 2010
Elbagerma et al.
Figure 13. Transmission Raman spectra in the 1700-1100 cm-1 region obtained for (a) salicylic acid, (b) 6-hydroxynicotinic acid, and (c) the
cocrystal.
Figure 14. Transmission Raman spectra in the 500-100 cm-1 region obtained for (a) salicylic acid, (b) 6-hydroxynicotinic acid, and (c) the
cocrystal.
Figure 15. DSC melting curves of 6-hydroxynicotinic acid (blue line), cocrystal (red line), and salicylic acid (green line).
maximum at 160 °C and the 6HNA has a melting endotherm
maximum at 300 °C.
acid cocrystals has been demonstrated in stoichiometric mix-
tures through the use of Raman spectroscopy (dispersive and
transmission), X-ray powder diffraction and thermal analysis.
Raman spectroscopy could be used to demonstrate a number
of important aspects regarding the nature of the interactions
in the cocrystal. Formation of the cocrystal causes broadening
4. Conclusions
The existence of salicylic acid-nicotinic acid, DL-phenyla-
lanine, 6HNA, and 3,4-dihydroxybenzoic acid with oxalic

Article
Crystal Growth & Design, Vol. 10, No. 5, 2010 2371
in the carbonyl band region that is diagnostic for the existence
of the cocrystals. Alterations in the energies of bands asso-
ciated with the carbonyl vibrations of the carboxyl group were
shown to be sensitive to fine details of the solids. The other
main interaction point is the -OH group as it becomes
deformed in the cocrystal. We have used a synthetic standard
to confirm that the ratios of cocrystal components were in a
1:1 molar ratio.
(14) Macleod, N. A.; Matousek, P. Appl. Spectrosc. 2008, 62, 276A–
304A.
(15) Scott, M. G.; Henry, Y. I.; Loren, Z.; Mones, B. J. Lipid Res. 1981,
l22, 24–36.
(16) Socrates, G. Infrared and Raman Characteristic Group Frequencies:
Tables and Charts, 3rd ed; John Wiley & Sons, Ltd: New York, 2001.
(17) Petrov, I.; Soptrajanov, B. Spectrochim. Acta Part A 1975, 31,
309–316.
(18) Villepin, J.; de Novak, A. Spectrochim. Acta Part A 1971, 27,
1259–1270.
TRS, which is well suited for rapid volumetric probing of
intact samples, is applied to study cocrystals for the first time
from which we can identify the presence of a different crystal
structure in the cocrystal compared with the two precursor
materials especially from low wavenumber Raman bands.
(19) Villepin, J.; de Novak, A. J. Mol. Struct. 1976, 30, 255.
(20) Villepin, J.; de Novak, A. Spectrochim. Acta Part A 1971, 34,
1009–1017.
(21) Mukherjee, K. M.; Misra, T. N. J. Raman Spectrosc. 1996, 27,
595–600.
(22) Koczo, P.; Dobrowolski, J. C. Z.; Lewandowski, W.; Mazurek,
A. P. J. Mol. Struct. 2003, 655, 89–95.
(23) Sala, O.; Gonc-alves, N. S.; Noda, L. K. J. Mol. Struct. 2001,
565-566, 411–416.
(24) Volovsek, V.; Colombo, L.; Furic, K. J, Raman Spectrosc. 1983, 14,
347–352.
(25) Rajkumar, B.; Ramakrishnan, V. Spectrochim. Acta Part A 2002,
58, 1923–1934.
References
(1) Aakeroy, C. B.; Salmon, D. J. CrystEngComm 2005, 7, 439–448.
(2) Morissette, S. L.; Almarsson, O.; Peterson, M. L.; Remenar, J. F.;
Read, M. J.; Lemmo, A. V.; Ellis, S.; Cima, M. J.; Gardner, C. R.
Adv. Drug. Delivery Rev. 2004, 56, 275–300.
(3) Almarsson, O.; Zaworotko, M. J. Chem. Commun. 2004, 17, 1889–
1896.
(4) Park, A.; Chyall, L. J.; Dunlap, J.; Schertz, C.; Jonaitis, D.; Stahly,
B. C.; Bates, S.; Shipplett, R.; Childs, S. Expert. Opin. Drug.
Discovery 2007, 2, 145–154.
(26) Ramachandran, E.; Natarajan, S. Cryst. Res. Technol. 2007, 42 (6),
617–620.
(27) Ravikumar, B.; Rajaram, R. K.; Ramakrishnan, V. J. Raman
Spectrosc. 2006, 37, 597–605.
(28) Briget Mary, M.; Sasirekha, V.; Ramakrishnan, V. Spectrochim.
Acta Part A 2005, 62, 446–452.
(29) Frost, R. L. Anal. Chim. Acta 2004, 517, 207–214.
(30) Godzisz, D.; Ilczyszyn, M.; Ilczyszyn, M. M. Spectrochim Acta
Part A 2003, 59, 681–693.
(31) Mohacek-Grosev, V.; Grdadolnik, J.; Stare, J.; Hadzi, D. J. Raman
Spectrosc. 2009, 40 (11), 1605–1614.
(32) Kalinowska, M.; Swiszocka, R.; Borawskab, M.; Piekut, J.;
Lewandowski, W. Spectrochim. Acta Part A 2008, 70, 126–135.
(33) Clavijo, E.; Menendez, J. R.; Aroca, R. J. Raman Spectrosc. 2008,
39, 1178–1182.
(34) Szorcsik, A.; Nagy, L.; Scopelliti, M.; Deak, A.; Pellerito, L.;
Hegetschweiler, K. J. Organomet. Chem. 2005, 690, 2243–2253.
(35) Quintal, S. M.; Nogueira, H. I.; Felix, V.; Drew, M. G. Polyhedron
2002, 21, 2783–2791.
(5) Storey, R.; Docherty, R.; Higginson, P.; Dallman, C.; Gilmore, C.;
Barr, G.; Dong, W. Crystallogr. Rev. 2004, 10, 45–56.
(6) Morissette, S. L.; Soukasene, S.; Levinson, D.; Cima., M. J.;
Almarsson, O. Proc. Natl. Acad. Sci. U. S. A. 2003, 100, 2180–84.
(7) Peterson, M. L.; Morissette, S. L.; McNulty, C.; Goldsweig, A.;
Shaw, P.; LeQuesne, M.; Monagle, J.; Encina, N.; Marchionna,
J. M.; Johnson, A.; Gonzalez-Zugasti, J.; Lemmo, A. V.; Ellis, S. J.;
Cima,M.J.;Almarsson,O.J. Am. Chem. Soc. 2002,124, 10958–10959.
(8) Hilfiker, R.; Berghausen, J.; Blatter, F.; Burkhard, A.; De Paul,
S. M.; Freiermuth, B.; Geoffroy, A.; Hofmeier, U.; Marcolli, C.;
Siebenhaar, B.; Szelagiewicz, M.; Vit, A.; Von Raumer, M. J.
Therm. Anal. Calorim. 2003, 73, 429–40.
(9) Bolton, B. A.; Prasad, P. Pharm. Sci. 1984, 73, 1849–1851.
(10) Brittain, H. G. Cryst. Growth Des. 2009, 9, 2492–2499.
(11) Brittain, H. G. Cryst. Growth Des. 2009, 9, 3497–3503.
(12) Brittain, H. G. Cryst. Growth Des. 2010, 10, 1990-2003.
(13) Elbagerma, M. A.; Edwards, H. G. M.; Munshi, T.; Scowen, I. J.
Anal. Bioanal. Chem. 2009, DOI 10.1007/s00216-009-3375-7.
(36) Yin-Hua, H.; Zhi-Han, G.; Yun-Long, F. Chin. J. Struct. Chem.
2008, 27 (9), 1073–1078.
(37) Xu, N.; Shi, W.; Liao, D.; Yan, S. Inorg. Chem. Commun. 2007, 10,
1218–1221.