Synthesis, spectroscopic characterization and structural studies of a new proton transfer (H-bonded) complex of o-phenylenediamine with l-tartaric acid

Article abstract of DOI:10.1016/j.molstruc.2013.07.015

A proton transfer or H-bonded (CT) complex of o-phenylenediamine (OPD) as donor with l-tartaric acid (TART) as acceptor was synthesized and characterized by spectral techniques such as FTIR, ¹H NMR, elemental analysis, TGA-TDA, X-ray crystallography and spectrophotometric studies. The structural investigations exhibit that the cation [OPD⁺] and anion [TART ^-] are linked together through strong N⁺-Ha O^- type hydrogen bonds due to transfer of proton from acceptor to donor. Formed H-bonded complex exhibits well resolved proton transfer bands in the regions where neither donor nor acceptor has any absorption. The stoichiometry of the H-bonded complex (HBC) was found to be 1:1, determined by straight line methods. Spectrophotometric studies have been performed at room temperature and Benesi-Hildebrand equation was used to determine formation constant (K _CT), molar extinction coefficient (ε_CT) and also transition energy (E_CT) of the H-bonded complex. Spectrophotomeric and crystallographic studies have ascertained the formation of 1:1 H-bonded complex. Thermal analysis (TGA-DTA) was also used to confirm the thermal fragmentation and the stability of the synthesized H-bonded complex.

Full text of DOI:10.1016/j.molstruc.2013.07.015

Journal of Molecular Structure 1050 (2013) 122–127
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Synthesis, spectroscopic characterization and structural studies of a new
proton transfer (H-bonded) complex of o-phenylenediamine
with L-tartaric acid
⇑
Ishaat M. Khan , Afaq Ahmad
Department of Chemistry, Faculty of Science, Aligarh Muslim University, Aligarh 202002, India
h i g h l i g h t s
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
A proton transfer complex is synthesized and characterized by spectral techniques.
Bonding modes and structural information are ascertained from crystallographic studies.
Single-crystal X-ray studies confirmed the H-bonded network.
Thermal analysis (TGA–DTA) was also used to confirm the thermal fragmentation and the stability of the CT complex.
Benesi–Hildebrand equation was used to determine KCT, eCT, and ECT of the CT complex.
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 23 May 2013
Received in revised form 5 July 2013
Accepted 10 July 2013
Available online 18 July 2013
A proton transfer or H-bonded (CT) complex of o-phenylenediamine (OPD) as donor with L-tartaric acid
(TART) as acceptor was synthesized and characterized by spectral techniques such as FTIR, 1H NMR,
elemental analysis, TGA–TDA, X-ray crystallography and spectrophotometric studies. The structural
investigations exhibit that the cation [OPD+] and anion [TARTꢁ] are linked together through strong
N+–Hꢂ ꢂ ꢂOꢁ type hydrogen bonds due to transfer of proton from acceptor to donor. Formed H-bonded
complex exhibits well resolved proton transfer bands in the regions where neither donor nor acceptor
has any absorption. The stoichiometry of the H-bonded complex (HBC) was found to be 1:1, determined
by straight line methods. Spectrophotometric studies have been performed at room temperature and
Benesi–Hildebrand equation was used to determine formation constant (KCT), molar extinction coeffi-
Keywords:
O-phenylenediamine
L-tartaric acid
Hydrogen bond
1H NMR
cient (eCT) and also transition energy (ECT) of the H-bonded complex. Spectrophotomeric and crystallo-
graphic studies have ascertained the formation of 1:1 H-bonded complex. Thermal analysis (TGA–DTA)
was also used to confirm the thermal fragmentation and the stability of the synthesized H-bonded
Formation constant
FTIR
complex.
Ó 2013 Published by Elsevier B.V.
1. Introduction
and exhibit non-linear optical properties [22–25]. In complexes
with organic bases, tartaric acid can interact in three ways: (i) as
L
(+)-(2R,3R)-Tartaric acid is
a
naturally occurring chiral
an acid, without proton transfer to the basic proton-acceptor cen-
ter, (ii) as a semi-tartrate anion, in which the proton from one car-
boxyl group is transferred to the basic center, and (iii) as tartrate
di-anion. The diastereomeric salts often differ in solubility, making
tartaric acid one of the most frequently used resolving agents [1].
The interaction pathways strongly depend on the basicity of the or-
ganic bases, the number of the interacting centers, and the molec-
ular structure of the bases used [26,27]. (2R,3R)-Tartaric acid has
been used in stereoselective synthesis in the acidic form or with
protected hydroxyl or carboxylic groups. Recently, investigations
have been performed on the crystal structure and spectroscopic
dicarboxylic acid [1]. It has two kinds of different proton-donor
centers comprising of two carboxylic and two hydroxyl groups
[2]. Studies of molecular packing and hydrogen bonding in the
crystals of tartaric acid and its derivatives are relevant for the
development of new approaches in crystal engineering [3–6].
Tartaric acid forms a broad range of hydrogen-bonded complexes
with amines, betaines and amino acids, and salts of tartaric acid
with amines show either 1:1 or 1:2 stoichiometry [7–20]. Some
of such complexes can undergo ferroelectric phase transition [21]
properties of several complexes of betaines with L(+)-(TART) and
with meso-TART, whose energy and enthalpy calculations evalu-
ated have been by the DFT calculations [10,15,16,28]. In view of
⇑
Corresponding author. Tel.: +91 5712703515 (O), mobile: +91 9412174753.
E-mail addresses: drishaatamu@gmail.com (I.M. Khan), drafaqahmad7@gmail.
com (A. Ahmad).
0022-2860/$ - see front matter Ó 2013 Published by Elsevier B.V.
http://dx.doi.org/10.1016/j.molstruc.2013.07.015

I.M. Khan, A. Ahmad / Journal of Molecular Structure 1050 (2013) 122–127
123
the above mentioned potentials of
L
-tartaric acid, many H-bonded
1.2
(C)
CT complexes of it and other acceptors with organic bases were re-
ported [29–42] and some of them were deposited in the CCDC [43].
Here-in, we report for the first time, to the best of our knowl-
edge, new 1:1 H-bonded complex [(OPD)+(TART)ꢁ] of o-phenylene-
diamine with tartaric acid. The 1:1 H-bonded complex was
synthesized and characterized by absorption spectra, elemental
analysis, FTIR, 1H NMR, and single-crystal X-ray studies. The aim
of the work is to determine the reaction stoichiometry, formation
(C) Donor
(B) CT Complex
(A) Acceptor
1.0
0.8
0.6
0.4
0.2
0.0
(B)
constant (KCT), molar extinction coefficient (
eCT), free energy
(A)
(D
G°), and also transition energy (ECT) of the H-bonded complex
by using Benesi–Hildebrand equation. H-bonded complex was
investigated in both solution and solid state and the nature of these
interactions was interpreted using above mentioned spectral
techniques.
250
300
350
400
450
Wavelength, nm
Fig. 1. Electronic absorption spectra of (A)
L
-tartaric acid, Acceptor (1 ꢃ 10ꢁ4 M);
(B) Donor–Acceptor CT complex and (C) o-phenylenediamine, Donor (1 ꢃ 10ꢁ4 M),
2. Experimental
in methanol at room temperature.
2.1. Materials
0.00024
0.00022
0.00020
0.00018
0.00016
0.00014
0.00012
0.00010
0.00008
0.00006
0.00004
0.00002
Analytical grade o-phenylenediamine and L-tartaric acid were
employed for the synthesis of H-bonded complexes. Methanol, ace-
tonitrile and DMSO were taken as HPLC grade.
2.2. Synthesis of H-bonded complex
The solid [(OPD)+(TART)ꢁ] was prepared by mixing the satu-
rated solution of OPD (0.108 g, 1 mmol) in acetonitrile (25 ml) with
saturated solution of TART (0.636 g, 1 mmol) also in acetonitrile. A
pink color solution was formed upon the mixing and continuous
stirring of these two reactants produced pink precipitate. The pre-
cipitate was filtered off and washed several times with small
amounts (5 ml) of acetonitrile and dried under vacuum over CaCl2.
The melting point and transition energy of H-bonded complex
were found to be >300 °C and 4.6061 eV, respectively. The result
of the elemental analysis of the H-bonded complex is given as:
(theoretical values are shown in brackets): [(OPD)+(TART)ꢁ],
(C10H14N2O6) H-bonded complex (M/w: 258.23 g): C, 46.51%
(47.00%); H, 5.42% (5.40%); N, 32.57% (32.77%).
0
1
2
3
4
5
6
7
8
1/[D] x 103
Fig. 2. Benesi–Hildebrand plots of charge transfer complex of o-phenylenediamine
with -tartaric acid, [D]o/A vs. 1/[A]o in methanol at room temperature.
L
2.3. Single crystal growth
Table 1
The electronic absorption spectral data for [(OPD)+(TART)ꢁ] CT complex of OPD with
TART in methanol at room temperature.
A saturated solution of the synthesized title complex [(OPD)+
(TART)ꢁ] was prepared in methanol, stirred well for about 1 h
and heated slightly to dissolve well. Then the solution was filtered
through a Whatmann: 41 grade filter paper to remove the sus-
pended impurities. The clear filtrate was collected in 50 ml conical
flask and kept unperturbed in a dust free chamber for the growth
of single crystal of the [(OPD)+(TART)ꢁ]. Well defined, bright col-
ored, transparent and needle shaped crystals were harvested at
the end of the fifth day.
Concentration
of donor
Concentration
of acceptor
(M)
Absorbance
at kCT
238 nm
Formation
constant
Molar
extinction
coefficient
(ꢃ10ꢁ4 M)
(KCT
)
l molꢁ1
(
eCT)
l cmꢁ1 molꢁ1
1.50
2.00
2.77
10.00
15.00
1 ꢃ 10ꢁ4
0.481
0.588
0.769
1.609
2.023
1.337 ꢃ 103 2.842 ꢃ 105
2.4. Spectrophotometric analyses
When 3 ml solution each of the acceptor and the donor were
mixed, a charge transfer complex was formed. The wavelength of
maximum absorption of the resulting solution was determined.
The H-bonded complex of the 1:1 reaction mixture was kept over
night at room temperature to form stable complexes, were ana-
lyzed as shown in Fig. 3. The maximum wavelength of the charge
transfer complex was determined by spectrophotometer to be
270 nm. The concentration of acceptor (TART) in the reaction mix-
ture was maintained at 1.0 ꢃ 10ꢁ4 M, whereas the concentration of
the donor (OPD) changed over a wide range of concentrations
(1.5 ꢃ 10ꢁ4–15 ꢃ 10ꢁ4 M) to maintain the conditions [D] ꢄ [A]o
or [D] ꢅ [A]o for applying Benesi–Hildebrand, which is valid under
this condition, to calculate the formation constant, KCT, and the
absorptivity, eCT, values for H-bonded complex in methanol. The
stoichiometry of the H-bonded complex was also obtained from
the Benesi–Hildebrand plots.
2.5. Spectral and crystal structure Analyses
The electronic absorption spectra of the donor (O-phenylenedi-
amine), acceptor (L-tartaric acid) and the resulting H-bonded

124
I.M. Khan, A. Ahmad / Journal of Molecular Structure 1050 (2013) 122–127
Table 2
Table 3
Crystallographic and data collection parameters for [(OPD)+(TART)ꢁ] CT complex of
OPD with TART.
Selected bond lengths (Å), bond angles (°) and torsion angles (°) in crystal structure of
CT complex.
CCDC no.
939527
C10H14N2O6
258.23
Prism
100(2)
Bond
Distance
Bond
Distance
Empirical formula
Formula weight (Mr)
Crystal description
Temperature (K)
Wavelength (Å)
Crystal system
N2 C2
O6 C10
O4 C9
O2 C7
O5 C10
O3 C8
O1 C7
C8 C9
C8 C7
C6 C1
1.416(4)
1.265(4)
1.425(4)
1.216(4)
1.246(4)
1.413(4)
1.302(4)
1.521(4)
1.532(4)
1.387(4)
C5 C4
C9 C10
C2 C3
C2 C1
C4 C3
C3 H3
N1 C1
C6 C5
1.381(4)
1.531(4)
1.385(4)
1.398(4)
1.382(5)
0.9300
0.71073
Triclinic
P ꢁ 1
Space group
Unit cell dimensions (Å)
1.462(4)
1.386(4)
a = 7.114(5)
b = 9.189(5)
c = 9.628(5)
a
= 70.734(5)
Bond
Angle
Bond
Angle
b = 69.464(5)
c
= 81.825(5)
O3 C8 C9
O3 C8 C7
C9 C8 C7
C5 C6 C1
C5 C4 C3
C4 C3 C2
C6 C1 C2
C6 C1 N1
111.8(2)
109.2(2)
109.9(2)
120.4(3)
120.9(3)
121.2(3)
121.0(3)
118.4(3)
C4 C5 C6
O4 C9 C8
O4 C9 C10
C8 C9 C10
C3 C2 C1
C3 C2 N2
C1 C2 N2
C2 C1 N1
118.8(3)
110.2(2)
110.6(2)
111.9(2)
117.7(3)
120.3(3)
122.0(3)
120.6(3)
Unit cell volume
Z
556.1(6) Å3
2
Calculated density (mg/m3)
Absorption coefficient (mmꢁ1
F(000)
1.542
0.129
272
)
Crystal size (mm3)
H, k, max
0.24 ꢃ 0.22 ꢃ 0.20
8, 11, 12
h range (°)
2.37–26.49
3214
Bond
Torsion Angle
Bond
Torsion Angle
Reflections collected/unique
Min and max transmission
Refinement method
Goodness of fit on F2
0.9698, 0.9747
Full matrix least squares on F2
1.009
0.0648, 0.1607
0.0951, 0.1963
C1 C6 C5 C4
O3 C8 C9 O4
C7 C8 C9 O4
O3 C8 C9 C10
C7 C8 C9 C10
C6 C5 C4 C3
N2 C2 C1 N1
O3 C8 C7 O2
C9 C8 C7 O2
O3 C8 C7 O1
C8 C9 C10 O6
O4 C9 C10 O6
ꢁ0.2(4)
ꢁ64.3(3)
57.1(3)
C5 C4 C3 C2 C1
C2 C3 C4 N2 C2
C3 C4 C5 C6 C1
C2 C5 C6 C1 N1
C3 C2 C1 C6
N2 C2 C1 C6
C3 C2 C1 N1
C9 C8 C7 O1
O4 C9 C10 O5
C8 C9 C10 O5
ꢁ0.2(5)
ꢁ0.4(4)
ꢁ177.6(3)
ꢁ0.5(4)
R1, wR2 [I > 2
r
(I)]
59.2(3)
R1, wR2 (all data)
ꢁ179.4(3)
ꢁ180.0(3)
0.8(4)
0.6(4)
ꢁ2.6(4)
ꢁ4.4(4)
ꢁ127.4(3)
175.5(2)
53.7(4)
177.9(3)
ꢁ179.7(3)
52.6(3)
complex in methanol, was recorded in the region of 700–200 nm
using a Intra 10 UV–Visible spectrophotometer with a 1 cm quartz
cell paths length are shown in Fig. 1. 1H NMR spectra of the reac-
tants as well as the crystal of [(OPD)+(TART)ꢁ] were recorded in
DMSO on Bruker DRX-300 NMR spectrometer. The elemental anal-
yses were carried out using an Elementar Vario EL III Carlo Erba
1108. The FTIR spectra were recorded employing spectroscopic
2020 FTIR spectrometer using the KBr pellet technique. A single
crystal of the H-bonded compound was mounted on a glass capil-
lary and data were collected using graphite-monochromated Mo
ꢁ3.3(4)
ꢁ126.6(3)
177.0(3)
Table 4
H-bond geometries for the CT complex (Å, °).
D–Hꢂ ꢂ ꢂA
D–H
Hꢂ ꢂ ꢂA
Dꢂ ꢂ ꢂA
D–Hꢂ ꢂ ꢂA
N1–H3ꢂ ꢂ ꢂO4
O1–H11ꢂ ꢂ ꢂO6
N1–H2ꢂ ꢂ ꢂO5
O3–H8ꢂ ꢂ ꢂN2
0.934
1.033
0.979
0.859
1.924
1.457
1.749
1.944
2.845
2.489
2.711
2.801
168.47
177.06
166.43
175.02
K
a radiation (k = 0.71073 Å) on Bruker SMART APEX CCD diffrac-
tometer at 293 K and the thermal analysis (TGA and DTA) were car-
ried out under nitrogen atmosphere with a heating rate of
20 °C/min for TGA and DTA using Shimadzu model DTG-60H
Thermal Analyzer.
The data integration and reduction were processed with SAINT
[44] software. An empirical absorption correction was applied to
the collected reflections with SADABS using XPREP [44]. All the
structures were solved by the direct method using SIR-97 [45]
and were refined on F2 by the full-matrix least-squares technique
using the SHELXL-97 [45] program package. A summary of crystal-
lographic data collection and refinement parameters are given in
Table 2.
to the O3 of the alcoholic (OH) group of TART
(N2–H8–O3 = 1.944 Å) The ORTEP view, hydrogen bonding packing
diagrams and extended hydrogen bonding network are shown in
Figs. 6–8, respectively. The crystal data and refinement parameters
are provided in Table 2, and selected bond lengths, bond angles and
H-bond geometries for the H-bonded complex are given in Tables 3
and 4. These hydrogen bonding interactions are comparable or
even stronger to those reported for proton transfer complexes
[46–48] exhibiting the high stability of the H-bonded complex. It
is clear from the crystal structure that two rings are parallel to each
other and provided ring to ring (pi–pi) interaction which also con-
solidates the structure of H-bonded complex. Besides the intermo-
lecular H-bonding, there also exists some intramolecular H-
bonding forces viz. O5ꢂ ꢂ ꢂH9–O4 = 2.118 Å, O1–H11ꢂ ꢂ ꢂO2 = 2.420 Å
and (N1ꢂ ꢂ ꢂH6ꢂ ꢂ ꢂN2 = 2.568 Å).
3. Result and discussion
3.1. X-ray Crystallographic studies of H-bonded complex
The formation of the H-bonded complex has been confirmed by
the single crystal X-ray studies. The X-ray data exhibits that the
OPD and TART are joined through various H-bonding interactions
and the crystal lattice is costabilized via pi-pi interaction which ex-
ist between parallel aromatic rings of OPD.
3.2. Observation of CT bands
The structure of the complex clearly indicates that the N1 of one
of the NH2 groups of OPD is protonated and strong hydrogen bond-
ing interaction is formed between N1 with O5 of TART (N1–H2–
O5 = 2.711 Å). The other N2 of NH2 group of OPD is also bonded
The spectrophotometric data were used to calculate both
formation constant (KCT), and extinction coefficient (eCT) of the
H-bonded complex in the defined solvent based on Benesi–Hilde-
brand equation [49,50].

I.M. Khan, A. Ahmad / Journal of Molecular Structure 1050 (2013) 122–127
125
½Aꢆo=A ¼ 1=KCTeCT ꢂ 1=½Dꢆo þ 1=eCT
where [D]o and [A]o are the initial concentration of the donor and
acceptor, respectively and A is the absorbance of the CT band. The
concentration of the donor in the reaction mixture was kept greater
than acceptor, ([D]o) ꢄ [A]o) [49,50], and changed over a wide range
of concentration from 1.5 ꢃ 10ꢁ4 M to 15 ꢃ 10ꢁ4 M while concen-
tration of the acceptor (TART) in each of the reaction mixture was
kept fixed at 1.0 ꢃ 10ꢁ4 M. These produced solutions with donor:
acceptor molar ratios varying from 1.5:1 to 15:1 are reported in
Table 1. The electronic absorption spectra of 1 ꢃ 10ꢁ4 M OPD,
10ꢁ4 M TART and H-bonded complex were recorded against meth-
anol as references are shown in Fig. 1. To obtain the electronic
absorption spectra of H-bonded complex, solutions of donor and
acceptor were prepared in the same solvents. The straight line
was obtained when [A]o/A was Plotted against 1/[D]o which sup-
ports the formation of the charge transfer complex as shown in
Fig 2 [49,50]. In this plot, the slope and intercept equals 1/KCT eCT
and 1/eCT respectively. It was observed that the correlation
coefficient (r) value for the straight line was found to be 0.999.
The formation constant and molar extinction coefficient of the
H-bonded complex were obtained by calculation of intercept on
y-axis and slope, and were found to be 1.337 ꢃ 103 l molꢁ1 and
2.842 ꢃ 105 l cmꢁ1 molꢁ1 respectively. It is observed that new
absorption peaks appear in UV region. In some cases multiple peaks
were obtained and the longest wavelength peak was considered as
H-bonded peak [51–53], which is found at 270 nm. The change of
the absorption intensities to higher values for complex on addition
of the donor are reported in Table 1. These measurements are based
on the H-bonded absorption bands in the spectra of the systems and
are mentioned in Figs. 1 and 2.
Fig. 4. 1H NMR spectrum of charge transfer complex of o-phenylenediamine with
L-
tartaric acid.
3.3. FTIR spectra
The FTIR absorption spectra of the o-phenylenediamine, L-tar-
taric acid and their CT complex employed in the range 4000–
400 cmꢁ1 frequency are shown in Fig. 3. The FTIR spectrum of the
H-bonded complex shows the characteristic peaks of the free donor
and acceptor, except some peaks being disappeared and some shift
to the lower side of frequencies. These FTIR assignments are based
on the comparison of the spectrum of the formed H-bonded com-
plex with the spectra of the free acceptor and donor. The formed
H-bonded complex shows changes in their band intensities as well
as in the values compared to those of the free OPD and TART shifts
in the frequencies. The formation of H-bonded complex between
Fig. 5. TGA–DTA curves of charge transfer complex of o-phenylenediamine (donor)
and -tartaric acid (acceptor).
L
donor and acceptor is strongly evidenced by the presence of the
main characteristic infra red bands of the OPD and TART in the spec-
trum of the product. However, the bands of the donor and acceptor
are shifted. It is observed that the proton of one of COOH from TART
migrated to the nitrogen atom of amine groups of o-phenylenedi-
amine, which contains lone pair of electrons. The intensity of COOH
peak was reduced in spectrum of H-bonded complex, which ap-
pears at 1600 cmꢁ1 (strong) whereas this was observed at
1734 cmꢁ1 (strong) in free TART. The phenolic
m (OH) vibrations
of the free acceptor (TART) occur with medium and broad band
range 3416–3300 cmꢁ1 whereas this was observed at 3422 cmꢁ1
(weak) in the spectrum of the H-bonded complex and this new peak
is also assigned to the N+–H–Oꢁ species between the nitrogen atom
of one of two NH2 group of the donor and oxygen of acceptor. This is
due to the ring current influence of the magnetic anisotropy of
nitrogen atoms and the field effect of the electronic dipoles located
on the nitrogen atoms. However, the stretching vibrations of
m (N–
H) and d (N–H) appear at (3368 cmꢁ1), medium and strong,
(1592 cmꢁ1) strong in free donor, respectively. It can be said that
the donation process has been carried out by –NH2 group in case
of TART. This behavior is in accordance with the proton migration
from the acceptor to the donor [54,55], which gives an additional
Fig. 3. FTIR of (A) o-phenylenediamine (OPD); (B)
charge transfer complex of o-phenylenediamine and
L
-tartaric acid (TART); and (C)
-tartaric acid.
L

126
I.M. Khan, A. Ahmad / Journal of Molecular Structure 1050 (2013) 122–127
3.4. 1H NMR Spectra
The 1H NMR spectrum of the proton-transfer complex recorded
in DMSO is given as Fig. 4. The peak corresponding to the proton of
the solvent (i.e., DMSO) is present at d = 2.00 ppm in the spectrum
of the H-bonded complex. All the protons of OPD and TART indicate
their signals at the appropriate positions in the spectrum of the H-
bonded complex. The aromatic protons of the protonated OPD moi-
ety appear at d = 6.82 and 6.72 ppm whereas in free OPD the same
were observed at d = 6.651 ppm while those corresponding to
—NHþ3 group of the OPD occur at d = 3.30 ppm indicating proton-
ation of the donor. The proton transfer occurs from –COOH group
of the TART to the –NH2 group of the OPD part as is clear from
the crystal structure of the H-bonded complex. This type of transfer
is similar to that reported in earlier work [46,56]. The resonance
signal due to the –NH2 group in OPD is observed at 5.17 ppm.
The additional peaks at 7.9 ppm are indicative of the OH group
which is undisturbed in the TART part. The peak due to the undis-
sociated –COOH function in TART appears at 2.5 ppm. It was
concluded that the above studies are in line with the crystallo-
graphic investigation.
Fig. 6. ORTEP view of crystal of [(OPD)+(TART)ꢁ] CT complex showing the
numbering and bonding length scheme.
3.5. Study of thermogram of the H-bonded complex of o-
phenylenediamine and L-tartaric acid
Thermogravimetric (TG) and differential thermogravimetric
(DTG) analysis were carried out for H-bonded complex of OPD
and TART under on N2 flow in order to confirm its formula, struc-
ture and thermal stability. The thermogram for H-bonded complex
is depicted in Fig. 5. The degradation data clearly support the pro-
posed molecular structure of the H-bonded complex. However, it is
difficult to propose a definite decomposition reaction.
It was observed that the thermal decomposition of the H-
bonded complex of OPD and TART exhibits various degradation
steps differing in weight loss. The thermogram for the H-bonded
complex (Fig. 5) shows two stages of decomposition. The first step
occurs within the range 150–250 °C which corresponds to 31.78%
weight loss of the acceptor (TART). The second step is the decom-
position of the donor (OPD) which occurs within the range 280–
500 °C with weight loss 67.37% as decomposition of OPD in its CT
complex, with picric acid [56]. It was also observed that H-bonded
complex has been completed with endothermic process, as indi-
cated by absorption of 45.99 and 109.07 J/g in the 1st and 2nd
steps of its degradation, respectively. This has been attributed that
the H-bonded complex is more stable than its constituents.
Fig. 7. Packing diagram, in which molecules are bonded through hydrogen bonding
between C–Hꢂ ꢂ ꢂO and also pi–pi interaction.
evidence for the involvement of the nitrogen atoms of the donor
and the COOH group of TART.
These changes strongly support the formation of H-bonded
complex and could be attributed to the expected symmetry and
electronic structure modifications in both donor and acceptor units
in the formed product relative to the free molecules.
Fig. 8. H-bonding scheme in extended structure of [(OPD)+(TART)ꢁ] CT complex.

I.M. Khan, A. Ahmad / Journal of Molecular Structure 1050 (2013) 122–127
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4. Conclusions
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The formation of 1:1 hydrogen bonded proton or H-bonded
complex of OPD and TART was studied spectrophotometrically in
methanol as well as in solid phase at room temperature. The for-
mation constant and molar extinction coefficient of the H-bonded
Complex, were estimated employing Benesi–Hildebrand equation,
and also, the stoichiometry of the H-bonded complex was found to
be 1:1 consistent with the formula [(OPD)+(TART)ꢁ]. FTIR, 1H NMR
and electronic absorption spectra provide evidence for the exis-
tence of new bands of the H-bonded complex, which indicate a
H-bonded interaction associated with proton migration from the
acceptor to the donor followed by intermolecular hydrogen.
X-ray data confirm the formation of 1:1 H-bonded complex and
presence of extensive hydrogen bonding interactions. TGA–DTA
studies show thermal stability of the H-bonded complex as well
as show good agreement with the proposed stoichiometry by ele-
mental analysis.
Acknowledgements
Thanks are due to the Chairman, Department of Chemistry,
Aligarh Muslim University, Aligarh for providing research facilities
at physical chemistry Lab.
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