Synthesis and spectrophotometric studies of charge transfer complexes of p-nitroaniline with benzoic acid in different polar solvents

Article abstract of DOI:10.1016/j.molstruc.2014.05.076

The charge transfer complexes of the donor p-nitroaniline (PNA) with the π-acceptor benzoic acid (BEA) have been studied spectrophotometrically in various solvents such as acetone, ethanol, and methanol at room temperature using an absorption spectrophotometer. The outcome suggests that the formation of the CT-complex is comparatively high in less polar solvent. The stoichiometry of the CT-complex was found to be 1:1. The physical parameters of the CT-complex were evaluated by the Benesi-Hildebrand equation. The data are discussed in terms of the formation constant (K_CT), molar extinction coefficient (_CT), Standard Gibbs free energy (ΔG⁰), oscillator strength (f), transition dipole moment (μ_EN), resonance energy (R_N) and ionization potential (I_D). The formation constant (K_CT) of the complex was depends upon the nature of electron acceptor, donor, and polarity of solvents used. It is also observed that a charge transfer molecular complex is stabilized by hydrogen bonding. The formation of the complex has been confirmed by UV-visible, FT-IR, ¹H NMR and TGA/DTA. The structure of the CT-complex is [(PNA)⁺ (BEA)^-]. A general mechanism for its formation of the complex has also been proposed.

Full text of DOI:10.1016/j.molstruc.2014.05.076

Journal of Molecular Structure 1074 (2014) 408–415
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Synthesis and spectrophotometric studies of charge transfer complexes
of p-nitroaniline with benzoic acid in different polar solvents
⇑
Neeti Singh, Afaq Ahmad
Department of Chemistry, Faculty of Science, Aligarh Muslim University, Aligarh 202002, India
g r a p h i c a l a b s t r a c t
h i g h l i g h t s
The charge – transfer complex was prepared and characterized by different techniques such as FTIR,
1HNMR, TGA-DTA. Spectrophotometric studies were also performed in various solvents with different
polarity to calculate thermodynamic parameters. Absorption spectra of benzoic acid (1 ꢁ 10ꢂ2 M) in
acetone with addition of p-nitro aniline concentrations ranging 0.01–0.05 M are shown with increasing
concentrations bottom to top.
ꢀ
A charge transfer or proton transfer
complex is synthesized and
confirmed by spectral investigations.
Spectrophotometric studies were
made in different polar solvents.
Benesi–Hildebrand equation in
various solvents was used to
ꢀ
ꢀ
0.25
determine KCT, eCT, and ECT of the CT
complex.
Thermal analysis (TGA–DTA) was also
used to confirm the thermal
fragmentation and the stability of the
CT complex.
Absorbance maxima 410
0.20
E
ꢀ
D
0.15
C
B
0.10
0.05
0.00
A
400
450
500
550
600
Wavelength (nm)
a r t i c l e i n f o
a b s t r a c t
Article history:
The charge transfer complexes of the donor p-nitroaniline (PNA) with the p-acceptor benzoic acid (BEA)
Received 23 February 2014
Received in revised form 25 May 2014
Accepted 29 May 2014
have been studied spectrophotometrically in various solvents such as acetone, ethanol, and methanol at
room temperature using an absorption spectrophotometer. The outcome suggests that the formation of
the CT-complex is comparatively high in less polar solvent. The stoichiometry of the CT-complex was
found to be 1:1. The physical parameters of the CT-complex were evaluated by the Benesi–Hildebrand
equation. The data are discussed in terms of the formation constant (KCT), molar extinction coefficient
Available online 11 June 2014
Keywords:
(eCT), Standard Gibbs free energy (D lEN), resonance
G0), oscillator strength (f), transition dipole moment (
Charge transfer complex
p-Nitroaniline (PNA)
Benzoic acid (BEA)
UV–visible
energy (RN) and ionization potential (ID). The formation constant (KCT) of the complex was depends upon
the nature of electron acceptor, donor, and polarity of solvents used. It is also observed that a charge
transfer molecular complex is stabilized by hydrogen bonding. The formation of the complex has been
confirmed by UV–visible, FT-IR, 1H NMR and TGA/DTA. The structure of the CT-complex is [(PNA)+
(BEA)ꢂ]. A general mechanism for its formation of the complex has also been proposed.
Ó 2014 Elsevier B.V. All rights reserved.
FT-IR
1H NMR
⇑
Corresponding author. Tel.: +91 9411983056.
E-mail addresses: neetisingh72@gmail.com (N. Singh), afaqahmad212@gmail.
com (A. Ahmad).
http://dx.doi.org/10.1016/j.molstruc.2014.05.076
0022-2860/Ó 2014 Elsevier B.V. All rights reserved.

N. Singh, A. Ahmad / Journal of Molecular Structure 1074 (2014) 408–415
409
NMR spectrum of CT-complex is measured in CDCl3 using Bruker
Advance II 400 NMR spectrometer and the thermal analysis (TGA
and DTA) were carried out under nitrogen atmosphere with a flow
rate of 30 ml minꢂ1 and a heating rate of 25 °C minꢂ1 in the tem-
perature range 20–400 °C for TGA and DTA using Shimadzu model
DTG-60H thermal analyzers.
Introduction
Molecular interactions between electron donors and electron
acceptors are generally associated with the formation of intensely
colored charge-transfer complexes, which absorb radiation in the
visible region [1]. Charge transfer complexes are formed between
electron donors having sufficiently low ionization potential, and
electron acceptors having sufficiently high electron affinity. The
transfer of an electron from a donor to an acceptor is readily pos-
sible in the charge transfer process [2]. Also protonation of the
donor to acidic acceptor are generally rout for the ion pair adducts
[3–10]. Benzoic acid and its derivatives have been reported to yield
charge-transfer complexes [11]. Such a complexation reaction is
usually simple, fast, reproducible and reliable.
This paper presents studies of the charge-transfer interaction
between p-nitroaniline and benzoic acid in both liquid and solid
states. The aim of the work is to determine the reaction stoichiom-
etry, nature of bonding between BEA and PNA, and also some phys-
ical parameters. In addition, the nature and structure of the
reaction product (CT-complex) in both solution and solid states
can be estimated using the spectroscopic techniques like FT-IR,
1H NMR, TGA–DTA and UV–Vis electronic absorption to obtain
the stoichiometry, molecular structure and nature of interaction
for the CT-complexes [12–15].
Results and discussion
Observation of CT-bands
A 3 ml volume of donor and acceptor were scanned separately
through a spectrophotometric titration [16] at room temperature
with their wavelength of maximum absorption 230 nm for benzoic
acid, 400 nm for p-nitroaniline in acetone, 340 nm for blank sol-
vent (acetone) and 410 nm for CT-complex of .01 M PNA and
.01 M BEA in acetone are shown in Fig. 1. The reaction mixture of
donor (10 ml) and acceptor (10 ml) in different solvents viz, ace-
tone, ethanol and methanol formed a yellow colored charge trans-
fer complex. The concentration of the donor in the reaction
mixture was kept greater than acceptor, [D0] ꢃ [A0] [17,18] and
changed over a wide range of concentration from 0.01 M to
0.05 M while concentration of p-acceptor (benzoic acid) was kept
fixed [17] at 0.01 M in each solvents, these produced solutions with
donor: acceptor molar ratios varying from 1:1 to 5:1. These con-
centrations ratios were used to straight line diagram for determi-
nation of the formation constants of CT-complex as separate
shown in Table 3.
Experimental
Materials
The spectrum of solution of 0.01 M BEA, and 0.01 M PNA in dif-
ferent solvents were recorded with solvents as reference, the lon-
gest wavelength peak was considered as CT-peak [19]. The
change of the absorption intensity to higher value for all complexes
in this study was detected with added donor and was investigated
as shown in Table 3. These measurements were based on the
CT-absorption bands exhibited by the spectra of the organiza-
tions described on above and are represented in Figs. 2–4. In all
systems studied, the absorption spectra are of similar nature
except for the position of the absorption maxima ðkCT Þ of the com-
plexes. The CT-complex absorption spectra were analyzed by
fitting to the Gaussian function;
Analytical grade chemicals were used throughout p-nitroaniline
(PNA) and Benzoic acid (BEA) was obtained from, CDH. Methanol
(Merck), Ethanol (Merck) and Acetone (Merck) were all analytical
grade (AR) and used without further purification.
Preparation of standard solutions
Solutions of donor of different concentrations, 0.01 M, 0.02 M,
0.03 M, 0.04 M, and 0.05 M, were prepared in different volumetric
flask by dissolving accurately weighed p-nitroaniline in different
solvents such as acetone, ethanol and methanol.
A standard solution of acceptor, benzoic acid (0.01 M concentra-
tion), was prepared by dissolving accurately weighed of acceptor in
above solvents in different volumetric flasks.
p
½ꢂ2ðxꢂxcÞ2=w2
ꢄ
y ¼ y0 þ ½A=w
ð
p
=2Þꢄe
Synthesis of solid CT–complex
0.19
0.18
0.17
0.16
0.15
0.14
0.13
0.12
0.11
0.10
0.09
0.08
0.07
0.06
0.05
0.04
0.03
0.02
0.01
Analar R grade samples of p-nitroaniline and benzoic acid were
employed for the synthesis of the title compound. Equimolar solu-
tions of the two reactants were separately prepared in methanol
and mixed together. The resulting solution was stirred well for
about thirty minutes.
The precipitated adduct was filtered off and repeatedly
recrystallised from methanol to enhance the degree of purity of
the synthesized compound.
C
B
A
Analyses
D
The electronic absorption spectra of the donor p-nitroaniline,
acceptor benzoic acid and the resulting complex in acetone, etha-
nol and methanol were recorded in the visible range 400–600 nm
using a spectrophotometer ELICO SL 177 scanning mini spectro-
photometer with a 1 cm quartz cell path length. The FT-IR spectra
of the reactants and the resulting CT-complex were recorded with
the help of FT-IR spectrometer INTERSPEC-2020 (spectra lab UK)
measured in KBr pellets. The nuclear magnetic resonance, 1H
200 250 300 350 400 450 500 550 600
Wavelength (nm)
Fig. 1. Absorption spectra of (A) .01 M Benzoic acid. (B) Blank solvent (Acetone). (C)
.01 M p-nitro aniline. (D) CT-complex of .01 M PNA and .01 M BEA in acetone.

410
N. Singh, A. Ahmad / Journal of Molecular Structure 1074 (2014) 408–415
Table 1
Gaussian curve analysis for the CT-complex in spectrum of BEA with PNA in different polar solvents.
Systems
Solvent
A
W
vC
Y0
BEA + PNA
BEA + PNA
BEA + PNA
Acetone
Ethanol
Methanol
2.75 0.730
5.33 3.730
3.24 0.278
52.53 12.49
154.78 68.35
42.69 3.95
405.78 7.47
385.65 46.77
419.21 1.67
0.00976 0.0015
0.0044 0.00318
0.0107 0.00142
Table 2
CT-complex absorption maxima (kCT), transition energies (h
m
CT), of the BEA + PNA complexes, experimentally determined values of ionization potentials (ID), oscillator strength
(f), dipole moments (
lEN), and resonance energies (RN) of complexes.
f ꢁ 105
Systems
Solvent
kCT (nm)
hvCT (eV)
ID (eV)
lEN (Debye)
[RN] (eV)
BEA + PNA
BEA + PNA
BEA + PNA
Acetone
Ethanol
Methanol
405.78
385.65
419.21
3.06
3.22
2.96
9.52
9.72
9.40
4.62
1.34
1.20
0.429
0.426
0.770
0.0016
0.0016
0.0056
Table 3
Data for spectrophotometric determination of stoichiometry, absorption maxima (kCT), and association constants (KCT), molar absorptivities (eCT), of CT-complex of BEA and PNA in
acetone, ethanol, and methanol at 298 K.
Systems
Solvent
Acetone
Temp (K)
298
[D]0 in M
[A]0 in M
Absorbance at kCT (nm)
kCT (nm)
KCT (l molꢂ1
)
eCT (1 molꢂ1 cmꢂ1
)
BEA + PNA
0.01
0.02
0.03
0.04
0.05
0.01
0.064
0.104
0.139
0.176
0.218
410
18,550
40.8
BEA + PNA
BEA + PNA
Ethanol
298
298
0.01
0.02
0.03
0.04
0.05
0.01
0.01
0.043
0.073
0.103
0.129
0.161
415
420
11,870
6410
40.1
131
Methanol
0.01
0.02
0.03
0.04
0.05
0.080
0.138
0.197
0.265
0.397
0.17
0.16
0.15
0.25
0.20
0.15
0.10
0.05
0.00
E
Absorbance maxima 415
0.14
0.13
0.12
0.11
0.10
0.09
0.08
0.07
0.06
0.05
0.04
0.03
0.02
0.01
0.00
Absorbance maxima 410
E
D
C
D
C
B
A
B
A
400
450
500
550
600
400
450
500
550
600
Wavelength (nm)
Wavelength (nm)
Fig. 3. Absorption spectra of benzoic acid (1 ꢁ 10ꢂ2M) in ethanol with addition of
p-nitro aniline concentrations ranging 0.01–0.05 M are shown with increasing
concentrations bottom to top.
Fig. 2. Absorption spectra of benzoic acid (1 ꢁ 10ꢂ2 M) in acetone with addition of
p-nitro aniline concentrations ranging 0.01–0.05 M are shown with increasing
concentrations bottom to top.
where x and y denote wavelength and absorbance, respectively. The
results of the Gaussian analysis for all systems under study are
shown in Table 1. The wavelengths at these new absorption max-
Determination of Ionization potentials of the donor
The ionization potentials of the donor (ID) in the charge transfer
complexes were calculated using empirical equation derived from
Aloisi and Piganatro [20]
ima ðkCT
¼ vCÞ and the corresponding transition energies (hv) are
summarized in Table 2

N. Singh, A. Ahmad / Journal of Molecular Structure 1074 (2014) 408–415
411
0.4
0.3
0.2
0.1
0.0
Determination of standard free energy changes (
D
G0) and energy (ECT
)
of the
p–
pꢆ interaction between donor and acceptor
E
The standard free energy changes of complexation (
estimated from the association constants by following the equa-
tion derived by Martin et al. [22].
D
G0) were
Absorbance maxima 420
D
C
D
G0 ¼ ꢂ2:303 RT log KCT
ð6Þ
where
D
G0 is the free energy change of the CT-complexes (kJ molꢂ1),
R is the gas constant (8.314 J molꢂ1 K), T is the temperature in Kelvin
degrees (273 + °C) and KCT is the association constant of the com-
plexes (l molꢂ1) in different solvents at room temperature.
B
A
The energy (ECT) of the
p–
p⁄ interaction between the donor
(PNA), and acceptor (BEA), is calculated using the following
equation derived by Briegleb [23].
400
450
500
550
600
ECT ¼ 1243:667=kCT nm
ð7Þ
Wavelength (nm)
Fig. 4. Absorption spectra of benzoic acid (1 ꢁ 10ꢂ2 M) in methanol with addition
of p-nitroaniline concentrations ranging 0.01–0.05 M are shown with increasing
concentrations bottom to top.
The calculated values of (ECT) given in Table 4, where kCT is the
wavelength of the CT band.
IDðeVÞ ¼ 5:76 þ 1:53 ꢁ 10ꢂ4mCT
where vCT is the wave number in cmꢂ1 of the complex were deter-
mined in different solvents, viz, acetone, ethanol and methanol.
ð1Þ
Spectrophotometric study of formation constants of the charge
transfer Complexes of in different polar solvents
Stoichiometries and the formation constants of the charge
transfer complex of p-nitroaniline with benzoic acid have been
determined in different polar solvents, acetone, ethanol and meth-
anol at room temperature using Benesi–Hildebrand equation
[24,25]. The spectrophotometric data were used to estimate the
values of formation constants, (KCT) of the complexes. The changes
in the absorbance upon addition of PNA of a solution of BEA of
fixed concentration follow the Benesi–Hildebrand [24,25] equation
in the form.
Determination of oscillator strength, (f), and transition dipole
moment, (lEN
)
From the CT-absorption spectra, one can determine oscillator
strength. The value of oscillator strength f is estimated using the
formula
Z
f ¼ 4:32 ꢁ 10ꢂ9
eCT d
v
ð2Þ
½Aꢄ0=A ¼ ð1=KCT eCT Þ ꢁ 1=½Dꢄ0 þ 1=eCT
ð8Þ
R
where eCTdv is the area under the curve of the extinction coeffi-
cient of the absorption band in question vs. frequency. To a first
estimate,
where [D]0 and [A]0 are the concentrations of the p-nitroaniline
donor, and benzoic acid acceptor, respectively, A is the absorbance
of the donor–acceptor mixture at kCT, against the solvents as refer-
ence, (KCT) is the formation constant and eCT is the molar extinction
coefficient, is not quite that of complex. Eq. (8) [24,25] is valid
under the condition [D]0 ꢃ [A]0 [17,18] for 1:1 donor–acceptor
complexes. The concentration of the p-nitroaniline (PNA) was var-
ied over a wide range from 0.01 M to 0.05 M while the concentra-
f ¼ 4:32 ꢁ 10ꢂ9
eCT DV1=2
ð3Þ
where eCT is the maximum extinction coefficient of the band and
v1/2 is the half-width, i.e., the width of the band at half the max-
D
imum extinction. The observed oscillator strengths of the CT-bands
are summarized in Table 2.
tion of
p-acceptor BEA was kept fixed at 0.01 M in each reaction
mixture. These produced solutions with a donor: acceptor molar
ratios varying from 1:1 to 5:1 experimental data were given in
Table 3.
The extinction coefficient is associated to the transition dipole by
lEN ¼ 0:0952½
where
e
CT Dv1=2
=
D
v 1=2
ꢄ
ð4Þ
The Benesi–Hildebrand [24,25] method produced good results.
The intensity in the visible region of the absorption bands, mea-
sured against the solvent as reference, increases with increased
in the polarity and addition of the PNA. The typical absorbance
data for charge transfer complexes of PNA with BEA in different
polar solvents at room temperature were reported in Tables 1
and 3. In all systems, linear plots, were obtained according to Eq.
(7) [24,25] and are shown in Figs. 5–7. Formation constant for
the complexes in different polar solvents at room temperature
determined from the BH plots are summarized in Table 3. The cor-
relation coefficients of all such plots were above 0.99. Plots of [A]0/
A against 1/[D]0 were found to be linear in all systems in Figs. 5–7
showing 1:1 charge transfer complexes, i.e. the straight lines are
D
m
ꢅ
m
at eCT. The estimated values of (
lEN) for the complex
are reported in Table 2.
Determination of resonance energy (RN)
Briegleb and Czekalla [21] theoretically derived the relation
eCT ¼ 7:7 ꢁ 10ꢂ4=½h
mCT =½RNꢄ ꢂ 3:5ꢄ
ð5Þ
where eCT is the molar extinction coefficient of the complex at the
maximum of the CT absorption, vCT is the frequency of the CT-peak
and (RN) is the resonance energy of the complex in the ground state,
which, indeed is a contributing factor to the stability constant of the
complex (a ground state property). The values of (RN) for the com-
plexes under study have been given in Table 2.
obtained with the slopes, 1=KCT . These results suggest the forma-
eCT
tion of 1:1 CT-complexes. From slope 1=KCT and intercept, 1/eCT
,
eCT
KCT and eCT of the CT–complex was calculated in different polar
solvents.

412
N. Singh, A. Ahmad / Journal of Molecular Structure 1074 (2014) 408–415
Table 4
Association constant (KCT), correlation coefficients (r) and standard free energy changes (D
G0), transition energy (ECT), and absorption maxima (kCT) of BEA + PNA complexes
obtained from Benesi–Hildebrand plots.
Systems
Solvent
KCT (l molꢂ1
)
kCT (nm)
ꢂ
D
G0 (kJ molꢂ1
)
r
ECT (eV)
ID (eV)
BEA + PNA
BEA + PNA
BEA + PNA
Acetone
Ethanol
Methanol
18,550
11,870
6410
410
415
420
24.306
23.222
21.682
0.995
0.998
0.993
3.03
2.99
2.96
9.49
9.44
9.40
0.24
0.22
0.20
0.18
0.16
0.14
0.12
0.10
0.08
0.06
0.14
0.12
0.10
0.08
0.06
0.04
0.02
B-H plot in Acetone
B-H Plot in Methanol
20
40
60
80
100
20
40
60
80
100
1/[D]0
1/[D]0
Fig. 5. Relation between [A]0/A and 1/[D]0 of BEA + PNA in acetone.
Fig. 7. Relation between [A]0/A and 1/[D]0 of BEA + PNA in methanol.
0.16
B-H plot in Ethanol
0.14
0.12
0.10
0.08
0.06
0.04
20
40
60
80
100
1/[D]0
Fig. 6. Relation between [A]0/A and 1/[D]0 of BEA + PNA in ethanol.
Fig. 8. FTIR spectrum of (A) Complex of BEA and PNA, (B) acceptor BEA and (C)
donor PNA.
Effect of solvents on the formation of CT-complexes
polar solvent [26]. This is supposed to refer to the decrease of
energy of electrostatic interactions within the complex. Dissocia-
tion of the complexes into D+–Aꢂ radicals have been found to occur
in the ground state [27]. It means the formation of the CT-complex
should be strong in less polar solvent than more polar solvent.
The influence of solvent polarity on spectroscopic and thermo-
dynamic properties of molecular electron-donor–acceptor (EDA)
complexes is discussed. The data given in Table 3, show that BEA
interacts more strongly with PNA in acetone among the other
two solvents. The experimentally determined values of oscillator
strength, (f) in acetone (4.62 ꢁ 10ꢂ5), ethanol (1.34 ꢁ 10ꢂ5) and
methanol (1.20 ꢁ 10ꢂ5), and the values of transition dipole
The experimental results of the CT interaction between BEA
with PNA in different polar solvents reveal the values of association
constant KCT, in acetone (18,550 1 molꢂ1), ethanol (11,870 1 molꢂ1
)
and methanol (6410 1 molꢂ1) and the values of molar extinction
coefficient eCT in acetone (40.8 1 molꢂ1 cmꢂ1), ethanol
(40.1 1 molꢂ1 cmꢂ1) and methanol (131 1 molꢂ1 cmꢂ1). The spec-
troscopic properties were markedly affected by the variation in sol-
vent polarity in which measurements were carried out. In the
present investigation, the KCT values increase significantly from
methanol to acetone with decreasing solvent polarity. Moreover,
the increase in KCT values with decreasing solvent polarity may also
be due to the fact that, the CT-complex should be stabilized in less

N. Singh, A. Ahmad / Journal of Molecular Structure 1074 (2014) 408–415
413
Table 5
(0.0056 eV), given in Table 2, indicate that complex should be sta-
ble in less polar solvent (acetone).
Characteristic infrared frequencies (cmꢂ1) and tentative assignments for BEA, PNA
and their complex.
The parameters, thus obtained are represented in Table 4, and
these values show that complexation is thermodynamically
favored proved. The free energy change of the complexation also
reveals that the CT-complex formation between donor (PNA) and
BEA
PNA
Complex
Assignments
(OAH), BEA, hydrogen bonding b/w
AOHAHANH
(NAH), PNA
mas (CAH), CH3 + CH3
(CAH), aromatic
(+NH)
3067w,br
–
3480s
3356s,br
3220s,br
3484s
3361s
3223s
–
3076w
2834w
2679w
1688s
1633vs
1589ms
–
m
D
G0 in
m
acceptor (BEA) is of exothermic in nature. The values of
3004w,br
2833br
2674br
acetone (ꢂ24.306 kJ molꢂ1), ethanol (ꢂ16.204 kJ molꢂ1), and
methanol (ꢂ15.291 kJ molꢂ1) given in Table 4, generally become
more negative as the association constants for molecular complex
increases.
3108w
1923w
1747w
1635ms
–
m
m
1690vs
1603
1579
AC@O
m
m
s
s
m
m
as(NO2)PNA
(C@C), aromatic
1591vs
–
d def (NAH), +NH2 ring
Breathing bands
FT-IR spectra of CT-complex and reactants
–
–
1474s
1442s
1398vs
1302w
1474vs
1423ms
1320vs
1292vs
CAH deformation
1496ms
1420s
1321
FT-IR spectra of the p-nitro aniline (donor), benzoic acid (accep-
tor) and their CT-complex were shown in Fig. 8. While their assign-
ments of the characteristic FT-IR spectral bands were reported in
Table 5. The formation of the charge transfer complex during the
reaction of PNA with BEA is strongly evidenced by the presence
of the main characteristic infrared bands of the donor and acceptor
in the spectrum of the product. However, the bands of the donor
are shifted to lower frequencies while that of the acceptor are
shifted to the higher frequencies. There are changes in their inten-
sities compared with that of the free donor and acceptor. This shift
has been attributed to the charge transfer from donor to acceptor
upon the complexation. These changes may be attributed to the
expected symmetry and electronic structure modifications in both
donor and acceptor units in the product thus formed relative to the
free molecules. The AOH peak has disappeared in the spectrum of
the CT-complex, whereas in free BEA this was observed at
3004 cmꢂ1 (weak, broad). The disappearance of the AOH peak in
the CT-complex has been ascribed to the proton transfer from
the acceptor to the donor or transfer of lone pair electrons the
donor to acceptor leading to an intermolecular hydrogen bonding
as reported previously [15,28], so as to the new band observed at
m
m
(CAC), msNO2, PNA
as(CAN)
m
s
–
1289
m
s
–
–
m
m
(CAO)
1182ms
1127ms
1071vs
928ms
809w
1183w
1110br
–
1181ms
1110ms
1070m
s(CAN)
998s
842sharp
1030
999w
m
s
d (CAH) in plane bending
drock, +NH2
935s
844s
CH2 rock skeletal
Vibrations
–
754vs
698vs
534ms
490ms
–
753m
CAH out of plane
Bending
706ms
–
662ms
543sh
495br
–
666m
s
CAH wagging
547w
428s
418sharp
420ms
CNC deformation
S, strong, w, weak; m, medium, sh, shoulder, v, very; vs, very strong, br, broad;
stretching; s, symmetrical stretching; as, asymmetrical stretching.
m,
m
m
moment, (lEN) in acetone (0.0429 Debyes) ethanol (0.426 Debyes)
and methanol (0.770 Debyes), and values of resonance energy (RN)
in acetone (0.0016 eV), ethanol (0.0016 eV), and methanol
3484 cmꢂ1 (strong) and is due to the
t
(N+AHꢇ ꢇ ꢇOꢂ) while the
Fig. 9. 1H NMR spectrum of CT complex.

414
N. Singh, A. Ahmad / Journal of Molecular Structure 1074 (2014) 408–415
COOH
NH2
+
NO2
O
C
O-
+NH3
NO2
Scheme 1. Mechanism for interaction between p-nitro aniline with benzoic acid.
NAH stretching vibration is kept at 3361 cmꢂ1 (strong) and
3223 cmꢂ1 (strong) which is normally kept at 3300 cmꢂ1 (med-
ium) and 3199 cmꢂ1 (medium weak). The band at 3076 cmꢂ1
(weak) is due to the aromatic CAH stretching vibration. The
ANH2 deformation mode is observed by the absorption at
1633 cmꢂ1 (very strong) in CT complex, whereas in free PNA this
was observed at 1635 cmꢂ1 (medium strong). This band overlaps
with the aromatic C@C stretching vibrations. Thus, it has been
understood that the shift to the lower frequency of donor in the
spectrum of the CT-complex is due to the formation of the CT-com-
plex [28]. The ANO2 group are observed at 1589 cmꢂ1 (medium
strong) in the CT complex, whereas in free PNA this was observed
at 1591 cmꢂ1 (very strong). The absorption at 1633 cmꢂ1and
1474 cmꢂ1are due to the aromatic C@C stretching vibrations. The
Fig. 10. TGA–DTA curves (A) p-nitro aniline, (B) benzoic acid and (C) complex of
BEA + PNA.
CAH in and out plane vibration is observed at 1070 cmꢂ1
,
753 cmꢂ1 and 935 cmꢂ1 respectively.
p-nitro aniline. The doublet ‘peaks at d = 8.12 ppm and triplet
peaks at d = 7.62 ppm are assigned to the rest of protons are in
the same kind in the benzoic acid moiety in the CT-complex. The
formation of a hydrogen bond bridge between ANH proton of
p-nitro aniline and AOH proton of benzoic acid is supported by
the significant downfield shift of the NH peaks relative to pure
compounds provide convincing evidence for the association of
molecules [30]. Mechanism and structure of the CT-complex of
acceptor and donor is given in Scheme 1.
1H NMR spectrum of CT-complex
1H NMR spectrum of the CT-complex were studied in CDCl3 and
shown in Fig. 9. In the nuclear magnetic resonance 1H NMR spec-
trum of CT-complex the proton of carboxylic group of benzoic acid
is absent while in free benzoic acid this proton occurred at
d = 12.09 ppm and p-nitro aniline ring system were specified in
the region d = 8.05–6.60 ppm for p-nitro aniline. All the observed
peaks in the spectrum of the individual reactants are also
celebrated in the spectrum of CT-complex except AOH peak, sug-
gesting that the disappearance of AOH peak in the spectrum of
CT-complex is due to the formation of +NAH3 ion through hydro-
gen bonding between AOH group of benzoic acid, ANH2 group of
p-nitro aniline [29]. The AOH proton is assigned at d = 12.09 ppm
in free benzoic acid, which has been united in the bunch of
aromatic protons of p-nitro aniline moiety in the region
d = 8.05–6.60 ppm. This up field shift in frequency has been attrib-
uted to an increase in electron density on the benzoic acid part of
the CT-complex due to transfer of protons from benzoic acid to
Comparative study of thermograms for benzoic acid, p-nitroaniline
and their CT-complex
The comparative study of thermograms for benzoic acid,
p-nitroaniline and their CT-complex were carried out by DTG-
60H thermal analysis in a nitrogen atmosphere with a flow rate
of 30 ml minꢂ1 and heating rate 25 °C minꢂ1 in the temperature
range 20–400 °C showing in Fig. 10(a–c). The reference was a
10 mg alumina powder. Thermo gravimetric (TGA) and differential
thermo gravimetric (DTA) analysis were carried out using in the
temperature range of 0–400 °C using 10.299 mg, 7.914 mg, and

N. Singh, A. Ahmad / Journal of Molecular Structure 1074 (2014) 408–415
415
4.926 mg for benzoic acid, p-nitroaniline, CT-complex, respectively
in order to confirm the charge transfer interaction and thermal
stability of the CT-complex.
spectrophotometer. Financial assistance from the UGC, New Delhi
extended through the Women-PDF fellowship is also gratefully
acknowledged. The authors also thank the learned referee for mak-
ing valuable comments.
The thermogram of the donor (PNA) exhibit it decomposes in
one step at 256 °C with weight loss of close to 89.75% and acceptor
shows two decomposition steps at 180 °C and 315 °C with a weight
loss of approximately 89.038% and 12.001%, respectively, with a
complete loss in each case, at higher decomposition temperature
for donor and acceptor in CT complex compared with free donor
and acceptor (shown in Fig. 10). The CT complex of BEA and PNA
exhibits two decomposition steps at 178 °C and 247 °C with weight
loss 98.620% and 3.370% respectively. The decomposition at 247 °C
with weight losses 3.370% is probably due to the donor (PNA). This
has been attributed to the formation of CT-complex and its
stability compared to its constituents.
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D
G0), have been esti-
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Acknowledgements
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Author thanks Dr. Zafar A. Siddiqui Chairman of Chemistry
Department, Aligarh Muslim University, India, for furnishing the
facilities of instruments of FT-IR Spectrometer, UV–Visible