Spectroscopic investigations of the charge-transfer Interaction between the drug reserpine and different acceptors: Towards understanding of drug-receptor mechanism

Article abstract of DOI:10.1016/j.saa.2013.06.046

The study of the charge-transfer Interaction of the drugs may be useful in understanding the drug-receptor Interactions and the mechanism of drug action. Structural and thermal stability of charge-transfer (CT) complexes formed between the drug reserpine (Res) as a donor and quinol (QL), picric acid (PA), tetracyanoquinodimethane (TCNQ) or dichlorodicyanobenzoquinone (DDQ) as acceptors were reported. Elemental analysis, electronic absorption, spectrophotometric titration, iR, Raman, ¹H NMR and X-ray powder diffraction (XRD) were used to characterize the new products. The thermal stability of the synthesized CT complexes was investigated using thermogravimetric (TG) analyses, and the morphology and particle size of these complexes were obtained from scanning electron microscopy (SEM). The stoichiometry of the complexes (donor: acceptor molar ratio) was determined to be 1: 1 for all complexes. Accordingly the formed CT complexes could be formulated as [(Res)(QL)], [(Res)(PA)], [(Res)(TCNQ)] and [(Res)(DDQ)]. it was found that the obtained CT complexes are nanoscale, semi-crystalline particles, thermally stable and formed through spontaneous reaction. The results obtained herein are satisfactory for estimation of drug Res in the pharmaceutical form.

Full text of DOI:10.1016/j.saa.2013.06.046

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 115 (2013) 309–323
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Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Spectroscopic investigations of the charge-transfer interaction between
the drug reserpine and different acceptors: Towards understanding
of drug–receptor mechanism
a
Hala H. Eldaroti a, , Suad A. Gadir , Moamen S. Refat b,c, Abdel Majid A. Adam b
⇑
a Department of Chemistry, Faculty of Education, Alzaeim Alazhari University, Khartoum, Sudan
b Department of Chemistry, Faculty of Science, Taif University, Al-Haweiah, P.O. Box 888, 21974 Taif, Saudi Arabia
c Department of Chemistry, Faculty of Science, Port Said University, Egypt
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
ꢀ
ꢀ
ꢀ
ꢀ
Reserpine (Res) is a biologically active
naturally occurring drug.
CT-bands
2.0
Four new charge-transfer complexes
of Res were reported for the first time.
Bonding modes were ascertained
from various spectral techniques.
The obtained complexes are
O
O
CH3
CH3
O
1.5
N
O
O
H3C
O
N
O
CH3
NH
C
1.0
0.5
0.0
O
CH3
NC
NC
H3C
O
nanoscale and thermally stable.
CN
Res-TCNQ CT complex
300
400
500
Wavelength (nm)
Electronic absorption spectrum of Res-TCNQ CT complex
a r t i c l e i n f o
a b s t r a c t
Article history:
The study of the charge-transfer interaction of the drugs may be useful in understanding the drug–
receptor interactions and the mechanism of drug action. Structural and thermal stability of charge-
transfer (CT) complexes formed between the drug reserpine (Res) as a donor and quinol (QL), picric acid
(PA), tetracyanoquinodimethane (TCNQ) or dichlorodicyanobenzoquinone (DDQ) as acceptors were
reported. Elemental analysis, electronic absorption, spectrophotometric titration, IR, Raman, 1H NMR
and X-ray powder diffraction (XRD) were used to characterize the new products. The thermal stability
of the synthesized CT complexes was investigated using thermogravimetric (TG) analyses, and the mor-
phology and particle size of these complexes were obtained from scanning electron microscopy (SEM).
The stoichiometry of the complexes (donor:acceptor molar ratio) was determined to be 1:1 for all com-
plexes. Accordingly the formed CT complexes could be formulated as [(Res)(QL)], [(Res)(PA)],
[(Res)(TCNQ)] and [(Res)(DDQ)]. It was found that the obtained CT complexes are nanoscale, semi-crys-
talline particles, thermally stable and formed through spontaneous reaction. The results obtained herein
are satisfactory for estimation of drug Res in the pharmaceutical form.
Received 28 March 2013
Received in revised form 6 June 2013
Accepted 12 June 2013
Available online 28 June 2013
Keywords:
Reserpine
Charge-transfer complexes
XRD
SEM
Thermal analysis
Ó 2013 Elsevier B.V. All rights reserved.
Introduction
the reaction between acceptors and drugs or biological com-
pounds. This interest stems from the significant physical and
Considerable attention has recently been devoted to the
formation of stable charge-transfer (CT) complexes that result from
chemical properties of these complexes. The CT complexation is
an important technique that is cheaper, simpler, and more efficient
than the methods of drug determination described in the literature
[1]. The study of the CT complexes of drugs may be useful in under-
standing the drug–receptor interactions and the mechanism of
⇑
Corresponding author. Tel.: +249 9 12998623.
E-mail address: halahassan_1972@yahoo.com (H.H. Eldaroti).
1386-1425/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.saa.2013.06.046

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H.H. Eldaroti et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 115 (2013) 309–323
6
5
purchased from Merck Chemical Company and were used without
further purification. Commercially available spectroscopic grade
solvents (BDH) were also used as received.
O
O
CH3
CH3
O
9
N4
O
7
21
8
10
11
20 19
18
H3C
2
3
O
13
N
H
1
O
12
14 15
Instrumental measurements
17
CH3
16
O
O
CH3
H3C
O
The elemental analyses of the carbon and hydrogen contents
were performed by the microanalysis facility at Cairo University,
Egypt, using a Perkin–Elmer CHN 2400 (USA). The electronic
absorption spectra of methanolic solutions of the donor, acceptors
and resulting CT complexes were recorded over a wavelength
range of 200–800 nm using a Perkin–Elmer Lambda 25 UV/Vis
double-beam spectrophotometer at Taif University, Saudi Arabia.
The instrument was equipped with a quartz cell with a 1.0 cm path
length. The mid-infrared (IR) spectra (KBr discs) within the range
of 4000–400 cmꢂ1 for the solid CT complexes were recorded on a
Shimadzu FT-IR spectrophotometer with 30 scans at 2 cmꢂ1 reso-
lution. The Raman laser spectra of the samples were measured
Scheme 1. Chemical structure of reserpine.
drug action [2–4]. Furthermore, the crystalline CT complexes have
a vital role in biological systems such as antimicrobial activity and
DNA-binding. Literature shows that the CT complexes exhibit po-
tential antimicrobial properties against Gram-positive and Gram-
negative bacteria as well as fungi [5–10].
Herein, the CT interaction between the drug reserpine and four
acceptors are investigated. Reserpine (Res; C33H40N2O9, Scheme 1)
is an indole alkaloid antipsychotic and antihypertensive that exists
at room temperature as a white or pale-buff to yellow odorless
powder [11]. It is practically insoluble in water; freely soluble in
chloroform, methylene chloride, and glacial acetic acid; soluble
in benzene and ethyl acetate; and slightly soluble in methanol, eth-
anol, acetone, ether, and weak solutions of acetic and citric acids. It
is stable under normal storage conditions but is subject to oxida-
tion and hydrolysis. Res acquire a yellow color with pronounced
fluorescence, especially after addition of acid or exposure to light.
When heated to decomposition, it emits toxic fumes of nitrogen
oxides [12]. Res is a biologically active naturally occurring drug
produced by several members of the genus Rauwolfia, a climbing
shrub indigenous to southern and Southeast Asia. Extracts of Rau-
wolfia serpentine have been used medicinally in ancient India for
centuries. In traditional Hindu medicine, the roots of Rauwolfia ser-
pentine was brewed as a tea and were used in humans to treat
hypertension, insanity, fever, snakebite, insomnia and cholera.
The purified alkaloid, reserpine, was isolated in 1952 from the died
root of Rauwolfia serpentine and is considered the first modern drug
for the treatment of hypertension [13]. Drug Res is now largely
used to lower blood pressure, reduce the heart rate, relief of
psychotic symptoms and as a tranquilizer and sedative in humans
[14–17]. It has also been used as a radioprotective agent and
experimentally as a contraceptive [18,19].
on
a Bruker FT-Raman spectrophotometer equipped with a
50 mW laser at Taif University, Saudi Arabia. 1H NMR spectra were
collected by the Analytical Center at King Abdul Aziz University,
Saudi Arabia, on a Bruker DRX-250 spectrometer operating at
250.13 MHz with a dual 5 mm probe head. The measurements
were performed at ambient temperature using DMSO-d6 (dimeth-
ylsulfoxide, d6) as a solvent and TMS (tetramethylsilane) as an
internal reference. The 1H NMR data are expressed in parts per mil-
lion (ppm) and are internally referenced to the residual proton
impurity in the DMSO solvent. Thermogravimetric analysis (TGA)
was performed under an air atmosphere between room tempera-
ture and 800 °C at a heating rate of 10 °C/min using a Shimadzu
TGA-50H thermal analyzer at the Central Lab at Ain Shams Univer-
sity, Egypt. The X-ray diffraction patterns for the obtained CT com-
plexes were collected on a PANalytical X’Pert PRO X-ray powder
diffractometer at the Central Lab at Ain Shams University, Egypt.
The instrument was equipped with a Ge(III) monochromator, and
a Cu Ka1 X-ray source with a wavelength of 0.154056 nm was
used. Scanning electron microscopy (SEM) images were collected
on a Jeol JSM-6390 instrument at Taif University, Saudi Arabia.
The instrument was operated at an accelerating voltage of 20 kV.
Procedures
To provide basic data that can be used to understand of
drug–receptor mechanism, the CT complexes of Res with quinol
(QL), picric acid (PA), tetracyanoquinodimethane (TCNQ) and
dichlorodicyanobenzoquinone (DDQ) were synthesized and spec-
troscopically investigated. The newly synthesized CT complexes
have been structurally characterized via elemental analysis; infra-
red (IR), Raman, 1H NMR and electronic absorption spectroscopy;
powder X-ray diffraction; and scanning electron microscopy (SEM)
to interpret the behavior of the interactions. The spectroscopic and
physical data were analyzed in terms of formation constant (KCT),
Reaction procedure
The solid CT complexes of Res with QL, PA, TCNQ or DDQ were
prepared by mixing equimolar amounts of Res with each acceptor
in methanol (10 ml). The solutions were stirred for about 20 min,
and allowed to evaporate slowly at room temperature, which
resulted in the precipitation of the solid CT complexes. The resul-
tant complexes were filtered off, washed well with little amounts
of methanol, and then collected and dried under vacuum over anhy-
drous calcium chloride for 24 h. Elemental analyses (C and H) of the
Res CT complexes were performed, and the obtained results are as
follows: [(Res)(QL)]; C39H46N2O11; Mol. wt. = 718.79; Dark brown;
Calc.:%C, 65.11;%H, 6.40; Found:%C, 64.89;%H, 6.38. [(Res)(PA)];
molar extinction coefficient (eCT), standard free energy (
lator strength (f), transition dipole moment ( ), resonance energy
(RN) and ionization potential (ID). The thermal behavior of the ob-
DG°), oscil-
l
tained complexes and the kinetic and thermodynamic parameters
(Eꢁ, A,
D D D
Sꢁ, Hꢁ and Gꢁ) have also been investigated.
C39H43N5O16; Mol. wt. = 837.78; Yellow; Calc.:%C, 55. 86;%H, 5.13;
Found:%C, 55.71;%H, 4.96. [(Res)(TCNQ)]; 45H44N6 O9; Mol.
C
wt. = 812.87; Dark red; Calc.:%C, 66.43;%H, 5.41; Found: %C,
66.73;%H, 5.29. [(Res)(DDQ)]; C41H40Cl2N4O11; Mol. wt. = 835. 68;
Dark brown; Calc.:%C, 58.87;%H, 4.79; Found:%C, 58.50;%H, 4.91.
Experimental
Chemicals
Preparation of standard stock solutions of the donor and acceptors
Stock solutions of the Res and acceptors at a concentration of
5.0 ꢃ 10ꢂ3 M were freshly prepared before each series of measure-
ments by dissolving precisely weighed amounts in the appropriate
volume of the methanol solvent. The stock solutions were pro-
tected from light.
All chemical used were of high grade. Reserpine (Res; Methyl (3b,
6b, 17 , 18b, 20 )-11,17-dimethoxy-18-[(3,4,5-trimethoxybenz
a
a
oyl) oxy]yohimban-16-carbox-ylate, C33H40N2O9) and
p-acceptors
of quinol (QL), picric acid (PA), 7,70,8,80-tetracyanoquinodimethane
(TCNQ) or 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) were

H.H. Eldaroti et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 115 (2013) 309–323
311
Spectrophotometric titration procedure
QL; Benzene-1,4-diol (quinol).
Spectrophotometric titration measurements were carried out
for the reactions of Res with QL, PA, TCNQ and DDQ against meth-
anol as a blank at wavelengths of 294, 354, 300 and 395 nm,
respectively. A 0.25, 0.50, 0.75, 1.00, 1.50, 2.0, 2.50, 3.00, 3.50 or
4.00 mL aliquot of a standard solution (5.0 ꢃ 10ꢂ4 M) of the appro-
PA; 2,4,6-trinitrophenol (picric acid).
TCNQ; 7,70,8,80-tetracyanoquinodimethane.
DDQ; 2,3-dichloro-5,6-dicyano-1,4-benzoquinone.
[(Res)(QL)] complex; 11,17-dimethoxy-16-methylcarboxy-18-
[(3,4,5-trimeth- oxybenzoyl)oxy]yohimban-4-ium-hydroxyph
enolate.
[(Res)(PA)] complex; 11,17-dimethoxy-16-methylcarboxy-18-
[(3,4,5-trimeth-oxybenzoyl)oxy]yohimban-4-ium-picrate.
[(Res)(TCNQ)] complex; cyano[4-(dicyanomethylene)cyclohe
xa-2,5-dien-1-ylidene]ethanenitriliummethyl-11,17-dimetho
xy-18-[(3,4,5-trimethoxy-benzo-yl) oxy]yohimban-1-ide-16-
carboxylate.
[(Res)(DDQ)] complex; 4,5-dichloro-2-cyano-3,6-dioxocyclohe xa-
1,4-diene-1-carbonitrilium methyl-11,17-dimethoxy-18-[(3,4,5-
trimethoxybenzoyl)oxy] yohimban-1-ide-16-carboxy late.
priate acceptor in methanol was added to 1.00 ml of 5.0 ꢃ 10ꢂ4
M
Res, which was also dissolved in methanol. The total volume of the
mixture was 5 mL. The concentration of Res (Cd) in the reaction
mixture was maintained at 5.0 ꢃ 10ꢂ4 M, whereas the concentra-
tion of the acceptors (Ca) changed over a wide range of concentra-
tions (0.25 ꢃ 10ꢂ4–4.00 ꢃ 10ꢂ4 M) to produce solutions with an
acceptor molar ratio that varied from 4:1 to 1:4. The stoichiometry
of the Res CT complexes was obtained from the determination of
the conventional spectrophotometric molar ratio according to
known methods [20] using a plot of the absorbance of each CT
complex as a function of the Cd:Ca ratio. Modified Benesi–Hilde-
brand plots [21,22] were constructed to allow the calculation of
the formation constant, KCT, and the absorptivity, eCT, values for
each CT complex in this work.
Results and discussion
Electronic absorption spectra
Systematic IUPAC nomenclature
Fig. 1 shows the electronic spectra of the Res, acceptors and the
formed CT complexes. These spectra revealed the characterization
of the real absorption bands that are attributed to the CT interac-
tions. These bands are observed at (275, 294) nm, 354 nm, (300,
336) nm and (290, 395) nm for the [(Res)(QL)], [(Res)(PA)],
[(Res)(TCNQ)] and [(Res)(DDQ)] complexes, respectively. These
peak absorbance values that appeared in the spectra assigned to
The systematic IUPAC names of the drug Res, acceptors and the
formed CT complexes are;
Res; Methyl(3b, 16b, 17a, 18b, 20a)-11,17-dimethoxy-18-[(3,
4,5-trimethoxy-benzoyl)oxy]yohimban-16-carboxylate.
275 and 294 nm (CT-bands)
354 nm (CT-band)
1.5
1.5
1.0
1.0
Res-QL
Res-PA
0.5
0.5
QL
PA
Res
Res
CT complex
CT complex
0.0
250
0.0
300
350
400
300
400
500
600
Wavelength (nm)
Wavelength (nm)
300 and 336 nm (CT-bands)
290 and 395 nm (CT-bands)
2.0
1.5
1.0
0.5
0.0
2.0
1.5
1.0
0.5
0.0
Res-DDQ
Res-TCNQ
DDQ
TCNQ
Res
Res
CT complex
CT complex
300
400
500
300
400
500
Wavelength (nm)
Wavelength (nm)
Fig. 1. Electronic absorption spectra of Res-QL, Res-PA, Res-TCNQ and Res-DDQ charge-transfer systems in methanol solvent.

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3.5
3.0
2.5
2.0
1.5
1.0
0.5
4
3
2
1
Res-PA
Res-QL
294 nm
354 nm
0
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
ml added of QL
ml added of PA
4
3
2
1
0.34
0.32
0.30
0.28
0.26
0.24
0.22
0.20
Res-TCNQ
Res-DDQ
300 nm
395 nm
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
ml added of TCNQ
ml added of DDQ
Fig. 2. Spectrophotometric titration curves for Res charger-transfer systems in methanol solvent at the detectable peaks.
data reveal that the [(Res)(TCNQ)] complex shows a higher KCT va-
lue compared with the other complexes, reflecting the relatively
higher powerful electron acceptance ability for TCNQ. The TCNQ
acceptor has four strong withdrawing cyano groups in conjugation
with an aromatic ring, which causes high delocalization leads to a
great increase in the Lewis acidity of the acceptor TCNQ, and hence
the higher value of KCT for [(Res)(TCNQ)] complex compared with
the other complexes. The data also reveal that the [(Res)(PA)] com-
plex shows a higher value of e. The e values of Res complexes in
decreasing order is [(Res)(PA)] > [(Res)(TCNQ)] > [(Res)(QL)] >
[(Res)(DDQ)].
the formed CT complexes were measured and plotted as a function
of the Cd:Ca ratio according to a known method. Spectrophotometric
titration plots based on these characterized absorption bands
(Fig. 2) confirmed the complex formation at a ratio (Res:acceptor)
of 1:1 in all cases. Based on the obtained data, the formed CT com-
plexes were formulated as [(Res)(QL)], [(Res)(PA)], [(Res)(TCNQ)]
and [(Res)(DDQ)]. The formation of 1:1 complex was strongly sup-
ported by IR, 1H NMR and elemental as well as thermal analysis.
The formation constant (KCT) and the molar absorptivity (e) of
the Res CT complexes were calculated by applying the 1:1 Bene-
si–Hildebrand equation in the following equation:
ðCaCdÞ=A ¼ 1=K
e
þ ðCa þ CdÞ=
e
ð1Þ
Calculation of the spectroscopic and physical data
where Ca and Cd are the initial concentrations of the electron accep-
tor (QL, PA, TCNQ or DDQ) and the electron donor (Res), respec-
tively, and A is the absorbance of the strongly detected CT band.
Plotting the (Ca Cd)/A values versus the corresponding (Ca + Cd)
values for the formed Res CT complexes, straight lines were
obtained supporting our finding of the formation of 1:1 complexes.
In the plots, the slope and the intercept equal 1/e and 1/Ke, respec-
tively. The Benesi–Hildebrand plots are shown in Fig. 3 and the
values of both KCT and e associated with the complexes are given
in Table 1. These complexes exhibit high values for both the
formation constants (KCT) and the extinction coefficients (e). The
high values of KCT reflect the high stabilities of the formed CT com-
plexes. The equilibrium constants are strongly dependent on the
nature of the used acceptor including the type of electron with-
drawing subsitituents to it such as nitro and halo groups [23]. The
The spectroscopic and physical data, such as the standard free
energy (DG°), the oscillator strength (f), the transition dipole mo-
ment ( ), the resonance energy (RN), and the ionization potential
l
(IP), were estimated for complexes dissolved in methanol at
25 °C. The oscillator strength (f) is a dimensionless quantity used
to express the transition probability of the CT band. From the CT
absorption spectra, the oscillator strength (f) can be estimated
using the approximate formula [24]:
Z
f ¼ 4:319 ꢃ 10ꢂ9
eCT dm
ð2Þ
R
where
eCT dm is the area under the curve of the extinction coeffi-
cient of the absorption band in question plotted as a function of
frequency. To a first approximation,

H.H. Eldaroti et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 115 (2013) 309–323
313
1.4
1.2
1.0
0.8
0.6
0.4
1.0
Res-QL
Res-PA
0.9
0.8
0.7
0.6
0.5
100 150 200 250 300 350 400 450 500 550
100 150 200 250 300 350 400 450 500 550
(Cd +Cao)x10-6
o
(Cd +Cao)x10-6
o
14
12
10
8
Res-DDQ
Res-TCNQ
1.0
0.8
0.6
0.4
6
4
2
0
100 150 200 250 300 350 400 450 500 550
100 150 200 250 300 350 400 450 500 550
(Cd +Cao)x10-6
o
(Cd +Cao)x10-6
o
Fig. 3. The 1:1 Benesi–Hildebrand plot of Res charger-transfer systems in methanol solvent at the detectable peaks.
f ¼ 4:319 ꢃ 10ꢂ9eCT m1=2
ð3Þ
sured by its ionization potential, which is the energy required to re-
move an electron from the highest occupied molecular orbital.
Briegleb and Czekalla [27] theoretically derived the following rela-
tionship to obtain the resonance energy (RN):
where eCT is the maximum extinction coefficient of the CT band, and
m
is the half-bandwidth in cmꢂ1 (i.e., the bandwidth at half of the
½
maximum extinction coefficient value). The transition dipole
eCT ¼ 7:7 ꢃ 10ꢂ4=½h
m
CT =½RNꢄ ꢂ 3:5ꢄ
ð6Þ
moments (
l
) of the Res CT complexes have been calculated from
Eq. (4) [25]:
where eCT is the molar absorptivity coefficient of the CT complex at
the maximum of the CT absorption, mCT is the frequency of the CT
peak, and RN is the resonance energy of the complex in the ground
state, which contributes to the stability constant of the complex (a
ground-state property). The energy values (ECT) of the n ? pꢁ and
1=2
l
¼ 0:0958½eCT m1=2
=mmax
ꢄ
ð4Þ
The transition dipole moment can be used to determine if a
particular transition is allowed; the transition from a bonding
p
orbital to an antibonding pꢁ orbital is allowed because the integral
that defines the transition dipole moment is nonzero. The ioniza-
tion potentials (IP) of the Res donor in the CT complexes were cal-
culated using the empirical equation derived by Aloisi and
Pignataro represented in Eq. (5) [26]:
p–
pꢁ interactions between the donor (Res) and the acceptors were
calculated using the equation derived by Briegleb [28]:
ECT ¼ ðhmCT Þ ¼ ð1243:667=kCT
where kCT is the wavelength of the CT band. The standard free en-
Þ
ð7Þ
IPðeVÞ ¼ 5:76 þ 1:53 ꢃ 10ꢂ4mCT
where mCT is the wavenumber in cmꢂ1 that corresponds to the CT
band formed from the interaction between the donor and the
acceptor. The electron-donating power of a donor molecule is mea-
ð5Þ
ergy of complexation ( G°) for each complex was calculated from
D
the formation constants using the equation [29]:
D
Gꢅ ¼ ꢂ2:303RT log KCT
ð8Þ
Table 1
Spectroscopic and physical data of the Res CT complexes.
Complex
kmax (nm)
ECT (eV)
K (L molꢂ1
)
emax (L molꢂ1 cmꢂ1
)
f
l
Ip
D
RN
D
G° (25 °C) (kJ molꢂ1
)
[(Res)(QL)]
[(Res)(PA)]
[(Res)(TCNQ)]
[(Res)(DDQ)]
294
354
300
395
4.23
3.52
4.15
3.15
3.21 ꢃ 104
2.87 ꢃ 104
5.82 ꢃ 104
1.25 ꢃ 104
5.32 ꢃ 104
8.77 ꢃ 104
5.99 ꢃ 104
3.32 ꢃ 104
46.58
37.88
64.65
28.72
49.16
53.38
64.20
15.53
10.96
10.08
10.86
9.63
32.70
1.20
1.00
1.18
0.84
ꢂ32,999
ꢂ31,137
ꢂ32,894
ꢂ34,792

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H.H. Eldaroti et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 115 (2013) 309–323
Table 2
Infrared frequenciesa (cmꢂ1) and tentative assignments for Res, acceptors and the formed CT complexes.
Res
QL
PA
TCNQ
DDQ
[(Res)(acceptor)] CT complexes
Assignmentsb
QL
PA
TCNQ
DDQ
3440 s
3462 s
3287 br, s
2984 m
3462 s
3411 ms
3419 ms
m
m
m
(NAH)
(OAH)
(CAH); automatic
3262 br
3031 m
3416 br
3103 ms
3296 ms, br
3036 vw
2985 ms
2940 vs
3137 ms
3050 s
2969 mw
2851 mw
3325 w
3218 br
3295 ms, br
3195 ms, br
2984 m
2840 s
2857 m
2980 sh
2872 w
2856 w
2730 br, mw
2945 vw
2854 w
2790 w
3049 m
2984 m
m
s(CAH) + mas(CAH); Res, QL
Hydrogen bonding
2220 s
2250 vw
2231 ms
2188 ms
2226
m
m
(C„N); TCNQ, DDQ
(C@O); Res, Complex
1732 s
1720 s
1625 m
-
1785 vs
1770 sh, vs
1785 vs
1770 sh, vs
1642 m
1788 vs
1767 vs sh
1789 vs
1765 sh
1596 mw
1788 vs
1698 mw
1568 ms
1673 vs
1552 vs
ddef(NAH); Res, Complex
m
m
1632 vs
1608 vs
1529 vs
1432 s
1636 m
as(NO2);PA
(C@O) + (C@C) + m(CAN)
1588 ms
1500 ms
1455 vs
1410 s
1377 m
1332 ms
1275 ms
1250 ms
1225 m
1120 s
1518 vs
1540 vs
-
1513 s
1471 s
1610 m, sh
1534 m, br
1452 m
1541 ms
m
d(CAH) deformation
1451 s
1449 ms
1408 w
1359 m
1304 ms
1257 m
1175 s
1450 ms
1406 m
1305 m
1258 m
1218 m
1174 ms
1128 ms
1075 m
1037 m
987 ms
935 m
m
m
m
(CAH); Res, complex
(C@C) (in-ring), aromatic
(CAC) + (CAO) + s(CAN)
1477 vs
1366 ms
1244 vs
1222 vs
1210 vs
1164 ms
1097 m
1343 ms
1150 ms
1086 s
1352 ms
1117 ms
1044 vw
1358 w
1267 s
1172 vs
1072 w
1010 vw
1380 ms
1307 ms
1252 m
1216 s
1173 s
1126 s
1362 m
1306 s
1258 s
1173 s
1126 s
m
m
CAH in-plane bending
m
sNO2, PA
1125 s
975 ms
800 ms
763 ms
740 m
827 s
917 vs
781 s
732 s
860 vs
473 vs
893 s
800 vs
720 s
615 ms
527 vw
457 ms
983 s
985 s
986 s
914 ms
861 s
728 m
476 ms
drock, NH
(CAC1); DDQ, complex
759 vs
616 m
525 ms
935 ms
827 ms
760 ms
730 ms
935 ms
914 ms, sh
868 mw
730 ms
653 ms
m
913 sh
820 m
730 m
CAH out of plane bending
Skeletal vibrations
CNC deformation
d(ONO);PA
703 s
652 sh
522 ms
a
s, strong; w, weak; m, medium; sh, shoulder; v, very; vs, very strong; br, broad.
b
m
, stretching; ms, symmetrical stretching; mas, asymmetrical stretching; d, bending.
where
D
G° is the free energy change of the CT complexes (kJ molꢂ1),
and 5, respectively. The formation of CT complexes during the
reaction of Res with QL, PA, TCNQ or DDQ is strongly supported
by observing of main infrared bands of the Res donor and acceptors
in the product spectra. However, the bands of the donor and accep-
tors in the complexes spectra reveal small in frequency and
changes in their band intensities compared with those of the free
donor and acceptors. This result could be attributed to the ex-
pected changes in symmetry and electronic structure changes
upon the formation of the CT complexes. The IR spectra of the
[(Res)(QL)] and [(Res)(PA)] complexes are characterized by a broad
medium-to-weak band that appears between 2400 and 2800 cmꢂ1
(2730 cmꢂ1 for the QL complex; 2790 cmꢂ1 for the PA complex),
which does not appear in the spectra of the free Res donor or those
of the QL and PA acceptors. These broadened peaks are attributed
to the stretching vibration of the intermolecular hydrogen bond
formed through the transfer of proton from the acidic center on
the QL and PA acceptors to the basic center of the Res donor (lone
pair of electrons on the N (4) atom) to form +NH based on acid–
base theory [32–38].
R is the gas constant (8.314 J molꢂ1 Kꢂ1), T is the absolute tempera-
ture in K, and KCT is the formation constant of the complex (L molꢂ1
at room temperature.
)
The calculated spectroscopic and physical values (f,
l, IP, RN and
D
G°) for the Res CT complexes using these Eqs. (2)–(8) are pre-
sented in Table 1. The [(Res)(TCNQ)] complex exhibits considerably
higher values of both oscillator strength (f) and transition dipole
moment (l) compared to the other complexes, indicating a strong
interaction between the drug Res and the TCNQ acceptor. TCNQ is a
strong electron acceptor to form stable CT complexes with the do-
nors. One important aspect of characterizing CT complexes is the
calculation of the ionization potential (IP) of the donor. The calcu-
lated IP value for the highest filled molecular orbital that partici-
pates in the CT interaction of the Res is approximately (10.38).
Further evidence for the nature of CT interactions is the calculation
of the standard free energy change (DG°). The obtained values of
D
G° for the [(Res)(QL)], [(Res)(PA)], [(Res)(TCNQ)] and [(Res)
(DDQ)] are ꢂ33, ꢂ31, ꢂ33 and ꢂ35 kJ molꢂ1, respectively; these
negative values indicate that the interaction between the Res and
the acceptors is spontaneous [30,31].
The IR and Raman spectra of the [(Res)(TCNQ)] and [(Res)(DDQ)]
complexes indicated that the band that results from the m(C„N)
vibration of the free TCNQ and DDQ acceptors changed in frequen-
cies and decrease in intensities in the complexes upon CT complex-
Interpretation of IR and Raman spectra
ation. Free TCNQ shows one
in its complex; (C„N) occurs at lower wavenumber values (IR/Ra-
man; 2188/2230 cmꢂ1). In free DDQ,
(C„N) occurs at 2231 and
2250 cmꢂ1, while in the complex it moved to (IR/Raman; 2226/
2258 cmꢂ1). It clear that
(C„N) of TCNQ and DDQ are decreased
m
(C„N) vibration at 2220 cmꢂ1, while
m
Infrared and Raman spectral studies shed light on the donation
location in the donor species, and the differences occur in the
spectra of the obtained CT complexes. The IR absorption spectra
of the Res solid CT complexes were registered in the frequency
range 4000–400 cmꢂ1 using KBr disc and their peak assignments
for the important characteristic bands are given in Table 2. The full
IR and Raman spectra of the Res CT complexes are shown in Figs. 4
m
m
upon complexation. The characteristic band of (ANH) group ob-
served at 3440 cmꢂ1 in the free Res donor [39–41], is shifted in
the two complexes and its intensities are affected. The outlined

H.H. Eldaroti et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 115 (2013) 309–323
315
[(Res)(QL)] complex
[(Res)(PA)] complex
3500
3000
2500
2000
1500
1000
500
3500
3000
2500
2000
1500
1000
500
Wavenumbers (cm-1)
Wavenumbers (cm-1)
[(Res)(TCNQ)] complex
[(Res)(DDQ)] complex
3500
3000
2500
2000
1500
1000
500
3500
3000
2500
2000
1500
1000
500
Wavenumbers (cm-1)
Wavenumbers (cm-1)
Fig. 4. Infrared spectra of Res charge-transfer complexes.
changes in
m
(NH) upon complexation clearly confirm the involve-
NMR spectra of the Res complexes were measured in DMSO-d6 at
room temperature and are given in Fig. 6. The chemical shifts (d) of
the different types of protons of the CT complexes are;
ment of the nitrogen atom of the (ANH) group in the Res donor
through the CT interaction process. These observations clearly indi-
cate that the (ANH) group in the Res donor and the (AC„N) group
in the TCNQ or DDQ acceptors participated in the complexation
process. Because TCNQ and DDQ lacks acidic centers, the molecular
complexes can be concluded to form through
charge migration from the HOMO of the donor to the LUMO of the
acceptor. The
pꢁ CT complex is formed via the benzene ring
(electron-rich group) of the Res and the TCNQ or DDQ reagents
(electron acceptor) [42]. The cyano group (AC„N) is an electron-
withdrawing group that exists in TCNQ and DDQ in a conjugated
bonding system. The CN groups in TCNQ and DDQ withdraw elec-
trons from the aromatic ring, and such a process will make the aro-
matic ring an electron-accepting region. The pꢁ–CN electron density
appears to increase and more easily accept a proton from the donor
because of the electron-withdrawing process and the conjugated
electron system. So, the interactions mode between Res and the
TCNQ and DDQ acceptors also occur through the migration of a
H+ ion to one of the cyano groups in the acceptors to form a positive
ion (AC„N+H) that associates with the anion to form ion pairs
[38,43,44].
[(Res)(QL)] complex; d = 1.06, 1.12, 1.28, 1.31, 1.80 (5s, 15H,
5CH3 at C(11, 17,) and 3,4,5-trimethoxybenzoate), 1.91–1.97 ((dd,
2H, J = 9.2, 9.2 Hz (CH2) at C(14)), 2.67 (d, 2H, J = 3.6 Hz, CH2 at
C(6)), 2.70 (d, 2H, J = 3.2 Hz, CH2 at C(21)), 2.79 (d, 2H, J = 3.6 Hz,
CH2 at C(5)), 2.88 (t, 1H, J = 5.2, 4.4 Hz, CH at C(3)), 3.31 (s, 3H,
COOCH3), 3.38–3.40 (dd appears t, 2H, 2CH at C(16 and 19)), 3.53
(s, 1H, NH+ Hydrogen bonded), 3.63–3.68 (dd, 2H, 2CH at C(17 and
18)), 5.05–5.19(2 m, 2H, 2CH at C(15 and20)), 6.06(s, 1H, indolering
protonsatC(7)), 6.37–6.39(2d, 2H, J = 9.2, 9.8 Hz, indoleringprotons
at C(4 and 5)), 6.52 (s, 4H, quinol ring protons), 6.55 (s, 2H, protons at
C(2 and 6) benzoate ring, 8.59 (s, 2H, ArAOH, phenolic OH).
[(Res)(PA)] complex; d = 1.06, 1.12, 1.28, 1.31, 1.80 (5s, 15H,
5CH3 at C(11, 17,) and 3,4,5-trimethoxybenzoate), 1.91–1.97 ((dd,
2H, J = 9.2, 9.2 Hz (CH2) at C(14)), 2.66 (d, 2H, J = 3.6 Hz, CH2 at
C(6)), 2.70 (d, 2H, J = 3.2 Hz, CH2 at C(21)), 2.79 (d, 2H, J = 3.6 Hz,
CH2 at C(5)), 2.88 (t, 1H, J = 5.2, 4.4 Hz, CH at C(3)), 3.39 (s, 3H,
COOCH3), 3.52 (s, 1H, NH+ Hydrogen bonded), 3.62–3.68 (dd, 2H,
2CH at C(16 and 19)), 4.83–4.87 (dd, 2H, 2CH at C(17 and 18)),
5.05–5.19 (2 m, 2H, 2CH at C(15 and 20)), 6.95 (s, 1H, indole ring
protons at C(7)), 7.08–7.20 (2d, 2H, J = 9.2, 9.8 Hz, indole ring
protons at C(4 and 5)), 7.28 (s, 2H, protons at C(2 and 6) benzoate
ring, 8.59 (s, 2H, picric acid protons).
p ?
pꢁ and/or n ? pꢁ
p
?
Interpretation of 1H NMR spectra
The nuclear magnetic resonance spectra present the persuasive
confirmation of the complexation pathway. Thus, the 400 MHz 1H
[(Res)(TCNQ)] complex; d = 1.06, 1.12, 1.29, 1.31, 1.80 (5s, 15H,
5CH3 at C(11, 17,) and 3,4,5-trimethoxybenzoate), 1.91–1.97

316
H.H. Eldaroti et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 115 (2013) 309–323
[(Res)(QL)] complex
[(Res)(PA)] complex
0.015
0.012
0.009
0.006
0.003
0.000
0.06
0.05
0.04
0.03
0.02
0.01
0.00
3500
3000
2500
2000
1500
1000
500
0
3500
3000
2500
2000
1500
1000
500
0
Wavenumbers (cm-1)
Wavenumbers (cm-1)
[(Res)(DDQ)] complex
[(Res)(TCNQ)] complex
0.025
0.020
0.015
0.010
0.005
0.000
0.020
0.015
0.010
0.005
0.000
3500
3000
2500
2000
1500
1000
500
0
3500
3000
2500
2000
1500
1000
500
0
-1
-1
Wavenumbers (cm )
Wavenumbers (cm )
Fig. 5. Raman spectra of Res charge-transfer complexes.
((dd, 2H, J = 8.8, 8.4 Hz (CH2) at C(14)), 2.67 (d, 2H, J = 3.6 Hz, CH2
at C(6)), 2.70 (d, 2H, J = 3.2 Hz, CH2 at C(21)), 2.79 (d, 2H,
J = 3.6 Hz, CH2 at C(5)), 2.88 (t, 1H, J = 5.2, 4.4 Hz, CH at C(3)),
3.53 (s, 1H, NH+ Hydrogen bonded), 3.63–3.67 (dd, 2H, 2CH at
C(16 and 19)), 4.83(dd, 2H, 2CH at C(17 and 18)), 4.88 (s, 3H,
COOCH3), 5.05–5.19 (2 m, 2H, 2CH at C(15 and 20)), 6.80 (s, 1H, in-
dole ring protons at C(7)), 7.03–7.18 (2d, 2H, J = 9.2, 9.8 Hz, indole
ring protons at C(4 and 5)), 7.28 (s, 2H, protons at C(2 and 6) ben-
zoate ring, 7.77 (s, 4H, ArAH, TCNQ protons).
[(Res)(DDQ)] complex; d = 1.06, 1.12, 1.28, 1.31, 1.72 (5s, 15H,
5CH3 at C(11, 17,) and 3,4,5-trimethoxybenzoate), 1.91–1.97 ((dd,
2H, J = 8.8, 8.4 Hz (CH2) at C(14)), 2.66 (d, 2H, J = 3.6 Hz, CH2 at
C(6)), 2.70 (d, 2H, J = 3.2 Hz, CH2 at C(21)), 2.79 (d, 2H, J = 3.6 Hz,
CH2 at C(5)), 2.87 (t, 1H, J = 5.2, 4.4 Hz, CH at C(3)), 3.52 (s, 1H,
NH+ Hydrogen bonded), 3.62–3.67 (dd, 2H, 2CH at C(16 and 19)),
4.83–4.87 (dd, 2H, 2CH at C(17 and 18)), 4.87 (s, 3H, COOCH3),
5.05–5.19 (2 m, 2H, 2CH at C(15 and 20)), 6.80 (s, 1H, indole ring
protons at C(7)), 7.03–7.18 (2d, 2H, J = 9.2, 9.8 Hz, indole ring pro-
tons at C(4 and 5)), 7.28 (s, 2H, protons at C(2 and 6) benzoate ring.
In the [(Res)(QL)] complex, the phenolic proton (AOH) signal,
which is observed at approximately 8.50 ppm in the spectrum of
the QL acceptor, decreased in intensity with a downfield shift for
the non-hydrogen-bonded one (8.59 ppm) in the spectrum of the
CT complex due to deprotonation from acceptor-to-donor. Besides,
the appearance of a new weak broad band at 3.53 ppm, due to the
formation of (+NH), indicate the involvement of the N (4) atom of
Res donor and (AOH) group of QL acceptor in chelating through
the transfer of the phenolic proton of QL acceptor to the N (4) atom
of Res donor. It has been found in the [(Res)(PA)] complex that, the
peak at d = 11.94 ppm, which is assigned to the AOH proton of the
free picric acid acceptor [45], was absent in the spectrum of this
complex, which is attributed to the formation of the CT complex.
Instead, a new peak is observed at 3.52 ppm in the complex, which
is not detected in the spectrum of the free donor, attributing to the
proton of (+NH). Together, these data confirm the formation of
intermolecular hydrogen bond between PA and Res [46]. The mode
of chelation concerning [(Res)(TCNQ)] complex was sustained by
the presence of a new signal at 3.53, which is assigned to the pro-
ton of (+NH), and the disappearance of the distinguish signal of H
proton of ANH which exist in the free Res as broad single peak
at d = 7.70 ppm [47]. These observations indicate that the (ANH)
and (AC„N) groups are primarily involved in the formation of
the CT complex between Res and TCNQ. The migration of the H+
ion from the (ANH) group in the Res donor to one of the four cyano
groups in the TCNQ acceptor resulted in the formation of a positive
ion (AC„N+H), which is associated with the anion Nꢂ. It is clearly
obvious in [(Res)(DDQ] complex that, the new signal observed at
3.52 in the spectrum of this complex, which is attributed to the
proton of (+NH), confirm that the (ANH) and (AC„N) groups are
primarily involved in the formation of the CT complex.
Suggested structure of Res CT complexes
Based on the results obtained from elemental analyses, spectro-
photometric titration, infrared and 1H NMR spectra, the suggested

H.H. Eldaroti et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 115 (2013) 309–323
317
Fig. 6. 1H NMR spectrum of (A) [(Res)(QL)], (B) [(Res)(PA)], (C) [(Res)(TCNQ)] and (D) [(Res)(DDQ)] complexes.
OH
NO2
O2N
H
O
O
CH3
CH3
O
O
N
O
O
H
NO2
O
O
O
CH3
CH3
O
O
O
H3C
N
O
N
H
H3C
CH3
O
N
H
O
O
O
CH3
CH3
H3C
O
O
O
CH3
H3C
Scheme 2. Suggested structure of [(Res)(QL)] complex.
Scheme 3. Suggested structure of [(Res)(PA)] complex.
structure for the [(Res)(QL)], [(Res)(PA)], [(Res)(TCNQ)] and
[(Res)(DDQ)] complexes are given in Schemes 2–5, respectively.
Res donor and acceptors and thermal stability of the new CT com-
plexes, the thermogravimetric analysis of the Res CT complexes
were carried out over the temperature range of 25–800 °C under
an air atmosphere using 10.15, 10.56, 8.50 and 9.75 mg samples
for [(Res)(QL)], [(Res)(PA)], [(Res)(TCNQ)] and [(Res)(DDQ)] com-
plexes, respectively. The TG curves were redrawn as mg mass loss
versus temperature. Fig. 7 shows the thermograms for Res CT com-
plexes and thermal analyses data are listed in Table 3. The overall
Thermal analysis studies
The thermogravimetric analysis (TG) provided information
about the thermal stabilities of the prepared CT complexes and
about the differences in the physical behavior of the starting and
resulting compounds. In order to verify CT interaction between

318
H.H. Eldaroti et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 115 (2013) 309–323
of the complex began at ꢆ141 °C and finished at ꢆ600 °C. The
thermal decomposition of the complex occurs completely in three
steps within the range of 25–800 °C. The first mass loss step occurs
between 25 and 295 °C corresponds to the loss of C2H2, 2CO2 and
2NO2 molecules with a weight loss of 29.53%. The second degrada-
tion step at 295–481 °C is attributed to the loss of 10C2H2 and 6CO2
molecules. This step is associated with a total weight loss of
53.25%, which is in good agreement with the calculated value
(52.87%). The final decomposition step was detected within the
range of 481–800 °C corresponding to the loss of 2C2H2 and 10H2
molecules, representing a weight loss of 9.55%, with remaining a
few carbon atoms as a residual. The thermal analysis curve of [(Re-
s)(PA)] complex indicates that decomposition takes places in two
clear decomposition steps within the 25–800 °C temperature
range. The first decomposition step within the temperature range
25–320 °C has a weight loss about 64.52% very close to the
expected theoretical value of 64.46% and is attributed to the forma-
tion of 6C2H2, 6CO2 and 4NO2 molecules. The second decomposi-
tion stage existed within the 320–800 °C temperature range
(obs. = 24.19%, calc. = 23.99%), which is assigned by the removal
of 6C2H2, CO2, NH3 and 8H2 molecules, then leaving residual car-
bon as final fragment. The thermal TG curve of the [(Res)(TCNQ)]
complex indicates that the decomposition occurs in four main
stages in the temperature range of 25–800 °C. The first decomposi-
tion step within the temperature range 25–143 °C corresponding
to loss of 2HCN and CO2 molecules representing a weight loss of
10.08% very close to the expected theoretical value of 10.09%.
The second decomposition step found within the temperature
O
O
CH3
CH3
O
N
O
H3C
O
N
O
CH3
NH
O
O
CH3
NC
C
H3C
O
NC
CN
Scheme 4. Suggested structure of [(Res)(TCNQ)] complex.
O
O
CH3
CH3
O
N
O
O
H3C
Cl
O
N
O
CH3
O
NH
O
CH3
H3C
O
CN
Cl
O
Scheme 5. Suggested structure of [(Res)(DDQ)] complex.
loss of mass from the TG curves is 92.32% for [(Res)(QL)], 88.71% for
[(Res)(PA)], 76.79% for [(Res)(TCNQ)] and 91.06% for [(Res)(DDQ)].
The obtained data indicate that the [(Res)(QL)] complex is ther-
mally stable in the 25–141 °C temperature range. Decomposition
10
8
10
8
6
6
4
4
2
2
[(Res)(PA)] complex
[(Res)(QL)] complex
0
0
0
100
200
300
400
500
600
700
800
0
100
200
300
400
500
600
700
800
Temp [ OC]
Temp [ OC]
10
8
8
6
4
2
6
4
2
[(Res)(TCNQ)] complex
[(Res)(DDQ)] complex
0
0
100
200
300
400
500
600
700
800
0
100
200
300
400
Temp [ OC]
500
600
700
800
Temp [ OC]
Fig. 7. TG curves of Res charge-transfer complexes.

H.H. Eldaroti et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 115 (2013) 309–323
319
Table 3
Thermal decomposition data for the Res CT complexes.
Complex
Stage
TG range (°C)
Weight loss (%)
Found
Evolved moiety
Calculated
[(Res)(QL)] C39H46N2O11
I
II
25–295
295–481
481–800
–
29.53
53.25
9.55
28.66
52.87
10.02
8.35
C2H2 + 2CO2 + 2NO2
10C2H2 + 6CO2
2C2H2 + 10H2
III
Residue
7.68
Residual carbon
[(Res)(PA)] C39H43N5O16
[(Res)(TCNQ)] C45H44N6O9
I
II
25–320
320–800
–
64.52
24.19
11.29
64.46
23.99
11.46
6C2H2 + 6CO2 + 4NO2
6C2H2 + CO2 + NH3 + 8H2
Residual carbon
Residue
I
II
III
IV
25–143
143–321
321–576
576–800
–
10.08
15.12
34.82
16.76
23.21
10.09
15.01
34.94
16.24
23.62
2HCN + CO2
2HCN + C2H2 + NO2 + CO2
5C2H2 + 5CO2 + NO2
4C2H2 + 2CO2 + 10H2
Residual carbon
Residue
[(Res)(DDQ)] C41H40Cl2N4O11
I
II
III
IV
25–222
222–312
312–601
601–800
–
11.70
29.67
31.83
17.86
8.93
11.73
30.04
32.31
17.23
8.62
Cl2 + HCN
HCN + 3C2H2 + 3CO2 + NO2
6C2H2 + 3CO2 + NO2
4C2H2 + CO2 + 6H2
Residue
Residual carbon
range 143–321 °C which is assigned by the removal of 2HCN, C2H2,
NO2 and CO2 molecules representing a weight loss of 15.12% very
close to the expected theoretical value of 15.01%. The third decom-
position step within the temperature range 321–576 °C has an
extremely large scale of weight loss for about 34.82% may be
attributed to the liberation of 5C2H2, 5CO2 and NO2 molecules.
The last decomposition step has a weight loss about 16.76%, which
are reasonably by the loss of 4C2H2, 2CO2 and 10H2 molecules. Few
-0.6
-0.8
-1.0
-1.2
-1.4
-12.4
-12.8
-13.2
-13.6
-14.0
[(Res)(QL)] complex, (HM)
[(Res)(QL)] complex, (CR)
-1.6
-12
-10
-8
-6
-4
-2
0
2
3.12
3.15
3.18
3.21
3.24
3.27
1000/T (K -1
)
θ (Κ)
-12.0
-0.2
-0.3
-0.4
-0.5
-0.6
-0.7
-0.8
-12.2
-12.4
-12.6
-12.8
-13.0
-13.2
-13.4
[(Res)(PA)] complex, (HM)
[(Res)(PA)] complex, (CR)
-10
-5
0
5
10
15
2.00
2.02
2.04
2.06
2.08
2.10
1000/T (K-1
)
θ (Κ)
Fig. 8. Kinetic curves for Res charge-transfer complexes (CR; Coats–Redfern equation, HM; Horowitz–Metzger (HM) equation).

320
H.H. Eldaroti et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 115 (2013) 309–323
-11.4
-11.6
-11.8
-12.0
-12.2
-12.4
-12.6
-0.3
-0.4
-0.5
-0.6
-0.7
-0.8
[(Res)(TCNQ)] complex, (CR)
[(Res)(TCNQ)] complex, (HM)
2.950 2.975 3.000 3.025 3.050 3.075 3.100
-8
-6
-4
-2
0
2
4
6
8
10
12
1000/T (K-1)
θ (Κ)
-12.4
-12.6
-12.8
-13.0
-13.2
-0.3
-0.4
-0.5
-0.6
-0.7
[(Res)(DDQ)] complex, (HM)
[(Res)(DDQ)] complex, (CR)
-10
-5
0
5
10
15
20
1.88
1.90
1.92
1.94
1.96
1.98
2.00
2.02
1000/T (K-1)
θ (Κ)
Fig. 8 (continued)
Table 4
Kinetic parameters determined using the Coats–Redfern (CR) and Horowitz–Metzger (HM).
a
Complexes
Stage
Method
Parameters
r
Eꢁ
A
D
Sꢁ
D
Hꢁ
D
Gꢁ
[(Res)(QL)]
1st
1st
1st
1st
CR
HM
6.98 ꢃ 104
1.18 ꢃ 109
ꢂ7.19 ꢃ 101
6.71 ꢃ 104
9.03 ꢃ 104
0.99580
0.99340
7.51 ꢃ 104
2.98 ꢃ 1010
ꢂ4.51 ꢃ 101
7.24 ꢃ 104
8.70 ꢃ 104
[(Res)(PA)]
CR
HM
8.57 ꢃ 104
9.66 ꢃ 104
3.42 ꢃ 109
2.77 ꢃ 1010
ꢂ6.46 ꢃ 101
ꢂ4.72 ꢃ 101
8.25 ꢃ 104
8.73 ꢃ 104
1.08 ꢃ 105
1.06 ꢃ 105
0.99840
0.99670
[(Res)(TCNQ)]
[(Res)(DDQ)]
CR
HM
1.16 ꢃ 105
1.32 ꢃ 1017
8.23 ꢃ 101
1.13 ꢃ 105
8.68 ꢃ 104
0.99730
0.99700
1.25 ꢃ 105
1.11 ꢃ 1019
1.19 ꢃ 102
1.22 ꢃ 105
8.42 ꢃ 104
CR
HM
1.10 ꢃ 105
1.18 ꢃ 105
5.59 ꢃ 109
6.98 ꢃ 1010
ꢂ6.24 ꢃ 101
ꢂ4.14 ꢃ 101
1.06 ꢃ 105
1.14 ꢃ 105
1.36 ꢃ 105
1.35 ꢃ 105
0.99670
0.99920
a
Units of parameters: E in kJ molꢂ1, A in sꢂ1
,
D
S in J molꢂ1 Kꢂ1
,
D
H and
D
G in kJ molꢂ1
.
carbon atoms remain in the final step as a final residual. The
[(Res)(DDQ)] complex was thermally decomposed in four
successive decomposition steps within the temperature range of
25–800 °C. The first-to-fourth decomposition steps occur at
25–143, 143–321, 321–576 and 576–800 °C, with weight loss of
11.70%, 29.67%, 31.83% and 17.86%, respectively. The total percent-
age of weight loss observed 91.06% matched with the calculated
weight loss 91.31% and corresponds to the loss of C41H40Cl2N4O11
(organic moiety). The decomposition of this complex ended with
a carbon as a final residue.
Kinetic studies
In recent years, there has been increasing interest in determin-
ing the rate-dependent parameters of solid-state non-isothermal
decomposition reactions by analysis of TG curves. Several
equations have been proposed as means of analyzing a TG curve
and obtaining values for kinetic parameters. Two major different
methods were used to evaluate the kinetic thermodynamic param-
eters: the Coats–Redfern method [48] and the Horowitz–Metzger
method [49].

H.H. Eldaroti et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 115 (2013) 309–323
321
For convenience of integration, the lower limit T1 is usually taken as
zero. After integration, this equation can be represented as
Ln½ꢂ lnð1 ꢂ
where
a
Þ=T2ꢄ ¼ ꢂEꢁ=RT þ ln½AR=
u
Eꢁꢄ
ð10Þ
a
is the fraction of the sample decomposed at time t, T is the
derivative peak temperature, A is the frequency factor, R is the gas
constant, Eꢁ is the activation energy, and
u
is the linear heating rate.
A plot of the left-hand side (LHS) against 1/T was constructed and
was found to be linear. Eꢁ is the energy of activation in kJ molꢂ1
and was calculated from the slope. The A (sꢂ1) value was calculated
from the intercept. The entropy of activation,
was calculated using the equation:
D )
Sꢁ, in (J Kꢂ1 molꢂ1
D
Sꢁ ¼ RlnðAh=kTsÞ
ð11Þ
Fig. 9. X-ray diffraction pattern for [(Res)(QL)] and [(Res)(PA)] complexes.
where k is the Boltzmann constant, h is Planck’s constant, and Ts is
the DTG peak temperature.
Table 5
XRD spectral data of [(Res)(QL)] and [(Res)(PA)] complexes.
Horowitz–Metzger equation
The Horowitz–Metzger equation (Eq. (12)) was written in the
following form:
Complex
2h (°)
d value
b;
Relative
Particle size
(nm)
(Å)
FWHM
intensity (%)
log½logðwa=wcÞꢄ ¼ Eꢁh=2:303RT2 ꢂ log 2:303
ð12Þ
[(Res)(QL)] 21.153 4.197
[(Res)(PA)] 22.632 3.926
0.210
0.325
100
100
7.016
4.831
s
where h = T ꢂ Ts, w = w ꢂ w, w is the mass loss at the completion
c
a
a
of the reaction, and w is the mass loss at time t.
Coats–Redfern equation
The Coats–Redfern Eq. (9), which is an atypical integral method,
can be represented as:
The plot of log [log (w /w )] versus h was constructed and was
a
c
observed to be linear, and Eꢁ was calculated from its slope. The pre-
exponential factor, A, was calculated from the following equation:
Z
Z
ꢁ
n
da
=ð1 ꢂ
a
Þ ¼ ðA=
u
Þ
eꢂE =RT dT
ð9Þ
Eꢁh=RT2s ¼ A=½
u
expðꢂEꢁ=RTsÞꢄ
ð13Þ
0!1
T1!T2
Fig. 10. SEM images and EDX spectrum of (A) [(Res)(QL)], (B) [(Res)(PA)], (C) [(Res)(TCNQ)] and (D) [(Res)(DDQ)] complexes.

322
H.H. Eldaroti et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 115 (2013) 309–323
From the TG curves, the activation energy, Eꢁ, the entropy of activa-
tion,
Sꢁ, the enthalpy of activation, Hꢁ, and the Gibbs free energy,
Gꢁ, were calculated from;
SEM and EDX studies
D
D
D
The morphological features of the Res CT complexes were
investigated using scanning electron microscopy (SEM). SEM pro-
vides general information about the microstructure, surface mor-
phology, the particle size, the chemical composition, and the
porous structures of the surfaces. In addition, the chemical compo-
sitions of the complexes were determined using energy-dispersive
X-ray diffraction (EDX). Fig. 10 shows the SEM surface images of
the [(Res)(QL)], [(Res)(PA)], [(Res)(TCNQ)] and [(Res)(DDQ)] com-
plexes along with their EDX spectra. The formed complexes
showed a homogeneous distribution of each acceptor. The sizes
of the particles are quite different with different acceptors. The
morphological phases of these complexes have a homogeneous
matrix, as indicated by the uniformity and similarity of the parti-
cles of the synthesized CT complexes. The complexes are semi-
crystalline, and some single-phase formations exhibit well-defined
shapes. The peaks refer to all elements that constitute the mole-
cules of these complexes; these elements were clearly identified,
and the results confirmed the proposed structures.
D
Hꢁ ¼ Eꢁ ꢂ RT and
D
Gꢁ ¼
D
Hꢁ ꢂ T
D
Sꢁ
The kinetic thermodynamic parameters for the decomposition
of the Res CT complexes; the activation energy (Eꢁ), the frequency
factor (A), the enthalpy of activation (Hꢁ), the entropy of activation
(Sꢁ) and the free energy of activation (Gꢁ) were evaluated graphi-
cally (Fig. 8) by employing the Coats–Redfern and Horowitz–
Metzger methods, and the evaluated data are listed in Table 4.
The results indicate that the kinetic data obtained from the two
methods are comparable and in agreement with each other. The
activation energy (Eꢁ) of the complexes is expected to increase with
increasing thermal stability of complexes. Hence, the Eꢁ value for
[(Res)(TCNQ)] complex exhibit a higher activation energy value
than other complexes, which indicates the higher thermal stability
of the [(Res)(TCNQ)] complex. Comparing the Eꢁ values for the
main decomposition stage of the Res CT complexes gave the order
TCNQ > DDQ > PA > QL for the different acceptors. These differ-
ences may be caused by the reactivity of the complexes and the
electronic configuration of the acceptor when attached to Res do-
nor. The calculated Eꢁ values using the Coats–Redfern and Horo-
witz–Metzger methods for the main decomposition stage of the
complexes are found to be 7.25 ꢃ 104 kJ molꢂ1 for [(Res)(QL)],
9.12 ꢃ 104 kJ molꢂ1 for [(Res)(PA)], 1.21 ꢃ 105 kJ molꢂ1 for
[(Res)(TCNQ)] and 1.14 ꢃ 105 kJ molꢂ1 for [(Res)(DDQ)] complex.
Conclusion
Reserpine (Res) is a biologically active naturally occurring drug
used medicinally to lower blood pressure. This paper described the
CT complexes of Res with four acceptors (QL, PA, TCNQ and DDQ).
The synthesized CT complexes were characterized using various
spectroscopic techniques including UV–Visible, IR, Raman, 1H
NMR spectroscopy and X-ray diffraction as well as scanning elec-
tron microscopy and thermogravimetric (TG) analyses. It is ob-
served that the reaction stoichiometry is 1:1, and the resulting
CT complexes were shown to have the general formula:
[(Res)(acceptor)]. The obtained complexes are nanoscale, semi-
crystalline material, thermally stable and spontaneous. Physical
and kinetic parameters such as formation constant (KCT), molar
extinction coefficient (eCT) and other spectroscopic data have been
estimated. The results obtained are satisfactory for estimation of
drug Res in the pharmaceutical form.
The negative values of
and [(Res)(DDQ)] complexes indicate that the reaction rate is
slower than normal [50]. The
Sꢁ values of the Res CT arranged
D
Sꢁ observed for [(Res)(QL)], [(Res)(PA)]
D
in decreasing order is [(Res)(TCNQ)] > [(Res)(DDQ)] > [(Re-
s)(PA)] > [(Res)(QL)]. The satisfactory values for the correlation
coefficients from the Arrhenius plots of the thermal decomposition
steps were observed to be r ꢆ 1 in all cases, which indicates a good
fit with the linear function and reasonable agreement between the
experimental data and the kinetic parameters. The thermal decom-
position process of all of the Res CT complexes is spontaneous, i.e.,
the complexes are thermally stable.
X-ray powder diffraction studies
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