Spectroscopic studies on the formation of molecular complexes of sulfamethoxazole with novel 2,3,5-trichloro-6-alkoxy-1,4-benzoquinones

Article abstract of DOI:10.1016/j.molstruc.2012.09.062

UV-Vis, ¹H NMR, FT-IR, LC-MS and fluorescence spectral techniques were employed to investigate the mechanism of interaction of sulfamethoxazole with alkoxy substituted 2,3,5-trichloro-1,4-benzoquinones and to characterize the reaction products. The interactions of these quinones with sulfamethoxazole (SULF) were found to proceed through the formation of donor-acceptor complex, containing radical anion and its conversion to the product. Fluorescence quenching studies indicated that the interaction between the donor and the acceptors are spontaneous. Correlation of association constants of the CT complexes with Taft's polar and steric constants indicated that polar factor plays a significant role in governing the reactivity. The results indicated that the electronic effects of the substituents play significant role in governing the reactivity of the quinones when compared to steric factor.

Full text of DOI:10.1016/j.molstruc.2012.09.062

Journal of Molecular Structure 1033 (2013) 312–320
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Spectroscopic studies on the formation of molecular complexes
of sulfamethoxazole with novel 2,3,5-trichloro-6-alkoxy-1,4-benzoquinones
⇑
K. Ganesh, C. Balraj, A. Satheshkumar, K.P. Elango
Department of Chemistry, Gandhigram Rural Institute (Deemed University), Gandhigram 624 302, India
h i g h l i g h t s
"
"
"
New 2,3,5-trichloro-6-alkoxy-1,4-benzoquinones were employed as acceptors in CT interaction.
The mechanism/structure of the products were characterized using various spectral techniques.
Electronic effects of the substituents determine electron accepting property of quinones.
a r t i c l e i n f o
a b s t r a c t
UV–Vis, 1H NMR, FT-IR, LC–MS and fluorescence spectral techniques were employed to investigate the
mechanism of interaction of sulfamethoxazole with alkoxy substituted 2,3,5-trichloro-1,4-benzoquinon-
es and to characterize the reaction products. The interactions of these quinones with sulfamethoxazole
(SULF) were found to proceed through the formation of donor–acceptor complex, containing radical
anion and its conversion to the product. Fluorescence quenching studies indicated that the interaction
between the donor and the acceptors are spontaneous. Correlation of association constants of the CT
complexes with Taft’s polar and steric constants indicated that polar factor plays a significant role in gov-
erning the reactivity. The results indicated that the electronic effects of the substituents play significant
role in governing the reactivity of the quinones when compared to steric factor.
Article history:
Received 7 July 2012
Received in revised form 23 September
2012
Accepted 24 September 2012
Available online 2 October 2012
Keywords:
Charge transfer
Sulfamethoxazole
Substituent effect
Fluorescence
Ó 2012 Elsevier B.V. All rights reserved.
Theoretical studies
Taft correlation
1. Introduction
to determine whether CT phenomenon at any level involved, the
ability of the donor drugs and related compounds to form charge
In recent years considerable attention has been given to study
the charge transfer (CT) or electron donor acceptor (EDA) com-
transfer complexes with acceptors should be studied.
Quinones are important class of organic molecules which nota-
bly plays an essential role in oxido-reduction reactions such as
respiration and photosynthesis [17]. Quinones are capable of
accepting one or two electrons to form the corresponding radical
anion (QÅꢀ) and hydroquinone dianion (Q2ꢀ). The biological activity
of the quinones is reportedly due to the redox chemistry of the qui-
none system [18]. It is well known that a modification in the direct
substitution pattern on the quinone ring impact its capability to
accept electrons and thus its capacity to mediate biological reac-
tions [19–21]. The key electron acceptors in the photosynthetic
processes are menoquinones, plastoquinones and ubiquinones
are exist in the form of substituted 1,4-benzoquinone module
[22,23]. Also, these naturally occurring quinones possess variable
number of alkoxy substituents in their units. Although good
amount of work, on the study of charge transfer (CT) complexes
of number of quinones with variety of donors, has been carried
out [24–27], reports related to quinones with systematic variation
of substituents is rare in literature.
plexes formed between organic donor compounds and
r or p
acceptors [1–5]. This growing importance is due its interesting
optical and electronic properties [6]. CT phenomenon has enor-
mous applications in the field of organic light emitting diodes
[7], organic photovoltaics [8], sensors [9], organic field effect tran-
sistor [10], nonlinear optics [11], magnetic materials [11], organic
solar cells [12], xerogel nanoparticles [13] and in the quantitative
estimation of drugs [14]. Drug–receptor mechanism can also be ex-
plained by CT phenomenon in addition to weak interactions such
as hydrogen bonding and hydrophobic interactions together with
van der Waal’s forces like dipole–dipole, dipole induced dipole
and dispersion interactions. Understanding the drug–receptor
mechanism is a complex issue, but collection of data of the interac-
tions like CT, hydrogen bonding even in organic solvents can shed
some light on drug–receptor mechanism [15,16]. As a primary step
⇑
Corresponding author. Tel.: +91 451 245 2371; fax: +91 451 245 4466.
E-mail address: drkpelango@rediffmail.com (K.P. Elango).
0022-2860/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molstruc.2012.09.062

K. Ganesh et al. / Journal of Molecular Structure 1033 (2013) 312–320
313
Thus, the mechanism of interaction of quinone with drugs, in
general, is a research topic of significant interest and hence the
present study. The objective, therefore, of the present article is to
study the spectral, thermodynamic and kinetic aspects of the inter-
action of 1,4-benzoquinones with variable alkyl chain of alkoxy
groups with sulfamethoxazole (SULF) drug with an aim to investi-
gate the mechanism of the interaction and to characterize the
structure of the products formed in these interactions. In general,
drugs are poly-functional organic molecules and the present study
aims at to investigate the actual site of attack during the formation
of charge transfer complexes. Such a study would undeniably shed
some light on to the mechanism of the drug action in real pharma-
cokinetic study. Sulfamethoxazole is chemically known as 4-ami-
no-N-(5-methylisoxazol-3-yl)-benzene sulfonamide which is used
as an antibacterial agent and to treat urinary tract infections
[27,28]. It can also used in the treatment of sinusitis, toxoplasmosis
and pneumocystis pneumonia which affects primarily patients
with HIV. Though these chosen 1,4-benzoquinones are known to
organic chemists as intermediates, it is the first systematic attempt
to utilize them as acceptors with the drug. Such a structural varia-
tion of the quinones would certainly helps to tune the redox chem-
istry of them and hence its biological activity. Attempts have also
been made to investigate the effect of substituents on the CT inter-
action using the techniques of correlation analysis by employing
Taft’s substituent constants.
Solutions for the spectroscopic measurements were prepared
by dissolving accurately weighed amounts of donor (D) and accep-
tor (A) in the appropriate volume of solvent immediately before
running the spectra. The electronic absorption spectra were re-
corded on a JASCO (V 630, Japan) UV–Vis double beam spectropho-
tometer using 1 cm matched quartz cells by using corresponding
pure solvent as reference. The temperature of the cell holder was
controlled with a water flow (0.2 °C). The steady state fluorescence
spectra were obtained on a JASCO (FP 6200, Japan) spectrofluorim-
eter. The emission slit width (5 nm) and the scan rate (250 nm)
was kept constant for all of the experiments. FT-IR spectra were re-
corded in a JASCO (FT-IR 460 Plus, Japan) spectrometer by using
KBr pellet. 1H NMR spectra were recorded at Madurai Kamaraj Uni-
versity, Madurai in a Brucker NMR spectrometer (300 MHz, Swit-
zerland) using TMS as internal standard and DMSO-d6 as solvent
at room temperature. The LCMS spectra were obtained from Uni-
versity of Hyderabad in a Shimadzu (LCMS 2010) spectrophotom-
eter with ionization potential at 70 eV (LCMS 2010, Japan).
Elemental analyses for CHN was performed at Sophisticated Ana-
lytical Instrument Facility, Cochin University of Science and Tech-
nology, Kochi (Elementar Vario EL III, Germany).
2.2. Synthesis and characterization of substituted quinones
1,4-Benzoquinones possessing varying number of chloro and
alkoxy substituents were synthesized and purified as reported
elsewhere [29]. An excess amount of corresponding sodium alkox-
ide was added into a stirred solution of chloranil in the respective
solvent at RT under N2 atm. The reaction mixture was stirred for
12 h at 70 °C or 90 °C and cooled to room temperature. Then the
reaction mixture was added into 200 ml of water and stirred for
1 h at RT. The crude material formed was filtered through a filter
paper and residue was purified using column chromatography (Sil-
ica gel 60–120 by using 5–10% ethyl acetate pet ether mixture).
The percentage yields of various quinones obtained by this method
are shown in Scheme 1.
2. Experimental
2.1. Material and methodology
The electron acceptors viz. alkoxy substituted 1,4-benzoquinon-
es were synthesized and purified by the reported method [29]. The
electron donor sulfamethoxazole was obtained as gift sample from
a locally available pharmaceutical company and was used as re-
ceived. The purity of the drug was checked by its melting point
(observed 169 °C; literature 169 °C), 1H NMR and FT-IR spectra.
Commercially available spectroscopic grade solvents (Merck, India)
were used without further purification. The structures of the drug
and the acceptors are shown below.
2.2.1. 2,3,5-Trichloro-6-methoxycyclohexa-2,5-diene-1,4-dione (MQ)
1H NMR (DMSO-d6, 300 MHz; Fig. 1Sa), d (ppm) 4.19 (s, 3H), FT-
IR (KBr, cmꢀ1): 1680 (C@O), 1668 (C@O), 1567 (C@C), UV–Vis in
O
H
N
ethanol (kmax): 419 nm (n–pꢁ), log
e 2.50, Anal. Calcd. for C7H3Cl3-
S
O3: C, 34.82; H, 1.25; found: C, 36.17; H, 1.74; m.p. 172 °C.
O
N
O
2.2.2. 2,3,5-Trichloro-6-ethoxycyclohexa-2,5-diene-1,4-dione (EQ)
1H NMR (DMSO-d6, 300 MHz, Fig. 1Sb), d (ppm) 1.39 (t,
J = 7.20 Hz, 3H), 4.45–4.53 (m, 2H);FT-IR (KBr, cmꢀ1): 1676, 1608
NH2
(C@O); UV–Vis in ethanol (kmax): 419 nm, log
for C8H5Cl3O3: C, 37.61; H, 1.97; found: C, 36.87; H, 1.84; m.p.
e 2.62, Anal. Calcd.
Chemical structure of Sulfamethoxazole
1
112 °C.
O
2.2.3. 2,3,5-Trichloro-6-isopropoxycyclohexa-2,5-diene-1,4-dione
(IPQ)
O
Cl
Cl
1
1H NMR (DMSO-d6, 300 MHz, Fig. 1Sc), d (ppm) 1.34 (d,
R
6
2
J = 6.00 Hz, 6H), 4.98–5.10 (m, 1H); FT-IR (KBr, cmꢀ1): 1670, 1648
(C@O), UV–Vis in ethanol (kmax): 419 nm, log
e 2.62, Anal. Calcd.
3
5
for C9H7Cl3O3: C, 40.11; H, 2.62; found: C, 40.81; H, 2.56; m.p.
74 °C.
4
Cl
2.2.4. 2,3,5-Trichloro-6-butoxycyclohexa-2,5-diene-1,4-dione (BQ)
1H NMR (DMSO-d6, 300 MHz, Fig. 1Sd),
d (ppm) 0.9 (t,
2
O
J = 6.00 Hz, 3H), 1.35–1.48 (m, 2H), 1.63–1.74 (m, 2H), 4.45 (t,
Where R= Methyl (MQ); Ethyl (EQ); Isopropyl (IPQ)
and n-butyl (BQ)
J = 6.30 Hz, 2H); FT-IR (KBr, cmꢀ1): 1672, 1649 (C@O), UV–Vis in
ethanol (kmax): 419 nm, log
e 2.62, Anal. Calcd. for C10H9Cl3O3: C,
42.36; H, 3.20; found: C, 42.14; H, 3.18; m.p. 88 °C.

314
K. Ganesh et al. / Journal of Molecular Structure 1033 (2013) 312–320
O
Cl
Cl
O
O
O
Cl
Cl
O
Cl
O
Cl
O
O
3 (64%)
Cl
Cl
Cl
Cl
1 (72%)
H5
C2
,
O
O
a
a
h
2
N
.
q
O
O
e
l
o
n
1
Cl
Cl
O
t
2
e
Cl
Cl
O
3
Cl
Cl
O
4 (67%)
2 (68%)
Scheme 1. Preparation method for alkoxy substituted quinones.
2.3. Characterization of reaction products
2.4. Kinetic procedure
In all the cases, the reaction products were obtained by allowing
the reactants to react for 24 h under stoichiometric conditions
(4 mmol of SULF and 4 mmol of corresponding acceptors in etha-
nol) and subjected to MPLC separation. The products were charac-
terized using analytical (CHN) and spectral techniques viz. 1H NMR
[(Figs. 3Sa, 3Sb, 3Sc, and 3Sd); supplemental information], FT-IR
[(Figs. 4Sa, 4Sb, 4Sc and 4Sd; supplemental information)] and
LC–MS [(Figs. 5Sa, 5Sb, and 5Sc; supplemental information)].
The kinetics of the interaction of SULF with the acceptors (MQ,
EQ, IPQ and BQ) different quinones was followed at three different
temperatures in ethanol solvent under pseudo-first-order condi-
tions, keeping [D]ꢂ[A]. The increase in absorbance of the new peak
at 340 nm for with elapse of time was recorded. The pseudo-first-
order rate constants (k1) were calculated from the gradients of log
ꢀ(A –At) against time plots, where A and At represent the absor-
1
1
bance at infinity and time t, respectively. The second order rate
constants were calculated by dividing k1 by [D] [5].
2.3.1. SULF-MQ [2,5-dichloro-6-methoxy-3-(4-amino-N-(5-methyl-
isoxazol-3-yl)-benzene sulfonamide) cyclohexa-2,5-diene-1,4-dione]
1H NMR (DMSO-d6, 300 MHz), d (ppm) 2.29 (s, 3H), 4.17 (s, 3H),
6.14 (s, 1H), 7.14 (d, J = 8.70 Hz, 2H), 7.72 (d, J = 8.70 Hz, 2H), 9.62
(s, 1H), 11.33 (s, 3H). FT-IR (KBr, cmꢀ1): 3249 (NAH), 1671 (C@O),
1265, 1037 (AOCH3); LCMS: Calcd. For C17H13Cl2N3O6S: 458.2;
found: 458.4; Anal. Calcd. for C17H13Cl2N3O6S: C, 44.55; H, 2.86;
N, 9.17; found: C, 44.75; H, 2.70; N, 9.06.
2.5. Theoretical studies
The geometrical optimization of the donor SULF and the qui-
nones MQ, EQ, IPQ and BQ was performed by using Density Func-
tional Theory method with the Backle3LYP hybrid functional, of 6-
31G basis sets with the Gaussian 03 Revision D.01 program pack-
age, 2004.
2.3.2. SULF-EQ [2,5-dichloro-6-ethoxy-3-(4-amino-N-(5-methyl-
isoxazol-3-yl)-benzene sulfonamide) cyclohexa-2,5-diene-1,4-dione]
1H NMR (DMSO-d6, 300 MHz), d (ppm) 1.31(t, 3H), 2.29 (s, 3H),
4.47 (m, 2H), 6.14 (s, 1H), 7.15 (d, J = 8.70 Hz, 2H), 7.72 (d,
J = 8.70 Hz, 2H), 9.61 (s, 1H), 11.33 (s, 3H). FT-IR (KBr, cmꢀ1):
3224 (NAH), 1671 (C@O), 1278, 1035 (AOCH3); LCMS: Calcd. For
3. Results and discussion
3.1. Stoichiometry of the interaction
The stoichiometry of the CT complex formed, in all the cases,
was determined by applying Job’s continuous variation method
[30]. In all the cases (SULF-MQ, SULF-EQ, SULF-IPQ, SULF-BQ) the
symmetrical curve with a maximum at 0.5 mol fraction indicated
the formation of a 1:1 (D:A) CT complex [(Fig. 6Sa) Supplemental
information]. The photometric titration measurements were also
performed for the determination of the stoichiometry in these
interactions. The results of the photometric titration studies
[(Fig. 6Sb) Supplemental information] confirmed the observed
stoichiometry of the interaction [31]. The stoichiometry of the
SULF-MQ, SULF-EQ, SULF-IPQ, SULF-BQ systems is further con-
firmed by Jobs continuous variation method using emission studies
also [32] [(Fig. 6Sc) Supplemental information].
C18H15Cl2N3O6S: 472.3; found: 472.3; Anal. Calcd. for C18H15Cl2N3-
O6S: C, 45.77; H, 3.20; N, 8.90; found: C, 45.75; H, 3.17; N, 8.87.
2.3.3. SULF-IPQ [2,5-dichloro-6-isopropoxy-3-(4-amino-N-(5-methyl-
isoxazol-3-yl)-benzene sulfonamide) cyclohexa-2,5-diene-1,4-dione]
1H NMR (DMSO-d6, 300 MHz), d (ppm) 1.32 (d, J = 6.00 Hz, 6H),
2.29 (s, 3H), 4.99 (m, 1H), 6.14 (s, 1H), 7.16 (d, J = 8.70 Hz, 2H), 7.72
(d, J = 8.70 Hz, 2H), 9.61 (s, 1H), 11.33 (s, 3H). FT-IR (KBr, cmꢀ1):
3222 (NAH), 1670 (C@O), 1274, 1025 (AOCH3); LCMS: Calcd. For
C19H17Cl2N3O6S: 486.3; found: 486.3; Anal. Calcd. for C19H17Cl2N3-
O6S: C, 46.92; H, 3.52; N, 8.64; found: C, 46.87; H, 3.49; N, 8.63.
2.3.4. SULF-BQ [2,5-dichloro-6-butoxy-3-(4-amino-N-(5-methyliso-
xazol-3-yl)-benzene sulfonamide) cyclohexa-2,5-diene-1,4-dione]
1H NMR (DMSO-d6, 300 MHz), d (ppm) 0.90 (t, 3H), 1.40 (m,
2H), 1.64(m, 2H), 2.29 (s, 3H), 4.45 (t, 2H), 6.15 (s, 1H), 7.15 (d,
J = 8.70 Hz, 2H), 7.72 (d, J = 8.70 Hz, 2H), 9.61 (s, 1H), 11.33 (s,
3H). FT-IR (KBr, cmꢀ1): 3220 (NAH), 1670 (C@O), 1263, 1033
(AOCH3); LCMS: Calcd. For C20H19Cl2N3O6S: 499.0; found: 499.0;
Anal. Calcd. for C20H19Cl2N3O6S: C, 48.01; H, 3.83; N, 8.40; found:
C, 47.97; H, 3.81; N, 8.36.
3.2. Characterization of reaction products
The FT-IR spectra of the pure SULF, MQ, EQ, IPQ, BQ and their
products (SULF-MQ, SULF-EQ, SULF-IPQ and SULF-BQ) were re-
corded and the peak assignments for important peaks are given
in [(Tables 1Sa-1Sd, Supplemental Information)]. The results indi-
cated that the shifts in positions of some of the peaks could be
attributed to the symmetry and electronic structure modifications

K. Ganesh et al. / Journal of Molecular Structure 1033 (2013) 312–320
315
in both donor and acceptor units in the formed product relative to
the free molecules.
0.4
0.3
0.2
0.1
0.0
C
For a representative case, in SULF-MQ system, some of the sig-
nificant shifts are: the peak due to asymmetric and symmetric
ANH2 vibrations of the free SULF occurred at 3469 cmꢀ1 and
3376 cmꢀ1 respectively and in SULF-MQ product it appeared at
3249 cmꢀ1 indicating the participation of NAH moiety in product
formation. The v(C@O), v(OACH3) and v(CACl) stretching vibrations
400
500
600
700
800
in the free MQ species appeared at 1684, 1270 and 909 cmꢀ1
respectively. In the product these stretching vibrations occurred
at 1671, 1265 and 896 cmꢀ1, respectively. Such a bathochromic
shift could be indicative of a higher charge density on the carbonyl
group of the MQ molecule [26].
Wavelength (nm)
A
As a representative case, the 1H NMR spectra of the donor SULF
and its reaction product with MQ were recorded in DMSO d6 and
are given in [(Figs. 2S and 3Sa Supplemental Information)] respec-
tively. Using the proton NMR technique, we can identify the nature
of interaction between the donor and acceptor in the resulted
product. Pure SULF molecule exhibited the characteristic peak
due to aromatic ANH2 group at 6.087 ppm and in the product it ap-
peared at 9.620 ppm as ANH group. This observation indicated
that the product between SULF and MQ is formed with the elimi-
nation of HCl molecule. It is also interesting to note that the
ANH protons, in the corresponding product, appeared at 9.620,
9.614, 9.610 and 9.613 ppm for MQ, EQ, IPQ and BQ systems,
respectively. This indicated that the order of the strength of the
acceptors is MQ > EQ > BQ > IPQ. This is because of the fact that
greater is the electron accepting power of the acceptor, greater is
the acidity of the ANH proton directly attached to the quinone
ring. The aromatic protons of pure SULF molecule lie in the range
of 6.562–7.475 ppm and in the product they lie in the range of
7.144–7.754 ppm. The ANH proton of the pure SULF occurs at
10.927 ppm and in the product it occurs at 11.339 ppm. The down
field shifts of most of the signals are corresponding due to the
interaction occurred between the donor and the acceptor [5,33].
The LCMS spectrum, of SULF-MQ product was recorded and it
exhibited the molecular ion peak at m/z 458 (Fig. 5a) which con-
firms the chemical reaction between the donor and acceptor lead-
ing to the product. In the SULF-EQ and SULF-IPQ cases, the
molecular ion peaks at m/z 472 and 486 respectively confirmed
the product formation between SULF and the corresponding accep-
tors. [(Fig. 5sa, 5sb and 5sc. Supplemental information)].
P
D
300
400
500
600
700
800
Wavelength (nm)
Fig. 1. Electronic absorption spectra of SULF with MQ in ethanol at 298 K (D):
Donor; A: Acceptor; C: complex; P: product) Inset: clear isosbestic point at 456 nm.
concurrent increase in intensity of a band at 418 nm. A clear isos-
bestic point is observed at 456 nm. The position of the absorption
maximum of the peak at 418 nm has also been blue shifted with
elapse of time. These observations indicated that the initial reac-
tants were converted into final product via radical ion formation.
For comparison the electronic spectrum of the product is also
shown in Fig. 1. In other cases (SULF-EQ, IPQ and BQ systems), also
on mixing the ethanolic solutions of the reactants, similar pattern
of electronic spectra were observed. The electronic spectra of all
the systems are shown in [(Figs. 7Sa, 7Sb and 7Sc) Supplemental
Information].
The kinetics of the interaction of SULF with the acceptors has
been followed by monitoring the increase in absorbance of the
new peak in ethanol as a function of time under pseudo-first-order
conditions i.e. [D] > [A]. The pseudo-first-order rate constant (k1)
values for the formation of the product as a function of [D] and
[A] are collected in Table 1. It is evident from the results that in
all the cases, the rate constant is independent of initial concentra-
tion of [A] indicating first order dependence on [A]. In all the cases
the plot of log k1 versus log [D] is linear with a slope of unity
(r > 0.995; slope range 0.845–0.912) [(Fig. 8S) Supplemental Infor-
mation] indicating unit order dependence on [D]. This was further
supported by the constancy in k2 values [34].
3.3. Interaction of sulfamethoxazole with the acceptors
The electronic spectra of MQ in the presence of large excess of
donor i.e. [D]/[A] > 100 were recorded as a function of time in eth-
anol (Fig. 1). Immediately after mixing ethanolic solutions of color-
less SULF and pale yellow colored MQ, yields a pink colored
solution whose electronic spectrum showed absorption band in
the 460–600 nm range. This is the characteristic absorption bands
of quinone radical ion [34–36]. It is observed that with elapse of
time the intensity of these bands decreased (Fig. 1 inset) with a
The pseudo first-order rate constants, for all the systems, were
measured at three different temperatures and the thermodynamic
parameters computed are collected in Table. 2. A large negative va-
lue of entropy of activation indicated the involvement of polar
Table 1
Effect of concentration of donor and acceptors on the rate of the interaction at 298 K.
[D] (10ꢀ2 M)
[A] (10ꢀ4 M)
k1 (10ꢀ4), (sꢀ1)
k2 (sꢀ1 molꢀ1 dm3)
SULF-MQ
SULF-EQ
SULF- IPQ
SULF-BQ
SULF-MQ
SULF-EQ
SULF-IPQ
SULF-BQ
0.5
0.7
0.9
1.1
1.1
1.1
1.1
5.5
5.5
5.5
5.5
4.4
3.3
2.2
1.65
2.45
3.12
3.76
3.06
3.06
3.06
1.47
2.08
2.76
3.25
2.74
2.70
2.71
1.18
1.65
2.12
2.54
1.89
1.89
1.89
1.42
1.95
2.61
3.17
2.29
2.29
2.29
3.4
3.5
3.4
3.4
2.9
3.0
3.0
3.0
2.3
2.3
2.3
2.3
2.8
2.8
2.9
2.8

316
K. Ganesh et al. / Journal of Molecular Structure 1033 (2013) 312–320
Table 2
the new bands were measured using constant acceptor concentra-
tion in ethanol solvent and varying concentrations of the donor but
always [D]ꢂ[A]. Representative spectra for SULF-MQ system is
shown in [(Fig. 9S supplemental information]. The nature of the
spectra indicated that the interactions between the donor and
the acceptor are of CT type. The formation constants (K) and molar
Kinetic and thermodynamic parameters for the interaction of SULF with the acceptors
in ethanol.
System
k (nm)
k1 (10ꢀ4) (sꢀ1
)
D
H#
ꢀ
D
S#
D
G#
298
305
313 K
SULF-MQ
SULF-EQ
SULF-IPQ
SULF-BQ
355
355
355
355
3.76
3.26
2.54
3.17
4.70
4.47
3.90
4.56
5.60
5.72
4.72
5.12
17
26
29
23
250
223
216
235
93
93
93
93
extinction coefficients (
e) of the CT complexes were determined
spectrophotometrically using the Scott equation [37].
½Dꢃ½Aꢃ=d ꢄ ½Dꢃ= þ ð1=K
e
eÞ
ð1Þ
D
H#, kJ molꢀ1 S#, J Kꢀ1 molꢀ1 G#, kJ molꢀ1
; D ; D .
where [D] and [A] are the initial molar concentration of the donor
and acceptor, respectively and d the absorbance. The values of K
transition state. This may be due to the fact that there is some
charge separation in the transformation of the reactants to
product. Also the negative entropy of activation indicated a greater
degree of ordering in the transition state than in the initial state,
due to an increase in solvation during the activation process.
Based on the foregoing results and discussions the following
plausible mechanism for the interaction SULF with acceptors has
been proposed (Scheme 2).
and
e are determined from the gradient and intercept of the linear
plot of [D][A]/d against [D]. Representative Scott plots are shown in
[(Fig. 10S) Supplemental Information] and the values of K and e thus
determined are given in Table 3. The observed high values of K sug-
gested that the formed CT complexes are of a strong type [34] and
the linearity of the Scott plots further supports this result.
3.5. Fluorescence studies
3.4. Characteristics of the CT complexes
The nature and magnitude of the interaction of drugs with
receptors play an important role in the pharmacokinetics of the
drugs. CT interaction is one of the non covalent binding forces in
In all the cases, an attempt was made to characterize the CT
complexes formed in these reactions. For that the absorbance of
O
N
O
N
R
O
O
Cl
Cl
NH
O
NH
O
R
O
S
S
+
O
O
O
O
Cl
Cl
Cl
O
Cl
H2N
H2N
R= Me, Et, i-Pr and n-Bu
CT- Complex
O
O
N
R
O
N
O
O
R
O
O
O
Cl
NH
NH
O
O
Cl
Cl
S
S
O
+
O
Cl
Cl
Cl
H
H2N
HN
radical ion pair
-HCl
R
O
H
N
O
O
O
S
N
Cl
O
O
N
Cl
H
Scheme 2. Mechanism of interaction of SULF with the acceptors.

K. Ganesh et al. / Journal of Molecular Structure 1033 (2013) 312–320
317
Table 3
Spectral properties of the CT complexes formed between sulfamethoxazole and the acceptors in ethanol at 298 K.
Property
SULF-MQ
SULF-EQ
SULF-IPQ
SULF-BQ
Formation constant K (dm3 molꢀ1) Abs method
Extinction coefficient log e (dm3 molꢀ1 cmꢀ1) Abs method
Association constant Kf (mol Lꢀ1) emission method
Stern–Volmer constant KSV emission method
2980
3.806
3615
3891
2600
3.725
2083
2001
2040
3.641
1268
1643
2140
3.698
1982
1991
60
120
100
80
60
40
20
0
D
D
50
a
a
b
40
b
c
d
e
c
d
30
e
f
f
20
10
0
320 360 400 440 480 520 560 600 640 680
300
400
500
600
700
Wavelength (nm)
Wavelength (nm)
Fig. 2. Variation of fluorescence spectra of SULF-MQ system in ethanol at fixed
concentration [D] = {2.8125 ꢅ 10ꢀ3 (curve D)} and variable concentration of
[A] ꢅ 10ꢀ4 = {1.3875 (curve a), 2.775 (curve b), 4.1625 (curve c), 5.55 (curve d),
6.9375 (curve e), 8.325 (curve f)} mol Lꢀ1 at 298 K.
Fig. 4. Variation of fluorescence spectra of SULF-IPQ system in ethanol at fixed
concentration [D] = {2.8125 ꢅ 10ꢀ3 (curve D)} and variable concentration of
[A] ꢅ 10ꢀ4 = {1.3875 (curve a), 2.775 (curve b), 4.1625 (curve c), 5.55 (curve d),
6.9375 (curve e), 8.325 (curve f)} mol Lꢀ1 at 298 K.
140
160
140
120
100
80
D
D
120
a
100
a
b
c
b
80
c
d
60
e
f
g
d
60
e
f
40
40
g
20
20
0
0
300
400
500
600
700
300
400
500
600
700
Wavelength (nm)
Wavelength (nm)
Fig. 3. Variation of fluorescence spectra of SULF-EQ system in ethanol at fixed
concentration [D] = {2.8125 ꢅ 10ꢀ3 (curve D)} and variable concentration of
[A] ꢅ 10ꢀ4 = {1.3875 (curve a), 2.775 (curve b), 4.1625 (curve c), 5.55 (curve d),
6.9375 (curve e), 8.325 (curve f), 9.7125 (curve g)} mol Lꢀ1at 298 K.
Fig. 5. Variation of fluorescence spectra of SULF-BQ system in ethanol at fixed
concentration [D] = {2.8125 ꢅ 10ꢀ3 (curve D)} and variable concentration of
[A] ꢅ 10–4 = {1.3875 (curve a), 2.775 (curve b), 4.1625 (curve c), 5.55 (curve d),
6.9375 (curve e), 8.325 (curve f), 9.7125 (curve g)} mol Lꢀ1 at 298 K.

318
K. Ganesh et al. / Journal of Molecular Structure 1033 (2013) 312–320
the drug–receptor mechanism. In the present study, an attempt
was made to study the CT interaction of SULF with MQ, EQ, IPQ,
and BQ by means of fluorescence study. Fluorescence spectra were
recorded at room temperature in ethanol in the range of 300–
700 nm using an excitation at 270 nm for SULF. It was observed
that the fluorescence of SULF was quenched by these acceptors
as a result of formation of CT complex. The experimental results
indicated that the quenching efficiency increased with increasing
concentration of the electron acceptor (Figs. 2–5) and with increas-
ing time. The fraction of acceptors bound to the donor was deter-
mined by using the following equation.
5
4
3
2
SULF-MQ r=0.99
SULF-EQ r=0.99
SULF-IPQ r=0.97
SULF-BQ r=0.97
h ¼ ðFo ꢀ FÞ=Fo
ð2Þ
where F and Fo denote the fluorescence intensities of donor in the
presence of acceptor and in the absence of acceptor, respectively.
From the resulting values of h, the association constant Kf for
SULF-MQ, SULF-EQ, SULF-IPQ and SULF-BQ systems was computed
using the method described by Ward [38]. It has been shown that
for equivalent and independent binding sites
1
0.0000
0.0002
0.0004
0.0006
0.0008
0.0010
1=ð1 ꢀ hÞKf ¼ ½AT ꢃ=h ꢀ n½DT ꢃ
ð3Þ
[Q]
Fig. 6. Stern–Volmer plot for the fluorescence quenching of SULF with MQ, EQ, IPQ
and BQ in ethanol at 298 K.
where n is the number of binding sites, [AT] is the total acceptor
concentration and [DT] is the total donor concentration. For all the
cases the plots 1/(1ꢀh) versus [AT]/h are linear (r > 0.97) indicating
that under the experimental conditions all the binding sites are
equivalent and independent. The value of Kf obtained, from the
plots, for SULF-MQ, SULF-EQ, SULF-IPQ and SULF-BQ systems are
found to be 3.6 ꢅ 103, 2.1 ꢅ 103, 1.2 ꢅ 103 and 1.9 ꢅ 103 mol Lꢀ1
,
0.4
respectively. The standard Gibbs energy change
D
Go was calculated
SULF-MQ r=0.99
from the Kf values using the relation
D
Go = ꢀ2.303 RT log10 Kf. The
SULF-EQ r=0.99
D
Go values for SULF-MQ, SULF-EQ, SULF-IPQ and SULF-BQ systems
SULF-IPQ r=0.98
SULF-BQ r=0.98
0.2
0.0
were found to be ꢀ19.6, ꢀ18.3, ꢀ17.1 and ꢀ18.1 kJ molꢀ1 respec-
tively, indicating that the interaction between the drug and the
acceptors is spontaneous in nature.
Fluorescence quenching can occur by different mechanisms viz.
static or dynamic or both. The Stern–Volmer equation (Eq. (4)) is
useful in understanding the mechanism of fluorescence quenching.
-0.2
-0.4
-0.6
-0.8
Fo=F ¼ 1 þ KSV½Qꢃ
ð4Þ
where Fo is the initial fluorescence intensity measured in the ab-
sence of quencher and F is that in the presence of quencher concen-
tration [Q]. The Stern–Volmer constant KSV is obtained by plotting
Fo/F against [Q]. In all the cases, linear Stern–Volmer relationship
observed (Fig. 6) indicated that either static or dynamic quenching
is dominant [39,40].
The relationship between the fluorescence quenching intensity
and the concentration of quenchers can be described by the follow-
ing equation.
-3.8
-3.6
-3.4
log [Q]
-3.2
-3.0
Fig. 7. Plot of log (Fo–F/F) versus log [Q] of SULF with MQ, EQ, IPQ and BQ in ethanol
at 298 K.
log10ðFo ꢀ FÞ=F ¼ log KA þ nlog10½Qꢃ
ð5Þ
where KA is the binding constant and n is the number of binding
sites per donor molecule [41]. In the present study, in all the cases,
a plot of log10 (FoꢀF)/F versus log10 [Q] is linear (r > 0.981, Fig. 7)
and the binding constant values computed are collected in Table 4.
The results indicated the magnitude of binding constant is in the
order of SULF-MQ > SULF-EQ > SULF-BQ > SULF-IPQ. These observa-
tions are in corroboration with the results of absorption spectral
and kinetic studies as enumerated earlier in this paper. That is the
formation and rate constant computed using absorption spectral
data follows the same order (Tables 2 and 3). The value of n, for
these systems, is nearly constant in ethanol indicating the presence
of equivalent binding sites.
Table 4
Binding constants (KA) and number of binding sites (n) for SULF-MQ, SULF-EQ, SULF-
IPQ and SULF-BQ systems in ethanol solvent.
Acceptors
KA (molꢀ1 L)
n
SULF-MQ
SULF-EQ
SULF-IPQ
SULF-BQ
3.2 ꢅ 103
2.9 ꢅ 103
1.5 ꢅ 103
1.6 ꢅ 103
1.1
1.0
1.0
0.9

K. Ganesh et al. / Journal of Molecular Structure 1033 (2013) 312–320
319
SULF
HOMO (-6.4167 eV)
MQ
LUMO (-4.4017 eV)
EQ
LUMO (-4.3522 eV)
IPQ
LUMO (-4.3370 eV)
BQ
LUMO (-4.3326 eV)
Fig. 8. The optimized structures of the SULF, MQ, EQ, IPQ and BQ along with frontier orbitals.
-4.0
-4.5
-5.0
-5.5
-6.0
-6.5
the donor along with HOMO and the acceptors along with LUMO
are depicted in Fig. 8. In the case of SULF, the HOMO is concen-
trated on the ANH2 group and in the case of the acceptors the
LUMO resides on the carbonyl group of the quinone ring. The
energies of the frontier orbitals of the donor and the acceptors
along with the energy corresponds to the CT transition,
BQ
IPQ
EQ
MQ
LUMO
D
E = (HOMOSULF ꢀ LUMOacceptor) [43,44], for all the systems, are
shown in Fig. 9. It is evident from the figure that the E depends
D
on the nature of the substituent present in the quinone.
3.7. Correlation analysis
A preliminary attempt has been made to correlate the results
(i.e. association constants, Kf) with Taft’s polar and steric constants
[45]. The results of the correlation analysis are shown below:
HOMO
Kf ¼ 12310rꢁ þ 3529 ðr ¼ 0:99; n ¼ 4Þ
ð6Þ
ð7Þ
Kf ¼ ꢀ3498Es þ 3050 ðr ¼ 0:83; n ¼ 4Þ
A good correlation obtained, between the association constant
of the CT complexes and the Taft’s polar constant (Eq. (6)), indi-
cated that the electronic effects of the substituents play signifi-
cant role in governing the reactivity of the quinones when
compared to steric factor which showed poor correlation (Eq.
(7)). The positive correlation observed between Kf and rꢁ (Eq.
(6)) indicated that with an increase in electron releasing power
of the alkoxy substituent, the quinone becomes increasingly a
weaker acceptor.
Fig. 9. Energy of HOMO of the donor and LUMO of the acceptors.
3.6. Theoretical calculations
To understand the foregoing experimental observations on the
CT complex formed between SULF and the acceptors, we have
performed the optimization of structures using Density Functional
Theory. Computations have been performed using the Gaussian 03
Revision D.01 program package [42]. The optimized geometry of
The above observations indicated that the strength of the accep-
tor decrease in the order:

320
K. Ganesh et al. / Journal of Molecular Structure 1033 (2013) 312–320
O
O
O
O
O
Cl
Cl
O
Cl
Cl
O
Cl
Cl
O
Cl
Cl
>
>
>
Cl
Cl
Cl
Cl
O
O
O
O
BQ
IPQ
EQ
MQ
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The charge transfer properties of these 1,4-benzoquinones
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