Synthesis, characterization, structural optimization using density functional theory and superoxide ion scavenging activity of some Schiff bases

Article abstract of DOI:10.1016/j.molstruc.2007.02.031

Four Schiff bases, 2-[(4-nitro-phenylimino)-methyl]-phenol (NPIMP-2), 4-[(4-nitro-phenylimino)-methyl]-phenol (NPIMP-4), (1H-indol-3-ylmethylene)-(4-nitro-phenyl)-amine (IYNPA), (3-nitro-benzylidene)-(4-nitro-phenyl)-amine (NBNPA) with experimental clue of binding with superoxide ion have been synthesized by the condensation of p-nitroaniline with o-hydroxy benzaldehyde, p-hydroxybenzaldehyde, indole-3-carboxaldehyde and m-nitrobenzaldehyde respectively. These were characterized by IR, ¹H NMR, CHN, mass spectroscopy and UV-visible spectroscopy. The Schiff bases thus synthesized exhibited evidence for their binding with superoxide ion in the form of a high intensity charge transfer band beyond 500 nm in their respective UV-vis spectra on the addition of two drops of N/10 NaOH to their respective 10^-2 M solutions in DMSO. On further addition of two drops of N/10 HCl solution, the charge transfer band vanished and the original spectral pattern of the Schiff bases in DMSO was observed. Out of the four Schiff bases, the binding of NPIMP-4 with O₂^- was observed to be reversible throughout both the cycles of measurements, but the remaining Schiff bases, namely NPIMP-2, IYNPA and NBNPA were found to show reversibility in only one cycle. The variable temperature ¹H NMR (RT-175 °C) in DMSO-d₆ revealed reversible intramolecular proton transfer in NPIMP-2 and NPIMP-4 leading to existence of several tautomeric structures. Besides these synthetic and spectroscopic studies, the density functional theory (DFT) calculations were performed for two Schiff bases i.e., NPIMP-2 and NPIMP-4. These calculations provided some important information about the relative stability of various tautomeric forms of NPIMP-2 and NPIMP-4.

Full text of DOI:10.1016/j.molstruc.2007.02.031

Available online at www.sciencedirect.com
Journal of Molecular Structure 873 (2008) 5–16
www.elsevier.com/locate/molstruc
Synthesis, characterization, structural optimization using
density functional theory and superoxide ion scavenging activity
of some Schiff bases
a,
a
a
b
*
K.K. Upadhyay , Ajit Kumar , Shalini Upadhyay , P.C. Mishra
a
Department of Chemistry, Faculty of Science, Banaras Hindu University, Varanasi 221005, India
Department of Physics, Faculty of Science, Banaras Hindu University, Varanasi 221005, India
b
Received 3 October 2006; received in revised form 30 January 2007; accepted 1 February 2007
Available online 2 March 2007
Abstract
Four Schiff bases, 2-[(4-nitro-phenylimino)-methyl]-phenol (NPIMP-2), 4-[(4-nitro-phenylimino)-methyl]-phenol (NPIMP-4), (1H-
indol-3-ylmethylene)-(4-nitro-phenyl)-amine (IYNPA), (3-nitro-benzylidene)-(4-nitro-phenyl)-amine (NBNPA) with experimental clue
of binding with superoxide ion have been synthesized by the condensation of p-nitroaniline with o-hydroxy benzaldehyde, p-hydroxy-
1
benzaldehyde, indole-3-carboxaldehyde and m-nitrobenzaldehyde respectively. These were characterized by IR, H NMR, CHN, mass
spectroscopy and UV–visible spectroscopy. The Schiff bases thus synthesized exhibited evidence for their binding with superoxide ion in
the form of a high intensity charge transfer band beyond 500 nm in their respective UV–vis spectra on the addition of two drops of N/10
À2
NaOH to their respective 10 M solutions in DMSO. On further addition of two drops of N/10 HCl solution, the charge transfer band
vanished and the original spectral pattern of the Schiff bases in DMSO was observed. Out of the four Schiff bases, the binding of NPIMP-
À
4
with O2 was observed to be reversible throughout both the cycles of measurements, but the remaining Schiff bases, namely NPIMP-2,
1
IYNPA and NBNPA were found to show reversibility in only one cycle. The variable temperature H NMR (RT-175 °C) in DMSO-d
6
revealed reversible intramolecular proton transfer in NPIMP-2 and NPIMP-4 leading to existence of several tautomeric structures.
Besides these synthetic and spectroscopic studies, the density functional theory (DFT) calculations were performed for two Schiff bases
i.e., NPIMP-2 and NPIMP-4. These calculations provided some important information about the relative stability of various tautomeric
forms of NPIMP-2 and NPIMP-4.
Ó 2007 Elsevier B.V. All rights reserved.
Keywords: Superoxide ion; Schiff base; DFT; Variable temperature NMR
1
. Introduction
industrial waste water [12]. The Schiff bases in their poly-
meric form like polyazomethine ether have been found to
be potential candidate and as non-linear optical (NLO)
materials [13]. Metal complexes specially the mononuclear
and dinuclear complexes of Cu(II) and Zn(II) derived from
Schiff bases have been used as working models of the active
site of SOD enzyme by many workers [14–17]. Interest-
ingly, no Schiff base itself has been found to be effective
against the superoxide ion so far. Literature reports of var-
ious applications of Schiff bases and our earlier interest in
the field [17,18] prompted us to synthesize some Schiff
bases with complimentary groups and to study them as
The Schiff bases form a promising area of research due
to their wide spectrum applications, particularly as corro-
sion inhibitors [1], catalysts for activation of small mole-
cules [2,3] and in biological systems [4–11]. Besides these
applications, Schiff bases derived from the condensation
of oligosalicyaldehyde and a variety of aromatic amines
have been exploited as carriers of metal ions found in the
*
Corresponding author. Tel.: +91 0542 2307321x101.
E-mail address: drkaushalbhu@yahoo.co.in (K.K. Upadhyay).
0
022-2860/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2007.02.031

6
K.K. Upadhyay et al. / Journal of Molecular Structure 873 (2008) 5–16
possible superoxide ion scavengers. Out of the four Schiff
bases reported in this communication, NPIMP-4 was
already reported in the literature [13] as a potential NLO
material in its polymeric form. But its reported synthesis
ical shifts are reported in d values in the units of ppm rel-
ative to the tetramethylsilane (TMS) as the internal
standard. Besides the room temperature (RT) spectra the
1
variable temperature H NMR spectra have also been
[
13] involved a longer time duration (12 h) and use of an
recorded in the temperature range RT-50 °C in CDCl3
and RT-175 °C in DMSO-d6 for two Schiff bases i.e.,
NPIMP-2 and NPIMP-4. The same procedure for the
remaining two Schiff bases i.e., IYNPA and NBNPA was
obnoxious solvent i.e., DMF, while the present study
involved a much shorter time (4 h) and use of an unharmful
solvent i.e., ethyl acetate for its synthesis. The general
structure of the synthesized Schiff bases may be given as
follows,
not successful as these were insoluble in CDCl and decom-
3
posed in DMSO-d at higher temperatures. The UV–vis
6
spectra were obtained using a computer controlled Spe-
cord-200 spectrophotometer.
O
N
N
CH
X
2.2. Reagents and chemicals
O
p-Nitroaniline was obtained from SISCO research
laboratory, Mumbai, India. Sodium hydroxide pellets
were purchased from Qualigens, Mumbai, India, while
other required chemicals were obtained from Aldrich
and were used as such without any further purification.
Ethyl acetate used as solvent for the synthesis of Schiff
bases was purchased from E. Merck (India) Limited.
The dimethyl sulphoxide (DMSO) of spectroscopic
grade was purchased from Central Drug House (P)
Ltd., Delhi.
where X =
OH
O
O
N
N
H
HO
NPIMP-2
NPIMP-4
IYNPA
NBNPA
2
.3. Preparation of Schiff bases
The Schiff bases being reported in the present communi-
cation are likely to exist in various tautomeric forms hence
in order to have a deeper insight about the structural
details, we performed density functional theory calcula-
tions on two Schiff bases i.e., NPIMP-2 and NPIMP-4.
The density functional theory has been used by several
workers in recent years in order to solve the mechanistic
details of various chemical/biochemical reactions [19].
The existence of various tautomeric forms and their rela-
tive stability in a number of biochemically important sys-
tems has been established using density functional theory
earlier [20,21].
For the preparation of Schiff bases, a general procedure
was adopted. In this procedure 4.0 mmol of p-nitroaniline
was dissolved in 50 ml of ethyl acetate. To this solution,
4
.0 mmol of respective aldehyde solution in ethyl acetate
was added with constant stirring over a time period of
0 min, followed by their boiling under reflux for 4 h. After
1
refluxing, the contents of the respective round bottom
flasks were poured in four different petridishes. The sol-
vents were allowed to evaporate at room temperature
which led to micro-crystalline products followed by their
recrystallisation from alcohol-water mixture 40:60 v/v.
The synthesis of the Schiff bases in the present study can
be expressed conveniently through as follows (Scheme 1)
where X =
2
. Experimental
2
.1. Apparatus
OH
Melting points were obtained on a capillary melting
O
point apparatus and are uncorrected. The IR Spectra of
Schiff bases as their mull in nujol were recorded on JAS-
CO-FTIR Spectrophotometer. The H NMR Spectra were
N
1
N
O
HO
H
recorded on JEOL AL 300 FT NMR spectrometer. Chem-
NPIMP-2
NPIMP-4
IYNPA
NBNPA
O
O
H
X
O
Refluxed For 4 hours
in ethyl acetate
N
NH2
+
O
N
N
CH
X
O
Scheme 1. General method for the preparation of Schiff bases.

K.K. Upadhyay et al. / Journal of Molecular Structure 873 (2008) 5–16
7
2.4. Interaction of Schiff base solutions in DMSO with N/10
NaOH and N/10 HCl
NPIMP-2 while both the groups absorb at different wave
numbers in rest of the Schiff bases. The slight lowering of
mC@N in NPIMP-2 may be a consequence of electron releas-
ing nature of OH but this does not seem to be the sole fac-
tor because in NPIMP-4 which is a position isomer of
À2
The 10 M solutions of respective Schiff bases were pre-
pared in DMSO and their UV–vis spectra were recorded.
This was followed by the addition of two drops of N/10
NaOH to each solution of Schiff bases. Proper time was
given along with shaking of the vessels to develop colour
and then UV–vis spectra were again recorded. The samples
were taken out and two drops of N/10 HCl were added to
each solution. Full time was given with proper shaking of
the vessels so that interaction of HCl could take place.
All the measurements involved DMSO as the reference
standard.
À1
NPIMP-2, the mC@N was observed at 1644 cm . Hence
the distance of electron releasing group from the azome-
thine and its ability to get involved in H-bonding with
azomethine also seemed to be an important factor for mC@N
absorption in the Schiff bases studied here. The unexpected
À1
higher mC@N absorption value at 1695 cm in NBNPA
may be a consequence of the presence of an extra –NO2
group present on the aromatic ring at the meta position
to the azomethine group. However, the m
for IYNPA
C@N
À1
was observed at 1630 cm . The peak observed in the range
À1
3
100–3060 cm in all the Schiff bases was assigned to aro-
2
.5. Density functional theory (DFT) calculations
matic mC–H. IR spectra of NPIMP-2 showed strong evi-
dence of intramolecular H-bonding by the frequency at
All the possible tautomeric structures for NPIMP-2 and
À1
2760 cm . This observation was found to be in accordance
NPIMP-4 were optimized using density functional theory
DFT) at the B3LYP/6-31G** level [22–24] employing the
Windows versions of Gaussian 98(G98W) and Gaussian
with the literature report on similar systems [28]. On the
other hand, NPIMP-4 showed evidence for intermolecular
(
À1
H-bonding due to the presence of a peak at 3366 cm [27]
0
3(G03W) [25,26] programs.
for Ar–OH. All the Schiff bases showed absorption of m
of azomethine group in the range 2973–2924 cm in accor-
C–H
À1
3
. Results and discussions
dance with a recent report [29] for the similar systems.
3
.1. IR spectra
The major IR spectral bands along with other important
3.2. NMR spectra
analytical data have been given in Table 1.
À1
The presence of a peak in the range 880–837 cm in all
the Schiff bases was assigned to mC–N absorption. The Schiff
bases with electron releasing –OH groups i.e., NPIMP-2
and NPIMP-4 exhibited mC–N absorption at comparatively
The peak positions with their corresponding integration
for different Schiff bases at RT have been given in Table 2.
The NMR spectral data for two Schiff bases i.e., IYNPA
and NBNPA could not be recorded in CDCl due to their
3
À1
1
lower wave numbers i.e., 854 and 837 cm , respectively.
poor solubility in this solvent. The H NMR spectrum of
The asymmetric and symmetric absorptions for the –NO2
NPIMP-2 is almost the same in DMSO as in CDCl , the
3
group were observed in the ranges 1564–1520 and 1377–
only difference being in terms of the chemical shifts of dif-
À1
1
200 cm respectively [27]. All the Schiff bases showed
ferent groups in DMSO and CDCl . A comparative down-
3
strong absorption bands due to the out of plane C–H bend-
ing of the aromatic ring in the range 905–675 cm , while
field shift of azomethine hydrogen in DMSO may be a
consequence of interaction of this H-atom with the O-atom
of DMSO. The chemical shift for the Ar–OH suggested an
intramolecular H-bonding (Fig. 1) between the hydrogen
atom of Ar–OH group and nitrogen atom of azomethine
À1
the corresponding in plane bending vibrations were
À1
observed at 1300–1028 cm . The aromatic skeletal vibra-
À1
tions seemed to absorb at 1599 cm along with mC@N in
Table 1
Melting point, elemental analysis, mass spectral and IR spectral data
À1
Schiff
bases
Molecular
formula
M.P.
(°C)
Mass
(M + H)
Elemental analysis found (calcd.) (%)
Major IR peaks (cm
)
C
H
N
NPIMP-2
NPIMP-4
IYNPA
C
C
C
C
13
H
13
H
15
H
13
H
10
10
11
N
N
N
2
2
3
O
O
O
3
3
2
162 243.2
63.90 (64.46) 4.19 (4.16) 11.63 (11.56) 3075, 2963, 2760, 1599, 1564, 1464, 1377, 1271, 1177, 1150,
109, 1028, 905, 854, 812, 764, 685
1
98–110 243.2
135 266.2
65.10 (64.46) 4.34 (4.16) 11.24 (11.56) 3366, 3200, 3060, 2926, 1644, 1597, 1472, 1460, 1377, 1300,
161, 1113, 837, 720, 675,
1
67.38 (67.92) 4.16 (4.18) 16.19 (15.84) 3459, 3350, 3100, 2973, 1630, 1600, 1520, 1460, 1377, 1302,
111, 880, 787, 723
1
NBNPA
9
N
3
O
4
115 272.1
56.79 (57.57) 3.13 (3.34) 14.87 (15.49) 3077, 2924, 1696, 1599, 1522, 1348, 1200, 1109, 905, 879, 856,
14, 754, 733, 711, 694
8

8
K.K. Upadhyay et al. / Journal of Molecular Structure 873 (2008) 5–16
H
C
O
N
N
O
H
O
Fig. 1. Intramolecular hydrogen bonding in NPIMP-2.
group in CDCl as well as in DMSO. This observation is in
3
accordance with an earlier report [12] on similar systems.
The VT-NMR of NPIMP-2 in DMSO in the tempera-
ture range RT-175 °C revealed some interesting aspects
about the structural details of this Schiff base. At the room
temperature, NPIMP-2 existed in the tautomeric form [a]
(Fig. 2). As the temperature was increased, the intramolec-
ular H-bonding started rupturing and NPIMP-2 started
moving towards tautomeric form [c] and [d] through [b]
1
(
Fig. 2). The H NMR spectrum of NPIMP-2 at 100 °C
showed broadening as well as shortening of Ar–OH signal
as compared to that of room temperature. At the same
time, this also gave rise to new signals at 10.2, 6.7 and
6.3 d ppm for Ar–OH non-hydrogen bonded, @NAOH
and –NH–Ar–NO respectively. With further increase of
2
temperature, the Ar–OH signal got broadened further
and upfield shifted. At the same time, the signal for
@
NAOH and –NH–Ar–NO got further intensified and
2
finally at 175 °C the Ar–OH was shifted to 11.5 d ppm.
The other signals for Ar–OH (non-hydrogen bonded),
@
NAOH and –NH–Ar–NO were clearly visible at 10.2,
2
6
.7 and 6.0 d ppm respectively suggesting that NPIMP-2
existed in all its tautomeric forms shown in Fig. 2 at
75 °C in DMSO. On reversal of temperature to the RT,
1
the structure shown in Fig. 1 was not restored; instead,
the spectrum suggested a mixture in which all the tautom-
ers coexisted. At the same time, intermolecular H-bonded
form of NPIMP-2 also existed because a new signal at
1
0.7 d ppm was seen along with the other signals in the
1
reverse H NMR spectrum of NPIMP-2 at RT.
1
The H NMR spectrum of NPIMP-4 (Fig. 3) in CDCl
3
exhibited some difference from that in DMSO. The most
prominent difference was the absence of the signal for the
Ar–OH in CDCl ; instead, two additional signals centered
3
at 5.9 (sharp) and 4.3 (broad) d ppm were observed and
assigned to @N–OH and –NH–Ar–NO , respectively.
2
1
The H VT NMR of NPIMP-4 in CDCl in the tempera-
3
ture range RT-50 °C did not show any significant change
in any signal excepting upfield shifting of @N–OH and –
NH–Ar–NO which may be a consequence of breaking of
2
intermolecular H-bonding. Hence it was concluded that
in CDCl , NPIMP-4 existed in the forms [b] and [c]. The
3
1
reverse H NMR spectrum of NPIMP-4 was same as its
initial RT spectrum.
1
The H NMR spectrum of NPIMP-4 (Fig. 3) in DMSO
at RT showed a broad signal centered at 10.6 d ppm (Ar–
OH) and a signal with multiplet tendency at 6.6 d ppm (–
NH–Ar–NO ) besides the other signals. Hence it was con-
2
cluded that at RT, NPIMP-4 existed in the forms [a] and [b]

K.K. Upadhyay et al. / Journal of Molecular Structure 873 (2008) 5–16
9
a
d
b
O
O
O
CH
O
N
H
N
C
H
N
N
O
H
O
c
H
C
O
OH
O
N
H
N
C
N
N
H
O
O
O
1
Fig. 2. Tautomeric forms for NPIMP-2 on the basis of H NMR spectral studies.
a H
b
O
O
O
CH
N
N
O
CH
NH
N
O
O
c
OH
O
CH
N
N
O
1
Fig. 3. Tautomeric forms for NPIMP-4 on the basis of H NMR spectral studies.
Table 3
UV–vis spectral data of Schiff bases
Schiff bases
Major peaks in solution (nm)
Chloroform
432, 407, 404, 387, 385, 383, 376, 372, 371, 368, 364, 362,
59, 354, 350, 347, 343, 338, 335, 332, 326, 323, 317, 310,
07, 302, 298, 266, 264
412, 409, 382, 378, 370, 366, 358, 354, 351, 340, 337, 330,
Dimethyl sulphoxide
447, 370, 318, 269
NPIMP-2
3
3
NPIMP-4
444, 436, 432, 428, 425, 416, 414, 399, 392, 390, 383, 363, 355, 352,
337, 333, 330, 327, 320, 310, 297, 283, 277, 267, 257
3
2
27,324, 317, 310, 305, 298, 295, 288, 285, 279, 272, 260,
54, 251, 245
IYNPA
NBNPA
440, 436, 426, 423, 400, 390, 386, 384, 381, 378, 375, 372, 370, 367,
3
64, 358, 353, 348, 345, 342, 339, 335, 329, 325, 321, 318, 303, 292
446, 437, 420, 415, 406, 401, 399, 396, 389, 387, 385, 381, 372, 361,
3
40, 334, 330, 329, 326, 321, 316, 311, 307, 303, 300, 294, 292, 241
Table 4
UV–vis spectral data for the interaction of Schiff bases with NaOH and HCl in DMSO
Schiff bases
kmax (nm)
Ist cycle
IInd cycle
DMSO
Two drop N/10 NaOH
Two drop N/10 HCl
Two drop N/10 NaOH
Two drop N/10 HCl
NPIMP-2
NPIMP-4
IYNPA
446.65
443.87
439.48
445.47
590.62
508.57
501.63
532.53
444.01
444.50
448.57
446.29
527.92
508.05
470.67
506.91
458.82
444.49
448.79
439.48
NBNPA

10
K.K. Upadhyay et al. / Journal of Molecular Structure 873 (2008) 5–16
in DMSO. The Ar–OH signal showed a vanishing tendency
could not be substantiated as it was done in CDCl . The
reverse H NMR spectrum of NPIMP-4 was same as its
3
1
at 50 °C and it totally vanished at 75 °C. The –NH–Ar–
NO signal shifted upfield and up to 175 °C no Ar–OH
initial RT spectrum. The peak positions for different types
2
1
was observed. At the same time the, –NH–Ar–NO kept
of H-atom in the H NMR spectra of IYNPA and NBNPA
2
on shifting upfield with continuous broadening with reduc-
tion in its intensity, indicating that at high temperatures
have been given in Table 2. The aldimine proton for
NBNPA absorbed at most downfield d ppm (10.2), which
(
75–175 °C) the NPIMP-4 existed totally in the form [b].
may be a consequence of presence of an extra –NO group
at the m-position in this Schiff base. In the H NMR spec-
trum of IYNPA at RT, the –NH proton was observed at
12.1 d ppm which corresponded to the literature report
for similar systems [30].
2
1
Hence it was concluded that in DMSO the NPIMP-4
existed in the form [a] and [b] up to 50 °C temperature
but beyond this temperature it was the form [b] which
totally dominated. The presence of form [c] in DMSO
1
1
1
1
Fig. 4. (a) H NMR spectra of NPIMP-2 in CDCl
3
, (b) H NMR spectra of NPIMP-2 in DMSO, (c) H NMR spectra of NPIMP-4 in CDCl
3
, (d) H
NMR spectra of NPIMP-4 in DMSO.

K.K. Upadhyay et al. / Journal of Molecular Structure 873 (2008) 5–16
11
Fig. 4 (continued)

12
K.K. Upadhyay et al. / Journal of Molecular Structure 873 (2008) 5–16
Fig. 4 (continued)
3
3
.3. UV–visible spectroscopic studies
CDCl by the absence of Ar–OH signal and appearance
of new signals for –NH–Ar–NO2 and @N–OH as dis-
cussed in Section 3.2.
3
.3.1. UV–visible spectra
The major UV–vis spectral bands for the Schiff bases
The absorption in the UV–vis spectra of NPIMP-2 and
NPIMP-4 in the chloroform showed a band in the range
have been given in Table 3. The bands observed above
*
*
4
00 nm in the UV–vis spectra were assigned to n–p tran-
of 340–350 nm. It was assigned to a p–p transition of the
sition. Comparatively higher kmax values than the
ACH@NA group in accordance with the literature report
[13]. This bathochromic shift for the above type of band
than their expected value 220–230 nm [27] may be a con-
sequence of presence of electronically complimentary
groups across ACH@NA group [13]. This band showed
further red shift in DMSO that may be a consequence
reported ones for the Ar–NO [27] may be a consequence
2
of enhanced conjugation involving this moiety [27]. It was
*
observed that n-p bands of the Schiff bases show red
shifts of 15 and 32 nm for NPIMP-2 and NPIMP-4 in
DMSO as compared to their peaks in chloroform. These
red shifts may be a consequence of the higher polarity of
DMSO than that of chloroform as reported in the litera-
ture [31]. In chloroform, the UV–vis spectrum of NPIMP-
of higher polarity of DMSO than that of CHCl as men-
3
tioned above [31]. Similar bands were also observed for
the remaining two Schiff bases i.e., IYNPA and NBNPA
in their solutions in DMSO. Although they do not con-
tain the complimentary groups across ACH@NA,
IYNPA contains an indole moiety having a fused pyrrole
ring which is equivalent to a phenolic moiety [32]. Thus
IYNPA has similarity with NPIMP-2 and NPIMP-4.
NBNPA does not contain electronically complimentary
4
showed blue shift (kmax 412 nm) in comparison to that
of NPIMP-2 (kmax 432 nm). It may be understood in
terms of decrease of conjugation [31] in NPIMP-4 and
shifting of equilibrium in favour of [b] and [c] (Fig. 3),
It is further supported by the presence of a band at
2
45 nm and those above 400 nm which are supposed to
correspond to the p-substituted enone moiety as reported
groups at the first sight but out of the two –NO groups,
2
in the literature for similar systems [27]. This observation
is supported by the H NMR spectrum of NPIMP-4 in
one is present at the m-position with that to one of the
benzene rings across ACH@NA group, hence its electron
1

K.K. Upadhyay et al. / Journal of Molecular Structure 873 (2008) 5–16
13
Fig. 4 (continued)

14
K.K. Upadhyay et al. / Journal of Molecular Structure 873 (2008) 5–16
withdrawing effect is not exhibited [32] and ultimately the
M effect of phenyl group operates through one side of
produced bathochromic shifts irrespective of the structure
of the Schiff bases. It was followed by change of the kmax val-
ues to or very near the original values on the addition of two
drops of N/10 HCl. It is worth mentioning that out of the
four Schiff bases, only one i.e., NPIMP-4 showed the revers-
ibility in both the cycles of measurements, while the remain-
ing Schiff bases showed the reversibility in only one cycle
(Table 5).
+
the ACH@NA group. In this way, here also we virtually
have complimentary groups, across the ACH@NA group.
The bands observed in the range 250–300 nm were
assigned as corresponding to the B-band of benzene
which were common to all the Schiff bases. The compar-
atively higher values for B-bands than the literature
report [27] may be understood once again as a conse-
quence of the higher polarity of the solvent in which these
studies were performed.
Two probable pathways may be suggested for the inter-
action of Schiff bases with NaOH and HCl leading to bath-
ochromic and hypsochromic shifts as follows,
3
.3.2. Interaction of schiff bases with NaOH and HCl in
DMSO
The UV–vis spectral data for the interaction of Schiff
H O
HCl
SH
2
-
+
[
i] SH
+
NaOH
S Na
SH + NaCl
Charge transfer complex
HCl
DMSO
[ii] NaOH + O
2
NaO
2
bases with NaOH and HCl in DMSO are given in Table 4.
A perusal of the data showed that addition of two drops of
N/10 NaOH to the solutions of the Schiff bases in DMSO
2 2
SH + NaCl + H O + ½ O
H
H
O
H
O
N
H
N
O
N
N
O
O
O
(i)
(i)
O
O
H
N
N
O
O
N
N
H
H
O
O
(ii)
(ii)
O
H
O
N
N
O
OH
N
N
OH
O
(iii)
(iii)
OH
H
N
N
O
OH
O
N
N
O
O
(iv)
(iv)
NPIMP-2
NPIMP-4
Fig. 5. Tautomeric forms for NPIMP-2 and NPIMP-4 on the basis of DFT calculations.

K.K. Upadhyay et al. / Journal of Molecular Structure 873 (2008) 5–16
15
Out of the above two pathways, the first one appears to be
unlikely because literature reports suggest that the superox-
ide ion is generated by the reaction of NaOH and dissolved
oxygen in DMSO [33,34]. Hence, in the present context,
the second pathway seems to be operative and a probable
formation of the charge transfer complex between the super-
oxide ion and the Schiff bases may be responsible for the
bathochromic shift in kmax as observed in the case of nitoblu-
etetrazolium chloride [33]. The addition of HCl reduces the
pH of the medium and this may lead to rupture of charge
transfer complex of superoxide ion with any one of the Schiff
bases which ultimately gets converted into water and oxygen
through hydrogen peroxide in accordance with the literature
report [35]. Out of the four Schiff bases three showed revers-
ibility in only one cycle may be understood in terms of their
probable disintegration in second cycle.
Due to difference of energy between tautomeric forms,
the H NMR spectral patterns of these Schiff bases are dif-
ferent at RT and elevated temperatures. The UV–vis spec-
tral measurements indicated a probable binding between
the Schiff bases particularly NPIMP-4, and the superoxide
ion under the experimental conditions.
1
Acknowledgement
Authors are thankful to Prof. K.D.S. Yadav, Head,
Department of Chemistry, DDU University, Gorakhpur,
India for making available the facility of UV–vis
spectrophotometer.
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[
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ꢀ
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1
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