Experimental and theoretical study of antioxidative properties of some salicylaldehyde and vanillic Schiff bases

Article abstract of DOI:10.1039/c5ra02134k

The antioxidative capacity and structure-activity relationships of ten Schiff bases were investigated experimentally and theoretically. All compounds contain the aniline moiety, while the aldehyde part is either salicylaldehyde or vanillin. The DPPH assay was used to test the potential antioxidative activity of these compounds, and DFT study was used to investigate their electronic structures and provide insight into their structure-activity relationships. The effect of the position of the hydroxy, as well other groups present, on the antioxidative activity was examined. The possible radical scavenging mechanism was determined in polar (water and methanol), and nonpolar (benzene) solvents. Based on the experimental and computational results, compounds 7 and 8 exhibit the highest radical scavenging properties.

Full text of DOI:10.1039/c5ra02134k

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Experimental and theoretical study of antioxidative
properties of some salicylaldehyde and vanillic
Cite this: RSC Adv., 2015, 5, 24094
Schiff bases†
a
Zorica D. Petrovic, Jelena Đorovic,b Dusica Simijonovic,a Vladimir P. Petrovica
*
´
´
ˇ
´
´
and Zoran Markovicbc
´
The antioxidative capacity and structure–activity relationships of ten Schiff bases were investigated
experimentally and theoretically. All compounds contain the aniline moiety, while the aldehyde part is
either salicylaldehyde or vanillin. The DPPH assay was used to test the potential antioxidative activity of
these compounds, and DFT study was used to investigate their electronic structures and provide insight
into their structure–activity relationships. The effect of the position of the hydroxy, as well other groups
present, on the antioxidative activity was examined. The possible radical scavenging mechanism was
determined in polar (water and methanol), and nonpolar (benzene) solvents. Based on the experimental
and computational results, compounds 7 and 8 exhibit the highest radical scavenging properties.
Received 3rd February 2015
Accepted 24th February 2015
DOI: 10.1039/c5ra02134k
www.rsc.org/advances
The ability to scavenge free radicals is a common feature of
phenolic compounds. Antioxidative activity of phenolic Schiff
Introduction
bases (SB–OH) is directly related to their ability to release
hydrogen atoms. A few different mechanisms of free radical
scavenging are known: hydrogen atom transfer (HAT), single
electron transfer followed by proton transfer (SET-PT), and
sequential proton loss electron transfer (SPLET).9 All these
mechanisms have the same net result, i.e. the formation of
corresponding phenoxy radical.10
Schiff bases are compounds that were rst obtained in the
condensation reactions of aromatic amines and aldehydes
(1864).1 They are also known as imines or azomethines.2 A wide
range of these attractive compounds, with the general formula
RHC]N–R1 (R and R1 can be alkyl, aryl, cycloalkyl or hetero-
cyclic groups), have been synthesized to date. Schiff bases are of
great importance in the eld of coordination chemistry because
they are able to form stable complexes with metal ions.3 The
chemical and biological signicance of Schiff bases can be
attributed to the presence of a lone electron pair in the sp2
hybridized orbital of the nitrogen atom of the azomethine
group.4 These imines are used in the elds of organic synthesis,
chemical catalysis and analysis, medicine, pharmacy, as well as
other new technologies.5 The antitumor, antiviral, antifungal
and antibacterial properties of these compounds means they
have found applications in medicine and pharmacy.6 Due to
these biological properties, Schiff bases are used as basic
materials for the synthesis of many drugs.7 It has also been
reported that Schiff bases of salicylaldehydes show some anti-
microbial activity.8
HAT mechanism is the only one which consists of one step in
which hydrogen atom is transferred to free radical.11
SB–OH / SBOc + Hc
(1)
SET-PT and SPLET mechanisms consist of two steps. In SET-
PT mechanism, the rst step is characterized by process in
which one electron is lost and radical cation is created, whereas
in the second step radical cation is deprotonated and corre-
sponding radical is formed.9b,12
SB–OH / SB–OHc+ + eꢀ
SB–OHc+ / SB–Oc + H+
(2.1)
(2.2)
In SPLET mechanism, the rst step is deprotonation of
parent molecule. In the second step the anion formed loses an
electron and corresponding radical is formed.13
aFaculty of Science, University of Kragujevac, Department of Chemistry,
´
RadojaDomanovica 12, 34000 Kragujevac, Republic of Serbia. E-mail: zorica@kg.ac.
rs; Fax: +381 34335040
bBioengineering Research and Development Center, 34000 Kragujevac, Republic of
SB–OH / SB–Oꢀ + H+
SB–Oꢀ / SB–Oc + eꢀ
(3.1)
(3.2)
Serbia
cDepartment of Chemical-Technological Sciences, State University of Novi Pazar, Vuka
ˇ ´
Karadzica bb, 36300 Novi Pazar, Republic of Serbia
† Electronic supplementary information (ESI) available: Characterization of
compounds 1–10: bond lengths and angles, NBO charges and spin density, 1H
NMR spectra, melting points. See DOI: 10.1039/c5ra02134k
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These mechanisms are described by thermodynamical
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parameters: bond dissociation enthalpy (BDE) related to eqn
(1), ionization potential (IP) related to eqn (2.1), proton disso-
ciation enthalpy (PDE) related to eqn (2.2), proton affinity (PA)
related to eqn (3.1), and electron transfer enthalpy (ETE) related
to eqn (3.2).
It is known that some phenolic Schiff bases act as effective
antioxidants and potential drugs that can prevent disease
caused by free radical damage.14 However, the antioxidative
activity of this class of polyfunctional compounds deserves
further investigation. Also, further advance in analysis of their
structure–activity relationship, particularly how the position of
the hydroxy group effects the reactivity of these phenolic
compounds towards radicals is needed. In this sense, we put
under consideration some salicylaldehyde and vanillic Schiff
bases using experimental and theoretical tools.
Fig. 1 General structural formula of the investigated Schiff bases 1–10.
p-hydroxy group in the ring A (8–10), or o-hydroxy group in the
ring B (7 and 8), relatively to the positions 1 and 10, Fig. 1. Less
active compounds (2–5) are hydroxy substituted only in o-posi-
tion of the ring A, while compound 6 bears additional hydroxy
group in m-position of the ring B. The only difference between
compounds 1 and 7 is substitution in the ring B, p- and
o-respectively. On the basis of the IC50 values, it can be
concluded that o-hydroxy position in the ring B is responsible
for radical scavenging of these compounds. Common for the
Schiff bases 8–10 is that hydroxy group is present in p- and
methoxy group in m-position in the ring A. Taking into account
IC50 for these molecules, as well as for the compound 1, one can
conclude that presence of p-hydroxy group in the ring A
contributes more to antiradical activity than equivalent substi-
tution in the ring B. Observation that appearance of hydroxy
group in the p-position of the ring A, as well as in o-position of
the ring B contributes to the highest extent to the radical
scavenging activity is additionally supported by the fact that the
most active compound 8 possess hydroxy groups in both posi-
tions. We note in passing that substitution of the ring B by
electron donor or acceptor functional groups, had negligible
impact on the antioxidative activity.
The low activity of compounds 2–5 towards radical scav-
enging activity can be rationalised on the basis that only present
hydroxy group in o-position in the ring A can form intra-
molecular hydrogen bond with nitrogen, and thus will be pre-
vented to interact with DPPH. Although Schiff base 6, in
addition to the hydroxy group in o-position in the ring A, has
another one in the m-position in the ring B, it is reasonable to
expect that radical obtained in m-position will not be stabilised
by delocalisation of its unpaired electron over the entire mole-
cule, but only over ring B. On the other hand, in active
compounds, this stabilisation through delocalisation over both
rings is possible.
The rst part of this work is devoted to investigation of
antioxidative capacity of ten phenolic Schiff bases, depending
on substitution on the both phenyl rings – aldehyde and
aniline. To full this, DPPH assay is selected as method. Choice
was made due to its well-known application in determination of
the antioxidative activity of compounds,15 and due to fact that it
can be used in prediction of activity against reactive oxygen
species present in the living cells.16
Polarity of solvents plays signicant role and species which
mechanism to overcome. Bearing in mind this, further impor-
tant aim of this paper was to estimate the solvent effects to the
reaction enthalpies. To complete this, water and methanol were
used as polar, whereas benzene was used as nonpolar solvent.
To our best knowledge, this type of compounds, which can be
considered as imine analogues of good antioxidant resveratrol,
has not been subjected to this kind of study.
Results and discussion
In the reaction of aldehyde (salicylaldehyde or vanillin) and
aromatic amine (aniline, 4-uoroaniline, 4-nitroaniline, tolui-
dine, 2-hydroxyaniline, 3-hydroxyaniline or 4-hydroxyaniline) in
methanol, a series of ten Schiff bases was synthesized (1–10,
Fig. 1), wherein compound 10 is newly synthesized. The selec-
tion of these compounds was based on their structural char-
acteristics, such as positions of hydroxy and methoxy groups in
rings A and B.
DPPH test
All of the obtained compounds were subjected to evaluation of
their antioxidative activity in DPPH test, Tables 1 and S1 ESI.† It
was found that compounds 2–6 are poor radical scavengers,
while 1, 9, and 10 turned to be more active. Schiff bases 7 and 8
interact well with DPPH radical and exhibit high, slightly lower,
activity than the reference compound NDGA. On the basis of
Performed DPPH test provided insight into potential anti-
oxidative activity of the investigated compounds. However, to
obtain full insight in the structure–activity relationship of the
investigated Schiff bases, further investigation on the electronic
structure of these compounds was necessary. For the sake of
completeness, DFT study of the compounds subjected to this
examination was performed.
this, compounds
antioxidants.
7 and 8 can be considered as good
Obtained results in this study suggest that position of the
hydroxy groups in the Schiff bases plays decisive role in the
antioxidative activity. Namely, the active compounds possess
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Table 1 Calculated and experimental properties of investigated Schiff bases
Compound
HOMO (eV)
LUMO (eV)
HOMO–LUMO gap (eV)
DEiso (kJ molꢀ1
)
IC50 (mM)
Methanol
1
2
3
4
5
6
7
8
9
10
ꢀ0.274
ꢀ0.293
ꢀ0.280
ꢀ0.285
ꢀ0.285
ꢀ0.283
ꢀ0.280
ꢀ0.269
ꢀ0.273
ꢀ0.273
ꢀ0.039
ꢀ0.075
ꢀ0.040
ꢀ0.041
ꢀ0.042
ꢀ0.043
ꢀ0.046
ꢀ0.041
ꢀ0.033
ꢀ0.034
0.234
0.218
0.239
0.244
0.243
0.240
0.234
0.227
0.240
0.239
ꢀ13.038
41.638
42.042
43.145
42.827
1.896
ꢀ3.678
ꢀ11.227
ꢀ13.385
ꢀ13.188
561.3 (ref. 17)
117.4 (ref. 18)
186.3a
>500a
>500a
>500a
>500a
>500a
18.8a
5.3a
468.2 (ref. 17)
27.4 (ref. 17)
406.9 (ref. 18)
98.5 (ref. 18)
86.2a
68.8a
Water
1
2
3
4
5
6
7
8
9
ꢀ0.274
ꢀ0.293
ꢀ0.280
ꢀ0.285
ꢀ0.285
ꢀ0.283
ꢀ0.280
ꢀ0.269
ꢀ0.273
ꢀ0.273
ꢀ0.040
ꢀ0.075
ꢀ0.041
ꢀ0.042
ꢀ0.042
ꢀ0.043
ꢀ0.046
ꢀ0.041
ꢀ0.033
ꢀ0.034
0.234
0.218
0.239
0.244
0.243
0.240
0.234
0.227
0.240
0.239
ꢀ13.015
41.310
41.656
42.775
42.486
1.961
ꢀ3.946
ꢀ11.479
ꢀ13.592
ꢀ13.422
10
Benzene
1
2
3
4
5
6
7
8
9
ꢀ0.272
ꢀ0.295
ꢀ0.278
ꢀ0.283
ꢀ0.284
ꢀ0.281
ꢀ0.279
ꢀ0.266
ꢀ0.270
ꢀ0.271
ꢀ0.037
ꢀ0.073
ꢀ0.038
ꢀ0.040
ꢀ0.042
ꢀ0.042
ꢀ0.047
ꢀ0.040
ꢀ0.029
ꢀ0.032
0.235
0.221
0.239
0.243
0.242
0.240
0.232
0.226
0.240
0.239
ꢀ13.301
48.283
49.522
50.509
49.611
1.050
1.124
ꢀ6.238
ꢀ9.005
ꢀ8.325
10
a
IC50 values obtained in this study.
o-position of the ring A and nitrogen from C]N is conrmed.
On the basis of the lengths of these intramolecular hydrogen
bonds (Tables S2 and S3†) they can be considered as strong.
Density functional theory
All geometrical and conformational isomers of the investigated
Schiff bases were determined, and their energies calculated.
The most stable isomers of all compounds are presented in
Fig. S1 of ESI.†
HOMO and LUMO
To verify the quality of structures predicted by theoretical The HOMO and LUMO are delocalized through the entire
calculations, available crystal structures (those for compounds molecule for all studied Schiff bases (Fig. S2†). The energies of
1 and 7)19 were compared to their optimized structures (Tables HOMO and LUMO are very important parameter in dening the
S2 and S3,† respectively). As expected, the obtained geometrical reactivity of molecules, because they usually take part in
parameters for all solvents used are in mutual, excellent chemical reactions. The molecule which has the lower EHOMO
agreement. Furthermore, the calculations reproduced the has weak electron donating ability. On the other hand, the
experimental bond lengths and angles, as well as dihedral higher EHOMO implies that the molecule is a good electron
angles very well. Some deviations between experimental and donor.20 The EHOMO values for the Schiff bases are shown in
calculated structural characteristics are, certainly, a conse- Table 1. The molecules with hydroxy group in p-position and
quence of the fact that experimental values refer to the solid methoxy group in m-position in ring A show the largest EHOMO
state, whereas calculated values refer to the solution. As the values of ꢀ0.269 eV for compound 8, and ꢀ0.273 eV for
skeleton of other molecules under investigation is identical compounds 9 and 10. In contrast, molecules with hydroxy group
with these two, we can assume that their geometries are in o-position in ring A (compounds 2–7) show decreased HOMO
also well determined. Furthermore, assumption that Schiff values. Consequently, these compounds have weaker electron
bases 1–7 form hydrogen bond between hydroxy group in donating ability than other Schiff bases. The results obtained in
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Table 2 Calculated thermodynamical parameters (kJ molꢀ1) of anti-
oxidative mechanisms for Schiff bases
the present work indicate that the existence of structure that
resembles vanillin in A-ring is important for the increased
energy of HOMO orbitals, and thereby better antioxidative
potential of these compounds. The o-OH group in ring B
contributes to the increase of EHOMO, and cannot be neglected.
All these results are in accordance with the experimental data
for IC50 (Table 1).
The HOMO–LUMO gap determines chemical reactivity. This
energy is directly related to the easiness of excitation of inves-
tigated molecules. Data from Table 1 for HOMO–LUMO gap also
suggest that 8 has the highest antioxidative potential since it
has the lowest value.
HAT
BDE
A
SET-PT
IP
SPLET
PA
PDE
A
ETE
A
B
B
0
A
B
B
Methanol
1
2
3
4
5
6
7
8
9
10
407
406
406
407
407
407
396
353
351
351
351
542
624
559
584
586
566
555
533
542
541
56
ꢀ28
38
13
11
32
31
11
0
183
168
181
180
179
178
167
140
146
146
149
414
428
416
417
419
419
419
403
395
396
392
366
360
364
ꢀ9
ꢀ4
22
152
152
171
404
399
384
Stabilization energies (DEiso
)
Calculated DEiso values are presented in Table 1. On the basis of
obtained values it is possible to nd relative stability for the
involved hydroxy and methoxy groups of investigated Schiff
bases. Applying stabilization energy is a simple method to
predict the antioxidative potential to scavenge free radicals.
Obtained results in this study conrm the importance of
p-hydroxy group in the ring A, and o-hydroxy group in the ring B,
in the stabilization of radical species obtained aer hydrogen
atom abstraction. The presence of an additional methoxy group
decreased DEiso for compounds 8–10, because of the fact that
oxygen atoms can donate lone electron pairs to stabilize the
corresponding semiquinone free radical.
0
Water
1
2
3
4
5
6
7
8
9
10
412
411
411
412
412
412
401
358
356
356
356
522
603
539
564
566
546
535
513
522
521
71
ꢀ12
53
29
27
47
47
26
15
16
16
191
177
189
189
188
187
176
150
155
155
159
401
415
403
404
405
406
406
389
381
382
379
371
365
369
7
11
37
161
161
179
391
386
371
Benzene
Bond dissociation enthalpy and proton affinity
1
2
3
4
5
6
7
8
9
10
415
352
636
661
650
665
639
661
651
621
630
633
189
163
174
161
186
165
159
148
136
134
126
465
434
464
462
457
458
437
409
422
418
416
359
390
360
363
368
368
373
360
345
349
345
414
415
416
415
417
400
359
357
357
Homolytic O–H bond cleavage of the investigated molecules
yields corresponding radicals. The calculated BDE values are
presented in Table 2. It is obvious that stability of the obtained
semiquinone radicals plays a very important role in deter-
mining the antioxidative activity of Schiff bases. Distribution of
spin density provides a reliable representation of reactivity and
stability of free radicals.21 The radicals with good spin delocal-
ization are formed more easily and they are more stable than
those with localized spin density. SOMOs for the most stable
radicals of investigated Schiff bases are presented in (Fig. 2) and
distribution of the spin density for all radicals in methanol are
presented in Fig. S3.† In all radicals formed by homolytic
cleavage of the OH group in p-position, either in the rings A or B,
spin density is delocalized over the involved oxygen, and o- and
p-carbon atoms of the corresponding aromatic ring. Additional
stabilization is achieved by delocalization across the double CN
bond, as well as across o- and p-carbon atoms of the adjacent
ring. As a consequence Schiff bases with hydroxy groups in
p-position have the smallest BDE values (compounds 1, 8–10).
Table 2 shows that BDE values of Schiff bases with hydroxy
group in p-position are lower than those with hydroxy groups in
m- and o-positions. The main reason for somewhat higher BDE
values of m-hydroxy groups lies in the fact that less stable
radicals are formed, and that there is less delocalization of spin
density via CN double bond and another aromatic ring (6 in
Fig. 2). As for hydroxy groups in o-position in the A ring, rela-
367
367
370
115
125
159
426
429
456
350
347
324
Heterolytic cleavage of the O–H bonds of Schiff bases leads to
formation of the corresponding anions. Geometrical parame-
ters for anions of Schiff bases 1 and 7 are listed in Tables S2 and
S3.† In comparison to the parent molecules, there are signi-
cant changes in bond lengths in the ring where anion is formed,
suggesting decrease of aromaticity of this ring. All anions, with
corresponding charge distribution obtained by NBO analysis,
are presented in Fig. S4.† The charge distribution shows that
anions formed in the ring A are slightly better delocalized
compared to those formed in the ring B. The decrease of
negative charge on o- and p-oxygen atoms is a consequence of
good delocalization of negative charge over the ring A and CN
double bond, and across o- and p-carbon atoms of the adjacent
ring (Fig. S3†).
Ionization potential
tively strong intramolecular hydrogen bond with nitrogen is The ionization potential (IP) illustrates the easiness of electron
also responsible for signicantly higher BDE values.
donation of phenolic compounds. It is well known that
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is thermodynamically more probable. All thermodynamical
parameters for the investigated Schiff bases were calculated
using M05-2X/6-311+G(d,p) in water, methanol, and benzene
(Table 2). The obtained results show that compounds 2–6 have
signicantly higher BDE, IP, and PA values than other
compounds under investigation. The values of thermodynam-
ical parameters undoubtedly show that negligible antioxidative
activity can be expected for these compounds. These results are
in agreement with experimental IC50 values Table 1. Mutual
comparison of BDE, IP, and PA values in Table 2 for other
compounds (1, 7–10) reveals that IP values are the largest,
indicating that SET-PT mechanism is not a favourable reaction
path in all investigated solvents.
On the other hand, in polar solvents PA values are signi-
cantly lower than the corresponding BDEs. It means that the
SPLET mechanism is a dominant reaction pathway in polar
medium. Low PA values show that Schiff bases can easily
undergo heterolytic dissociation of OH bonds and yield the
corresponding phenoxide anions. Taking this into account, it is
reasonable to expect that SPLET mechanism prevails under
physiological conditions (pH of 7.4). BDE and PA values in Table
2 show that HAT and SPLET mechanisms are competitive in
nonpolar solvent.
Conclusions
In this study, antioxidative capacity of some phenolic Schiff
bases has been examined, using experimental and theoretical
tools. Two of investigated compounds, one salicylaldehyde (7)
and one vanillic (8) Schiff base, can be considered as good
radical scavengers. p-Hydroxy group in ring A, as well as o-in the
ring B, are responsible for the antioxidative activity of these
compounds. Furthermore, DFT examination showed that pres-
ence hydroxy groups in respective positions inuences the
increase of EHOMO, lowers HOMO–LUMO gap, and in that way
contributes to better antioxidative potential of these
compounds. Low activity of Schiff bases 2–6 is assigned to the
intramolecular hydrogen bond formation between o-hydroxy
group in the ring A and nitrogen from C]N, which conse-
quences signicantly higher values for the thermodinamical
parameters for these compounds. Based on the same data for
the molecules which exhibit radical scavenging activity (1, 7–10),
SET-PT mechanism is not expected in all investigated solvents.
In polar medium, SPLET mechanism prevails, while HAT and
SPLET mechanisms are competitive in nonpolar solvent. Taking
into account that Schiff bases 7 and 8 interact well with DPPH
radical, and fact that DPPH assay may indicate activity of
compounds towards reactive oxygen species present in the living
cells, these compounds can be considered as good antioxidants
and will be further investigated in vitro/vivo, for example on the
cancer cell lines.
Fig. 2 SOMOs for the most stable radicals issued from the investi-
gated Schiff bases.
molecules with lower IP values are more active. The obtained IP
values of investigated compounds are presented in Table 2.
Comparison of the IP values from Table 2 with IP values of other
Schiff bases,22 showed that the values obtained in this study are
generally somewhat lower. This is due to using different
approaches for calculating these parameters. On the basis of
our results, molecules with only one o-hydroxy group in the ring
A (compounds 2–5) have generally higher IP values. Introducing
another hydroxy group in the ring B in m-, o-, or p-position
(compounds 6, 7, and 1), additionally reduces the IP value. This
is consistent with the well-known fact that position of the
hydroxy group in the molecule plays a very important role for
the electron donating capacity. The IP values decrease in
molecules with additional methoxy group in m-position
(compounds 9 and 10). Moreover, introduction of another
hydroxy group in ring B in o-position, additionally reduces the
IP values (compound 8). This implies that this compound has
increased electron donor capacity, which facilitates the forma-
tion of the radical cation.
Antioxidative mechanisms
Experimental
Materials and reagents
On the basis of values of thermodynamical parameters (BDE, IP,
PDE, PA, and ETE) prevailing antioxidative mechanism of Schiff
bases in a corresponding solvent can be predicted.23 The lowest The compounds salicylaldehyde, vanillin, aniline, 4-uoroani-
value for some parameter points out which reaction mechanism line, 4-nitroaniline, toluidine, 2-hydroxyaniline, 3-hydroxyaniline,
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4-hydroxyaniline, nordihydroguaeretic acid (NDGA), and
The local and global minima were conrmed to be real
2,2-diphenyl-1-picrylhydrazyl (DPPH) were obtained from Aldrich minima by frequency analysis (no imaginary frequencies were
Chemical Co. All common chemicals were of reagent grade. The obtained). To evaluate the impact of different solvents (water,
NMR spectra were run in DMSO on a Varian Gemini 200 MHz methanol, and benzene) the continuum solvation model CPCM
spectrometer. Melting points were determined on a Mel-Temp was used.29 The solvent effects were taken into account in all
capillary melting points apparatus, model 1001. Elemental geometry optimizations and energy calculations. Water and
microanalyse for carbon, hydrogen, and nitrogen were performed benzene were used to mimic aqueous and lipid environments,
at the Faculty of Chemistry, University of Belgrade.
whereas methanol was selected because the experiments were
performed in this solvent. The NBO analysis of all species was
performed at the M05-2X/6-311+G(d,p) level of theory.30 The
NBO analysis describes a structure by a set of localized bonding,
antibonding, and Rydberg orbitals. Also, this analysis provides
explanation of stabilizing and destabilizing interactions
between occupied and unoccupied orbitals.
Synthesis of Schiff bases
Schiff bases (1–10) were prepared according to procedure in the
literature with some modications.24 In our case, aldehyde
(salicylaldehyde, vanillin) (1 mmol), corresponding aromatic
amine (aniline, 4-uoroaniline, 4-nitroaniline, toluidine,
2-hydroxyaniline, 3-hydroxyaniline, 4-hydroxyaniline) (1 mmol),
and 3 mL of methanol were placed in ask and stirred at 70 ꢁC
for 3 h. Aer completion of the reaction, the solvent was evap-
orated, and nal product was obtained by recrystallization from
ethanol. Schiff bases were obtained in 90–97% yield. All Schiff
bases (1–10) were characterized with melting point and 1H NMR
spectra (ESI,† compounds 1–10).
BDE, IP, PDE, PA and ETE values were determined from total
enthalpies of the individual species, as follows:
BDE ¼ H(SB–Oc) + H(Hc) ꢀ H(SB–OH)
IP ¼ H(SB–OHc+) + H(eꢀ) ꢀ H(SB–OH)
PDE ¼ H(SB–Oc) + H(H+) ꢀ H(SB–OHc+)
PA ¼ H(SB–Oꢀ) + H(H+) ꢀ H(SB–OH)
ETE ¼ H(SB–Oc) + H(eꢀ) ꢀ H(SB–Oꢀ)
(4)
(5)
(6)
(7)
(8)
The corresponding data for the new compound (E)-4-((4-
uorophenylimino)methyl)-2-methoxy-phenol (10) are presented
1
ꢁ
here: colourless crystals – mp 144–146 C; H NMR (200 MHz,
DMSO-d6): d ¼ 3.84 (s, 3H, ꢀOCH3), 6.89 (d, J ¼ 8.10 Hz, 1H,
Ar-H), 7.16–7.28 (m, 5H, Ar-H), 7.32 (dd, J ¼ 8.3, 1.9 Hz, 1H,
Ar-H), 7.52 (d, J ¼ 1.80 Hz, 1H, Ar-H), 8.44 (s, 1H, CH]N), 9.77
(s, 3 –OH); 13C NMR (50 MHz, DMSO) d ¼ 56.7, 111.8, 111.4,
116.1, 116.5, 116.6, 117.2, 123.6, 127.5, 145.3, 149.21, 151.1,
153.9, 162.1; C14H12FNO2 (FW ¼ 245.25): C, 68.56; N, 5.71; H,
4.93%; found: C, 68.15; N, 5.68; H, 4.81%.
The values for solvation enthalpies of proton and electron
were taken from literature.31 All reaction enthalpies dened in
eqn (4)–(8) were calculated at 298 K.
The radical stability was determined by the calculation of
stabilization energies (DEiso), as shown in eqn (9), where Ph–OH
and Ph–Oc stand for the molecule of phenol and phenoxy radical.
DPPH free radical scavenging assay
DEiso ¼ (H(SB–Oc) + H(Ph–OH)) ꢀ (H(SB–OH) + H(Ph–Oc))
In this study, the free radical scavenging activity of the exam-
ined Schiff bases was performed using the DPPH method,
according to ref. 25. In brief, 1 mL (0.1 mm) of DPPH solution in
methanol was mixed with an equal volume of the tested
compound (20 mL of compound solution in DMSO and 980 mL of
methanol). The reaction mixture is le at room temperature for
30 and 60 min. Aer incubation the absorbance was measured
at 517 nm. As control solution, methanol was used. IC50 values
represent the concentration necessary to obtain 50% of a
maximum scavenging capacity. NDGA was used as positive
control with 96% activity at 0.1 mM.
(9)
Acknowledgements
This work was supported by the Ministry of Education, Science
and Technological Development of the Republic of Serbia
(projects no. 172016, 174028).
Notes and references
Computational details
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Gaussian program package.26 The equilibrium geometries of all
Schiff bases and corresponding radicals, radical cations, and
anions were optimized by the empirical exchange-correlation
M05-2X functional27 and split-valence basis set 6-311+G(d,p).
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This functional has been successfully used for solution of
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