1H NMR, 13C NMR and mass spectral studies of some Schiff bases derived from 3-amino-1,2,4-triazole

Article abstract of DOI:10.1016/j.saa.2009.08.042

Heterocyclic Schiff bases derived from 3-amino-1,2,4-triazole and different substituted aromatic aldehydes are prepared and subjected to ¹H NMR, ¹³C NMR and mass spectral analyses. ¹H NMR spectra in DMSO exhibit a sharp si

Full text of DOI:10.1016/j.saa.2009.08.042

Spectrochimica Acta Part A 74 (2009) 902–910
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
1H NMR, 13C NMR and mass spectral studies of some Schiff bases
derived from 3-amino-1,2,4-triazole
Y.M. Issa∗, H.B. Hassib, H.E. Abdelaal
Department of Chemistry, Faculty of Science, Cairo University, Giza, Egypt
a r t i c l e i n f o
a b s t r a c t
Article history:
Heterocyclic Schiff bases derived from 3-amino-1,2,4-triazole and different substituted aromatic alde-
hydes are prepared and subjected to 1H NMR, 13C NMR and mass spectral analyses. 1H NMR spectra in
DMSO exhibit a sharp singlet within the 9.35–8.90 ppm region which corresponds to the azomethine
proton. The position of this signal is largely dependent on the nature of the substituents on the benzal
moiety. It is observed that the shape, position and the integration value of the signal of the aromatic pro-
ton of the triazole ring (5C) are clearly affected by the rate of exchange, relaxation time, concentration of
solution as well as the solvent used. 13C NMR is taken as substantial support for the results reached from
1H NMR studies. The mass spectral results are taken as a tool to confirm the structure of the investigated
compounds. The base peak (100%), mostly the M-1 peak, indicates the facile loss of hydrogen radical. The
fragmentation pattern of the unsubstituted Schiff base is taken as the general scheme. Differences in the
other schemes result from the effect of the electronegativity of the substituents attached to the aromatic
ring.
Received 30 May 2009
Received in revised form 19 July 2009
Accepted 7 August 2009
Keywords:
NMR
Mass spectra
1,2,4-Triazole
Schiff bases
Substituent effect
© 2009 Elsevier B.V. All rights reserved.
1. Introduction
Some new liquid crystalline 2,5-disubstituted 1,3,4-thiadiazole
derivatives incorporating a central group (–CH N–) have been syn-
3-Amino-1,2,4-triazole has been reported as being a carcino-
genic reagent, but its ring has shown a well known anti-microbial
effect. It has been proved that the incorporation of this molecule
into the biologically active azomethine linkage (–CH N–) produces
compounds with high pharmacological activity [1]. Heterocyclic
Schiff bases of 2-aminobenzothiazole, benzoxazol, thiazole and tri-
azole were prepared and characterized by their melting points,
elemental analysis, IR, UV and NMR spectra [2]. It has been proven
that they all have a planar configuration and an extended con-
jugation. 1H and 13C NMR study and AMI calculations of some
azobenzenes and N-benzylideneanilines have been studied [3].
A correlation between NMR chemical shifts and the Hammet
ꢀ-constants of substituents and the resonance mechanism of trans-
ferring the substituents were found to dominate. 13C and 17O
NMR spectra were reported for three series of Schiff bases: 2-
(aminomethylene)-cyclohexanone (1), salicydeneamine (2) and
N-[(2-hydroxy-1-naphthalenyl) methylene] amine (3) [4]. The data
showed that Schiff bases (1) exist in keto enamine form, (2) in enol
imine form and (3) as an equilibrium mixture of both forms. The
tautomeric equilibria were found to be shifted towards the enol
imine form in non-polar solvents and with increase in temperature.
thesized [5]. Their structures have been characterized with elemen-
tal analysis, IR, super 1H NMR and MS, and their properties were
determined using DSC and texture. Recently, new Schiff base hydra-
zones bearing 3-(4-pyridine)-5-mercapto-1,2,4-triazole were pre-
pared and subjected to biological and computational studies [6].
Microwave-assisted synthesis, characterization and theoretical
calculations of the first example of free and metallophthalocya-
nines from salen type, Schiff base, derivative bearing thiophen and
triazole rings were studied [7]. The newly prepared compounds
have been characterized by elemental analyses, IR, 1H NMR, MS
and UV–vis spectroscopy.
A
series of metal complexes of Co(II), Ni(II), Cu(II) and
Zn(II) have been synthesized with newly biologically active
ligands prepared by the condensation of 4-amino-5-mercapto-
3-methyl-S-triazole, 4-amino-3-ethyl-5-mercapto-S-triazole with
2-acetylpyridine. The structures of the complexes have been pro-
posed by elemental analysis, spectroscopic data i.e. IR, 1H NMR,
electronic and magnetic measurements. Antibacterial activities
are also reported [8]. Synthesis and spectral studies of 5-[(1,2,4-
triazzolyl-azo]-2,4-dihydrxybenzaldehyde (TA) and its Schiff base
with 1,3-diaminopropane (TAAP) and 1,6-diaminohexane (TAAH)
were reported [9]. The authors discussed the analytical application
of the prepared compounds for spectrophotometric microdeter-
mination of cobalt (II). Some radiochemical studies were also
presented.
∗
Corresponding author. Tel.: +20 2 35868094; fax: +20 2 35728843.
E-mail address: yousrymi@yahoo.com (Y.M. Issa).
1386-1425/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2009.08.042

Y.M. Issa et al. / Spectrochimica Acta Part A 74 (2009) 902–910
903
Table 1
Physical properties and elemental analysis of the investigated Schiff bases.
Compound
X
M.P. (◦C)
Color
Analysis
Calc. (Found)
%C
%H
%N
a
b
c
d
e
f
H
194
190
195
178
193
235
238
205
White
62.78
(62.50)
64.43
(64.30)
59.38
(59.40)
57.35
(56.70)
57.35
(57.50)
61.38
(61.25)
49.77
(51.50)
52.27
4.64
(4.40)
5.36
(5.20)
4.94
(5.20)
4.24
(4.20)
4.24
(4.50)
6.09
(6.19)
3.22
(4.50)
3.38
32.54
(32.11)
30.07
(29.98)
27.71
(28.03)
29.73
(27.00)
29.73
(29.52)
32.53
(32.15)
32.26
(31.20)
27.10
p-CH3
p-OCH3
o-OH
p-OH
p-N(CH3)2
p-NO2
p-Cl
White
Pale yellow
Yellow
Yellow
Bright yellow
Orange yellow
Pale yellow
g
h
(53.00)
(3.70)
(27.51)
The present article deals with the synthesis and spectral char-
following structural formulae:
acterization of some 1,2,4-triazole Schiff bases using 1H NMR, 13
NMR, EI-MS, CI-MS.
C
2. Experimental
All chemicals used were of the highest purity grade purchased
from either Fluka (Germany) or Aldrich (USA). The heterocyclic
Schiff Bases under study were prepared by the reflux of equimolar
amounts of 3-amino-1,2,4-triazole and the appropriate aromatic
aldehyde in 100 ml absolute ethanol for about 4 h. The products
obtained were cooled, filtered off and crystallized from absolute
ethanol. The prepared 3-amino-1,2,4-triazole Schiff bases have the
where: x = H (a), p-CH3 (b), p-OCH3 (c), o-OH (d), p-OH (e), p-NMe2
(f), p-NO2 (g), and p-Cl (h).
The physical properties and the elemental analysis results of
the prepared Schiff bases are given in Table 1. The 1H NMR spec-
tra were recorded on one of the following Bruker spectrometers: a
WP-200 or an AC-200 (200 MHz for proton), an AM-300 (300 MHz
Table 2
1H NMR chemical shift values (ı) of 3-amino-1,2,4-triazole Schiff bases (Ic-Ih) in different solvents.
Compound
c
Solvent
ı (ppm)
H2
H5
H7
H9
H13
H10
H12
7.06
7.08
7.03
6.99
7.04
6.86*
7.06
H11
3.84
3.83
.85
3.87
3.87
3.77*
3.89
DMSO
DMSO + D2O
CD3CN
CDCl3
CD3OD
13.9
–
–
–
–
–
8.49
8.24
–
9.13
9.13
9.17
9.24
9.08
5.29*
9.19
7.97
7.95
7.95
7.95
7.96
7.32*
8.95
7.93
7.93
7.92
7.91
7.93
7.29*
7.95
7.11
7.12
7.05
7.03
7.08
6.89*
7.51
–
8.17
And
8.38
(CD3)CO
12.98
d
e
DMSO
DMSO + D2O
CD3OD
14.11
–
–
.8.5
9.42
9.40
9.32
9.36
7.79
7.76
7.59
7.45
7.44
7.44
7.43
7.45
6.96
6.98
6.98
6.99
12.53
–
–
6.96
6.98
6.98
6.90
.8.40
8.32
8.10
CDCl3
12.35
10.95
DMSO
13.95
13.85
–
8.47
9.07
7.84
7.81
6.90
6.88
10.43
10.22
–
–
–
DMSO + D2O
CD3OD
8.43
8.17
And
9.07
9.05
4.91*
7.85
7.88
7.33*
7.83
7.83
7.29*
6.91
6.93
6.82*
6.88
6.88
6.77*
–
f
DMSO
CD3OD
CDCl3
13.76
–
11.02
8.40
Broad
7.90
8.99
8.94
9.17
7.79
7.84
7.88
7.77
7.80
7.84
6.81
6.81
6.76
6.77
6.78
6.72
3.03
3.08
3.10
g
DMSO
DMSO + D2O
14.18
–
8.58
8.42
9.35
9.29
8.27
8.36
8.23
8.31
8.38
8.24
8.34
8.20
–
–
h
DMSO
DMSO + D2O
CD3OD
14.04
8.53
8.35
8.35
And
9.21
9.22
9.16
5.82*
8.04
8.05
8.01
7.49*
7.99
7.96
7.96
7.48*
7.63
7.64
7.56
7.45*
7.58
7.59
7.51
7.34*
–
–
–
–
–
–
X = p-OMe (c), o-OH (d), p-OH (e), p-NMe2 (f), p-NO2 (g), p-Cl (h).
*
The signals of the hydrolysis product in presence of D2O.

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Y.M. Issa et al. / Spectrochimica Acta Part A 74 (2009) 902–910
Fig. 1. 1H NMR spectra of Ie in DMSO at different temperature 312 K (a), 315 K (b),
372 K (c) and 298 K (d).
for proton), or a WP-360 (360 MHz for proton). 13C NMR spectra
were obtained using a high-power broad-band proton decoupling.
The probe was usually at ambient temperature. All low temper-
ature experiments were recorded on the AM-300 instrument. All
the spectra are reported in ppm with respect to tetramethylsilane
(TMS, ı = 0.00 ppm), and were referenced vs TMS or the appropri-
ate solvent peak. All deuterated solvents (CD3CN, DMSO, CD3OD)
were purchased from Aldrich in sealed ampoules that were opened
immediately prior to the experiment.
Fig. 2. 1NMR spectra of Ie in DMSO (a), DMSO + H2O (b), DMSO conc. (c), and CD3OD
(d).
Mass spectra were recorded with one of the following Kratos
double-focusing instruments at the Penn State University (USA)
mass spectrometry facility: a MS-950 mass spectrometer in the
electron-impact mode (reported as EI-MS), and a MS-25 mass spec-
trometer for chemical ionization (reported as CI-MS).
responds to the azomethine proton of the p-NO2 derivative (Ig),
which has the highest electron affinity; while the upperfield value
(ı = 8.9 ppm) is of the p-NMe2 substituent (If) with the highest
donating power. Thus, it can be concluded that the position of
the azomethine proton is strongly affected by the electronegative
character of the substituents on the benzal ring. As the electron-
affinity of the substituent increases, the azomethine proton is
shifted downfield due to increased deshielding effect.
3. Results and discussion
3.1. 1H NMR and 13C NMR spectral studies
The chemical shift of the aromatic proton of the triazole ring
(C5) is observed within the 8.59–8.17 ppm region of spectrum.
The shape, the position, and the integration value of this signal
are clearly affected by the rate of exchange; the relaxation time,
the concentration of the solution, as well as the solvent used. In
most cases (especially when using DMSO), the signal correspond-
ing to this proton appears as a broad peak with ∼half its integration
value; while the other half integration value is revealed by a peak
at ∼7.92–7.80 ppm. In most cases, this latter peak can hardly be
distinguished from the aromatic protons of the benzal ring (H9
The 1H NMR main signals-chemical shifts of the investigated
heterocyclic Schiff bases (Ic-Ih) recorded in DMSO are given in
Table 2. In each case, D2O was then added to check for the
hydrolysable protons. The chemical shifts of the different types of
protons of the investigated Schiff bases in different solvents (when-
ever possible) are reported in Table 2.
A sharp singlet is observed within the 9.36–8.90 ppm region
of spectrum, which corresponds to the azomethine proton. It is
remarkable that the downfield chemical shift (ı = 9.35 ppm) cor-

Y.M. Issa et al. / Spectrochimica Acta Part A 74 (2009) 902–910
905
Scheme 1. Fragmentation pattern of unsubstituted 1,2,4-triazole Schiff base (a).
Fig. 3. CI-MS of Ic.

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Y.M. Issa et al. / Spectrochimica Acta Part A 74 (2009) 902–910
and H13). Accordingly, when this signal is merged into the dou-
blet corresponding to those two protons, its integration is added
to the value equal to the two aromatic protons. This phenomenon
may be attributed to the presence of a rapid tautomeric equilibrium
between the following two structures:
age. It is worthwhile mentioning that, the mesomeric effects can
only be transferred to the heterocyclic ring in planar molecules.
This confirms the planarity of the investigated Schiff bases.
The NH peak of the triazole ring appears as a broad sig-
nal strongly shifted downfield (ı ∼ 14.18–11.02 ppm), indicating
In CD3OD or DMSO + H2O, the latter peak is shifted upfield and
appears as a sharp singlet with an integration of one proton at
ı ∼ 8.42–8.17 ppm. This can be explained on the basis that, when
the rate of exchange is low; the probe can see both tautomers
and the observed peak is an average between both forms. How-
ever, when the rate of exchange is rapid (e.g. in presence of H2O),
only one tautomeric form prevails and can be detected. It can also
be revealed from Table 2 that this signal is shifted to lower field
values by strong electron-withdrawing substituents (p-NO2, and
p-Cl); and is shifted to higher fields with electron-donating sub-
stituents (p-NMe2, p-OCH3, and p-OH). Such observations indicate
that the electronic effects of the substituents on the benzal ring are
transferred to the heterocyclic ring through the azomethine link-
the possibility of its involvement in either an intramolecular or
an intermolecular hydrogen bonding with the solvent. This is
attributed to the fact that hydrogen bonding decreases the electron
density around the proton, and thus moves the proton absorption
to lower field [10]. The position of this signal is solvent depen-
dent. The upfield shift (ı ∼ 12.3 ppm) observed in deuterated CDCl3
(non-polar solvent) is in accordance with the presence of inter-
molecular hydrogen bonding. The appearance of the NH signal as
a doublet in some spectra and as a singlet in others; is in agree-
ment with the presence of a rapid tautomeric equilibrium which
is dependent on the rate of exchange, as well as, the concentration
of the solution. The disappearance of the NH signal from the 1H
NMR spectra recorded in D2O or in CD3OD confirms its ionizable
Scheme 2. Fragmentation pattern of p-methoxy 1,2,4-triazole Schiff base (c).

Y.M. Issa et al. / Spectrochimica Acta Part A 74 (2009) 902–910
907
character. The relation between the chemical shift of this signal and
the substituents on the benzal ring is the same as in the case of the
azomethine proton (H7) and the aromatic proton of the triazole ring
(H5). This fact further confirms the direct effect of the substituents
of the benzal ring on the proton of the heterocyclic ring, giving a
proof of the planarity of the molecule.
The four aromatic protons of the investigated Schiff bases appear
as two doublets each with an integration value corresponding to
two protons. These doublets appear at chemical shift values of
8.28–7.79 ppm corresponding to H9 and H13, and 8.38–6.77 ppm
for H10 and H12. It has to be mentioned that, when there is a large
chemical shift difference between the two protons of each set, the
peaks appear as clear distinct doublets with a quite wide separa-
tion, as in the case p-NMe2 (Id). In the case where the four protons
are nearly in the same chemical environment, there is only a small
chemical shift difference and the peaks appear as almost a quartet
as in the case of the p-NO2 (Ie).
firms its participation in intramolecular hydrogen bonding where
it becomes more deshielded than in the case of the p-OH derivative.
In the nonpolar deuterated chloroform (CDCl3) solvent, the signal
corresponding to the o-OH proton is shifted upfield (ı 10.95 ppm).
On raising the temperature of the DMSO solution of Ie (p-OH),
from 312 K up to 372 K, the peaks at 13.90 ppm and 10.43 ppm; cor-
responding to the NH and OH protons, are slightly shifted upfield
and started to broaden until they nearly disappeared and could
not be picked (Fig. 1). Also, it is remarkable that the peak cor-
responding to the CH proton of the triazole ring (H5) started to
sharpen with increasing the temperature giving the correct inte-
gration of one proton; indicating the prevalence of one tautomer at
high temperatures. On lowering the temperature, the disappeared
peaks started to sharpen again, while the one corresponding to H5
appeared broad again.
The 13C NMR of the heterocyclic Schiff bases Ic-Ih in DMSO and
CD3OD are used as complementary technique for confirming the
structure of the prepared Schiff bases. The 13C NMR spectra of Ic
is characterized by very sharp peaks at 114.48 and 131.16 ppm,
corresponding to C10 and C12, and C9 and C13 of the aromatic
ring, respectively. The downfield peaks at 163.39 and 162.71 ppm
are attributed to C7 and C11, respectively. The downfield shift of
these signals is due to the descreening effect of the electronega-
The 1H NMR spectra of Ib (o-OH) and Ie (p-OH) in DMSO
showed the hydroxyl proton as a broad signal at ı 12.53 ppm and ı
10.43 ppm, respectively; with an integration value corresponding
to one proton in each case. In CD3OD and D2O, the peak corre-
sponding to this proton disappears. The strong downfield shift of
the hydroxyl proton signal in the case of the o-OH derivative, con-
Scheme 3. Fragmentation pattern of unsubstituted 1,2,4-triazole Schiff base (a).

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Y.M. Issa et al. / Spectrochimica Acta Part A 74 (2009) 902–910
bons C10 and C12 are significantly shifted downfield (ı 124.04 ppm).
This is the new signal of the diminishing peak at 123.66 ppm, indi-
cating the presence of two tautomeric forms of the compound in
solution. The peak at 130.12 ppm is assigned to C9 and C13. The
peaks at 128.50, 140.85, 144.78, 149.27 and 161.97 ppm correspond
to C8, C3, C5, C11, and C7, respectively.
The chemical shift of the four aromatic carbon atoms in case of
Ih is very close indicating a very similar chemical environment.
The spectra of all investigated Schiff bases in CD3OD revealed the
same peaks as in the case of DMSO with a slight downfield shift.
tive nitrogen and oxygen atoms, respectively. The signal observed
at ı 127.99 ppm corresponds to C8. The upfield peak at 55.46 ppm
is attributed to the 13C of the CH3 group of the p-OCH3 substituent.
In the case of Ie, the spectrum showed the same sharp peaks with
a close chemical shift values as those in the case of Ic.
The 13C NMR spectrum of Id (o-OH), is also characterized by
a number of sharp peaks which are in good agreement with the
expected chemical shift values. The upfield peaks at 116.69, 119.48,
and 119.18 ppm correspond to the aromatic carbon atoms C12
,
C
10, and C11, respectively. The downfield peaks at 164.85 and
134.16 ppm are assigned to C9 and C8. The peaks corresponding
to C3 and C5 are broad and may or may not be easily picked.
In the case of p-NMe2, Schiff base derivative (If), the peaks at
39.62 and 111.48 ppm are the only observed sharp peaks in the
spectrum. These peaks correspond to the methyl groups of the
NMe2 and to the aromatic carbon C10 and C12, respectively. The rest
of the signals appeared broad, but at the expected chemical shift
values. The 13C chemical shifts corresponding to the aromatic car-
3.2. Mass spectral studies
The fragmentation pattern of the unsubstituted Schiff base (Ia)
is taken as a general scheme showing the main fragmentation
paths involved (Scheme 1). Differences of fragmentations patterns
of Schiff bases Ib-Ih is attributed to the effect of the electronega-
tivity of the substituents attached to the aromatic ring. The mass
Scheme 4. Fragmentation pattern of p-chloro 1,2,4-triazole Schiff base (c).

Y.M. Issa et al. / Spectrochimica Acta Part A 74 (2009) 902–910
909
spectral pattern of Ia (Fig. 2) shows a strong molecular ion peak (m/z
M-68 (m/z 118, 12%) corresponds to the stable ion:
172, 22.5%) which loses a hydrogen radical (path (A)) to give the
base peak at M-1 (m/z 171, 100%). The loss of a neutral molecule of
HCN (path (C), Scheme 1) through break of 4N–5C and 1N–2N bonds
is a general feature of the prepared Schiff bases. This step gives the
characteristic peak at m/z 145 (4%) and is usually followed by loss
of HN2 (path (F)) or CHN2 (path (D)); giving the peaks at m/z 116
(6%) and 104 (11%), respectively. It is evident from Scheme 1 that
cleavage at 6N–3C bond (path (B)) gives the predominant peak at
m/z 104 through the loss of C2H2N3 (m/z 68). This peak corresponds
to the stable ion:
This ion gives the characteristic peaks at m/z 91 (9.7%) and 103
(6.5%) through loss HCN molecule or CH3 radical, respectively. The
peak at m/z 91 (C6H5CH2)+ is a tropylium rather than a benzylic
cation (Scheme 2). The observed peak at m/z 65 (6%), results from
the elimination of a neutral acetylene molecule from the tropylium
cation. The loss of the CH3 radical from the ionic fragment at m/z
118 (m/z 103, 6.5%) is more favorable than from the molecular ion
at m/z 186 (m/z 171, 2.6%).
The EI-MS of the p-OCH3 Schiff base derivative (Ic) shows all the
expected prominent peaks following the same previously proposed
fragmentation paths for Ia. As the case of Ib the loss of a CH3 radical
from the OCH3 group is confirmed by the fragment at M-15 (m/z
187, 4.3%). The OCH3 group either cleaves from the M+• or from the
prominent ion at m/z 134 giving the ions at m/z 171 (5%) or 103
(13%), respectively.
The CI-MS of Ic (Fig. 3) confirms its molecular formula, as well
as, the proposed major fragmentation mechanism. In chemical ion-
ization spectra, since the ions that are chemically produced do not
have the high excess of energy associated with the ionization by EI,
This ion then looses either NH or HCN giving the characteristic
peaks at m/z 89 (9%) and m/z 77 (9%), respectively. These latter
fragments are responsible of the prominent peaks at m/z 63 (6.5%)
and 51 (6%), respectively; through loss of an acetylene molecule.
The EI-MS of Ib (p-CH3) and Ic (p-OCH3) show all the expected
prominent peaks following the same previously proposed fragmen-
tation paths, with but few exceptions related to the presence of CH3
and OCH3 attached to the benzal ring, respectively. In cases of Ib,
the base peak is the M-1 (m/z 185, 100%) and the parent peak at
Scheme 5. Fragmentation pattern of p-nitro 1,2,4-triazole Schiff base (g).

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Y.M. Issa et al. / Spectrochimica Acta Part A 74 (2009) 902–910
they undergo less fragmentation. Fig. 3 shows the quasi-molecular
ion MH+ (m/z 203, 100%) as the base peak and confirms the ease
loss of the H+ by the intense peaks at m/z 202 and 201. The peaks at
because of its high electron-donating power which stabilizes the
positively charged ionic fragments. It is clear that the most stable
fragment is the one at m/z 146 (54%), which corresponds to (Mp-
1)—68.
m/z 243 and 245 correspond to M++C3H5 (M+41) and M++C3H7
+
+
(M+43), respectively. The radicals C3H5 and C3H7 belong to the
isobutane gas used in the CI-MS. The other peaks corresponding
to m/z 75, 134, 103 and 77 are of just low intensities.
4. Conclusion
1H NMR spectra of the heterocyclic Schiff bases in DMSO show
a sharp singlet within the 9.35–8.90 ppm region of the spectrum
which corresponds to the azomethine proton. The position of this
signal is found to be largely dependent on the electronegative char-
acter of the substituent on the benzal ring. As the electron-affinity of
the substituent increases, the azomethine proton is shifted down-
field due to increased deshielding effect. The shape, the position
and the integration value of the signal of the aromatic proton of the
triazole ring (C5) ion appears to be affected by the rate of exchange,
the relaxation time, the concentration of the solution, as well as
the solvent used. The signal appears as a broad peak with ∼half its
integration value; while the other half integration value is revealed
by a peak at ∼7.92–7.80 ppm. This phenomenon is attributed to
the presence of a rapid tautomeric equilibrium between two struc-
tures. The 13C NMR measurements are used as complementary tool
for confirming the structure of the prepared Schiff bases.
The mass spectral studies are used to give more insight on the
structure of the investigated Schiff bases. A general fragmentation
pattern is proposed from which it is evident that primary cleavages
occur at the hetero-carbon bond. The major cleavage pathways may
take place through loss of a neutral HCN molecule, followed by
elimination of either HN2 or CHN2, or through of C2H2N2. It is also
evident from the spectra that the base peak (100%) is, mostly, the
M-1 peak, indicating the facile loss of hydrogen radical. The base
peak for the o-OH Schiff base derivative is that corresponding to
the M-OH.
In the case of the p-OH Schiff base derivative, Ie, the loss of the
OH group is not a favorable step (m/z 171, 1.63%). Thus it follows the
general fragmentation patter as for the unsubstituted Schiff base.
On the contrary to the p-OH Schiff base; the o-OH compound (Id),
is very easily cleaved from the ring giving the most stable ion at
m/z 171, 100%. This step is followed by several available cleavages
at m/z 104 (5%), 77 (18%), 63 (25%) and 51 (15%); as in the case of
Ia. The peaks at m/z 120 (8%), 161 (1.4%) and 132 (5%) confirm the
proposed fragmentation paths (B, C, and C, F), Scheme 3.
It is apparent from the spectral presentation of the p-Cl Schiff
base derivative (Ih), that there is one chlorine atom in the molecule,
as the intensity of the M+2 peak (m/z 208, 8.4%) is almost 1/3 the
intensity of the parent peak (m/z 206, 26.4%). This is also confirmed
by the relative abundance of the base peak (m/z 205, 100%) which
corresponds to M-1 and the peak at m/z 207, 35% (noting that part
of this ratio belongs to the 13C isotope contribution of the par-
ent peak). The intensity of the fragment peak at m/z 181 (2%) is
about 1/3 the intensity of the fragment formed through loss of HCN
molecule (path (C), Scheme 4) at m/z 179 (6%). These peaks corre-
spond to fragments containing one chlorine atom each. This fact is
also confirmed for the ions producing the peaks at M-68 (4.3%) and
125 (1.9%); and at m/z 111 (4.5%) and 113 (1.9%). The fragment at
M-Cl (m/z 171, 2.2%) has no M+2 peak, confirming the absence of
chlorine. Also, the fragments at m/z 89 (5.7%) and m/z 75 (5.8%) has
no M+2 (Scheme 4).
In the case of the p-NO2 Schiff base derivative, Ig, the major step
in the fragmentation pattern is the loss of the NO2 radical from the
molecular ion (M-46). This step may take place through loss of an
oxygen radical from the M-1 ion (m/z 200, 6.6%), followed by loss of
a neutral NO molecule giving the prominent peak at m/z 170, 41%.
The diagnostic peak at m/z 30 (13%), confirms the loss of the NO+ ion
which is a characteristic feature of the nitro compounds [10]. Also,
the peak at m/z 46 (4%) corresponds to NO2+. The observed addi-
tional peaks resemble those resulting from the molecular ion of
the unsubstituted Schiff base. The presence of the fragments bear-
ing NO2 is less favored because of its electron-withdrawal effect
which destabilizes the positively charged fragments (Scheme 5).
On the contrary to the p-NO2 and the o-OH Schiff bases, the
p-NMe2 group is very difficulty cleaved from the benzene ring. In
fact the most stable fragments are those bearing the NMe2 group;
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