Pyridazine-based Schiff base ligands with N4OxS2 (x = 2, 4) donor set atoms: Synthesis, characterization, spectral studies and 13C chemical shifts computed by the GIAO-DFT and CSGT-DFT methodologies

Article abstract of DOI:10.1016/j.saa.2009.09.053

Six pyridazine-based Schiff base ligands, H₂Lⁿ (n = 1-5) and H₄L, with N₄O₂S₂ and N₄O₄S₂ donor set atoms, respectively, were prepared by condensation reacti

Full text of DOI:10.1016/j.saa.2009.09.053

Spectrochimica Acta Part A 75 (2010) 127–133
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Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Pyridazine-based Schiff base ligands with N4OxS2 (x = 2, 4) donor set atoms:
Synthesis, characterization, spectral studies and 13C chemical shifts computed
by the GIAO-DFT and CSGT-DFT methodologies
Hamid Khanmohammadi∗, Malihe Erfantalab
Department of Chemistry, Arak University, Arak 38156, Iran
a r t i c l e i n f o
a b s t r a c t
Six pyridazine-based Schiff base ligands, H2Ln (n = 1–5) and H4L, with N4O2S2 and N4O4S2 donor set
atoms, respectively, were prepared by condensation reaction of 3,6-bis-((2-aminoethyl)thio)pyridazine
with various salicyladehyde derivatives in ethanol and under solvent-free polyphosphate ester catalyzed
conditions. The acid–base properties of H2L2 and H2L3 in DMSO/water (1:1) solution have been studied by
spectrophotometric method at 25 ◦C. Optimized geometries of all compounds were also obtained at the
B3LYP level of theory. Additionally, the 13C chemical shielding of gas phase H2L1 and H2L2 were studied
by the gauge independent atomic orbital (GIAO) and continuous set of gauge transformations (CSGT)
methods at the level of density functional theory (DFT). The 6-311++G* basis set was utilized for all of
the atoms.
Article history:
Received 7 April 2009
Received in revised form
16 September 2009
Accepted 25 September 2009
Keywords:
Solvent-free
Schiff base
Pyridazine
Acid–base properties
GIAO
© 2009 Elsevier B.V. All rights reserved.
CSGT
NMR
1. Introduction
either by the solvent-free under microwave irradiation [20,21] or
by the solvent-free acid-catalyzed conditions [22,23].
Schiff base compounds have attracted considerable attention
due to their impressive and useful chemical and physical prop-
erties [1–4]. The instant and enduring popularity of Schiff base
compounds undoubtedly stem from the ease with which they can
be synthesized and their wide ranging complexing ability [5–7].
On the other hand, a wide variety of Schiff base ligands and their
metal complexes have been extensively investigated because of
their potential applicability as catalyst [8–11], their magnetic prop-
erties [3,12] and so on.
Among the various Schiff base ligands, the heterocycle-coupled
salicylaldimines have received special attention because of their
mixed soft-hard donors (O, N and S donor sites), versatile coor-
dination behavior [13–15] and their pharmacological properties
[16–19]. Although the most straightforward protocol for synthesis
of salicylaldimines is the condensation reaction of salicylaldehyde,
or its derivatives, with the appropriate amines in high boiling point
volatile organic solvents, this protocol often provides only low to
moderate yields of products followed by extensive recrystallization
and/or chromatography. This has led to the recent disclosure of sev-
eral improved reaction protocols for synthesis of salicylaldimines,
With the assistance of solvent-free acid-catalyzed conditions, it
was found that the condensation reaction of salicylaldehyde and
amines could proceed fast and efficiently with high yield. In addi-
tion, the absence of solvent reduces the risk of hazardous explosions
when the reactions take place in a mild condition.
In order to promote conditions that would favor the
formation and interception of bis-imine pyridazine-based sali-
cylaldimine ligands we have now investigate the condensation
reaction of 3,6-bis((aminoethyl)thio)pyridazine (PTA) with sali-
cylaldehyde, 5-Br-salicyladehyde, 3-OH-salicylaldehyde, 3-MeO-
salicylaldehyde, 3,5-di-(t-Bu)-butylsalicylaldehyde and 5-MeO-3-
(t-Bu)-salicylaldehyde in boiling ethanol and also under solvent-
free polyphosphate ester (PPE)-catalyzed condition (Scheme 1).
The high yields and overall low waste generation of PPE-catalyzed
solvent-free approach, gives this attractive green chemistry met-
rics along with remarkable versatility. The solid products were
purified simply and characterized by spectroscopic methods (1H
1
NMR, 13C{ H} NMR, UV–vis and IR) as well as microanaly-
sis.
The acid–base properties of the prepared compounds have also
studied by monitoring of changes in the electronic absorption spec-
tra in DMSO/water (1:1) solution at pH range of 2–11, as far as
possible. The results indicated each two identical protons of the
soluble compounds are liberated at basic condition.
∗
Corresponding author. Tel.: +98 861 2777401; fax: +98 861 4173406.
E-mail address: h-khanmohammadi@araku.ac.ir (H. Khanmohammadi).
1386-1425/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2009.09.053

128
H. Khanmohammadi, M. Erfantalab / Spectrochimica Acta Part A 75 (2010) 127–133
Scheme 1. The synthesized compounds.
On the other hand, the comparison between experimental and
using a Unicom Galaxy Series FT-IR 5000 spectrophotometer
(400–4000 cm−1).
theoretical NMR data may be helpful in making correct assignments
and understanding the relationship between chemical shielding
and molecular structure. Therefore, nowadays the Gauge Inde-
pendent Atomic Orbitals/Density Functional Theory (GIAO/DFT)
[24,25] and Continuous Set of Gauge Transformations/Density
Functional Theory (CSGT/DFT) [26,27] approaches are widely used
for the calculation of chemical shifts for a variety of compounds
[28–31]. According to our knowledge, there are only few computa-
tional NMR studies on heterocycle-based Schiff base compounds
[32,33]. At this work, the 13C NMR isotropic chemical shield-
ings of gas phase H2L1 and H2L2 was also systematically studied
by the GIAO and CSGT methods at the level of density func-
tional theory (DFT) with 6-311++G* basis set. The computed 13C
NMR chemical shieldings are compared with the experimental
data.
2.2. Procedure for pH dependence study of the electronic
absorption spectra of H2Ln n = 1–5
To record electronic absorption spectra as a function of pH, the
pH of the DMSO/water (1/1) solution containing each ligands, 25 mL
of 1.0 × 10−5 M (for H2L1, H2L2 and H2L4) and 2.0 × 10−5 M (for
H2L3, H2L5), was adjusted in the range of 2–11 by adding hydrochlo-
ric acid and potassium hydroxide solutions, as far as possible. Then,
the solution transferred through the flow cell and the absorbance
at 190–800 nm was recorded at 1 nm increments. All experiments
were carried out at the temperature (25 0.5) ◦C. After each pH
adjustment, the spectra were recorded and data were collected for
subsequent treatment. The calibration of the pH meter was done
in the usual way using two standard buffer solutions (Metrohm) in
aqueous media.
2. Experimental
2.3. Theoretical calculations
2.1. General information
The structures and NMR chemical shifts of the compounds were
calculated using Gaussian 03 [37] series of programs. A start-
ing molecular mechanics structure for the DFT calculations was
obtained using the HyperChem 5.02 program [38]. The geometry
of all compounds were fully optimized at the B3LYP/6-31G* level.
Vibrational frequency analyzes, calculated at the same level of the-
ory, indicate that the optimized structures are at the stationary
points corresponding to local minima without any imaginary fre-
quency. The calculations of 13C NMR chemical shieldings of H2L1
and H2L2 were performed using GIAO/DFT [24,25] and CSGT/DFT
[26,27] methods at 6-311++G* basis set. The 13C NMR chemical shift
was referenced to TMS.
All chemicals and solvents were of reagent grade and purchased
commercially. 3,5-di-(t-Bu)-butylsalicylaldehyde [34], 5-MeO-3-
(t-Bu)-salicylaldehyde [34], 3,6-bis((aminoethyl)thio)pyridazine
(PTA) [35] and polyphosphate ester (PPE) [36] were prepared as
1
described previously. 1H and 13C{ H} NMR spectra were obtained
with a Bruker Avance 300 MHz spectrometer. Electronic spec-
tral measurements were carried out using Perkin-Elmer Lamda
spectrophotometer in the range 200–600 nm. The pH measure-
ments were made using a Metrohm 691 pH meter equipped
with a glass calomel combined electrode. Elemental analyses
were performed on an Elementar Vario EL III elemental ana-
lyzer. Infrared spectra were recorded as pressed KBr discs,

H. Khanmohammadi, M. Erfantalab / Spectrochimica Acta Part A 75 (2010) 127–133
129
1
13.44 (s, 2H). 13C{ H} NMR (CDCl3, 300 MHz, RT, ppm): ıC 168.3,
158.2, 151.7, 148.5, 125.9, 124.5, 123.1, 118.1, 114.3, 57.7, 56.1, 30.8.
ꢀmax (nm) (ε(M−1 cm−1)): 268 (30170), 329 (4890) in DMSO.
Table 1
Bis-imine isolated products.
Compounds
Solvent free, yield (%)
Ethanol, yield (%)
H2L1
H2L2
H2L3
H2L4
H2L5
H4L
82
92
90
83
76
75
70
80
76
70
56
60
2.5.4.
3,6-Bis((2-aminoethyl-5-MeO-3-t-Bu-salicyliden)thio)pyridazine
(H2L4)
1H NMR (d6-DMSO, 300 MHz, RT, ppm): ıH 1.34 (s, 18H), 3.60
(t, 4H), 3.88 (t, 4H), 3.69 (s, 3H), 6.86 (dd, 4H), 7.50 (s, 2H), 8.53 (s,
2H), 13.59 (s, 2H). 13C{ H} NMR (d6-DMSO, 300 MHz, RT, ppm): ıC
1
2.4. General procedure for the solvent-free PPE-catalyzed
synthesis of the compounds
168.1, 158.5, 154.5, 151.2, 138.2, 126.5, 118.2, 118.1, 112.4, 57.8,
55.8, 34.9, 30.2, 29.5. ꢀmax (nm) (ε(M−1 cm−1)): 268 (30650), 346
(9300) in DMSO.
In
a
typical
solvent-free
experiment,
the
3,6-
bis((aminoethyl)thio)pyridazine (0.5 mmol) was added to the
mixture of aldehyde (1 mmol) and PPE. The mixture was agitated
at 40 ◦C (ca. 2 min), affording an oil (ranging from yellow to orange
in colour). On further grinding (ca. 15 min) the solid product was
formed. The product was washed with cooled n-hexane/ethanol
(4/1), and dried in air. If required, analytically pure product can be
obtained by recrystallization.
2.5.5.
3,6-Bis((2-aminoethyl-3,5-di-t-Bu-salicyliden)thio)pyridazine
(H2L5)
1H NMR (d6-DMSO, 300 MHz, RT, ppm) ıH: 1.32 (s, 18H), 1.46
(s, 18H), 3.69 (t, 4H), 4.01 (t, 4H), 7.12 (s, 2H), 7.16 (br, s, 2H), 7.42
(d, 2H, J = 2.10 Hz), 8.46 (s, 2H), 13.40 (br, 2H). 13C{ H} NMR (d6-
1
DMSO, 300 MHz, RT, ppm) ıC: 167.5, 158.2, 153.5, 150.4, 137.2,
126.0, 120.1 117.6, 114.4, 57.4, 35.1, 34.2, 31.7, 30.1, 29.5. ꢀmax
(nm) (ε(M−1 cm−1)): 267 (23700), 327 (5920) in DMSO.
2.5. General procedure for the synthesis of the compounds in
ethanol
2.5.6. 3,6-Bis((2-aminoethyl-3-OH-salicyliden)thio)pyridazine
(H4L)
A solution of 3,6-bis((aminoethyl)thio)pyridazine (1 mmol) in
absolute EtOH (10 ml) was added to a stirring solution of aldehyde
(2 mmol) in absolute EtOH (30 ml) at 50 ◦C over a period of 10 min.
The solution was heated in water bath for 3 h at 70 ◦C with stirring,
then cooled and let to stand at 0 ◦C. The obtained solid was filtered
off, washed with cooled n-hexane/ethanol (4/1), and dried in air.
The characterization data of the synthesized compounds are given
in Tables 1 and 2.
1H NMR (d6-DMSO, 300 MHz, RT, ppm) ıH: 3.59 (t, 4H), 3.93 (t,
4H), 6.66 (m, 2H), 6.84 (m, 4H), 7.50 (s, 2H), 8.52 (s, 2H), 13.20 (br,
2H). 13C{ H} NMR (d6-DMSO, 300 MHz, RT, ppm) ıC: 167.6, 158.5,
1
151.4, 146.3, 126.6, 122.4, 118.6, 118.3, 118.2, 56.8, 30.8. ꢀmax (nm)
(ε(M−1 cm−1)): 272 (28200), 332 (2900) in DMSO.
3. Result and discussion
2.5.1. 3,6-Bis((2-aminoethylsalicyliden)thio)pyridazine (H2L1)
1H NMR (d6-DMSO, 300 MHz, RT, ppm): ıH 3.59 (t, 4H), 3.90 (t,
4H), 6.88 (m, 4H), 7.31 (m, 2H), 7.46 (m, 4H), 8.57 (s, 2H), 13.22 (br,
The main goal of this work is the synthesis and characterization
of six bis-imine pyridazine-based compounds (four of which are
previously unpublished) by (i) a solvent-free PPE-catalyzed method
and (ii) using organic solvent, ethanol, as a reaction medium. All
prepared compounds are air stable solid and soluble in DMSO and
DMF at ambient temperature. The characterization data are given
in Section 2, Tables 1 and 2.
2H). 13C{ H} NMR (d6-DMSO, 300 MHz, RT, ppm): ıC 167.4, 161.0,
1
158.6, 132.9, 132.1, 126.6, 119.01, 119.04, 116.9, 57.7, 30.8. ꢀmax
(nm) (ε(M−1 cm−1)): 269 (21230), 317 (5330) in DMSO.
2.5.2. 3,6-Bis((2-aminoethyl-5-Br-salicyliden)thio)pyridazine
(H2L2)
An overview of the synthetic details is summarized in Scheme 1.
Typically in the solvent-free PPE-catalyzed reaction, the bis-imine
product is formed in near-quantitative yield by low intensity grind-
ing of the diamine with two molar equivalents of the aldehyde using
a pestle and mortar over a period of ca. 15 min, associated with con-
stant agitation of the mixture. During the reaction the product was
formed as a yellow to orange solid. As the product solidifies from
the reaction mixture, care needs to be taken to prevent it coating
small quantities of the starting materials.
1H NMR (d6-DMSO, 300 MHz, RT, ppm): ıH 3.58 (t, 4H), 3.93 (t,
4H), 6.83 (d, 2H, J = 8.80 Hz), 7.45 (dd, 2H, J = 8.80 Hz and 2.40 Hz),
7.47 (s, 2H), 7.66 (d, 2H, J = 2.40 Hz), 8.55 (s, 2H), 13.14 (br, 2H).
1
13C{ H} NMR (d6-DMSO, 300 MHz, RT, ppm): ıC 166.1, 160.4,
158.5, 135.3, 133.9, 126.6, 120.7, 119.5, 109.6, 57.7, 30.6. ꢀmax (nm)
(ε(M−1 cm−1)): 263 (29650), 322 (9290) in DMSO.
2.5.3. 3,6-Bis((2-aminoethyl-3-MeO-salicyliden)thio)pyridazine
(H2L3)
Analysis of the bulk product by 1H and 13C NMR show only
the pure products (typically >90%) and the unreacted start-
ing materials. Potential limitations in the solvent-free method
1H NMR (CDCl3, 300 MHz, RT, ppm): ıH 3.58 (t, 4H), 4.04 (t, 4H),
3.77 (s, 6H), 6.80 (m, 2H), 7.01 (m, 4H), 7.49 (s, 2H), 8.55 (s, 2H),
may be overcome using ultra high intensity grinding in
a
Table 2
The characterization data of the prepared compounds.
Compounds
Color
Mol. formula
mp ◦
C
Analysis (%) found (calculated)
C
H
N
S
H2L1
H2L2
H2L3
H2L4
H2L5
H4L
Yellow
Yellow
Orange
Cream
Yellow
Orange
C22H22N4O2S2
C22H20Br2N4O2S2
C24H26N4O2S2
109–110
178–180
142–143
80–81
150–151
156–158
60.25 (60.1)
44.5 (44.31)
57.7 (57.81)
62.8 (62.91)
68.9 (68.84)
56.21 (56.15)
5.06 (4.8)
3.6 (3.38)
5.4 (5.26)
7.0 (6.93)
8.4 (8.24)
4.9 (4.71)
12.78 (12.5)
9.5 (9.39)
11.3 (11.24)
9.3 (9.17)
8.2 (8.45)
11.8 (11.91)
14.62 (14.7)
10.9 (10.75)
12.7 (12.86)
10.3 (10.50)
9.6 (9.67)
C32H42N4O4S2
C38H54N4O2S2
C22H22N4O4S2
13.8 (13.63)

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H. Khanmohammadi, M. Erfantalab / Spectrochimica Acta Part A 75 (2010) 127–133
Table 3
Tentative assignments of some selected IRa frequencies (cm−1) and UV–vis data of the prepared compounds.
Compound
ꢁ(C–O)
ꢁ(Ar–H)
ꢁ(C N/C C)
ꢁ(Phenol ring)
ꢁ(N–N)
ꢀmax (nm) (ε(M−1 cm−1)) in DMSO
H2L1
H2L2
H2L3
H2L4
H2L5
H4L
1281
1271
1271
1278
1273
1252
3065
3050
3062
3040
3030
3085
1635, 1610, 1497
1634, 1604, 1479
1632, 1601, 1472
1636, 1601, 1470
1632, 1595, 1470
1643, 1586, 1460
1578
1570
1466
1451
1441
1540
1232
1248
1260
1237
1252
1220
269 (21230), 317 (5330)
263 (29650), 322 (9290)
268 (30170), 329 (4890)
268 (30650), 346 (9300)
267 (23700), 327 (5920)
272 (28200), 332 (2900)
a
KBr disc.
ball mill. We predict that the implementation of a spinning
disk/cylinder reactor will also lead to improved conversion
without additional purification steps. Traces of PPE can be
Fig. 1. UV spectra of H2Ln (n = 1–5) in DMSO at 25 ◦C.
Fig. 4. The pH-absorbance curve of H2L2 (385 nm band).
Fig. 2. The electronic absorption spectra of protonated H2L4 in DMSO/water solution
of pH 2–5.
Fig. 5. The electronic absorption spectra of ligands: (a) H2L2 and (b) H2L3, in
DMSO/water solution of different pH.
Fig. 3. The electronic absorption spectra of protonated H2L2 in DMSO/water solution
of pH 2–5.

H. Khanmohammadi, M. Erfantalab / Spectrochimica Acta Part A 75 (2010) 127–133
131
Fig. 6. B3LYP/6-31G* geometry optimized structure of compounds.
removed from the product by washing with small amount
3.1. IR spectra of the compounds
cooled n-hexane/ethanol (4/1). The obtained results are shown in
Table 1.
The positions of some of the prominent bands in the IR spectra of
the prepared compounds and their assignments based on extensive
data available for related compounds [39,40] are given in Table 3.
The IR spectra of all compounds show bands at 1630–1660 and
1240–1280 cm−1 assigned for ꢁs and ꢁas of the C N and C–O groups,
respectively. The band in the IR spectrum of H2L4 at approximately
826 cm−1 can be assigned to the C–Br stretching mode.
The second method implemented in this study utilizes ethanol
as an alternative solvent system without additional purifica-
tion steps. Typically, the diamine and two molar equivalents
of the aldehyde were heated in a minimal volume (≈40 ml) of
ethanol and stirred until the reaction was complete, as deter-
mined by TLC. The pure product precipitates out of the reaction
medium on cooling and can be collected by filtration. For com-
parison purposes the obtained results are shown in Table 1. It
was found that significantly lower yields (8–20%) were obtained
using in boiling ethanol rather than the solvent-free PPE-catalyzed
method.
3.2. NMR spectra of the compounds
1
The 1H NMR and 13C{ H} NMR spectral results, obtained for all
compounds at ambient temperature in d6-DMSO and/or CDCl3, are

132
H. Khanmohammadi, M. Erfantalab / Spectrochimica Acta Part A 75 (2010) 127–133
Table 4
given in Section 2. All compounds show a slightly broad singlet in
the region ıH 13.10–13.60 ppm assigned to OH proton, as was con-
firmed by deuterium exchange when D2O was added to solutions.
The CH N imine protons exhibit a singlet resonance in the region
ıH 8.40–8.60 ppm. The 1H NMR spectra of all compounds show two
set of triplets (ıH 3.58–3.61 and 3.90–4.05 ppm) assigned to the CH2
protons (3JH–H ≈ 5.50 Hz).
Selected carbon chemical shifts (ı, ppm) in d6-DMSO and calculated magnetic
isotropic shielding tensors (s) for H2L1 and H2L2.
3.3. Electronic spectra of the compounds
The electronic spectra of H2Ln (n = 1–5) in DMSO display main
features at 265–272 and 315–345 nm (Fig. 1 and Table 1). The
intense absorption band at short wavelengths may be assigned
for ␲–␲* aromatic rings transitions [41]. The weak broad absorp-
tion band at 315–345 nm may be assigned to the n–␲* and ␲–␲*
electronic transition associated with the C N linkages [41].
.
Atoms
H2L1, X = H
H2L2, X = Br
ꢂC (Cal)
ıC (Exp)
ꢂC (Cal)
ıC (Exp)
C1
57.87a
59.09b
12.72
126.6
C1
61.14
61.76
8.49
126.6
3.4. The electronic absorption spectra of the prepared compounds
in DMSO and in buffer solutions of different pH values
C2
158.6
30.8
C2
158.5
30.6
13.92
10.71
141.70
138.90
117.52
114.11
16.59
17.23
47.13
48.80
19.04
20.17
54.06
55.73
All of the prepared compounds contain available active cen-
tres for proton attacks and are appropriate for use as pH probes.
In the light of previous studies of the acid–base properties
of salicylaldimines [42], H2Ln (n = 1–5) ligands are to be con-
sidered as bi-basic acids while H4L would be tetra-basic acid.
Protonation–deprotonation equilibrium of H2Ln (n = 1–5) ligands
are shown as follows:
C3
138.65
135.37
118.39
115.86
20.04
20.73
21.62
22.01
56.33
C3
C4
57.7
C4
57.7
C5
167.4
161.0
116.9
C5
166.1
109.6
160.4
120.7
C10
C11
C7
C10
C11
K
K
2
1
H2Ln ꢀHLn− + H+, HLn−ꢀLn2− + H+
59.20
n=1–5
The absorption spectra of H2Ln (n = 1–5) in DMSO/water (1:1)
solution of pH 2–11 were investigated in the UV–vis region
(200–600 nm). The obtained spectra indicate that the absorbance
and position of the absorption bands changed with pH of the
medium according to the following:
a
Computed data at GIAO, B3LYP/6-311++G* for all atoms.
Computed data at CSGT, B3LYP/6-311++G* for all atoms.
b
closely related to the structure of these compounds in solution. The
resulting optimized structures and atom numbering are displayed
in Fig. 6. The selected bond lengths, bond angles and torsion angles
of the minimum energy structures are given in Table 4.
1. The extinction of the all bands of protonated forms of H2L1,
H2L4, H2L5 and H4L decreases with increasing pH. Unfortu-
nately, the monitoring of electronic absorption spectra at pH > 5
in DMSO/water (1:1) solution of these compounds is impossible
because of formation an unsoluble precipitate (Fig. 2).
2. Absorption spectra of H2L2 and H2L3, in solution having
pH < 5, showed two absorption bands with ꢀmax ≈ 265 nm and
ꢀmax ≈ 325 nm corresponding to protonated forms of H2Ln (n = 2,
3) (Fig. 3).
The 13C NMR chemical shifts of selected carbons were calcu-
lated on the optimized structures of H2L1 and H2L2 using GIAO/DFT
and CSGT/DFT methods with 6-311++G* basis set for all atoms. Cal-
culated and measured 13C chemical shifts of selected atoms are
tabulated in Tables 3 and 4.
The following conclusions can be straightforwardly derived
based on presented data. On an absolute scale, the computed NMR
chemical shifts for H2L1 and H2L2 at the DFT level of theory are
in acceptable agreement with the experimental data. Differences
between the calculated and measured values may be a result of
The extinction of these bands decrease with increasing pH
values of solution while additional absorption band appeared at
pH > 5 with ꢀmax ≈ 385 nm corresponding to the (HLn)− and/or
(Ln)2− forms. The extinction of newly appeared band was increase
with increasing pH values of the solution until pH 11. The
changes of absorbance with pH of the solution (Fig. 4) lead to
the appearance of isobestic point at ≈350 nm for both H2L2 and
H2L3 (Fig. 5) indicating the existence of an acid–base equilib-
rium between two different forms of these compounds as given
above.
solvent interactions. Regarding the method for achievement of 13
C
chemical shifts, for the present case, at the B3LYP level, the CSGT
algorithm is slightly superior to GIAO. The relation between the
experimental 13C chemical shifts and the computed (CSGT/DFT
method) magnetic isotropic shielding tensors for H2L1 and H2L2
are shown in Fig. 7. The correlation is linear and it is described by
the equation:
ıexp = a + bꢂ
4. Computational studies
Parameters a and b for H2L1 and H2L2 are
In spite of our many efforts, single crystals suitable for X-
ray crystallography have not been obtained for newly prepared
compounds. The structural parameters of all compounds were
assessed by undertaking density functional theory calculations.
This was done by undertaking a full geometry optimization at the
B3LYP/6-31G* level of theory. The calculations predicate a sym-
metric structure (Cs configuration) for H2L1 and H2L2, which are
ı13 = −0.9802ꢂ + 177.4(R2 = 0.9758) for H2L1
C
and
ı13 = −1.0094ꢂ + 177.4(R2 = 0.9553) for H2L2
C

H. Khanmohammadi, M. Erfantalab / Spectrochimica Acta Part A 75 (2010) 127–133
133
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Acknowledgment
We are grateful to the Arak University for financial support of
this work.
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