DFT and experimental determination of conformations of 2- (ethoxycarbonylmethoxy)-5-(arylazo)benzaldehydes and their oximes

Article abstract of DOI:10.1002/mrc.3874

2-(Ethoxycarbonylmethoxy)-5-(arylazo)benzaldehydes 1-4 and their oximes 5-8 were synthesized and characterized by IR, ¹H and ¹³C NMR spectroscopy. The favoured conformations of aldehydes 1-4 and oximes 5-8 were predicted theoreticall

Full text of DOI:10.1002/mrc.3874

MRC Letters
Received: 5 May 2012
Revised: 16 August 2012
Accepted: 27 August 2012
Published online in Wiley Online Library
(wileyonlinelibrary.com) DOI 10.1002/mrc.3874
DFT and experimental determination of
conformations of 2-(ethoxycarbonylmethoxy)-
5-(arylazo)benzaldehydes and their oximes
A. Manimekalaia* and R. Balachandera
2-(Ethoxycarbonylmethoxy)-5-(arylazo)benzaldehydes 1–4 and their oximes 5–8 were synthesized and characterized by IR, 1H
and 13C NMR spectroscopy. The favoured conformations of aldehydes 1–4 and oximes 5–8 were predicted theoretically.
1
Selected geometrical parameters and charges were derived from optimized structures. IR, H and 13C NMR data were also
computed using Gaussian-03 package and compared with the observed values. 15N and 17O chemical shifts were also
determined theoretically. Copyright © 2012 John Wiley & Sons, Ltd.
Supporting information may be found in the online version of this article.
Keywords: 2-(ethoxycarbonylmethoxy)-5-(arylazo)benzaldehydes and their oximes; conformations; computational studies
can be used as colorimetric receptor for some ions. Further,
these aldehydes and their oximes can be used to synthesize
Introduction
Azobenzene and derivatives of azobenzene can be used as
intermediates as well as ligands to synthesize several organic
compounds, coordination complexes,[1–4] polymers[5,6] and
dendrimers[7,8] and play important role in biological activities[9,10]
due to redox behaviour. Azobenzene chromophore has elicited
considerable interest because of its novel photoisomerization
and photocyclization reaction[11] as well as its technological
applications in the dye and pigment industry.[12] Generally,
organic chromophores and polymers are strongly fluorescent in
dilute solution, and aggregate formation leads to reduction in
fluorescent efficiency in solid state. Several reports in literature
reveal enhancement of fluorescence[13,14] and photoreactiv-
ity[15,16] from self-assembled aggregates of azobenzene units
present in the compound. Photoswitchable multivalent sugar
ligands incorporating azobenzene group[17] and photoresponsive
azobenzene bridged crown ethers[18,19] were synthesized, and
their photochemical behaviour, binding ability of alkali metal
cations and ion transport across the cell membrane have been
investigated in detail. Materials containing azobenzene units
have also been intensively studied because they are a promising
system for optical applications such as halographic and digital
storage, liquid crystal command surfaces and nonlinear optical
devices.[20] Second-order nonlinear optical materials consisting
of azo dyes poled in polymer matrices have been synthesized
and their NLO behaviour was studied in detail.[21,22] Because
of the wide applicability of azobenzene chromophores, we
thought that synthesis and conformational study of some simple
azobenzene chromophores containing active groups such as –
OH and –CHO are of considerable interest because several com-
pounds can be easily synthesized from these starting materials.
Recently, we have synthesized and determined the conformations
of some 5-arylazosalicylaldehydes and their oximes by theoretical
methods and spectral studies.[23] These aldehydes can be used as
starting materials to synthesize several Schiff bases, which in turn
several coordination complexes, fused macrocycles, oxazolidines,
pyrimidines and so on. In continuation of this work, some
2-(ethoxycarbonylmethoxy)-5-(arylazo)benzaldehydes 1–4 and
their oximes 5–8 were synthesized and characterized in the
present study, and their conformations have been predicted
from theoretical studies. Selected geometrical properties, IR
frequencies, 1H, 13C, 15N and 17O chemical shifts were determined
by the Density Functional Theory (DFT) method and analyzed.
Results and Discussion
2-(Ethoxycarbonylmethoxy)-5-(phenylazo)benzaldehyde (1),
2-(ethoxycarbonylmethoxy)-5-(p-methylphenylazo)benzaldehyde (2),
2-(ethoxycarbonylmethoxy)-5-(p-fluorophenylazo)benzaldehyde
(3), 2-(ethoxycarbonylmethoxy)-5-(p-nitrophenylazo)benzaldehyde
(4), 2-(ethoxycarbonylmethoxy)-5-(phenylazo)benzaldoxime (5),
2-(ethoxycarbonylmethoxy)-5-(p-methylphenylazo)benzaldoxime (6),
2-(ethoxycarbonylmethoxy)-5-(p-fluorophenylazo)benzaldoxime
(7) and 2-(ethoxycarbonylmethoxy)-5-(p-nitrophenylazo) benzaldoxime
(8) were synthesized according to the Scheme 1 and characterized
by IR, 1H, 13C, 1H-1H and 1H-13C COSY spectra. Representative NMR
spectra are provided as supporting information (Figs. S1 and S2).
The labelling of the atoms followed in the present study was also
indicated in the Scheme 1.
*
Correspondence to: A. Manimekalai, Department of Chemistry, Annamalai
University, Annamalainagar 608 002, India. E-mail: profmeka1@gmail.com
a Department of Chemistry, Annamalai University, Annamalainagar 608 002,
India
Magn. Reson. Chem. (2012)
Copyright © 2012 John Wiley & Sons, Ltd.

A. Manimekalai and R. Balachander
Scheme 1. Steps involved in the synthesis of aldehydes 1–4 and their oximes 5–8.
IR spectral analysis
NMR spectral analysis
The sharp peaks at approximately 1680 cmꢀ1 in the IR spectra of
compounds 1–4 are due to nC=O of aldehydic group. The sharp
peaks for the COO group in compounds 1–8 appeared at approx-
imately 1740 cmꢀ1. In compounds 5–8, the nC=N mode observed
at approximately 1600 cmꢀ1 and the stretching vibration of the
OH (nOH) group was observed in the region 3400 cmꢀ1. In
compounds 1–8, the strong peaks for N = N group are observed
at approximately 1300 cmꢀ1. Aromatic C–H stretching vibration
appeared at approximately 2920 cmꢀ1. In compounds 4 and 8,
peaks for the nNO2 group are observed at approximately 1520
and 1340 cmꢀ1. The peaks at approximately 1090 cmꢀ1 are attrib-
uted to the nC–O mode. The sharp peaks at approximately
960 cmꢀ1 in compounds 5–8 are due to the nN–O mode. The IR
data of compounds 1–8 are listed in Table 1.
The triplet and the quartet centred at approximately 1.3 and
4.3 ppm in H NMR spectra of compounds 1–8 are assigned to
1
the methyl and methylene protons of the ethyl group attached
to the COO moiety. A singlet observed at approximately
4.8 ppm is assigned to methylene protons attached to the
oxygen atom O(15). The low-frequency signals at approximately
14 and 61 ppm in 13C NMR spectra of compounds 1–8 are due
to the methyl [C(23)] and methylene carbons [C(22)] of the ethyl
group attached to oxygen atom O(21). The methylene carbon
attached to oxygen O(15) [C(18)] resonates at approximately
66 ppm. The remaining signals for the aromatic system in the
1H and 13C NMR spectra were assigned based on their positions,
integrals and multiplicities and on comparison of these signals
with those of arylazosalicylaldehydes and their oximes.[23]
Table 1. IR spectral data (cmꢀ1) of 1–8
Assignments
1
2
3
4
5
6
7
8
nOH
naromatic(CH)
nCOO
nC=O
nC=N
nC=C
nN=N
dOH
—
2923
1740
1682
—
—
2922
1740
1683
—
—
2924
1736
1685
—
—
2921
1737
1688
—
3251
2923
1752
—
3417
2921
1744
—
3416
2921
1765
—
3431
2923
1752
—
1605
1605 1497
1316
1218
1105
953
1594
1594 1482
1269
1223
1104
968
1596
1596 1496
1267
1201
1110
965
1604
1492 1456
1343
1234
1107
962
1594 1481
1301
1246
1095
—
1602 1483
1289
1204
1097
—
1592 1483
1301
1244
1094
—
1598 1484
1283
1211
1097
—
nC–O
nN–O
Aromatic CH, out-of-plane
bending vibration
825
824
852
858
828
822
841
861
771
727
821
825
770
763
712
817
690
641
751
686
690
640
—
690
nNO2
—
—
—
1524 1345
—
—
—
1527 1343
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DFT and experimental
Further, these assignments were confirmed by the correlations
observed in the 1H–1H and 1H–13C COSY spectra. The 1H and
13C chemical shifts obtained in this manner are listed in Tables 2
and 3, respectively.
Conformational analysis
The configuration of azo double bond in compounds 1–8 is
expected to be same as that of the parent arylazosalicylaldehydes
and their oximes [E configuration][23] because they are derived
from arylazosalicylaldehydes only. However, the orientation of
aldehydic and oxime moieties may differ from their parent
compounds because intramolecular hydrogen bonding stabilizes
the orientation of these moieties in the parent compounds and
no such stabilization in compounds 1–8. Thus, there are eight
important possible conformations for the aldehydes 1–4 as
shown in Fig. 1. In conformations A, B, E, F, G and H, the carbonyl
oxygen of aldehydic group, that is, O(17), is opposite to that of
alkoxy oxygen O(15). However, in conformations C and D, the
carbonyl oxygen is on the same side as that of alkoxy oxygen.
The orientation of carbonyl group of ethoxycarbonyl moiety in
conformations A and C is different from the conformations
B and D. The carbonyl oxygen of ethoxycarbonyl group, that
is, O(20), is on the same side as that of alkoxy oxygen O(15) in
conformations A and C, whereas it is on opposite side in confor-
mations B and D. In conformations E and F, the methylene carbon
C(18) is on the same side as that of aldehydic moiety, and they dif-
fer in the orientation of carbonyl group of ethoxycarbonyl moiety
only. The orientations of methylene carbon [C(22)] of ethoxycar-
bonyl moiety in conformation G and carbonyl group of ethoxycar-
bonyl moiety in conformation H are different from conformation
A. It is seen from Fig. 1 that severe interaction exists between
aldehydic moiety and ethoxycarbonyl/ethoxycarbonylmethoxy
moiety in conformations C, D, E and F and hence not favoured
in the present study. Further, severe steric interaction exists
between carbonyl oxygen of ethoxycarbonyl moiety with the
hydrogen H(3) in conformation H and between C(18) methylene
moiety with C(22) methylene moiety in conformation G; hence,
these conformations are ruled out in the present study. Among
the conformations A and B, the steric interaction between alkoxy
oxygen and ethoxycarbonyl moiety is expected to be slightly
higher in conformation B than in A. Therefore, the favoured
conformation is expected to be A in aldehydes 1–4.
To confirm the favoured conformation, DFT calculations
(geometry optimizations) were performed for the aldehyde 1
according to the DFT method using B3LYP/6-31 G(d,p) basis set
available in Gaussian-03 package[24] for the possible conforma-
tions shown in Fig. 1. The optimized structures derived from
conformations E and H are similar to the optimized structure
derived from conformation A with slight distortion and F is
similar to conformation B (Fig. S3), thus indicating that the
methylene carbon C(16) is opposite to that of aldehydic group,
and carbonyl oxygen of ethoxycarbonyl moiety is away from H
(3). The relative energies determined for other conformations
are found to be 0.0 (A), 0.146 (B), 4.177 (C), 3.192 (D) and 8.876
(G) kcal molꢀ1. Thus, the theoretical study eliminates conforma-
tions C, D and G predicts A as the stable conformation. However,
the small energy difference between conformers A and B suggests
that the aldehyde 1 exists as an equilibrium mixture of two
rotamers A (56%) and B (44%). The other aldehydes 2–4 also
exist as an equilibrium mixture of two rotamers A and B only.
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A. Manimekalai and R. Balachander
Table 3. 13C Chemical shifts (ppm) of 1–8
Compd. C(3) C(4) C(6) C(16) C(18) C(19) C(22) C(23) C(24)
C(1)
C(2) C(5) C(9) C(10) and C(14) C(11) and C(13) C(12)
1
2
3
4
5
6
7
8
113.0 129.6 123.6 189.0 65.8 167.7 61.8 14.1
—
125.7 161.6 147.2 152.4
125.6 161.4 147.2 150.5
125.7 161.6 147.0 148.9
125.7 162.5 146.9 155.4
ꢂ123.1 157.8 147.4 152.4
122.8 157.6 147.4 150.6
122.2 157.7 147.2 149.1
122.4 158.7 147.2 155.7
122.9
122.9
124.9
123.4
123.0
122.8
124.8
123.3
129.1
129.8
116.1
124.7
129.0
129.7
116.1
124.7
131.1
141.8
165.4
148.7
130.8
141.4
165.3
148.6
112.9 129.5 123.5 189.1 65.8 167.8 61.8 14.1 21.5
113.0 129.6 123.5 189.0 65.8 167.7 61.8 14.1
113.1 130.3 124.2 188.7 65.8 167.5 61.9 14.1
112.5 126.3 121.9 146.1 66.2 168.1 61.8 14.2
—
—
—
112.4 125.7 121.6 146.0 66.0 168.1 61.7 14.1 21.5
112.4 125.8 121.4 145.9 66.0 168.1 61.7 14.2
112.4 126.7 121.8 145.7 65.9 167.9 61.8 14.2
—
—
Figure 1. Possible conformations of aldehydes 1–4.
To confirm the orientation of aldehydic group potential
energy scan was also carried out for the C(1)–C(16) bond in
conformation A for aldehyde 1. The dihedral angle C(6)–C(1)–
C(16)–O(17) was varied in steps of 10ꢁ, and the scan diagram is
reproduced in Fig. 2. From this study, it is confirmed that the
carbonyl oxygen of aldehydic moiety, that is, O(17), is opposite
to that of alkoxy oxygen O(15) (conformation A) only. In the
favoured conformation, there are three rotameric forms possible
for the ethyl group as shown in Fig. 3. In conformation A0, the
methyl group is anti to carbonyl group, whereas in B0 and C0, it
is gauche to carbonyl group. The two gauche conformations B0
and C0 are equivalent. To predict the conformation of the ethyl
group, geometry optimizations were done for anti and gauche
conformations shown in Fig. 3 for aldehydes 1–4. The energies
determined are displayed in Table 4. As shown in Table 4, it is
concluded that both rotamers contribute to the equilibrium,
that is, aldehydes 1–4 exist as an equilibrium mixture of A0, B0
and C0 only.
In oximes 5–8, one can expect that the C = N of oxime moiety
must be away from alkoxy oxygen similar to the corresponding
aldehydes 1–4. Because oximes are derived from aldehydes,
the four important possible conformations derived from
conformations A and B of aldehydes as shown in Fig. 4 alone
are considered for a discussion in the present study for the
oximes. The oxime moiety adopts E configuration in conforma-
tions A and C and Z configuration in conformations B and D.
The conformers A and C differ in the conformation of the
COOCH2CH3 group only. So also conformers B and D. Generally,
the E configuration of oxime is favoured over the Z configuration
and hence the favoured conformation for oximes 5–8 are
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DFT and experimental
Table 4. Relative energies of anti and gauche forms in aldehydes 1–4
Energies (kcal molꢀ1
)
Compound
anti (A0)
gauche (B0)
1
2
3
4
0.0
0.0
0.0
0.0
0.086
0.088
0.087
0.095
calculations are corrected using the scale factor 0.9613, and the
corrected data are compared (Table S2) with the observed values.
There is a close agreement between the IR frequencies calculated
by theoretical method with the experimental ones. The 1H,
13C, 15N and 17O chemical shifts in conformations A and B in
aldehydes 1–4 and conformations A and C in oximes 5–8 were
also determined theoretically by the DFT method in CDCl3 using
the basis set B3LYP/6-311 + G(2d,p) GIAO and the solvation model
PCM (SCRF = PCM). The 1H and 13C shifts relative to reference
material TMS are determined from the shielding tensors using the
scale factor 31.8821 and 182.4656, respectively. The 15N and 17O
shifts relative to NH3 and H2O are determined from the shielding
tensors using the scale factor 258.4 and 320.0, respectively.
The scale factors are based on the reference compounds used as
well as on the basis set used for DFT calculation. Because
conformations A and B contribute to the equilibrium in aldehydes,
the theoretical chemical shifts (1H and 13C) for 1–4 are calculated
using the following equation:
Figure 2. PES diagram of aldehyde 1.
predicted to be an equilibrium mixture of A and C only similar to
their parent aldehydes. Single crystal measurements made for
the oxime 5 reveal conformation A in the solid state.[25]
To confirm the favoured conformation, computational calcula-
tions were performed according to the DFT method using B3LYP/
6-31 G(d,p) basis set for oxime 5, and the relative energies deter-
mined are found to be 0.0 (A), 1.883 (B), 0.126 (C) and 1.945 (D)
kcal molꢀ1. Thus, the theoretical study also confirms that the ox-
ime 5 exists as an equilibrium mixture of conformers A (ꢂ53%)
and C (ꢂ43%) only. To determine the rotational barrier for E/Z
isomerization, a Potential Energy Scan (PES) study has been carried
out in conformation A for the oxime 5. From the scan diagram
dcal ¼ xAdA þ ð1 ꢀ xAÞdB
where xA is the mole fraction of conformer A in the equilibrium and
dA and dB are the calculated chemical shifts in conformations A and
B, respectively. Similarly, in oximes, the theoretical chemical shifts
[1H and 13C] are calculated using the following equation:
(Fig. 5), the rotational barrier is estimated to be 55.254kcal molꢀ1
.
Stabilization energies of E form over Z form for other oximes 6–8
were calculated by determining energies of conformations A and
B only, and the energies determined are given in Table 5. The sta-
bilizing energies are nearly equal and are independent on the na-
ture of the substituents present in the phenyl ring. The optimized
structures of 1–8 are provided as supporting information (Fig. S4).
dcal ¼ xAdA þ ð1 ꢀ xAÞdC
where xA is the mole fraction of conformer A and dA and dC are the
calculated chemical shifts in conformations A and C, respectively.
The chemical shifts thus calculated in this manner are compared
with the experimental values. The experimental 13C chemical shifts
are closer to the theoretical values (Table S3), and the correlation
between experimental and theoretical 13C values is shown in Fig.
S5. Theoretically determined 1H chemical shifts are generally higher
when compared with experimental values (Table S4).
Molecular properties
From the optimized structures, geometrical parameters in confor-
mation A were derived (Table S1). The torsional angles reveal that
the aromatic rings adopt a trans configuration about the N = N
bond and are perfectly coplanar with the dihedral angle of 180ꢁ.
The IR frequencies in conformation A were determined
theoretically by the DFT method using the basis set B3LYP/
6-31 G(d,p). The frequencies obtained from the computational
From the theoretically determined 15N and 17O chemical shifts
for 1–8 (Table S5), it is seen that considerable deshielding occurs
on N(7) due to the presence of electron withdrawing NO2 group
and shielding due to the presence of other substituents. These
Figure 3. Possible conformations of the ethyl group in 1–4.
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A. Manimekalai and R. Balachander
Table 5. Energies of E and Z configurations of oximes 5–8
Energies (kcal molꢀ1
Compound E configuration Z configuration Stabilizing energies
)
5
6
7
8
ꢀ704771.607
(ꢀ1123.125)
ꢀ729445.908
(ꢀ1162.446)
ꢀ767040.630
(ꢀ1222.357)
ꢀ833097.300
(ꢀ1327.624)
ꢀ704769.726
(ꢀ1123.122)
ꢀ729444.026
(ꢀ1162.443)
ꢀ767038.747
(ꢀ1222.354)
ꢀ833095.417
(ꢀ1327.622)
ꢀ1.883
(ꢀ0.003)
ꢀ1.882
(ꢀ0.003)
ꢀ1.883
(ꢀ0.003)
ꢀ1.882
(ꢀ0.003)
Values within parentheses are the energies in atomic unit (a.u)
Conclusions
Structures of 2-(ethoxycarbonylmethoxy)-5-(arylazo)benzaldehydes
1–4 and their oximes 5–8 were analyzed by IR, NMR and theoret-
ical methods. The aromatic rings adopt a trans configuration
about N = N bond, in which the two rings are perfectly coplanar,
and oxime group adopts E configuration. Theoretically derived IR
frequencies, 1H and 13C chemical shifts are compared with exper-
imentally determined values.
Experimental
Synthesis of 2-(ethoxycarbonylmethoxy)-5-(arylazo)benzal-
dehydes (1–4)
5-Arylazosalicylaldehydes were prepared according to the proce-
dure reported by Odabasoglu et al.[26] A mixture of 5-arylazosalicy-
laldehyde (ꢂ0.23 g, 1 mmol), ethylbromoacetate (0.11 ml, 1 mmol)
and potassium carbonate (0.5 g) in acetonitrile (15ml) was
stirred for 24 h. The reaction mixture was filtered, and the solvent
was evaporated to obtain 2-(ethoxycarbonylmethoxy)-5-(arylazo)
benzaldehydes. The pure product was obtained by recrystalliza-
tion from ethanol: (1) aryl = phenyl, m.p. 86ꢁC, yield = 70%; (2)
aryl = p-tolyl, m.p. 101ꢁC, yield = 72%; (3) aryl=p-fluorophenyl, m.p.
114ꢁC, yield = 72%; and (4) aryl=p-nitrophenyl, m.p. 125ꢁC, yield =
75%.
Figure 4. Possible conformations of oximes 5–8.
Synthesis of 2-(ethoxycarbonylmethoxy)-5-(arylazo)benzal-
doximes (5–8)
A mixture of 2-(ethoxycarbonylmethoxy)-5-(arylazo)benzalde-
hydes (ꢂ0.32 g, 1 mmol) and sodium acetate (0.5 g) was dissolved
in boiling ethanol and hydroxylamine hydrochloride (0.13 g,
2 mmol) was added. The mixture was refluxed for 3 h. The reac-
tion mixture was poured into water. The 2-(ethoxycarbonyl-
methoxy)-5-(arylazo)benzaldoxime separated out was filtered
and recrystallized from ethanol: (5) aryl = phenyl, m.p. 153ꢁC,
yield = 60%; (6) aryl = p-tolyl, m.p. 160ꢁC, yield = 60%; (7) aryl =
p-fluorophenyl, m.p. 175ꢁC, yield = 65%; (8) aryl = p-nitro, m.p.
180ꢁC, yield = 65%.
Figure 5. PES diagram of oxime 5.
observations are in accordance with that of the mesomeric effect
of the substituents (NO2, F) and the hyperconjugative effect of
the methyl substituent. The introduction of substituents (CH3,
F and NO2) at the para position of the phenyl ring, however,
shields N(8) considerably. Charges were also determined by
NBO analysis and Mulliken method (Table S6).
Spectral measurements
1H and 13C NMR spectra were recorded on a Bruker AMX 500
NMR spectrometer operating at 500.03 and 125.73 MHz for 1H
and 13C, respectively. Solutions were prepared by dissolving
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DFT and experimental
10 (1H) and 50 mg (13C) of the compound in 0.5 ml of the solvent
(CDCl3). All NMR measurements were made on 5-mm NMR tubes.
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1
The spectral parameters for H were as follows: spectral width,
5387.9–10330.6 Hz; acquisition time, 1.59–2.50 s; digital resolu-
tion, 0.2–0.4 Hz; and number of scans, 16–32. For 13C, the spectral
parameters were as follows: spectral width, 24038.5–30030.0 Hz;
acquisition time, 0.50–0.68 s; digital resolution, 0.73–1.00 Hz; and
number of scans, 1024–3382. HOMOCOSY phase-sensitive HSQC
spectra were recorded on a Bruker DRX 500 NMR spectrometer
using standard parameters. Avatar-330 FT-IR spectrophotometer
was used for recording IR spectra (KBr pellet).
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Computational study
Geometry optimizations were carried out according to density
functional theory available in Gaussian-03 package using
B3LYP/6-31 G(d,p) basis set.[24] The 1H, 13C, 15N and 17O chemical
shifts were determined theoretically by the DFT method in CDCl3
using the solvation model PCM (SCRF = PCM) and the basis set
B3LYP/6-311 + G(2d,p) GIAO. IR frequencies and NBO charges
were also determined using the basis set B3LYP/6-31 G(d,p),
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Acknowledgement
The authors thank the NMR Research Centre, Indian Institute of
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