An efficient and simple approach for the synthesis of pyranopyrazoles using imidazole (catalytic) in aqueous medium, and the vibrational spectroscopic studies on 6-amino-4-(4′-methoxyphenyl)-5-cyano-3-methyl-1-phenyl-1,4- dihydropyrano[2,3-c]pyrazole using density functional theory

Article abstract of DOI:10.1016/j.saa.2011.06.033

We describe a one-pot four component synthesis of pyranopyroles from aryl aldehydes, ethyl acetoacetate, malononitrile and hydrazine hydrate in the presence of catalytic amounts of an organocatalyst imidazole in water as medium. A plausible mechanism for the formation of imidazole catalyzed pyranopyrazoles has been envisaged. This method is rapid, simple, provides products in good yield, and is eco-friendly. In addition, based on the optimized geometry, the frequency and intensity of the vibrational bands of 6-amino-4-(4′- methoxyphenyl)-5-cyano-3-methyl-1-phenyl-1,4-dihydropyrano[2,3-c]pyrazole were obtained by the density functional theory (DFT) calculations using 6-31G(d,p) basis set. The vibrational frequencies were calculated and the scaled values have been compared with experimental FT-IR and FT-Raman spectra. The observed and the calculated frequencies are found to be in good agreement.

Full text of DOI:10.1016/j.saa.2011.06.033

Spectrochimica Acta Part A 81 (2011) 431–440
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
An efficient and simple approach for the synthesis of pyranopyrazoles using
imidazole (catalytic) in aqueous medium, and the vibrational spectroscopic
studies on 6-amino-4-(4ꢀ-methoxyphenyl)-5-cyano-3-methyl-1-phenyl-1,4-
dihydropyrano[2,3-c]pyrazole using density functional
theory
Aisha Siddekhab, Aatika Nizama, M.A. Pashaa,∗
a Department of Studies in Chemistry, Central College Campus, Bangalore University, Bangalore 560001, India
b Department of Chemistry, Smt. V.H.D. Central Institute of Home Science, Bangalore 560001, India
a r t i c l e i n f o
a b s t r a c t
Article history:
We describe
a one-pot four component synthesis of pyranopyroles from aryl aldehydes, ethyl
Received 23 November 2010
Received in revised form 10 June 2011
Accepted 16 June 2011
acetoacetate, malononitrile and hydrazine hydrate in the presence of catalytic amounts of an
organocatalyst imidazole in water as medium. A plausible mechanism for the formation of imida-
zole catalyzed pyranopyrazoles has been envisaged. This method is rapid, simple, provides products
in good yield, and is eco-friendly. In addition, based on the optimized geometry, the frequency and
intensity of the vibrational bands of 6-amino-4-(4ꢀ-methoxyphenyl)-5-cyano-3-methyl-1-phenyl-1,4-
dihydropyrano[2,3-c]pyrazole were obtained by the density functional theory (DFT) calculations using
6-31G(d,p) basis set. The vibrational frequencies were calculated and the scaled values have been com-
pared with experimental FT-IR and FT-Raman spectra. The observed and the calculated frequencies are
found to be in good agreement.
Keywords:
Pyranopyrazoles
Imidazole
Aldehydes
Hydrazine hydrate
Ethyl acetoacetate
Vibrational spectroscopic analysis
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
green chemistry. A number of classic reactions which were car-
ried out strictly under anhydrous conditions, in hazardous and
Pyranopyrazoles are fused heterocyclic compounds that possess
many biological properties such as fungicidal [1], bacterici-
dal [2], vasodilatory activities [3] and act as anticancer agents
[4]. They also find application as pharmaceutical ingredi-
ents and biodegradable agrochemicals [5–8]. Apart from this,
pyrano[2,3-c]pyrazoles have been shown to act as potential
toxic organic solvents, can also be carried out in water with
proper use of catalysts and reaction conditions. MCRs which
are carried out in water as a medium offer better environ-
mental protection, hence, are considered as clean and green
reactions.
Pyranopyrazole was first synthesized by the reaction between
3-methyl-1-phenylpyrazolin-5-one and tetracyanoethylene [5].
Sharanin et al. later reported a one-pot three component reac-
tion between pyrazolone, aromatic aldehyde and malononitrile
in the presence of triethyl amine as a catalyst in ethanol to
get pyranopyrazoles [7]. Another method has been reported by
the condensation of N-methylpiperidone, pyrazoline-5-one and
malononitrile in absolute ethanol [12]. Nadia et al. reported a
method using a mixture of 5-methyl-2,4-dihydro-3H-pyrazol-
3-one, malononitrile and different aromatic aldehydes in the
presence of ammonium acetate in ethanol [4]. A four compo-
nent reaction of aldehydes, ethyl acetoacetate, malononitrile and
hydrazine hydrate in water and catalytic ␤-cyclodextrin was
reported by Vasuki and Kandhasamy [8] and the same reac-
tion is carried out by Shestopalov et al. using catalytic amounts
of triethyl amine in ethanol [13]. However, all the reported
insecticidal [9a] and molluscicidal agents [9b]. As
a result,
considerable attention has been focused on the development
of new methodologies for the synthesis of these heterocy-
cles.
Multi-component reactions (MCRs), are those reactions in
which three or more reactants react together to give the product
in a single step under suitable reaction conditions [10]. MCRs offer
the advantage of simplicity and synthetic efficiency over conven-
tional chemical reactions. The MCRs have the additional advantages
of selectivity, synthetic convergency, and atom-economy [11a,b].
Further, use of water as a solvent is an active area of research in
∗
Corresponding author. Tel.: +91 80 22961337.
E-mail addresses: mafpasha@gmail.com, m af pasha@ymail.com (M.A. Pasha).
1386-1425/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2011.06.033

432
A. Siddekha et al. / Spectrochimica Acta Part A 81 (2011) 431–440
Scheme 1.
Table 1
methods are associated with disadvantages such as: use of
expensive and environmentally hazardous reagents, low yields of
products, drastic reaction conditions and tedious work-up proce-
dures.
Synthesis of pyranopyrazoles from various aromatic aldehydes, malononitrile, ethyl
acetoacetate and hydrazine hydrate.
Entry
Aromatic aldehydes (1)
Producta (5)c
Time (min)
Yield (%)b
OMe
In continuation of our work on the synthesis of biologically
active heterocyclic organic compounds [14a–e]; herein, we are
reporting a simple and efficient synthesis of pyranopyrazoles
by a four component reaction of aromatic aldehydes (1), ethyl
acetoacetate (2), malononitrile (3) and hydrazine hydrate (4) in
the presence of an organocatalyst imidazole in water at 80 ◦C
(Scheme 1). The method is environmentally friendly as it is car-
ried out in water, involves the use of an inexpensive ecofriendly
organocatalyst and gives high yield of the products within
30 min.
O
CN
H
HN
N
MeO
O
NH
a
20
20
88
89
O
H
CN
HN
N
O
NH
b
NO
O
2. Results and discussion
CN
NH
H
HN
For a detailed exploration, the reaction between p-anisaldehyde
and ethyl acetoacetate, malononitrile and hydrazine hydrate in
the presence of imidazole was considered for the optimiza-
tion of amount of catalyst used; and to study the effect of
solvents on the rate of the reaction. Initially, when the reac-
tion was carried out in the absence of any catalyst, no product
was found even after 30 min. Addition of 0.2 mmol imidazole
afforded the product in low yield, an increase in the amount
of imidazole to 0.5 mmol gave 6-amino-4-(4ꢀ-methoxyphenyl)-5-
cyano-3-methyl-1-phenyl-1,4-dihydropyrano[2,3-c]pyrazole (5a)
in 88% yield; further increase in the amount of catalyst did not
increase the product yield. Next, in order to investigate the effect
of solvents, the above reaction was carried out in conventional
organic solvents such as DCM, CH3CN, ethanol and in water. Water
as a solvent provided the best yields compared to DCM, CH3CN and
ethanol.
N
O2N
O
Cl
c
20
20
85
90
O
H
CN
NH
HN
N
Cl
O
d
OH
O
H
CN
HN
N
HO
O
NH
e
f
20
30
90
89
O
MeO
H
CN
NH
HN
N
OMe
O
O
MeO
In order to establish the generality, the catalyst was successfully
applied to the reaction of various araldehydes with ethyl acetoac-
etate, malononitrile and hydrazine hydrate and the results of this
study are presented in Table 1. It is clear from this table that,
excellent product yields were obtained with aryl and heteroaryl
aldehydes. Furthermore, the reaction is compatible in the presence
of various functional groups such as –Cl, –OCH3, –NO2 and –OH.
H
CN
NH
HN
N
OMe
O
O
g
30
30
85
90
O N
H
CN
NH
HN
NO2
O
N
O
h
i
CH3
O
Cl
H
CN
NH
HN
HN
N
N
Cl
O
30
30
85
89
O
O
H3C
CN
NH
CN
NH2
H
O
O
j
N
N
a
All the products are known and were identified by either comparison of their IR
spectra, or by their melting points or on TLC with the authentic samples prepared
by known methods.
O
H
b
Isolated yield.
c
Compounds 5a, 5b and 5i were characterized by 1H NMR spectral analysis.
Fig. 1. Structure of 6-amino-4-(4ꢀ-methoxyphenyl)-5-cyano-3-methyl-1-phenyl-
1,4-dihydropyrano[2,3-c]pyrazole (C15H14N4O2).

A. Siddekha et al. / Spectrochimica Acta Part A 81 (2011) 431–440
433
Scheme 2. A plausible mechanism for the formation of pyranopyrazoles.
Fig. 2. Numbering system adopted for the optimized structure of 6-amino-4-(4ꢀ-methoxyphenyl)-5-cyano-3-methyl-1-phenyl-1,4-dihydropyrano[2,3-c]pyrazole.

434
A. Siddekha et al. / Spectrochimica Acta Part A 81 (2011) 431–440
Table 2
Table 3
Optimized bond lengths and bond angles of 6-amino-4-(4ꢀ-methoxyphenyl)-5-
Optimized dihedral angles of 6-amino-4-(4ꢀ-methoxyphenyl)-5-cyano-3-methyl-1-
cyano-3-methyl-1-phenyl-1,4-dihydropyrano[2,3-c]pyrazole.
phenyl-1,4-dihydropyrano[2,3-c]pyrazole.
Bond
Length
Angle
(◦)
Dihedral angle
(◦)
Dihedral angle
(◦)
C1–C2
C1–C5
C1–H30
C2–C3
C2–H31
C3–C4
1.396
1.401
1.083
1.397
1.087
1.401
1.530
1.087
1.390
1.365
1.401
1.539
1.504
1.100
1.368
1.419
1.370
1.426
1.367
1.373
1.349
1.363
1.419
1.098
1.098
1.091
1.366
1.007
1.334
1.496
1.167
1.010
1.010
1.096
1.092
1.095
1.085
C2–C1–C5
C2–C1–H30
C1–C2–C3
C1–C2–H31
C5–C1–H30
C1–C5–O11
C1–C5–H34
C3–C2–H31
C2–C3–C4
C2–C3–C6
C4–C3–C6
C3–C4–H32
C3–C4–H34
C3–C6–C7
119.6
119.4
121.6
119.0
121.0
124.7
119.5
119.5
118.0
121.5
120.5
119.5
121.3
112.0
113.8
106.6
119.2
120.0
115.8
118.3
118.6
106.8
107.9
125.1
118.8
109.6
121.9
134.7
116.1
126.3
123.8
177.9
103.4
109.1
128.5
111.5
128.1
109.9
116.2
116.0
113.9
H30–C1–C2–C3
C5–C1–C2–H31
H30–C1–C2–H31
C2–C1–C5–O11
C2–C1–C5–C34
H30–C1–C5–O11
H30–C1–C5–C34
C1–C2–C3–C4
179.8
C7–C6–C8–C18
−178.7
118.2
−62.09
176.2
−1.01
−3.30
179.4
179.75
−1.026
−0.05
179.18
−179.7
0.26
−179.8
0.13
179.8
0.058
−0.11
−179.9
0.19
H33–C6–C8–C10
H33–C6–C8–C18
C6–C7–C9–N20
C6–C7–C9–O29
C19–C7–C9–N20
C19–C7–C9–O29
C6–C8–C10–N16
C6–C8–C10–O29
C18–C8–C10–N16
C18–C8–C10–O29
C6–C8–C18–N17
C6–C8–C18–C24
C10–C8–C18–N17
C10–C8–C18–C24
C7–C9–N20–H21
C7–C9–N20–H22
O29–C9–N20–H21
O29–C9–N20–H22
C7–C9–O29–C10
N20–C9–O29–C10
C8–C10–N16–N17
C8–C10–N16–H23
O29–C10–N16–N17
O29–C10–N16–H23
C8–C10–O29–C9
N16–C10–O29–C9
C5–O11–C12–H13
C5–O11–C12–H14
C5–O11–C12–C15
C10–N16–N17–C18
H23–N16–N17–C18
N16–N17–C18–C8
N16–N17–C18–C24
C8–C18–C24–H25
C8–C18–C24–H26
C8–C18–C24–H27
N17–C18–C24–H25
N17–C18–C24–H26
N17–C18–C24–H27
C3–C6
C4–H32
C4–C34
C5–O11
C5–C34
C6–C7
C6–C8
C6–H33
C7–C9
C1–C2–C3–C6
179.2
179.9
H31–C2–C3–C4
H31–C2–C3–C6
C2–C3–C4–H32
C2–C3–C4–C34
C6–C3–C4–H32
C6–C3–C4–C34
C2–C3–C6–C7
C2–C3–C6–C8
C2–C3–C6–H33
C4–C3–C6–C7
−1.0841
179.7
−0.12
0.63
0.01
C3–C6–C8
−179.2
−71.72
49.55
170.5
107.3
−131.4
−10.49
0.02
−180.
14.32
155.5
−168.1
−26.91
1.682
−176.0
0.07
179.1
−179.2
−0.23
−0.71
178.4
60.89
−61.53
179.7
−0.06
−179.1
0.03
C7–C19
C8–C10
C8–C18
C9–N20
C9–O29
C10–N16
C10–O29
O11–C12
C12–H13
C12–H14
C12–H15
N16–N17
N16–H23
N17–C18
C18–C24
C19–N28
N20–H21
N20–H22
C24–H25
C24–H26
C24–H 27
C34–H35
C3–C6–H33
H32–C4–H34
C4–H34–C5
O11–C5–H34
C5–O11–C12
C5–H34–H35
C7–C6–C8
C4–C3–C6–C8
C4–C3–C6–H33
C3–C4–C34–C5
C3–C4–C34–H35
H32–C4–C34–C5
H32–C4–C34–H35
C1–C5–O11–C12
C34–C5–O11–C12
C1–C5–C34–C4
C1–C5–C34–H35
O11–C5–C34–C4
O11–C5–C34–H35
C3–C6–C7–C9
C7–C6–H33
C6–C7–C9
179.9
−179.8
0.15
C6–C7–C19
C8–C6–H33
C6–C8–C10
C6–C8–C18
C9–C7–C19
C7–C9–O20
C7–C9–O29
C7–C19–N28
C10–C8–C18
C8–C10–N16
N8–N10–O29
N8–C18–N17
N8–C18–N24
N20–C9–O29
C9–N20–H21
C9–N20–H22
C9–O29–C10
0.72
−179.5
0.01
−180.0
−179.8
0.25
124.6
−55.90
−0.64
178.9
−118.4
61.10
−122.6
57.2
C3–C6–C7–C19
C8–C6–C7–C9
−180.0
62.10
−177.9
−57.39
−117.9
2.06
C8–C6–C7–C19
H33–C6–C7–C9
H33–C6–C7–C19
C3–C6–C8–C10
C3–C6–C8–C18
C7–C6–C8–C10
1.54
122.6
yield of the products and time taken for completion of the reactions
is summarized in Table 1.
3. Experimental
3.2. Mechanism
All chemicals are commercial and were used without further
purification. Progress of the reactions was monitored using Silica
gel-H TLC plates. The synthesized compounds were character-
ized by 1H NMR spectral analysis; by comparing the products
on TLC or by the comparison of melting points with products
prepared by known methods. NMR spectra were recorded on
a Brucker spectrophotometer. FT-IR spectra were recorded on
a Bruker Optics Alpha-P FT-IR spectrophotometer with attenu-
ated total reflectance (ATR) module. The FT-Raman were recorded
in solid phase on Bruker Optics MultiRAM FT-Raman spec-
trophotometer using Nd-YAG laser operating at 1064 nm as an
excitation source at the resolution of 4 cm−1. Vibrational spec-
troscopic studies of 6-amino-4-(4ꢀ-methoxyphenyl)-5-cyano-3-
methyl-1-phenyl-1,4-dihydropyrano[2,3-c]pyrazole (5a) was done
by recording FT-IR, FT-Raman spectra in the region 4000–400 cm−1
and 3500–100 cm−1, respectively.
A plausible mechanism for the formation of imidazole catalyzed
pyranopyrazoles is envisaged. The formation of pyranopyrazoles
catalyzed by imidazole may involve the protonation of ethyl
acetoacetate by imidazole, followed by intermolecular attack by
hydrazine hydrate. Subsequent loss of water, and intramolecular
nucleophillic attack by –NH2 group on the carbonyl carbon to give
5-methyl-2,4-dihydro-pyrazol-3-one (I). Similarly protonation of
aldehyde by imidazole and reaction with 3-imino-acrylonitrile (II)
may afford 2-benzylidene-malononitrile (III). Addition of I to III in
the presence of imidazole followed by rearrangement may give the
expected pyranopyrazole as shown in Scheme 2.
4. Vibrational spectroscopic studies on
6-amino-4-(4ꢀ-methoxyphenyl)-5-cyano-3-methyl-1-
phenyl-1,4-dihydropyrano[2,3-c]pyrazole
(5a)
3.1. Typical procedure for the preparation of pyranopyrazole
Determination of the molecular structure of the organic
molecules provides insight into the important molecular prop-
erties. Knowledge of the electronic structure and the spectral
properties help in understanding the biological activity. Density
functional theory (DFT) is a quantum mechanical theory used
The aromatic aldehyde (1 mmol), malononitrile (1 mmol), ethyl
acetoacetate (1 mmol) hydrazine hydrate (1 mmol) and imidazole
(0.5 mmol) were taken in water (5 ml) and heated on a preheated
hot plate at 80 ◦C for 20–30 min. Various aromatic aldehydes used,

A. Siddekha et al. / Spectrochimica Acta Part A 81 (2011) 431–440
435
Table 4
Comparison of the calculated and experimental vibrational frequencies of 6-amino-4-(4ꢀ-methoxyphenyl)-5-cyano-3-methyl-1-phenyl-1,4-dihydropyrano[2,3-c]pyrazole.
Mode
Calculated
Freq (cm−1
Experimental
%PED
Approximate character
)
IR intensity
Raman
IR freq (cm−1
)
Raman freq
(cm−1
)
intensity
(cm−1
)
(cm−1
)
11
13
521
552
6.60
3.09
0.69
523
565
511
567
ꢀ N28C19C7C6–25, ꢀ
O11C1C34C5–17
ı C1C5C34–17, ı
C12O11C5–10, ꢀ
N28C19C7C6–12
ı C4C34C5–18, ı
C2C1C5–24
Torsion of nitrile group
Phenyl ring def
2.24
14
15
19
618
631
671
2.09
7.69
1.40
7.81
0.97
0.50
614
630
668
613
634
678
In plane bending
phenyl ring
NCC bending of nitrile
group
ı N28C19C7–10, ı
C19C7C9–20
ꢀ N17N16C10C8–18, ꢀ
H26C24C18C8–11, ꢀ
C18N17N16C10–16
ꢀ C4C34C5C1–10, ꢀ
C2C1C5C34–10, ꢀ
C3C2C1C5–24,
ꢁ C1C5–11,
ꢁ O11C5–15,
ı C3C2C1–10
ꢁ N17C18–10, ı
C18N17N16–14, ı
H25C24H27–11, ꢀ
N25C24C18C8–21, ꢀ
H27C24C18C8–20
ꢁ O29C9–32, ı
Pyrazole ring torsion
20
22
30
697
760
966
1.13
5.33
22.6
1.03
3.05
2.86
721
750
971
706
748
936
Phenyl ring torsion
OC stretch ether
Linkage + CC
Stretching
Methyl ring wagging
attached to pyrazole
ring
32
34
36
42
1006
1035
1092
1164
49.27
48.8
3.10
7.29
0.97
2.49
2.87
5.03
1001
1029
1073
1169
1003
1.57
O–C stretch of pyran
ring
O–C stretch of ether
attached to phenyl ring
NN stretch of pyrazole
ring
H22N20C9–23
ꢁ O11C12–72
1083
1169
ꢁ N17N16–41
ı H14C12H13–15
Out of plane
bending + torsion of
methyl group attached
to ether linkage
ꢀ H13C12O11C5–28, ꢀ
H14C12O11C5–29
ꢁ C3C2–15, ı
45
49
1235
1291
1.36
19.3
7.14
1.69
1223
1301
1222
1313
CC stretching in phenyl
ring + HCC asym
bending
H33C6C3–31
ı H30C1C2–13, ı
H31C2C3–14, ı
H32C4C34–14, ı
H35C34C5–11
Phenyl ring bending
51
53
56
1362
1384
1436
2.2
8.16
1328vw
1392
1363vw
1390
H27C24H26–25,
H26C24H25–22,
H25C24H27–23
ꢁ N17C18–26, ı
C18N17N16–11, ı
H23N16N17–18
ı H27C24H26–44
HCH in plane bending
methyl group attached
to pyrazole
NC stretch + CNN bend
in pyrazole ring (def)
60.59
5.54
37.58
19.98
1442
1392
HCC sym bending
methyl group attached
to pyrazole ring
ı H26C24H25–31
ꢀ H26C24C18C8–1
ı H25C24H27––37, ı
H26C24H25–15
57
1440
5.9
20.37
1443
1441
HCC asym bending CH3
group attached to
pyrazole ring
59
62
1460
1513
31.29
7.15
1465
1512
1440
1549
ı H14C12H13–39
Methyl group bending
(methoxy group)
NC stretch + HNN
bending in pyrazole
ring + OC stretching
pyran ring
CC stretch of pyran
ring + out of plane NH2
bend
173.13
12.52
ꢁ N16C10–31, ꢁ
O29C10–17, ı
H23N16N17–15, ı
N16C10C8–14
ꢁ C10C8–23, ı
H21N20H22–24
65
66
67
1595
1605
1640
9.63
4.64
1582
1597
1641
1542
1584
1628
86.17
444.70
82.16
55.44
ꢁ C4 C34–30,
ꢁ C2C1–11,
ꢁ C3C2–11
ꢁ C7 C9–36, ꢁ
C10C8–11,
Phenyl ring breathing
Ring breathing of pyran
ring + NH2 bend
ꢁ N20C9–11, ı
H21N20H22–13

436
A. Siddekha et al. / Spectrochimica Acta Part A 81 (2011) 431–440
Table 4 (Continued)
Mode
Calculated
Experimental
%PED
Approximate character
Freq (cm−1
)
IR intensity
Raman
IR freq (cm−1
)
Raman freq
(cm−1
(cm−1
)
intensity
)
(cm−1
)
68
2222
66.95
192.72
2191vs
2191vs
ꢁ N28C19–89, ꢁ C19
C7–11
CN sym stretch of
nitrile group
69
70
2884
2898
22.49
56.53
65.68
123.49
2856m
2836
2839m
2882
ꢁ C6H33 100
ꢁ C12H13–45, ꢁ
C12H14–47
CH stretch
CH sym stretch of
methyl group attached
to ether linkage
CH sym stretch of
methyl group attached
pyrazole ring
CH asym stretch of
methyl group attached
to ether linkage.
CH asym stretch
phenyl ring
CH sym stretch phenyl
ring
NH sym stretch of NH2
group attached to
pyran ring
71
72
2929
2957
17.17
40.72
122.53
48.80
2925
2962
2937
2935
ꢁ C24 H25, ␯ C24
H26–17, ꢁ C24 H27–30
ꢁ C12H13–51, ꢁ
C12H14–49
76
79
80
3054
3097
3445
11.02
11.28
63.14
59.77
88.90
175.11
3053
3104
3481
3050
3078
3482
ꢁ C4H32 95
ꢁ C1–H30 95
ꢁ N20H21–57, ꢁ
N20–H22–43
81
82
3545
3562
119.74
42.35
165.22
78.67
3565
3587
ꢁ N16–H23–100
NH stretch of pyrazole
ring
NH asym stretch of
NH2 group attached to
pyran ring
ꢁ N20H21 43,
ꢁN20–H22 57
ı, bend; ꢀ, torsion; ꢁ, stretch; asym, asymmetric; sym, symmetric; def, deformation.
to investigate the electronic structure in the ground state of
many-body systems, in particular atoms and molecules. Raman
spectroscopy has recently been proved to be a valuable tool in the
investigation of complex molecules of biological interest [15–17].
Recently, vibrational spectral studies and theoretical computa-
tions related to natural coumarin derivatives [18,19], ginkgolide
[20], cresyl violet perchlorate [21], and molecules like 2,4-dichloro-
6-nitrophenol [22] have appeared in the literature. Atalay et al.
[23,24] have reported the theoretical studies on molecular struc-
ture and vibrational spectra of 2-amino-5-phenyl-1,3,4-thiadiazole
and 1-amino-5-benzoyl-4-phenylpyrimidin-2(1H). Yakuphanoglu
et al. [25] and Sekerci et al. [26] have successfully used
Hartree–Fock and density functional methods to study the spec-
tral properties of molecules like N-phenyl-Nꢀ-(2-thienylmethylene)
hydrazine and 1-(thiophen-2-yl-methyl)-2-(thiophen-2-yl)-1H-
benzimidazole.
In this paper, we report the interpretation of vibra-
tional spectra and computations related to the structure of
6-amino-4-(4ꢀ-methoxyphenyl)-5-cyano-3-methyl-1-phenyl-1,4-
dihydropyrano[2,3-c]pyrazole (5a, Fig. 1) using density functional
theory.
4.1. Computational details
Density functional theory calculations were carried out using
Gaussian 09 package [27]. Equilibrium structure and vibrational
frequencies were optimized using Becke-3-Lee-Yang-Parr (B3LYP)
functional with 6-31g** basis set [28,29]. Frequency calculations
were carried out on the optimized structure using the program
available in the Gaussian software itself. Equilibrium symmetry
predicted is C1 with energy of −1392.97 Hartrees. Potential energy
distribution was carried out using VEDA 4.0 program [30]. Opti-
0.00
-0.01
-0.02
-0.03
-0.04
4000
3500
3000
2500
2000
1500
1000
500
Wavenumber cm-1
Fig. 3. FT-IR spectrum of 6-amino-4-(4ꢀ-methoxyphenyl)-5-cyano-3-methyl-1-phenyl-1,4-dihydropyrano[2,3-c]pyrazole in 400–4000 cm−1
.

A. Siddekha et al. / Spectrochimica Acta Part A 81 (2011) 431–440
437
0.00
-0.01
-0.02
-0.03
-0.04
1800
1600
1400
1200
1000
800
600
Wavenumber cm-1
Fig. 4. FT-IR spectrum of 6-amino-4-(4ꢀ-methoxyphenyl)-5-cyano-3-methyl-1-phenyl-1,4-dihydropyrano[2,3-c]pyrazole in 1800–400 cm−1
.
0.00
-0.01
-0.02
-0.03
-0.04
4000
3500
3000
2500
2000
Wavenumber cm-1
Fig. 5. FT-IR spectrum of 6-amino-4-(4ꢀ-methoxyphenyl)-5-cyano-3-methyl-1-phenyl-1,4-dihydropyrano[2,3-c]pyrazole in 4000–1800 cm−1
.
0.04
0.03
0.02
0.01
0.00
3500
3000
2500
1500
1000
Wavenumber cm-1
Fig. 6. FT-Raman spectrum of 6-amino-4-(4ꢀ-methoxyphenyl)-5-cyano-3-methyl-1-phenyl-1,4-dihydropyrano[2,3-c]pyrazole in 3700–600 cm−1
.
mized geometry of the molecule and the numbering system is given
in Fig. 2, optimized bond lengths and bond angles are presented in
Table 2 and optimized dihedral angles are given in Table 3.
of fundamental vibrations are possibe. Predicted equilibrium sym-
metry for this molecule is C1 with energy of −1392.97 hartrees.
A detailed vibrational analysis has been carried out and assign-
ments of the observed fundamental bands have been proposed
on the basis of peak positions and the relative intensities. The
PED contribution to each of the observed frequencies gives the
reliability and precision of the spectral analysis. The calculated
harmonic-vibrational frequencies and the observed FT-IR and FT-
5. Vibrational analysis
6-Amino-4-(4ꢀ-methoxyphenyl)-5-cyano-3-methyl-1-phenyl-
1,4-dihydropyrano[2,3-c]pyrazole has 35 atoms, hence, 99 modes

438
A. Siddekha et al. / Spectrochimica Acta Part A 81 (2011) 431–440
0.04
0.03
0.02
0.01
0.00
1800
1600
1400
1200
1000
800
600
400
200
Wavenumber cm-1
Fig. 7. FT-Raman spectrum of 6-amino-4-(4ꢀ-methoxyphenyl)-5-cyano-3-methyl-1-phenyl-1,4-dihydropyrano[2,3-c]pyrazole in 1800–100 cm−1
.
0.00
3500
3000
2500
2000
wavenumber cm-1
Fig. 8. FT-Raman spectrum of 6-amino-4-(4ꢀ-methoxyphenyl)-5-cyano-3-methyl-1-phenyl-1,4-dihydropyrano[2,3-c]pyrazole in 3700–1800 cm−1
.
consistent with the predicted values of 2898 cm−1, 2929 cm−1
.
Raman frequencies for various modes of vibrations are presented
in Table 4. A comparison of the calculated frequencies with the
experimental values indicates the overestimation of the calculated
vibrational modes due to neglect of anharmonicity in real system.
Hence, computed frequencies were multiplied with scale factor
0.9614 to offset the systematic error caused by neglecting anhar-
monicity giving a good agreement with the observed frequencies.
These calculations provide a valuable insight into the vibrational
spectrum and molecular parameters.
C–H asymmetric stretching vibrations computed at 2957 cm−1
and 2984 cm−1 match with the 2963 cm−1, 3005 cm−1 in IR and
2957 cm−1, 3007 cm−1 in Raman spectrum.
The symmetric and asymmetric bending vibrations of methyl
group are normally expected in the regions 1465–1440 cm−1 and
1390–1370 cm−1, respectively. The asymmetric bending of methyl
group observed at 1465 cm−1 (IR) and 1440 cm−1 (Raman) are
in good agreement with the theoretically calculated value of
1460 cm−1. The symmetric bending calculated at 1436 cm−1 is
observed as a medium band at 1442 cm−1 (IR) and 1392 cm−1
(Raman). Out of plane bending with torsion of methyl group
attached to ether linkage is calculated at 1164 cm−1 and is experi-
mentally seen at 1169 cm−1 in IR and Raman.
6-Amino-4-(4ꢀ-methoxyphenyl)-5-cyano-3-methyl-1-
phenyl-1,4-dihydropyrano[2,3-c]pyrazole (5a) has
ring with methyl substituent, pyran ring has
an amino group and phenyl ring with
a
pyrazole
nitrile,
a p-methoxy
a
a
a
substituent. The pyran (C10–C8–C6–C7–C9–O29) and the
pyrazole(C10–C8–C18–N16–N17) are fused together and lie
in one plane. The phenyl ring (C1–C2–C3–C4–C34–C5) lies perpen-
dicular to the plane of the fused rings ( C3–C6–C7 = 112◦, Fig. 2).
Hence, the vibrational spectral analysis was performed based on
the characteristic vibrations of the methyl group, amino group,
nitrile group, methoxy group, and a phenyl ring. The FT-IR and
FT-Raman spectra are shown in Figs. 3–8.
5.2. NH2 and NH group vibrations
The NH2 wagging was computed at 496 cm−1 and is observed
at 519 cm−1 in FT-IR and at 512 cm−1 in FT-Raman. NH sym stretch
of NH2 group attached to pyran ring is calculated at 3445 cm−1 is
in good agreement with the experimental values of 3481 cm−1 and
3482 cm−1 in FT-IR and FT-Raman, respectively. NH asym stretch
of NH2 group attached to pyran ring which is calculated to appear
at 3562 cm−1 is observed only in FT-IR at 3581 cm−1 and not in FT-
Raman. NH stretch of pyrazole ring is calculated as 3545 cm−1 is
5.1. Methyl group vibrations
The molecule under consideration has two methyl groups, one
attached to the pyrazole ring and the other to the side chain phenyl
ring as methoxy substituent. The asymmetric stretching mode of
methyl group is expected to be around 2980 cm−1 and symmetric
stretching at 2870 cm−1 [31,32]. In our molecule, C–H symmet-
also seen only in FT-IR at 3565 cm−1
.
5.3. C N, C–N, N–N and the nitrile group
ric stretch of methyl group vibrations recorded at 2836 cm−1
2925 cm−1 (FT-IR) and 2882 cm−1, 2937 cm−1 (FT-Raman) are
,
C
N and C–N stretching vibrations generally occur in the region
1382–1266 cm−1, and it is difficult to assign these bands as they

A. Siddekha et al. / Spectrochimica Acta Part A 81 (2011) 431–440
439
get mixed with several other bands. In our study, the pyrazole ring
contains the C N and C–N bonds, which show stretching vibrations
at 1392, 1512 cm−1 in IR and at 1390, 1549 cm−1 in Raman. These
are consistent with the calculated frequencies at 1384 cm−1 and
1513 cm−1 along with the bending vibrations.
A characteristic band for the nitrile group attached to the pyran
ring is observed at 2191 cm−1 in both FT-IR and FT-Raman spec-
tra, which is in very good agreement with the computed value of
2222 cm−1. Torsion of nitrile group calculated at 521 cm−1 agrees
with the observed values of 523 cm−1 in IR and 511 cm−1 in Raman.
N–N Stretch of pyrazole ring was calculated at 1092 cm−1 and
is consistent with the experimental values of 1073 cm−1 in IR and
1083 cm−1 in Raman spectrum.
be in good agreement with the experimental values for both FT-
IR and FT-Raman. These results confirm the validity of potential
energy distribution to each of the observed frequencies.
Organic molecules possessing similar functional groups have
been synthesized and their structures have been studied using DFT
calculations and vibrational spectral analyses [34,35]. It has been
found by us that, the vibrational frequency values reported for
the corresponding functional groups are consistent with the val-
ues which we have observed for the functional groups present in
pyranopyrazoles.
Acknowledgments
Aisha Siddekha gratefully acknowledges, The Indian Academy
of Sciences, Bangalore, Indian National Science Academy, New
Delhi, The National Academy of Sciences, Allahabad, for the short
term Fellowship and The Indian Institute of Science, Bangalore,
for helping her in the FT-Raman and FT-IR spectral analysis dur-
ing Summer Research Fellowship programme during May to July
2010. The authors would also like to acknowledge the Sophisticated
Instruments Facility for recording the NMR spectra and Archives
Department, Indian Institute of Science, Bangalore, for editing the
manuscript.
5.4. C–H vibrations
Generally, C–H stretching, C–H in-plane bending and C–H out-
of-plane bending vibrations appear in the range 3100–3000 cm−1
,
1300–1000 cm−1 and 1000–750 cm−1 [33]. HCH in plane bending of
methyl group attached to pyrazole ring is calculated at 1362 cm−1 is
experimentally observed at 1328 cm−1 (IR) and 1363 cm−1 (Raman)
as very weak bands. CH stretch of hydrogen attached to pyran ring
is observed at 2856 cm−1 in IR and at 2839 cm−1 in Raman spectrum
matches the predicted frequency of 2884 cm−1
.
5.5. C–O group vibrations
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A convenient and efficient protocol has been developed for the
synthesis of pyranopyrazoles in high yields using catalytic amounts
of imidazole as an organocatalyst, in water. We have demonstrated
that, the synthesis is milder, fast and does not require any tedious
work-up procedure. An effort has been made in the present study
to optimize the structure of 6-amino-4-(4ꢀ-methoxyphenyl)-5-
cyano-3-methyl-1-phenyl-1,4-dihydropyrano[2,3-c]pyrazole (5a,
C15H14N4O2). The geometric parameters and vibrational frequen-
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440
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