Nickel nanoparticles catalyzed knoevenagel condensation of aromatic aldehydes with barbituric acids and 2-thiobarbituric acids

Article abstract of DOI:10.1007/s10562-010-0376-2

An efficient route for the Knoevenagel condensation of aromatic aldehydes with barbituric acids and 2-thiobarbituric acids in the presence of polyvinyl pyrrolidone (PVP) stabilized Ni nanoparticles in ethylene glycol has been reported. A range of biologically important arylidene barbiturates were obtained in high yields (82-97%) in a very short reaction time. Graphical Abstract: A novel and highly efficient PVP-stabilized Ni nanoparticles catalyzed synthesis of arylidene barbiturates and arylidene 2-thiobarbiturates has been described.

Full text of DOI:10.1007/s10562-010-0376-2

Catal Lett (2010) 138:104–110
DOI 10.1007/s10562-010-0376-2
Nickel Nanoparticles Catalyzed Knoevenagel Condensation
of Aromatic Aldehydes with Barbituric Acids
and 2-Thiobarbituric Acids
•
Jitender M. Khurana Kanika Vij
Received: 6 February 2010 / Accepted: 15 May 2010 / Published online: 4 June 2010
Ó Springer Science+Business Media, LLC 2010
Abstract An efficient route for the Knoevenagel con-
densation of aromatic aldehydes with barbituric acids and
2-thiobarbituric acids in the presence of polyvinyl pyrrol-
idone (PVP) stabilized Ni nanoparticles in ethylene glycol
has been reported. A range of biologically important ary-
lidene barbiturates were obtained in high yields (82–97%)
in a very short reaction time.
[5]. Various methods have been reported for the Knoeve-
nagel condensation of carbonyl compounds with barbituric
acids or 2-thio barbituric acids like homogeneous systems
[6–8], solvent-free grinding using NH2SO3H [9], micro-
wave irradiation [10], infra red promoted [11], ionic liquid
mediated condensation [12], various acid and base cata-
lyzed condensation reactions including dry condensation
with acidic clay catalysts [13–20]. Recently, Ni supported
on SiO2 has been used for the selective catalytic Knoeve-
nagel condensation [21]. However, some of these methods
have been associated with shortcomings such as longer
reaction times, use of expensive catalysts, formation of
undesirable di-substituted product, requirement of excess
of solvent, lower yields, etc. Therefore, in view of the
importance of barbituric acid derivatives, especially
5-arylidene barbiturates, development of a facile and prac-
tically efficient methodology for the target Knoevenagel
condensation is desired.
Keywords PVP stabilized nickel nanoparticles ꢀ
Knoevenagel condensation ꢀ Aromatic aldehydes ꢀ
Barbituric acids ꢀ Ethylene glycol
1 Introduction
The Knoevenagel condensation of aldehydes with active
methylene compounds is an important and widely
employed method for carbon–carbon bond formation in
organic synthesis with numerous applications in the syn-
thesis of fine chemicals, hetero Diels–Alder reaction and in
synthesis of carbocyclic as well as heterocyclic compounds
of biological significance. Barbituric acids and their
derivatives have been widely used as a sedative, hypnotic,
anaesthetic, anticonvulsant, antiosteoporosis, as well as
antitumor agents [1]. Arylidene barbituric acids as well as
their 2-thio analogues are useful as intermediates in syn-
thesis of heterocyclic compounds [1], benzyl barbituric
derivatives [2], oxadiazaflavines [3], and unsymmetrical
disulphides [4]. Additionally, some of them have been
recently studied as non-linear optical materials and dyes
Transition-metal nanoparticles have gained tremendous
importance in the last two decades as they exhibit inter-
esting electrical, optical, magnetic and chemical properties,
especially catalytic properties (owing to their large surface
area) which cannot be achieved by their bulk counterparts
[22–31]. Metal nanoparticles offer new possibilities for
designing and constructing structure controllable catalysts,
which can be synthesized in a solvent or even be deposited
on a support without obvious aggregation [32–34]. A
variety of chemical methods have been developed for the
preparation of metal nanoparticles [35] including those
wherein, polymers carrying functional groups have been
used as specific stabilizers for the solution synthesis of
nanoparticles as they prevent irreversible aggregation of
nanometer-sized metal particles due to steric repulsion [36,
37] and also make them ‘‘soluble’’ in solvents. Amongst
these, the modified polyol method has been conveniently
J. M. Khurana (&) ꢀ K. Vij
Department of Chemistry, University of Delhi, Delhi 110007,
India
e-mail: jmkhurana1@yahoo.co.in
123

Nickel Nanoparticles Catalyzed Knoevenagel Condensation of Aromatic Aldehydes
105
utilized for the heterogeneous nucleation and growth of
nickel nanoparticles [38, 39]. Metal nanoparticles includ-
ing monolayer protected ones, exhibit excellent catalytic
activities for a variety of organic reactions [40–45].
components using ethanol. Subsequently, a drop of meth-
anol dispersion of the nickel nanoparticles was placed on
carbon coated Cu grid (mesh size 300). The ethylene glycol
dispersion, as obtained directly from polyol process, was
used as such for carrying out QELS analysis and for UV–
Vis spectral analysis. IR spectra of the sample; isolated
upon centrifugation followed by washing with acetone; was
done using KBr pellets. Sample for the X-ray diffraction
study was obtained by depositing a thin coating of the
isolated nickel nanoparticles (dispersed in absolute etha-
nol) onto a glass plate followed vacuum drying.
Recently, there has been growing interest in using nickel
nanoparticles in organic synthesis and these are foreseen as
an active area of research in the future because of their
potent catalytic activity, possible processability, easy
preparation, high stability, ease of recyclability as well as
greener compared to traditional Raney Ni catalyst. A great
deal of work has demonstrated that the Ni(0)-catalyzed
C–C bond forming reaction is an extremely powerful tool
in organic synthesis [46]. However, an increasing number
of examples are available in literature wherein Ni nano-
particles alone have been used as a catalyst during an
organic transformation [47–55]. Such remarkable proper-
ties associated with Ni nanoparticles spurred us to study the
possibility of Knoevenagel condensation of aromatic
aldehydes with barbituric acids and 2-thiobarbituric acids
in the presence of Ni nanoparticles as the active catalysts.
2.3 General Procedure for the Synthesis of 5-Arylidene
Barbituric Acids and 2-Thio Analogues (IIIa-x)
Aldehyde (I, 2 mmol) was added to a well stirred disper-
sion of nickel nanoparticles in ethylene glycol (2 mL/0.1 g
of I: 1.5 mol%); as obtained directly from the modified
polyol process; in a 25 mL round bottomed flask. To it,
barbituric acid (II, 2 mmol) was added and the contents
were stirred in an oil-bath maintained at 50 °C till a solid
product separates out. Completion of the reaction was
monitored by TLC using methanol: chloroform (10:90) as
the eluant. All the reactions were invariably complete in
10–15 min. Upon completion, the reaction mixture was
diluted with water (*25 mL) when a solid separated out.
The solid product was filtered at pump using Whatman
type 1 filter paper, washed with water, dried and recrys-
tallised from hot glacial acetic acid. 5-arylidene barbitu-
rates (and 2-thio analogues) were obtained as identified by
their mp and spectral data.
2 Experimental
2.1 General
All the chemicals used were of research grade and were
used without further purification. The size and morphology
of nickel nanoparticles were characterized with the help of
transmission electron microscopy (JEOL 2100-F operating
at 200 kV) and quasi-electron light scattering (QELS)
analysis (PHOTOCOR-FC-1135P). EDAX spectrum was
obtained using Cu grids on TECNAI G2 U-TWIN
(300 kV). IR spectra were recorded on Perkin-Elmer FT-IR
SPECTRUM-2000. X-ray diffraction pattern was obtained
on BRUKER D8. 1H NMR spectra were recorded on
Brucker Spectrospin AVANCE (300 MHz) with DMSO-d6
as the solvent and TMS as the internal standard. Melting
points were recorded on a Tropical Labequip apparatus and
are uncorrected. All the nanoparticle characterization data
matched satisfactorily with the reported ones [39].
2.4 General Procedure for the Recyclability
of the Catalyst
Aldehyde (Ia, 2 mmol) was added to nickel nanoparticle
dispersion in ethylene glycol (2 mL/0.1 g I; 1.5 mol%) in
25 mL round bottomed flask. Barbituric acid (II, 2 mmol)
was added to the reaction mixture and it was stirred. Upon
complete formation of IIIa as monitored by TLC, 10.0 mL
of ethyl acetate were added to the reaction mixture. The
reaction mixture was stirred until complete dissolution of
IIIa in ethyl acetate was observed. The two layers were
then separated. The ethylene glycol layer was sonicated for
5–10 min and reused for the same experiment for over five
cycles.
2.2 Preparation of Ni Nanoparticles
Nickel nanoparticles were prepared by the modified polyol
process reported in the literature [39]. 20.0 mL of a
2 9 10-4 mol L-1 solution of NiCl2ꢀ6H2O in ethylene
glycol was reduced with NaBH4 (0.180 g) in presence of
PVP (Ni2?:PVP: 1:5 wt%) at 140 °C. The sample for TEM
and EDAX analysis was obtained by the addition of ace-
tone to the nickel nanoparticles dispersion in ethylene
glycol, followed by centrifugation (6000 rpm). The parti-
cles, so obtained, were washed free of any residual
2.5 Spectral Data
IIIa: 5-(4-Chlorobenzylidene)barbituric acid: M.P. =
297 °C (lit. [11] 298.5 °C); IR (KBr, cm-1) tmax = 3214,
3089, 2849, 1755, 1703, 1674, 1575. H NMR (300 MHz,
1
DMSO) d: 7.53 (d, 2H, J = 8.4 Hz), 8.08 (d, 2H,
123

106
J. M. Khurana, K. Vij
2Ni2þðegÞ þ BH4ꢁðSÞ þ 2H2OðegÞ þ 2n PVPðegÞ
J = 8.7 Hz), 8.25 (s, 1H), 11.23 (s, 1H, NH), 11.38 (s, 1H,
NH).
! 2NiðPVPÞnðSÞ þ 2H2ðgÞ þ 4HþðegÞ þ BO2ꢁðegÞ
IIIm: 5-[4-(N,N-Dimethylamino)benzylidene]-2-thio-
barbituric acid: M.P.C300 °C (lit. [12][300 °C); IR (KBr,
cm-1) tmax = 3455, 3120, 2918, 1638, 1490, 1461. 1H
NMR (300 MHz, DMSO) d: 3.16 (s, 6H), 6.83 (d, 2H,
Ar–H), 8.16 (s, 1H), 8.46 (d, 2H, Ar–H), 12.04 (s, 1H, NH),
12.14 (s, 1H, NH).
The metal dispersion so obtained was characterized by
TEM analysis which revealed the formation of coated Ni
nanoparticles with average diameter 11 nm (Fig. 1a, b).
This result was further supported by QELS analysis which
showed a maximum population distribution centered
around 10–11 nm (Fig. 1c). Metallic nature of nanoparti-
cles was confirmed by UV–Vis spectral data (Fig. 2) and
by the energy-dispersive spectrum (EDAX) as shown in
Fig. 3. IR spectra of the sample showed the presence of
PVP along with the presence of ethylene glycol which
remained adsorbed onto the particles despite repeated
washing (Fig. 4). XRD analysis further supported the
results (Fig. 5). Three peaks for Ni at 2h = 44.5°, 51.8°,
76.4°, corresponding to the (111), (200), (222) lattice
planes, were observed, revealing that the resultant particles
were pure elemental Ni with a face-centered cubic (FCC)
structure. The particle diameter was estimated by the
Scherrer’s equation to be about 6.9 nm. No obvious peak of
nickel oxide or hydroxide was detected. The results are in
complete agreement with the reported data [39].
IIIq: 5-(4-Hydroxybenzylidene)-1,3-dimethylbarbituric
acid: M.P. = 297 °C (lit.[18] 297 °C); IR (KBr, cm-1
)
tmax = 3209, 2987, 1669, 1642, 1609, 1530. 1H NMR
(300 MHz, DMSO) d: 3.20 (s, 3H), 3.22 (s, 3H), 6.88 (d,
2H, Ar–H), 8.27 (d, 2H, Ar–H), 8.31 (s, 1H), 10.81 (s, 1H,
OH).
IIIv:
5-(4-Hydroxybenzylidene)-1,3-diphenyl-2-thio-
barbituric acid: M.P. = 272 °C; IR (KBr, cm-1
)
tmax = 3340, 2925, 1655, 1603, 1571, 1560, 1532. 1H
NMR (300 MHz, DMSO) d: 6.87 (d, 2H, Ar–H), 7.28-7.47
(m, 10H, N-Ph), 8.32 (d, 2H, Ar–H), 8.36 (s, 1H), 11.00 (s,
1H, OH). 13C NMR (300 MHz, DMSO) d: 114.93, 115.80,
124.00, 127.95, 128.57, 128.89, 129.05, 138.92, 140.54,
158.04, 163.96, 180.77. MS (ESI): 400 [M]?.
IIIw: 5-[4-(N,N-Dimethyl)benzylidene]-1,3-diphenyl-2-
thiobarbituric acid: M.P. = 296 °C (lit. [300 °C); IR
(KBr, cm-1) tmax = 3448, 2923, 1659, 1610, 1506, 1421.
1H NMR (300 MHz, DMSO) d: 2.99 (s, 3H), 3.03 (s, 3H),
6.80 (d, 2H, Ar–H), 7.15-7.68 (m, 10H, N-Ph), 8.27 (s,
1H), 8.39 (d, 2H, Ar–H).
Ni nanoparticles were used in the form of dispersion in
ethylene glycol as obtained by modified polyol process.
Additionally, ethylene glycol served as a suitable solvent
for the currently probed transformation as well, based on
the solubility difference of the product from the starting
materials, leading to separation of product from the
reaction mixture upon completion, thereby facilitating
easy isolation of solid product from the reaction mixture
simply by filtration. As part of our program aimed at
developing new and environmentally benign synthetic
methodologies with Ni nanoparticles, a reaction of
4-chlorobenzaldehyde (Ia) was attempted with barbituric
acid (IIa) in presence of Ni nanoparticles under different
conditions. It was observed that 4-chlorobenzaldehyde
(1 equiv.) reacted with barbituric acid (1 equiv.) in presence
of Ni nanoparticles, dispersed in ethylene glycol (2 mL/
0.1 g I; 1.5 mol%), at 50 °C to afford 4-chlorobenzylidene
barbiturate (IIIa) in 10 min in 93% yield. Subsequently,
reactions of other aldehydes also with barbituric acid (IIa)
gave the corresponding arylidene barbiturates (IIIa-IIIj).
Reactions of aldehydes with 2-thiobarbituric acid (IIb)
yielded arylidene 2-thiobarbiturates (IIIk-o). Similarly,
reactions of aldehydes with N,N-dimethyl barbituric
acid (IIc) and N,N-diphenyl 2-thiobarbituric acid (IId)
also gave corresponding arylidene barbiturates (IIIp-x)
(Scheme 1).
IIIx:
5-(4-Chlorobenzylidene)-1,3-diphenyl-2-thio-
barbituric acid: M.P. = 256–258 °C; IR (KBr, cm-1
)
tmax = 3423, 2926, 1710, 1682, 1566, 1547, 1489. 1H
NMR (300 MHz, DMSO) d: 7.15 (d, 2H, Ar–H), 7.20-7.55
(m, 10H, N-Ph), 8.09 (d, 2H, Ar–H), 8.43 (s, 1H). 13C
NMR (300 MHz, DMSO) d: 120.46, 124.43, 127.72,
128.72, 128.83, 134.75, 139.34, 141.53, 162.22, 177.13.
MS (ESI): 418 [M]?, 420.
3 Results and Discussion
In this manuscript, we report a simple, easy and convenient
protocol for the formation of 5-arylidene barbiturates and
their 2-thio-analogues (III) by a condensation reaction
between aromatic aldehydes (I) and barbituric acids or
2-thiobarbituric acids (II) catalyzed efficiently by PVP-sta-
bilized Ni nanoparticles in ethylene glycol. Nickel nano-
particles were prepared by modified polyol process which is
a promising technique for synthesizing nickel nanoparticles
with excellent catalytic properties and high stability [39].
The Ni2? salt was reduced with sodium borohydride in
presence of PVP in ethylene glycol which led to the forma-
tion of highly monodispersed Ni nanoparticles as:
All the reactions were performed at 50 °C to slightly
increase the solubility of barbituric acids in ethylene gly-
col. Reaction time observed for all the transformations was
123

Nickel Nanoparticles Catalyzed Knoevenagel Condensation of Aromatic Aldehydes
107
Fig. 1 a TEM image of coated
Ni nanoparticles; b HRTEM
image of Ni nanoparticles. The
scale bar corresponds to 20 nm
in the images; and c QELS data
of Ni nanoparticles: Plot of
population distribution in
percentile versus size
distribution in nanometers (nm)
Fig. 2 a Comparative UV–Vis
spectra. Curve A: UV–Vis
spectra of PVP-Ni nanoparticles
in ethylene glycol, Curve B:
UV–Vis spectra of NiCl2ꢀ6H2O
in ethylene glycol and b
UV–Vis spectra of
PVP-stabilized Ni nanoparticles
in ethylene glycol
10–15 min. In order to confirm the effective involvement
of Ni nanoparticles during this transformation, a blank
reaction of Ia with barbituric acid (IIa) was performed in
ethylene glycol alone (containing traces of PVP) at 50 °C.
In the absence of Ni nanoparticles, the reaction was
incomplete even after 10 h of stirring though formation of
a small amount of IIIa (38%) was observed. Reactions
with varying amounts of PVP were attempted. It is clear
that PVP does not play a substantial role during the reac-
tion, as is evident from the reaction time and yield. How-
ever, its presence can not be ignored due to its superior
stabilizing properties. The reaction of Ia with barbituric
acid was also attempted with Ni powder (size \150
micron) in ethylene glycol at 50 °C. The reaction was only
123

108
J. M. Khurana, K. Vij
700
600
500
400
300
200
100
0
(111)
(200)
(222)
30
40
50
60
70
80
2 Theta (degrees)
Fig. 5 X-ray diffraction pattern of PVP-Ni nanoparticles
Fig. 3 Energy-dispersive spectrum of Ni nanoparticles with use of a
Cu grid
O
O
N
X
X
45% complete even after stirring for 24 h. All these results
have been summarized in the form of Table 1.
N
Ar
N
PVP-Ni nanoparticles
ArCHO
EG, 50°C
The condensation could be achieved not only with aryl
aldehydes but also with a,b-unsaturated aldehydes (Ih) as
well as with heteroaromatic aldehydes (Ii). One common
trait among all the synthesized arylidene barbiturates (III)
was their distinct strong color. The scope and generality of
this condensation has been illustrated by screening a vari-
ety of aromatic aldehydes (I) with diverse substitution
pattern and barbituric acids (II). All the results have been
summarized in Table 2.
(Ia-k)
O
N
Y
O
Y
X
(II)
X
(III)
a-j X=H, Y=O
k-o X=H, Y=S
a X=H, Y=O
b X=H, Y=S
c X=CH3, Y=O
d X=Ph, Y=S
p-u X=CH3, Y=O
v-x X=Ph, Y=S
Scheme 1 Ni nano particles catalyzed knoevenagel condensation of
aldehydes and barbituric acid
Fig. 4 Curve A: IR spectra of
PVP-Ni nanoparticles; Curve B:
IR spectra of PVP
123

Nickel Nanoparticles Catalyzed Knoevenagel Condensation of Aromatic Aldehydes
109
completely immiscible with ethylene glycol at room tem-
perature. Upon sonication, the ethylene glycol layer con-
taining the Ni nanoparticles could be reused for the next
cycle. The catalyst retained optimum activity till three
cycles after which drop in yield was observed. Also, QELS
analysis after second catalytic cycle showed no change in
size. However, an increase in size (around 30 nm) after
third cycle was observed along with presence of smaller
sized particles (Fig. 6a–c).
Table 1 Reaction condition optimization for the synthesis of 4-
chlorobenzylidene barbiturate (IIIa)
S.No. Catalytic system
Yield (IIIa, %) Time (min)
1
2
3
4
5
6
Ethylene glycol alone
Ni: PVP :: 1: 0
Ni: PVP :: 1: 1
Ni: PVP :: 1:5
38
82
87
93
86
[10 ha
15
10
10
Ni: PVP :: 1: 10
10
Ni powder (size \ 150 micron) 45
[10 h
Thus, we have developed a rapid, clean and highly
efficient methodology for the Knoevenagel condensation of
aromatic aldehydes with barbituric acids and 2-thiobarbi-
turic acids catalyzed by PVP stabilized Ni nanoparticles to
give 5-arylidene barbituric acids and their 2-thio analogues
as the desired products in short time span and in quanti-
tative yields by a simple and economical protocol. The
reaction time, yield and handling highlight the efficiency of
this novel protocol.
Reaction conditions: Ia (1 equiv.), IIa (1 equiv.), PVP-stabilized Ni
nanoparticles dispersion in ethylene glycol (2mL/0.1 g Ia), 50 °C
a
Solvent—2.0 mL ethylene glycol
Interestingly, the catalyst could be recycled easily sim-
ply by solvent extraction of the product from the reaction
mixture. For this, ethyl acetate was used as it was
Table 2 Nickel nanoparticles catalyzed synthesis of 5-arylidene barbiturates (IIIa-x) by Knoevenagel condensation of aromatic aldehydes with
barbituric acid
Entry
Ar
II
X
Product
Yield (%)
M.P. (°C)
Y
Obs.
Lit. [6–20]
1
4-ClC6H4 (Ia)
H
O
O
O
O
O
O
O
O
O
O
S
IIIa
IIIb
IIIc
IIId
IIIe
IIIf
IIIg
IIIh
IIIi
93
94
91
97
94.5
92.5
89
90
87
88
85
87
92
83
84
82
87
89
91
86
84
90
93
85
297
298.5 [11]
292–293 [11]
256 [14]
2
4-BrC6H4 (Ib)
H
289
3
C6H5 (Ic)
H
239
4
4-(CH3)2NC6H4 (Id)
4-HOC6H4 (Ie)
4-CH3C6H4 (If)
4-FC6H4 (Ig)
H
268–272
[300
272
274–276 [11]
[300 [18]
278 [11]
5
H
6
H
7
H
[300
268
309–310 [11]
270 [10]
8
C6H5CH=CH (Ih)
2-Furanyl (Ii)
H
9
H
275
257–259 [14]
268(d) [14]
264–266 [12]
290–291 [12]
254.4 [12]
305(d) [17]
[300 [15]
172 [14]
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
2-Naphthyl (Ij)
C6H5CH=CH (Ih)
4-ClC6H4 (Ia)
H
IIIj
266(d)
260
H
IIIk
IIIl
H
S
267
4-(CH3)2NC6H4 (Id)
4-CH3C6H4 (If)
4-HOC6H4 (Ie)
4-ClC6H4 (Ia)
H
S
IIIm
IIIn
IIIo
IIIp
IIIq
IIIr
IIIs
IIIt
255
H
S
[300
[300
169–171
297
H
S
CH3
CH3
CH3
CH3
CH3
CH3
Ph
Ph
Ph
O
O
O
O
O
O
S
4-HOC6H4 (Ie)
4-BrC6H4 (Ib)
297 [18]
184
–
4-(CH3)2NC6H4 (Id)
4-(HO)-3-(CH3O)C6H3 (Ik)
C6H5CH=CH (Ih)
4-HOC6H4 (Ie)
4-(CH3)2NC6H4 (Id)
4-ClC6H4 (Ia)
237
240–242 [19]
243 [19]
239–242
200
IIIu
IIIv
IIIw
IIIx
202–203 [19]
–
272
S
[300
256–258
[300 [20]
–
S
Aldehyde (1 equiv.) reacts with barbituric acid (1 equiv.) in presence of PVP-Ni nanoparticles in ethylene glycol (2 mL/0.1 g I) at 50 °C;
10–15 min reaction time
123

110
J. M. Khurana, K. Vij
Fig. 6 a Reusability of Ni
nanoparticles: Plot of Yield (%)
of IIIa versus Number of cycles;
b QELS data of Ni
nanoparticles after second
cycle, and c QELS data of Ni
nanoparticles after third cycle
(b)
(c)
(a)
Recyclability of Ni nanoparticles
100
80
60
40
20
0
1
2
3
4
5
No. of cycles
22. Alivisatos AP (1996) Science 271:933
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In conclusion, we have described a novel and highly effi-
cient protocol for the synthesis of structurally complex and
diverse 5-arylidene barbiturates and their corresponding
2-thio derivatives catalyzed effectively by PVP stabilized
nickel nanoparticles in ethylene glycol at 50 °C. Owing to
its several advantages over other methods, this method has
proved to be a highly promising one, enjoying high prac-
tical utility.
Acknowledgements KV is thankful to CSIR, New Delhi, India for
the award of Junior Research Fellowship.
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