2
42
B.P.S. Gautam et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 129 (2014) 241–254
and their nitrogen analogues have been reported for their antitu-
mor activities [6–8]. Quinones that act against animal tumours
are believed to function as bio-reductive alkylating agents [9–11]
and play important roles in biological functions including a role
in oxidative phosphorylation and electron transfer [12,13]. Besides,
they have found many technical applications and also represent
important synthetic building blocks. Quinones [14–17] and the
parent p-quinols [18] are found as structural units in a variety of
bioactive natural compounds showing antigerminative, antifungal,
antibacterial, antiviral and anticancer activities. Hence, they are
useful intermediates for organic synthesis with particular interest
into the production of biologically active compounds [19–22], e.g.
trimethyl-1,4-benzoquinone is a key reagent in the synthesis of
Synthesis of 2,5-dichloro-3,6-bis-methylamino-[1,4]benzoquinone
For the synthesis of 2,5-dichloro-3,6-bis-methylamino-
[1,4]benzoquinone (dmdb), sodium acetate (1.148 g, 14 mmol)
was added pinch by pinch in 20 min to a reaction mixture of
chloranil (0.492 g, 2 mmol) added dropwise methylamine hydro-
chloride (0.270 g, 4 mmol) in 15 ml absolute ethanol with constant
stirring for 15 min at room temperature. The color of the reaction
mixture slowly changed from yellow to dark brown. The stirring
was continued for 1 h and refluxed for 4 h. The resulting dark
brown precipitated product was filtered off, washed three times
with water, ethanol–water mixture followed by ethanol and dried
under vacuo over calcium chloride.
vitamin E and 2-methyl-1,4-naphthoquinone (vitamin K
important additive in animal feed, is also utilized for construction
of vitamin K [23]. From the point of view of electron transfer reac-
tions, benzoquinones play an important role in biological systems
12,24–26]. 1,4-Benzoquinone is one of the most important and
3
), an
Computational details
1
To optimize the structure of 2,5-dichloro-3,6-bis-methylamino-
[
1
,4-benzoquinone, the following procedure was adopted, initially
fundamental p-electron systems because of its high electron affin-
ity and photoreactivity [27,28]. Molecules with the quinoid struc-
ture form one of the most attractive classes of compounds in
organic chemistry. The chemistry of quinones is largely dependent
on the substituents being either on the quinonic or on adjacent
rings. This is reflected in their chemical reactivity, especially in het-
erocyclic quinones [29]. DFT method is shown to predict accurately
molecular properties such as optimized structure, vibrational fre-
quencies, energy, charge distributions on atoms of a molecule and
point group symmetry.
the structure of p-diamino benzene was optimized. In the opti-
mized structure of p-diamino benzene one methyl group is at-
tached at N11 position. Out of the two possible conformers one
with the lower energy was taken and another methyl group at
the position N13 was added. Now by taking different orientations
of the ACH
energy conformer one oxygen atom is attached at the position C
and another at the position C and the structure was again opti-
mized. To this minimum energy conformer one–one chlorine atom
is attached at the position C and C and the structure was further
optimized at the B3LYP/6-311++G level. The geometry was fully
optimized without any constraint. Since there is C axis therefore
= 16 conformers are possible. But because of symmetry regions
3
group geometries were optimized. With the minimum
5
2
1
4
ꢂꢂ
In the present paper we have synthesized and characterized
2
,5-dichloro-3,6-bis-methylamino-[1,4]benzoquinone
(abbrevi-
4
ated as dmdb) which consists of
p-conjugated double bonds and
4
2
is capable of forming molecular complexes like chloranilic acid.
Further, since dmdb molecule possesses alternate double bonds,
it is expected to be a good candidate for molecular conductors. In
this paper we present the results of the IR and Raman spectral
study and vibrational analysis in light of the DFT calculations at
the total numbers of conformers are 10 only and the lowest energy
conformer is considered.
The optimized molecular geometries, APT charges, natural
charges and fundamental vibrational wave numbers along with
their corresponding intensities in IR spectrum, Raman activities
and depolarization ratios of the Raman bands for the lowest energy
conformer of dmdb molecule were computed by using Gaussian 09
package program [30] with molecular visualization program [31]
on the personal computer at B3LYP method at 6-311++G(d,p)
ꢂꢂ
the B3LYP/6-311++G level using the Gaussian 09 package for sup-
porting experimental results and vibrational assignments. In addi-
tion molecular structure, charges on atomic sites, Natural Bond
Orbital (NBO) analysis, HOMO–LUMO analysis, molecular electro-
static potential (MESP) and thermodynamic functions have been
computed and analyzed for dmdb (lowest energy conformer C-I).
[
32–36] calculation level. In order to obtain the reasonable fre-
quency matching, scale factors proposed by Rauhut and Pulay
37] were employed. The assignments of all the normal modes of
[
vibration have been made on the basis of the calculated potential
energy distributions (PEDs). For the calculation of the PEDs the
vibrational problem was set up in terms of internal coordinates
using gar2ped software. The observed IR and Raman frequencies
corresponding to the fundamental modes have been correlated to
the calculated fundamental frequencies in light of the PEDs. The
molecular electrostatic potential (MESP) surface, which is used
for predicting sites and relative reactivity towards electrophilic at-
tack and in studies of biological recognition and hydrogen bonding
interactions, has been plotted and the NBO analysis has been car-
ried out in order to investigate intra-molecular charge-transfer
Experimental
Material and instrumentation
All the chemicals used under the present investigation were of
analytical reagent (A R) grade. 2,3,5,6-Tetrachloro-1,4-benzoqui-
none was purchased from the Merck and used as such without
further purification. The elemental analysis was done on a Per-
kin–Elmer CHN analyzer model-240C. H NMR spectra were
recorded on a JEOL AL 300 MHz multinuclear NMR spectrometer
1
6
using d -DMSO as solvent and TMS as an internal reference. The
(
CT) interactions, rehybridisation and delocalization of electron
IR spectrum of the solid sample was recorded on a JASCO FTIR
spectrometer model-5300 in KBr pellet in the spectral range
density (ED) within the molecule.
ꢁ
1
4
000–400 cm . Following experimental parameters were used
ꢁ1
for recording the IR spectra: resolution – 4 cm ; gain-20; scan-
00. The Raman spectrum of the molecule has been recorded in
Results and discussions
1
ꢁ1
the region 4000–400 cm , on a Jobin Yvon HORIBA HR 800 Raman
spectrometer using 488 nm line of an Ar+ laser for excitation with
Characterization of the 2,5-dichloro-3,6-bis-methylamino-
[1,4]benzoquinone
ꢁ1
the following parameters: laser spot size: 1 cm , resolu-
ꢁ
1
tion ꢃ1 cm , power at the sample <10 mW, integration time:
Physical methods
ꢁ1
1
–
0 s, one window covers ꢃ800 cm , accuracy of measurements
The compound is insoluble in common organic solvents like
ethanol, chloroform and dichloromethane, etc. but on heating it
ꢁ1
2 cm , slit-width fixed at the entrance of laser 200 lm.