Structural conformations and density functional study on the intramolecular charge transfer based on vibrational spectra of 2,4-dihydroxy-N′-(4- methoxybenzylidene)benzohydrazide

Article abstract of DOI:10.1016/j.saa.2013.01.067

The NIR-FT Raman and FT-IR spectra of 2,4-dihydroxy-N′- (methoxybenzylidene) benzohydrazide (DMBBH) molecule have been recorded and analyzed. Density functional theory (DFT) calculations at the B3LYP/6-31G(d) level has been used to compute energies of different conformers of DMBBH to find out their stability, the optimized geometry of the most stable conformer and its vibrational spectrum. Information about the size, shape, charge density distribution and site of chemical reactivity of the molecules has been obtained by mapping electron density isosurface with electrostatic potential surfaces (ESP). A complete vibrational analysis has been attempted on the basis of experimental infrared and Raman spectra and calculated vibrational modes and potential energy distribution over the internal coordinates. The vibrational analysis confirm the charge transfer interaction between the phenyl rings through CNN skeleton. The broadening of the band at 1631 cm^-1 and the appearance of the band at 1556 cm^-1 strongly suggests the existence of proton equilibrium.

Full text of DOI:10.1016/j.saa.2013.01.067

Accepted Manuscript
Structural conformations and density functional study on the intramolecular
charge transfer based on vibrational spectra of 2,4-dihydroxy-N’ -(4-methoxy‐
benzylidene)benzohydrazide
D.M. Suresh, D. Sajan, Yun-Peng Diao, Ivan Němec, I. Hubert Joe, V. Bena
Jothy
PII:
S1386-1425(13)00105-4
http://dx.doi.org/10.1016/j.saa.2013.01.067
SAA 10217
DOI:
Reference:
To appear in:
Spectrochimica Acta Part A: Molecular and Biomo‐
lecular Spectroscopy
Received Date:
Revised Date:
Accepted Date:
5 October 2012
2 January 2013
18 January 2013
Please cite this article as: D.M. Suresh, D. Sajan, Y-P. Diao, I. Němec, I. Hubert Joe, V. Bena Jothy, Structural
conformations and density functional study on the intramolecular charge transfer based on vibrational spectra of
,4-dihydroxy-N’ -(4-methoxybenzylidene)benzohydrazide, Spectrochimica Acta Part A: Molecular and
2
Biomolecular Spectroscopy (2013), doi: http://dx.doi.org/10.1016/j.saa.2013.01.067
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Structural conformations and density functional study on the intramolecular
charge transfer based on vibrational spectra of
2,4-dihydroxy-N'-(4-methoxybenzylidene)benzohydrazide
a,f
b,e*,
c#
d
e
D.M .Suresh , D. Sajan
and
Yun-Peng Diao ,Ivan Němec , I. Hubert Joe
f £
V. Bena Jothy
a
Government Arts College, Stone Hill House, Udhagamandalam , The Nilgiris -
43 002, Tamilnadu, India\
6
b
Department of Physics, Bishop Moore College, Mavelikara, Alappuzha-690 110,
Kerala, India.
c
School of Pharmacy, Dalian Medical University, Dalian 116044, People’s
Republic of China
d
Department of Inorganic Chemistry, Faculty of Science, Charles University in
Prague, Albertov 2030, 128 40, Prague 2, Czech Republic
Centre for Molecular and Biophysics Research, Department of Physics, Mar
e
Ivanios College, Thiruvananthapuram-695 015, Kerala, India
Department of Physics, Women's Christian College, Nagercoil, Tamil Nadu -
f
6
29001
*Corresponding author: Tel.:+91-9447000266; Fax: +91-479-2303230
*
#
E-mail address: dsajand@gmail.com, diaoyiwen@126.com,
£
benaezhil@yahoo.com
ABSTRACT
The NIR-FT Raman and FT-IR spectra of 2,4-dihydroxy-N'-
methoxybenzylidene) benzohydrazide (DMBBH) molecule have been recorded
and analysed. Density functional theory (DFT) calculations at the B3LYP/6-
1G(d) level has been used to compute energies of different conformers of
(
3
DMBBH to find out their stability, the optimized geometry of the most stable
conformer and its vibrational spectrum. Information about the size, shape, charge
density distribution and site of chemical reactivity of the molecules has been
obtained by mapping electron density isosurface with electrostatic potential
1

surfaces (ESP). A complete vibrational analysis has been attempted on the basis
of experimental infrared and Raman spectra and calculated vibrational modes and
potential energy distribution over the internal coordinates. The vibrational
analysis confirm the charge transfer interaction between the phenyl rings through
-1
›
C=N-N‹ skeleton. The broadening of the band at 1631cm and the appearance of
-1
the band at 1556 cm strongly suggests the existence of proton equilibrium.
Keywords:, FT-IR ,FT-Raman spectra, DFT, NBO, Electrostatic potential
1. Introduction
Schiff base compounds, in addition to their role as versatile ligands in the
field of coordination chemistry, can also act as important intermediates in many
enzymatic reactions [1,2]. Due to the presence of an imine (-N=CH-) group these
compounds are susceptible to hydrolysis and transamination by nucleophiles [3].
The synthesis and physicochemical characterization of substituted aromatic
Schiff base have received considerable attention since many of these compounds
exhibit thermochromic and photochromic properties in the solid state by
intramolecular proton transfer associated with a change in the π-electron
configuration [4,5] .Some Schiff base metal complexes have been found to have
pharmacological and antitumor properties [6,7], and can be used as model for
biological systems [8]. The resistance reversal properties of some Schiff base
compounds and metal complexes, their in vitro toxicity against cancer cells and
2

efficiency in slowing leukemia cell growth have also been reported in the
literature [9]. Though the crystal structure of the title compound, 2,4-dihydroxy-
N'-(4-methoxybenzylidene) benzohydrazide (DMBBH) has been reported [10],
the vibrational spectral features of DMBBH crystal have not been subject of
detailed analysis so far. NIR-FT Raman spectra combined with quantum
chemical computations have recently been effectively applied in the vibrational
analysis of drug molecules [11,12]
,
biological compounds [13], natural products
[14] and NLO active compounds [15-17], since fluorescence free Raman spectra
and computed results help with unambiguous identification of vibrational modes
and provide deeper insight into the bonding and structural features of complex
organic molecular systems. The present work reports the detailed vibrational
spectral analysis of 2,4-dihydroxy-N'-(4-methoxybenzylidene) benzohydrazide
molecule (Figure 1) to elucidate the correlation between the molecular structure
and prediction of normal modes, to understand the structural and bonding
features, photon transfer equilibrium ,the intramolecular interactions and factors
influencing the vibrational spectrum of the molecule supported using the scaled
quantum mechanical (SQM) force field technique based on density functional
theory.
2. Experimental
2.1 Synthesis of the compound
3

4-Methoxybenzaldehyde and 2,4-dihydroxybenzoic acid hydrazide were
dissolved in ethanol and the mixture was stirred at room temperature to give a
clear colourless solution. Crystals of 2,4-dihydroxy-N'-(4-methoxybenzylidene)
benzohydrazide were formed by slow evaporation of the solvent over a period of
three days at room temperature.
2.2. IR and Raman measurements
FT-IR
spectra of the title compound were recorded on Thermo
Nicolet Magna 760 FTIR spectrometer by DRIFTS (Diffuse Reflectance Infrared
Fourier Transform Spectroscopy) technique. The samples were mixed with KBr
-1
and scanned in 400-4000 cm wavenumber range (Happ-Genzel apodization, 2
-
1
cm resolution) using Pike Technology EasiDiff accessory. The Raman spectra
of polycrystalline samples and ethanol solution were collected on the same
spectrometer equipped with Thermo Nicolet Nexus FT-Raman module. The
-1
measurements were carried out in the range of 100-3700 cm (Happ-Genzel
-1
apodization, 2 cm resolution, 1064 nm Nd:YVO laser excitation, 450 mW
4
power at the sample). Both IR and Raman spectra were processed using the
OMNIC software [18].
3. Computational details
All ab initio molecular orbital calculations were carried out using the
Gaussian’03 program package [19]. Initially, the structure of DMBBH molecule
4

were optimized, and then the vibrational wavenumbers were calculated with the
B3LYP method using the 6-31G(d) basis set. At the optimized structure of the
DMBBH, no imaginary frequency modes were obtained, proving that a true
minimum on the potential energy surface was found. The optimum geometry was
determined by minimizing the energy with respect to all geometrical parameters
without imposing molecular symmetry constraints. The inclusion of‘d’
polarization and double-zeta function in the split valance basis set is expected to
produce a marked improvement in the calculated geometry [20]. The DFT hybrid
B3LYP functional tends to overestimate the fundamental modes in comparison to
the other DFT method, therefore scaling factor s have to be used for obtaining a
considerably better agreement with experimental data. Isoelectronic molecular
electrostatic potential surfaces (MEPSs) and electron density surfaces were
calculated using 6-311G(d,p) basis set with (83,83,52) grid points with an
average resolution of 0.333 Bohr. Structures resulting from the plot of electron
density surface mapped with electrostatic potential surface depict the shape, size,
charge density and the site of chemical reactivity of a molecule. These surfaces
were plotted by the computer software Gauss View 5.0. The vibrational modes
were assigned on the basis of PED analysis using VEDA 4 program [21] and its
visualization interface. Normal coordinate analysis of the title molecule has been
carried out to obtain a more complete description of the molecular motions
involved in the fundamentals. The calculated vibrational wavenumbers were
5

scaled [22] with the scale factors, which accounts for systematic errors caused by
the basis set incompleteness, neglect of electron correlation and vibrational
anharmonicity. The Raman activities (S ) calculated by the Gaussian-03 program
i
have been converted to relative Raman intensities (I ) using the following
i
relationship derived from the basic theory of Raman scattering [23].
4
f (ν −ν ) S
o
i
i
Ii =
(1)
⎡
⎛ − hcν ⎞⎤
⎟
⎥
i
ν 1− exp⎜
i
⎢
⎣
kT
⎝
⎠⎦
-
1
where ν is the exciting frequency (in cm units), ν is the vibrational
o
i
th
wavenumber of the i normal mode, h c and k are universal constants, and ƒ is
the suitably chosen common scaling factor for all the peak intensities.
4. Results and discussion
4.1 Crystal structure
As it was previously reported [10], DMBBH crystallizes in orthorhombic
space group, Pna2 . The cell dimensions are a=12.494(3) Å, b=5.196(1) Å,
l
3
c=20.825(4) Å and V=1351.9(5) Å . The title molecule displays a trans
configuration with respect to the C=N bond. The dihedral angle between the two
benzene rings is 15.0(2)˚. The molecular conformation is stabilized by the
intramolecular N-H···O interaction. In the crystal, molecules are linked through
6

intermolecular O-H···N and O-H···O hydrogen bonds, forming layers parallel to
the ab plane.
4.2. Conformational analysis
With B3LYP/6-31G(d) level the conformational analysis was done by
changing the torsional angle θ by 10º steps between θ = 0º (syn form) and θ =
80º (anti form) to analyze the stability of the molecule. For each conformer, the
dihedral angle was held fixed while the remainder of the molecule was optimized
the rigid rotor approximation was not applied). The two conformers of DMBBH
1
(
molecule, which have been optimized at B3LYP/6-31G(d) level, are shown in
Fig.1(a) & (b). The optimized energy of conformers are predicted at -2.60025 x
6
6
1
0 KJ/mol (conformer 1) and -2.60024 x 10 KJ/mol
(conformer 2). The
relative energy is determined at 0.00001 KJ/mol for conformer 2 (Fig.1a &1b).
The results show that conformer2 is more stable than the conformer1.
Figure-1
Potential energy surface scan studies have been carried out in order to
ascertain its trans Fig.1b) form due to paucity in reliable experimental structural
data and for the popular notion that the trans form is more stable [10]. The results
obtained in PES scan studies by varying the torsional perturbation of τ C –C –
1
14
N –N with step angle of 10° are plotted in figure -2. The calculated molecular
16
18
6
energy at 180.0° torsion angle, (τC –C –N –N ) is about -2.60024 x 10
1
14
16
18
KJ/mol which is larger than that of the cis conformation.
7

•
Figure-2
The energy barrier of rotation around the O -C bond was calculated for
31
24
the B3LYP with 6-31G(d) basis sets. The dihedral angles of C -C -O -C
25
24
31
32
(
around O -C bond) were varied from 0° to 180° by steps of 10°. The energy
31
24
profile obtained from DFT method (Fig-3) as a function of φ has shown the
minimum at 0° and the maximum at 180°. The minimum positions were
corresponding to the trans conformer and the maximum positions were
corresponding to the cis conformer.
•
Figure-3
The O -H group favour planar conformation (figure -4a) due to the
7
13
energy minimum at 0/360˚ predicted by energy profile. This is because of the
Van der Waals’ repulsion between H and H . The PES scan results reveal that
17
13
the molecule exists in trans-configuration, which is important for its bioactivity.
From the graph (figure-4b) the energy minimum for O -H P –orbital oxygen
9
10
z
lone pair conjugation of O and π -system of resorcinol group is maximum
9
leading to the conclusion that possibility of perpendicular conformation of O -H
9
10
with respect to the resorcinol group is rejected. The local minimum at 180˚ is less
favored compared to planar conformation which is due to the possible steric
effect arising from the Van der Waals’ repulsion between H and H
8
10
…
..
…..
10
(
H8 H =2.28 Å) and H and H (H
H =2.29Å).
11
10
11
10
4.3 Natural bond orbital (NBO) analysis
8

The NBO analysis was performed with the NBO 5.0 program [24]. A short
outline of the NBO segments used and their structural meaning is presented
below. NBO theory allows the assignment of the hybridization of atomic lone
pairs and of the atoms involved in bond orbitals. These are the important data in
spectral interpretation since the frequency ordering is related to the bond hybrid
composition. The NBO analysis allows us to estimate the energy of the molecule
with the same geometry but in the absence of electronic delocalization.
Moreover, only the steric and electrostatic interactions through the ELewis are
taken into account. The most important interactions between filled (donor)
Lewis-type NBOs and empty (acceptor) non-Lewis NBOs are reported in Table
1
. The stabilization energy E associated with delocalization is estimated using
ij
the second-order perturbation theory as:
2
2
σ F σ
Fij
ΔE
E
(
2
)
= −n
= −n
σ
(2)
σ
σ
*
−ε
ε
σ
2
2
where σ F σ or F is the Fock matrix ( Kohn–Sham matrix) element between
ij
the i and j NBO orbitals,
and
*
are the energies of σ and σ* NBO's, and n
σ
ε
σ
ε
σ
is the population of the donor σ orbital.
The second-order perturbation theory analysis of Fock matrix in NBO
shows strong intramolecular hyperconjugative interactions, which are presented
in Table 1. The intramolecular hyperconjugative interactions are formed by the
9

*
orbital overlap between
π (C–C) and π (C–C) bond orbitals which
results intramolecular charge transfer (ICT) causing stabilization of the system
*
[
25,26]. The hyperconjugative interaction between LP N
π (C -O )
[
1
16
14
15
-1
*
-1
2
43.1 KJmol ], LP N
π (N -C )[ 121.1 KJmol ], LP O → π *( C -C ) [
1
16
18
19
2
9
4
5
-1
-1
1
32.6 KJmol ], LP O → π * ( C -C ) [ 129.3 KJmol ] and LP O → σ *(
2
31
23
25
2
15
-1
C -N ) [ 116.6 KJmol ] have higher values than the other delocalizations.
14
16
•
Table-1
4.4 Geometry and Electronic Structure
The optimized structural parameters of DMBBH are listed in Table 2. The
corresponding values obtained from X-ray diffraction pattern are also given in
the table for comparison. From the Table 2 it can be seen that there are small
deviations in the computed geometric parameters from those obtained from the
XRD data [10]. These deviations can be attributed to the fact that the theoretical
calculations have been carried out with isolated molecules in the gaseous phase
and the experimental values correspond to molecules in the crystalline state. As
evident from the observed N -C bond distance of 1.270 Å, this is consistent
18
19
with an N=C double bond. The C -C -N -N torsion angle [178.5°] shows a
21
19
18
16
trans configuration about the C -N bond. The C -O and C -O bond lengths of
19
18
4
9
6
7
1.366 Å are consistent with the C-O single bond distances reported in analogous
structures [7,27]. The molecular conformation is stabilized by an intramolecular
1
0

N-H···O hydrogen bond interaction [2.7191 Å]. A short intramolecular hydrogen
bond associated with the charge flow through the system of conjugated double
bond is denoted as resonance–assisted hydrogen bonding [28] and a
delocalization parameter, Q, defined as Q= (d - d ) + (d - d ), where d d d and
1
4
3
2
1, 2,
3
d are C -O [1.366 Å], C -C [1.405 Å] , C -C [1.483 Å] and C -N [1.340 Å]
4
6
7
6
1
1
14
14
16
bond lengths respectively. The positive sign of Q value [0.104 Å] in the title
compound is a consequence of the electron withdrawing of resorcinol substituent
group and is in agreement with other crystallographic structures [29]. In the
crystal, molecules are linked through intermolecular O-H···N and O-H···O
hydrogen bonds, forming layers parallel to the ab plane.
•
Table-2
The net charges of the atoms in the title compound calculated at the
B3LYP/6-31G (d) level are shown in the figure-5. The oxygen and nitrogen
atoms in the molecule bear negative charges; the phenyl ring carbon atoms with
substituent (i.e. C , C , C , C and C ), the carbonyl carbon C and bridging C
4
6
1
21
24
14
19
atom bear positively charges while the remaining carbon atoms of the phenyl
rings bear negative charges. The large electron density at the phenolic O atoms
(
O and O ) suggests possible protonation at O and O . Due to this charge
7
9
7
9
distribution, the dipole of the molecule becomes 5.663 Debye.
Figure-5
•
1
1

The orbital energy level analysis for DMBBH at the B3LYP/6-31G(d)
level shows EHOMO (highest occupied molecular orbital) and ELUMO (lowest
unoccupied molecular orbital) values of -0.31156 eV and -0.20753 eV,
respectively. The magnitude of the HOMO-LUMO energy separation could
indicate the reactivity pattern of the molecule. The charge densities for the
HOMO and LUMO are shown in Figure-6. For the HOMO and LUMO in
DMBBH there is very little charge density on the hydrogen atoms. The HOMO is
located on the C -C , C -C ,C -C and C -C bonds of the phenyl ring (p-
21
22
21
26 24
25
24
23
methoxybenzylidene) as well as on the oxygen atoms of hydroxyl (O and O ),
6
9
carbonyl oxygen (O ), methoxy group (O ) and the nitrogen(N and N ) atoms,
15
31
16
18
with only minor population on the other phenyl ring (resorcinol). The LUMO in
DMBBH, however, populates on the carbon atoms (C ,C and C ) of the
2
2
24
26
phenyl ring(p-methoxybenzylidene), C ,C ,C and C atoms of the phenyl ring
2
4
5
6
(
(
(
resorcinol), carbonyl group (C =O ), methoxy group (O ) and the nitrogen
14
15
31
N ) atoms, but with only minor population on the oxygen atoms of hydroxyl
18
O and O ). The population of LUMO on the bonding between C -C , C -C ,
6
9
22
23
19
21
C -C and C -C forms antibonding orbitals. Minor population is located on the
1
14
4
3
oxygen atoms on the methoxy groups. The difference between the orbital
energies corresponding to HOMO-1 (-0.34296 eV) and HOMO-2 (-0.35718 eV)
is much higher than 0.05 eV indicating that the HOMO-1 and HOMO-2 are
1
2

nondegenerate in DMBBH. Similar conclusion can be drawn from the LUMO+1
and LUMO+2 orbital energy calculations.
•
Figure-6
According to molecular orbital energy, HOMO and LUMO are the two
important factors influencing the bioactivity. The interaction between the title
compound and the receptor can be dominated by π-π or hydrophobic interaction
among these frontier molecular orbitals. The negative charges mainly located on
O , O , O N16, N and O are likely to interact with positive part of the
7
9
15,
18
31
receptor. On the contrary, the most positively charged part will interact quite
easily with negatively charged part of the receptor. These interactions can play
crucial role in bioactivity.
4.5 Molecular electrostatic potential
The ESP, V(r), at a point r due to a molecular system with nuclear charges
Z } located at {R } and electron density ρ(r) is given by
{
A
A
N
3
ZA
ρ(r′)d r′
V (r) =
−
(3)
∑
A−1
∫
r − RA
r − r′
The graphical representation of the molecular electrostatic potential surface
MEP or ESP), as described by Kollman and Singh[30] is a series of values
representing the evaluation of the interaction energy between a positively charged
proton) probe and points on a solvent accessible surface as defined by
(
(
1
3

Connolly[31]. As implemented within the Gauss View ver.4.1 program, areas of
high electron density, representing a strong attraction between the proton and the
points on the molecular surface, have the brightest red color and areas of lowest
electron density; have deep blue to indigo color, indicating the regions of
maximum repulsion.
Molecular electrostatic potential (ESP) at a point in the space around a
molecule gives an indication of the net electrostatic effect produced at that point
by the total charge distribution (electron + nuclei) of the molecule and correlates
with dipole moments, electro negativity, partial charges and chemical reactivity
of the molecules. It provides a visual method to understand the relative polarity
of the molecule. An electron density isosurface mapped with electrostatic
potential surface depicts the size, shape, charge density and site of chemical
reactivity of the molecules. The different values of the electrostatic potential at
the surface are represented by different colours; red represents regions of most
negative electrostatic potential, blue represents regions of most positive
electrostatic potential and green represents regions of zero potential. The
potential increases in the order red < orange < yellow < green < blue. Projections
of these surfaces along the molecular plane are given in Figure- 7. This figure
provides a visual representation of the chemically active sites and comparative
reactivity of atoms.
•
Figure-7
1
4

It may be seen that, in both the molecules, a region of zero potential
envelopes the π-system of the benzene rings, leaving a more electrophilic region
in the plane of the hydrogen atoms. The shape of the electrostatic potential
surfaces at sites close to the polar carbonyl group in the molecule and the
hydrazide group is influenced by the stereo structure and the charge density
distribution. These sites show regions of most negative electrostatic potential and
high activity of the carbonyl and hydrazide group. In contrast, regions close to
the other two polar atoms – oxygen of the phenyl ring and oxygen of the methoxy
group – show regions of mildly negative and zero potential, respectively.
Another quantity of utility in the analysis of intrinsic reactivity is the local
ionization potential (IP), I(r). This is defined as the sum over orbital electron
densities, ρ (r) times the absolute orbital energies, ε and divided by the total
i
i
electron density ρ(r) ,
occupied molecular orbitals
ρ (r)εi
ρ(r)
i
I(r) =
∑
i
The local IP is intended to reflect the relative ease of electron removal at any
location around a molecule [32-34]. Thus, as we use it here, it is a measure of the
susceptibility of a molecular region to electrophilic attack (reactivity). The
surface mapped values of the IP, as derived from Gauss View ver.4.1 program
are shown in Fig.7. The graphical convention used (Fig.7,) indicates a range of
1
5

relative values, with red representing regions of highest likelihood of
electrophilic attack, and blue, at the other extreme, areas of lowest likelihood
(most nucleophilic regions).
4.6 Vibrational spectral analysis
The vibrational analysis of DMBBH is performed on the basis of the
characteristic vibrations of resorcinol, hydrazide and methoxybenzilidine groups.
Theoretical calculations were performed using density functional theory with
B3LYP/6-31G(d) basis set. The computed vibrational wavenumbers and the
atomic displacements corresponding to different normal modes are used for
identifying the vibrational modes unambiguously. The detailed vibrational
assignments of fundamental modes along with the calculated infrared and Raman
intensities and normal modes description (characterized by PED) are reported in
Table 3. For visual comparison, the observed and simulated FT-IR and FT-
Raman spectra are presented in figures-8 & 9. The calculated spectra are found to
be close to the experimental values with reasonable accuracy. H-atoms involved
in intermolecular H-bonds shows deviation in the simulated spectra, which is
attributed to the fact that the theoretical spectrum was obtained in gas phase
without considering the inter-molecular H-bonding effects. Vibrational spectral
analysis of the phenyl rings are different as expected due to their substitution and
they are comprehensively studied according to Wilson’s numbering convention
[35].
1
6

•
•
Table-3
Figure-8&9
4.6.1 Resorcinol group vibrations
The major vibrational modes of resorcinol are derived from phenyl ring
and hydroxyl groups .The hydroxyl stretching and bending bands can be
identified by their broadness and strength of the band which is dependent on the
extend of hydrogen bonding. Vibrational absorption spectra provide information
about the local properties of hydrogen-bonded groups. The line shapes of the
high-wavenumber O–H stretching mode involved in the hydrogen bonds are
often very complex and highly congested, reflecting the complicated underlying
microscopic interactions, and consequently, the dynamics and line broadening
…
mechanisms of hydrogen bonds. Formation of a hydrogen bond X-H Y between
a hydrogen donor X and hydrogen acceptor Y leads to pronounced changes in the
high-wavenumber X-H stretching modes [36-38]. The non-hydrogen bonded or
-
1
free hydroxyl group absorbs strongly in the 3700-3600 cm region while the
existence of intermolecular hydrogen bond can lower the O-H stretching
-1
wavenumber in the 3550-3200 cm region with the increase in IR intensity and
broadness. In IR spectrum the hydroxyl stretching bands splits into two bands at
−1
−1
3
200 cm and 3250 cm corresponding to O -H and O -H stretching
7
13
9
10
vibrations. The DFT computations give the wavenumber of these bands at 3581
−1
−1
cm (100 % ) and 3587 cm (100 % ) for O -H and O -H stretching
7
13
9
10
1
7

vibrations .The appearance of the red shift of the O -H and O -H stretching
9
10
7
13
wavenumber is clearly due to the formation of a intermolecular and H ...O -H
17
7
13
intramolecular hydrogen bond respectively. Interaction of lone pairs of oxygen
electron donor) with the O-H antibonding σ* orbital leads to an increase of
(
electron populations in this orbital, followed by a weakening of the O-H bond
which results in the lowering of O-H stretching wavenumber. The computed
results suggest the possibility of intra molecular effects. The H-O---H distances
…
being: H13 H =2.785 Å. The intramolecular interactions are obviously weaker
17
and the above interactions throw off the expected values of band positions,
computed using DFT. The above mentioned interactions are expected to weaken
the bonds, thereby reducing O -H stretching force constant which is manifested
7
13
as the lowering of the experimental stretching wavenumber from the computed
values. Also the observed O -H10 and O -H stretching wavenumbers are
9
7
13
…
..
lowered from the computed wavenumbers due to the intermolecular O–H O
hydrogen bond interaction, justified by XRD data. The in plane bending O-H
deformation vibration usually appears as a strong band in the region 1440-1260
-
1
cm in the IR spectrum, which gets shifted to higher wavenumbers due to the
presence of hydrogen bonding. The O-H in-plane bending vibration is observed
-1
at 1463 cm as medium IR band together with CH bending modes. The O-H in
3
−1
plane bending mode appears in our calculations at 1473 cm with a composition
of 60 % δHOC and δCH3 asy(22 %). The O-H out of plane bending vibration
1
8

-1
-1
gives rise to a broad band in the region 800-600 cm .The weak band at 790 cm
in the infrared spectrum is attributed to the O-H out of plane bending mode. The
-1
band correlated with O-H torsional mode was identified at 463 cm in infrared
spectra.
In resorcinol group, the phenyl ring is asymmetrically tri substituted. The
C-H stretching wavenumber of tri substituted benzene is expected in the region
-1
-1
3
000 - 3100 cm [36-38]. These are 3091, 3078 and 3008 cm vibration, in
which only the C-H bonds of the Ph ring participate. The selection rule allows all
2
the three bands 2, 20a and 20b modes of C-H stretching [35]. But 2 and 20b is
absent or mixed with very broad band of hydroxyl stretch in that region which
cannot be distinctly observed .The ring mode 20a can be observed in IR at 3074
-
1
-1
cm and in Raman at 3076 cm .
For asymmetrically tri substituted benzene and its derivatives, the modes 1,
8a, 8b, 19a, 19b and 14, are allowed for tangential C-C stretch [35]. The mode 8b
-
1
appears in calculation at 1609 cm with a composition of 51% 8b. The
-1
experimental value of this wavenumber is 1609 cm in Raman spectra, which
indicates an excellent agreement with the calculations. The 8a modes appear at
-1
lower wavenumber of 1582 cm with a composition of 50% 8b. The ring mode
−1
-1
8
a can be observed as very strong bands at 1576 cm (IR) and 1574 cm (Raman
and solution Raman). The 19b mode is found having considerable contribution to
-1
this mode, appearing at 1500 cm , which is mixed with 11% ν and 33 % δCCH,
CC
1
9

and cannot be distinctly observed experimentally. The 19a mode is attributed to
-1
1
433 cm , with significant contribution of 41% ν .The mode 19a is found as
CC
-1
medium bands at 1424 cm in IR , Raman and solution Raman spectrum. Mode
-1
1
4 was assigned to the wavenumber of 1307 cm , since it has a composition of
-1
5
7 % ν and 26 % δ . The weak band observed at 1335 cm (IR) corresponds
CC
HOC
to the 14 mode.
The normal vibrations 3, 15 and 18b are classified as C-H in-plane bending
−1
-1
vibrations [35]. The strong bands observed at 1258 cm (IR), 1257 cm (solution
−1
Raman) and 1259 cm (Raman) are assigned to mode 3. The medium IR band
−1
appearing at 1193 cm is attributed to vibrational mode 15. The mode 18b is
coupled with the CCC bending mode and is found active in the Raman spectrum
−1
−1
at 1073 cm as a medium band and in the IR spectrum at 1074 cm . The normal
modes 10a, 10b and 11 which allow C-H out-of-plane bending vibrations, are
−1
expected in the region 960-780 cm [37-39] .The mode 10a is observed in the IR
−1
−1
at 950 cm and the calculated wavenumber is 958 cm (32% PED). The weak
−1
−1
bands observed in the IR at 807 cm and in the Raman spectrum at 808 cm are
assigned to 10b mode. The radial skeletal (1, 6a and 6b) and the out-of-plane
skeletal (4,16a and 16b) vibrations are listed in Table 3.
4.6.2 Hydrazide group vibrations
The vibrational analysis of hydrazide is performed based on the
characteristic vibrations of amide group, C-N and N-N groups. The secondary
2
0

amides exhibit bands due to the N-H stretching and deformation vibrations. The
positions of the NH stretching and deformation bands are dependent on the
strength of hydrogen bond formed. The free NH stretching modes of secondary
-1
amides are generally observed in the region 3550-3250 cm for the N-H
-1
stretching mode and a weak band at 3100-3070 cm for the overtone of the N-H
bending [36-40]. The N-H stretching modes are observed as a medium broad
-1
band in the IR spectrum at 3160 cm . The DFT computations give the
-1
wavenumber of this band at 3352 cm (100% N–H) for the N-H stretch. The red-
shifting of the N-H stretching wavenumbers are due to the formation of inter- and
intramolecular N–H….O hydrogen bonds. The calculated geometry H ….O
17
7
(
1.922 Å),N ….O (2.719 Å) and N –H ….O (132.82˚) supports the
11
19
16
17
7
intramolecular N–H….O hydrogen bonding. From Table 1 it is evident that the
ED in N -H antibonding σ* orbital was significantly increased (0.8750 e) by
16
17
the strong hydrogen bond between N-H group (N -H ) and lone pair of O in
16
17
7
the molecule providing unambiguous evidence about the weakening of the bond,
its elongation (116.1 pm) and concomitant red shift of the N-H stretching
(2)
wavenumber. The stabilization energy E associated with hyperconjugative
interaction LP (O ) → σ*(N -H ) is obtained as (37.45 kJ/mol). It is noteworthy
7
16
17
…
..
that the N–H bond order correlates with the calculated N O distance, by
……
increasing the hydrogen bond strength, reducing the N O distance, the N-H
bond order decreases.
2
1

The N-H in- plane deformation vibrations normally appear in the region
-
1
-1
1
650-1550 cm [36-39]. The weak band at 1536 cm in IR spectra corresponds to
the N-H in-plane deformation mode, which is coupled with the CCH bending, as
revealed by PED calculations. The DFT computation gives the wavenumber at
-1
1
548 cm has 47 % N-H in-plane bending character because of its association
with the 14 % CCH bending modes.The N-H out-of-plane bending mode can be
-1
-1
observed as a weak band in IR and Raman at 572 cm and 571 cm
-1
respectively, whereas the computed value is 596 cm (325). The C-N stretching
-1
vibrations generally occur in the region 1170-1040 cm . The C-N stretching
-1
wavenumber was calculated to be 1024 cm (27 % PED) and the corresponding
-
1
IR band is found at 1024 cm , which is coupled with N-N stretching mode (37 %
PED).
Carbonyl vibrations are considered to give rise to characteristic bands in
the vibrational spectrum and, for this reason, such bands have been subject of
extensive studies [36-40]. The intensity of these bands can increase owing to the
conjugation or formation of hydrogen bonds. The increase of conjugation,
therefore, leads to an intensification of the Raman lines as well as increased
infrared band intensities. Carbonyl stretching vibrations generally appear in the
-1
-1
region 1750-1600 cm . The broad, intense IR band at 1631 cm corresponds to
the C=O stretching vibration. The DFT computation gives the C=O stretching
-
1
wavenumber of this mode as 1695 cm (63 % PED). The carbonyl stretching
2
2

mode is simultaneously influenced by the conjugation of C=O with amide
nitrogen and mesomeric effect. When a nitrogen, oxygen or chlorine is attached
to a carbonyl carbon, the nonbonding electrons on the attached atom N can
-
+
rearrange and donate electrons to the carbonyl oxygen O=C-N ↔ O -C=N . This
weakens the C=O force constant depending on the electron donation tendency of
N. In DMBBH it is a dominant effect, as the C=O stretching wavenumber is
-1
lower than that of ketones (1715 cm ), even though the nitrogen is more
electronegative than a carbon. This is justified by DFT calculation also. The
unusual lowering of the carbonyl stretching wavenumber is also contributed by
the electron releasing effect of the C=O bond in the acceptor subunit due to
intramolecular charge transfer, as reported earlier [15-18] in addition to the
intermolecular effect and π-conjugation. This mechanism plays an important role
in the biological activity of DMBBH. The broadening of the band at about
-1
-1
1
631cm and the appearance of the band at 1556 cm strongly suggests the
existence of proton equilibrium.
In phenyl hydrazones the possibility of obtaining a charge transfer
interaction is related to the absence of even the slight amount of strain in the
hydrazo group, which must be perfectly planar to allow conjugation of the group.
The C=N stretching mode can be used as a good probe for evaluating the bonding
configuration around the amino N atom and the electronic distribution of the
aromatic amine compounds [40-41]. The C=N stretching appears in the region
2
3

-1
1
670-1630 cm [39]. The C=N stretching wavenumber is found to have
-1
considerable contribution to this mode and is appearing at 1624 cm , which is
mixed with 32 % νC18=N19 and 40 % νC-C . This cannot be distinctly resolved
experimentally. The downshifting of C=N stretching wavenumber is due to the
charge transfer interaction between the phenyl rings through ›C=N-N‹ skeleton.
The charge transfer interaction is related to the presence of at least one H atom
bonded to the atoms of the phenylhydrazone skeleton (›C=N-N‹). The C=N
-1
stretching vibration is observed as a medium band at 1240 cm (Raman) and at
-1
1
241cm (Solution Raman). According to PED calculations, vibrational mode
C=N has only 20 % C=N stretching contribution and 16 % C-C stretching
character.
4.6.3 p-Methoxybenzylidene group vibrations
The methoxy group vibrations are observed as intense bands in both IR
and Raman, but with large variation from the normal values of methyl groups.
This must be due to the electronic effect caused by the presence of oxygen atoms,
which causes a deviation from the expected values [38-39]. The vibrational
modes of methoxy groups in DMBBH are known to be influenced by a variety of
interesting interactions such as electronic effects and conjugation. For methoxy-
substituted phenyl groups, the conjugation of the lone pair of oxygen with the pz
orbital of the aryl ring and the back donation are two competitive effects whose
2
4

relative weights determine the conformation dependent molecular properties [15-
8]. The internal rotation of the OCH group can be described in terms of an
1
3
angle φ between the plane of the ring and the plane containing C , O and C
ar
me
atoms. Two effects influence the internal rotation about the C -O axis are (i) the
ar
steric repulsion between the methyl hydrogen and the hydrogen at the adjacent
position of the ring and (ii) the conjugation of the oxygen atom with ring. During
the methoxy rotation, the conjugation of the oxygen lone pair with the aryl pz
orbital changes, reaching a maximum when the carbon atom of the methoxy
group lies in the plane of the ring(φ = 0˚). In DMBBH the methoxy carbon atom
lies in the ring plane (φ = 0.001˚), the induction effect is prominent. Induction
produces stronger polarization of C-H bonds along with the increase of C-H force
constant and charge on the hydrogen atom and with the decrease of C-H bond
length [42]. This will cause the conjugation of the oxygen lone pair with the aryl
pz orbital changes, reaching a maximum when the carbon atom of the methoxy
group lies in the plane of the ring (φ = 0˚).
The methoxy asymmetric and symmetric stretch bands are usually
-1
-1
observed around 2960 cm and 2846 cm respectively [36-39,43]. DFT
-1
-1
-1
computations exhibit three vibrations at 3016 cm , 2949 cm and 2892 cm
respectively. As indicated by the PED, these three modes involve 100%. The
-
1
asymmetric stretching mode is observed as weak bands at 2988 cm in both IR
and Raman spectrum. The asymmetric in phase stretching vibration is active in
2
5

-1
-1
IR as a weak band at 2954 cm . The weak band at 2912 cm (Raman) and at
-
1
2
2
912 cm (IR) is assigned to methyl symmetric stretching mode. The mode at
-
1
902 cm has 100 % ν (CH ) character. This composition suggests that we can
s
3
assign confidently this mode to the symmetric stretching of the three methyl C-H
bonds.
The asymmetric and symmetric bending vibrations of methyl groups
-1
-1
normally appear in the region 1470-1440 cm and 1390-1370 cm respectively
-1
[
36-39]. The weak band at 1454 cm in infrared spectrum and corresponding
-1
Raman band at 1453 cm is attributed to the CH asymmetric bending mode. The
3
-1
-1
weak band around 1436 cm in IR and 1436 cm in Raman spectrum
corresponds to the symmetric bending mode. As revealed by the PED, vibrational
mode δCH has only 82 % symmetric bending mode. The rocking vibrations of
3
the CH group in DMBBH appear as mixed vibrations. These modes usually
3
-1
appear in the region 1170-1100 cm [36-39]. The medium bands observed in IR
-1
and Raman spectrum at 1172 cm are attributed to the CH rocking mode, which
3
is coupled with aromatic CCH bending mode.
The selection allows for C-H attached modes 7b, 2, 20a and 20b for p-
disubstituted ring [30]. The computed eigen vector distribution using DFT shows
that, for p-disubstituted ring, mode 20b is found to be active. The weak bands
−1
−1
observed at 3067 cm (IR) and at 3065 cm (Raman) is assigned to mode 7b.
The normal vibrations 3, 9a, 18a and 15 are classified as C–H in-plane bending
2
6

−1
vibrations. The medium band observed at 1193 cm in the IR spectra is assigned
to mode 3. The medium bands appearing in both IR and Raman spectrum at 1152
−1
cm corresponds to vibrational mode 9a. The mode 18a is coupled with the C–C
−1
stretching mode and is found active in the IR spectrum at 1104 cm as a medium
-1
band. Weak band at 984 cm in Raman corresponds to the mode 15. Normal
vibration 1 of phenyl ring is usually referred to as substituent sensitive vibration.
-1
For heavy substitution the modes found in the region 900-800 cm are strongly
-
1
IR active [35]. This is confirmed by the medium band in IR spectra at 826 cm .
The C–H out-of-plane bending vibrations are expected in the region 960–780
−1
−1
cm . The mode 17a is observed in the IR at 930 cm . The weak bands observed
−1
in the IR at 918 cm are assigned to 10a mode. The radial skeletal (6a and 6b)
and the out-of-plane skeletal (16a and 16b) are listed in Table 3.
The normal modes 8a, 8b, 19a, 19b and 14 are classified as the C-C
stretching vibrations. The mode 8a is found at a higher wavenumber than 8b and
−
1
they appear in the range 1610 -1580 cm [35-39]. According to the calculated
−1
PED, the very strong band at 1600 cm in the Raman spectrum is assigned to
mode 8, which is described as the in-phase stretching vibrations of the C=C
bonds in the aromatic ring ν(C -C ) + ν(C -C ). This mode corresponds to the
2
5
26
22
23
so called mode 8a in the benzene ring [35]. The 8b mode appears as very strong
−
1
-1
bands at 1556 cm (IR), 1559 cm ( solution Raman) and its strong counterpart
−1
at 1558 cm in Raman. According to PED calculations, vibrational mode 8b has
2
7

only 55 % C–C stretching contribution and 10 % C-H in-plane bending
−1
character. The strong band observed at 1514 cm in IR is assigned to the 19a
−1
mode and the medium counterpart in the Raman spectrum occurs at 1514 cm .
As revealed by the calculated PED, vibrational mode 19a has only 15 % C–C
stretching contribution and 47 % C-H in plane bending character. The mode 19b
−1
is observed only in the IR spectrum as a weak band at 1390 cm . The 19b mode
-1
is attributed to 1410 cm , with significant contribution of, 40 % ν and 30 % C-
CC
−1
H in plane bending character. The very strong band at 1279 cm in the IR and an
−1
intense band at 1278 cm in the Raman spectrum are attributed to vibrational
mode 14 of C–C stretching. The 14 mode appears in the calculation at 1293
−1
cm with a composition of 56 % C-C stretch and 28 % C-H in plane bend .The
b, 19b and 14 modes appear simultaneously in both the IR and Raman spectra
providing evidence for charge transfer interaction.
8
5. Molecular Docking Study of DMBH with Phospholipase A2
Based on the Glide energy and Glide score the poses were ranked. The binding
mode (figure 1-3) and hydrogen bond interactions (table) were computed. There
are six subsites were reported for phospholipase A Based on the combination of
2
.
subsites, the target complexes were chosen and subjected for docking with the
DMBH. The results confirm that the DMBH prefers the subsite 4, 5 and 6 and is
similar to the indomethacin [3] binding mode with human pancreatic secretary
PLA . It could be confirmed by the interactions in table 1 and the corresponding
2
2
8

cartoorn diagram of the DMBH-PLA complex. In order to confirm the binding
2
preference of DMBH to subsites 4, 5 and 6, the molecular docking is performed
with the PLA – inhibitor complexes (monomer and dimer form) with specificity
2
to the subsites 1, 2 and 3. Here, in both the cases, the DMBH failed to interact
with the active site amino acids H47 and D48 and shows unfavourable score and
energy comparing to the binding with subsites 4, 5 and 6. Hence, the results
confirms that DMBH can able to inhibit Phospholipase A2 and exhibit anti-
inflammatory and anticoagulant activity thereby indirectly involve in tumor
suppression.
5
. Conclusions
FT-Raman and IR spectra of the Schiff base compound, 2,4-dihydroxy-N'-
methoxybenzylidene) benzohydrazide have been recorded and analyzed. The
(
detailed interpretation of the vibrational spectra has been carried out with the aid
of normal coordinate analysis (NCA) following the scaled quantum mechanical
force field methodology. The various intramolecular interactions that is
responsible for the stabilization of the molecule was revealed by natural bond
orbital analysis. Information about the size, shape, charge density distribution and
site of chemical reactivity of the molecules has been obtained by mapping
electron density isosurface with electrostatic potential surfaces (ESP). A
complete vibrational analysis has been attempted on the basis of experimental
infrared and Raman spectra, calculated frequency, intensity of the vibrational
2
9