Synthesis, spectroscopic and structural evaluation of ethyl 2-cyano-3-{5-[(4-nitro-benzoyl)-hydrazonomethyl]-1H-pyrrol-2-yl}-acrylate using experimental and theoretical approaches

Article abstract of DOI:10.1016/j.molstruc.2013.07.001

A new C-vinylpyrrole containing aroylhydrazone, ethyl 2-cyano-3-{5-[(4- nitro-benzoyl)-hydrazonom-ethyl]-1H-pyrrol-2-yl}-acrylate (ECNBHPA) derived from ethyl 2-cyano-3-(5-formyl-1H-pyrrol-2-yl)-acrylate and 4-nitro-benzohydrazide has been characterized by various spectroscopic techniques (¹H NMR, ¹³C NMR, Mass, UV-Visible, Emission, FT-IR). TD-DFT has been used to calculate the various electronic excitations and their nature within molecule. The emission spectra of ECNBHPA show photo-luminescent behavior of title molecule. Natural bond orbital (NBO) analysis has been carried out to explore the various conjugative/hyperconjugative interactions within molecule and their second order stabilization energy (E⁽²⁾). A combined experimental and theoretical vibrational analysis designates presence of the classical hydrogen bonding N1 AH29- ? N9 between pyrrole NAH as proton donor and N atom of cyanide as proton acceptor. To determine the strength and nature of hydrogen bonding, topological parameters at bond critical points (BCP) have been analyzed by 'Quantum theory of atoms in molecules' (QTAIM) in detail. The result of hydrogen bonding is obvious in ¹H NMR, FT-IR and ESP map as down field chemical shift, vibrational red shift and absence of blue colur relative to pyrrole NAH proton, respectively. Global electrophilicity index (o = 6.573 eV) shows that title molecule behaves as a strong electro-phile. The local reactivity descriptors analyses such as Fukui functions (?,f _k~), local softnesses (s￡,s^) and electrophilicity indices (cojJ",co^) have been performed to determine the reactive sites within molecule. The first hyperpolarizability (?₀) of ECNBHPA has been computed to evaluate the non-linear optical (NLO) response of the investigated molecule.

Full text of DOI:10.1016/j.molstruc.2013.07.001

Accepted Manuscript
Synthesis, spectroscopic and structural evaluation of Ethyl 2–cyano–3–{5–[(4–
nitro–benzoyl)–hydrazonomethyl]–1H–pyrrol–2–yl}–acrylate using experi‐
mental and theoretical approaches
R.N. Singh, Amit Kumar, Poonam Rawat, Anchal Srivastsva
PII:
S0022-2860(13)00605-4
DOI:
http://dx.doi.org/10.1016/j.molstruc.2013.07.001
MOLSTR 19875
Reference:
To appear in:
Journal of Molecular Structure
Received Date:
Revised Date:
Accepted Date:
15 April 2013
30 June 2013
1 July 2013
Please cite this article as: R.N. Singh, A. Kumar, P. Rawat, A. Srivastsva, Synthesis, spectroscopic and structural
evaluation of Ethyl 2–cyano–3–{5–[(4–nitro–benzoyl)–hydrazonomethyl]–1H–pyrrol–2–yl}–acrylate using
experimental and theoretical approaches, Journal of Molecular Structure (2013), doi: http://dx.doi.org/10.1016/
j.molstruc.2013.07.001
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Synthesis, spectroscopic and structural evaluation of Ethyl 2–cyano–3–{5–[(4–
nitro–benzoyl)–hydrazonomethyl]–1H–pyrrol–2–yl}–acrylate using
experimental and theoretical approaches
a
a
a
b
R.N. Singh *, Amit Kumar , Poonam Rawat , Anchal Srivastsva
a
Department of Chemistry, University of Lucknow, Lucknow 226007, U.P., India
b
Department of Physics, University of Lucknow, Lucknow 226007, U.P., India
*
Corresponding author e–mail: rnsvk.chemistry@gmail.com;
Tel.: +919451308205

Abstract
A new C–vinylpyrrole containing aroylhydrazone, ethyl 2–cyano–3–{5–[(4–nitro–
benzoyl)–hydrazonomethyl]–1H–pyrrol–2–yl}–acrylate (ECNBHPA) derived from ethyl 2–
cyano–3–(5–formyl–1H–pyrrol–2–yl)–acrylate and 4–nitro–benzohydrazide has been
1
13
characterized by various spectroscopic techniques ( H NMR, C NMR, Mass, UV–Visible,
Emission, FT–IR). TD–DFT has been used to calculate the various electronic excitations and
their nature within molecule. The emission spectra of ECNBHPA show photoluminescent
behavior of title molecule. Natural bond orbital (NBO) analysis has been carried out to explore
the various conjugative / hyperconjugative interactions within molecule and their second order
(2)
stabilization energy (E ). A combined experimental and theoretical vibrational analysis
designates presence of the classical hydrogen bonding N1–H29···N9 between pyrrole N–H as
proton donor and N atom of cyanide as proton acceptor. To determine the strength and nature of
hydrogen bonding, topological parameters at bond critical points (BCP) have been analyzed by
‘
Quantum theory of Atoms in molecules’ (QTAIM) in detail. The result of hydrogen bonding is
1
obvious in H NMR, FT–IR and ESP map as down field chemical shift, vibrational red shift and
absence of blue colour relative to pyrrole N–H proton, respectively. Global electrophilicity index
(
ω = 6.573 eV) shows that title molecule behaves as a strong electrophile. The local reactivity
+
–
+
–
k k k k
descriptors analyses such as Fukui functions (f , f ), local softnesses (s , s ) and
+
–
electrophilicity indices (ω , ω ) have been performed to determine the reactive sites within
k
k
0
molecule. The first hyperpolarizability (β ) of ECNBHPA has been computed to evaluate the
non–linear optical (NLO) response of the investigated molecule.
Keywords: TD–DFT, NBO analysis, hyperconjugation, hydrogen bonding, QTAIM analysis,
first hyperpolarizability

1
. Introduction
Aroylhydrazones (>C=N–NH–CO–) are special group of compounds in the Schiff bases
family due to their wide applications in chemistry and biology and characterized by the presence
2
3
two inter–linked nitrogen atoms as Nsp –Nsp . They are good synthons for the target syntheses
of 1,3,4–oxadiazine, 1,2,4–triazine, pyrazole derivatives [1] and 1,3,4–oxadiazolines, 2–
azetidinones, 4–thiazolidinones via heterocyclic transformations [2, 3]. Several aroylhydrazones
have wide applications in the field of analytical chemistry as a selective metal extracting agent as
well as in spectrophotometric determination of certain transition metals [4–6]. However, the
most valuable property of aroylhydrazones is their great physiological activity due to the
presence of the active pharmacophore (>C=N–NH–CO–) and provides a wide range of
application in medicinal and pharmaceutical fields with various biological applications.
Therefore, a number of hydrazone derivatives have been used for the treatment of diseases such
as convulsant [7], malaria [8], HIV, cancer [9], microbial [10–12] and tuberculosis [13–15].
Tuberculosis activity is attributed to the formation of stable chelates with transition metals
present in the cell. Thus many vital enzymatic reactions catalyzed by these transition metals
cannot take place in the presence of hydrazones [16, 17]. Many of hydrazones are used as
prospective new materials for the development of potential chemosensors [18], opto–electronic
[
19] and nonlinear optical (NLO) [20, 21] applications.
Materials with excellent optical non–linearity and certain spectral characteristics are
required for high–level technologies such as data storage, information processing,
telecommunications and optical switching. Organic materials having π–extended frameworks
with alternating single (σ) and double/triple bonds exhibit photoluminescence and enjoy many

applications as organic light emitting devices (OLEDs) [22]. Organic compounds are also
important for photonic devices because of their optical non–linear properties [23].
Photoluminescence, the phenomenon of luminescence or light emission caused by the absorption
of optical radiation, is widely used to characterize many chemical, physical and biological
processes [24, 25]. There are numerous optical properties of the photoluminescence material that
are of interest as excitation wavelength, emission wavelength, excitation absorption, emission
intensity, polarization and decay time. The photoluminescence is classified into two categories as
fluorescence and phosphorescence (delayed luminescence), which are involved with emitted
radiation <10ns, >10ns, respectively after excitation [26]. In both cases, the emitted radiation is
generally of a longer wavelength than the wavelengths imposed on the photoluminescent
material, and this conversion from excitation wavelength to a range of emission wavelengths is
known as bispectral convertance [26]. The fluorescent property of pyrrole(4–
nitrophenyl)hydrazone derivatives are also well reported in literature [27].
Hydrogen bonds are of versatile importance in fields of chemistry and biochemistry, which
governs chemical reactions, supramolecular structures, molecular assemblies and life processes.
Intra and intermolecular hydrogen bonds are classified depending upon the nature of atoms
involved in the hydrogen bridges [28–30].
The B3LYP hybrid functional has been shown to successfully predict a wide range of
molecular properties. DFT / B3LYP methods is appropriate for calculation of binding energies
of hydrogen bonded complexes because this adequately takes into account of electron correlation
contributions, which are especially important in systems containing extensive electron
conjugation and / or electron lone pairs [31].

The functionalized C–vinylpyrroles are prospective new materials for molecular optical
switches, nano–devices, photo– and electro–conducting applications and also used as ligands for
new photo catalysts, biologically active complexes [32, 33]. Therefore, ECNBHPA has been
synthesized, characterized using spectroscopic and structural investigation. In present paper, we
report the spectroscopic, structural and hyperpolarizability properties. Furthermore, quantum
chemical calculations have also been performed to determine the intramolecular conjugative and
hyperconjugative interactions, hydrogen bonding and chemical reactivity of ECNBHPA.
2
. Experimental Details
All the chemicals were used of analytical grade. Ethyl 2–cyano–3–(5–formyl–1H–pyrrol–
–yl)–acrylate (1) was prepared by an earlier reported method [34]. 4–Nitro–benzohydrazide was
2
prepared by refluxing the reaction mixture of ethyl 4–nitro–benzoate and hydrazine hydrate in
methanol. The Mass spectrum was recorded on JEOL–Acc TDF JMS–T100LC, Accu TOF mass
1
spectrometer. The H NMR spectrum of ECNBHPA was recorded in DMSO–d
6
on Bruker
DRX–300 spectrometer using TMS as an internal reference. The FT–IR spectrum was recorded
in KBr medium on a Bruker spectrometer. The UV–Visible absorption spectrum of ECNBHPA,
–
5
(
1×10 M in DMSO) was recorded on V-670 JASCO spectrophotometer. The emission
–
5
spectrum of ECNBHPA, (1×10 M in DMSO) was recorded on PL Spectrometer LS-55, Perkin
Elmer, U.K.

Synthesis of Ethyl 2–cyano–3–{5–[(4–nitro–benzoyl)–hydrazonomethyl]–1H–pyrrol–2–yl}–
acrylate:
To the equimolar solution of ethyl 2–cyano–3–(5–formyl–1H–pyrrol–2–yl)–acrylate
(
0.250 g, 1.1464 mmol) and 4–nitro–benzohydrazide (0.2076 g, 1.1464 mmol) in 25 ml MeOH
was added 0.01 ml of conc. HCl acid as catalyst. The reaction mixture was stirred at room
temperature for 10 hours and dark yellow color precipitate was obtained. The precipitate was
filtered off, washed with MeOH and dried in air afforded ECNBHPA, as yellow solid. M.p.:
285–288 °C. Anal. calcd. for C18
H
15
N
5
O
5
: C 56.68%, H 3.97%, N 18.37%; obs.: C 56.72%, H
+
3
.94%, N 18.34%. MS (m/z): calcd. 381.1074, obs. 382.2 [M +1].
3. Quantum chemical calculations
The quantum chemical calculations have been carried out using Gaussian 09 program
1
package [35] to predict the molecular structure, energies of the optimized structures, H NMR
chemical shifts, vibrational wavenumbers and chemical reactivity of molecule at B3LYP
’
functional and 6–31G(d,p) basis set, which invokes Becke s three parameter (local, non local,
Hartree–Fock) hybrid exchange functional (B3) [36], with Lee–Yang–Parr correlational
functional (LYP) [37]. IR spectrum was simulated using line shape of Lorentzian curves type
-1
and the FWHM (The full width at half-maximum) of each peak was 8 cm . The basis set 6–
1G(d,p) with ‘d’ polarization functions on heavy atoms and ‘p’ polarization functions on
3
hydrogen atoms are used for better description of polar bonds of molecule [38, 39]. It should be
emphasized that ‘p’ polarization functions on hydrogen atoms are used for reproducing the out of

1
plane vibrations involving hydrogen atoms. The H NMR chemical shifts are calculated using
gauge including atomic orbital (GIAO) approach using IEFPCM model in appropriate solvent.
Time dependent density functional theory (TD–DFT) is used to find the various electronic
excitations and their nature within molecule. The optimized geometrical parameters are used in
the vibrational wavenumbers calculation to characterize all stationary points as minima and their
harmonic vibrational wavenumbers are positive. Potential energy distribution along internal
coordinates is calculated by Gar2ped software [40]. To estimate the thermodynamic parameters
as enthalpy (H) and Gibbs free energy (G), the thermal corrections to these parameters are added
to the calculated total energies. The reaction energies, standard enthalpies ∆H = (∆G - T∆S) and
Gibbs free energies ∆G = (∆H - T∆S) of ECNBHPA was derived from frequency calculations at
0
the B3LYP/6-31G (d,p) level of theory. The reaction entropies (∆S ) of ECNBHPA was
evaluated using a thermodynamic equation ∆S = (∆H - ∆G)/T.
4
4
4
. Results and discussion
.1 Thermochemistry
For simplicity, the reactants ethyl 2–cyano–3–(5–formyl–1H–pyrrol–2–yl)–acrylate and
–nitro–benzohydrazide are abbreviated as (1), (2) and product ethyl 2–cyano–3–{5–[(4–nitro–
benzoyl)–hydrazonomethyl]–1H–pyrrol–2–yl}–acrylate, byproduct water as ECNBHPA, (4),
respectively. The calculated thermodynamic parameters as Enthalpy (H / a.u.), Gibbs free energy
(
G / a.u.) and Entropy [S / (cal/mol–K)] of (1), (2), ECNBHPA, (4) and their change for
Reaction, at 25 °C are listed in Supplementary material (S Table 1). The calculated positive
value of enthalpy change (∆H) and negative value of entropy change (∆S) show that both energy

factor and entropy factor are unfavorable. It is to be noticed that net spontaneity of the chemical
reaction is depend upon the Gibbs free energy change of reaction (∆G). For uncatalyzed reaction,
the calculated positive value of (∆G) shows that this reaction is non–spontaneous. As we know
that catalyst plays an important role to investigate spontaneity of the reaction because catalyst
affects the reaction rate, it lower down the activation barrier energy, and speed up the reaction
but not equilibrium position. Therefore, this reaction is carried out in presence of acid as
catalyst.
4
.2 Molecular geometry
Optimized geometry of all the reactants (1, 2), product ECNBHPA and byproduct water
(
4) involved in chemical reaction is shown graphically in Scheme 1.
The optimized geometry for ground state lower energy conformer I of ECNBHPA is
shown in Figure 1. Selected optimized geometrical parameters of ECNBHPA, calculated at
B3LYP/6–31G (d,p) are listed in Supplementary material (S Table 2). The molecule possesses
C
1
symmetry. Another two E–conformers of ECNBHPA have been obtained around the dihedral
angle C C C N (conformer II) and C C C C (conformer III). The E–configuration in
4
5
15 16
3
2
6
7
6 4 2
conformer I has been obtained with respect to the pyrrole and –NH–CO–C H NO group located
on the opposite side of the Schiff base C15=N16 bond. The E–configuration about Schiff base
C15=N16 bond is not only observed in our theoretical study but also reported in crystal structure
of various aroylhydrazone derivatives [41, 42]. The energy of Conformer I and II are calculated
as –1344.7258, -1344.7248 a.u. respectively. They have energy difference 0.628 kcal/mol. The
asymmetry of N1–C2 and N1–C5 bonds i.e difference between their bond lengths can be

explained due to the presence of the two different groups as ‘ethyl 2–cyano–acrylate’ and ‘(4–
nitro–benzoyl)–hydrazonomethyl’ at C2 and C5 of pyrrole ring, respectively. Another conformer
III with E–configuration about vinyl C6=C7 bond with respect to the higher priority group
ethoxycarbonyl and pyrrole has been also obtained. The energy of conformer III is calculated as
–
1344.7240 a.u. The energy difference between conformer I and III is 1.129 kcal/mol. In
ECNBHPA, due to the presence of intramolecular hydrogen bonding (N1–H29···N9) proton
donor pyrrole N─H bond is elongated than free N─H bond in other pyrrole derivatives.
1
13
4
.3. H and C NMR Spectroscopy
The geometry of ECNBHPA, together with that of tetramethylsilane (TMS) is fully
1
13
optimized. H and C NMR chemical shifts were calculated with GIAO approach using
B3LYP/6–31G (d,p) method [43-44]. The GIAO method shows much lower sensitivity with
respect to the choice of basis set. Besides being much less sensitive to basis set choice, the GIAO
approach represents a very convenient starting point for the development of computational
schemes which include electron correlation effects on calculated shielding [45-48]. Chemical
X
shift of any ‘x’ proton (δ ) is equal to the difference between isotropic magnetic shielding (IMS)
1
of TMS and proton (x). It is defined by the equation: δ
X
= IMSTMS – IMS
X
. The experimental H
1
3
and C NMR spectrum of ECNBHPA in DMSO–d
6
are given in Figure 2, 3, respectively. The
1
13
calculated and experimental H and C NMR chemical shifts (δ / ppm) of ECNBHPA in

1
DMSO–d
quartet chemical shift in range 4.249–4.319 ppm and triplet in range 1.272–1.319 ppm designate
presence of the ester –CH , –CH group, respectively. The vinylic CH=C and ArCONH protons
6
solvent at 25°C are listed in Table 1. In the H NMR spectrum of ECNBHPA, a
2
3
are observed as a singlet at 9.900, 12.751 ppm, respectively. The observed chemical shift as a
singlet at 7.557 ppm indicates presence of hydrazone linkage (CH=N–NH–) in the molecule. In
order to compare the chemical shifts, correlation graph between the experimental and calculated
1
H NMR chemical shifts is shown in Supplementary material (S Figure 1). The correlation graph
1
follows the linear equation: y = 1.23894 x – 1.21244, where ‘x’ is the calculated H NMR
1
chemical shift, ‘y’ is the experimental H NMR chemical shift (δ in ppm). The correlation value
1
(
R = 0.91995) shows that experimental H NMR data is consistent with the calculated data from
optimized structure of probed molecule.
1
3
Additional support for molecular structure of ECNBHPA is provided by its C NMR
spectrum, in which chemical shift of carbon atoms at 143.39 ppm (–CH=N–NH) and 160.46 ppm
1
(
Ar–CONH–) substantiates the aroylhydrazone character of molecule assigned from the H NMR
data. In order to compare the chemical shifts, correlation graph between the experimental and
13
calculated C NMR chemical shifts is shown in Supplementary material (S Figure 2). The
correlation graph follows the linear equation: y = 0.9827 x – 0.9823, where ‘x’ is the calculated
1
3
13
C NMR chemical shift, ‘y’ is the experimental C NMR chemical shift (δ in ppm). The
correlation value (R = 0.9745) shows that there is good agreement between experimental and
1
3
calculated C NMR chemical shifts. The mass spectrum of ECNBHPA has been given in Figure
4
.
4
.4 Electronic absorption (UV–Visible) and emission (photoluminescence) spectroscopy

The nature of excitations in the observed UV–Visible spectrum of the title compound has
been studied by the time dependent density functional theory (TD–DFT). TD-DFT is being used
today in an ever-increasing range of applications to widely-varying systems in chemistry,
biology, solid-state physics, and materials science. TDDFT has emerged as a reliable standard
tool for the theoretical treatment of electronic excitation spectra and recent works demonstrate
the good accuracy for a wide range of systems [49-55]. The calculated and experimental
electronic excitations of high oscillatory strength are listed in Table 2. The experimental UV–
Visible spectrum of ECNBHPA is shown in Figure 5. The HOMO–LUMO energy gap for
ECNBHPA, calculated at B3LYP/6–31G(d,p) method is given below
HOMO energy = –5.9634 eV,
LUMO energy = –2.9451 eV,
HOMO–LUMO energy gap = 3.0183 eV.
A combined experimental and theoretical UV–Visible spectrum analysis of ECNBHPA indicates
that the first observed electronic excitation at λex.max = 266 nm agrees well with the aggregate of
three electronic excitations calculated at λex.max = 264.70 nm, f = 0.2911; λex.max = 273.76 nm, f =
0
.2041; and λex.max = 279.71 nm, f = 0.1279. The second observed electronic excitation at λex.max
440 nm corresponds to the aggregate of two electronic excitations calculated at λex.max = 397.89
=
nm, f = 0.6967; λex.max = 470.19 nm, f = 0.3597. Therefore, the first observed λex.max at 266 nm is
in good agreement with the calculated λex.max, whereas the second observed λex.max at 440 nm is
blue shifted up to ~30 nm compared with the calculated λex.max values in the theoretical UV–
Visible spectrum. On the basis of molecular orbital coefficients analyses, the nature of these
electronic excitations is assigned as n→π*.The optical properties of ECNBHPA have been

explored using electronic absorption and photoluminescence spectra. Photoluminescence (PL) is
a process in which a substance absorbs photons (electromagnetic radiation) and then re–radiates
photons. Quantum mechanically, PL can be described as an excitation to a higher energy
state and then a return to a lower energy state accompanied by the emission of a photon. The
emitted radiation is generally of a longer wavelength than the wavelengths imposed on the
photoluminescent material and difference between wavelength absorption maxima and
wavelength emission maxima is known as the Stokes shift. For steady–state measurements, it is
not possible to distinguish between the two types of photoluminescence phenomenon as
fluorescence and phosphorescence. The time–resolved emission spectroscopy is a very important
tool for distinguishing between these two types of luminescence and providing an insight into the
molecules involved in this process. This is outside the scope of this paper and this work is
limited to the steady–state photoluminescence measurements. The photoluminescence spectra of
ECNBHPA in DMSO excited at λex.max = 266, 440 nm is shown in Figure 6. The experimental
photoluminescence (PL) spectral data of ECNBHPA are listed in Table 3. The most striking
feature is that ECNBHPA gives an intense photoluminescence emission at (λem.max = 545 nm) in
green region upon irradiation by ultraviolet light (λex.max = 266 nm), whereas another intense
photoluminescence emission at (λem.max = 483 nm) in blue region upon irradiation by visible light
(λex.max = 440 nm). Therefore, the emission spectrum shows that title compound is a good
photoluminescent material due to intense emission in visible (green and blue) region with
Stoke’s shift 279, 43 nm, respectively.
4
.5 Natural bond orbitals (NBO) analysis

A useful aspect of the NBOs provides an accurate method for studying intramolecular bonding
and interaction among bonds and also gives an efficient basis for investigating charge transfer or
conjugative interaction in various molecular systems [56-59].
Second–order perturbation theory analysis of the Fock matrix in NBO basis for
ECNBHPA is presented in Supplementary material (S Table 3). The interactions π(C2–
1
C3)→π*(C4–C5), π(C4–C5)→π*(C2–C3) and n (N1)→π*(C2–C3) / π*(C4–C5) are responsible
for conjugation of respective π–bonds in pyrrole ring. The electron density at the conjugated π
bonds (1.67837–1.66823) and π* bonds (0.42076–0.42962) of pyrrole ring indicate strong π–
electron delocalization within ring leading to a maximum stabilization of energy ~22.68
kcal/mol. In same manner, the interactions π(C20–C21)→π*(C22–C23) / π*(C24–C25), π(C22–
C23)→π*(C20–C21) / π*(C24–C25) and π(C24–C25)→π*(C20–C21) / π*(C22–C23) are
responsible for the conjugation of respective π–bonds in benzene ring and stabilized the
molecule with maximum stabilization of energy ~22.25 kcal/mol. The interactions π(C2–
C3)→π*(C6–C7), π(C6–C7)→π*(C2–C3) are responsible for the conjugation of bonds C2–C3,
C6–C7 with C2–C6 and stabilized the molecule up to ~24.55 kcal/mol. In the same manner, the
interactions π(C4–C5)→π*(C15–N16), π(C15–N16)→π*(C4–C5) are responsible for the
conjugation of these bonds with C5–C15. It is to be noticed that the charge transfer interactions
are formed by the orbital overlap between bonding (π) and antibonding (π*) orbitals, which
results in intra–molecular charge transfer (ICT) causing stabilization of the system. The
movement of π–electron cloud from donor to acceptor i.e. intramolecular charge transfer (ICT)
can make the molecule more polarized and the ICT must be responsible for the NLO properties
of molecule. Therefore, ECNBHPA may be used for non–linear optical materials.

The primary hyperconjugative interactions n
O12), n (O19)→σ*(N17–C18) / σ*(C18–C20) and other interactions n
(N17)→π*(C15–N16) / π*(C18–O19) are stabilized the molecule to a greater extent ~50.68
1
2
(N9)→σ*(C7–C8), n (O11)→σ*(C10–
2
2
(O12)→π*(C10–O11),
n
1
kcal/mol. The secondary hyperconjugative interactions associated with pyrrole ring such as
σ(N1–C2)→σ*(C5–C15), σ(N1–C5)→σ*(C2–C6), σ(C2–C3)→σ*(C2–C6) and associated with
benzene ring as σ(C18–C20)→σ*(C24–C25), σ(C24–C25)→σ*(C18–C20) are stabilized to the
molecule with energy ~4.54 kcal/mol.
Selected Lewis orbitals (occupied bond or lone pair) of ECNBHPA with their NBO
hybrid orbitals are listed in Supplementary material (Table 4). The NBO hybrid orbitals analysis
shows that all the N–H / C–N bond orbitals are polarized towards the nitrogen atom (ED = 57.40
–
74.63 % at N), whereas C–O / N–O bond orbitals towards oxygen atom (ED = 51.40 – 68.74 %
at O). The electron density distribution (occupancy) around the imino group (>N–H) mainly
influences the polarity of the compound. Therefore, they consist with the maximum electron
density on the oxygen and nitrogen atoms, which is responsible for the polarity of molecule.
4
.6 Vibrational analysis
The experimental and theoretical (selected) vibrational modes of ECNBHPA, calculated
at B3LYP/6–31G(d,p) method and their assignments using PED are given in Supplementary
material (S Table 5). Comparison between experimental and theoretical IR spectra in the region
–
1
4
000–400 cm is shown in Supplementary material (S Figure 3). The calculated vibrational
wavenumbers are higher than their experimental values for the majority of the normal modes.
Two factors may be responsible for the discrepancies between the experimental and computed

wavenumbers. The first is caused by the environment (gas and solid phase) and the second is due
to the fact that the experimental values are an anharmonic wavenumbers while the calculated
values are harmonic ones. Therefore, calculated wavenumbers are scaled down using scaling
factor 0.9608 [60], to discard the anharmonicity present in real system. The observed
wavenumbers are assigned by comparing the calculated wavenumbers using potential energy
distribution (PED) analysis of the various vibrational modes.
4
.6.1 N–H Vibrations
In the FT–IR spectrum, the N–H stretch of pyrrole (νN1–H29) is observed at 3423 cm ,
–
1
–
1
whereas this is calculated at 3451 cm for the investigated molecule ECNBHPA. The observed
–
1
pyrrole νN1–H29 deviates from the free νN–H of pyrrole at 3475 cm , reported in literature [61].
–
1
Therefore, the red shift of 52 cm in the observed νN1–H29 of pyrrole compared with the reported
free νN–H indicates the involvement of the pyrrole N1–H29 group in hydrogen bonding. The
theoretical IR spectrum shows that N–H wagging mode of pyrrole (ωN1–H29) is moderately active
–
1
–1
at 627 cm , whereas this is observed at 653 cm . The N–H stretch of –CH=N–NH– part of
–
1
–1
molecule (νN17–H39) is observed at 3283 cm , whereas this is calculated at 3376 cm in
–
1
theoretical IR spectrum. The rocking and wagging mode of N17–H39 assigned at 1478, 460 cm
–
1
agrees with the observed band at 1497, 500 cm , experimentally.
4.6.2 C–H Vibrations
–
1
A weak band for ester asymmetric stretch νasCH
2
calculated at 2987 cm matches with
–
1
–1
the observed wavenumber at 2957 cm . The observed band at 2957 cm also matches with the
reported band in literature in the region 3000±50 [62, 63]. The calculated ester symmetric stretch

–
1
ν
s
CH
symmetric C–H stretching vibrations of CH
modes for ester CH assigned at 1470, 1350 cm matches well with the reported bands in
literature in the region 1455±55, 1350±85, respectively [63]. The calculated νasCH and ν CH at
011, 2939 cm correspond to the reported band in literature in the region 2985±25, 2970±30 for
asymmetric, symmetric C–H stretching vibrations of CH , respectively [63]. The calculated ester
2
at 2945 cm corresponds to the reported band in literature in the region 2965±30, for
2
[63]. The scissoring and wagging deformation
–
1
2
3
s
3
–
1
3
3
–
1
–1
ν
3
asCH at 3011 cm also corresponds to the observed wavenumber at 2984 cm , experimentally.
–
1
In theoretical IR–spectrum, the wavenumbers at 1441, 1384 cm display the δas and δ
s
–
1
deformation modes of ester methyl, respectively. The calculated weak band at 3070 cm for C–
–
1
H stretching vibrations of benzene agrees well with the observed wavenumber at 3077 cm . The
–
1
Schiff base C–H stretching vibration (νC15–H38) is assigned at 2947 cm , whereas this is observed
–
1
at 2850 cm in FT–IR spectrum. The observed Schiff base C–H stretching vibration is
responsible for the formation of hydrazone linkage in ECNBHPA.
4
.6.3 C N Vibrations
In the theoretical IR spectrum of ECNBHPA, the calculated wavenumber at 2229 cm
designates the presence of the C N stretching vibration and agrees well with the observed
–
1
–
1
wavenumber at 2218 cm . The free C N stretching vibrations are reported in literature in the
–
1
region 2240–2260 cm [64]. Therefore, the red shift in the observed C N stretching compared
with the free C N stretching indicates the involvement of the C N group in intramolecular
hydrogen bonding.
4
.6.4 C=O and C–O Vibrations

–
1
The ester carbonyl stretching vibration νC=O is expected in the region 1750–1715 cm
–
1
[
64] and in the present study this mode is assigned at 1727 cm using DFT calculation. The
–
1
experimental FT–IR spectrum gives this mode at 1714 cm . The “C–O stretching vibrations” of
esters actually consists of two asymmetrical coupled vibrations as O–C(=O)–C, O–C–C and
–
1
–1
these bands occur in the region 1300–1000 cm [64]. The calculated wavenumber at 1084 cm ,
with 16% contribution in PED demonstrates to the O–C–C stretching vibration (νO12–C13) and
–
1
corresponds to the observed wavenumber at 1054 cm , experimentally. In theoretical IR
spectrum, a combination band of the ‘O–C(=O)–C stretching vibration (νC10–O12)’ and ‘ester O–
–
1
C–O deformation’ at 1236 cm is observed at same wavenumber in experimental FT–IR
spectrum.
4.6.5 C=C, C–C Vibrations
–
1
The calculated C=C stretches of benzene at 1588 cm matches well with the reported
–
1
band in literature in the region 1600–1585 cm [64]. The C=C stretches of pyrrole are assigned
–
1
–1
at 1533, 1463, 1388 cm , whereas these DFT modes are observed at 1524, 1434, 1368 cm ,
–
1
respectively in the experimental FT–IR spectrum. In DFT calculation, a band at 989 cm
–
1
displays the trigonal deformation of benzene ring (δ–R), whereas this is observed at 943 cm .
–
1
The calculated C–C–H deformation associated with pyrrole at 1037 cm agrees with the
–
1
observed wavenumber at 1013 cm . The observed stretching vibrations of vinyl group (νC6=C7) at
–
1
–1
1
1
552 cm matches with the calculated wavenumber at 1577 cm in PED. The DFT mode at
–
1
008 cm designates to the C–C stretching vibration of ester (νC13–C14). A weak band for ring
–
1
puckering vibration (a tortional mode) of benzene is assigned at 655 cm , in theoretical IR

–
1
spectrum. A DFT mode for asymmetric deformation mode of benzene ring (δas–R1) at 701 cm
–
1
agrees with the observed band at 713 cm , experimentally.
4.6.6 C=N, C–N and N–N vibrations
–
1
For ECNBHPA Schiff base υC=N mode assigned at 1611 cm matches with the observed
–
1
wavenumber at 1593 cm , experimentally. The presence of the υ
band confirms the
C=N
–
1
hydrazone linkage in the investigated molecule ECNBHPA. The calculated band at 1611 cm
–
1
for υC=N also matches well with the reported band at 1602 cm in literature [65]. The observed
–
1
C–N stretching vibration as νC18–N17 at 1187 cm matches well with the calculated wavenumber
–
1
at 1206 cm with 12% contribution in PED. The C–N stretching vibration is also active in the
–
1
region 1275 ± 55 cm , reported in literature [66]. The DFT mode for N–N stretching vibration
–
1
–1
(
ν
N16–N17) at 1130 cm corresponds to the observed wavenumber at 1130 cm .
4
.7 Quantum theory of Atoms in Molecules (QTAIM) analysis
Geometrical as well as topological parameters are useful tool to characterize the strength
and nature of hydrogen bond. The existence of hydrogen bond could be supported further by
Koch and Popelier criteria [67] based on ‘Atoms in Molecules’ theory (i) The existence of bond
critical point for the ‘proton (H)···acceptor (A)’ contact as a confirmation of the existence of
H A
hydrogen bonding / interaction (ii) The value of electron density (ρ … ) should be within the
2
range 0.0020–0.0400 a.u. (iii) The corresponding Laplacian (V ρBCP) should be within the range
0
.024–0.139 a.u. if hydrogen bond exist. According to Rozas et al., [68] the interactions may be
2
classified as follows: (i) Strong H–bonds are characterized by (V ρBCP) < 0 and HBCP < 0 and

2
their covalent character is established (ii) Medium H–bonds are characterized by (V ρBCP) > 0
and HBCP < 0 and their partially covalent character is established (iii) Weak H–bonds are
2
characterized by (V ρBCP) > 0 and HBCP > 0 and they are mainly electrostatic in nature. The van
der Waals interactions are characterized by the distance between interacting atoms is greater than
the sum of van der Waals radii of these atoms.
Molecular graph of ECNBHPA using AIM program at B3LYP/6–31G(d,p) level is
shown in Supplementary material (S Figure 4). Topological as well as geometrical parameters
for bonds of interacting atoms in ECNBHPA are given in Supplementary material (S Table 6, 7),
H A
respectively. For the intramolecular interaction N1–H29···N9, electron density (ρ … ) and its
2
Laplacian (V ρBCP) follows the Koch and Popelier criteria and the distance between interacting
atoms (dH29···N9 = 2.45 Å) is less than the sum of van der Waals radii of these atoms (r
H N
+ r =
2
.75 Å). Therefore, the homonuclear interaction N1–H29···N9 is a classical hydrogen bond. The
2
nature of N1–H29···N9 hydrogen bond is weak due to (V ρBCP) > 0 and HBCP > 0. In this article,
QTAIM theory is used to estimate hydrogen bond energy (E). Espinosa proposed proportionality
between hydrogen bond energy (E) and potential energy density (VBCP) at H···N contact: E = ½
(
VBCP) [69]. According to this equation, the energy of intramolecular hydrogen bond (N1–
H29···N9) is calculated as –2.2251 kcal/mol.
4.8 Chemical reactivity
The chemical reactivity of molecule is described in three ways as: (i) using molecular
electrostatic potential (ESP) map (ii) Molar refractivity (MR) (iii) Global and local electronic
reactivity descriptors.

4.8.1. Molecular electrostatic potential (ESP) map
The molecular electrostatic potential (ESP) map is widely used as a reactivity map to display
the most probable regions for electrophilic / nucleophilic attack of charged point–like reagents
on organic molecules. The negative electrostatic potential corresponds to an attraction of the
proton by the concentrated electron density in the molecule (red color on the ESP surface),
whereas the positive electrostatic potential corresponds to repulsion of the proton by atomic
nuclei in regions where low electron density exists and the nuclear charge is incompletely
shielded (blue color on the ESP surface) [70, 71]. ESP provides a visual method to understand
the relative polarity of a molecule. It also serves as a useful quantity to explain electronegativity,
partial charges, site of chemical reactivity, structure–activity relationship, hydrogen bonding and
other interactions of molecules including biomolecules and drugs [72-74]. In the present study,
ESP map of ECNBHPA has been obtained using B3LYP/6–31G(d,p) and shown in
Supplementary material (S Figure 5).
The different values of electrostatic potential at the surface are represented by different
colors as red represents the region of most electronegative electrostatic potential, blue represents
region of most positive electrostatic potential and green represents region of zero potential.
Electrostatic potential increases in the order red < orange < yellow < green < blue. The ESP
surface has three projections, one in molecular plane and two in perpendicular planes. The color
code of MEP map is in the range between −0.0156 a.u. (deepest red) to +0.0156 (deepest blue).

The regions of negative electrostatic potential are usually associated with the lone pair of
electronegative N and O atoms. In MEP map of ECNBHPA the reduction in the electrostatic
potential around N9 nitrogen atom of cyanide group and H29 hydrogen atom of pyrrole N–H
indicate the equalization of electrostatic potentials as result of intramolecular hydrogen bonding
N1–H29···N9.
4.8.2. Molar refractivity (MR)
Molar refractivity (MR) is an important property used in quantitative structure property
relationship (QSPR). It is directly related to the refractive index, molecular weight and density of
steric bulk and responsible for the lipophilicity and binding property of investigated system. It
can be calculated by the Lorentz–Lorentz equation [75-77] and defined as:
2
2
MR = [(n – 1) (n + 2)]. (MW / ρ) = 1.333 πNα
Where, n – refractive index, MW – molecular weight, ρ – density, (MW / ρ) – molar volume, N –
Avogadro Number, α – polarizability of molecular system. This equation holds for both liquid
and solid state of system. For a radiation of infinite wavelength, the molar refractivity represents
the real volume of the molecules. Molar refractivity is related, not only to the volume of the
molecules but also to the London dispersive forces that act in the drug–receptor interaction.
Using this equation, the molar refractivity (MR) of investigated molecule ECNBHPA is
calculated as 61.57 esu, which responsible for binding property of hydrazone molecule for
treatment of different diseases. Molar refractivity based binding property of 4–Quinolinyl and 9–
Acrydinyl hydrazones as potent antimalarial agents active against C–Q resistant clone K1
Plasmodium falciparum strain is also well reported in literature [78].

4
.8.3 Electronic reactivity descriptors
4
.8.3.1. Global reactivity descriptors
The chemical reactivity and site selectivity of the molecular systems have been determined on
the basis of Koopman’s theorem [79]. Global reactivity descriptors as electronegativity (χ) = –
1
/2(εLUMO + εHOMO), chemical potential (µ) = 1/2 (εLUMO + εHOMO), global hardness (η) = 1/2
2
(
ε
LUMO – εHOMO), global softness (S) = 1/2η and electrophilicity index (ω) = µ /2η are highly
successful in predicting global reactivity trends [80-84]. According to Parr et al., electrophilicity
index (ω) is a global reactivity index similar to the chemical hardness and chemical potential.
This is positive and definite quantity. This new reactivity index measures the stabilization in
energy when the system acquires an additional electronic charge (∆N) from the environment. The
direction of the charge transfer is completely determined by the electronic chemical potential of
the molecule because an electrophile is a chemical species capable of accepting electrons from
the environments and its energy must decrease upon accepting electronic charge. Therefore, its
electronic chemical potential must be negative. The energies of frontier molecular orbitals
,
(εHOMO, εLUMO), energy gap (εLUMO – εHOMO) electronegativity (χ), chemical potential (µ), global
hardness (η), global softness (S), global electrophilicity index (ω) for (1), (2), ECNBHPA and
ECT for reactant system [(1)↔(2)] are listed in Supplementary material (S Table 8). The global

elecrophilicity index (ω = 6.5733 eV) for ECNBHPA shows that it behaves as a strong
electrophile.
4
.8.3.2 Electrophilicity-Based Charge Transfer (ECT) Descriptor.
Associated with the definition of global electrophilicity, there is an additional and useful
relationship that accounts for the maximum electronic charge ∆Nmax that the electrophile may
accept from the environment. Here, the environment may be represented by either external
effects coming, for instance, from the interaction with the solvent or more simply as field effects
coming from the presence of substituent groups in the molecule. ∆Nmax has been defined as
(
A
∆Nmax) = -µ/η
If we consider the two systems A and B approaching each other, the amount of charge
transfer between them can be written in terms of electrophilicity, that is, electrophilicity-based
A B
charge transfer (ECT) can be written as (ECT) = (∆Nmax) – (∆Nmax) [85] is defined as the
difference between the ∆Nmax values of interacting molecules. If we consider two molecules A
and B approach to each other (i) if ECT > 0, charge flow from B to A (ii) if ECT < 0, charge
flow from A to B. ECT is calculated as 0.3137 i.e ECT > 0 for reactant system [(1)↔(2)], which
indicates that charge flows from (2) to (1). Therefore, (1) acts as electron acceptor (electrophile)
and (2) as electron donor (nucleophile). The high value of elecrophilicity index for (1) than (2)
also favors their electrophilic and nucleophilic behavior, respectively. It is tacitly assumed that
the electrophilicity effect dominates the electronegativity effect.
4
.8.3 .3 Local reactivity descriptors

+
+
+
Selected electrophilic reactivity descriptors (f
k
, s
k k
, ω ) for reactant (1) and
–
–
–
nucleophilic reactivity descriptors (f
given in Table 4. The maximum values of local electrophilic reactivity descriptors (f
at aldehyde carbon C6 of reactant (1) indicates that this is the most electrophilic site. The
k k k
, s , ω ) for reactant (2), using Hirshfeld charges are
+
+
+
k
k k
, s , ω )
–
–
–
k k k
nucleophilic reactivity descriptors (f , s , ω ) analysis of reactant (2) shows that N13 is the
most nucleophilic site. Therefore, the nucleophilic attack of N13 site of reactant (2) at the most
electrophilic site C6 of reactant (1) confirms the formation of product molecule ECNBHPA or
Schiff base linkage (C15=N16) in hydrazone. Selected reactivity descriptors as Fukui functions
+
–
+
–
+
–
(
f
k
, f
k k k k k
), local softnesses (s , s ), local electrophilicity indices (ω , ω ) for ECNBHPA, using
Mulliken charges are given in Table 5. The maximum values of local electrophilic reactivity
+
+
+
k k k
descriptors (f , s , ω ) at vinyl carbon (C6) of ECNBHPA indicate that this site is more prone
to nucleophilic attack and favor formation of the new unsymmetrical dipyrromethanes by attack
of 2–unsubstituted pyrrole nucleophile at (C6). In the same way, for ECNBHPA the maximum
–
–
–
values of the nucleophilic reactivity descriptors (f
k k k
, s , ω ) at nitrogen atom of –CH=N–NH–
part of molecule (N17) indicate that this site is more prone to electrophilic attack.
4
0 0
.9 Static dipole moment (µ ), mean polarizability (|α |), anisotropy of polarizability (∆α) and
first hyperpolarizability (β
0
)
In order to investigate the relationship between molecular structure and NLO response, first
hyperpolarizability (β ) [86-90] of this novel molecular system, and related properties (|α | and
0
0

∆α) are calculated using B3LYP/6–31G(d,p), based on the finite–field approach and their
calculated values are given in Supplementary material (S Table 9).
Large value of particular component of the polarizability and hyperpolarizability
indicate a substantial delocalization of charge in these directions. Since the value of the
polarizability (|α
0 0
|), first hyperpolarizability (β ) of Gaussian 09 output are reported in atomic
unit (a.u.) and these values are converted into electrostatic unit (esu) using converting factors
–
24
–30
as (for α : 1a.u. = 0.1482 x10 esu; for β : 1a.u = 0.008639 x10 esu). The first
0
0
–
30
0
hyperpolarizability (β ) of ECNBHPA is calculated as 43.82 x10 esu. In this study, p–
Nitroaniline (p–NA) is chosen as a reference molecule because there were no experimental
values for the title compound. The p–NA is one of the prototypical molecules used in the study
–30
of the NLO properties of molecular systems (for p–NA, β = 8.5 x10 esu) [91]. Therefore,
investigated molecule will show non–linear optical response and might be used as non–linear
optical (NLO) material.
5
. Conclusions
In this study the analysis of synthesized molecule ECNBHPA by spectroscopic and
1
13
theoretical techniques has been presented. The calculated H and C NMR chemical shifts are in
agreement with the experimental chemical shifts. A combined experimental and theoretical UV–
Visible spectral analysis indicates that observed wavelength absorption maxima (λex.max) have
blue shifts compared to the calculated λex.max values. The molecular orbital coefficients analysis
*
suggests that the nature of all the electronic excitations is n→ . The emission spectrum shows
that title compound is a good photoluminescent material due to intense emission in visible (green

and blue) region at λem.max = 545, 483 nm, respectively. The NBO analysis indicates the
maximum conjugation stabilization energy ~22.68 kcal/mol through π→π* interactions in
pyrrole ring. The interaction n→π* also stabilize the molecule to a greater extent 50.68 kcal/mol.
The red shift in the wavenumber of both proton donor as pyrrole N–H and proton acceptor as
C
N confirms presence of the intramolecular hydrogen bonding N1–H29···N9. AIM
calculation also confirms presence of the intramolecular interaction as N1–H29···N9 due to
2
existence of the bond critical point at H29···N9 contact. The criteria of topological (V ρ
> 0
BCP
and HBCP > 0) and geometrical parameters indicates that the intramolecular interaction N1–
H29···N9 is a weak hydrogen bonding having calculated energy –2.2251 kcal/mol. The global
elecrophilicity index (ω = 6.5733 eV) for ECNBHPA shows that title molecule behaves as a
+
+
+
k k k
strong electrophile. The electrophilic reactivity descriptor (f , s , ω ) analysis for ECNBHPA
indicates that the investigated molecule might be used as precursor for the target synthesis of
new unsymmetrical dipyrromethane derivatives. The compound exhibits strong effective
intramolecular charge transfer (ICT) due to movement of π–electron cloud from donor to
–
30
0
acceptor and shows high hyperpolarizability. The first hyperpolarizability (β = 43.82 x10 esu)
shows that title molecule is an attractive material for non–linear optical (NLO) applications.
Acknowledgement
Authors are thankful to the CSIR for providing financial support

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