Synthesis, molecular structure, multiple interactions and chemical reactivity analysis of a novel ethyl 2-cyano-3-[5-(hydrazinooxalyl- hydrazonomethyl)-1H-pyrrol-2-yl]-acrylate and its dimer: A combined experimental and theoretical (DFT and QTAIM) approach

Article abstract of DOI:10.1016/j.molstruc.2013.01.011

A detailed spectroscopic analyses of a newly synthesized ethyl 2-cyano-3-[5-(hydrazinooxalyl-hydrazonomethyl)-1H-pyrrol-2-yl]-acrylate (3) have been carried out using ¹H and ¹³C NMR, UV-Visible, FT-IR and mass spectroscopic techniques. All the quantum chemical calculations have been carried out using DFT level of theory, B3LYP functional and 6-31G(d,p) as basis set. The ¹H and ¹³C NMR chemical shifts are calculated using gauge including atomic orbitals (GIAOs) approach in DMSO-d ₆. TD-DFT is used to calculate the energy (E), oscillatory strength (f) and wavelength absorption maxima (λ_max) of various electronic transitions and their nature within molecule. Natural bond orbital (NBO) analysis is carried out to investigate the various intra and intermolecular interactions in dimer and their corresponding second order stabilization energy (E⁽²⁾). A combined theoretical and experimental vibrational analysis confirms the existence of dimer and the binding energy of dimer is calculated as 9.21 kcal/mol using DFT calculations. To determine the energy and nature of different interactions topological parameters at bond critical points (BCPs) have been analyzed by Bader's 'atoms in molecules' (AIMs) theory in detail. Electrophilic charge transfer (ECT) is calculated to investigate the relative electrophilic or nucleophilic behavior of reactant molecules involved in chemical reaction. Global electrophilicity index (ω = 5.5836 eV) shows that title molecule (3) is a strong electrophile. The local reactivity descriptors such as Fukui functions (fk+,fk-), local softness (sk+,sk-) and electrophilicity indices (ωk+,ωk-) analyses are performed to determine the reactive sites within molecule.

Full text of DOI:10.1016/j.molstruc.2013.01.011

Journal of Molecular Structure 1037 (2013) 420–430
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Synthesis, molecular structure, multiple interactions and chemical reactivity
analysis of a novel ethyl 2-cyano-3-[5-(hydrazinooxalyl–hydrazonomethyl)-
1H-pyrrol-2-yl]-acrylate and its dimer: A combined experimental and
theoretical (DFT and QTAIM) approach
⇑
R.N. Singh , Amit Kumar, R.K. Tiwari, Poonam Rawat
Department of Chemistry, University of Lucknow, Lucknow 226 007, UP, India
h i g h l i g h t s
"
"
"
"
"
FT-IR spectrum of the compound recorded and compared with the theoretical results.
The hydrogen bonds are characterized using AIM calculations.
Electronic transitions of compound were predicted using TD-DFT method.
The 1H-NMR spectrum has also been studied.
The electronic descriptors have been used to explain chemical reactivity.
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 9 October 2012
Received in revised form 4 January 2013
Accepted 4 January 2013
Available online 18 January 2013
A detailed spectroscopic analyses of a newly synthesized ethyl 2-cyano-3-[5-(hydrazinooxalyl–hydrazo-
nomethyl)-1H-pyrrol-2-yl]-acrylate (3) have been carried out using 1H and 13C NMR, UV–Visible, FT-IR
and mass spectroscopic techniques. All the quantum chemical calculations have been carried out using
DFT level of theory, B3LYP functional and 6-31G(d,p) as basis set. The 1H and 13C NMR chemical shifts
are calculated using gauge including atomic orbitals (GIAOs) approach in DMSO-d6. TD-DFT is used to cal-
culate the energy (E), oscillatory strength (f) and wavelength absorption maxima (kmax) of various elec-
tronic transitions and their nature within molecule. Natural bond orbital (NBO) analysis is carried out to
investigate the various intra and intermolecular interactions in dimer and their corresponding second
order stabilization energy (E(2)). A combined theoretical and experimental vibrational analysis confirms
the existence of dimer and the binding energy of dimer is calculated as 9.21 kcal/mol using DFT calcula-
tions. To determine the energy and nature of different interactions topological parameters at bond critical
points (BCPs) have been analyzed by Bader’s ‘atoms in molecules’ (AIMs) theory in detail. Electrophilic
charge transfer (ECT) is calculated to investigate the relative electrophilic or nucleophilic behavior of
Keywords:
TD-DFT
NBO analysis
Multiple interactions
QTAIM analysis
Reactivity descriptor
reactant molecules involved in chemical reaction. Global electrophilicity index (x = 5.5836 eV) shows
that title molecule (3) is a strong electrophile. The local reactivity descriptors such as Fukui functions
ðfkþ; fkꢀÞ, local softness ðskþ; skꢀÞ and electrophilicity indices ðxkþ
;
xꢀk Þ analyses are performed to determine
the reactive sites within molecule.
Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction
carbonyls [3] and particular in hydrazone Schiff base ligands [4].
The biological activity of hydrazone compounds is due to the pres-
ence of the active (ACOANHAN@CHA) pharmacophore and these
compounds form a significant category of compounds in medicinal
and pharmaceutical chemistry with various biological applications
such as anticonvulsant [5], antidepressant [6], anti-inflammatory
[7], antimalarial [8], antimycobacterial [9], anticancer [10] and
antimicrobial [11–14] activities. All these physiological activities
are attributed to the formation of stable chelate complexes with
Hydrazone compounds are not only effective organic com-
pounds but they are also versatile intermediates for the syntheses
of N-alkyl hydrazides, 1,3,4-oxadiazolines, 2-azetidinones and 4-
thiazolidinones [1,2]. They are used as functional groups in metal
⇑
Corresponding author. Tel.: +91 9451308205.
E-mail address: rnsvk.chemistry@gmail.com (R.N. Singh).
0022-2860/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molstruc.2013.01.011

R.N. Singh et al. / Journal of Molecular Structure 1037 (2013) 420–430
421
transition metals, which catalyze physiological processes [15,16].
They also act as herbicides, insecticides, nematocides, rodenticides,
plant growth regulators and sterilants for houseflies [15–17]. In
analytical chemistry, hydrazones are used for the different colori-
metric or fluorimetric determinations [18,19]. They have strong
coordinating ability towards different metal ions [20,21]. Hydra-
zones are used as prospective new materials for the development
of potential chemosensors [22], opto-electronic applications [23]
and nonlinear optical (NLO) properties [24,25].
Hydrogen bonds are of versatile importance in fields of chemis-
try and biochemistry. They govern chemical reactions, supramolec-
ular structures, molecular assemblies and life processes. Intra and
intermolecular hydrogen bonds are classified in two categories
depending upon the nature of atoms involved in the hydrogen
bridges (i) classical or conventional hydrogen bonds XAHꢁ ꢁ ꢁY,
where, X and Y are more electronegative atoms as O, N, F, Cl, S
and (ii) non-conventional or improper hydrogen bonds such as
CAHꢁ ꢁ ꢁY, where, Y = O, N, F, Cl. Non-conventional hydrogen bonds
are further classified in three categories depending on nature of
hydrogen bond donor and acceptor involved such as (i) those in
which the nature of hydrogen bond donor is non-conventional,
as a CAH group [26], (ii) the nature of hydrogen bond acceptor is
2.1. Synthesis of ethyl 2-cyano-3-[5-(hydrazinooxalyl–hydrazonomethyl)-
1H-pyrrol-2-yl]-acrylate (3)
Ethyl
2-cyano-3-(5-formyl-1H-pyrrol-2-yl)-acrylate
(1)
(0.250 g, 1.1464 mmol) was dissolved in methanol and oxalyl dihy-
drazide (2) (0.1354 g, 1.1464 mmol) in hot water. The solution of
(2) and 0.01 ml of conc. HCl as catalyst were added drop-wise in
solution of (1) and the reaction mixture was refluxed while stirring
for 8 h. The formed yellow color precipitate was filtered off using
vacuum filtration, washed with methanol and dried in air, afforded
(0.150 g, 41.13%) of (3), as yellow color solid. Anal. calcd. for
C
13H14N6O4: C 49.04%, H 4.43%, N 26.41%, obs.: C 49.18%, H
4.48%, N 26.52%. MS (m/z): obs. 319.24 [M++1], calcd. 318.10765.
3. Quantum chemical calculations
All the quantum chemical calculations have been carried out
with Gaussian 09 program package [42] to predict the molecular
structure, 1H, 13C NMR chemical shifts, vibrational wavenumbers
and energy of the optimized structures using B3LYP functional
and 6-31G(d,p) basis set, which invokes Becke’s three parameter
(local, non-local, Hartree–Fock) hybrid exchange functional (B3)
[43], with Lee–Yang–Parr correlational functional (LYP) [44]. The
basis set 6-31G(d,p) with ‘d’ polarization functions on heavy atoms
and ‘p’ polarization functions on hydrogen atoms are used for bet-
ter description of polar bonds of molecule [45,46]. It should be
emphasized that ‘p’ polarization functions on hydrogen atoms
are used for reproducing the out of plane vibrations involving
hydrogen atoms. TD-DFT was used to find the various electronic
transitions and their nature within molecule. The optimized geo-
metrical parameters were 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 [47]. To estimate the enthalpy (H) and Gibbs free
energy (G) values, thermal corrections to the enthalpy and Gibbs
free energy are added to the calculated total energies.
non-conventional, as a C atom or a
p-system [27] and (iii) both
the donor and acceptor are non-conventional groups [28].
Cyanovinyl was employed first by Fisher [26] as protecting group
in formyl pyrrole for the synthesis of 2,5-diformyl-3,4-dimethylpyr-
role and later by Woodward [29] in the synthesis of chlorophyll. The
C-vinylpyrrole fragment is found to be reactive for the target synthe-
sis of conjugated and fused heterocycles similar to natural pyrrole
assemblies [30,31]. The functionalized C-vinylpyrroles are prospec-
tive 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 [32–34].
In observation of above applications cyanovinyl containing
ethyl 2-cyano-3-[5-(hydrazinooxalyl–hydrazonomethyl)-1H-pyr-
rol-2-yl]-acrylate (3) has been synthesized. The product was char-
acterized using 1H and 13C NMR, UV–Visible, FT-IR and mass
spectroscopic techniques. Quantum chemical calculations have
been carried out using DFT to determine the thermodynamic
parameters and the nature of the reaction. The 1H and 13C NMR
chemical shifts and vibrational analysis provided the clue on exis-
tence of intramolecular H-bonding. To investigate the strength and
nature of intra and intermolecular H-bonding, topological and
energetic parameters at bond critical points (BCPs) have been ana-
lyzed using ‘quantum theory of atoms in molecules’ (QTAIMs). Nat-
ural bond orbitals (NBOs) analysis has been carried out to
investigate the intramolecular conjugative and hyperconjugative
interactions within molecule and their second order stabilization
energy (E(2)). The nature of chemical reactivity and site selectivity
of this molecule has been determined on the basis of Global and
Local reactivity descriptors [35–40].
4. Results and discussions
4.1. Thermodynamic properties
For simplicity, the reactants ethyl 2-cyano-3-(5-formyl-1H-pyr-
rol-2-yl)-acrylate and oxalyldihydrazide are abbreviated as (1), (2)
and product ethyl 2-cyano-3-[5-hydrazinooxalyl–hydrazonometh-
yl)-1H-pyrrol-2-yl]-acrylate, byproduct water as (3), (4), respec-
tively. The calculated thermodynamic parameters as Enthalpy (H/
a.u.), Gibbs free energy (G/a.u.) and Entropy [S/(cal/mol K)] of (1),
(2), (3), (4) and their change for Reaction, at 25 °C are listed in Ta-
ble 1. At 25 °C, for non-catalyzed reaction the calculated positive
value of
DH, DG shows that thermodynamically the reaction is
endothermic (i.e. energetically unfavorable) and non-spontaneous.
Thermodynamic relation between equilibrium constant (Keq) and
2. Experimental section
Gibbs free energy change of reaction (DG) at temperature (T) is gi-
D
ven as Keq = eꢀ G/RT. At 25 °C, the equilibrium constant (Keq) for this
reaction is calculated as 0.0530 i.e. Keq ꢃ 1. This indicates that the
reaction is favored in backward direction and does not favor the
All the chemicals were used of analytical grade. Ethyl 2-cyano-
3-(5-formyl-1H-pyrrol-2-yl)-acrylate (1) was prepared by an ear-
lier reported method [41]. Oxalic acid dihydrazide was prepared
by stirring the equimolar reaction mixture of diethyl oxalate and
hydrazine hydrate in ethanol. The Mass spectrum was recorded
on JEOL-Acc TDF JMS-T100LC, Accu TOF mass spectrometer. The
1H NMR spectrum of (3) was recorded in DMSO-d6 on Bruker
DRX-300 spectrometer using TMS as an internal reference. The
FT-IR spectrum was recorded in KBr medium on a Bruker spec-
Table 1
Calculated thermodynamic parameters: Enthalpy (H/a.u.), Gibbs free energy (G/a.u.)
and Entropy [S/(cal/mol-K)] of (1), (2), (3), (4) and their change for Reaction, at 25 °C.
(1)
(2)
(3)
(4)
Reaction
H
G
S
ꢀ760.154972 ꢀ449.134085 ꢀ1132.8944 ꢀ76.394588
ꢀ760.21544 ꢀ449.179337 ꢀ1132.976 ꢀ76.416024
D
D
D
H
G
S
0.000085
0.002772
ꢀ5.653
trometer. The UV–Visible absorption spectrum of (3), (1 ꢂ 10ꢀ5
M
127.264
95.241
171.736
45.116
in DMSO) was recorded on ELICO SL-164 spectrophotometer.

422
R.N. Singh et al. / Journal of Molecular Structure 1037 (2013) 420–430
Table 2
Table 6
Experimental and calculated 1H NMR chemical shifts (d/ppm) of (3) in DMSO-d6 as
the solvent at (25 °C).
Geometrical parameters for various intra and intermolecular interactions in dimer of
(3): bond distance (Å), bond angle (°) and sum of van der Wall radii of interacting
atoms (Å).
Atom
dcalcd
dexp
Assignment
Interactions
dDAH
dHꢁ ꢁ ꢁA
dDꢁ ꢁ ꢁA
DAHꢁ ꢁ ꢁA
(rH + rA)
H24
H25
H26
H27
H28
H29
H30
H31
H32
H33
H34
H35
10.3229
7.016
12.962
6.438
6.738
8.381
4.249–4.319
(s, 1H, pyrroleANH)
(d, 2H, pyrroleACH)
(d, 2H, pyrroleACH)
(s, 1H, vinylACH@C)
(q, J = 7.00 Hz, 2H, esterACH2)
N17AH34ꢁ ꢁ ꢁO58
N54AH71ꢁ ꢁ ꢁO21
N22AH35ꢁ ꢁ ꢁO19
N59AH72ꢁ ꢁ ꢁO56
C15AH33ꢁ ꢁ ꢁO58
C52AH70ꢁ ꢁ ꢁO21
N1AH24ꢁ ꢁ ꢁN9
1.02428
1.02428
1.01325
1.01325
1.09224
1.09224
1.01402
1.01402
2.00471
2.00469
2.20473
2.20475
2.32963
2.32937
2.46732
2.46734
2.97375
2.97381
2.65111
2.65112
3.23947
3.23945
3.32171
3.32173
156.81787
156.83234
104.79458
104.79375
139.57455
139.59992
141.53314
141.53307
2.72
2.72
2.72
2.72
2.72
2.72
2.75
2.75
6.7681
7.9079
4.3046
4.3044
1.2574
1.4751
1.4751
7.6878
10.0289
7.9292
1.273–1.320
(t, J = 7.05 Hz, 3H, esterACH3)
N38AH61ꢁ ꢁ ꢁN46
7.434
12.539
11.911
(s, 1H, ACH@N)
(s, 1H, hydrazideAC@NANH)
(s, 1H, hydrazideANHANH2)
(s, 1H, ANH2)
(s, 1H, ANH2)
The optimized geometry for ground state lower energy conformer
of monomer (3) and its dimer are shown in Fig. 1 and 2, respec-
tively. Selected optimized geometrical parameters from dimer of
(3), calculated at B3LYP/6-31G(d,p) are listed in Supplementary
material (S Table 1). The molecule possesses C1 symmetry. The
asymmetry of the N1AC2 and N1AC5 bonds i.e. difference between
bond lengths can be explained due to the presence of the two dif-
ferent groups as cyanovinyl and hydrazone at C2, C5 of pyrrole
ring, respectively. The E-configuration about the vinyl C6@C7 bond
with respect to the ester and pyrrole give lower energy conformer.
The molecule also exist in E-configuration with respect to the pyr-
role and ANHACOACOANHANH2 group located on the opposite
side of the C15@N16 bond. In dimer, heteronuclear intermolecular
classical hydrogen bonding (NAHꢁ ꢁ ꢁO) between hydrazide (NAH)
and carbonyl (C@O) oxygen form double hydrogen bonded dimer.
In cyclic ester dimer both monomer unit exist in E-configuration
about C15@N16 bond, where (NAH) bond as proton donor and
(C@O) bond as proton acceptor. According to the Etter terminology
[48] the cyclic acid hydrazide dimer form the ten membered ring
denoted as R22ð10Þ. The superscript designates the number of
acceptor centers and subscript the number of donors in the motif.
In dimer, due to the classical intermolecular hydrogen bonding
(NAHꢁ ꢁ ꢁO) both proton donor (NAH bond) and proton acceptor
(C@O bond) are elongated by 0.0030 Å and 0.0048 Å, respectively,
whereas due to the non-conventional intermolecular hydrogen
bonding (CAHꢁ ꢁ ꢁO) the CAH bond is contracted by 0.0034 Å. Total
energy of the ground state lower energy conformer of monomer (3)
and its dimer is calculated as ꢀ1133.198189, ꢀ2266.396377 a.u.,
respectively. Therefore, the binding energy of dimer is calculated
as 9.21 kcal/mol after correction in energy due to basis set super-
position error (BSSE) [49].
H36
H37
3.2972
3.2972
4.844
4.556
Table 3
Experimental and calculated 13C NMR chemical shifts (d/ppm) of (3) in DMSO-d6 as
the solvent at (25 °C).
Atom
dcalcd
dexp
Assignment
C2
C3
C4
C5
C6
128.69
126.12
116.5
135.95
141.35
125.74
122.74
118.90
128.78
136.16
(pyrrole ring)
(pyrrole ring)
(pyrrole ring)
(pyrrole ring)
(VinylicACH@CCN)
(ACCN)
C7
93.15
115.36
160.74
65.547
16.747
134.67
150.69
158.35
116.32
125.78
160.55
52.92
C8
(ACN)
C10
C13
C14
C15
C18
C20
(esterACOO)
(esterACH2)
11.67
(ester CH3)
137.39
143.36
152.61
(hydrazoneACH@NANH
(amidicANHACOCONH)
(amidicANHACOCONH)
Table 4
Comparison between experimental and calculated electronic transitions of (3): E/eV,
oscillatory strength (f), (kmax/nm) at TD-DFT/B3LYP/6-31G(d,p) level.
S. no. Excitation E (eV)
(f)
kmax calcd. kmax exp. Assignment
1
2
H ? L
HA4 ? L
2.9873 0.9026 415.04
4.5736 0.4314 271.09
402
251
p
p
?
?
pꢄ
pꢄ
formation of product (3). Therefore, reaction was carried out at re-
flux temperature and in the presence of acid as catalyst.
4.3. NMR and mass spectroscopy
4.2. Molecular geometry
The geometry of the title compound, together with that of tet-
ramethylsilane (TMS) is fully optimized. 1H and 13C NMR chemical
shifts are calculated with GIAO approach at B3LYP/6-31G(d,p)
method [50]. Chemical shift of any ‘x’ proton (dX) is equal to the
Optimized geometry of all the reactants (1,2) and product (3,4)
involved in chemical reaction is shown graphically in Scheme 1.
Table 5
Topological parameters for various intra and intermolecular interactions in dimer of (3): electron density (
density (GBCP), electron potential energy density (VBCP), total electron energy density (HBCp), estimated interaction energy (Eint) at bond critical point (BCP).
q
BCP), Laplacian of electron density (r2qBCP), electron kinetic energy
Interactions
qBCP
r
2qBCP
GBCP
VBCP
HBCP
Eint
N17AH34ꢁ ꢁ ꢁO58
N54AH71ꢁ ꢁ ꢁO21
N22AH35ꢁ ꢁ ꢁO19
N59AH72ꢁ ꢁ ꢁO56
C15AH33ꢁ ꢁ ꢁO58
C52AH70ꢁ ꢁ ꢁO21
N1AH24ꢁ ꢁ ꢁN9
0.02048
0.02048
0.01950
0.01950
0.01327
0.01327
0.01162
0.01162
0.08255
0.08255
0.06591
0.06590
0.03975
0.03977
0.04223
0.04223
0.01624
0.01624
0.01830
0.01830
0.00971
0.00971
0.00872
0.00872
ꢀ0.01601
ꢀ0.01602
ꢀ0.01596
ꢀ0.01596
ꢀ0.00949
ꢀ0.00949
ꢀ0.00688
ꢀ0.00688
0.000225
0.000228
0.002337
0.002337
0.000224
0.000222
0.001839
0.001838
ꢀ5.02631
ꢀ5.02662
ꢀ5.00857
ꢀ5.00838
ꢀ2.97765
ꢀ2.97969
ꢀ2.15903
ꢀ2.15895
N38AH61ꢁ ꢁ ꢁN46
qBCP
,
r
2qBCP, GBCP, VBCP, HBCP in eV and Eint in (kcal/mol).

R.N. Singh et al. / Journal of Molecular Structure 1037 (2013) 420–430
423
Table 7
Calculated eHOMO
,
e
LUMO, energy band gap (eL
–
e
H), chemical potential (
l
), electronegativity (
v
), global hardness (
g
), global softness (S) and global electrophilicity index (
x
) for (1),
(2), and (3), Elecrophilicity based charge transfer (ECT) for reactant system [(1) M (2)].
eH
eL
(eL
–eH
)
v
l
g
S
x
ECT
(1)
(2)
(3)
ꢀ6.5133
ꢀ6.7803
ꢀ5.9011
ꢀ2.8547
ꢀ1.1461
ꢀ2.6370
3.6585
5.6341
3.2640
4.6840
3.9632
4.2690
ꢀ4.6840
ꢀ3.9632
ꢀ4.2690
1.8292
2.8170
1.6320
0.2733
0.1774
0.3063
5.9969
2.7878
5.5836
1.1537
eH, eL, (eH–eL), v, l, g, x
(in eV) and S (in eVꢀ1).
Table 8
Selected electrophilic reactivity descriptors ðfkþ; sþk ; xþk Þ for reactant (1) and nucleophilic reactivity descriptors ðfkꢀ; sꢀk ; xꢀk Þ for reactant (2) using Hirshfeld atomic charges.
Reactant (1)
Reactant (2)
fkþ
skþ
xkþ
fkꢀ
Sites
Sites
skꢀ
xkꢀ
From Mulliken atomic charges
C6
C12
0.063177
0.05621
0.017268
0.015363
0.378871
0.33709
N5
N6
N7
N8
0.056658
0.119492
0.066731
0.131873
0.010056
0.021207
0.011843
0.023405
0.157954
0.333126
0.186036
0.367642
From Hirshfeld atomic charges
C6
C12
0.117083
0.038475
0.032001
0.010516
0.702143
0.230733
N5
N6
N7
N8
0.120217
0.241103
0.135814
0.263275
0.021336
0.042791
0.024104
0.046726
0.335147
0.672159
0.378629
0.733971
fkþ; fkꢀ (in e); skþ; skꢀ (in eVꢀ1) and xkþ xꢀk (in eV).
;
Table 9
Selected reactivity descriptors as Fukui functions ðfkþ; fkꢀÞ, local softnesses ðskþ; sꢀk Þ, local electrophilicity indices ðxkþ; xꢀk Þ for (3).
fkþ
skþ
xkþ
fkꢀ
Sites
Sites
skꢀ
xkꢀ
From Mulliken atomic charges
C6
C7
C10
C15
C18
C20
0.057402
0.041069
0.045228
0.044804
0.040774
0.025841
0.017586
0.012582
0.013856
0.013726
0.012492
0.007917
0.320512
0.229314
0.252536
0.250169
0.227667
0.144287
N1
0.000496
0.033004
0.00441
0.000152
0.010111
0.001351
0.004522
0.002769
0.184282
0.024624
0.082426
N17
N22
N23
0.014762
From Hirshfeld atomic charges
C6
C7
C10
C15
C18
C20
0.100417
0.058778
0.030737
0.080058
0.042448
0.018155
0.030764
0.018007
0.009417
0.024527
0.013004
0.005562
0.560691
0.328195
0.171624
0.447014
0.237014
0.101371
N1
0.023362
0.070759
0.018756
0.032425
0.007157
0.021678
0.005746
0.009934
0.130445
0.395092
0.104727
0.181049
N17
N22
N23
fkþ; fkꢀ (in e); skþ; skꢀ (in eVꢀ1) and xkþ xꢀk (in eV).
;
difference between isotropic magnetic shielding (IMS) of TMS and
proton (x). It is defined by the equation: dX = IMSTMS–IMSX. The
experimental and calculated 1H and 13C NMR chemical shifts (d/
ppm) of (3) in DMSO-d6 as the solvent at 25 °C are given in Tables
2 and 3 and shown in Fig. 3 and 4 respectively. The experimental
1H NMR data was consistent with the calculated values from as-
signed structure. In 1H NMR spectrum of the compound, the
ACH2 proton was observed as quartet in range 4.249–4.319 ppm.
The CH3 protons appeared as singlet at 1.32 ppm. The AN@CHA
proton was observed as a singlet at 7.43 ppm indicating hydrazone
exists as E geometric isomer about the C@N double. The vinylic
ACH@CA proton was observed as a singlet at 8.381 ppm. The
@NANHACOACOANHANH2 protons were observed as singlet at
12.539, 11.911 ppm, respectively. In order to compare the chemi-
cal shifts, correlation graphs between the experimental and calcu-
lated 1H NMR chemical shifts was drawn and shown in
Supplementary material (S Fig. 1). The correlation graph follows
the linear equation: y = 1.2456x – 0.4481, where ‘x’ is the calcu-
lated 1H NMR chemical shift, ‘y’ is the experimental 1H NMR chem-
ical shift (d in ppm). The correlation coefficients (R = 0.89) shows
that there is an agreement between experimental and calculated
chemical shifts.
Additional support for the structure of the synthesized com-
pound was provided by its 13C NMR spectrum, in which chemical
shift values of the carbon atoms at around 150–158 ppm (hydra-
zide C@O), and 136 ppm (imine N@CH) corroborated the hydra-
zide character deduced from the 1H NMR data. Similarly, in
order to compare the chemical shifts, the correlation graph of
13C NMR are given in Supplementary material (S Fig. 2) shows
good agreement between experimental and calculated chemical
shifts with correlation coefficients (R = 0.97) and follows the lin-
ear equation: y = 0.99152x ꢀ 0.16491, where ‘x’ is the calculated
13C NMR chemical shift, ‘y’ is the experimental 13C NMR chemical
shift (d in ppm).
The Experimental Mass spectrum is given in Fig. 5. The Mass
spectrum of compound showed intense peak at m/z: 319 due to
[M+1] having in agreement with its molecular formula weight
(318). Other peaks at 266, 215, 178 and 123 are due to the frag-
mentations and rearrangements which are given in the Supple-
mentary material (S Fig. 3).

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R.N. Singh et al. / Journal of Molecular Structure 1037 (2013) 420–430
Fig. 1. Optimized geometry for ground state lower energy conformer of monomer (3).
Fig. 2. Optimized geometry for dimer of (3).
4.4. UV–Visible spectroscopy
molecular orbital coefficients analysis and molecular orbital plots,
the nature of both H ? L and HA4 ? L electronic excitations are
assigned to be
The nature of the transitions observed in the UV–Visible spec-
trum of the title compound has been studied by TD-DFT. The ob-
served and calculated electronic transitions of higher oscillatory
strengths are listed in Table 4. The experimental UV–Visible spec-
trum of (3) is shown in Fig. 6. HOMO–LUMO and their vicinal
molecular orbital plots involved in the higher oscillator strength
electronic excitations are shown in Supplementary material (S
Fig. 4) for (3). The HOMO–LUMO energy gap for (3), calculated to
be as below:
p ? .
pꢄ
4.5. Natural bond orbitals (NBOs) analysis
NBO provides an accurate method for studying interaction and
also gives an efficient basis for investigating charge transfer or con-
jugative interaction in various molecular systems [51]. The large
value of second order stabilization energy (E(2)) shows that the
interaction is more intense between electron donors and electron
acceptors, i.e., the more donating tendency from electron donors
to electron acceptors and the greater the extent of conjugation of
the whole system. The delocalization of electron density between
occupied Lewis-type (bond or lone pair) NBO orbitals and formally
unoccupied (antibond or Rydgberg) non-Lewis NBO orbitals corre-
spond to a stabilizing donor–acceptor interaction. In order to char-
acterize the intra and intermolecular interactions quantitatively, a
second-order perturbation theory is applied that gives the energy
lowering associated with such interactions. For each donor NBO(i)
and acceptor NBO(j), the strength of various types of interactions
or stabilization energy (E(2)) associated with electron delocaliza-
tion between donor and acceptor is estimated by the second order
energy lowering equation and it is described below [52,53].
HOMO energy ¼ ꢀ5:9011 eV
LUMO energy ¼ ꢀ2:6370 eV
HOMO—LUMOenergy gap ¼ 3:2640 eV
TD-DFT calculations at B3LYP/6-31G(d,p) method predict two
electronic excitations at kmax = 415.04 nm, kmax = 271.09 nm, are
in agreement with the observed electronic excitations at kmax
402 and 251 nm, respectively. TD-DFT calculations show that the
experimental bands at 402 and 251 nm originate mainly due to
H ? L, HA4 ? L excitations, respectively. On the basis of calculated

R.N. Singh et al. / Journal of Molecular Structure 1037 (2013) 420–430
425
Fig. 3. Experimental 1H NMR spectrum of (3).
bonding (p
) and antibonding (pꢄ) orbitals, which results in intra-
2
ðFijÞ
E2 ¼ q
ð1Þ
i ej
ꢀ
ei
molecular charge transfer (ICT) causing stabilization of the system.
The primary hyperconjugative interactions n1(O19) ? rꢄ
(N22AH35) are responsible for intramolecular hydrogen bonding
N22AH35ꢁ ꢁ ꢁO19. Another primary hyperconjugative interactions
n1(N9) ? rꢄ(C7AC8) and n2(O11) ? rꢄ(C10AO12) are stabilized
to the molecule up to ꢅ32.35 kcal/mol. The secondary hyperconju-
gative interactions associated with the pyrrole ring such as
where qi is the population of donor orbital or donor orbital occu-
pancy; ei j are orbital energies of donor and acceptor NBO orbitals,
respectively; Fij is the off-diagonal Fock or Kohn–Sham matrix ele-
ment between i and j NBO orbitals.
Various intra and intermolecular interactions are generated due
to the different types of ‘‘orbital–orbital’’/‘‘loan pair-orbital’’ over-
,
e
r
(N1AC2) ? rꢄ(C5AC15),
r
(N1AC5) ? rꢄ(C2AC6),
r(C2AC3) ?
rꢄ(C4AH26), (C3AC4) ? rꢄ(C5AC15),
r
r
(C4AC5) ? rꢄ(C3AH25)
lap. The
tion in ring is involved due to the
primary hyperconjugative interactions due to the various types of
orbital overlaps such as
rꢄ, n ? rꢄ and secondary
rꢄ orbital overlap
p
-conjugation and resonance due to
p-electron delocaliza-
are stabilized to the molecule within range 3.77–5.49 kcal/mol.
Second order perturbation theory analysis of the Fock matrix in
NBO basis for dimer of (3): (i) from monomer unit 1 to unit 2 and
(ii) from monomerunit 2 to unit 1, are given in Supplementary mate-
rial (S Table 3). In dimer, the primary hyperconjugative interactions
from monomer unit (1) to unit (2) due to n1(O21)/n2(O21) ? rꢄ
(N54AH71) stabilized the molecule up to ꢅ7.74 kcal/mol and
responsible for the presence of classical hydrogen bonding
N54AH71ꢁ ꢁ ꢁO21. Another weak intermolecular interaction
n1(O21)/n2(O21) ? rꢄ(C52AH70) indicates the presence of non-
conventional hydrogen bonding C52AH70ꢁ ꢁ ꢁO21 and stabilized
the molecule up to ꢅ1.58 kcal/mol. In the same way, intermolecular
primary hyperconjugative interactions n1(O58)/n2(O58) ? rꢄ
(N17AH34) and n1(O58)/n2(O58) ? rꢄ(C15AH33) confirm the
presence of conventional hydrogen bonding N17AH34ꢁ ꢁ ꢁO58 and
non-conventional hydrogen bonding C15AH33ꢁ ꢁ ꢁO58, respectively.
Selected Lewis orbitals (occupied bond or lone pair) of (3)
with their valence hybrids are listed in Supplementary material
(S Table 4). The valence hybrids analysis of NBO orbitals shows that
all the NAH/CAN and CAO bond orbitals are polarized towards the
nitrogen (ED = 57.40–74.61% at N), oxygen (ED = 64.54–64.88% at
p
?
pꢄ interactions, whereas the
r
? , p ?
pꢄ
hyperconjugative interactions due to the
r ?
[54].
Second-order perturbation theory analysis of the Fock matrix in
NBO basis for monomer unit 1 within dimer of (3) is presented in
Supplementary material (S Table 2). The interactions
p
(C2AC3) ? pꢄ(C4AC5) and (C4AC5) ? pꢄ(C2AC3) are responsi-
p
ble for the conjugation of respective
p
p
-bonds in pyrrole ring. The
bonds (1.66728–1.67727)
electron density at the conjugated
and pꢄ bonds (0.42118–0.43114) of pyrrole ring indicate strong
p
-electron delocalization within ring leading to a maximum stabil-
ization of energy up to ꢅ22.89 kcal/mol. The interactions
(C2AC3) ? pꢄ(C6AC7), (C6AC7) ? pꢄ(C2AC3) are responsible
p
p
for the conjugation of bonds C2AC3 and C6AC7 with C2AC6 and
stabilized the molecule up to ꢅ24.75 kcal/mol. In the same man-
ner, the interactions
p p(C15AN16) ?
(C4AC5) ? pꢄ(C15AN16),
pꢄ(C4AC5) are responsible for the conjugation of bonds C4AC5
and C15AN16 with C5AC15. It is to be noticed that the charge
transfer interactions are formed by the orbital overlap between

426
R.N. Singh et al. / Journal of Molecular Structure 1037 (2013) 420–430
Fig. 4. Experimental 13C NMR spectrum of (3).
O), respectively. The electron density distribution (occupancy)
around the imino group (˜NAH) 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.
numbers using potential energy distribution (PED) analysis of the
various vibrational modes.
NAH Vibrations
In the experimental FT-IR spectrum of (3), the NAH stretch of
hydrazide (
mN22AH35) and pyrrole (mN1AH24) are observed at 3421,
4.6. Vibrational assignments
3316 cmꢀ1, whereas these are calculated at 3460, 3451 cmꢀ1
,
respectively. It is to be noted that the calculated closest wavenum-
bers at 3460, 3451 cmꢀ1 are merged and appeared only one peak in
the theoretical IR-spectrum. The NAH stretch of hydrazide
The experimental and theoretical (selected) vibrational wave-
numbers of dimer of (3), calculated at B3LYP/6-31G(d,p) method
and their assignments using PED are given in Supplementary mate-
rial (S Table 5). Comparison among experimental and theoretical
(monomer and dimer) IR spectra in the region 4000–400 cmꢀ1 is
shown in Fig. 7. The total number of atoms (n) in monomer (3)
and its dimer are 37, 74 respectively. Therefore, they give 105
and 216, (3n ꢀ 6) vibrational modes, respectively. The calculated
vibrational wavenumbers are higher than their experimental val-
ues 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 wave-
numbers are scaled down using scaling factor 0.9608 [55], to dis-
card the anharmonicity present in real system. The observed
wavenumbers are assigned by comparing the calculated wave-
(m
N17AH34) is observed at 3201 cmꢀ1, whereas it is calculated at
3337 cmꢀ1 in monomer and 3293 cmꢀ1 in dimer. Therefore, the
observed wavenumber at 3201 cmꢀ1 are in good agreement with
the calculated wavenumber of dimer and indicates the involve-
ment of N17AH34 group in hydrogen bonding in the solid phase.
In theoretical IR-spectrum, the calculated wavenumbers at 3400,
3321 cmꢀ1 are assigned to the asymmetric and symmetric stretch-
ing vibration of NH2 group, respectively. The asymmetric and sym-
metric stretching vibrations of NH2 group are also reported in
literature at 3477, 3274 cmꢀ1, respectively [56].
C@O and CAO Vibrations
In the experimental FT-IR spectrum, the stretching vibration of
ester carbonyl (m
C10@O11) is observed at 1699 cmꢀ1, whereas it is

R.N. Singh et al. / Journal of Molecular Structure 1037 (2013) 420–430
427
Fig. 5. Experimental DART Mass spectrum of (3).
calculated at 1723 cmꢀ1. The stretching vibration of hydrazide car-
bonyl (
C20@O21) is observed at 1659 cmꢀ1, whereas it is calculated
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0.00
-0.05
m
at 1681 cmꢀ1 in dimer and 1738 cmꢀ1 in monomer. Therefore, the
observed wavenumber at 1659 cmꢀ1 agrees well with the calcu-
lated wavenumber of dimer and indicate the involvement of
C20@O21 carbonyl group in hydrogen bonding. The ‘‘CAO stretch-
ing vibrations’’ of esters actually consists of two asymmetrical cou-
pled vibrations as OAC(@O)AC and OACAC and these bands occur
in the region 1300–1000 cmꢀ1 [57]. An observed ester CAC(@O)AO
stretching at 1198 cmꢀ1 corresponds to the calculated wavenum-
ber at 1235 cmꢀ1. The wavenumber calculated at 1082 cmꢀ1 is
demonstrated to the OACAC stretching vibration of ester group
and corresponds to the observed wavenumber at 1059 cmꢀ1 in
the experimental FT-IR spectrum. The calculated wavenumber at
826 cmꢀ1 is attributed to the ester CAOAC deformation mode with
8% contribution in PED and observed at 791 cmꢀ1
.
200
250
300
350
400
450
500
550
600
CAH Vibration
Wavelength (nm)
Fig. 6. Experimental UV–Visible spectrum of (3).
According to the Internal coordinate system recommended by
Pulay et al. [58], CH2 group associate with six types of vibrational
frequencies namely: symmetric stretch, asymmetric stretch, scis-
soring, rocking, wagging and twisting. The scissoring and rocking
deformations belong to polarized in-plane vibration, whereas wag-
ging and twisting deformations belong to depolarized out-of-plane
vibration. The calculated wavenumber at 2986 cmꢀ1 is assigned to
the asymmetric CAH stretching vibrations of ester CH2 group. The

428
R.N. Singh et al. / Journal of Molecular Structure 1037 (2013) 420–430
Experimental
Monomer
Dimer
500
1000
1500
2000
2500
3000
3500
4000
-1
Wavenumbers (cm )
Fig. 7. Comparison among experimental and theoretical (monomer and dimer) IR spectra for (3).
Scheme 1. Optimized geometry of reactants (1, 2), product (3) and byproduct water (4).
symmetric CAH stretching vibration of ester CH2 is observed at
2853 cmꢀ1, whereas it is calculated at 2945 cmꢀ1. The calculated
wavenumbers at 1470, 1246 cmꢀ1 are demonstrated to the scissor-
ing and twisting mode of ester CH2 group, respectively. The ob-
served wagging mode of ester CH2 at 1367 cmꢀ1 corresponds to
the calculated wavenumber at 1349 cmꢀ1 in the theoretical FT-IR
spectrum. According to the Internal coordinate system recom-
mended by Pulay et al. [58], CH3 group associate with different
vibrational modes namely: symmetric stretch, asymmetric stretch,
symmetric deformation, asymmetric deformation and rocking
mode. The calculated wavenumber at 2938 cmꢀ1 is attributed to
the symmetric stretching of ester methyl (Me). The calculated
wavenumber at 3019 cmꢀ1 is demonstrated to the asymmetric
stretching vibration of Me and it is observed at 2954 cmꢀ1 in the
experimental FT-IR spectrum.
is attributed to the stretching vibration of vinyl C@C and corre-
spond to the calculated wavenumber at 1574 cmꢀ1 with 17% con-
tribution in PED.
C@N and NAN vibration
The observed C@N stretching vibration (m
C@N) at 1591 cmꢀ1
matches well with the calculated wavenumber at 1599 cmꢀ1. In
FT-IR spectrum, the presence of the C@N band confirms the pres-
ence of hydrazone linkage in the investigated molecule (3). The ob-
served band at 1595 cmꢀ1 correlates with the reported
wavenumber at 1602 cmꢀ1 [59], 1655 cmꢀ1 [60]. The NAN stretch-
ing vibration is observed at 1096 cmꢀ1, whereas it is calculated at
1131 cmꢀ1 in theoretical IR-spectrum with 9% contribution in PED.
The NAN stretching vibration is also reported in literature at
1108 cmꢀ1 [59], 1107 cmꢀ1 [60].
CAC Vibration
The observed CAC stretch of pyrrole at 1407 cmꢀ1 agrees well
with the calculated wavenumber at 1406 cmꢀ1 with 10% contribu-
tion in PED. An observed combination band of ‘CAC stretches of
pyrrole ring and N53AH70AC52 deformations’ at 1325 cmꢀ1
matches with the calculated wavenumber at 1337 cmꢀ1. The ob-
served CACAC deformations associated with pyrrole ring at 779,
630 cmꢀ1 correspond to the calculated wavenumber at 796,
632 cmꢀ1, respectively. The observed wavenumber at 1549 cmꢀ1
4.7. Quantum theory of atoms in molecules (QTAIMs) analysis
Geometrical as well as topological parameters are useful tool to
characterize the strength of hydrogen bond [61]. They are fre-
quently considered as insufficient, therefore, existence of hydrogen
bond could be supported further by Koch and Popelier criteria [62]
based on ‘atoms in molecules’ theory and has been classified in de-
tail by Rozas et al. [63].

R.N. Singh et al. / Journal of Molecular Structure 1037 (2013) 420–430
429
Molecular graph of the dimer is shown in Supplementary mate-
rial (S Fig. 5). Topological as well as geometrical parameters for
bonds of interacting atoms in dimer are given in Table 5 and 6,
respectively. In dimer, for all the interactions electron density
indicate that this site is more prone to nucleophilic attack and fa-
vor the formation of the unsymmetrical dipyrromethane by attack
of 2-unsubstituted pyrrole nucleophile at C6. In the same way, the
maximum values of local nucleophilic reactivity descriptors
ðfkꢀ; sꢀk ; xꢀk Þ at hydrazide nitrogen atom (N17) for (3) indicate that
this site is more prone to electrophilic attack.
(q
H. . .A) and its Laplacian (
2qBCP) are in the range 0.01162–
r
0.02048, 0.03977–0.08255 a.u., respectively. Therefore, all these
interactions follow the Koch and Popelier criteria. There exist var-
ious types of interactions as (i) N17AH34ꢁ ꢁ ꢁO58, N54AH71ꢁ ꢁ ꢁO21
are weak intermolecular classical hydrogen bonds, (ii)
C15AH33ꢁ ꢁ ꢁO58, C52AH70ꢁ ꢁ ꢁO21 are weak intermolecular non-
conventional hydrogen bonds and (iii) N22AH35ꢁ ꢁ ꢁO19,
N59AH72ꢁ ꢁ ꢁO56, N1AH24ꢁ ꢁ ꢁN9, N38AH61ꢁ ꢁ ꢁN46 are weak intra-
molecular classical hydrogen bonds. The various type of interac-
tions visualized in molecular graph are classified on the basis of
geometrical, topological and energetic parameters. Espinosa pro-
posed proportionality between hydrogen bond energy (E) and po-
tential energy density (VBCP) at Hꢁ ꢁ ꢁO contact: E = 1/2 (VBCP) [64].
According to AIM calculations, the strength of various intra and
intermolecular hydrogen bond in this molecule are in the following
order as N17AH34ꢁ ꢁ ꢁO58 ꢆ N54AH71ꢁ ꢁ ꢁO21˜N22AH35ꢁ ꢁ ꢁO19
ꢆ N59AH72ꢁ ꢁꢁO56˜C15AH33ꢁ ꢁꢁO58 ꢆ C52AH70ꢁ ꢁꢁO21˜N1AH24ꢁꢁꢁ
N9 ꢆ N38AH61ꢁ ꢁ ꢁN46.
5. Conclusions
The title compound (3) is synthesized and characterized by var-
ious spectroscopic techniques. The calculated 1H NMR and 13C
NMR chemical shifts are in agreement with the observed chemical
shifts experimentally. The TD-DFT/B3LYP calculations show that
observed wavelength absorption maxima (kmax) have some blue
shifts compared with the calculated kmax. A combined molecular
orbital coefficients analysis and molecular orbital plots suggest
that the nature of electronic excitations is
p ?
pꢄ. Natural bond
orbitals (NBOs) analysis exhibits the presence of various intra
and intermolecular interactions and hydrogen bonding such as
N22AH35ꢁ ꢁ ꢁO19/N59AH72ꢁ ꢁ ꢁO56,
N17AH34ꢁ ꢁ ꢁO58/
N54AH71ꢁ ꢁ ꢁO21 and C15AH33ꢁ ꢁ ꢁO58/C52AH70ꢁ ꢁ ꢁO21 in dimer
and their corresponding second order stabilization energy (E(2)).
A combined experimental and theoretical vibrational analysis con-
firms the existence of dimer through heteronuclear hydrogen
bonding (NAHꢁ ꢁ ꢁO/CAHꢁ ꢁ ꢁO) between hydrazide NAH, Schiff base
CAH as proton donor and carbonyl oxygen as proton acceptor. On
the basis of topological, geometrical and energetic parameters
using AIM calculations, the nature of all the hydrogen bonds is
weak and their energy is calculated within range 2.97765–
5.02662 kcal/mol. The ectrophilic reactivity descriptors for reac-
tant (1) and nucleophilic reactivity descriptors for reactant (2)
are in agreement for the formation of product molecule (3). The
4.8. Chemical reactivity
Global reactivity descriptors
The chemical reactivity and site selectivity of the molecular sys-
tems have been determined on the basis of Koopman’s theorem
[35]. Global reactivity descriptors as electronegativity (
2(eLUMO HOMO), chemical potential ( ) = 1/2 (eLUMO HOMO), glo-
bal hardness ( ) = 1/2 (eLUMO HOMO), global softness (S) = 1/2
and electrophilicity index ( ) = are highly successful in pre-
2/2
dicting global reactivity trends [35–41]. The energies of frontier
molecular orbitals (eHOMO LUMO), energy gap (eLUMO HOMO), elec-
tronegativity ( ), chemical potential ( ), global hardness ( ), global
softness (S), global electrophilicity index ( ) for (1), (2), (3) and
ECT for reactant system [(1) M (2)] are listed in Table 7. The global
elecrophilicity index ( = 5.4425 eV) for (3) shows that the prod-
v) = ꢀ1/
+
e
l
+ e
g
ꢀ
e
l
g
x
g
global elecrophilicity index (x = 5.5836 eV) for (3) shows that the
,
e
ꢀ
e
product to be a strong electrophile. The electrophilic reactivity
descriptors analyses of (3) indicate that the investigated molecule
might be used as precursor for the target syntheses of unsymmet-
rical dipyrromethane derivatives.
v
l
g
x
x
uct to be as a strong electrophile.
Acknowledgement
Electrophilic charge transfer (ECT) = (
D
Nmax)A–(DNmax)B [65] is
defined as the difference between the
DNmax values of interacting
The authors are thankful to DST and CSIR funding agencies for
research work.
molecules. If we consider two molecules A and B approach to each
other (i) if ECT > 0, charge flow from B to A and (ii) if ECT < 0,
charge flow from A to B. ECT is calculated as 1.1537 for reactant
system [(1) M (2)], which indicates that charge flows from (2) to
(1). Therefore, (1) acts as electron acceptor (electrophile) and (2)
as electron donor (nucleophile). The low value of chemical poten-
tial and high value of electrophilicity index for (1) favor its electro-
philic behavior. In the same way, the high value of chemical
potential and low value of electrophilicity index for (2) also favor
its nucleophilic behavior.
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.molstruc.2013.01.
011.
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Selected electrophilic reactivity descriptors ðfkþ; skþ; xkþÞ for reac-
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