Synthesis, multiple interactions and spectroscopy analysis of ethyl-4-[(benzoyl)-hydrazonomethyl]-3,5-dimethyl-1H-pyrrole-2-carboxylate: Through experimental and quantum chemical approaches

Article abstract of DOI:10.1016/j.molstruc.2012.10.065

The carboxaldehydes of pyrrole form important class of precursors for synthesis of acid hydrazide-hydrazones and their derivatives which show the various applications. A novel hydrazide-hydrazone of pyrrole: Ethyl-4-[(benzoyl)-hydrazonomethyl]-3,5-dimethy

Full text of DOI:10.1016/j.molstruc.2012.10.065

Journal of Molecular Structure 1035 (2013) 295–306
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Synthesis, multiple interactions and spectroscopy analysis of
ethyl-4-[(benzoyl)-hydrazonomethyl]-3,5-dimethyl-1H-pyrrole-2-carboxylate:
Through experimental and quantum chemical approaches
⇑
R.N. Singh , Amit Kumar, R.K. Tiwari, Poonam Rawat
Department of Chemistry, University of Lucknow, University Road, Lucknow 226 007, India
h i g h l i g h t s
"
"
"
"
"
FT-IR spectrum of the studied compound is recorded and compared with the theoretical results.
The 1H NMR spectrum has also been calculated using GIAO method.
By using TD-DFT/6-31G method, electronic transitions for title compound are predicted.
Dimer formation has been evaluated with the help of DFT and QTAIM theory.
Chemical reactivity has been explained with the aid of global and local electronic descriptors.
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 17 June 2012
Received in revised form 31 October 2012
Accepted 31 October 2012
Available online 23 November 2012
The carboxaldehydes of pyrrole form important class of precursors for synthesis of acid hydrazide-
hydrazones and their derivatives which show the various applications. A novel hydrazide-hydrazone of
pyrrole: Ethyl-4-[(benzoyl)-hydrazonomethyl]-3,5-dimethyl-1H-pyrrole-2-carboxylate (3) has been
synthesized by condensation of ethyl-4-formyl-3,5-dimetyl-1H-pyrrole-2-carboxylate and benzohydraz-
ide. The product was characterized by spectroscopy methods (FT-IR, 1H NMR, UV–Vis and DART Mass)
and quantum chemical calculations. The calculated thermodynamic parameters show that the formation
of (3) is exothermic and spontaneous at room temperature. The local electronic descriptors analysis and
molecular electrostatic potential surface were used to predict the sites and nature of interactions which
indicate the dimer formation through multiple interaction through NAHꢀ ꢀ ꢀO and C–Hꢀ ꢀ ꢀO. The vibra-
Keywords:
Pyrrole hydrazide-hydrazone
Multiple interactions
Hydrogen bonded dimer
AIM calculation
tional analysis shows red shifts in vNAH and vC@O as result of dimer formation. Topological and energetic
parameters generated by AIM show the nature of interactions in dimer. The interaction energies of dimer
formation using DFT and AIM calculations are found to be 14.19 and 15.19 kcal/mol respectively. Elliptic-
ity analysis confirms the presence of resonance assisted heteronuclear H-bonding in dimer.
Ó 2012 Elsevier B.V. All rights reserved.
Reactivity descriptors
1. Introduction
been of significant interest [10,11]. The chemical stability of hydra-
zones and their high melting point have recently made them
The carboxaldehydes of pyrrole form important class of precur-
sors for synthesis of hydrazones and their derivatives. Pyrrole frag-
ment is a constituent of many biological systems. The pyrrole-
attractive as prospective new material for opto-electronic applica-
tions [12]. Phenyl hydrazones exhibit a series of good organic non-
linear optical (NLO) properties [13,14]. Hydrazones having frame
ACH@NANHA constitute an important class of compounds for
[2 + 2] cycloadditions, 1, 3 dipolar cycloadditions and have been
turned into a valuable tool for the synthesis of azetidinones, pyra-
zoles, respectively [15,16] and various other N, O or S containing
heterocyclic compounds such as oxadiazolines, thiazolidinones
and triazolines [17–23].
Hydrogen bonds are of versatile importance in fields of
chemistry and bio-chemistry governing molecular assemblies,
supramolecular structures, chemical reactions and life processes.
Intra- and intermolecular hydrogen bonds are classified in two
a-carboxaldehyde has been used for the development of potential
chemosensors [1]. Hydrazones are an important class of com-
pounds due to their various properties and applications. They are
used as antimicrobial, antitubercular [2–6] and antidiabetic agents
[7]. They have strong coordinating ability towards different metal
ions [8,9]. In addition, aroyl hydrazones and their mode of chela-
tion with transition metal ions present in the living system have
⇑
Corresponding author. Tel.: +91 9451308205.
E-mail address: rnsvk.chemistry@gmail.com (R.N. Singh).
0022-2860/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molstruc.2012.10.065

296
R.N. Singh et al. / Journal of Molecular Structure 1035 (2013) 295–306
categories (i) classical or conventional hydrogen bonds and (ii)
non-conventional or improper hydrogen bonds depending upon
the nature of atoms or hydrogen bond donor and acceptor involved
in the hydrogen bridges. Type of non-conventional hydrogen bonds
are (i) those in which the nature of hydrogen bond donor is non-
conventional as CAH group [24] (ii) the nature of hydrogen bond
The absence of imaginary frequency verified that stationary points
correspond to minima of the potential energy surfaces. The time
dependent density functional (TD-DFT) is carried out to find the
electronic transitions. The optimized geometrical parameters are
used in the calculation of vibrational wavenumbers to characterize
all stationary points as minima and their harmonic vibrational
wavenumbers are positive. All molecular structures are visualized
using software Chemcraft [33] and Gauss-View [34]. Potential en-
ergy distribution along internal coordinates is calculated by
Gar2ped software [35]. Internal coordinate system recommended
by Pulay et al. is used for the assignment of vibrational modes
[36]. To estimate the enthalpy and Gibbs free energy values, ther-
mal corrections to enthalpy and Gibbs free energy calculated at
B3LYP/6-31G(d,p) level were added to the calculated total ener-
gies. The wave function file generated at B3LYP/6-31G(d,p) basis
set from the optimized coordinate was used to calculate the topo-
logical parameters by AIM 2000 1.0 software [37]. Bader’s theory of
atoms in molecules (AIM) theory efficiently describes the nature
and strength of various types of hydrogen-bonded interactions.
One of the advantages of the AIM theory is its ability to provide
information on changes in the electron density distribution as re-
sult of either bond formation or complex formation. The basis set
reliability and stability in the values of AIM parameters have been
studied and found that they are almost independent of basis set in
case of used functional B3LYP in both DFT and AIM calculations
[38].
acceptor is non-conventional as C atom or p-system [25] (iii) both
the donor and acceptor are non-conventional groups [26]. The title
compound (3) was synthesized and characterized due to the
various applications of hydrazones. In the present paper we report
the molecular dimer structure of (3) through multiple interactions
using quantum chemical calculations and experimental FT-IR
spectrum. Quantum chemical calculations also provide further
information on chemical reactivity of the title compound.
2. Experimental details
Benzohydrazide was prepared by refluxing the equimolar reac-
tion mixture of ethylbenzoate and hydrazine hydrate in methanol.
Ethyl-4-formyl-3,5-dimetyl-1H-pyrrole-2-carboxylate was pre-
pared by an earlier reported method [27]. The 1H NMR spectrum
of (3) was recorded in DMSO-d6 on Bruker DRX-300 spectrometer
using TMS as an internal reference. The UV–Vis absorption spec-
trum of (3) (1 ꢁ 10ꢂ5 M in DMSO) was recorded on ELICO SL-164
spectrophotometer. The FT-IR spectrum was recorded in KBr med-
ium on a Bruker spectrometer. The DART-Mass spectrum was re-
corded on JMS-T100LC, Accu TOF mass spectrometer.
4. Results and discussion
2.1. Synthesis of ethyl-4-[(benzoyl)-hydrazonomethyl]-3,5-dimethyl-
1H-pyrrole-2-carboxylate (3)
4.1. Thermodynamic properties
An ice cold solution of benzohydrazide (0.1394 g, 1.0251 mmol)
in 20 ml methanol was added dropwise with stirring in 15 ml solu-
For simplicity, reactants ethyl-4-formyl-3,5-dimetyl-1H-pyr-
role-2-carboxylate, benzohydrazide, product ethyl-4-[(benzoyl)-
tion
of
Ethyl-4-formyl-3,5-dimetyl-1H-pyrrole-2-carboxylate
hydrazonomethyl]-3,5-dimethyl-1H-pyrrole-2-carboxylate
and
(0.200 g, 1.0251 mmol) in methanol. Now, the reaction mixture
was stirred for 32 h at room temperature, the precipitate of red col-
or was appeared. The precipitate was filtered off, washed with
methanol and dried in air. Yield: 41.83%. Compound decomposes
water as a byproduct are abbreviated as 1, 2, 3 and 4 respectively.
Calculated Enthalpy (H), Gibbs free energy (G) and Entropy (S) of
reactants (1, 2) and products (3, 4) at room temperature is listed
in Table 1. For reaction, enthalpy change (
D
HReaction), Gibbs free en-
SReaction) are calcu-
above 250 °C without melting. 1H NMR DMSO-d6,
d (ppm):
ergy change ( GReaction) and entropy change (
D
D
1.278–1.323 (t, 3H, J = 6.75, CH3 ester); 2.369 (s, 3H, pyrrole-
CH3); 2.409 (s, 3H, pyrrole-CH3); 4.208–4.275 (q, 2H, J = 6.70,
CH2 ester); 7.859–7.889 (m, 3H, ArAH); 8.363–8.396 (d, 2H,
J = 9.90, ArAH); 8.884 (s, 1H, CH@N); 11.049 (s, 1H, hydrazide-
NH); 11.623 (s, 1H, pyrrole-NH). UV–Vis absorption spectrum
(1 ꢁ 10ꢂ5 M in DMSO): kmax (nm): 226 nm, 308 nm. DART Mass:
Observed m/z 314.1953 [M++1], Calc. 313.1426 amu.
lated from thermodynamic parameters of reactants and products
at room temperature and found to be ꢂ3.41, ꢂ1.63 kcal/mol and
ꢂ5.94 cal molꢂ1 Kꢂ1 respectively. The negative values for
DHReaction
and
DGReaction show that the reaction is exothermic and spontane-
ous at room temperature. Thermodynamic relation between equi-
librium constant (Keq) and Gibbs free energy change of reaction
D
(D
GReaction) at temperature (T) is given as Keq = eꢂ G/RT. The value
of equilibrium constant (Keq) determines the nature of reaction:
(i) if Keq > 1, reaction is favored in forward direction, (ii) if Keq = 0,
reaction is in equilibrium state, and (iii) if Keq < 1, reaction is fa-
vored in backward direction at temperature (T). Using above equa-
tion, equilibrium constant (Keq) for title reaction is calculated as
60.27 at room temperature. Therefore, reaction is favored in for-
ward direction at room temperature. This confirms the formation
of product (3) at room temperature.
3. Computational methods
The theoretical calculations for the synthesized compound were
carried out with Gaussian 03 program package [28] to predict the
molecular structure, 1H NMR chemical shifts and vibrational wave-
numbers using density functional theory (DFT), B3LYP functional
and 6-31G(d,p) basis set. B3LYP invokes Becke’s three parameter
(local, non local, Hartree–Fock) hybrid exchange functional (B3)
[29] with Lee–Yang–Parr correlation functional (LYP) [30]. The ba-
sis 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 [31,32]. It should be emphasized that
‘p’ polarization functions on hydrogen atoms are used for repro-
ducing the out of plane vibrations involving hydrogen atoms. Con-
vergence criterion in which both the maximum force and
displacement are smaller than the cutoff values of 0.000450 and
0.001800 and r.m.s. force and displacement are less than the cut-
off values of 0.000300 and 0.001200 are used in the calculations.
The binding energy of dimer is calculated as 14.19 kcal/mol
using DFT calculation. The calculated hydrogen binding energy of
Table 1
Calculated Enthalpy (H), Gibbs free energy (G) and Entropy (S) of reactants (1,2) and
products (3,4) at room temperature, using B3LYP/6-31G(d,p) at (25 °C).
Thermodynamic parameters
1
2
3
4
H (a.u.)
G (a.u.)
ꢂ669.12
ꢂ669.18
124.94
ꢂ456.12
ꢂ456.16
92.98
ꢂ1048.85
ꢂ1048.93
166.86
ꢂ76.39
ꢂ76.41
45.11
S (kcal molꢂ1 Kꢂ1
)

R.N. Singh et al. / Journal of Molecular Structure 1035 (2013) 295–306
297
Table 2
Calculated Enthalpy (H), Gibbs free energy (G), Entropy (S), their change and equilibrium constant (Keq) of conversion from monomer to dimer at (25 °C).
Parameters
Enthalpy (H) a.u.
Gibbs free energy (G) a.u.
Entropy (S) (cal molꢂ1 Kꢂ1
)
D
H (kcal/mol)
D
G (kcal/mol)
D
S (cal molꢂ1 Kꢂ1
ꢂ34.45
)
Keq
60.27
2 ꢁ Monomer
ꢂ2097.71
ꢂ2097.73
ꢂ2097.87
ꢂ2097.87
333.73
299.28
ꢂ12.70
ꢂ2.42
Dimer
dimer formation has been corrected for the basis set superposition
error (BSSE) via the standard counterpoise method [39] and found
to be 10.63 kcal/mol. The changes in thermodynamic parameters
during the dimer formation in gaseous phase have the negative
values of
DH (kcal/mol), DG (kcal/mol) and D )
S (cal molꢂ1 Kꢂ1
and incorporated in Table 2, indicating that the dimer formation
is exothermic and spontaneous. The high value of equilibrium con-
stant (Keq = 60.27) between monomer and dimer indicates that di-
mer formation is more preferred and as result even anti conformer
gets converted to syn and finally changes into dimer.
4.2. Molecular geometry
Geometry of all the reactants and products involved in chemical
reaction was computed at B3LYP/6-31G(d,p) level. The product
molecule (3) has many stable conformers but global minimum
and another having next low-lying minimum conformers have
been incorporated in considering the dimer formation and compar-
ing with experimental spectral values in the discussion. The re-
ported optimized ground state syn and anti stable conformers are
as the global minimum and the next low-lying minimum on the
Fig. 2. Optimized geometry of dimer for syn-conformer using B3LYP/6-31G(d,p).
potential energy surface. Molecular geometry was drawn consider-
ing both obtained spectral data of this synthesized compound (3)
and the structural pattern reported in single crystal structure of
the other hydrazide-hydrazone obtained from the reaction of acid
hydrazides with aromatic aldehydes [40]. The conformers were
determined from drawn geometry by systematic search using po-
tential energy surface scan around different dihedral angle, but
the global minima and another next low-lying minimum resulted
around the dihedral angle
s C3C2C9O20. The idea from reported lit-
erature on singe crystals, determining method and the calculated
energy values for conformers of product (3) provided the basis
for principle involved in discussion. Optimized geometry of the
ground state syn and anti conformer is shown in Fig. 1, with atom
numbering. They have energy ꢂ1049.2206 and ꢂ1049.2202 a.u.
respectively at room temperature. They have energy difference
0.2397 kcal/mol and exist in ratio 59.42%, 40.48% respectively at
room temperature. The asymmetry in the N1AC2 and N1AC5
bonds arises due to the presence of opposite nature groups such
as ethoxycarbonyl (electron withdrawing) group at C2 and methyl
(electron donating) group at C5 of pyrrole ring. It leads to the
elongation of N1AC2 bond. These effects are not only observed
in quantum calculation but also reflect in crystal structure of
the ethyl-3,5-dimethyl-1H-pyrrole-2-carboxylate [41], methyl 4-
p-tolyl-1H-pyrrole-2-carboxylate [42] and other pyrrole-2-carbox-
ylate derivatives [43]. Optimized geometry of dimer is shown in
Fig. 1. Optimized geometry of syn and anti conformer of (3) using B3LYP/6-
31G(d,p).

298
R.N. Singh et al. / Journal of Molecular Structure 1035 (2013) 295–306
Table 3
Selected optimized geometrical parameters from molecular dimer of (3) calculated at B3LYP/6-31G(d, p): bond length (Å), bond angle (°) and dihedral angle (°).
Bond length
(Å)
Bond angle
(°)
Dihedral angle
(°)
R(1,2)
R(1,5)
R(1,24)
R(2,3)
R(2,9)
R(3,4)
R(3,8)
R(4,5)
R(4,7)
1.3888
1.3474
1.0203
1.3983
1.4485
1.4251
1.5010
1.4103
1.4456
1.4916
1.0920
1.2891
1.0998
1.0968
1.2320
1.3503
1.3638
1.3839
1.0168
1.2206
1.5039
1.4025
1.4014
1.3951
1.0863
1.3955
1.0860
1.3973
1.0861
1.3922
1.5164
1.0943
1.0942
1.9074
1.9073
2.59585
2.59574
2.88753
2.88741
3.5009
3.5008
A(2,1,5)
110.7402
124.8666
124.3931
107.6694
117.9231
134.4074
106.3910
128.3132
125.2957
107.8297
125.5677
126.6019
107.3698
122.3102
130.3199
110.3442
122.4269
116.6984
120.8746
124.0002
114.1186
121.8812
116.5603
120.6972
119.1853
119.1889
123.3290
114.2605
122.4061
123.7543
120.0825
119.8538
120.1151
120.0421
120.4187
116.3813
107.4846
160.0303
160.0251
139.7314
139.7288
D(5,1,2,3)
D(2,1,5,4)
D(1,2,3,4)
D(1,2,9,21)
D(2,3,4,5)
D(2,3,8,29)
D(3,4,5,1)
D(3,4,7,10)
ꢂ0.0133
0.0046
0.0161
ꢂ179.882
ꢂ0.0135
ꢂ119.213
0.0056
A(2,1,24)
A(5,1,24)
A(1,2,3)
A(1,2,9)
A(3,2,9)
A(2,3,4)
A(2,3,8)
A(4,3,8)
A(3,4,5)
A(3,4,7)
A(5,4,7)
A(1,5,4)
A(1,5,6)
ꢂ179.970
D(1,5,6,25)
2.3165
R(5,6)
D(4,7,10,11)
D(28,7,10,11)
D(2,9,20,57)
D(21,9,20,57)
D(2,9,21,22)
D(20,9,21,22)
D(7,10,11,12)
D(7,10,11,32)
(10,11,12,13)
(10,11,12,14)
(32,11,12,13)
(32,11,12,14)
(11,12,14,15)
(11,12,14,19)
(13,12,14,15)
(13,12,14,19)
(12,14,15,16)
(12,14,19,18)
(14,15,16,17)
(15,16,17,18)
(16,17,18,19)
(17,18,19,14)
D(9,21,22,23)
(21,22,23,41)
D(2,1,24,53)
D(9,20,57,54)
D(6,25,53,51)
D(60,65,20,9)
ꢂ179.686
0.4789
ꢂ0.1615
179.8210
179.9848
0.0006
ꢂ172.757
ꢂ3.8726
1.4964
ꢂ179.252
ꢂ167.387
11.8646
27.0989
ꢂ154.645
ꢂ153.642
24.6149
179.3526
ꢂ179.85
ꢂ0.0259
ꢂ0.7158
0.3393
R(6,25)
R(7,10)
R(7,28)
R(8,29)
R(9,20)
R(9,21)
R(10,11)
R(11,12)
R(11,32)
R(12,13)
R(12,14)
R(14,15)
R(14,19)
R(15,16)
R(15,33)
R(16,17)
R(16,34)
R(17,18)
R(17,35)
R(18,19)
R(22,23)
R(22,38)
R(23,40)
R(24,53)
R(20,57)
R(25,53)
R(20,65)
R(1,53)
R(54,20)
R(6,53)
R(60,20)
A(4,5,6)
A(5,6,25)
A(4,7,10)
A(4,7,28)
A(10,7,28)
A(2,9,20)
A(2,9,21)
A(20,9,21)
A(7,10,11)
A(10,11,12)
A(10,11,32)
A(12,11,32)
A(11,12,13)
A(11,12,14)
A(13,12,14)
A(12,14,15)
A(15,16,17)
A(16,17,18)
A(17,18,19)
A(17,18,36)
A(14,19,18)
A(9,21,22)
A(21,22,23)
A(1,24,53)
A(54,57,20)
A(6,25,53)
A(60,65,20)
0.7827
179.8415
60.3452
179.1416
ꢂ179.439
ꢂ177.351
ꢂ177.339
Table 4
Experimental and calculated GIAO 1H NMR chemical shifts of (3) using B3LYP/6-
31G(d,p) in DMSO-d6 as the solvent dExp
Fig. 2, with atom numbering. Selected optimized geometrical
parameters of dimer of (3) are listed in Table 3. In dimer, heteronu-
clear intermolecular hydrogen bonding (NAHꢀ ꢀ ꢀO) between pyrro-
lic (NAH) and carbonyl (C@O) oxygen of ester form two hydrogen
bonds. In intermolecular hydrogen bonds, the NAH bond acts as
proton donor and C@O bond as proton acceptor. According to the
Etter terminology [44], the cyclic ester dimer form the ten member
pseudo ring denoted as R22ð10Þ or more extended sixteen member
pseudo ring R22ð16Þ. The superscript designates the number of
acceptor centers and the subscript, the number of donors in
the motif. In dimer, due to intermolecular hydrogen-bond
formation both proton donor (NAH bond) and proton acceptor
(C@O bond) are elongated from 1.0105 Å to 1.0203 Å, 1.2238 Å to
1.2320 Å respectively. Total energy of the monomer (syn-con-
former) and its dimer are calculated as ꢂ1049.22060088,
ꢂ2098.46381691 a.u. respectively.
.
Atom
no.
dCal
dExp
Assignment
H24
H25
H26
H27
H28
H29
H30
H31
H32
H33
9.182 11.623
2.148 2.409
2.638
(s, 1H, pyrrole-NH)
(s, 3H, pyrrole a-CH3)
2.589
7.950 8.884
2.304 2.369
2.139
(s, 1H, proton of CH@N)
(s, 3H, pyrrole b-CH3)
2.366
8.204 11.049
7.978 8.363–
8.396
(s, 1H, hydrazide-NH)
(d, 1H, o-H of benzene ring with J = 9.90 Hz)
H34
7.810 7.859–
7.889
(m, 3H, m- and p-protons of benzene ring)
H35
H36
H37
7.917
7.849
8.273 8.363–
8.396
(d, 1H, o-H of benzene ring with J = 9.90,
ArAH)
4.3. 1H NMR spectroscopy
H38
4.299 4.208–
4.275
4.320
1.209 1.278–
1.323
1.472
(q, 2H, J = 6.70, CH2 ester)
Isotropic magnetic shielding of optimized TMS and product are
calculated with GIAO approach using B3LYP functional and 6-
31G(d,p) basis set [45]. 1H NMR Chemical shift of any proton is
the difference between isotropic magnetic shielding of TMS and
that proton. Experimental and calculated 1H NMR chemical shifts
of the title compound are listed in Table 4. The experimental 1H
NMR spectrum is given in Fig. 3. In order to compare the chemical
shifts, correlation graph between the experimental and calculated
H39
H40
(t, 3H, J = 6.75, CH3 ester)
H41
H42
1.485
Abbreviations: s = singlet, t = triplet, q = quartet, d = doublet, m and p meta and para
position hydrogen atom in benzene ring.

R.N. Singh et al. / Journal of Molecular Structure 1035 (2013) 295–306
299
Fig. 3. The experimental 1H NMR spectrum of the studied compound.
of the transitions observed in the UV–Vis spectrum of (3) has been
studied by the comparison between experimental and calculated
electronic transitions and are listed in Table 5. The experimental
and theoretical UV–Vis spectrum is shown in Fig. 5. The selected
molecular orbital plots involved in electronic excitations for (3)
are given in Fig. 6. The FMOs are important in determine the ability
of a molecule to absorb light and molecular reactivity. The vicinal
orbitals of HOMO and LUMO play the role of electron donor and
electron acceptor respectively. From the charge transfer viewpoint,
12
10
8
6
the high value of |eHOMO| means that the ionization is more diffi-
4
cult, that is to say, that molecule is hard to lose electron. The
HOMO–LUMO energy gap is an important stability index. The high
value of HOMO–LUMO energy gap of title molecule reflects the
chemical stability of the molecule.
2
0
0
2
4
6
8
10
HOMO energy ¼ ꢂ5:4757 eV
δ
exp (ppm)
LUMO energy ¼ ꢂ1:2188 eV
HOMO—LUMO energy gap ¼ 4:2569 eV
Fig. 4. Correlation graph between experimental and calculated 1H NMR chemical
shifts (d) in (ppm).
TD-DFT calculations using B3LYP/6-31G(d,p), predict three in-
tense electronic transitions at kmax = 241.06 nm, f = 0.4149;
kmax = 272.30 nm, f = 0.2528 and kmax = 327.23 nm, f = 0.8430. The
calculated electronic transitions at kmax = 241.06 nm and
kmax = 327.23 nm are in agreement with the experimental elec-
tronic transitions observed at kmax 226, 308 nm respectively. The
chemical shifts is shown in Fig. 4. The correlation graph follow the
linear equation (y = 0.7799x ꢂ 0.7991), where ‘y’ is the experimen-
tal 1H NMR chemical shift, ‘x’ is the calculated 1H NMR chemical
shift. The high value of correlation coefficient (R2 = 0.94) shows
that there is a good agreement between experimental and calcu-
lated chemical shifts.
calculated values for wavelength of maximum absorption (kmax
)
are also in agreement with the reported kmax in literature [46].
The experimental bands at 226 and 308 nm originate mainly due
to the H-1?L + 1, H ? L electronic excitation respectively.
According to the NBO analyses, H-1 ? L + 1, H ? L electronic
4.4. UV–Vis spectroscopy
The electronic transitions of the product were calculated using
time dependent density functional (TD-DFT) theory. The nature
excitations show
(C9AO20) transitions respectively. The results show that the
p
(C4AC5) ? pꢃ(C7AN10) and n(O13) ? pꢃ

300
R.N. Singh et al. / Journal of Molecular Structure 1035 (2013) 295–306
Table 5
Comparison between experimental and calculated electronic transitions: E (eV), oscillatory strength (f), kmax (nm) using TD-DFT/B3LYP/6-31G(d,p) in DMSO at (25 °C).
S. no.
Electronic transitions
E (eV)
Calculated kmax (nm)
Oscillator strength (f)
% Contribution
Assignment
Observed kmax (nm)
1
2
H ? L
H-4 ? L
3.78
4.55
327.23
272.3
0.8430
0.2528
44.89
2.19
p
p
?
?
pꢃ
pꢃ
308
H-2 ? L
1.45
H-1 ? L
32.75
9.99
39.62
1.35
H ? L + 1
H-1 ? L + 1
H-4 ? L
3
4
5.14
5.44
241.06
227.57
0.4149
0.0989
n ? pꢃ
pꢃ
226
p
?
H-2 ? L
1.62
H-2 ? L + 2
H-1 ? L + 2
1.28
38.35
Theoretical
Experimental
150
200
250
300
350
400
450
500
Wavenumbers (cm-1)
Fig. 5. Comparison between experimental and theoretical UV–Vis spectrum.
charge transfer (CT) occurs from the proton acceptor nonbonding
n(O) orbital of carbonyl to the proton donor rꢃ(NAH) orbital of pyrrole.
4.5. Vibrational spectroscopy
The experimental and theoretical (selected) vibrational wave-
numbers of dimer at B3LYP/6-31G(d,p) level and their assignments
using PED are given in Table 6. The calculated (monomer, dimer)
and the experimental FT-IR spectra of (3) in the region 4000–
400 cmꢂ1 are shown graphically in Fig. 7. The total number of
atoms in monomer and dimer are 42 and 84 which give 120 and
246, (3n ꢂ 6) vibrational modes for monomer and dimer respec-
tively. The calculated vibrational wavenumbers have higher values
than their experimental for most of the normal modes. Two factors
may be responsible for the discrepancies between the experimen-
tal and computed wavenumbers of this compound. The first is
caused by difference of the environment (gas and solid phase)
and the second is due to the fact that the experimental values
are anharmonic wavenumbers while the calculated values are har-
monic ones. Therefore, calculated wavenumbers at B3LYP/6-
31G(d,p) level are scaled down using single scaling factor 0.9608
[47], to discard the anharmonicity present in real system. The cor-
relation graph is shown in Fig. 8. The value of correlation coeffi-
cient found to be r2 = 0.99. The observed wavenumbers are in
good agreement with the calculated wavenumbers of dimer than
monomer. Therefore, observed wavenumbers are assigned using
dimer PED.
Fig. 6. Selected molecular orbital plots involved in electronic excitations for (3).
In the experimental FT-IR spectrum of (3), the NAH stretch of
calculated wavenumber of dimer. The observed value of mNH also
correlates with the earlier reported strong absorption at
3358 cmꢂ1 for hydrogen bonded pyrrole-2-carboxylic acid dimer
pyrrole (m
NH) is observed at 3301 cmꢂ1, whereas it is calculated
as 3341 cmꢂ1 in dimer and 3502 cmꢂ1 in monomer. The observed
wavenumber at 3301 cmꢂ1 is in good agreement with the
recorded in KBr pellet, but it deviates to the reported free mNH band

Table 6
Experimental and theoretical (selected) vibrational wavenumbers of dimer using B3LYP/6-31G(d,p) and their assignments [harmonic vibrational wavenumbers (cmꢂ1), IRint (Kmmolꢂ1)].
Mode no.
Wavenumbers
Intensity
Exp. waven-umbers
Assignment [PED (>5%)]
Unscaled
Scaled
245
244
241
239
238
231
228
226
222
220
218
215
213
211
209
208
206
203
199
197
195
194
189
188
182
180
177
173
171
168
165
164
161
157
153
149
148
145
143
137
135
134
129
124
121
120
105
103
101
97
3511
3477
3221
3207
3196
3178
3139
3132
3106
3081
3064
3057
3056
3037
3023
1782
1724
1685
1636
1607
1569
1562
1532
1524
1500
1498
1491
1486
1467
1429
1426
1415
1395
1358
1320
1269
1260
1223
1206
1171
1142
1130
1095
1058
1051
1047
931
3373
3341
3095
3081
3071
3054
3016
3009
2984
2961
2944
2937
2936
2918
2904
1712
1656
1619
1571
1544
1508
1501
1472
1464
1441
1439
1433
1428
1409
1372
1370
1360
1340
1304
1268
1219
1211
1175
1159
1125
1098
1086
1052
1016
1010
1006
895
25.09
2732.37
11.84
46.74
40.65
14.26
65.18
47.24
12.72
30.01
52.33
18.34
29.91
111.05
90.66
503.02
2001.56
311.13
9.44
454.78
934.33
243.1
57.69
52.27
11.99
77.93
101.07
41.91
405.4
96.19
30.83
72.15
44.63
42.81
1015.84
423.98
526.05
579.43
110.12
441.2
45.63
284.77
43.55
22.2
3391
3301
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
(N11H32)(53) ꢂ
m
(N70H72)(46)
(N1H24)(50) ꢂ
(C76H80)(61) ꢂ
(C17H35)(21) ꢂ
(C18H36)(20) ꢂ
(C15H33)(27) ꢂ
(C23H41)(19) ꢂ
(C23H40)(34) ꢂ
(C22H38)(26) ꢂ
m(N54H57)(49)
m(C19H37)(28) +
m(C81H84)(21) +
m(C79H83)(19) ꢂ
m(C16H34)(20) ꢂ
m(C23H42)(19) +
m(C43H45)(31) ꢂ
m(C22H39)(25) +
m
m
m
m
(C79H83)(6)
3110
2976
(C16H34)(17) ꢂ
m
m
m
(C77H82)(17) +
m
m
m
(C18H36)(6) ꢂ
m
m
(C79H83)(6)
(C75H78)(8)
m(C81H84)(6)
(C16H34)(17) +
(C77H82)(16) ꢂ
(C15H33)(8) +
(C75H78)(18) +
(C77H82)(13) +
(C17H35)(10) ꢂ
ꢂ C43C44)(7)
(C43H47)(8) + (C43H46)(8)
(C23H41)(5) ꢂ (C23H42)(5)
m
m
(C43H46)(16) ꢂ
m
m
m
(C43H47)(16) ꢂ (
(C23H41)(9) +
(C44H50)(13) +
s
m
(C23H42)(9) ꢂ
m
m
(C44H49)(13) ꢂ
m
m
(C8H30)(45) ꢂ
(C22H39)(25) +
(C6H26)(13) ꢂ (
(C43H45)(20) +
(C59H62)(24) ꢂ
(C61H69)(49) ꢂ
m
m
s
m
(C8H29)(42) +
(C22H38)(25) ꢂ
-H25O53)(13) ꢂ
(C43H47)(19) +
m
(C59H63)(6) ꢂ
m
(C59H62)(6)
(C44H49)(23)
(C6H25)(10) +
(C23H40)(14) ꢂ
(C8H30)(21)
2928
2863
m
m
(C44H50)(24) ꢂ
m
m
(C60H66)(12) +
m
(C6H27)(9) ꢂ
m
(C60H67)(9) ꢂ
m(C60H65)(9) ꢂ (das-Me1)(8) + (s-H65O20)(6)
m
(C43H46)(19) ꢂ
m
m
(C23H42)(13) ꢂ m
(C23H41)(13)
m
m
(C8H29)(23) +
(C7H28)(45)
m
(C59H63)(22) ꢂ m
(C71O73)(27) ꢂ (das-Me1)(15) + (
s
-C5C6)(5) + (das-Me1)(5)
(C9O20)(26) ꢂ (das-Me1)(8) + ( -C5C6)(6)
-C5C6)(31) ꢂ (das-Me1)(28) ꢂ (
(C79C81)(11) ꢂ (C77C81)(10) +
-C5C6)(27) ꢂ ( -Me1)(27) + (das-Me1)(9) ꢂ (
1669
1619
(C51O53)(26) ꢂ
m
s
(s
s
-C60C56)(10) + (das-Me1)(9) ꢂ
m
(C61N68)(6) +
m
(C7N10)(6)
m
m
m
(C74C75)(8) ꢂ
m
(C17C18)(8) +
m
(C16C17)(8)-
m
(C74C76)(7) ꢂ m(C14C15)(7) + m(C14C19)(6) ꢂ (das-R1)(5)
(s
q
s-C60C56)(9) + (
q-Me1)(8)
1500
(das-Me1)(56) ꢂ (das-Me1)(18) ꢂ (
s-C5C6)(11) ꢂ (
q
-Me1)(7)
(das-Me1)(49) ꢂ (
(das-Me1)(45) ꢂ (
(das-Me1)(53) ꢂ (
s-C5C6)(24) ꢂ (das-Me1)(15) + (
s-C5C6)(25) ꢂ (das-Me1)(14) + (
s-C5C6)(19) ꢂ (das-Me1)(17) + (
s
s
s
-C60C56)(8)
-C60C56)(8)
-C60C56)(6)
(s-C43C44)(34) + (das-Me1)(14) + (das-Me1)(8)
(das-Me1)(49) ꢂ (
(das-Me1)(43) ꢂ (
(das-Me1)(49) ꢂ (
(das-Me1)(38) ꢂ (
s-C5C6)(26) ꢂ (das-Me1)(15) + (
s-C5C6)(33) ꢂ (das-Me1)(13) + (
s-C5C6)(25) ꢂ (das-Me1)(15) + (
s-C5C6)(37) ꢂ (das-Me1)(12) + (
s
s
s
s
-C60C56)(8)
1433
1417
-C60C56)(10)
-C60C56)(8)
-C60C56)(12)
(
(
(
(
(
(
(
(
(
(
(
(
(
(
(
(
(
(
(
(
(
(
s
s
s
s
s
s
s
s
s
s
s
s
s
s
s
s
s
s
s
s
s
-C5C6)(28) + (
s
-H25O53)(21) ꢂ (das-Me1)(15) ꢂ (
s-H65O20)(11) ꢂ (
s
-C60C56)(9) + (ds-Me1)(6) + (das-Me1)(5)
-C5C6)(40) ꢂ (das-Me1)(34) ꢂ (
-C5C6)(42) ꢂ (das-Me1)(34) ꢂ (
-C5C6)(41) ꢂ (das-Me1)(35) ꢂ (
-C5C6)(44) ꢂ (das-Me1)(30) ꢂ (
-C5C6)(46) ꢂ (das-Me1)(28) ꢂ (
s
s
s
s
s
-C60C56)(14) + (das-Me1)(12)
-C60C56)(13) + (das-Me1)(11)
-C60C56)(13) + (das-Me1)(11)
-C60C56)(14) + (das-Me1)(10)
-C60C56)(15) + (das-Me1)(9)
1364
1254
1211
-C5C6)(51) ꢂ (
-C5C6)(52) ꢂ (
-C5C6)(45) ꢂ (
-C5C6)(26) ꢂ (
-C5C6)(51) ꢂ (
s
s
q
s
-C60C56)(16) ꢂ (das-Me1)(14) ꢂ (
q
q
-Me1)(8)
-Me1)(9)
-C60C56)(17) ꢂ (das-Me1)(14) ꢂ (
1134
1101
-Me1)(17) ꢂ (
s-C60C56)(14) + (
q
-Me1)(5)
-C60C56)(8) ꢂ (
-C60C56)(16) ꢂ (
-Me)(10) ꢂ (
-C60C56)(16) ꢂ (das-Me1)(15) ꢂ (
-C60C56)(17) ꢂ ( -Me1)(11) ꢂ (das-Me1)(10)
-Me1)(16) ꢂ ( -C60C56)(13) ꢂ (das-Me1)(8) + (
-C60C56)(16) ꢂ ( -Me1)(12) ꢂ (das-Me1)(11)
-C60C56)(17) ꢂ (das-Me1)(11) ꢂ ( -Me1)(10)
-Me1)(16) ꢂ ( -C60C56)(16) + ( -Me1)(5)
-C60C56)(16) ꢂ ( -Me1)(12) ꢂ (das-Me1)(8)
-C60C56)(9) ꢂ ( -Me1)(8) ꢂ (d-O53O48C51)(6)
-C60C56)(16) ꢂ ( -Me1)(15) ꢂ (das-Me1)(7) + (
-H57O20)(31) + ( -H24O53)(13)
-N1H24)(14) ꢂ (
-N54H57)(8) + (
-H57O20)(7) ꢂ (das-Me1)(5)
-C2C9)(30) ꢂ ( -C51C52)(16) ꢂ ( -C2C9)(13)
x
-C81H84)(7) + d(C15H34C16)(6) ꢂ (
-Me1)(16) ꢂ (das-Me1)(6) + ( -Me1)(5)
-C60C56)(6) + (dsc-C44C43O48)(6)-
-Me1)(7) + (das-Me1)(5)
q-Me1)(6) ꢂ (das-Me1)(5) + d(C74H78C75)(5) + d(C16H33C15)(5)
s
q
q
-C5C6)(18) + (
q
-Me)(11) ꢂ (
q
s
m
(C43C44)(6) + m(C22C23)(5)
1058
1032
-C5C6)(50) ꢂ (
-C5C6)(54) ꢂ (
-C5C6)(42) ꢂ (
-C5C6)(51) ꢂ (
-C5C6)(53) ꢂ (
-C5C6)(51) ꢂ (
-C5C6)(50) ꢂ (
-C5C6)(28) ꢂ (
-C5C6)(51) ꢂ (
s
s
q
s
s
q
q
s
s-H25O53)(5) + (q-Me1)(5)
66.97
15.95
56.4
973
879
q
q
q
s
q
903
867
824
822
763
747
868
833
791
790
733
718
8.65
s
s
s
q
13.37
83.92
115.46
11.67
12.8
844
831
774
q
q
q-Me1)(5)
96
90
88
x
-N54H57)(34) ꢂ (
(das-Me1)(16) ꢂ (
-C51C52)(30) + (
s
x
s
x
x
s
x
(x
x
(continued on next page)

302
R.N. Singh et al. / Journal of Molecular Structure 1035 (2013) 295–306
at higher wavenumber 3465 cmꢂ1, recorded in solution phase [48].
Therefore, solid state spectrum of (3) attributes to the vibration of
hydrogen bonded NAH group. In dimer, the stretching wavenum-
ber of hydrogen bond donor (NAH) is red-shifted due to its con-
ventional nature with respect to the hydrogen bond free
monomer. The observed NAH wagging mode of pyrrole at
774 cmꢂ1 correlates with the calculated wavenumber at
790 cmꢂ1 in theoretical IR-spectrum. The NAH stretch of acid
hydrazide part of molecule is observed at 3391 cmꢂ1, whereas it
is calculated at same wave number 3373 cmꢂ1 in both monomer
and dimer.
Three methyl groups are present in the molecule. They are
abbreviated as Me, Me1 and Me2. Me1 and Me2 are directly at-
tached to the C5 and C3 carbon of pyrrole ring. Me is attached to
the CH2 of ester group. In dimer, mCAH of Me1 is calculated at high-
er wavenumber 2937 cmꢂ1 (mode no. 215, intensity = 18.34) than
the monomer 2930 cmꢂ1 (mode no. 104, intensity 17.99). The
cause of blue shift in the stretching vibration of hydrogen bond
acceptor (CAH) is due to its non-conventional nature than the free
CAH vibration in monomer. According to Internal coordinate sys-
tem recommended by Pulay et al. [36], CH2 group associate six
stretching and bending vibrational frequencies namely: symmetric
stretch, asymmetric stretch, scissoring, rocking, wagging and
twisting. The scissoring and rocking deformations belong to polar-
ized in-plane vibration whereas wagging and twisting deforma-
tions belong to depolarized out-of-plane vibration. The
symmetric CH2 stretching vibrational band observed at
2928 cmꢂ1 is in an excellent agreement with the calculated wave-
number at 2944 cmꢂ1. The asymmetric CH2 stretching vibrational
band is assigned at 2984 cmꢂ1 but does not observed in experi-
mental FT-IR spectrum. The observed CAH stretching vibrations
of benzene ring, ester methyl and ester CH2 at 3110, 2976 and
2928 cmꢂ1 are in agreement with the calculated wavenumber at
3081, 3016 and 2944 cmꢂ1 respectively. The observed CAH
stretching vibration of hydrazone linkage CH@N at 2863 cmꢂ1 cor-
relates with the calculated wavenumber at 2904 cmꢂ1 in theoreti-
cal IR-spectrum. The observed rocking mode of methyl group at
1058, 1032 cmꢂ1 corresponds to the calculated wavenumber at
1086, 1052 cmꢂ1 respectively. Asymmetric deformation of pyrrole
Me1 group observed at 1500, 1433 cmꢂ1 agrees well with the cal-
culated wavenumber at 1508, 1433 cmꢂ1 respectively.
The CAC stretches and CACAH deformations of benzene ring
are assigned at 1571, 1159 cmꢂ1 respectively. The observed wag-
ging mode of C5AC6 at 655, 695 cmꢂ1 correlates with the calcu-
lated wavenumber at 667, 692 cmꢂ1 respectively. The observed
wagging mode of C2AC9 at 439, 511 cmꢂ1 agrees well with the cal-
culated wavenumber at 440, 512 cmꢂ1 respectively.
The stretching mode of ester carbonyl group (mC@O) is observed
at 1669 cmꢂ1, whereas it is calculated at 1656, 1689 cmꢂ1 in dimer
and monomer respectively. This shows that observed wavenumber
at 1669 cmꢂ1 is in good agreement with the calculated wavenum-
ber of dimer. The observed mC
@ also agrees well with the earlier
O
reported wavenumber at 1665 cmꢂ1 for dimer of syn-pyrrole-2-
carboxylic acid [48]. Therefore, mC stretching mode in (3) con-
@
O
firms the involvement of C@O group in intermolecular hydrogen
bonding. The reason for red shift in the stretching wavenumber
of hydrogen bond acceptor (C@O) group is due to its involvement
in dimer formation by hydrogen bond than free monomer. A com-
bination band of NAH and C@O stretching of acid hydrazide part of
molecule is calculated at 1712 cmꢂ1 with 27% contribution in PED.
The observed C7@N10 stretching vibration (
calculated at same wavenumber in theoretical IR spectrum. The
presence of mC confirms the hydrazone linkage in the product
m
C@N) at 1619 cmꢂ1 is
@
N
molecule (3). The obseved mC
@
N at 1619 cmꢂ1 also agrees well with
the reported absorption band at 1600 cmꢂ1 [49].

R.N. Singh et al. / Journal of Molecular Structure 1035 (2013) 295–306
303
120
100
80
60
40
20
0
Experimental
Monomer
Dimer
500
1000
1500
2000
2500
3000
3500
4000
Wavenumbers (cm-1)
Fig. 7. Correlation graph between experimental and theoretical wavenumbers.
Molecular graph of the dimer using AIM program at B3LYP/6-
31G(d,p) level is shown in Fig. 9. Geometrical parameter – bond
length (Å), topological parameters-electron density (
qBCP), Lapla-
cian of electron density (
r
2qBCP); energetic parameters – kinetic
electron energy density (GBCP), potential electron energy density
(VBCP), total electron energy density (HBCP), and estimated interac-
tion energy (Eint) at bond critical point (BCP) for bonds of interact-
ing atoms in dimer are given in Table 7. The tabulated parameters
clearly indicates that there are three types of interactions exist in
the dimer (i) O20ꢀ ꢀ ꢀH57 are O53ꢀ ꢀ ꢀH24 are classical (conventional)
moderate hydrogen bonds (ii) O21ꢀ ꢀ ꢀH31, O48ꢀ ꢀ ꢀH64, O20ꢀ ꢀ ꢀH65,
O53ꢀ ꢀ ꢀH25 are non-conventional (improper) weak hydrogen bonds
and (iii) N10ꢀ ꢀ ꢀC6, N68ꢀ ꢀ ꢀC60 are weak (van der Waal) interactions.
Espinosa proposed proportionality between hydrogen bond energy
(E) and potential energy density (VBCP) at Hꢀ ꢀ ꢀO contact: E = ½
(VBCP) [52]. The intermolecular interaction energy of dimer forma-
tion as per DFT calculation found to be 14.19 kcal/mol and after
BSSE correction 10.63 kcal/mol, whereas per AIM calculations it
is 15.19 kcal/mol.
Fig. 8. Correlation graph between experimental and calculated FT-IR
wavwnumbers.
The ellipticity (
character of bond. The
to the eigen values of Hessian and defined as by a relationship:
-electron delo-
e) at BCP is a sensitive index to monitor the
p-
e
is related to k1 and k2, which correspond
e
= (k1/k2) ꢂ 1. In order to investigate the effect of
p
4.6. Aim calculation
calization in bonds associated with N and O atoms of NAHꢀ ꢀ ꢀO het-
eronuclear intermolecular hydrogen bonds, the analysis of the
bond ellipticity is performed. In dimer, intermolecular hydrogen
bonds are associated with the 16 member pseudo ring
(C9AC2AC3AC4AC5AN1AH19ꢀ ꢀ ꢀO60AC52AC48AC46AC44AC47
Geometrical as well as topological parameters are useful tool to
characterize the strength of hydrogen bond. The geometrical crite-
ria for the existence of hydrogen bond are as follows: (i) The dis-
tance between proton (H) and acceptor (A) to be less than the
sum of their van der Waal’s radii of these atoms (ii) The ‘donor
(D)–proton (H)ꢀ ꢀ ꢀacceptor (A)’ angle to be greater than 90° (iii)
The elongation of ‘donor (D)-proton(H)’ bond length to be ob-
served. These criteria are frequently considered as insufficient,
therefore, the existence of hydrogen bond could be supported fur-
ther by Koch and Popelier criteria [50] based on ‘Atoms in Mole-
cules’ theory (i) The existence of bond critical point for the
‘proton (H)ꢀ ꢀ ꢀacceptor (A)’ contact (ii) The value of electron density
AN50AH56ꢀ ꢀ ꢀO14) and abbreviated as RP. The
e values for bonds
involved in pseudo ring (RP) are given in Table 8. In RP, the values
of for bonds O20AC9, C9AC2, C2AC3, C3AC4, C4AC5, C5AN1
e
associated with the N1AH24ꢀ ꢀ ꢀO53 and for bonds O53AC51,
C51AC52, C52AC55, C55AC58, C58AC56, C56AN54 associated
with the N54AH57ꢀ ꢀ ꢀO20 are in the range 0.1015–0.2890. These
values of
[53]. The
e
e
correspond to the aromatic bonds reported in literature
values confirm the presence of resonance assisted het-
eronuclear intermolecular hydrogen bonds because both N and O
(q
H. . .A) should be within the range 0.002–0.040 a.u. (iii) The
corresponding Laplacian (
2qBCP) should be within the range
0.024–0.139 a.u. According to Rozas et al. [51] the H-bonding
may be classified as (i) Strong H-bonds: having (
2qBCP) < 0 and
HBCP < 0, covalent in nature (ii) Medium H-bonds: having (
> 0 and HBCP < 0, partially covalent in nature (iii) Weak H-bonds:
having (
2qBCP) > 0 and HBCP > 0, mainly electrostatic in nature
atoms are interconnected by a system of
bonds.
p-conjugated double
r
r
r
2qBCP
)
4.7. Chemical reactivity
r
4.7.1. Global reactivity descriptors
and the distance between interacting atoms is greater than the
On the basis of Koopman’s theorem, global reactivity descrip-
tors electronegativity ( ), chemical potential ( ), global hardness
sum of van der Waal’s radii of these atoms.
v
l

304
R.N. Singh et al. / Journal of Molecular Structure 1035 (2013) 295–306
Fig. 9. Molecular graph of dimer: bond critical points (small red spheres), ring critical points (small yellow sphere), bond paths (pink lines) using AIM program. (For
interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Table 7
Geometrical parameter bond length (Å) and topological parameters for bonds of interacting atoms: electron density (
energy density (GBCP), potential electron energy density (VBCP), total electron energy density (HBCP), estimated interaction energy (Eint) at bond critical point (BCP).
q
BCP), Laplacian of electron density (r2qBCP), kinetic electron
Atoms involved in interactions
Bond distance (Å)
qBCP (a.u.)
r
2q(BCP) (a.u.)
GBCP (a.u.)
VBCP (a.u.)
HBCP (a.u.)
Bond degree
Eint (kcal/mol)
O20ꢀ ꢀ ꢀH57
O53ꢀ ꢀ ꢀH24
O21ꢀ ꢀ ꢀH31
O48ꢀ ꢀ ꢀH64
N10ꢀ ꢀ ꢀC6
1.9073
1.9074
2.3054
2.3053
3.0351
3.0351
2.5957
2.5958
0.0254
0.0254
0.0144
0.0144
0.0087
0.0087
0.0074
0.0074
0.0820
0.0819
0.0484
0.0484
0.0344
0.0344
0.0260
0.0260
0.0199
0.0199
0.0113
0.0113
0.0070
0.0070
0.0056
0.0056
ꢂ0.0194
ꢂ0.0194
ꢂ0.0105
ꢂ0.0105
ꢂ0.0055
ꢂ0.0055
ꢂ0.0048
ꢂ0.0048
0.0005
0.0005
0.0007
0.0007
0.0015
0.0015
0.0008
0.0008
0.0213
0.0213
0.0544
0.0544
0.1730
0.1730
0.1158
0.1158
ꢂ6.0
ꢂ6.0
ꢂ3.3
ꢂ3.3
ꢂ1.7
ꢂ1.7
ꢂ1.5
ꢂ1.5
N68ꢀ ꢀ ꢀC60
O20ꢀ ꢀ ꢀH65
O53ꢀ ꢀ ꢀH25
(g), global softness (S) and global electrophilicity index (x) are cal-
culated using the energies of frontier molecular orbitals eHOMO
,
eLUMO and given by Eqs. (1)–(5) [54,55]
Table 8
Ellipticity parameters for bonds of sixteen member pseudo ring (RP).
v
l
g
¼ 1=2ðeLUMO
¼ 1=2ðeLUMO
¼ 1=2ðeLUMO
þ
eHOMO
Þ
ð1Þ
Bond
e
Bond
e
¼
v
þ
eHOMO
Þ
ð2Þ
ð3Þ
ð4Þ
O20AC9
C9AC2
C2AC3
C3AC4
C4AC5
0.1015
0.2281
0.2890
0.2204
0.2521
0.1690
0.0308
0.0202
O53AC51
C51AC52
C52AC55
C55AC58
C58AC56
C56AN54
N54ꢀ ꢀ ꢀH57
H57ꢀ ꢀ ꢀO20
0.1015
0.2281
0.2890
0.2204
0.2521
0.1690
0.0308
0.0202
ꢂ
eHOMO
Þ
S ¼ 1=2
x
g
¼
l2=2
g
ð5Þ
C5AN1
N1ꢀ ꢀ ꢀH24
Electrophilicity index (
when the system acquires an additional electronic charge (
x
) measures the stabilization in energy
H24ꢀ ꢀ ꢀO53
D
Nmax
)

R.N. Singh et al. / Journal of Molecular Structure 1035 (2013) 295–306
305
Table 9
eHOMO
,
e
LUMO, energy gap (eLUMO
ꢂ
e
HOMO), chemical potential (
l
), electronegativity (
H) (eV)
v
), global hardness (
g), global softness (S) and global electrophilicity index (x).
Molecules
eH (eV)
eL (eV)
(
eL
ꢂ
e
v
(eV)
l
(eV)
g
(eV)
S (eVꢂ1
)
x (eV)
Reactant (1)
Reactant (2)
Product (3)
ꢂ6.16
ꢂ6.54
ꢂ5.47
ꢂ1.01
ꢂ0.86
ꢂ1.21
5.14
5.68
4.25
3.58
3.70
3.34
ꢂ3.58
ꢂ3.70
ꢂ3.34
2.57
2.84
2.12
0.19
0.17
0.23
2.50
2.41
2.63
Table 10
Electrophilic reactivity descriptors (fkþ; skþ; xþ) of (1) and nucleophilic reactivity descriptors (fkꢂ; skꢂ; xkꢂ) of (2) for selected atomic sites, using Hirshfeld atomic charges.
k
fkþ; (e)
sþk (eVꢂ1
)
xþk (eV)
fkꢂ (e)
Reactant (1)
Reactant (2)
sꢂk (eVꢂ1
)
xꢂk (eV)
C7
C10
0.1479
0.0921
0.0287
0.0179
0.3702
0.2305
N9
N10
0.0916
0.1105
0.0161
0.0194
0.2214
0.2670
Sþk ¼ Sfkþ; sk ¼ Sfꢂ; sk0 ¼ Sfk0
ð9Þ
ð10Þ
from the environment. The energies of frontier molecular orbitals
LUMO), energy gap (eLUMO HOMO), electronegativity ( ),
chemical potential ( ), global hardness ( ), global softness (S) and
global electrophilicity index ( ) for reactants 1, 2 and product (3)
are listed in Table 9. The calculated high value of ( ) for (3) shows
k
(
eHOMO
,
e
ꢂ
e
v
xkþ
¼
x
fkþ; xk
¼
x
fkꢂ; x0k
¼
x
fk0
l
g
x
where +, ꢂ, 0 signs show nucleophilic, electrophilic and radical at-
tack respectively.
x
Local electrophilic reactivity descriptors (fkþ; skþ; xkþ) for se-
lected atomic sites of (1) and nucleophilic reactivity descriptors
(fkꢂ; sꢂk ; xꢂk ) for selected atomic sites of (2) are listed in Table 10,
using Hirshfeld population analyses respectively. The maximum
values of (fkþ; skþ; xþk ) at C7 in (1) indicate that this is most reactive
site for nucleophilic attack. The maximum values of (fkꢂ; skꢂ; xkꢂ) at
C10 in (2) indicate that this is most nucleophilic site. Therefore,
formation of product (3) is confirmed by the attack of C10 site of
(2) on C7 site in (1).
that the title molecule to be a strong electrophile than reactants 1
and 2.
4.7.2. Local reactivity descriptors
Fukui functions (fkþ; fkꢂ; fk0), local softnesses (sþk ; skꢂ; s0k ) and lo-
cal electrophilicity indices (xþk xkꢂ k0) have been described ear-
; ; x
lier in literature [56,57]. Using Hirshfeld population analyses of
neutral, cation and anion state of molecule, Fukui Functions are
calculated at same calculation method B3LYP/6-31G(d,p) using fol-
lowing equations:
Fukui functions (fþ; fkꢂ), Local softnesses ðskþ; sꢂk Þ, Local electro-
philicity indices (xkþ k
;
xꢂk ) for selected atomic sites of (3) are listed
fkþ ¼ ½qðN þ 1Þ ꢂ qðNÞꢄ for nucleophilic attack
fk ¼ ½qðNÞ ꢂ qðN ꢂ 1Þꢄ for electrophilic attack
fk0 ¼ 1=2½qðN þ 1Þ þ qðN ꢂ 1Þꢄ for radical attack
ð6Þ
ð7Þ
ð8Þ
in Table 11 using Hirshfeld population analyses. The high values of
all the three local electrophilic reactivity descriptors (fkþ; skþ; xkþ
)
at C7 indicate that this site is more prone to nucleophilic attack.
The calculated local reactivity descriptors of synthesized molecule
(3) favor the formation of new heterocyclic compounds such as
azetidinones, oxadiazolines and thiazolidinones by attack of nucle-
ophilic part of the dipolar reagent on the C7 site and electrophilic
part of dipolar reagent on the N10 site of C7@N10 bond.
where N, N ꢂ 1, N + 1 are total electrons present in neutral, cation
and anion state of molecule respectively.
Local softnesses and electrophilicity indices are calculated using
the following equations:
5. Conclusions
Table 11
Electrophilic reactivity descriptors (fkþ; skþ; xkþ) and nucleophilic reactivity descrip-
tors (fkꢂ; skꢂ; xꢂk ) for selected atomic sites of (3), using Hirshfeld atomic charges.
In the present study, compound (3) has been synthesized and
characterized by 1H NMR, UV–Vis, FT-IR and mass spectroscopy.
Experimental electronic absorption spectra have some blue shifts
compared with theoretical data and NBO analysis suggests that
Atom no.
sꢂk (eVꢂ1
)
xꢂk (eV)
fkþ (e)
sþk (eVꢂ1
)
xþk (eV)
fkꢂ (e)
N1
C2
C3
C4
C5
C6
C7
C8
0.0259
0.0239
0.0366
0.0045
0.0184
0.0352
0.1011
0.0377
0.0332
0.0534
0.0303
0.0595
0.0863
0.0582 0.0060
0.0743 0.0055
0.0231 0.0084
0.0672 0.0010
0.0760 0.0042
0.0862 0.0081
0.0679 0.0233
0.0645 0.0087
0.0168 0.0076
0.0520 0.0123
0.0834 0.0069
0.0243 0.0137
0.0543 0.0199
0.0136
0.0171
0.0053
0.0154
0.0175
0.0198
0.0156
0.0148
0.0038
0.0119
0.0192
0.0056
0.0125
ꢂ0.0004
0.0014
0.0064
0.0100
0.0079
0.0047
0.0120
0.0034
0.0061
0.0062
0.0682
0.0630
0.0964
0.0118
0.0486
0.0928
0.2662
0.0993
0.0875
0.1407
0.0798
0.1567
0.2272
0.0768
0.1331
0.1498
0.2541
0.1600
0.1448
0.1246
0.0351
0.0575
0.0565
0.1531
0.1956
0.0608
0.1769
0.2002
0.2268
0.1787
0.1697
0.0443
0.1369
0.2196
0.0641
0.1430
ꢂ0.0053
0.0162
0.0732
0.1150
0.0902
0.0546
0.1372
0.0393
0.0698
0.0714
electronic transitions are
p ?
pꢃ and n ? pꢃ in nature. The exper-
imental vibrational wavenumbers agree well with the calculated
wavenumbers of dimer, support the existence of dimer in the solid
state. A combined experimental and theoretical vibrational analy-
sis shows that in conventional intermolecular hydrogen bridge
(NAHꢀ ꢀ ꢀO@C) the stretching wavenumber of both NAH and C@O
are red-shifted due to the bond elongation. In contrary, the stretch-
ing wavenumber of CAH in non-conventional intermolecular
hydrogen bridge (CAHꢀ ꢀ ꢀO@C) is blue-shifted due to the bond con-
traction. The energy of each heteronuclear intermolecular hydro-
gen bond (NAHꢀ ꢀ ꢀO) is calculated as ꢂ6.0916 kcal/mol. AIM
ellipticity analysis confirms the presence of resonance assisted
intermolecular hydrogen bonds in dimer. In addition, theoretical
results from reactivity descriptors are in complete agreement with
observed reactivity of such type of compounds. The local electronic
reactivity descriptors show that azomethine carbon (C7) is more
reactive site for nucleophilic attack and may be used as precursor
for the syntheses of new heterocyclic compounds such as azetidi-
nones, oxadiazolines and thiazolidinones.
C9
N10
N11
C12
O13
C14
C15
C16
C17
C18
C19
O20
O21
C22
C23
0.0291 ꢂ0.0020 0.0067
0.0505
0.0569
0.0965
0.0608
0.0550
0.0473
0.0133
0.0218
0.0214
0.0061 0.0116
0.0278 0.0131
0.0437 0.0222
0.0342 0.0140
0.0207 0.0126
0.0521 0.0109
0.0149 0.0030
0.0265 0.0050
0.0271 0.0049

306
R.N. Singh et al. / Journal of Molecular Structure 1035 (2013) 295–306
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