Synthesis, spectroscopic analysis and theoretical study of new pyrrole-isoxazoline derivatives

Article abstract of DOI:10.1016/j.molstruc.2016.09.042

In the present work, we have efficiently synthesized the pyrrole-isoxazoline derivatives (4a-d) by cyclization of substituted 4-chalconylpyrrole (3a-d) with hydroxylamine hydrochloride. The reactivity of substituted 4-chalconylpyrrole (3a-d), towards nucl

Full text of DOI:10.1016/j.molstruc.2016.09.042

Accepted Manuscript
Synthesis, spectroscopic analysis and theoretical study of new pyrrole-isoxazolines
derivatives
Poonam Rawat, R.N. Singh, Vikas Baboo, Priydarshni Niranjan, Himanshu Rani,
Rajat Saxena, Sartaj Ahmad
PII:
S0022-2860(16)30969-3
10.1016/j.molstruc.2016.09.042
MOLSTR 22954
DOI:
Reference:
To appear in: Journal of Molecular Structure
Received Date: 10 June 2016
Revised Date: 13 September 2016
Accepted Date: 16 September 2016
Please cite this article as: P. Rawat, R.N. Singh, V. Baboo, P. Niranjan, H. Rani, R. Saxena, S. Ahmad,
Synthesis, spectroscopic analysis and theoretical study of new pyrrole-isoxazolines derivatives, Journal
of Molecular Structure (2016), doi: 10.1016/j.molstruc.2016.09.042.
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ACCEPTED MANUSCRIPT

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Synthesis, Spectroscopic Analysis and Theoretical Study of New
pyrrole-isoxazolines derivatives
Poonam Rawat, R.N. Singh*, Vikas Baboo, Priydarshni Niranjan, Himanshu Rani, Rajat Saxena
and Sartaj Ahmad
Department of Chemistry, University of Lucknow, Lucknow, Uttar Pradesh, India 226 007
Abstract
In the present work, we have efficiently synthesized the pyrrole-isoxazoline derivatives (4a-d)
by cyclization of substituted 4-chalconylpyrrole (3a-d) with hydroxylamine hydrochloride. The
reactivity of substituted 4-chalconylpyrrole (3a-d), towards nucleophiles hydroxylamine
+
+
+
hydrochloride was evaluated on the basis of electrophilic reactivity descriptors (f , s , ω ) and
k
k
k
they were find to be high at unsaturated β carbon of chalconylpyrrole indicating its more
proneness to nucleophilic attack and thereby favoring the formation of reported new pyrrole-
isoxazoline compounds (4a-d). The structures of newly synthesized pyrrole-isoxazoline
1
derivatives were derived from IR, H NMR, Mass, UV-Vis and elemental analysis. All
experimental spectral data corroborate well with the calculated spectral data. The FT-IR analysis
shows red shifts in vN–H and vC=O stretching due to dimer formation through intermolecular
hydrogen bonding. On superposition error correction the intermolecular interaction energy for
(4a-d) is found to be 10.10, 9.99, 10.18, 11.01 and 11.19 kcal/mol. respectively. The calculated
-30
first hyperpolarizability (β ) values of (4a-d) molecules are in the range of 7.40-9.05 x 10 esu
0
1

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indicating their suitability for non–linear optical (NLO) applications. Experimental spectral
results, theoretical data, analysis of chalcone intermediates and pyrrole–isoxazolines find
usefulness in advancement of pyrrole-azole chemistry.
1
Keywords –Pyrrole-isoxazolines, H-NMR, UV-Visible, FT-IR, NLO
*
Corresponding Author – Prof. R. N. Singh
Tel.: +91 9451308205.
E-mail address: rnsvk.chemistry@gmail.com
2

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1
. Introduction
In the last few decades the chemistry of five-membered heterocycles particularly pyrroles,
pyrazolines and isoxazolines have received considerable attention owing to their synthetic and
effective biological importance [1]. Isoxazole is an azole having π–excessive five-member ring
with two different heteroatoms, oxygen and nitrogen in adjacent positions, next to each other at
1
, 2 positions [2-6]. Partially saturated analog of isoxazole is called isoxazoline and completely
saturated analog is isoxazolidine [7]. Isoxazolines continue to grow due to their role as building
block, applications in the fields of pharmaceutical and agrochemical chemistry, catalysis and
material science as well as crucial role in the theoretical development of heterocyclic chemistry
[
5-9]. Compounds possessing electron releasing groups such as dimethyl amino, methoxy and
hydroxyl substituent on both the rings at position 3 and 5 of isoxazolines, considerably enhanced
the antidepressant activity when compared to the isoxazolines having no substituent’s on the
rings [10, 11]. Various isoxazolines with different substituents have been synthesized with the
sole purpose of providing isoxazolines derivatives with unique properties for various
applications.
Encouraged due to diverse mentioned applications of isoxazoline derivatives, the main
goal of this study has been to synthesize, characterize and investigate the structure and properties
of pyrrole containing isoxazoline derivatives. In this work pyrrole-isoxazoline derivatives have
been synthesized via cyclization of substituted 4-chalconylpyrrole intermediates with
hydroxylamine hydrochloride in ethanol. The synthesized pyrrole–isoxazoline compounds may
find usefulness in medicinal chemistry and NLO applications. In addition to this the global and
local reactivity descriptors have been calculated to predict the formation of pyrrole–isoxazoline
3

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derivatives from pyrrole containing chalcones. The present paper covers the spectral analysis of
the synthesized compounds using both experimental and quantum chemical calculations.
Spectral analysis and theoretical results of pyrrole–isoxazolines are found to be responsible for
the advancement of pyrrole-azole chemistry.
2
.
Experimental detail
2
.1 Materials and Methods
Thin layer chromatography (TLC) was carried out using glass plates pre-coated with silica gel
Kieselgel 60 F256, 0.2 mm, mesh point, Merck) and visualized using UV-Visible light. Silica
(
gel (Flash Kiesegel 60 mesh point) was used for column chromatography. The 1H NMR
spectrum was recorded in MeOD on Bruker DRX–300 spectrometer using TMS as an internal
reference. The FT–IR spectrum was recorded in KBr pellets on a Bruker spectrometer. The UV–
-5
-6
Visible absorption spectrum (10 to 10 M in DMSO) was recorded on ELICO SL–164
spectrophotometer.
2
.2 a. General procedure for the synthesis of pyrrole-isoxazoline derivatives (4a-d)
To the solution of key intermediate (0.00034 mol) pyrrole containing chalcone (3a-d) [12, 13] in
ethanol, 2–4 drop of HCl was added to catalyze the reaction. Then the solution of (0.00034 mol)
hydroxylamine hydrochloride (HH) in 5 ml ethanol was added drop–wise to the stirring reaction
mixture and refluxed till the completion for the reaction with continuous stirring. Cool the
reaction mixture to the room temperature then pour into ice cool water. On standing overnight
precipitate was found, wash with water and dried in air. The completion of the reaction was
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checked with the help of TLC technique using ethyl acetate and hexane in the ratio of 3:7. The
crude compounds were separated through column chromatography (ethylacetate/hexane) to give
the analytically pure pyrrole-isoxazoline derivatives (4a-d).
2
.2.b General procedure for the synthesis of pyrrole containing chalcones (3a-d)
This pyrrole containing chalcones (3a-d) (1, 3-diarylprop-2-en-1-ones) (detailed general
procedure for the synthesis and characterization of 4-chalconylpyrrole intermediates are given in
Supplementary material) were prepared by reaction of ethyl 4-acetyl-3, 5-dimetyl-1H-pyrrole-2-
carboxylate (1) and aromatic aldehydes (2-chloro-benzaldehyde (2a) / 4-chloro-benzaldehyde
(2b) / 2-nitro-benzaldehyde (2c) / 3-nitro-benzaldehyde) in the presence of potassium hydroxide
and methanol through Claisen-Schmidt condensation.
2
1
.2.1 Synthesis of ethyl 4–(5–(2–chlorophenyl)–4,5–dihydroisoxazol–3–yl)–3,5–dimethyl–
H–pyrrole–2–carboxylate (3a)
Yield – 88 %, Melting Point – 120°C (decompose). Elemental analyses: Cal. C, 69.92; H, 6.79;
N, 8.58 and found C, 69.90; H, 6.79; N, 8.60.
2
1
.2.2 Synthesis of ethyl 4–(5–(4–chlorophenyl)–4,5–dihydroisoxazol–3–yl)–3,5–dimethyl–
H–pyrrole–2–carboxylate (3b)
Yield – 80%, Melting Point – 196 –197°C.Elemental analyses: Calculated C, 69.92; H, 6.79; N,
.58 and found C, 69.91; H, 6.78; N, 8.59.
8
5

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2
.2.3 Synthesis of ethyl 3,5–dimethyl–4–(5–(2–nitrophenyl)–4,5–dihydroisoxazol–3–yl)–1H–
pyrrole–2–carboxylate (3c)
Yield – 60%, Melting Point – 188–190°C.Elemental analyses: Calculated C, 60.50; H, 5.36; N,
1
1.76; found C, 60.52; H, 5.34; N, 11.74.
2
.2.4 Synthesis of ethyl 3,5–dimethyl–4–(5–(3–nitrophenyl)–4,5–dihydroisoxazol–3–yl)–1H–
pyrrole–2–carboxylate (3d)
Yield – 55%, Melting Point – 171–172°C. Elemental analyses: Calculated C, 60.50; H, 5.36; N,
1
1.76; found C, 60.51; H, 5.33; N, 11.75.
3
.
Computational methods
In order to investigate the geometries and spectroscopic properties of pyrrole-isoxazoline
derivatives, all quantum-chemical calculations were performed using Gaussian 09 program
package [14] at B3LYP/6-31G(d,p) level. The calculated wavenumbers were scaled down by
using scaling factors [15] and vibrational assignments of the normal modes were proposed on the
basis of the PED calculated using program GAR2PED [16] and internal coordinates were
1
defined using Pulay’s recommendations [17]. H NMR chemical shifts were calculated by
employing the Gauge-Included Atomic Orbital (GIAO) [18]. TD-DFT calculations were carried
out to obtain the theoretical excitation spectra. The global and local indexes of reactivity were
calculated with Koopman´s approximation [19, 20]. The hydrogen bonding energy was
computed as the difference in energy between the dimer on one hand, and the sum of isolated
monomers on the other hand. Basis set superposition error (BSSE) was corrected by the
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counterpoise procedure of Boys and Bernardi [21], as ∆E (BSSE) = E (complex) – E (monomer
Aunit) – E (Monomer Bunit). Visualization of molecules at their optimized geometries, molecular
electrostatic potential surfaces (MEP) was developed with the help of Gauss View [22] program
package.
4.
Results and discussion
4
.1 Evaluation of Molecular Structure, Conformational analysis, Chemical reactivity of
chalcone intermediate and pyrrole–isoxazoline
New isoxazoline derivatives (4a-d) were synthesized by cyclization of substituted chalcone
derivatives (3a-d) with hydroxylamine hydrochloride are shown in Scheme 1. The key
intermediates pyrrole chalcone (3a-3d) were prepared by treating ethyl 4-acetyl-3,5-dimetyl-1H-
pyrrole-2-carboxylate (1) with benzaldehyde derivatives (2a-d) in presence of sodium hydroxide
according to Claisen-Schmidt condensation method [12, 13]. The optimized geometry of lower
energy conformers of (3a-d) are given in Supplementary Figure FS1 and the major reactive site
in (3a-d) is shown in Figure 1. The chalcones (3a-d) were used as suitable precursor for the
synthesis of isoxazolines (4a-d). The chalcones (3a-d) on treatment with hydroxylamine
hydrochloride in ethanol, pyrrole-isoxazoline (4a-d) derivatives were obtained. New pyrrole-
isoxazoline derivative were obtained in good percentage yields. All the synthesized compounds
(4a-d) are stable at room temperature.
The new pyrrole isoxazolines (4a-d) were optimized after deriving the tentative structure
1
with the help of IR, HNMR and UV-VIS spectroscopy. Further conformational studies were
performed with help of potential energy surface analysis without imposing any symmetry
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constrains on the molecule. Potential energy surface scan studies have been performed to
understand the conformers’ stability which requires a careful scan of the conformational space
by using a reliable theoretical level. The potential energy of pyrrole isoxazolines (4a-d) were
determined by the variation in the total energy of the molecule with change in dihedral angles
(
C3C2C9O10) and (C3C4C7C16) at intervals of 10° by B3LYP/6-31G (d,p) method (Figure 2).
Two minima were observed in the potential energy curve around the dihedral angles
C3C2C9O10) of pyrrole-isoxazolines (4a-d) which correspond to the conformers IA/IB/ IC/ID
(
and IIA/IIB/IIC/IID, where the former is lower in energy than latter and thus represents the more
stable conformation to be called conformer IA/IB/IC/ID. The enthalpy difference between
conformer IA/IB/IC/ID and IIA/IIB/IIC/IID is 0.222, 0.119, 0.242 and 0.244 kcal/mol
respectively. The other two conformers around the dihedral angles (C3C4C7C16) of pyrrole-
isoxazoline derivatives (4a-d) correspond to conformer IA/IB/IC/ID and IIIA/IIIB/IIIC/IIID with
enthalpy difference 0.680, 0.841, 0.584 and 0.749 kcal/mol., respectively. All calculations have
been performed on stable conformers which would correspond to lower energies value i. e the
conformer IA/IB/IC/ID. All the geometrical parameters are given in Supplementary Table S1-S4.
The Molecular structures of lower energy conformer possess C1 point group symmetry. The
optimized structural parameters (bond lengths, bond angles, dihedral angle) of lower energy
conformer are given in Supplementary material Table S1-S4. The lower energy conformers of
new pyrrole-isoxazolines (4a-d) derivatives have been presented in Figure 3. The endocyclic
angles of the pyrrole rings add up to exactly 540°, indicating pyrrole ring is planar in all new
pyrrole-isoxazolines (4a-d) derivatives. The N1–C2 and N1–C5 bonds of pyrrole rings have
been observed to be asymmetric due to the presence of electron withdrawing ethoxycarbonyl
8

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group. This observation has also been shown in the other quantum calculations and crystal
structures of pyrrole-2-carboxylate [23, 24] and other pyrrole derivatives [25-26].
Electrostatic potential surface of (4a-b) is shown in Figure 4 which provides a visual
method to understand the reactivity of the molecules and correlates with dipole moment,
electronegativity, partial charges and site of chemical reactivity of the molecule. The red colour
region is present around carbonyl group of ester in (4a-d) and around NO group in 4c and 4d
2
shows the negative electrostatic potential. The blue colour around pyrrolic NH group shows most
positive electrostatic potential, whereas rest of the region of the molecule is shown by green
colour having moderate electrostatic potential. In (4a-d), nucleophilic region present around
ester carbonyl group and electrophilic region around the hydrogen atom of pyrrole N-H.
1
The structures of synthesized compounds were not only confirmed by IR, H NMR and
elemental analysis but also with the electronic descriptors. Conceptual DFT concentrates on the
extraction of chemically relevant concepts and principles from DFT. DFT has provided a sound
basis for the development of computational strategies for obtaining information about the
energetics, structure, and properties of (atoms and) molecules at much lower costs than
traditional ab-initio wave function techniques [19, 20, 27]. As per Sanderson’s electronegativity
equalization principle when atoms of different chemical potential unite to form a molecule their
chemical potential must equalize with its own characteristic chemical potential, to the extent that
the atoms retain their identity. The chemical potential characterizes the escaping tendency of
electrons from the equilibrium system. Systems (e.g. atoms or molecules) coming together must
attain at equilibrium, a common chemical potential. This chemical potential is none other than
the negative of the electronegativity. It is a constant, through all space, for the ground state of an
9

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atom, molecule or solid, and equals the slope of the Energy versus N curve at constant Potential
v (r),

∂E 
µ
=
= −χ



r
∂N

v(r )
χ
(N) = −(∂E / ∂N)V (r)
Chemical potential and electronegativity as per finite difference approximation is given as
follows:-
1
2
1
2
µ
χ
≈ −
(
I + A
)
≈
(
ε HOMO
+
ε LUMO
)
≈ −1/ 2(
ε
HOMO
−
ε
LUMO
)
Where I (ionization energy) and A (electron affinity) are replaced by the HOMO (highest
occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital) of the molecules
using Koopmans’ theorem.
The theoretical definition of chemical hardness is given as the second derivative of electronic
energy with respect to the number of electrons N, for a constant external potential V(r)
2
1
 ∂ E 
1  ∂
= 
2  ∂N 
µ

1
2
η
= 



≈
(
I − A
)

2
r
2
∂N
r


V (r )
V (r )
Finite difference approximation to chemical hardness gives,
1
η
≈
(
ε LUMO
−
ε HOMO
)
2
For Insulator and Semiconductor, hardness is half of the band gap.
1
0

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Softness is inversely proportional to hardness and given as follows
2
1
 ∂ N 
 ∂N 
S =
= 


= 



2

2η
∂E
∂µ


V (r)


V (r)
Electrophilicity Index another reactivity descriptor which measure of energy lowering due to
maximal electron flow between donor and acceptor. Electrophilicity index (
ω
) is calculated as
2
µ
ω
=
2η
Condensed Fukui Function describes the reactivity of an atom in a molecule. It is condensed
value around each atomic site into a single value (f ) for an atom k in a molecule that
k
characterizes the atomic contribution in a molecule. The f values are defined as
k
+
k
Nucleophilc attack
Electrophilic attack
Radical attack
f = q (N +1) − q (N)
k
k
−
f = q (N) − q (N −1)
k
k
k
o
f =
[
q (N + 1) − q (N −1) 2
]
k
k
k
To understand the formation of pyrrole-isoxazoline derivatives (4a-d) from pyrrole-chalcones
and other research goal, the calculation of the chemical reactivity parameters such as chemical
potential (
softness (S), electrophilicity / electron donor potential (
chalcones and pyrrole-isoxazolines were done. The calculated global and local reactivity
parameters of (3a-d 4a-d and hydroxylamine) derived from DFT have been listed in Table 1
and . The calculated value of electrophilic charge transfer (ECT) from hydroxylamine
hydrochloride to ethyl 4-[3-(2-Chloro-phenyl)-acryloyl]-3,5-dimethyl-1H-pyrrole-2-carboxylate
ꢀ
), electron affinity (EA), electronegativity (
χ
), chemical hardness (
η), chemical
ω
) and fukui function of pyrrole-
,
2
1
1

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(3a
)
/ ethyl 4-[3-(4-Chloro-phenyl)-acryloyl]-3,5-dimethyl-1H-pyrrole-2-carboxylate (3b) /
ethyl 3,5-Dimethyl-4-[3-(2-nitro-phenyl)-acryloyl]-1H-pyrrole-2-carboxylate (3c) and ethyl 3,5–
dimethyl–4–(3–(3–nitrophenyl)acryloyl)–1H–pyrrole–2–carboxylate (3d) is found to be = 0.757
/
0.840 / 1.196 / 1.182 eV, respectively indicating that charge flow from hydroxylamine
hydrochloride to (3a-d) molecules Therefore, (3a-d) molecules act as an electron acceptor and
.
hydroxylamine as an electron donor. The local electronic descriptors for (3a-d) molecules and
hydroxylamine hydrochloride have been listed in Table 2. The maximum values of local
+
+
+
electrophilic reactivity descriptor (f , s , ωk ) at C15 for (3a-d) molecules indicate that this site
k
k
is prone to nucleophilic attack. The calculated local reactivity descriptors for (3a-d) molecules
favor the formation of new pyrrole isoxazoline (4a-d) by the nucleophilic attack on the C15 site
of C14=C15 bonds of chalcone (3a-d) by hydroxylamine hydrochloride.
4
.2 Vibrational spectroscopy
Experimental and theoretical IR spectra of pyrole–isoxazoline derivatives in the region
–
1
–
4
00–4000 cm are shown in Figure 5. Theoretical (scaled) and experimental wavenumbers (cm
1
)
with their assignment are given in Table 3. DFT calculations for vibrational spectral
parameters were carried out for both the monomers and dimers but values of dimers were found
to be more closure to experimental values since dimer formation happens resulting lowering in
energy in solid state.
The N-H stretching frequencies occurs in a much wider range at lower frequencies, viz,
-
1
3
3
315-3085 cm in the solid state whereas in solution, a sharp band appears in the range 3435-
-1
400 cm . The frequencies shifts to lower region or red shift is observed due to hydrogen
1
2

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bonding with increase in IR intensity. In pyrole–isoxazoline derivatives the N-H and C=O
frequencies were observed at lower wavenumbers. Therefore, we computed the IR frequencies
for the dimer structure having two >N-H•••O=C< hydrogen bonds which agreeably correlate
with the observed red shift of the >N-H and >C=O stretches in the experimental FT-IR spectra.
These results clearly confirm the dimeric stable structure formation by hydrogen bonds >N-
H•••O=C< between monomeric units. In the experimental FT-IR spectrum of (4a-d), the N–H
-1
stretch of pyrrole (νNH) is observed at 3299, 3265, 3295, 3295 cm , whereas it is calculated as
-1
-1
3
346, 3345, 3345, 3342 cm in dimers and 3502, 3502, 3501, 3500 cm in monomers. This
-1
observation is correlates well with the earlier reported strong absorption band at 3358 cm for
hydrogen bonded pyrrole-2-carboxylic acid recorded in KBr pellet, but it deviates to the reported
-1
free νNH band at higher wavenumber 3465 cm , recorded in CCl solution [28, 29]. Therefore,
4
solid state spectrum of (4a-d) attributes to the vibration of hydrogen-bonded N–H group and the
intensity of N-H associated bands is also sharp and intense as absorbed in the experimental
spectra and theoretical calculations. In addition to this observation the N-H out of plane bending
δoop mode of pyrrole additionally confirms involvement of the N-H group in intermolecular
-1
interactions which are at 630, 622, 629, 633 cm corresponds to the calculated wavenumbers at
-1
6
29, 625, 628, 630 cm . These observed wavenumbers are in good agreement with the
calculated wavenumbers of dimers (Figure 6). The stretching mode (νC=O) of carbonyl group of
-1
ester in 4a-d, are observed at 1683, 1676, 1678, 1649 cm , corroborating well with calculated
-1
wavenumbers at 1684, 1698, 1686, 1654 cm in dimer but deviating in monomers at 1701, 1698,
-1
1
720, 1715 cm . The observed νC=O stretching vibration also agrees well with the earlier
-1
reported wavenumber at 1665 cm for ester containing pyrrole derivatives [30-38]. Therefore,
1
3

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νC=O stretching mode in (4a-d) confirms the involvement of C=O group in intermolecular
hydrogen bonding. The downshifting in the νC=O stretching mode in dimer confirms the
involvement of C=O group in intermolecular hydrogen bonding. In dimer, the stretching
wavenumber of hydrogen bond acceptor (C=O) is red–shifted (i.e. elongation of C=O bond) due
to its conventional nature than hydrogen bond free C=O group in monomer.
The stretching vibrations of C–H of phenyl ring, -CH and -CH - are observed in the
3
2
–
1
range of 3050–2900 cm . The stretching of C–H of Me was observed at 2979, 2924, 2962 and
–1
–1
2
988 cm , whereas calculated at 2979, 2926, 2979 and 2959 cm for (4a-d) molecules. The
–
1
symmetric deformation of -CH is observed at 1377, 1344, 1377 and 1375 cm , asymmetric
3
–
1
deformation observed at 1511, 1484, 1431 and 1441 cm , whereas in the calculated spectrum
–
1
symmetric deformation are assigned at 1373, 1374, 1375 and 1376 cm and asymmetric
–
1
deformation are assigned at 1498, 1498, 1433 and 1433 cm for (4a-d) molecules, respectively.
Experimental vibration belongs to –CH – group are in good agreement with calculated vibrations
2
are listed in Table 3. The identification of C–N vibrations is a very difficult task, since the
mixing of several bands is possible in this region. The calculated
ν(C–N) vibration assigned at
–
1
–1
–1
–1
1
285 cm for 4a, 1286 cm for 4b, 1284 cm for 4c and 1285 cm for 4d, whereas these
–
1
vibrations experimentally observed at 1275, 1272, 1282 and 1276 cm for (4a-d) molecules,
respectively. Other vibrations, related to rings, C–O, C–C, N–O, N–H and C–Cl with their
experimental and calculated wavenumbers are also tabulated in the given Table 3
.
1
4

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1
4
.3 H NMR spectroscopy
1
The experimental HNMR chemical shifts together with the calculated data for the stable
1
conformer of (4a-d) are given in Table 4. The experimental HNMR spectra of (4a-d) are given
in supplementary Figure FS2-FS5. The effect of solvent on the theoretical NMR parameters was
evaluated using the integral equation formalism polarizable continuum model (IEFPCM) model
1
in methanol solvent. The experimental values of the H chemical shift for N–H proton of pyrrole
in (4a-d) were found experimentally at
δ10.54,
δ9.98,
δ10.89 and
δ9.99 ppm, respectively,
whereas calculated by applying GIAO method [18] are at
δ
8.64, 8.36,
δ
δ
8.66 and 8.649 ppm,
δ
respectively. Rather surprisingly, the largest differences between the experimental and calculated
1
H chemical shifts of (4a-d) derivatives were found due to interaction of molecules with highly
polar solvent MeOD. The chemical shifts at
2.52 & 2.46 (4c) and 2.52 & 2.46 ppm (4d) have been assigned for CH groups attached at
and positions in pyrrole ring. The calculations give different isotropic shielding for methyl
δ2.51 (3H) & δ2.45 (3H) (4a), δ2.56 & δ2.50 (4b),
δ
δ
δ
δ
3
α
β
group proton which appears as singlet in our spectrum because a fixed geometry was used for
1
calculating H NMR spectrum. The chemical shifts for 4a
,
4b
4.21 whereas experimental values found in the range 4.32-4.23, 4.35-4.28, 4.34-
.27 and 4.37-4.30 ppm have been assigned to (–CH –) group protons attached to methyl group
, 4c and 4d calculated at δ4.29,
δ
4.31, δ4.22, δ
4
2
and appears as quartet in the spectrum, respectively. The calculated chemical shifts of (-CH3)
groups correspond to experimental values are given in Table 4. The signal at 2.56, 3.26, 3.30
and 3.29 ppm in 4a 4b 4c and 4d molecules have been assigned to the isoxazoline ring proton
–CH –), respectively.
δ
δ
δ
δ
,
,
(
2
1
5

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4
.4 UV-visible spectroscopy
The absorption of UV-Visible radiation by a molecule leads to transitions among the
electronic energy levels of the molecule. Its strength lies in its ability to measure the extent of
multiple bond or aromatic conjugation within molecule. To obtain the nature of the transitions,
electronic excitation energies and oscillatory strength the UV–visible spectra of hybrid
pyrrole-isoxazoline derivatives have been studied by the time–dependent density functional
theory (TD–DFT) [39]. The effect of solvent on the theoretical UV-Vis spectra were also
evaluated using the integral equation formalism polarizable continuum model (IEFPCM) model
in Dimethyl sulfoxide (DMSO) solvent. The observed and calculated electronic transitions of
high oscillatory strength for molecule (4a-d) are given in Table 5. The theoretical and
experimental UV–Vis spectra are shown in Figure 7. The calculated electronic transitions with
observed absorption maxima (in parentheses) for molecule ‘4a
’
λmax are assigned at 283 (298) &
38 (254) nm, for molecule ‘4b’ at 281 (310) & 254 (254) nm, for molecule ‘4c’ at 282 (278) &
39 (230) nm, and for molecule ‘4d’ at 283 (290), 277 (258) nm, respectively. All these
2
2
transition are assigned as π→π* and n→π* transitions. Chlorine is weak electron withdrawing
group whereas nitro group is strong electron withdrawing group, therefore, nitro group more
prominently influence the absorption peak to appear at longer wavelength in compound (4c-d).
4
.5 Static dipole moment (ꢀ ), mean polarizability (|α |), anisotropy of polarizability (∆α)
0
0
and first hyperpolarizability (β0)
Direct measurement of hyperpolarizability is difficult task but computational calculation
provides an alternate choice to investigate the extensive properties of materials. They determine
1
6

ACCEPTED MANUSCRIPT
the strength of molecular interactions (long–range intermolecular induction, dispersion forces,
etc.), cross sections of different scattering and collision processes, as well as the non–linear
optical (NLO) properties of the system [40]. The development of organic NLO materials for
device applications requires multidisciplinary effort involving both theoretical and experimental
studies. Quantum-chemical calculations have made an important contribution to the
understanding of the electronic polarization underlying the molecular NLO processes and the
establishment of structure–property relationships [41]. Organic molecules with large second–
order NLO are the subject of substantial research due to their potential applications in optical
modulation, molecular switching, optical memory, and frequency doubling [42, 43]. In this
–
30
study, p–Nitroaniline (p–NA, β0 = 11.54 x10 esu) is chosen as a reference material. The
calculated dipole moment (ꢀ0), mean polarizability (| 0|), anisotropy of polarizability (∆α) and
first hyperpolarizability (β0) for monomer and dimer at B3LYP/6–311+G (d,p) are listed in
α
-30
-30
-30
Table 6. The β0 values for (4a-d) are calculated as 7.40 x 10 , 9.05 x 10 , 8.49 x 10 x 9.05 x
-30
1
0
esu.
5. Conclusion
Pyrrole-isoxazoline derivatives (4a-d) have been synthesized from 4-chalconylpyrrole
intermediates and characterized by 1H-NMR, FT-IR and UV-Visible spectroscopy. Spectral
analysis and quantum chemical calculations provided the electronic structures of pyrrole-
isoxazoline derivatives (4a-d). The enthalpy difference between conformer IA/IB/IC/ ID and IIA
/
IIB/IIC/IID are 0.222, 0.119, 0.242 and 0.244 kcal/mol. The other two conformers around the
dihedral angles (C3C4C7C16) of (4a-d) correspond to conformer IIIA/ IIIB/IIIC/IIID and
1
7

ACCEPTED MANUSCRIPT
IVA/IVB/IVC/IVD with enthalpy difference between conformer IIIA/IIIB/IIIC/IIID and
IVA/IVB/IVC/IVD are 0.680, 0.841, 0.584 and 0.749 kcal/mol, respectively. Theoretical
electronic absorption spectra, experimental data and molecular orbital coefficients’ analysis
suggests that electronic transitions are π→π* and n→π* in nature. In the present study,
experimental and calculated vibrational wavenumber analysis confirms the existence of dimer by
involvement of heteronuclear association through pyrrolic (N─H) hydrogen and carbonyl (C=O)
oxygen of ester in solid state. The binding energy of intermolecular interaction of (4a-d) are
found to be 10.10, 9.99, 10,18, 11,01 and 11.19 kcal/mol, respectively after basis set
superposition error correction. Calculated and assigned vibrational frequencies validate
experimental IR spectra and support each other. The synthesized derivatives of pyrrole-
isoxazoline may be used as a precursor for the synthesis of new heterocyclic compounds. The
calculated first hyperpolarizability (β0) values of investigated molecules (4a-d) are in range of
7
.40-9.05 x 10-30 esu showing their suitability for non–linear optical application.
Acknowledgement
Authors are thankful to the DST New Delhi for providing research fund and IIT Kanpur for
spectral data recording.
1
8

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2
1

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Table Captions
Table 1. Calculated εHOMO
,
εLUMO, energy band gap ( H
), global softness (S), global electrophilicity index (
and electrophilic charge transfer (ECT) for reactants and products.
ε
- εL), chemical potential (ꢀ),
electronegativity (
χ), global hardness (
η
ω)
+
-
+
-
+
Table 2. Fukui functions (f , f ), local softnesses (s , ), local electrophilicity indices (ωk
,
k
k
k
SK
-
ωk ) for selected atomic sites of 3a-d using Hirshfield population analysis
Table 3. Calculated (selected) and experimental vibrational wavenumbers of synthesized
–
1
pyrrole–isoxazoline derivatives (4a-d) and their assignments: Wavenumbers (in cm )
1
Table 4. Experimental and calculated H–NMR chemical shifts of pyrrole–isoxazoline
derivatives (4a-d) using DFT/B3LYP/6–31G(d,p)
Table 5. Comparison between calculated and experimental electronic transitions: E (eV),
oscillatory strength (f), λmax (nm) using TD–DFT/B3LYP/6–31G(d,p) in (4a-d
)
Table 6. Calculated Dipole moment (µ ), Polarizability (|α0|), anisotropy of Polarizability (ꢁα),
0
First Hyperpolarizability (β0) in (4a-4d) and their components.
Figure captions
Scheme 1. Synthesis of pyrrole-isoxazoline derivatives (4a-d
Figure 1. Depicts the most reactive sites in pyrrole-chalcone derivatives (3a-d) for the formation
of pyrrole-isoxazoline derivatives (4a-d
Figure 2. The potential surface scans for (4a-d) around the dihedral angles (C3C2C9O10) and
C3C4C7C16)
)
)
(
2
2

ACCEPTED MANUSCRIPT
Figure 3. Optimized structures of conformers of pyrrole-isoxazoline derivatives (4a-d
)
Figure 4. The molecular electrostatic surface of (4a-d
Figure 5. Experimental and theoretical FT-IR spectra of pyrrole-isoxazoline derivatives (4a-d)
Figure 6 Dimers of (4a-d
)
)
Figure 7 (a) Theoretical (b) Experimental UV-Visible spectra of (4a-d)
Supplementary Data Captions
FS.1 The optimized geometry of pyrrole chalcone (3a-d
)
1
FS.2. Experimental H NMR of Molecule 4a
1
FS.3. Experimental H NMR of Molecule 4b
1
FS.4. Experimental H NMR of Molecule 4c
1
FS.5. Experimental H NMR of Molecule 4d
Table S1. The calculated geometrical parameters of compound 4a
Table S2. The calculated geometrical parameters of compound 4b
Table S3. The calculated geometrical parameters of compound 4c
Table S4. The calculated geometrical parameters of compound 4d
General procedure for the synthesis of 4-chalconylpyrrole intermediates
2
3

ACCEPTED MANUSCRIPT
Table 1. Calculated εHOMO, εLUMO, energy band gap (ε – ε ), chemical potential (ꢀ), electronegativity (χ), global
L
H
hardness (η), global softness (S), global electrophilicity index (ω) and electrophilic charge transfer (ECT) for reactants
and products.
Molecule
Reactant 3a
Reactant 3b
Reactant 3c
Reactant 3d
Hydroxylamine
Product 4a
Product 4b
Product 4c
εHOMO
-6.000
-6.072
-6.071
-6.117
-8.041
-5.567
-5.627
-5.624
-5.766
εLUMO
-1.970
-2.114
-2.52
-2.529
-0.821
-0.693
-0.748
-2.495
-2.410
εL – εH
4.030
3.958
3.546
3.588
7.219
4.873
4.878
3.129
3.355
ꢀ
χ
η
S
ω
ECT
3.990
4.093
4.298
4.323
4.431
3.130
3.187
4.059
4.088
-3.990
-4.093
-4.298
-4.323
-4.431
-3.130
-3.187
-4.059
-4.088
2.010
1.979
1.773
1.794
3.609
2.436
2.439
1.564
1.677
0.250
0.252
0.282
0.278
0.138
0.205
0.204
0.319
0.298
3.950
4.232
5.209
5.208
2.720
2.010
2.082
5.266
4.982
0.757
0.840
1.196
1.182
Product 4d

ACCEPTED MANUSCRIPT
+
-
+
-
Table 2 Fukui functions (f , f ), local softnesses (s , ), local electrophilicity indices
k
k
k
SK
+
-
(
ω , ω ) for selected atomic sites of 3a-d using Hirshfield population analysis
k k
+
-
+
-
+
-
Compounds
f
k
f
k
s
k
S
K
ω
k
ω
k
3a
C 13
C 14
C 15
0.0746
0.0881
0.1101
0.0089
0.0415
0.0496
0.0186
0.0220
0.0275
0.0022
0.0103
0.0124
0.2946
0.3479
0.4348
0.0351
0.1639
0.1959
3b
C13
C14
C15
0.0735
0.0870
0.1116
0.0090
0.0502
0.0519
0.0185
0.0219
0.0281
0.0022
0.0126
0.0130
0.3110
0.3681
0.4722
0.0380
0.2124
0.2196
3c
C13
C14
C15
0.0468
0.0750
0.0501
0.0175
-0.0099 0.0211
0.0447
0.0131
0.0049
-0.0027
0.0126
0.2437
0.3906
0.2609
0.0911
-0.0515
0.2328
0.0141
3d
C13
C14
C15
0.0500
0.0720
0.0651
0.0095
0.0276
0.0560
0.0139
0.0200
0.0180
0.0026
0.0076
0.0155
0.2604
0.3749
0.3390
0.0494
0.1437
0.2916

ACCEPTED MANUSCRIPT
Table 3 – Calculated (selected) and experimental vibrational wavenumbers of synthesized pyrrole–isoxazoline
–
1
derivatives (4a-d) and their assignments: Wavenumbers (in cm )
4a
4b
Calc.
4c
4d
Calc.
Assignment
S.No
.
Calc.
Exp.
Exp.
Calc.
Exp.
Exp.
M
D
M
D
M
D
M
D
1
2
3
4
5
6
7
8
9
.
.
.
.
.
.
.
.
.
3502
3079
2979
2928
1701
3346
3032
2930
2920
1684
3299
3290
2979
2927
1683
3502
3049
2976
2926
1698
3345 3265 3501
3102 3097
3295 3500
3290 3090
2962 2959
2932
3291 ν(N–H) Pyrrole
3199 ν(C–H) ring
2988 ν(C–H)
3
3
3
3
1
1
1
1
1
1
1
1
1
345
3342
3071
3017
3019
1654
1560
1537
1466
1461
1391
1375
1373
1343
085
017
009
686
603
604
569
536
506
466
455
409
2980
2979
2952 2924 2927
1681 1676 1720
1603
2926 ν(C–H)
1678 1715
1600 1613
1560 1561
1519 1539
1498
1649 ν(C=O)
1601 ν(C=O)+ν(C=C)
1553 ν(C=C)
1588
1539
1498
1468
1433
1427
1385
1374
1316
1286
1235
1215
1081
1064
1030
985
1602 1569
1539
1498
1467
1446
1421
1375
1373
1319
1285
1236
1217
1081
1537
1506
1426
1409
1375
1345
1320
1265
1253
1238
1219
1085
1550
1511
1474
1442
1587 1538 1538
1536 1484 1498
1528 ν(C=C)
1475 δas(–CH3)
δas(–CH3)
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
0.
1.
2.
3.
4.
5.
6.
7.
8.
9.
0.
1.
2.
3.
4.
5.
6.
7.
8.
9.
0.
1.
1506
1468
1468
1409 1434 1433
1431 1433
1426
1441 δas(–CH3)
1424 δas(–CH3)
1375 δ(–CH3)
1385
1422
1375
1344
1377 1376
1374
1377
1313
1275
1289 1344
1351 δsc(–CH3)
1304 δsc(–CH3)
1276 ν (C–N)+ω(–CH2–)
ω(–CH2–)
1
341
1280 1307 1342
1264 1272 1284
1185 1212 1233
1344 1326
1282 1285
1236
1
1
1
1
1
1
1
375
341
322
265
085
033
008
1290
1266
1264
1217
1085
1072
1033
964
1185
1097
1179
1217
1198 1214
1095 1081
1062 1068
1021 1029
985
1189 t(–CH2–)+ω(–CH2–)
1130 1102 1081
1071 1054
1097 ρ(–CH3)+δ (C=O)
s
1051 ν(C–Cl)+ ρ(–CH3)
1020 ρ(–CH3)
1011
986
892
869
845
821
1034
1011
935
1040
981
1097 1023 1030
890
870
850
794
987
892
868
847
821
778
724
723
ρ(–CH3)
9
8
8
8
7
7
6
6
29
90
48
21
95
38
82
61
870
892
893
ρ(–CH3)+ρ(–CH2–)
862 ρ(–CH2–)+ρ(–CH3)
ρ(–CH2–)+ρ(–CH3)
992 ρ(–CH2–)+ρ(–CH3)
779 ω(N–H)
892
890
869
873
869
870
870
848
842
801
774
726
696
846
819
762
735
725
672
819
821
821
796
762
740
724
672
738
744
761
724
762
738
764
722
725 ρ(–CH2–)+τ(ring)
τ(ring)
717
627
629
685
630
672 ω(C–C)
6
72
625
622
629
633 (δoop -NH)+τ(C–C)
6
28
630
Proposed assignment and potential energy distribution (PED) for vibrational modes: Types of vibrations: ν-stretching, δsc-scissoring, ρ-
rocking, ω-wagging, t-twisting, δ-deformation, δas-asymmetric deformation, δoop- out of plane deformation, τ-torsion.

ACCEPTED MANUSCRIPT
1
Table 4 – Experimental and calculated H–NMR chemical shifts of pyrrole–isoxazoline derivatives (4a-d)
using DFT/B3LYP/6–31G(d,p)
4
a
4b
4c
4d
Assignment
Pyrrole NH
δ
δ
δ
δ
δ
δ
δ
δ
Exp.
Calc.
Exp.
Calc.
Exp.
Calc.
Exp.
Calc.
H23 10.54
H24
8.64 H23 9.98 8.36 H23 10.89 8.66 H23 9.99 8.64
2.48 H24
2.61 H24
2.46 H24
2.47
H25
H26
H27
H28
H29
2.51
2.46 H25 2.56 2.59 H25 2.52 2.53 H25 2.52 2.47
α-CH3
2.16 H26
2.08 H27
1.81 H26
1.88 H27
2.19 H26
2.07 H27
2.16
2.07
2.45
2.00 H28 2.50 1.85 H28 2.46 1.90 H28 2.46 1.99
3.03 H29 3.06 H29 3.00 H29 3.03
4.29 H30 4.35- 4.31 H30 4.34- 4.22 H30 4.37- 4.21
4.29 H31 4.28 4.31 H31 4.27 4.22 H31 4.30 4.21
1.19 H32 1.34 1.07 H32 1.37 1.18 H32 1.34 1.19
1.43 H33 1.36 1.37 H33 1.38 1.42 H33 1.36 1.42
1.41 H34 1.39 1.36 H34 1.39 1.39 H34 1.39 1.41
5.78 H35 6.10 5.57 H35 7.26 6.27 H35 5.80 5.87
3.25 H36 3.26 3.07 H36 3.30 3.40 H36 3.29 3.42
4.08 H37 3.26 3.77 H37 3.30 4.31 H37 3.29 4.10
β-CH3
H30 4.32-
H31 4.23
Ester-CH2
Ester -CH3
H32
H33
H34
H35
H36
1.37
1.36
1.34
7.26
Pyrazoline -CH
Pyrazoline –CH2
H37 2.56
H38
7.74 H38 7.78- 7.69 H38
8.09 H38
8.16
H39 7.98-
H40
H43
7.46 H39 7.33 7.24 H39 7.66- 7.96 H39 7.78- 7.72
7
.34
7.32
7.58
Phenyl -CH
7.46 H40
7.48 H41
7.24 H40
7.08 H42
8.61 H41
7.73 H42
8.36
8.43

ACCEPTED MANUSCRIPT
Table 5 - Comparison between calculated and experimental electronic transitions: E (eV),
oscillatory strength (f), λmax (nm) using TD–DFT/B3LYP/6–31G(d,p) in (4a-4d)
S. Electronic E (eV) Oscillatory Calculated Observed
%
assignment
No. transitions
strength
f)
(λmax
)
(λmax
)
contribution
(
4
a
1
.
91 → 92
90 → 93
4.38
5.20
0.41
0.55
283
238
298
254
42.19
39.75
n→π*
π→π*
2.
4
b
1
.
91 → 92
91 → 93
4.41
4.87
0.27
0.15
281
254
310
254
40.46
19.53
n→π*
π→π*
2.
4
c
1
.
94 → 96
93 → 97
4.39
5.19
0.35
0.49
282
239
278
230
44.73
32.77
n→π*
π→π*
2.
4
d
1
.
94 → 96
91 → 95
4.38
4.47
0.40
0.23
283
277
290
258
34.98
36.98
n→π*
π→π*
2.