Synthesis, molecular structure, hydrogen-bonding, NBO and chemical reactivity analysis of a novel 1,9-bis(2-cyano-2-ethoxycarbonylvinyl)-5-(4- hydroxyphenyl)-dipyrromethane: A combined experimental and theoretical (DFT and QTAIM) approach

Article abstract of DOI:10.1016/j.saa.2013.04.121

The spectroscopic analysis of a newly synthesized 1,9-bis(2-cyano-2- ethoxycarbonylvinyl)-5-(4-hydroxyphenyl)-dipyrromethane (3) has been carried out using ¹H 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. Thermodynamic parameters (H, G, S) of all the reactants and products have been used to determine the nature of the chemical reaction. The chemical shift of pyrrolic NH in ¹H NMR spectrum appears at 9.4 ppm due to intramolecular hydrogen bonding. TD-DFT calculation shows the nature of electronic transitions as π → π* within the molecule. A combined experimental and theoretical vibrational analysis designates the existence of H-bonding between pyrrole N-H as proton donor and nitrogen of cyanide as proton acceptor, therefore, lowering in stretching vibration of NH and CN. To investigate the strength and nature of H-bonding, topological parameters at bond critical points (BCPs) are analyzed by 'Quantum theory of Atoms in molecules' (QTAIMs). Natural bond orbitals (NBO_s) analysis has been carried out to investigate the intramolecular conjugative and hyperconjugative interactions within molecule and their second order stabilization energy (E(²)). Global electrophilicity index (ω = 4.528 eV) shows that title molecule (3) is a strong electrophile. The maximum values of local electrophilic reactivity descriptors (f_k⁺; s_k⁺; ω_k⁺) at vinyl carbon (C6/C22) of (3) indicate that these sites are more prone to nucleophilic attacks.

Full text of DOI:10.1016/j.saa.2013.04.121

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 113 (2013) 378–385
Contents lists available at SciVerse ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Synthesis, molecular structure, hydrogen-bonding, NBO and chemical
reactivity analysis of a novel 1,9-bis(2-cyano-2-ethoxycarbonylvinyl)-
5-(4-hydroxyphenyl)-dipyrromethane: 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
g r a p h i c a l a b s t r a c t
A detailed spectroscopic analysis of a newly synthesized 1,9-bis(2-cyano-2-ethoxycarbonylvinyl)-5-(4-
hydroxyphenyl)-dipyrromethane (3) have been carried out using 1H NMR, UV–Visible, FT-IR and Mass
spectroscopic techniques. All the quantum chemical calculations (1H NMR, UV–Visible, FT-IR, NBOs,
QTAIM) are carried out using DFT level of theory, B3LYP functional and 6-31G(d,p) as basis set. A com-
bined experimental and theoretical vibrational analysis designates the existence of H-bonding between
pyrrole NAH as proton donor and nitrogen of cyanide as proton acceptor. To investigate the strength and
nature of H-bonding, topological parameters at bond critical points (BCPs) are analyzed by Bader’s ‘Quan-
ꢀ
ꢀ
ꢀ
FT-IR spectrum of the studied
compound was recorded and
compared with the theoretical result.
All the theoretical calculations were
made using DFT/B3LYP/6-31G(d,p)
method.
NBO analysis are performed to
determine the hyperconjugative
interactions.
QTAIM analysis are performed to
determine hydrogen bonding.
Chemical reactivity has been
explained with the aid of electronic
descriptors.
tum theory of Atoms in molecules’ in detail. Global electrophilicity index (
x = 4.5281 eV) shows that title
molecule (3) is a strong electrophile. Local reactivity descriptors as Fukui functions (fkþ; fkꢁ), local softness-
es (sþk ; skꢁ) and electrophilicity indices (xkþ xꢁk ) analyses are performed to find out the reactive sites
;
ꢀ
ꢀ
within molecule.
a r t i c l e i n f o
a b s t r a c t
Article history:
The spectroscopic analysis of
a newly synthesized 1,9-bis(2-cyano-2-ethoxycarbonylvinyl)-5-(4-
hydroxyphenyl)-dipyrromethane (3) has been carried out using 1H 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. Thermodynamic parameters (H, G, S) of all the
reactants and products have been used to determine the nature of the chemical reaction. The chemical
shift of pyrrolic NH in 1H NMR spectrum appears at 9.4 ppm due to intramolecular hydrogen bonding.
Received 24 January 2013
Received in revised form 22 April 2013
Accepted 29 April 2013
Available online 15 May 2013
Keywords:
Spectroscopic analysis
TD-DFT
TD-DFT calculation shows the nature of electronic transitions as
experimental and theoretical vibrational analysis designates the existence of H-bonding between pyrrole
NAH as proton donor and nitrogen of cyanide as proton acceptor, therefore, lowering in stretching
p ?
pꢂ within the molecule. A combined
⇑
Corresponding author. Tel.: +91 9451308205.
E-mail address: rnsvk.chemistry@gmail.com (R.N. Singh).
1386-1425/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.saa.2013.04.121

R.N. Singh et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 113 (2013) 378–385
379
Vibrational analysis
Hydrogen-bonding
QTAIM analysis
vibration of NH and CN. To investigate the strength and nature of H-bonding, topological parameters at
bond critical points (BCPs) are analyzed by ‘Quantum theory of Atoms in molecules’ (QTAIMs). Natural
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)). Global
Reactivity descriptors
electrophilicity index (
x = 4.528 eV) shows that title molecule (3) is a strong electrophile. The maximum
values of local electrophilic reactivity descriptors (fkþ; skþ; xkþ) at vinyl carbon (C6/C22) of (3) indicate that
these sites are more prone to nucleophilic attacks.
Ó 2013 Elsevier B.V. All rights reserved.
Introduction
Experimental details
Dipyrromethane and its derivatives are the building blocks for
the syntheses of a variety of calix[n] pyrroles, porphyrins [1–4],
polypyrrolicmacrocycles [5], hexaphyrin [6] and corroles [7,8].
The oxidized dipyrromethanes named as dipyrromethenes or
dipyrrins give monoanionic, conjugated, planar ligands that have
attracted attention in the metal organic framework and have
strong coordinating ability towards different metal ions as Li(I),
Zn(II), Ni(II), Pd(II/III), Cu(II) and Sn(II) [9–12]. They are reported
as versatile ligands for coordination chemistry and supramolecular
self-assembly with various transition metal ions [13,14]. The het-
eroleptic complexes and coordination polymers of dipyrrin are
used for developing novel magnetic and electronic materials [15–
19]. The dipyrrinato metal complexes of Ga(III), In(III) have shown
luminescent properties [20] and its several metal–organic frame-
works (MOFs) with Ag + salts generate strong optical absorption
materials [21]. Dipyrromethane based amido–imine hybrid macro-
cycles have shown oxoanions receptor property [22]. Dipyrrins are
also used as ligands for the syntheses of boron dipyrromethene
(BODIPY) [23–25], which are used extensively as molecular probes
and dyes.
Synthesis of 1,9-bis(2-cyano-2-ethoxycarbonylvinyl)-5-(4-
hydroxyphenyl)-dipyrromethane (3)
Ethyl 2-cyano-3-(1H-pyrrol-2-yl)-acrylate (1) was prepared by
an earlier reported method. To the solution of ethyl 2-cyano-3-
(1H-pyrrol-2-yl)-acrylate (0.200 g, 1.0522 mmol) and 4-hydroxy-
benzaldehyde (0.0642 g, 0.5261 mmol) in 20 ml dichloromethane,
p-toluene sulfonic acid (0.0002 g) as catalyst was added. The reac-
tion mixture was refluxed for 8 h, the color of reaction was chan-
ged to dark brown and completion of the reaction was analyzed
by thin layer chromatography (TLC). Reaction mixture was washed
with saturated aqueous solution of NaHCO3 and extracted with
CH2Cl2 (15 ml ꢃ 3). The organic layer was dried over MgSO4 and
solvent was removed under reduced pressure. Remaining solid
was purified by column chromatography on silica using hexane
and ethyl acetate and pure product (3) was obtained. Dark brown
color compound yielded: 0.1839 g, 72.20%; m.p.: 132–136 °C;
DART Mass for C27H24N4O5: Calc. 484.1748 amu, Found m/z
485.25 [M+H+].
Quantum chemical calculations
Hydrogen bonds are of versatile importance in fields of chemis-
try and biochemistry, as they govern chemical reactions, supramo-
lecular structures, molecular assemblies and life processes. Intra
and intermolecular hydrogen bonds are classified in two categories
depending upon the nature of changes in bond length during the
hydrogen bridges formation [26–28].
Cyanovinyl was employed first by Fisher [26,29] as protecting
group for formyl in pyrrole for the synthesis of 2,5-diformyl-3,4-
dimethylpyrrole and later by Woodward [30] in the synthesis of
chlorophyll. The C-vinylpyrrole fragment is found to be reactive
for the target synthesis of conjugated and fused heterocycles sim-
ilar to natural pyrrole assemblies [31,32]. The functionalized C-
vinylpyrroles are prospective new materials for molecular optical
switches, nanodevices, photo- and electro-conducting applications
and also used as ligands for new photo catalysts, biologically active
complexes [33–35].
All the quantum chemical calculations were carried out with
Gaussian 03 program package [42] using B3LYP functional and 6-
31G(d,p) basis set [43–45]. Potential energy distribution along
internal coordinates was calculated by Gar2ped software. Topolog-
ical parameters were calculated using software AIM2000 [46].
Results and discussion
Thermochemistry
Optimized geometry of the reactants ethyl 2-cyano-3-(1H-pyr-
rol-2-yl)-acrylate (1) and 4-hydroxybenzaldehyde (2) and product
1,9-bis(2-cyano-2-ethoxycarbonylvinyl)-5-(4-hydroxyphenyl)-
dipyrromethane (3) and byproduct water (4) involved in chemical
reaction are shown graphically in Scheme 1. The calculated ther-
modynamic 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 are listed in Table 1. The calculated negative
In observation of above applications of cyanovinyl contain-
ing
dipyrromethane-1,9-bis(2-cyano-2-ethoxycarbonylvinyl)-5-
(4-hydroxyphenyl)-dipyrromethane (3) has been synthesized
and characterized using 1H 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 NMR chem-
ical shifts and vibrational analysis indicated the existence of
intramolecular H-bonding. To investigate the strength and nat-
ure of intramolecular H-bonding, topological and energetic
parameters at bond critical points (BCPs) have been analyzed
using QTAIM. NBOs analysis has been carried out to investigate
the intramolecular conjugative and hyperconjugative interac-
tions within molecule and their second order stabilization en-
ergy. The nature of chemical reactivity and site selectivity of
this molecule has been determined on the basis of Global and
Local reactivity descriptors [36–41].
values of enthalpy change (
energy factor is favorable, whereas Gibbs free energy (G) factor is
unfavorable. The calculated positive value of ( G) shows that this
DH) and entropy change (DS) show that
D
reaction is non-spontaneous. At 25 °C, thermodynamic equilibrium
constant (KT) for this reaction is calculated as 2.7883 ꢃ 10ꢁ7 i.e.
Keq ꢄ 1 indicating that the reaction will require elevation of tem-
perature and presence of catalyst.
Molecular geometry
Optimized geometry for the ground state lower energy con-
former of (3) is shown in Fig. 1. Selected optimized geometrical
parameters of (3) are listed in S Table 1 of Supplementary material.

380
R.N. Singh et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 113 (2013) 378–385
Scheme 1. Optimized geometry of reactants (1 and 2), product (3) and byproduct water (4).
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
ꢁ646.8405 ꢁ420.6827 ꢁ1637.9773 ꢁ76.3945
ꢁ646.8971 ꢁ420.7230 ꢁ1638.0871 ꢁ76.4160
D
D
D
H
G
S
ꢁ0.0081
0.0142
ꢁ47.176
119.27
84.71
230.958
45.116
In (3) two pyrrole units arrange in anti form giving the lower en-
ergy ꢁ1638.4802 a.u. and C1 symmetry at 25 °C as anti forms are
reported in the crystal structure of dipyrromethane derivatives
[47,48]. The E-configuration about the vinyl C6@C7 and C22@C23
bond with respect to the ester and pyrrole give lower energy con-
former. The asymmetry in the N1AC2 and N1AC5 bond lengths is
explained due to the presence of the two different groups as cyano-
vinyl at C2 and substituted methylene group at C5 carbon atom of
pyrrole ring.
1H NMR spectroscopy
Fig. 1. Optimized geometry for the ground state lower energy conformer of (3).
1H NMR chemical shifts were calculated with GIAO approach
[49] and equation: dX = IMSTMS ꢁ IMSX. The experimental (Fig. 2)
and calculated 1H NMR chemical shifts (d, ppm) with assignment
for (3) are given in Table 2. In order to compare the chemical shifts,
correlation graphs between the experimental and calculated 1H
NMR chemical shifts are shown in S Fig. 1 of Supplementary mate-
rial. The correlation coefficients (R2 = 0.97284) shows that there is
good agreement between experimental and calculated chemical
shifts.
Table 2
Experimental and calculated 1H NMR chemical shifts (d, ppm) in MeOD as the solvent
at 25 °C for (3) with assignment.
Atom No.
dcalcd.
dexp.
Assignment
H37
H38
H39
H40
H41
H42
H43
H44
H45
H46
H47
H48
H49
H50
H51
H52
H53
H54
H55
H56
H57
H58
H59
H60
9.9008
6.9441
6.1525
7.8738
4.2458
4.2628
1.4291
1.2191
1.4365
5.4448
9.6731
6.4576
7.0167
7.8247
4.2331
4.2525
1.4294
1.4136
1.2122
7.1541
6.724
9.4014
s, 2 ꢃ 1H, pyrrole-NH
d, 2 ꢃ 1H, pyrrole-CH
d, 2 ꢃ 1H, pyrrole-CH
s, 2 ꢃ 1H, vinyl-CH
6.6567–6.6749
6.3382–6.3569
7.8908
4.3167–4.3878
q, J = 7.11 Hz, 2 ꢃ 2H, ester-CH2
UV–Visible spectroscopy
1.3640–1.4115
t, J = 7.125 Hz, 2 ꢃ 3H, ester-CH3
The nature of the transitions in UV–Visible spectrum of the title
compound has been studied by TD-DFT. The comparison between
experimental and theoretical UV–Visible spectra for (3) is shown
in Fig. 3. The experimental and calculated electronic transitions
parameters are listed in Table 3. TD-DFT calculations predict two
electronic transitions at 217 nm and 345 nm which are correspond
to the experimental electronic transitions observed at 214, 304 nm,
respectively. S Fig. 2 of Supplementary material shows that the
orbitals Hꢁ1 and H are localized over C34AC35 and C16AC31 of
benzene ring, whereas orbitals L+1 and L+6 are localized over
C10AO11 of carbonyl in ester and C22AC23 vinyl bond, respec-
tively. On the basis of molecular orbital coefficients analysis and
molecular orbital plots, the nature of these electronic excitations
5.1904
9.4014
6.6749–6.6567
6.3382–6.3569
7.8908
s, 1H, meso-CH
s, 2 ꢃ 1H, pyrrole-NH
s, 2 ꢃ 1H, vinyl-CH
4.3167–4.3878
q, J = 7.11 Hz, 2 ꢃ 2H, ester-CH2
1.3640–1.4115
7.1567–7.4212
t, J = 7.125 Hz, 2 ꢃ 3H, ester-CH3
m, 4H, benzene ring
6.9478
7.3625
4.409
are assigned to be
p p(C34AC35) ?
(C16AC31) ? pꢂ(C22AC23),
5.6308
s, 1H, AOH
pꢂ(C10AO11), respectively.

R.N. Singh et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 113 (2013) 378–385
381
Fig. 2. The experimental 1H NMR spectrum of (3) in MeOD solvent.
Natural bond orbitals (NBOs) analysis
The strength of various types of interactions or stabilization en-
ergy (E(2)) associated with electron delocalization between each
donor NBO(i) and acceptor NBO(j) is estimated by the second order
energy lowering equation [50–53]. Second-order perturbation the-
ory analysis of the Fock matrix in NBO basis for (3) is presented in S
Table
(C2AC3) ? pꢂ(C4AC5) and
ble for the conjugation of respective
electron density at the conjugated
bonds (0.369–0.420) of pyrrole ring indicate strong
2
of Supplementary material. The interactions
(C4AC5) ? pꢂ(C2AC3) are responsi-
-bonds in pyrrole ring. The
bonds (1.688–1.722) and pꢂ
-electron
p
p
p
p
p
delocalization within ring leading to a maximum stabilization of
energy up to ꢅ22.99 kcal/mol. The interaction n1(N1) ? pꢂ(-
C2AC3)/pꢂ(C4AC5) indicate the involvement of loan-pair of pyr-
role
interactions
N
atom with
p
-electron delocalization in ring. The
p
(C2AC3) ? pꢂ(C6AC7),
p
(C6AC7) ? pꢂ(C2AC3) are
responsible for the conjugation of bonds C2AC3 and C6AC7 with
C2AC6 and stabilized the molecule up to ꢅ27.34 kcal/mol. The
electron density at the conjugated
bonds (0.321–0.387) of benzene ring indicate strong
delocalization within ring leading to a maximum stabilization of
p
bonds (1.653–1.702) and pꢂ
Fig. 3. Comparison between experimental and theoretical UV–Visible spectra for
(3).
p-electron
ciated with the pyrrole ring such as
(N1AC5) ? rꢂ(C2AC6), (C2AC3) ? rꢂ(C2AC6),
rꢂ(C2AC6) and (C4AC5) ? rꢂ(N1AH37) stabilize the molecule
within range 3.26–5.42 kcal/mol.
r
(N1AC2) ? rꢂ(C5AC15),
(C3AC4) ?
energy up to ꢅ22.51 kcal/mol.
The primary hyperconjugative interactions n1(N9) ? rꢂ(-
C7AC8) and n2(O11) ? rꢂ(C10AO12) stabilize the molecule by
32.63 kcal/mol. The secondary hyperconjugative interactions asso-
r
r
r
r

382
R.N. Singh et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 113 (2013) 378–385
Table 3
Experimental and calculated electronic transitions for (3): E/eV, oscillatory strength (f), (kmax/nm) at TD-DFT/B3LYP/6-31G(d,p) level.
S. no.
Excitations
E (eV)
(f)
kmax calcd.
kmax obs.
Assignment
1
2
Hꢁ1 ? L+1
H ? L+6
3.5914
5.7059
0.7157
0.0441
345.22
217.29
304
214
p
p
(C34AC35) ? pꢂ(C10AO11)
(C16AC31) ? pꢂ(C22AC23)
Selected Lewis orbitals (occupied bond or lone pair) of (3) with
their valence hybrids are listed in S Table 3 of Supplementary
material. The valence hybrids analysis of NBO orbitals shows that
all the NAH/CAN and CAO bond orbitals are polarized towards
the nitrogen (ED = 57.50–73.94% at N), oxygen (ED = 65.52–
68.78% at O), respectively. The electron density distribution (occu-
pancy) around the loan pair of O and N atoms mainly influences
the polarity of the compound.
Vibrational assignments
The experimental and theoretical (selected) vibrational wave-
numbers of (3), calculated at B3LYP/6-31G(d,p) method and their
assignments using PED are given in Table 4. The total number of
atoms (n) in monomer (3) is 60, therefore this gives 174, (3nꢁ6)
vibrational modes. Calculated wavenumbers were scaled down
using scaling factor 0.9608 [54], to discard the anharmonicity pres-
Table 4
Experimental and theoretical [calculated at B3LYP/6–31G(d,p) level] vibrational wavenumbers of (3) and their assignments: Wavenumbers (V=cmꢁ1), intensity (K mmolꢁ1).
IRint
Assignment (PED) P 5%
V
V
V
unscal.
scaled
Exp.
3821
3598
3 584
3170
3133
3133
3066
3066
2311
2310
1794
1793
1672
1647
1645
1643
1586
1582
1559
3671
3456
3443
3045
3010
3010
2945
2945
2220
2219
1723
1722
1606
1582
1580
1578
1523
1519
1497
61.25
88.13
77.18
22.35
24.65
26.69
20.99
22.65
43.67
40.76
212.24
275.48 1701
56.28
290.59
580.65 1587
251.49
191
3409
3336
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
(O36H60)(100)
(N1H37)(99)
(N17H47)(99)
3036
2980
(C32H57)(93)–
(C14H44)(58)–
(C30H55)(59)–
(C13H41)(49)+
m
(C31H56)(5)
m
m
m
(C14H45)(19)–
(C30H53)(19)–
(C13H42)(49)
(C29H51)(48)
(C23C24)(13)
m
m
(C14H43)(17)
(C30H54)(17)
2928
2213
(C29H52)(50)+
m
(C24N25)(86)–
m
(C8N9)(87)–
(C10O11)(76)–
(C26O27)(76)–(d-C23C26)(6)–
(C34C35)(20)+ (C31C32)(19)–
(C6C7)(14)– (C2C6)(13)+ (C22C23)(9)–
m
(C7C8)(12)
(C10O12)(6)–(dip-C7C10)(5)
(C26O28)(6)
(C33C34)(11)+(das–R1)(11)–
(C21C22)(8)– (C32C33)(6)–(dip-C2C6)(5)+
(C6C7)(10)+ (C2C6)(10)–(dip-C21C22)(6)+ (C20C21)(6)
(C33C34)(11)– (C16C31)(7)+(das–R1)(6)+ (C6C7)(5)
(C4H39)(12)
(C22C23)(19)–(d-C18H47N17)(13)+ (C18C19)(11)–
m
m
m
m
m
(C16C31)(9)+(d-C16H56C31)(6)
(C2C3)(5)
m
m
m
m
m
(C22C23)(18)–
m
(C21C22)(17)–
(C16C35)(13)–
m
m
m
m
m
(C32C33)(17)+
m
m
(C6C7)(19)+(d-C2H37N1)(16)+m
346.64 1483
148.62 1453
m
m
(C6C7)(7)–
m
(C15C18)(6)
(d-C34H59C35)(14)–(d-C31H57C32)(13)–(d-C16H56C31)(13)+
m
(C16C31)(11)+
m
(C33O36)(9)–
m
(C33C34)(9)–
m(C32C33)(8)–(d-
C35H58C34)(8)
1532
1499
1520
1438
1405
1405
1351
1299
1289
1285
1199
1471
1440
1460
1381
1349
1349
1298
1248
1238
1234
1151
7.3
5.19
46.52
25.36
20
21.08
39.69
0.71
(dsc-CH2)(41)–(dsc-CH2)(24)–(das–Me)(11)+(das–Me)(8)+(das–Me)(3)+(dsc-C13C14O12)(2)–(
(das–Me)(67)+(das–Me)(24)–( -Me)(6)
(C2C3)(7)–(d-R)(7)+
(C20C21)(18)+(ds–Me)(12)+(d–C18H47N17)(7)–(d-C18H48C19)(6)–(d–R)(5)–(d-N17C22C21)(5)
q–Me)(2)
q
m
m
(C3C4)(13)–
(N17C21)(24)–m
m
(N1C5)(12)–
m
m(C5C15)(6)–m(C19C20)(6)+m(N17C18)(5)
1368
(x
-CH2)(46)–(ds–Me)(31)
1344
1253
(x-CH2)(45)–(ds–Me)(32)
(dip-C2C6)(28)+
(t-CH2)(86)–( –Me)(8)
(C26O28)(16)+(d-C18H47N17)(11)–
(C10O12)(22)– (C7C10)(16)–(d-O11O12C10)(8)+(
-C15H46)(9)+(d-C2H37N1)(8)+(d-C2H38C3)(8)–
(C2C3)(5)
(C13C14)(12)+(ddsc-C13C14O12)(9)-(
(O12C13)(12)+ (O28C29)(8)+ (C7C8)(7)–( –Me)(7)–
(C10O12)(5)+ (C23C24)(5)
(O28C29)(12)–(d-C22C23C24)(8)+
m(C3C4)(17)+m(C7C10)(7)–(d-C3H39C4)(7)+(d-C2H37N1)(5)–m(C7C8)(5)
q
568.13
898.49
579.88 1094
m
m
m
(C23C26)(9)–
m
(C15C18)(8)+(dip-O27C26O28)(5)+(
x
-CH2)(5)
m
x-CH2)(7)
(d-C3H39C4)(9)+(
(C2C6)(5)+
–Me)(16)–
q
m(N1C5)(7)+m(C10O12)(7)+m(C5C15)(6)–
m
m
1145
1129
1100
1084
0.84
(q
m
q–Me)(6)–
m(C10O12)(6)–m(C29C30)(5)–(q–Me)(5)
60.84
97.75
111.84 1018
86.93
47.82
41.95
21.62
12.94
55.96
35.11
44.46
41.22
15.51
1047
1047
m
m
m
m
m
q
m
(C7C10)(6)–(d-C22C23C24)(5)+(dip-C2C6)(5)–
m
1129
1078
1075
1050
1049
906
906
800
798
711
1084
1035
1032
1008
1007
870
870
768
766
683
m
(C23C24)(7)–
(C19C20)(24)
(C3C4)(24)
m
(O12 C13)(7)+(
q–Me)(6)–
m
(C23C26)(6)–
m
(C26O28)(5)+(dip-C21C22)(5)
(d-C18H48C19)(35)–(d-C19H49C20)(30)–
(d-C3H39C4)(35)–(d-C2H38C3)(30)+
m
m
994
879
757
663
m
m
m
m
(C13C14)(26)–
m
(O12C13)(22)–
(O28C29)(22)+
(O28C29)(15)+(
(O12C13)(14)+
-C3H38)(25)+(d–R)(9)+(d–C23C22C21)(5)
-C20H49)(19)
–R)(27)–(R1–Puckering)(10)+(doop-C15C18)(9)–(
–R)(8)+(doop-C33O36)(7)+(d–R)(6)
–R)(12)+(das–R1)(10)+-( -N1H37)(7)
m
(C29C30)(11)+
m
(O28C29)(9)
(O12C13)(9)
(C29C30)(27)–
m
m
(C13C14)(11)–
m
(O12C13)(16)–
(O28C29)(17)+
-C4H39)(34)+(
-C19H48)(33)+(x
m
m
x
q
m
–Me)(8)+
m
(C13C14)(8)–
m
(C29C30)(7)+(
q–Me)(6)
(C29C30)(8)+(
q
–Me)(7)–(
q–Me)(7)+ (C13C14)(7)
m
(
(
(
x
x
s
s–C21C22)(5)
697
636
669
611
(R1–Puckering)(33)+(
-N17H47)(28)+(
s
(x
s
x
Proposed assignment and potential energy distribution (PED) for vibrational modes: Types of vibrations:
m
– stretching,
q
– rocking,
x – wagging, d – deformation, ds –
symmetric deformation, das – asymmetric deformation, dip – in plane deformation, doop – out-of-plane deformation,
s – torsion. R – pyrrole ring, R1 – benzene ring, Me –
ester methyl.

R.N. Singh et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 113 (2013) 378–385
383
mental FT-IR spectrum. These observed vibrations of the CH2
groups are also correspond to the reported absorption bands in
the literature such as asymmetric stretches at 3000 50 cmꢁ1
,
and symmetric stretches at 2965 30 cmꢁ1 [57]. The calculated
wavenumber 1471 cmꢁ1 is assigned to the scissoring mode of
CH2 group. The calculated wavenumber at 1349 cmꢁ1 describes
the presence of wagging mode of CH2group and agrees well with
the observed wavenumber at 1344 cmꢁ1. These observed deforma-
tion modes of the CH2 groups are also correspond to the reported
absorption bands in the literature for scissoring modes at
1455 55 cmꢁ1 and wagging modes at 1350 85 cmꢁ1. The calcu-
lated CAH stretching vibration of benzene at 3045 cmꢁ1matches
well with the observed wavenumber at 3036 cmꢁ1. The calculated
wavenumber at 683 cmꢁ1 is assigned to the puckering vibration (a
torsional mode) of benzene ring and observed at 663 cmꢁ1 in the
experimental FT-IR spectrum.
Fig. 4. Comparison between experimental and theoretical IR spectra for (3).
ent in real system. Comparison among experimental and theoreti-
cal (selected) IR spectra for (3) in the region 4000–400 cmꢁ1 is
shown in Fig. 4.
C@O, CAO vibrations
The investigated molecule (3) contains two carbonyl groups
(C10@O11, C26@O27) and two vinyl groups (C6@C7, C22@C23)
due to the symmetrical nature of molecule. The stretching vibra-
NAH and OAH vibrations
tion of ester carbonyl group (m ,
C@O) is observed at 1701 cmꢁ1
In the FT-IR spectrum of (3), the NAH stretch of pyrrole (mNAH
)
is observed at 3336 cmꢁ1, whereas this is calculated at 3456 cmꢁ1
in one pyrrole unit and at 3443 cmꢁ1 in other pyrrole unit of the
investigated molecule. The observed mNAH for pyrrole at 3336 is
in good agreement with the earlier reported hydrogen bonded
mNAH at 3358 cmꢁ1 recorded in KBr pellet, but it deviates from
the reported free mNH band observed at higher wavenumber
3465 cmꢁ1, recorded in CCl4 solution [55]. The observed mNAH for
whereas this is calculated at 1722 cmꢁ1 in theoretical IR-spectrum.
The ‘‘CAO stretching vibrations’’ of esters actually consists of two
asymmetrical coupled vibrations as OAC(@O)AC and OACAC and
these bands occur in the region 1300–1000 cmꢁ1 [59]. The ester
OAC(@O)AC stretching vibration as
1238 cmꢁ1. The calculated wavenumber at 1234 cmꢁ1 is assigned
to a combination band of ester stretching C10AO12 and its deforma-
mC26AO28is assigned at
m
pyrrole also deviates from the free mNAH of pyrrole at 3475 cmꢁ1
,
tion O11AO12AC10 with 8% contribution in PED. The calculated
wavenumber at 1084 cmꢁ1is demonstrated to the OACAC stretch-
reported in literature [56]. Therefore, the red shift in the observed
mNAH of pyrrole compared with free mNAH indicates the involve-
ment of the NAH group in intramolecular hydrogen bonding. The
calculated wavenumbers at 611 cmꢁ1 is assigned to the wagging
mode of pyrrole (NAH) and matches well with the observed wave-
number at 608 cmꢁ1. The calculated wavenumber at 3671 cmꢁ1
ing vibration of ester groups as mO12AC13, mO28AC29 and corresponds
to the observed wavenumber at 1047 cmꢁ1 in the experimental FT-
IR spectrum.
demonstrates the presence of OAH stretch of phenol (mOAH) and
CAC vibrations
observed about 3409 cmꢁ1 in the experimental FT-IR spectrum. It
is to be noticed that NAH and OAH stretches are merged to each
other due to broadening of the peak.
The C@C stretches in benzene ring are assigned at 1606,
1578 cmꢁ1 and correspond to the observed band reported in liter-
ature in the region 1600–1585 cmꢁ1 [59]. The calculated wave-
numbers at 1580, 1519 cmꢁ1 exhibit presence of the C@C
CAH vibrations
stretches of vinyl groups (mC22@C23, mC6@C7) and observed at 1587,
Two ester methyl (Me) and two ester CH2 groups are present in
the investigated molecule (3). The theoretical vibrational analysis
displays the presence of asymmetric CAH stretching vibrations of
ester methyl group sat 3010 cmꢁ1 and it is observed at
2980 cmꢁ1 in the experimental FT-IR spectrum. These observed
vibrations of the methyl groups also correspond to the reported
absorption bands in the literature such as asymmetric stretches
at 2985 25 cmꢁ1, and symmetric stretches at 2920 80 cmꢁ1
[57]. The asymmetric and symmetric deformation modes of Me
are assigned at 1440, 1349 cmꢁ1, respectively. The symmetric
deformation mode of Me assigned at 1349 cmꢁ1 matches well with
the observed wavenumber at 1344 cmꢁ1. The rocking mode of Me
group calculated at 1084 cmꢁ1 with 7% contribution in PED corre-
sponds to the observed wavenumber at 1047 cmꢁ1. According to
Internal coordinate system recommended by Pulay et al. [58],
CH2 group associate with six types of vibrational frequencies
namely: symmetric stretch, asymmetric stretch, scissoring, rock-
ing, wagging and twisting. The scissoring and rocking deformations
belong to polarized in-plane vibration, whereas wagging and twist-
ing deformations belong to depolarized out-of-plane vibration. The
theoretical vibrational analysis indicates the presence of CAH
stretching vibration of ester CH2 group at 2945 cmꢁ1 and corre-
sponds to the observed wavenumber at 2928 cmꢁ1 in the experi-
1483 cmꢁ1, respectively in the experimental FT-IR spectrum. An
observed combination band of ‘pyrrole ring stretches and its defor-
mations’ at 1368 cmꢁ1corresponds with the calculated wavenum-
ber at 1381 cmꢁ1. The calculated wavenumber at 1460 cmꢁ1 also
describes the presence of the CAC stretch of pyrrole up to 13% con-
tribution in PED. The deformation modes associated with pyrrole
ring are calculated at 1298, 1151, 1035 cmꢁ1, whereas these are
observed at 1253, 1094, 1018 cmꢁ1, respectively. The calculated
wavenumber at 768 cmꢁ1 is assigned to the CAH wagging mode
of pyrrole and matches with the observed wavenumber at
757 cmꢁ1.The CAC stretches of ester calculated at 1008 cmꢁ1 cor-
responds to the observed wavenumber at 994 cmꢁ1
.
C„N vibrations
In the theoretical IR spectrum of (3), the calculated wavenum-
ber at 2220 cmꢁ1 designates the presence of the C„N stretching
vibration and agrees well with the observed wavenumber at
2213 cmꢁ1. The free C„N stretching vibrations are reported in lit-
erature in the region 2240–2260 cmꢁ1 [56]. Therefore, the red shift
in the observed C„N stretching compared with the free C„N
stretching indicates the involvement of the C„N group in intramo-
lecular hydrogen bonding.

384
R.N. Singh et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 113 (2013) 378–385
Quantum Theory of Atoms in Molecules (QTAIMs) analysis
A combined experimental and theoretical vibrational analysis des-
ignates the existence of intramolecular classical hydrogen bon-
dings (N1AH37ꢆꢆꢆN9 and N17AH47ꢆꢆꢆN25) between pyrrole NAH
as proton donor and cyanide as proton acceptor. QTAIM calculated
topological, geometrical and energetic parameters indicate the nat-
ure of intramolecular classical hydrogen bonds as weak with en-
ergy values ꢁ2.54 to ꢁ2.76 kcal/mol. The global elecrophilicity
Molecular graph of (3) using QTAIM program at B3LYP/6-
31G(d,p) level is shown in S Fig. 2 of Supplementary material.
Topological as well as geometrical parameters for bonds of inter-
acting atoms are given in S Tables 4 and 5 of Supplementary mate-
rial, respectively. The nature of interactions indicated in the
molecular graph depends upon the geometrical, topological and
index (x = 4.5281 eV) of (3) indicates its behavior as a strong elec-
energetic
parameters.
For
intramolecular
interactions
trophile. The electrophilic reactivity descriptors analyses of (3)
indicate that the investigated molecule might be used as precursor
for the target syntheses of unsymmetrical dipyrromethane
derivatives.
(N1AH37ꢆꢆꢆN9, N17AH47ꢆꢆꢆ N25), the electron density (
qH. . .A)
2
and its Laplacian (rqBCP) are in the range 0.0133–0.0143, 0.0470–
0.0498 a.u., respectively. Therefore, these interactions follow the
Koch and Popelier criteria [60]. The nature of these interactions
2
(intramolecular classical hydrogen bonds) is weak as (rqBCPÞ > 0
Acknowledgment
and HBCP > 0 at bond critical point. The energy of intramolecular
[61,62] classical hydrogen bonds N1AH37ꢆꢆꢆN9 and N17AH47ꢆꢆꢆ
N25 are calculated to be ꢁ2.76, ꢁ2.54 kcal/mol, respectively.
The authors are thankful to the CSIR for financial support.
Appendix A. Supplementary Material
Chemical reactivity
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.saa.2013.04.121.
Global reactivity descriptors
Global reactivity descriptors [37–41] electronegativity (
2 (eLUMO HOMO), chemical potential ( ) = 1/2 (eLUMO HOMO), glo-
bal hardness ( HOMO), global softness (S) = 1/2
and electrophilicity index ( , determined on the basis of
2/2
Nmax A
[63] for reactant system [(1) M (2)] are listed in S Ta-
ble 6 of Supplementary material. The global elecrophilicity index
= 4.53 eV) of (3) shows it as a strong electrophile. ECT calculated
v
) = ꢁ1/
+
e
l
+ e
g) = 1/2 (eLUMO
ꢁ
e
l
g
References
x
) =
g
Koopman’s theorem [36] for (1), (2), (3) and ECT = (
D
) -
[1] J.S. Lindsey, Metalloporphyrins Catalysed Oxidation, Kluwer Academic
Publishers, The Netherlands, 1994.
[2] S.-J. Hong, M.-H. Lee, C.-H. Lee, Bull. Korean Chem. Soc. 25 (2004) 1545–1550.
[3] W.-S. Cho, H.-Je Kim, B.J. Littler, M.A. Miller, C.-H. Lee, J.S. Lindsey, J. Org. Chem.
64 (1999) 7890–7901.
ꢁ ( Nmax B
D
)
(x
[4] R.L. Halterman, X. Mei, Tetrahedron Lett. 37 (1996) 6291–6294.
[5] A. Jasat, D. Dolphin, Chem. Rev. (1997) 2277–2282.
[6] G. Mori, H. Shinokubo, A. Osuka, Tetrahedron Lett. 49 (2008) 2170–2172.
[7] R. Guilard, D.T. Gryko, G. Canard, J.-M. Barbe, B. Koszarna, S. Brande‘s, M.
Tasior, Org. Lett. 4 (2002) 4491–4494.
[8] M. Schwalbe, D.K. Dogutan, S.A. Stoian, T.S. Teets, D.G. Nocera, Inorg. Chem. 50
(2011) 1368–1377.
[9] J.C. Wechsler, A.A.-S. Ali, E.E. Chapman, T.S. Cameron, A. Thompson, Inorg.
Chem. 46 (2007) 10947–10949.
[10] L. Yu, K. Muthukumaran, I.V. Sazanovich, C. Kirmaier, E. Hindin, J.R. Diers, P.D.
Boyle, D.F. Bocian, D. Holten, J.S. Lindsey, Inorg. Chem. 42 (2003) 6629–6647.
[11] C. Brückner, V. Karunaratne, S.J. Rettig, D. Dolphin, Can. J. Chem. 74 (1996)
2182–2193.
[12] S. ichi Tamaru, L. Yu, W.J. Youngblood, K. Muthukumaran, M. Taniguchi, J.S.
Lindsey, J. Org. Chem. 69 (2004) 765–777.
[13] A. Thompson, S.J. Retting, D. Dolphin, Chem. Commun. (1999) 631–632.
[14] S.R. Halper, S.M. Cohen, Angew Int. Ed. 43 (2004) 2385–2389.
[15] B. Moulton, M.J. Zaworotko, Chem. Rev. 101 (2001) 1629–1658.
[16] S.R. Halper, S.M. Cohen, Inorg. Chem. 44 (2005) 4139–4141.
[17] S.R. Halper, M.R. Malachowski, H.M. Delancy, S.M. Cohen, Inorg. Chem. 43
(2004) 1242–1245.
[18] L. Do, S.R. Halper, S.M. Cohen, Chem. Commun. (2004) 2662–2663.
[19] S.R. Halper, S.M. Cohen, Angew Int. Ed. 43 (2004) 2385–2388.
[20] V.S. Thoi, J.R. Stork, D. Magde, S.M. Cohen, Inorg. Chem. 45 (2005) 10688–
10697.
as 0.54264 for reactant system [(1) M (2)], indicates that charge
flows from (2) to (1).
Local reactivity descriptors
Selected nucleophilic reactivity descriptors (fkꢁ; skꢁ; xkꢁ) [37–41]
for reactant (1), using Mulliken atomic charges are given in S Ta-
ble 7 of Supplementary material. In reactant (1), the maximum val-
ues of the local nucleophilic reactivity descriptors (fkꢁ; skꢁ; xkꢁ) at C5
indicate that this site is prone to electrophilic attack. It is to be no-
ticed that the reactants (2), possess only one carbonyl functional
group (˜C@O), therefore for (2) there is no need to calculate the Lo-
cal electrophilic reactivity descriptors (fkþ; skþ; xkþ). Thus, Local reac-
tivity descriptors for reactant (1) confirm the formation of product
(3) by nucleophilic attack of the C5 carbon atom of (1) on the more
electrophilic carbonyl carbon (C11) of the reactant (2). Selected
electrophilic reactivity descriptors (fkþ; skþ; xkþ) for (3), using Mullik-
en atomic charges are given in S Table 8 of Supplementary mate-
rial. The maximum values of local electrophilic reactivity
descriptors (fkþ; skþ; xkþ) at vinyl carbons (C6/C22) of (3) indicate
that these sites are more prone to nucleophilic attack and favor
the formation of the unsymmetrical dipyrromethane.
[21] S.R. Halper, L. Do, J.R. Stork, S.M. Cohen, J. Am. Chem. Soc. 128 (2006) 15255–
15258.
[22] E.A. Katayev, N.V. Boev, V.N. Khrustalev, Y.A. Ustynyuk, Tananaev, J.L. Sessler,
J. Org. Chem. 72 (2007) 2886–2896.
[23] S. Goeb, R. Ziessel, Tetrahedron Lett. 49 (2008) 2569–2574.
[24] A. Haefele, G. Ulrich, P. Retailleau, R. Ziessel, Tetrahedron Lett. 49 (2008) 3716–
3721.
Conclusions
[25] K. Kim, C. Jo, S. Easwaramoorthi, J. Sung, D.H. Kim, D.G. Churchill, Inorg. Chem.
49 (2010) 4881–4894.
[26] Y. Gu, T. Kar, S. Scheiner, J. Am. Chem. Soc. 121 (1999) 9411–9422.
[27] F.H. Allen, V.J. Hoy, J.A.K. Howard, V.R. Thalladi, G.R. Desiraju, C.C. Wilson, G.J.
Mcintyre, J. Am. Chem. Soc. 119 (1997) 3477–3480.
[28] D. Philp, J.M.A. Robinson, J. Chem. Soc. Perkin Trans. 2 (1998) 1643–1650.
[29] H. Fischer, H. Hofelmann, Justus Liebigs, Ann. Chem. 533 (1938) 216.
[30] R.B. Woodward, Angew Chem. 72 (1960) 651.
[31] R.A. Jones, G.P. Bean, The Chemistry of Pyrroles, Academic Press, London, 1977.
525–528.
[32] S.J. Hong, M.H. Lee, C.H. Lee, Bull. Korean Chem. Soc. 25 (2004) 1545–1550.
[33] R.T. Hayes, M.R. Wasilewski, D. Gosztola, J. Am. Chem. Soc. 122 (2000) 5563–
5567.
[34] M. Harmjanz, H.S. Gill, M.J. Scott, J. Am. Chem. Soc. 122 (2000) 10476–10477.
[35] A. Sour, M.L. Boillot, E. Riviere, P. Lesot, Eur. J. Inorg. Chem. 1999 (1999) 2117–
2119.
The title compound (3) has been synthesized and characterized
by experimental and theoretical techniques. The calculated 1H
NMR chemical shifts are in agreement with the experimentally ob-
served chemical shifts. 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 suggests
that the nature of electronic excitations is
exhibits various types of intramolecular conjugative and hypercon-
jugative interactions being responsible for -electron delocaliza-
p ?
pꢂ. NBOS analysis
p
tion within pyrrole and benzene rings and stabilizes the
molecule with energy in the region from 3.19 to 44.04 kcal/mol.

R.N. Singh et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 113 (2013) 378–385
385
[36] R.G. Parr, W. Yang, Density functional Theory of Atoms and Molecules, Oxford
University Press, Oxford, New York, 1989.
[48] N. Maiti, J. Lee, D.G. Churchill, Y. Do, H.S. Shin, J. Chem. Crystallogr. 35 (2005)
949–955.
[37] R.G. Pearson, J. Org. Chem. 54 (1989) 1430–1432.
[49] K. Wolinski, J.F. Hinton, J.F. Pulay, J. Am. Chem. Soc. 112 (1990) 8251–8260.
[50] I.M. Snehalatha, C. Ravikumar, I. Joe Hubert, N. Sekar, V.S. Jayakumar,
Spectrochim. Acta Part A 72 (2009) 654–662.
[51] A.E. Reed, L.A. Curtiss, F. Weinhold, Chem. Rev. 88 (1988) 899–926.
[52] C.G. Liu, Z.M. Su, X.H. Guan, S. Muhammad, J. Phys. Chem. C 115 (2011) 23946–
23954.
[53] F. Weinhold, C.R. Landis, Valency and Bonding: A Natural Bond Orbital Donor–
Acceptor Perspective, Cambridge University Press, Cambridge, New York,
Melbourr, 2005. 215–274..
[54] J.A. Pople, H.B. Schlegel, R. Krishnan, D.J. Defrees, J.S. Binkley, M.J. Frisch, R.A.
Whiteside, R.F. Hout, W.J. Hehre, Int. J. Quantum Chem. 15 (1981) 269–278.
[55] A.T. Dubis, S.J. Grabowski, D.B. Romanowska, T. Misiaszek, J. Leszczynski, J.
Phys. Chem. A 106 (2002) 10613–10621.
[56] B.M. Giuliano, I. Reva, R. Fausto, J. Phys. Chem. A 114 (2010) 2506–2517.
[57] M.R. Anoop, P.S. Binil, S. Suma, M.R. Sundarsanakumar, S. Mary, H.T. Varghese,
C.Y. Panicker, J. Mol. Struct. 969 (2010) 233–237.
[58] P. Pulay, G. Fogarasi, F. Pang, J.E. Boggs, J. Am. Chem. Soc. 101 (1979) 2550–
2560.
[59] R.M. Silverstein, F.X. Webster, Spectrometric Identification of Organic
Compounds, sixth ed., John Wiley Sons Inc., New York, 1963.
[60] U. Koch, P. Popelier, J. Phys. Chem. A 99 (1995) 9747–9754.
[61] I. Rozas, I. Alkorta, J. Elguero, J. Am. Chem. Soc. 122 (2000) 11154–11161.
[62] E. Espinosa, E. Molins, C. Lecomte, Chem. Phys. Lett. 285 (1998) 170–173.
[63] J. Padmanabhan, R. Parthasarathi, V. Subramaniaan, P.K. Chattaraj, J. Phys.
Chem. A 111 (2007) 1358–1361.
[38] R.G. Parr, R.G. Pearson, J. Am. Chem. Soc. 105 (1983) 7512–7516.
[39] P. Geerlings, F.D. Proft, W. Langenaeker, Chem. Rev. 103 (2003) 1793–1874.
[40] R.G. Parr, L. Szentpály, S. Liu, J. Am. Chem. Soc. 121 (1999) 1922–1924.
[41] P.K. Chattaraj, U. Sarkar, D.R. Roy, Chem. Rev. 106 (2006) 2065–2091.
[42] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman,
J.A. Montgomery Jr., T. Vreven, K.N. Kudin, J.C. Burant, J.M. Millam, S.S. Iyengar,
J. Tomasi, V. Barone, B. Mennucci, M. Cossi, G. Scalmani, N. Rega, G.A.
Petersson, H. Nakatsuji, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa,
M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, M. Klene, X. Li, J.E. Knox,
H.P. Hratchian, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R.E.
Stratmann, O. Yazyev, A.J. Austin, R. Cammi, C. Pomelli, J.W. Ochterski, P.Y.
Ayala, K. Voth, G.A. Morokuma, P. Salvador, J.J. Dannenberg, V.G. Zakrzewski, S.
Dapprich, A.D. Daniels, M.C. Strain, O. Farkas, D.K. Malick, A.D. Rabuck, K.
Raghavachari, J.B. Foresman, J.V. Ortiz, Q. Cui, A.G. Baboul, S. Clifford, J.
Cioslowski, B.B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R.L.
Martin, D.J. Fox, T. Keith, M.A. Al-Laham, C.Y. Peng, A. Nanayakkara, M.
Challacombe, P.M.W. Gill, B. Johnson, W. Chen, M.W. Wong, C. Gonzalez, and
J.A. Pople, Gaussian 03, Revision C.02, Gaussian, Inc., Wallingford, CT, 2004.
[43] A.D. Becke, J. Chem. Phys. 98 (1993) 5648–5652.
[44] C.T. Lee, W.T. Yang, R.G.B. Parr, Phys. Rev. 37 (1988) 785–789.
[45] D.A. Petersson, M.A. Allaham, J. Chem. Phys. 94 (1991) 6081–6090.
[46] R.F.W. Bader, J.R. Cheeseman, In AIMPAC Ed., (2000).
[47] G.K. Patra, Y.D. Posner, I. Goldberg, Acta Cryst. E58 (2002) o530–o531.