Structural, vibrational spectra and normal coordinate analysis for two tautomers of 4(5)-(2′-furyl)-imidazole

Article abstract of DOI:10.1002/jrs.2482

We have synthesized both the 4 and 5 tautomeric forms of 4(5)-(2′-furyl)-imidazole (1) and investigated their molecular vibrations by infrared and Raman spectroscopies as well as by calculation based on the density functional theory (DFT) approach. Examination of the temperature dependence of IR intensity revealed the band characteristics of the 4 and 5 tautomersof(1).Comparisonofexperimentalandcalculatedchemicalshiftsinnucl earmagneticresonance(NMR)spectroscopy was made in order to identify the two tautomeric forms. The assignment of vibrational normal modes was performed, and the forcefieldobtainedreproduced theexperimentalvibrationalwavenumbers with a root mean-squaredeviation (RMSD)value of ca. 13 cm^-1 for both tautomers. The natural bond orbital (NBO) study reveals the characteristics of the electronic delocalization of the two tautomeric structures.

Full text of DOI:10.1002/jrs.2482

Research Article
Received: 17 April 2009
Accepted: 9 August 2009
Published online in Wiley Interscience: 19 October 2009
(
www.interscience.wiley.com) DOI 10.1002/jrs.2482
Structural, vibrational spectra and normal
coordinate analysis for two tautomers
ꢀ
of 4(5)-(2 -furyl)-imidazole
a
b†
c
a†
A. E. Ledesma, J. Zinczuk, J. J. L o´ pez Gonz a´ lez, A. Ben Altabef
a∗
and S. A. Brand a´ n
ꢀ
We have synthesized both the 4 and 5 tautomeric forms of 4(5)-(2 -furyl)-imidazole (1) and investigated their molecular
vibrations by infrared and Raman spectroscopies as well as by calculation based on the density functional theory (DFT)
approach. Examination of the temperature dependence of IR intensity revealed the band characteristics of the 4 and 5
tautomers of (1). Comparison of experimental and calculated chemical shifts in nuclear magnetic resonance (NMR) spectroscopy
was made in order to identify the two tautomeric forms. The assignment of vibrational normal modes was performed, and the
force field obtained reproduced the experimental vibrational wavenumbers with a root mean-square deviation (RMSD) value of
−
1
ca. 13 cm for both tautomers. The natural bond orbital (NBO) study reveals the characteristics of the electronic delocalization
of the two tautomeric structures. Copyright ꢀc 2009 John Wiley & Sons, Ltd.
Supporting information may be found in the online version of this article.
ꢁ
Keywords: 4(5)-(2 -furyl)-imidazole; DFT calculation; infrared spectrum; Raman spectrum; tautomeric structures; force field
ꢁ
ꢁ
Introduction
of the 4-(2 -furyl)-imidazole and 5-(2 -furyl)-imidazole molecules
were analyzed using abinitio Hartree–Fock (HF) and second-order
perturbation M o¨ ller–Plesset (MP2) calculations. In the present
work, we report an experimental and theoretical vibrational study
Since 1970, there have been a number of studies on the
tautomerism of imidazoles, and some aspects have been
reviewed.[
1–3]
In the solid and liquid states, the N-unsubstituted
ꢁ
of 4(5)-(2 -furyl)-imidazole by means B3LYP calculations using 6-
imidazole compounds frequently exist in the associated form and
they are subject to fast hydrogen exchange. In 4-substituted imi-
dazoles that contain a free N–H bond, this rapid exchange is the
responsible of the prototropic tautomerism exhibited by them.
∗
∗∗
3
1G and6-311++G basissets.Foracompleteassignmentofthe
compound, the DFT calculations were combined with the scaled
[
14–16]
quantum mechanical force field (SQMFF) methodology
in
1
order to fit the theoretical wavenumbers to the experimental ones.
The changes in the chemical shifts of the hydrogen and carbon
atoms in the tautomeric equilibrium were also studied. In addi-
tion, the electronic properties of both tautomers of the compound
were evaluated by natural bond orbital(NBO)
molecules (AIM)[
The H NMR spectrum of those compounds exhibits only two C–H
resonances with an integral ratio 2 : 1, which indicates that on the
[
4]
NMR timescale the H4 and H5 atoms are equivalent. In fact, the
imidazoles provide one of the best studied examples of annular
tautomerism, and the values of the tautomeric ratio KT have been
calculated for a variety of 4(5)-substituted imidazoles and the re-
sults summarized. The application of the Hammett equation to
heteroaromatic tautomerism by Charton[ has suggested qual-
itatively that the electron-donating groups (σm < 0) will favor
the 5-sustituted isomer whereas the electron-withdrawing groups
[
17–20]
and atoms in
21,22]
studies in order to analyze the nature and
[
2]
magnitude of the intramolecular interactions.
5,6]
∗
Correspondence to: S. A. Brand a´ n, Instituto de Química Física, Facultad de
(σm > 0) will favor the 4-substituted tautomer. Recently it was
Bioquímica, Química Y Farmacia, Universidad Nacional de Tucum a´ n, San
Lorenzo 456, T 4000 CAN, San Miguel de Tucum a´ n. Tucum a´ n, R. Argentina.
E-mail: sbrandan@fbqf.unt.edu.ar
found that new 4(5)-imidazole derivatives, their stereoisomers,
and their non-toxic pharmaceutically acceptable acid addition
salts exhibit selective aromatase-inhibiting properties.[ Many
studies in recent years have dealt with the synthesis and tau-
7]
†
Member of the Carrera de Investigador Científico, CONICET, R. Argentina.
[
8–10]
tomeric equilibrium related to aryl imidazoles.
Thus, Talbot
[
11]
et al.
using the loan laser system (LSF) have studied the elec-
a INQUINOA-CONICET, Instituto de Química Física, FBQyF, Universidad Nacional
tronic spectra of jet-cooled 4-phenylimidazole, its singly hydrated
complex, as well as its tautomer, 5-phenyl imidazole. Furthermore,
the phenyl ring of 4-phenylimidazole provides a suitable chro-
mophore to examine hydrogen bonding to the imidazole group,
de Tucum a´ n, San Lorenzo 456, T 4000 CAN, Tucum a´ n, R. Argentina
b InstitutodeQuímicaRosario(CONICET-UNR),FacultaddeCienciasBioquímicas
y Farmac e´ uticas, Suipacha 531, Santa F e´ , R. Argentina
[
12]
and 4-phenylimidazole is itself an enzyme inhibitor.
V a´ zquez
[
13]
et al.
have presented a theoretical structural and vibrational
c Departamento de Química Física y Analítica, Facultad de Ciencias Experimen-
tales, Universidad de Ja e´ n, 23071 Ja e´ n, Spain
ꢁ
study on (2 -furyl)-imidazole series, in which the two conformers
J. Raman Spectrosc. 2010, 41, 587–597
Copyright ꢀc 2009 John Wiley & Sons, Ltd.

A. E. Ledesma et al.
Experimental
14 spectrophotometer in KBr pellets using the RCII (VLT-2) cell.
−
1
The Raman spectrum (resolution 1 cm , 200 scans) was recorded
withaBrukerRF100/SspectrometerequippedwithaNd : YAGlaser
source(excitationline1064 nm, 800mWpower)andaGedetector.
Nuclear magnetic resonance (NMR) spectra were recorded for
dilute solutions in CDCl3 using a Bruker 400 FT spectrometer at
Synthesis
ꢁ
4
(5)-(2 -furyl)-imidazole (1) was synthesized using the following
precursors:
1
13
1
4
00 MHz for H and 100 MHz for C. The H NMR spectra were
Sulfonamide
also registered at 273, 263, 253, 243 K with the same spectrometer.
GC-MS spectra were recorded with a Perkin Elmer Autosystem
XL Gas Chromatograph Turbomass Mass Spectrometer, using a
mn o mn pi . od l)a. r (100% polidimethylsiloxane) column (SE-30, 25 m × 0.22
In a 250-ml flask with three mouths, 90 ml aqueous ammonium
hydroxide was placed (1.2 mol).
stirrer, 30 g of finely dividedp-toluenesulfonyl chloride (0.157 mol)
was added. The mixture was heated under reflux for 1 h. The
resulting product was filtered, recrystallized from water, and dried
[
23]
While stirring with magnetic
◦
in an oven. w: 25.5 g; m.p: 137–138 C; yield: 95%.
Computational details
The DFT calculations for the compound were performed using the
N-Tosylbenzaldimine
[26]
∗
∗∗
Gaussian03programpackage withthe6-31G and6-311++G
[
27,28]
basis sets and the hybrid B3LYP functional approach.
The
Under a nitrogen atmosphere, 25.5 g (0.15 mol) of sulfonamide
and 4 g (12.4 ml) (0.15 mol) of furfuraldehide purified by fractional
distillation were made to react in presence of Si(EtO)4 32.7 g
force fields in Cartesian coordinates were calculated for all
∗
conformations of the compound at B3LYP/6-31G and B3LYP/6-
∗∗
0.157 mol).[
24]
311++G levels. The resulting force fields were transformed to
(
The mixture was allowed to boil in a silicone oil
◦
[29,30]
‘
natural’ internal coordinates with the MOLVIB program.
bath at 164 C for 1h. The dark product was dissolved in 125 ml
of hot ethyl acetate with agitation and the resulting solution was
made colorless with activated charcoal. The filtrate was diluted
with 600 ml of hexane. The insoluble solid was separated, filtered,
The natural coordinates are shown in Table S1 (Supporting
[
31,32]
Information), which were defined as proposed by Pulay et al.
Following the SQMFF procedure,[
14–16]
the harmonic force field
was scaled using the recommended scaling factors of Rauhut and
and dried to give a yellowish crystalline solid. w: 18.97 g. yield:
◦
[14,15]
1
13
Pulay.
The calculated chemical shifts of the H NMR and
C
5
0.7%. m.p: 97.3–99.3 C. Finally, the product was purified by
◦
NMR for the two tautomers were obtained by means the GIAO
crystallization in AcOEt–hexane. m.p: 101.3–102.7 C.
method[ using the B3LYP/6-311++G level of theory. For all
33]
∗∗
structures, an NBO analysis was performed using the NBO 3.1
4
(5)-(2-furyl)-imidazole
[20]
program, while the topological properties and the nature and
magnitude of the intramolecular interactions were also studied by
means the AIM2000 program.[
A mixture of TosMIC (98%) 6.58 g (33 mmol), N-tosyl benzaldimine
22]
7
.50 g (30 mmol), K2CO3 9.10 g (66 mmol), and MeOH/DME 2 : 1
[
25]
(300 ml) was heated under reflux with stirring for 1 h.
It was
cooled down to room temperature (RT), and 100 of cold water
was added and then all the mixture stirred for 15 min. The
reddish solution was diluted with 500 ml of cold water; it was
extracted successively with 400 ml of ethyl ether and with 500 ml
of CH2Cl2. The organic phases were collected and concentrated
under reduced pressure. The residue (6.17 g) was dissolved in HCl
Result and Discussion
Geometrical optimization
ꢁ
For each tautomeric structure of 1, which are 4-(2 -furyl)-imidazole
ꢁ
(4-FI) and 5-(2 -furyl)-imidazole (5-FI), two different stables
3
N (500 ml), alkalinized with aqueous 50% NaOH (pH=8), and
conformations were obtained according to syn and anti position
of oxygen atom with respect to the C5–H9 and N1–H6 bonds,
respectively, named the syn and anti conformers, respectively. The
labelingoftheatomsforallconformersofbothtautomersisshown
in Fig. 1. We can see that the anti conformer of the 4-FI tautomer
is in equilibrium with the syn conformer of the 5-FI tautomer
due to the prototautomerism, and for this reason only two
structures would be expected for this compound. The theoretical
calculations predict that all molecules have Cs symmetry. The
calculated energy differences among both conformations of the
two tautomers of 1, with B3LYP/6-31G and 6-311++G levels,
are shown in Table 1. For both tautomers, the syn conformers have
lower energy and therefore are the most stable. Also, the energy
differencebetweenbothconformationsofeachtautomerishigher
for 4-FI than for the other tautomer. The calculated parameters
for 4-FI and 5-FI tautomers using the B3LYP level at different
basis sets compared with the experimental X-ray data for the
extracted with CH2Cl2 (2 × 150 ml). The collected organic phase
was washed with NaCl saturated solution, dried with MgSO4, and
filtered and concentrated under reduced pressure. The resulting
product (1.87 g, dark oil) was purified by column chromatography
(alumina 507 C, 100–125 mesh, pH: 7) with CH2Cl2 as solventusing
a Redifrac collector (fractions × 2000 drops). w: 0.61 g.
The product was dissolved in 50 ml of ether and stirred for 3 h
at RT with addition of NaOH 12.5 N (35 ml) solution. The aqueous
phase was neutralized with H2SO4 12 N, extracted with ether
∗
∗∗
(
3 × 20 ml), dried with MgSO4, filtered, and evaporated. A light
brown solid resulted. w: 284 mg, yield: 7%. On sublimation of the
◦
product (95 mg) at 100 C at 0.5 Torr, 90 mg of white solid was
obtained. m.p.: 112.8–113.4 C. On crystallization in CH2Cl2, the
m.p. rises to 115–116 C.
The Raman spectra of the solid as well as the FTIR spectra at RT
and low temperatures (LT) were registered. The infrared spectrum
of the solid 4(5)-(2 -furyl)-imidazole in KBr pellets from 4000 to
2
equipped with a globar source and a DGTS detector at a resolution
of 1 cm
◦
◦
ꢁ
ꢁ
[34]
2-(2 -furyl)-1H-imidazole compound are listed in Table 2. There
are no significant differences between the observed calculated
geometric parameters for all conformers of both tautomers. The
analysis of these parameters show that the calculated distances
and bond angles are very close to the experimental ones, with
−
1
00 cm at RT was recorded on an FTIR GX1 spectrophotometer,
−
1
and 64 scans. The infrared spectra between 4000
−
1
and 200 cm at LT were registered on an FTIR Digilab model
www.interscience.wiley.com/journal/jrs
Copyright ꢀc 2009 John Wiley & Sons, Ltd.
J. Raman Spectrosc. 2010, 41, 587–597

ꢁ
Vibrational spectroscopy of two tautomers of 4(5)-(2 -furyl)-1H-imidazole
ꢁ
Figure 1. Theoretical structures and atoms numbering of the 4 and 5 tautomers of 4(5)-(2 -furyl)-imidazole. The proton tautomerism is indicated with
arrows.
the root mean-square deviation (RMSD) values of 0.02 Å for the
distances and of 1.2 for the angles. The slight differences between
of the imidazole molecule. These chemical shift values indicate
that in 4(5)-(2 -furyl)-imidazole the H atom is held weakly by two
◦
ꢁ
the calculated N3–C4 and N3–C2 distances are attributed, in the
imidazole rings. At an intermediate rate of exchange, the H atom
is partially decoupled, and a broad N–H peak results. Figure S2
(Supporting Information) shows the C NMR spectrum of 1 in
ꢁ
first case, to the experimental value because in the 2-(2 -furyl)-
[
34]
13
1
H-imidazole compound
the resultant value is an average of
single and double bonds, while the theoretical value of the N3–C2
distance corresponds to a pure coordinate. Also, the calculated
N3–C4 distance (ca. 1.384 Å) is significantly smaller than the N–C
single bond (sum of the N and C covalent radii is 1.52 Å),[
suggesting a substantial degree of electron delocalization on the
imidazole ring.[ On the other hand, the C–C inter-ring bond
length is higher for both tautomers (4-FI and 5-FI) compared to
deuterated chloroform. The most intense peak of this spectrum
belongs to the C atom of the reference, while the following four
peaks belong to the C atoms of the furyl group. The peak at
135.62 ppm belongs to the C2 atom of the imidazole ring and the
two peaks of smaller intensity belong to the C5 and C4 atoms
of this ring. The calculated chemical shifts are in agreement with
the experimental ones, with RMSD values of 2.53 and 0.32 ppm
for the C and H atoms, respectively. The agreement with our
experimental data in these solvents is good, except for the H atom
of the H–N bond in chloroform. This is because the calculations
are for the gas phase, while the experimental values are for the
CDCl3 solution where the molecular interactions are important.
It is important to mention that the registered NMR spectra at RT
and LT were not sufficient to identify the tautomeric mixture of
35]
36]
ꢁ
13
1
the 2-(2 -furyl)-1H-imidazole compound, showing that both rings
aremoreseparatedinthosetautomersthaninthelatter.Moreover,
the larger calculated C C lengths for both tautomers justify the
lower conjugation effects between the furane and imidazole rings.
Significant changes are not observed for the bond angles.
NMR
1
, because the speed of exchange of protons is higher than the
1
Figure S1 (Supporting Information) shows the H NMR spectrum
response time of NMR.
of 1 in deuterated chloroform. Three signals corresponding to
three different H atoms of the imidazole ring are observed. The
corresponding calculated and experimental chemical shifts for the
H and C atoms appear in Table 3. The peaks at 6.43, 6.54, and
NBO and AIM analyses
The stability of the two tautomers of 1 was investigated by
[
17–19]
7
.39 ppm belong to three H atoms on 14, 15, and 16 positions of
NBO calculations.
E
The second-order perturbation energies
(
2)
thefuranerings, respectively, whilethepeaksat7.33and7.71 ppm
belong to two H atoms on position 9 and 7 of the imidazole rings,
respectively. These chemical shift values are in agreement with
(donor → acceptor) that involve the most important
(2)
delocalization stabilization energies (E , in kJ/mol) associated
ꢁ
with the main delocalizations for all structures of 4(5)-(2 -furyl)-
[
37,38]
those in the literature.
The peak belonging to H atom of the
imidazole are given in Table 4. We found that the contributions
∗
N–H bond appears at 11.37 ppm. A small shift of these peaks
towards lower fields implies the existence of some intermolecular
interaction between nonbonding electrons. As seen in Fig. S1
of the stabilization energies for the σN1-C5→ σ C4–C8 charge
transfers are higher for the 4-FI tautomer than the other tautomer,
while the opposite situation is observed in the stabilization
∗
(Supporting Information), the broad peak associated with the H
energies of the σN3-C4→ σ C4–C8 charge transfers for the 5-FI
atom of the N–H group of the imidazole ring appears at 11.37 ppm
because the H atom is exchanged between two imidazole rings via
the intermolecular hydrogen bond, and so the H atom of the N–H
bond peak is slightly shifted to lower fields with respect to that
tautomer.Moreover,aswasobservedforthemethylatedspeciesof
ꢁ
[39]
∗
∗
4-(2 -furyl)-imidazole,
the π C4 = C5→ π C8 = C10 and
∗
∗
π C11 = C12→ π C8 = C10 charge transfers are only observed
in the 4-FI tautomer. The parameters obtained for the atoms
J. Raman Spectrosc. 2010, 41, 587–597
Copyright ꢀc 2009 John Wiley & Sons, Ltd.
www.interscience.wiley.com/journal/jrs

A. E. Ledesma et al.
ꢁ
ꢁ
Table 1. Calculated total (ET) and relatives (ꢀE) energies and dipole moment (µ) for 4-(2 -furyl)-imidazole (4-FI) and 5-(2 -furyl)-1H-imidazole (5-FI)
∗
DFT/B3LYP/6-31G
Tautomer
4-FI
5-FI
Conformer
anti
3.94
syn
anti
3.98
syn
µ (Debye)
3.23
−455.0537620
0.0
3.37
−455.0534867
0.0
ET (Hartrees)
−455.0491050
−455.0513554
ꢀ
E (kJ/mol)
12.20
5.60
DFT/B3LYP/6-311++G∗∗
Tautomer
4-FI
5-FI
µ (Debye)
4.12
3.24
−455.1803045
0.0
4.17
−455.1772201
6.10
3.42
−455.1795573
0.0
ET (Hartrees)
−455.1753400
13.00
ꢀ
E (kJ/mol)
ꢁ
ꢁ
Table 2. Comparisonbetweenthecalculatedgeometricalparametersof4-(2 -furyl)-imidazole(4-FI)and5-(2 -furyl)-imidazolewiththeexperimental
one of 2-(2 -furyl)-1H-imidazole (5-FI)
ꢁ
4
-FI
5-FI
∗
6-311++G∗∗
∗
6-31G
6-311++G∗∗
6-31G
Parameter
Exp.#
anti
syn
anti
syn
anti
syn
anti
syn
Bond length (Å)
O13–C12
O13–C8
N3–C4
1.361
(3)
(2)
(3)
(3)
(2)
(3)
(3)
(3)
(3)
(3)
(4)
1.362
1.367
1.384
1.378
1.312
1.367
1.451
1.381
1.372
1.430
1.362
0.025
1.363
1.366
1.385
1.379
1.295
1.367
1.446
1.379
1.368
1.431
1.361
0.027
1.361
1.364
1.382
1.377
1.310
1.366
1.450
1.379
1.369
1.429
1.359
0.024
1.365
1.370
1.383
1.378
1.310
1.366
1.445
1.377
1.365
1.432
1.358
0.023
1.365
1.369
1.373
1.385
1.315
1.366
1.441
1.382
1.372
1.431
1.360
0.023
1.368
1.374
1.372
1.384
1.317
1.365
1.441
1.383
1.370
1.431
1.360
0.023
1.365
1.367
1.373
1.385
1.312
1.367
1.439
1.380
1.369
1.431
1.358
0.023
1.364
1.367
1.372
1.385
1.313
1.366
1.439
1.380
1.369
1.431
1.357
0.022
1.363
1.342
1.374
1.339
1.369
1.438
1.345
1.343
1.413
1.319
N1–C5
N3–C2
N1–C2
C4–C8
C4–C5
C8–C10
C10–C11
C11–C12
RMSD (Å)
◦
Bond angles ( )
C12–O13–C8
C2–N1–C5
106.6
(2)
(2)
(2)
(2)
(2)
(2)
(2)
(2)
(2)
(2)
(2)
(2)
(2)
107.3
106.7
107.3
105.4
111.9
121.2
110.3
105.1
109.8
132.9
117.3
106.4
106.3
110.4
107.4
107.1
107.2
107.1
107.4
105.3
111.8
122.1
110.9
104.3
109.6
134.2
116.2
106.5
106.5
110.2
107.4
107.5
105.6
105.7
111.6
123.7
108.7
108.4
109.2
134.2
116.6
107.1
106.6
110.6
107.2
105.5
111.9
122.3
110.1
105.3
109.5
133.1
117.4
106.6
106.0)
110.5
1.09
107.3
105.8
111.7
122.5
109.9
105.3
109.5
132.9
117.5
106.6
106.1
110.4
107.4
105.7
111.6
121.3
110.1
105.1
109.8
132.9
117.3
106.4
106.4
110.3
107.3
105.3
111.8
123.3
111.0
104.4
109.5
134.2
116.3
106.5
106.2
110.5
107.3
105.7
111.6
123.0
110.7
104.6
109.5
134.0
116.5
106.4
106.3
110.3
107.4
105.7
111.5
123.2
110.7
104.5
109.4
134.1
116.4
106.5
106.3
110.2
C4–N3–C2
N1–C2–N3
N3(N1)–C4–C8
N3–C4–C5
N1–C5–C4
C10–C8–O13
C10–C8–C4(5)
O13–C8–C4(5)
C8–C10–C11
C12–C11–C10
C11–C12–O13
◦
RMSD ( )
1.18
1.09
1.16
1.37
1.38
1.27
1.29
◦
Dihedral angles ( )
C10-C8-C4(C5)-N3(N1)
O13-C8- C4(C5)-N3(N1)
8.0
(4)
180
0
0
180
0
0
180
0
0
180
0
0
172.00
(17)
180
180
180
180
#
Ref. [34].
www.interscience.wiley.com/journal/jrs
Copyright ꢀc 2009 John Wiley & Sons, Ltd.
J. Raman Spectrosc. 2010, 41, 587–597

ꢁ
Vibrational spectroscopy of two tautomers of 4(5)-(2 -furyl)-1H-imidazole
a
ꢁ
ꢁ
Table 3. Calculated and experimental chemical shifts (in ppm) for 4-(2 -furyl)-imidazole (4-FI) and 5-(2 -furyl)-imidazole (5-FI)
δC
δH, multiplicity
δH
Position
Experimentalb
Calculatedc
Experimentalb
Calculatedc
1
1
5
4
2
1
8
0
104.31
111.32
114.68
131.61
135.62
141.23
148.99
–
103.55
|108.82
111.03
132.40
138.1
6.54 (d, J = 3.3 Hz, 1H)
6.43 (dd, J = 1.8, J = 3.3 Hz, 1H)
7.33 (d, J = 0.9 Hz, 1H)
6.29
6.17
6.98
1
7.71 (d, J = 0.9 Hz, 1H)
7.26
7.16
2
141.35
153.20
–
7.39 (dd, J = 0.9, J = 1.5 Hz, 1H)
N-H
11.37 (bs,1H)
7.86
a
∗∗
B3LYP/6-311++G , average value of the two tautomers.
b
c
In Cl3DC.
Ref to TMS.
Table 4. Stabilization energies (E(2), in kJ/mol) associated with main
ꢁ
delocalization for all studied structures of 4(5)-(2 -furyl)-imidazole
B3LYP/6-311++G∗∗
4
-FI
5-FI
E(2) (donor → acceptor)
anti
syn
anti
syn
∗
∗
∗
∗
σN1–C5→ σ C4–C8
16.47
2.72
17.14
2.84
4.93
7.98
–
σC2–N3→ σ N3–C4
2.30
16.85
19.65
84.77
65.75
67.84
57.18
–
σN3–C4→ σ C4–C8
6.02
6.69
15.42
14.42
79.88
84.02
72.94
52.25
70.31
435.60
–
σC4–C5→ σ C4–C8
14.30
89.70
62.57
56.64
58.19
70.18
425.02
689.32
–
14.42
87.78
62.95
62.37
66.38
71.44
327.96
538.01
481.70
∗
∗
∗
πC2–N3→ π C4–C5
πC4–C5→ π C2–N3
πC4–C5→ π C8–C10
∗
∗
πC8–C10→ π C4–C5
πC8–C10→ π C11–C12
ꢁ
Figure 2. Infrared spectra of 4(5)-(2 -furyl)-imidazole. Upper: theoretical
∗
∗
∗
∗
∗
π
π
π
C2–N3→ π C4–C5
367.17
–
∗∗
at B3LYP/6-311++G level. Lower: Experimental spectrum of the solid
C4–C5→ π C8–C10
substance.
∗
C11–C12→ π C8–C10
–
–
where the quality of the basis set was found to have no influence
on the topological results.[
48,49]
It is important to note that there
involved in those transitions, such as the natural charges and
the bond order expressed as the Wiberg bond index, are given
in Table S2 (Supporting Information), whereas the polarization
coefficients and percentages and population of the main orbitals
for all studied structures are presented in Table S3 (Supporting
Information). The results correlate well with the calculated short
C8–C10 distance. A possible explanation of this observation could
be the observed larger population in the bonding C4–C5 orbital
of the 4-FI tautomer in relation to other tautomer, or the larger
C4–C5 bond order value in this tautomer. Also, the intramolecular
interactions have been analyzed by using Bader’s topological
analysis of the charge electron density ρ(r). The localization of
the bond critical points (BCPs) in the ρ(r) and the values of the
Laplacian at these points are important for the characterization
of the molecular electronic structure in terms of the nature of
interaction and its magnitude. This BCP has the typical properties
of the closed-shell interaction. When the value of ρ(r) is relatively
is no BCP in all conformers of 1 with typical properties of the
closed-shell interaction. The analysis of the ring critical point (RCP)
isreportedinTableS4(SupportingInformation), andtheelectronic
2
densities in the BCPs as well as the large negative values of ∇ ρ(r)
for the C4–C5, C8–C10, C11–C12 distances in all conformers of 1
are in agreement with the obtained values for covalent bonding,
as expected (Table S5, Supporting Information). In this study,
two correlations in the topological properties of those BCPs are
observed. The first correlation is between the negative values
2
of ∇ ρ(r) and the charge electron density ρ(r) corresponding to
those C–C distances (Table S6, Supporting Information), while
2
the second one is between the ∇ ρ(r) and ρ(r) corresponding to
RCP of all conformers of 1 (Table S7, Supporting Information). The
relations are completely linear in both cases. It means that the
properties of the C–C BCPs may be very useful to estimate the
strength of the intramolecular C–C bond.
2
low, |λ1|/λ3 is <1 and the Laplacian of the electron density ∇ ρ(r)
Vibrational results
ispositive, whichindicatesthattheinteractionisdominatedbythe
contraction of charge away from the interatomic surface toward
The experimental infrared and Raman spectra at RT of 1 are
shown in Figs 2 and 3, respectively, revealing a rather complex
composition of the vibrational bands for this compound. The two
each nucleus.[
40–47]
In this study, only the 6-311++G basis
∗∗
set has been considered because there are numerous references
J. Raman Spectrosc. 2010, 41, 587–597
Copyright ꢀc 2009 John Wiley & Sons, Ltd.
www.interscience.wiley.com/journal/jrs

A. E. Ledesma et al.
ꢁ −1
Figure 5. Ramanspectraof4(5)-(2 -furyl)-imidazoleinthe500–10 cm re-
ꢁ
Figure 3. Raman spectra of 4(5)-(2 -furyl)-imidazole. Upper: theoretical
∗∗
∗∗
gion. Upper: Theoretical at B3LYP/6-311++G level. Lower: Experimental
at B3LYP/6-311++G level. Lower: Experimental spectrum of the solid
spectrum of the solid substance.
substance.
exhibit close similarity, as shown in Figs 2 and 3. Furthermore, the
analysis of the infrared spectrum at LT (100 K) allowed carrying
out an assignment with a good approach showing, e.g., that the
−
1
bands located at 1638, 1438, 1430, 1342, and 830 cm in the IR
spectrum at RT and LT are split. Figure 4 shows the experimental
−
1
spectrum at RT in the 1300–800 cm region compared with the
corresponding spectrum at 100 K and with the second derivative
curve of the spectrum at 100 K. It is important to observe that
the second derivative curve clearly indicates the peak positions
[
52]
of IR bands in this region, as was observed by Toyama et al.
in the tautomers of 4-methylimidazole. A reasonable explanation
for the observed changes at LT would be the presence in the
solid of the different conformers belonging to both tautomers.
According to the theoretical calculations, for the 5-FI tautomer
it is possible to observe bands assignable to each one of its two
conformers separately, whereas for the 4-FI tautomer only one
band is generally observed for both conformers. For example,
ꢁ
Figure 4. Infrared spectra of 4(5)-(2 -furyl)-imidazole. Infrared spectrum
(
−1
in the region between 800 and 700 cm (Fig. S3, Supporting
Information), the experimental spectra at 298 and 100 K show
a) and second derivative (b) recorded at 100 K and (c) 298 K.
−
1
−1
only one band at 750 cm (298 K) and 753 cm (100 K), which
would be assignable, in accordance with the calculations, to the
conformers of both tautomers of 1 exhibit Cs symmetry and have
2 vibration modes classified as 29A and 13A vibrations and all
are infrared and Raman active. The assignment of the vibrational
normal modes for the two conformers of the both tautomers is
shown in Table 5. An exhaustive comparison with the infrared
spectrum of the methylated species of the 4-FI tautomer[
as well as also with the experimental assignments of similar
was very useful for the assignment of the bands in
this compound. Thus, the observed IR bands at 1438, 1235, 917,
−
1
ꢁ ꢁꢁ
normal ν33 mode of the two conformers of 4-FI at 752 cm . On
the other hand, the bands observed at RT at 776 and 769 cm
and at LT at 777 and 771 cm , as well as by the calculations at
88 cm (syn) and 776 cm (anti), can also be assigned to ν33
4
−
1
−
1
−
1
−1
7
39]
for both conformers of 5-FI, although for the ν32 mode of the
−
conformer anti of this tautomer the value calculated at 789 cm
1
molecules[
50,51]
is overlapped with the band corresponding to the ν33 of the syn
conformer.Also,asimilarargumentcanbemadefortheν8 modein
−
1
−1
7
69, and 635 cm are absent in the spectrum of the methyled
the 1550 and 1500 cm region (Fig. S3, Supporting Information).
species of 4-FI, and for this reason they were assigned to the 5-
FI tautomer. The experimental vibrational spectrum fits fairly well
withthatpredictedbythetheoreticalmodelforthetwotautomers.
These results probably indicate the existence of a mixture of both
tautomers in the solid phase. Especially, in the case of both
conformers of the 4-FI tautomer, its theoretical vibrational spectra
For this mode, in the syn and anti conformers of 4-FI a single band
−
1
−1
isobservedat1516 cm inthespectrumatRTandat1521 cm in
the spectrum at 100 K, which can be compared with the calculated
−
1
values for those conformers of 4-FI at 1507 cm . On the other
hand, the two peaks of this band appear better resolved at RT at
−
1
−1
1538 and 1534 cm (298 K) and at 1542 and 1530 cm (100 K),
www.interscience.wiley.com/journal/jrs
Copyright ꢀc 2009 John Wiley & Sons, Ltd.
J. Raman Spectrosc. 2010, 41, 587–597

ꢁ
Vibrational spectroscopy of two tautomers of 4(5)-(2 -furyl)-1H-imidazole
J. Raman Spectrosc. 2010, 41, 587–597
Copyright ꢀc 2009 John Wiley & Sons, Ltd.
www.interscience.wiley.com/journal/jrs

A. E. Ledesma et al.
www.interscience.wiley.com/journal/jrs
Copyright ꢀc 2009 John Wiley & Sons, Ltd.
J. Raman Spectrosc. 2010, 41, 587–597

ꢁ
Vibrational spectroscopy of two tautomers of 4(5)-(2 -furyl)-1H-imidazole
and they could be associated with the calculated value for the
mode of the furan ring for the two tautomers. The pair of bands
−
1
−1
located at 1162 and 1067 cm , which is observed in the Raman
normal ν8 mode at 1521 and 1514 cm for the syn and anti
conformers of the 5-FI tautomer, respectively. The experimental
vibrational spectrum fits fairly well with that predicted by the
theoretical model as shown in Figs 2 and 3, and the calculated
initial RMSD values obtained from a comparison between the
−
1
spectrum as only one band at 1165 cm , is assigned, based on
prediction by the calculations for their intensity as well as on
comparison with the studied molecules,[
50,51]
to the stretching
C–O modes for both 4-FI tautomers.
∗
∗
theoretical (B3LYP/6-311++G ) and experimental wavenumbers
The in-plane C–H deformation modes are expected between
−
1
−1
for the anti and syn conformers are 53 and 47.7 cm for the 4-FI
tautomer and 68 cm for both conformers of the 5-FI tautomer.
1300 and 1000 cm , as predicted by the calculations and in
−
1
[50,51]
comparison with those observed for similar molecules.
In this
After the scaling process of the force constants, the RMSD values
case, the bands located in the infrared spectrum at 1307/1304,
1222, 1183, 1162, and 1006 cm and of which four only are
−
1
−1
decrease to 13 cm for all conformers of the studied isomeric
species. This reasonable standard deviation value, taking into
account the complexity of the analyzed sample and that the scale
observed in the Raman spectrum at 1308/1303, 1226, 1191, and
−
1
1010 cm are assigned to the five modes of the 4-FI tautomer,
factors were taken directly from those of Rauhut and Pulay,[
14,15]
−1
while the bands at 1251, 1235, 1204 and 1006 cm are assigned
allowed us to compare with an acceptable level of agreement
between the theoretical spectra and the experimental ones in
order to carry out a complete assignment of the vibrational
to the 5-FI tautomer. The in-plane N–H deformation band is
predicted by the calculations for both tautomers, with an average
−
1
−1
difference of 10 cm , for the IR band at 1071 cm (in the Raman
ꢁ
−1 −1
normal modes of 4(5)-(2 -furyl)-imidazole. The comparison of the
spectrumat1073 cm )andat1081 cm (intheRamanspectrum
at 1083 cm ), and they are assigned respectively to this mode for
−
1
theoretical infrared and Raman spectra for the two conformations
of both isomeric species with the corresponding experimental
spectrum at ambient temperature is shown in Figs 2 and 3.
the 4-FI and 5-FI tautomers.
−
1
Region below 1000 cm
Assignment of the bands
The pair of bands located in the infrared spectrum at RT at 971
and 964 cm (in the Raman spectrum at 971 and 966 cm )
−
1
−1
−
1
Region 4000–3000 cm
are assigned to the expected imidazol ring deformation (βR(i))
−
1
−1
The broad band observed in the infrared spectrum at 3442 cm
for the 5-FI tautomer, while the bands at 942 and 939 cm (in
the Raman spectrum observed as a single band at 943 cm 1) are
assigned to these modes for the 4-FI tautomer. According to the
prediction of the theoretical calculations and from comparison
−
is assigned to the N–H stretching mode for both tautomers of
ꢁ
4
3
(5)-(2 -furyl)-imidazole because they are expected at 3444 and
−
1
−1
443 cm . The group of bands in the 3150–3000 cm region
[
53]
was attributed to the C–H stretching modes as shown in Table 5.
with the wavenumbers reported for the furan molecule,
the
two furan ring deformation modes (β(R(f)) are assigned for both
−
1
−
1
tautomers to the IR and Raman bands at 892 and 885 cm
The five C–H out-of-plane deformations predicted by calcula-
.
Region 2000–1000 cm
This region is the most complex because the skeletal ring modes
are strongly coupled and for this reason it is difficult to assign the
bands of 5-FI tautomer. The band of relatively high intensity in the
infrared and Raman spectra at 1638 cm , which appears perfectly
split at LT, is assigned, by comparison with the theoretical results,
to the stretching C C modes. The other two stretching C
modes were assigned to the IR bands at 1538/1534 cm (in the
Raman spectra both at 1540 cm ) and 1438/1430 cm for the
[50,51]
tions and observed in the molecules previously studied
lie in
−1
the region between 870 and 700 cm . From these observations,
the shoulder at 868 cm and the observed bands in the infrared
spectrum at RT at 813, 800, 750, and 725 cm , of which only
four are observed in the Raman spectrum at 871, 800, 750, and
31 cm , are assigned to those modes of the 4-FI tautomer. For
−
1
−
1
−
1
−
1
C
7
−
−1
1
the 5-FI molecule, three of those bands are assigned to the same
wavenumbers as the previous species (868, 800, and 725 cm ),
−
1
−1
−
1
5
-FI tautomer and at 1516 and 1467 cm (in the Raman spectra
and the two remaining modes are assigned to the IR bands at 830
−
1
−1
−1
at 1520 and 1468 cm ) for the 4-FI tautomer. The second band
and 776 cm and the Raman bands at 826 and 775 cm .
−
1
−1
,
in the pair is split at LT at 1445/1441 cm and 1434/1431 cm
respectively. As predicted by the calculations, the band in the
infrared spectrum of the solid substance at 1491 cm (in the
Raman spectrum at 1492 cm ) is assigned to the C N stretching
modes. The theoretical calculations predict that the stretching
modes of the imidazole ring appear at different wavenumbers
for each one of the isomers, and accordingly the three stretching
C–N modes for the 4-FI tautomer are assigned to the IR bands at
RT at 1400, 1275 and 1050 cm and the Raman peaks at 1401
and 1277 cm 1, respectively. These modes corresponding to the
-FI tautomer are difficult to assign, given the complexity of the
potential energy distribution (PED) contributions for each one
of the conformers; only one of them is clearly predicted by the
For both tautomers, the two torsional modes of the imidazol
ring are predicted by the calculations to lie between 700 and
660 cm with significant differences among them. The bands
in the infrared spectrum at 702 and 652 cm and in the Raman
spectrumat693and657 cm areassociatedwiththesemodesfor
the 4-FI tautomer, while the corresponding bands are at 656 and
635 cm (655 and 636 cm ) in the 5-FI tautomer. As predicted
by the calculations, the two the furan ring torsional modes are
assigned to the IR bands at 621 and 591 cm (in the Raman
spectrum at 622 and 596 cm ) for the 4-FI tautomer and at
685 and 591 cm in the infrared and Raman spectra of the 5-FI
tautomer.
The theoretical calculations predict the N–H out-of-plane
deformation for each conformer of the tautomer species at
different wavenumbers and slightly displaced with respect to each
other. Therefore, the IR bands at 534 and 464 cm are assigned
to these modes for both conformers of the 5-FI tautomer, while
the band in the infrared spectrum at 498 cm for other tautomer
−
1
−1
−
1
−1
−
1
−
1
−1
−
1
−1
−
−1
−
1
5
−
1
calculations and it is assigned to the IR band at 1081 cm . The
shoulder located in the infrared spectrum at RT at 1378 cm
observed at low temperature as a weak band at the same
wavenumber and in the Raman spectrum at 1371 cm , and
−
1
,
−
1
−
1
[
53]
−1
in comparison with the furane molecule
and with the results
of the theoretical calculations, is assigned to the C–C stretching
is assigned to these modes.
J. Raman Spectrosc. 2010, 41, 587–597
Copyright ꢀc 2009 John Wiley & Sons, Ltd.
www.interscience.wiley.com/journal/jrs

A. E. Ledesma et al.
ꢁ
Table 6. Comparison of the force constants of the 4-FI and 5-FI with the corresponding to 2-(2 -furyl)-1H-imidazole
ꢁ
ꢁ
ꢁ
2
-(2 -Furyl)-1H-Imidazole
4-(2 -furyl)-Imidazole
5-(2 -furyl)-Imidazole
Description
anti
syn
anti
syn
anti
syn
f (C–C)int
f (C–C)
5.87
6.14
7.98
8.22
6.73
7.25
6.55
5.23
6.38
0.37
0.66
0.49
0.55
0.36
0.13
0.03
5.44
5.44
7.49
7.99
6.35
7.92
6.55
5.23
6.36
0.47
0.68
0.48
0.47
0.37
0.19
0.10
5.28
5.37
6.90
7.41
5.92
7.93
6.56
5.34
5.29
0.41
0.62
0.44
0.42
0.37
0.20
0.10
5.28
6.08
5.18
7.30
6.40
8.78
6.35
5.21
5.85
0.47
0.61
0.59
0.43
0.37
0.21
0.07
5.35
5.38
6.77
7.35
5.95
7.78
6.56
5.32
5.35
0.40
0.62
0.45
0.43
0.36
0.14
0.03
5.38
5.38
6.73
7.39
6.00
7.71
6.54
5.32
5.23
0.41
0.63
0.45
0.42
0.39
0.18
0.04
f (C C)imidazole
f (C C)furan
f (C–N)
f (C N)
f (N–H)
f (C–H)
f (C–O)
β
R
τR
f (β C–H)
f (β N–H)
f (γ C–H)
f (γ N–H)
f (τw Ring)
−
1
Below 460 cm the theoretical calculations predict for both Conclusions
tautomers six vibrational normal modes associated to the
ꢁ
-
The 4(5)-(2 -furyl)-imidazole molecule was synthesized and
inter-rings modes, such as γ (C–C)inter−rings, β(C–C)inter−rings, the
butterfly mode and τ (C–C)inter−rings (see Figure 5). However,
only two of them are observed in the infrared spectrum and
the remaining ones are in the Raman spectrum. The two
characterized by GC-MS as well as by infrared and Raman
spectroscopic techniques in the solid state and also by NMR in
solution. The following points may be noted:
-Thepresentworkshowstheexistenceofequilibriumbetween
the 4-FI and 5-FI tautomers of this molecule, which were
evaluated at lower temperature.
- The presence of 4-FI and 5-FI tautomers was detected in
the IR spectrum, and a complete assignment of the vibrational
modes was accomplished.
−
1
bands observed in the IR spectrum at 456 cm
and in the
−
1
Raman spectrum at 160 cm are assigned to β(C–C)inter−rings,
for both tautomers. The (C–C)inter−rings stretching is assigned
−
1
to the Raman band at 404 cm . The three bands observed in
−
1
the Raman spectrum at 336, 123 and 97 cm , according to
their wavenumbers and their corresponding intensities of the
-
The calculations suggest the existence of two molecular
anti and syn conformations for each of them, both of the Cs
calculations, as well as from comparison with the previously
studied molecules,[
50,51,54]
are attributed as follows: the first one
symmetry.
-
to the C–C out-of-plane deformation, the second to the butterfly
mode and the remaining one to the torsion inter-rings.
An SQM force field was obtained for anti and syn conformers
of 4-FI and 5-FI tautomeric structures after adjusting the the-
oretically obtained force constants to minimize the difference
between the observed and calculated wavenumbers.
Force field
Acknowledgements
We thank Prof. Tom Sundius for authorizing the use of the
MOLVIB Program. We also thank CONICET (Consejo Nacional
de Investigaciones Científicas y T e´ cnicas) and CIUNT (Consejo
de Investigaciones de la Universidad Nacional de Tucum a´ n) for
financial support.
Supporting information
The corresponding force constants for the experimentally studied
ꢁ
4
(5)-(2 -furyl)-imidazole were estimated using the scaling proce-
[
31,32]
dure of Pulay et al.,
as mentioned before. The force constants
appearing in Table 6 expressed in terms of simple valence internal
coordinates were calculated from the corresponding scaled force
fields. It is interesting to compare the principal force constants
calculated at the B3LYP/6-311++G∗∗ levels for both tautomers
Supporting information may be found in the online version of this
article.
ꢁ
[34]
with those corresponding to the 2-(2 -furyl)-1H-imidazole.
In
general, the calculated force constant values for the anti and syn References
conformers for both tautomers are approximately the same. The
[
1] K. Schofield,M. R. Grimment,B. R. T. Keene,HeteroaromaticNitrogen
most important difference is found for the C C, C–N, and C–O
Compounds, The Azoles, Cambridge University Press: London, New
York, 1976.
ꢁ
stretching force constants, which are higher for the 2-(2 -furyl)-1H-
imidazolecompound. Thisobservationcanbeexplainedbymeans
of the calculated bond lengths (Table 2) because they are larger
for these tautomers than those observed for the 2-(2 -furyl)-1H-
imidazole molecule. The values of the remaining force constants
do not present significant differences.
[2] J. Elguero, C. Marzin, R. Katritzky, P. Linda, Adv. Heterocycl. Chem.
Suppl. 1976, 1, 280.
[3] B. I. Khristich, Khim. Geterotsikl. Soedin. 1970, 1683.
ꢁ
[
4] M. R. Grimment, Sciences of Synthesis: Honben-Weyl Methods on
Molecular Transformation, vol 12, (Ed.: R. Neier), Georg Thieme
Verlog: New York, 2002, pp 325, Chapt. 3.
www.interscience.wiley.com/journal/jrs
Copyright ꢀc 2009 John Wiley & Sons, Ltd.
J. Raman Spectrosc. 2010, 41, 587–597

ꢁ
Vibrational spectroscopy of two tautomers of 4(5)-(2 -furyl)-1H-imidazole
[
[
[
[
[
5] M. Charton, J. Org. Chem. 1979, 44, 903.
6] M. Charton, J. Chem. Soc. B 1969, 1240.
7] PatentStorm LLC, United States Patent no., 5559141.
8] F. Bellina, S. Cauteruccio, R. Rossi, J. Org. Chem. 2007, 72(22), 8543.
9] S. A. Popov, R. V. Andreev, G. V. Romanenko, V. I. Ovcharenko,
V. A. Reznikov, J. Mol. Struct. 2004, 697(1–3), 49.
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 B.01, Gaussian, Inc.: Pittsburgh, PA, 2003.
[27] D. Becke, J. Chem. Phys. Rev. 1993, 98, 5648.
[28] C. Lee, W. Yang, R. G. Parr, Phys. Rev. 1988, B37, 785.
[29] T. Sundius, J. Mol. Struct. 1990, 218, 321.
[30] T. Sundius, Vib. Spectrosc. 2002, 29, 89.
[
10] K. Sztanke, S. Fidecka, E. K dzierska, Z. Karczmarzyk, K. Pihlaja,
D. Matosiuk, Eur. J. Med. Chem. 2005, 40(2), 127.
[
11] F. O. Talbot, L. C. Snoek, N. A. Macleod, P. Butz, J. P. Simons,
[31] P. Pulay, G. Fogarasi, F. Pang, E. Boggs, J. Am. Chem. Soc. 1979,
101(10), 2550.
R. T. Kroemer, Lasers for Science Facility Programme – Chemistry,
2
002.
[32] P. Pulay, G. Fogarasi, G. Pongor, J. E. Boggs, A. Vargha, J. Am. Chem.
[
[
12] J. P. Simons, E. G. Robertson, M. R. Hockridge, Science – Lasers for
Science Facility – Chemistry, 1999.
13] J. V a´ zquez, J. J. L o´ pez Gonz a´ lez, L. Ballester, J. Boggs, J. Mol. Struct.
Soc. 1983, 105, 7037.
[33] R. Ditchfield, Mol. Phys. 1974, 8, 397.
[34] A. E. Ledesma,S. A. Brand a´ n,J. Zinczuk,O. E. Piro,J. L o´ pezGonz a´ lez,
A. Ben Altabef., J. Phys. Org. Chem. 2008, 21(12), 1086.
[35] M. Winter, Web elements, http://www.webelements.com.
[36] H. Xile, K. Meyer, J. Am. Chem. Soc. 2004, 126, 16322.
[37] N. O. Pekmez, M. Can, A. Yildiz, Acta Chim. Slov. 2007, 54, 131.
[38] W. Brugel, HandbookofNMRSpectralParameters, vols. 1–3, Heyden:
Philadelphia, 1979.
1
997, 393, 97.
[
[
[
[
[
[
[
14] G. Rauhut, P. Pulay, J. Phys. Chem. 1995, 99, 3093.
15] G. Rauhut, P. Pulay, J. Phys. Chem. 1995, 99, 1457.
16] F. Kalincs a´ k, G. Pongor, Spectrochim. Acta A 2002, 58, 999.
17] A. E. Reed, L. A. Curtis, F. Weinhold, Chem. Rev. 1988, 88(6), 899.
18] J. P. Foster, F. Weinhold, J. Am. Chem. Soc. 1980, 102, 7211.
19] A. E. Reed, F. Weinhold, J. Chem. Phys. 1985, 83, 1736.
20] E. D. Gledening, J. K. Badenhoop, A. D. Reed, J. E. Carpenter,
F. F. Weinhold, NBO 3.1, Theoretical Chemistry Institute, University
of Wisconsin: Madison, WI, 1996.
[39] A. E. Ledesma,
Altabef,
S. A. Brand a´ n,
10.1016/j.molstruct.2009.01.058.
[40] R. F. W. Bader, J. Phys. Chem. A 1998, 102, 7314.
[41] P. L. A. Popelier, J. Phys. Chem. A 1998, 102, 1873.
[42] U. Koch, P. L. A. Popelier, J. Phys. Chem. 1995, 99, 9747.
J. Zinczuk,
J. L o´ pez
Mol.
Gonz a´ lez,
Struct. 2009,
A. Ben
DOI
J.
[21] R. F. W. Bader, Atoms in Molecules, A Quantum Theory, Oxford
University Press: Oxford, 1990, ISBN: 0198558651.
[
22] F. Biegler-K o¨ ning, J. Sch o¨ nbohm, D. Bayles, AIM2000; A program to
analyze and visualize atoms in molecules. J. Comput. Chem. 2001,
[43] G. L. Sosa, N. Peruchena, R. H. Contreras, E. A. Castro, J. Mol. Struct.
(THEOCHEM) 1997, 401, 77.
2
2, 545.
[44] G. L. Sosa, N. Peruchena, R. H. Contreras, E. A. Castro, J. Mol. Struct.
(THEOCHEM) 2002, 577, 219.
[
23] A. E. Vogel, Practical Organic Chemistry, (3rd edn), Longman.
London, 1997, p 823.
[45] S. Wojtulewski, S. J. Grabowski, J. Mol. Struct. 2003, 645, 287.
[46] S. Wojtulewski, S. J. Grabowski, J. Mol. Struct. 2003, 621, 285.
[47] S. J. Grabowski, Monatsh. Chem. 2002, 133, 1373.
[48] C. Vizioli, M. C. Ruiz de Az u´ a, C. G. Giribet, R. H. Contreras, L. Turi,
J. J. Dannenberg, I. D. Rae, J. A. Weigold, M. Malagoli, R. Zanasi,
P. Lazzeretti, J. Phys. Chem. 1994, 98, 8858.
[
[
24] B. E. Love, P. S. Raje, T. C. Williams, II Synlett 1994, 493.
25] R. Ten Have, M. Huisman, A. Meetsma, A. M. Van Leusen,
Tetrahedron 1997, 53(33), 11355.
[
26] 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, 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. Morokuma, G. A. Voth,
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,
[49] J. J. Dannenberg, Y. D. Rae, J. A. Weingold, M. Malagoli, R. Zanasi,
P. Lazzeretti, J. Phys. Chem. 1994, 98, 8858.
[50] J. Zinczuk, A. E. Ledesma, S. A. Brand a´ n, O. E. Piro, J. J. L o´ pez-
Gonz a´ lez, A. Ben Altabef, J. of Phys. Org. Chem 2009, 21, 1.
[51] A. E. Ledesma, J. Zinczuk, A. Ben Altabef, J. L o´ pez Gonz a´ lez,
S. A. Brand a´ n, J. Raman Spectrosc. 2009, DOI 10.1002/jrs.2219.
[52] A. Toyama, K. Ono, S. Hashimoto, H. Takeuchi, J.Phys.Chem.A 2002,
106, 3403.
[53] T. D. Klots, R. D. Chirrido, W. V. Stele, Spectrochim. Acta A 1994, 50,
765.
[54] S. T. King, J. Phys. Chem. 1970, 74, 2133.
J. Raman Spectrosc. 2010, 41, 587–597
Copyright ꢀc 2009 John Wiley & Sons, Ltd.
www.interscience.wiley.com/journal/jrs