X-ray crystallography and computational studies of the structure of bis-nitrone, 2,5-bis{[methyl(oxido)imino]phenyl}-furan

Article abstract of DOI:10.1016/j.molstruc.2010.05.029

Bis-nitrone, 2,5-bis{[methyl(oxido)imino]phenyl}-furan prepared by condensation of 2,5-diformylfuran with two equivalents of phenylhydroxylamine can theoretically exist in 10 different structural isomeric forms. The relative energies calculated with AUG-c

Full text of DOI:10.1016/j.molstruc.2010.05.029

Journal of Molecular Structure 977 (2010) 175–179
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
X-ray crystallography and computational studies of the structure of bis-nitrone,
2,5-bis{[methyl(oxido)imino]phenyl}-furan
a
b
b
a,
**
Ananda S. Amarasekara a, , Diamond Lewis , Shravana L. Nayani , Tatiana V. Timofeeva , Hau-Jun Fan
*
a Department of Chemistry, Prairie View A&M University, Prairie View, TX 77446, USA
b Department of Natural Sciences, New Mexico Highlands University, Las Vegas, NM 87701, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 7 April 2010
Received in revised form 19 May 2010
Accepted 19 May 2010
Available online 25 May 2010
Bis-nitrone, 2,5-bis{[methyl(oxido)imino]phenyl}-furan prepared by condensation of 2,5-diformylfuran
with two equivalents of phenylhydroxylamine can theoretically exist in 10 different structural isomeric
forms. The relative energies calculated with AUG-cc-pVDZ and 6-311+G(df, pd) basis sets are used to pre-
dict the most stable isomeric form for this molecule. The single crystal X-ray structure analysis confirms
this prediction. The bond lengths, angles, and dihedral angles obtained in the crystal structure are in good
agreement with the molecular parameters calculated using DFT B3LYP method employing 6-311+G(df,
pd) basis set. Population analysis using polar tensors (APT) indicates the traditional N+AOꢀ interpretation
of the nitrone function whereas Mülliken method fails to show the charge separated structure.
Published by Elsevier B.V.
Keywords:
Bis-nitrone
2,5-Diformylfuran
Crystal structure
Density functional theory
1. Introduction
in these studies. Our interest in the synthesis of 2,5-diformylfuran
[11] as a renewable resources based feedstock chemical, and appli-
Nitrones are an attractive class of compounds due to their wide
spectrum of applications as, 1,3-dipoles in organic synthesis [1],
spin traps in free radical probes [2], polymer stabilizers [3], and
metal chelating ligands [4]. These compounds are generally pre-
pared by condensation of aldehydes with hydroxylamines or by
oxidation of N-hydroxyl secondary amines. The geometries of
these nitrones are important in their ability to form metal com-
plexes, cycloaddition reactions, as well as to understand the hyper-
fine coupling in the EPR signals in spin trap applications [5].
Merino et al. has shown that, N-benzyl-C-(2-pyridyl) nitrone [6],
and N-[(2S)-2-(tert-butoxycarbonylamino)propylidene] benzyl-
amine N-oxide [7] exists in the Z-geometry, using X-ray crystallog-
raphy. In contrary to this observation, C-alkoxy nitrones like
C-ethoxy-C-phenyl-N-methylnitrone are known to exist as mix-
tures of E and Z isomers [8]. Due to the unique C@NAO moiety, nit-
rones are fascinating and challenging topics for computational
chemistry as well. Compounds with two nitrone groups, bis-nitro-
nes, are a rare class of compounds, and only a handful of bis-nitro-
nes are reported in literature [9,10], and these include the recently
reported, 6,60-bis{methyl(oxido)imino]methyl}-2,20-bipyridine [9],
which form stable complexes with Cu(II), Ni(II), Co(II), and Cr(III)
ions. Furthermore, the geometry of the free ligand is not reported
cations in the synthesis of polymeric materials [12] have led us to
the preparation of the corresponding bis-nitrone, 2,5-bis{[methy-
l(oxido)imino]phenyl}-furan. This symmetrical bis-nitrone can
theoretically exist in 10 different isomeric forms (1a–j) as shown
in Fig. 1. In this communication we will show the synthesis and
characterization of bis-nitrone, demonstrate the application of
computational methods for the prediction of the most stable iso-
meric form for this bis-nitrone, and compare the calculated bond
distances and bond angles with the X-ray crystallography data.
2. Experimental
2.1. Materials and physical measurements
2,5-Diformylfuran was prepared using our previously published
procedure [11]. N-Phenylhydroxylamine was freshly prepared by
ZnANH4Cl reduction of nitrobenzene [13]. 1H NMR spectra were
recorded in CDCl3 on a Varian Mercury plus spectrometer operat-
ing at 400 MHz and chemical shifts are given in ppm downfield
from TMS (d = 0.00). 13C NMR spectra in CDCl3 were recorded in
the same spectrometer operating at 100 MHz; chemical shifts were
measured relative to CDCl3 and converted to d(TMS) using
d(CDCl3) = 77.00. FT-IR spectra were recorded on a JASCO-470 PLUS
IR spectrometer using KBr pellets. Elemental analysis was per-
formed at QTI laboratories, New Jersey.
* Corresponding author. Tel.: +1 936 261 3107; fax: +1 936 261 3117.
** Corresponding author. Tel.: +1 936 261 3111; fax: +1 936 261 3117.
E-mail address: asamarasekara@pvamu.edu (A.S. Amarasekara).
0022-2860/$ - see front matter Published by Elsevier B.V.
doi:10.1016/j.molstruc.2010.05.029

176
A.S. Amarasekara et al. / Journal of Molecular Structure 977 (2010) 175–179
_
_
O
N
O
+
+
O
O
O
N
+
+
O
_
+
+
+
+
N
N
+
N
N
_
N
N
O
+
O
+
O
_
O
O
_
_
_
_
_
O
O
O
_
N
N
O
O
1b
1c
1d
1e
1a
1f
_
_
O
O
N
_
+
+
N
O
+
O
_
_
O
+
O
+
O
+
N
N
O
N
N
+
N
O
N
O
O
+
_
+
O
_
N
N
O
_
O
_
1g
1j
1h
1i
Fig. 1. Possible isomeric structures of 2,5-bis{[methyl(oxido)imino]phenyl}-furan (1).
2.2. Synthesis of 2,5-bis{[methyl(oxido)imino]phenyl}-furan (1)
tions were carried out by using the SHELXTL program [17]. Crystal-
lographic data for 1 have been deposited with the Cambridge
Crystallographic Data Center with deposition number CCDC
732,241. These data can be obtained free of charge via
www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cam-
bridge Crystallographic Data Centre, 12, Union Road, Cambridge
CB2 1EZ, UK; fax: +44 1223 336033).
A mixture of 2,5-diformylfuran [11] (0.186 g, 1.5 mmol) and N-
phenylhydroxylamine (0.327 mg, 3.0 mmol) in 10 mL of ethanol
was heated at 50 °C for 15 min. Then the solution was allowed to
stand at room temperature for 24 h, to give 2,5-bis{[methyl(oxi-
do)imino]phenyl}-furan. The product was recrystallized from
ethanol to give pale yellow crystals, 0.381 g, 83% yield, m.p. 194–
96 °C. Found: C, 70.35; H, 4.83; N, 8.90%. Calc. for C18H14N2O3: C,
70.32; H, 4.61; N, 9.15%.
3. Results and discussion
IR (KBr) 653, 684, 763, 819, 868, 1013, 1076, 1162, 1379, 1481,
3.1. IR and NMR spectroscopy
1543, 3031, 3158 cmꢀ1
.
1H NMR d 7.47–7.53 (m, 6H), 7.79–7.82 (m, 4H), 8.16 (s, 2H),
Bis-nitrone, 2,5-bis{[methyl(oxido)imino]phenyl}-furan (1) was
prepared by condensation of 2,5-diformylfuran with two equiva-
lents of N-phenylhydroxylamine, and was characterized by ele-
mental analysis, IR and NMR spectroscopy. The IR spectrum of 1
shows a strong peak at 1076 cmꢀ1 indicating the NAO absorptions
of the nitrone functions. The furan ring is indicated by the peaks at
653 and 684 cmꢀ1. Proton NMR spectrum of this compound shows
two singlets at 8.16 (s, 2H) and 8.17 (s, 2H) ppm corresponds to
furan ring protons and the ACH@NA protons of the nitrone groups.
The two multiplets, 7.47–7.53 (m, 6H), 7.79–7.82 (m, 4H) repre-
sents the phenyl groups of the compound. The 13C NMR spectrum
shows seven signals confirming the symmetrical nature of the
structure. The signals at 147.6 and 119.0 ppm can be assigned to
furan ring carbons C-2 and C-3, respectively, whereas the phenyl
ring carbons are observed at 121.3, 123.6, 129.5 and 130.6 ppm.
The two nitrone carbons are assigned to the signal at 148.8 ppm.
8.17 (s, 2H).
13C NMR d 119.0, 121.3, 123.6, 129.5, 130.6, 147.6, 148.8.
2.3. Theoretical calculations
All of the theoretical studies used density functional theory
(DFT) in Gaussian 03 package [14]. Geometry optimizations were
carried out with Becke hybrid exchange functional and the Lee–
YangꢀParr correlation function (B3LYP) [15] using Pople-style
basis sets (6-311+G(df, pd)) including diffuse (denoted by ‘‘+” for
Pople-style). Single point energy calculation was carried out using
AUG-cc-pVDZ. Frequency calculations were performed for all
structures, and this vibrational frequency analysis yielded no imag-
inary frequencies.
2.4. X-ray diffraction data
3.2. Theoretical calculations
The single crystals suitable for X-ray crystallography were
grown by dissolving the 2,5-bis{[methyl(oxido)imino]phenyl}-fur-
an (1) in methylenechloride, layering with pentane, and allowing
the undisturbed solution to stand at room temperature for three
days. X-ray crystallography data were collected on a Bruker SMART
Structural studies on bis-nitrone 1 enables us to explore the
geometric and electronic structure of a molecule with two conju-
gated nitrone moieties, as this compound can exist in 10 possible
isomeric forms as shown in Fig. 1. The relative energies calculated
for these isomeric structures (1a–j) using AUG-cc-pVDZ and 6-
311+G(df, pd) basis sets and the relative energies with zero-point
APEX II CCD diffractometer (Mo K
a-radiation, graphite monochro-
mator, and u scan modes) and corrected for absorption using
x
SADABS program [16]. Crystals of 1, C18.5H15ClN2O3, M = 348.77,
are monoclinic, space group P21/c, at 100 K: a = 15.632(2),
b = 9.9648(14), c = 10.4743(15) Å, b = 98.268(2)°, V = 1614.7(4) Å3,
energy correction ð
D
EꢁZPEÞ, enthalpy (
DH°) and free energies (DG°)
corrected to room temperature are shown in Table 1.
Out of the 10 isomeric structures studied, the lowest energy
conformation is from the structure 1b as shown in Table 1. This
minimized geometry of 1b showed a small curvature in the struc-
ture and flat form of this geometry (1b-flat) showed a slightly
Z = 4, dcalc = 1.435 g cmꢀ3
k(Mo K ) = 0.71073 Å,
,
l
(Mo K
a
) = 0.257 mmꢀ1, F(0 0 0) = 724,
a
x-scans 2h < 58°, number of reflections col-
lected = 16,639. The structure was solved by direct methods and
refined by full-matrix least squares technique with anisotropic dis-
placement parameters for non-hydrogen atoms. The crystal of 1
contains a dichloromethane solvent molecule disordering over
two sites related by the inversion center. The hydrogen atoms were
placed in calculated positions and refined in riding model with
fixed displacement parameters (Uiso(H) = 1.2Ueq(C)). All calcula-
higher values except in D
EꢁZPE @ 6-311+G(df, pd) calculation. How-
ever, the energy difference between 1b and its flat form 1b-flat is
minimal, which suggests the geometry curvature impact on the
p-delocalization system is minimal. The next two possible candi-
dates are 1i and 1j isomeric structures, which only differs by the
placement of phenyl group and O atom relative to the C@N bond.

A.S. Amarasekara et al. / Journal of Molecular Structure 977 (2010) 175–179
177
Table 1
The relative energies of 1a–j calculated using AUG-cc-pVDZ and 6-311+G(df, pd) basis sets (kcal/mol).
Isomeric structure
D
E° @ AUG-cc-pVDZ
D
E° @ 6-311+G(df, pd)
D
EꢁZPE @ 6-311+G(df, pd)
D
H° @ 6-311+G(df, pd)
DG°@ 6-311+G(df, pd)
1a
15.85
0.00
1.17
12.76
12.77
16.89
15.96
12.62
15.96
7.71
17.50
0.00
0.91
14.05
14.05
17.51
17.18
13.85
17.18
8.11
17.07
0.00
17.24
0.00
3.38
12.57
12.58
16.14
16.88
13.66
16.88
7.93
16.79
0.00
0.55
14.98
14.95
18.97
16.13
12.88
16.13
7.55
1b
1b-flat
1c
1d
1e
1f
1g
1h
1i
ꢀ0.11
13.46
13.47
17.13
16.73
13.52
16.73
7.87
1j
4.80
5.86
5.50
4.52
7.38
Examining the arrangements at the C@N bonds relative to the fur-
an ring, one can see that isomeric structures 1a–d have higher
symmetry compared to the structures 1e–j. However, structures
1a, and 1d have both nitrone moieties in the E-configuration. Addi-
tionally these two phenyl groups of 1d will actually overlap each
other in order to maintain a higher symmetry. Thus, the final opti-
mized geometry has to lower its symmetry and even lose its Cs
symmetry. The isomeric structure 1c has the negatively charged
oxygen atoms in close proximity, making it another high energy
structure. Separate frequency calculations were carried out using
the same method and no negative frequencies were found in the
lowest energy conformation 1b, which further confirms its global
energy minimum geometry. Though the 1b-flat shows two low
imaginary frequencies, both of these disappeared once a tighter
convergence criterion is used.
In order to give a better insight into the nature of the NAO
bonds in the system, a population analysis using atomic polar ten-
sors (APT) [18] was carried out for the symmetric structures 1a–d.
The popular atomic charge representation in common use is Mül-
liken population analysis. However, there are serious problems
with this method due to the fact that this approach is based on pro-
jecting the electron density onto some reference basis set, which is
related to the atomic orbital basis set. Here we will present the
population analysis for structures 1a–d, using APT (Table 2), which
is not required to have direct reference to the basis set, and is read-
ily available in the Gaussian output. Furthermore, Mülliken popu-
lation analysis data for 1a–d are also presented in Table 2, for
comparison with the APT results. Table 2 clearly shows the failure
of Mülliken interpretation of the NAO bond, which assigned nega-
tive charges to both N and O atoms. The APT population analysis
Table 2
The population analysis between NAO using atomic polar tensors (APT) and traditional Mülliken population analysis for isomeric structures 1a–d and PBN.
Mülliken population analysis APT population analysis
1a
1b
1c
1d
PBN
1a
1b
1c
1d
PBN
O1
N1
C
ꢀ0.246
ꢀ0.118
ꢀ0.270
ꢀ0.259
ꢀ0.193
0.279
ꢀ0.161
ꢀ0.195
0.302
ꢀ0.161
ꢀ0.195
0.302
ꢀ0.374
ꢀ0.094
0.346
ꢀ0.112
0.935
ꢀ0.0005
ꢀ0.047
0.166
0.169
ꢀ0.030
0.012
0.080
ꢀ0.029
0.011
0.080
ꢀ0.622
0.287
ꢀ0.014
Fig. 2. Front and side views of the single crystal X-ray structure of 1, representation of atoms by thermal ellipsoids (p = 50%). Dichloromethane solvent molecule is omitted for
clarity.

178
A.S. Amarasekara et al. / Journal of Molecular Structure 977 (2010) 175–179
suggests a good chelating site for the ligand to bind with metal
ions.
3.3. Crystal structure
The
D
E°, ð
D
EꢁZPEÞ,
DH°, and DG° calculations clearly shows 1b as
the most favored structural isomer and this is consistent with the
single crystal structure, as shown in the Fig. 2. The bond lengths,
bond, and dihedral angles in the structure 1b were calculated using
DFT B3LYP method with 6-311+G(df, pd) basis set and a comparison
of these calculated parameters with the experimental results ob-
tained in the crystallographic analysis are shown in the Table 3.
The calculated bond lengths of structure 1b are in excellent agree-
ment with the experimental results in comparison to the bond an-
gles. The differences in bond lengths are less than 0.022 Å, whereas
the discrepancy in the bond angles is 0.7°. The largest discrepancy
lies in dihedral angles, and is clearly shown in the dihedral angel
of C3AC2AC6AN1, which has an average value of 4.67° in the crys-
tal structure, whereas the calculated value is 0.21°. The C2AC6
bond, which is the link between the furan and C@N moiety is
1.4205 Å, while the C@N bond in the nitrone group has an average
length of 1.3175 Å, and both these values are in excellent agree-
ment with the calculated values. The calculated N1AO3 bond
length, 1.2741 Å is slightly lower than the experimentally observed
average NAO bond distance, which is 1.2965 Å. Both the calculated
and experimental bond lengths are much longer than the mean va-
lue for the NAO bond in nitrates, as reported in the Cambridge
structural database [19], which is 1.231 0.025 Å. However, the
calculated and experimental average NAO bond lengths of 1b are
much closer to the experimental gas-phase value for NAO bond
length in pyridine N-oxides (1.29 Å) [20]. The zwitterionic N+AOꢀ
form is widely used to represent the NAO bond in nitrone and
N-oxide compounds, nevertheless biradical and hypervalent cano-
nic forms are also suggested for nitrone compounds [21].
Fig. 3. The LUMO, HOMO, HOMO-1 and HOMO-2 orbital plot of 1b.
Table 3
The experimental and calculated bond lengths (Å), bond angles (°), and dihedral
angles (°) in 1b using DFT B3LYP method with 6-311+G(df, pd) basis set.
Experimental
Average
Calculated Difference
Bond distance (Å)
O1AC2
C2AC3
C3AC4
C2AC6
C6AN1
N1AO3
N1AC7
C7AC8
1.377
1.373
1.414
1.424
1.318
1.300
1.450
1.392
1.390
1.391
1.384
1.391
1.387
1.376
1.377
1.3765
1.3750
1.4140
1.4205
1.3175
1.2965
1.4515
1.3895
1.3885
1.3870
1.3855
1.3905
1.3875
1.3702
1.3821
1.4074
1.4132
1.3242
1.2741
1.4528
1.3907
1.3877
1.3918
1.3908
1.3896
1.3923
0.006
ꢀ0.007
0.007
1.417
1.317
1.293
1.453
1.387
1.387
1.383
1.387
1.390
1.388
0.007
ꢀ0.007
0.022
ꢀ0.001
ꢀ0.001
0.001
ꢀ0.005
ꢀ0.005
0.001
C8AC9
C9AC10
C10AC11
C11AC12
C12AC7
ꢀ0.005
Bond angle (°)
C(2)AO(1)AC(5)
O(3)AN(1)AC(6) 122.5
N(1)AC(6)AC(2) 122.2
106.7
106.7
122.4
122.0
116.8
107.2
ꢀ0.500
ꢀ0.700
ꢀ0.670
0.100
122.3
121.8
116.9
123.1
122.67
116.7
O(3)AN(1)AC(7) 116.7
Dihedral angle (°)
4. Conclusion
C3AC2AC6AN1
C2AC6AN1AC7
C6AN1AC7AC8
C7AC8AC9AC10
1.32
8.02
4.67
2.87
29.735
0.885
0.21
1.32
30.72
0.4
4.460
1.550
4.47
30.55
0.88
1.27
28.92
0.89
ꢀ0.985
In conclusion, we have correctly predicted the most energeti-
cally favored isomeric structure of 2,5-bis{[methyl(oxido)imino]-
phenyl}-furan and shown that molecular parameters calculated
using DFT B3LYP method with 6-311+G(df, pd) basis set are in good
agreement with crystallographic data of the compound. Population
analysis of the most favored isomeric structure using polar tensors
(APT) indicates the traditional N+AOꢀ interpretation of the nitrone
function whereas the molecular orbital plot indicates some hyper-
valent character in the nitrone nitrogen.
0.485
assigns negative charges for the O atoms, and positive charges for
N atoms for all four isomers studied, and clearly conform to the
typical organic chemistry interpretation of N+AOꢀ moiety.
Nitrone group charge densities of the common spin-trap
a-phe-
nyl-N-tert-butylnitrone (PBN) in the Z-geometry were also calcu-
lated using the same method for comparison, and these results
are shown in Table 2 as well. Mülliken population analysis of
PBN showed negative charges for both N and O, whereas the APT
results showed the correct charges for these atoms, again showing
the inadequacy of the Mülliken analysis in the case of nitrone func-
tion. Additionally, we have studied the molecular orbital picture of
the most stable isomer (1b) to further comprehend the electronic
nature of the NAO bond, and Fig. 3 shows the lowest unoccupied
molecular orbital (LUMO), highest occupied molecular orbital
(HOMO), and two orbitals (HOMO-1 and HOMO-2) below HOMO.
It is interesting to point out that, while both HOMO and LUMO
Acknowledgments
NMR spectrometer used in this study was obtained through a
NSF MRI Grant CHE-0421290. We are also grateful for support of
crystallography studies via NSF/DMR Grant 0420863. D.L. thanks
American Chemical Society’s SEED program for summer research
fellowship. H-J. F. acknowledge financial support from the Minority
Leader Program from US Air Force Research Lab, and US Depart-
ment of Education Minority Science and Engineering Improvement
Program.
orbitals of 1b are
p-bond in character, the N1AO3 bond shows a
strong anti-bonding character. However, the polarized NAO bond
is better illustrated with HOMO-1 and HOMO-2, particularly, the
HOMO-2 shows the second lone pair on the oxygen strongly polar-
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.molstruc.2010.05.029.
ized into the N ? O direction (Fig. 3). The electronic structure of
r
bond between NAO (not shown) shows even larger electron den-
sity polarization over the oxygen atoms. These results suggest that
the nitrogen atom possesses some hypervalent character in addi-
tion to the charge separated NAO bond proposed in the APT popu-
lation analysis. This extra electron density over oxygen atom
References
[1] K.B.G. Torssell, Nitrile Oxides, Nitrones, and Nitronates in Organic Synthesis,
VCH Publishers, Weinheim, 1988.

A.S. Amarasekara et al. / Journal of Molecular Structure 977 (2010) 175–179
179
[2] G. Durand, F. Choteau, B. Pucci, F.A. Villamena, J. Phys. Chem. A 112 (2008)
12498.
[3] J.R. Freedman, M.A. Young, US Patent, US 5153,169, Appl. 695,932, 1991; J.R.
Freedman, M.A. Young, Chem. Abstr., 118 (1991) 180177z.
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, H. Pomelli, J.W. Ochterski, P.Y. Ayala, K.
Morokuma, G.A. Voth, P. Salvador, J.J. Dannenberg, V. 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, A. Liu 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, J.A. Pople, Gaussian 03, Revision
D.01, Gaussian Inc., Pittsburgh, PA, 2005.
[4] F.A. Villamena, M.H. Dickman, D.R. Crist, Inorg. Chem. 37 (1998) 1446.
[5] G.M. Rosen, A. Beselman, P. Tsai, S. Pou, C. Mailer, K. Ichikawa, B.H. Robinson, R.
Nielsen, H.J. Halpern, A.D. MacKerell Jr., J. Org. Chem. 69 (2004) 1321.
[6] P. Merino, S. Anoro, T. Tejero., in: Fourth International Electronic Conference on
Synthetic Organic Chemistry (ECSOC-4), <www.mdpi.org/ecsoc-4.htm>, 2000.
[7] P. Merino, F.L. Merchan, T. Tejero, A. Lanaspa, Acta Crystallogr. C52 (1996) 218.
[8] D.E. Gallis, J.A. Warshaw, B.J. Acken, D.R. Crist, J. Org. Chem. 56 (1991) 6352.
[9] E.G. Petkovaa, R.D. Lampekaa, M.V. Gorichkob, G. Palamarchuk, V.V.
Dyakonenko, R.I. Zubatyuk, O.V. Shishkin, Z. Naturforsch. 63b (2008) 841.
[10] L. Vretik, H. Ritter, Macromolecules 36 (2003) 6340.
[15] A.D. Becke, J. Chem. Phys. 98 (1993) 5648.
[16] G.M. Sheldrick, SADABS, v. 2.01, Bruker/Siemens Area Detector Absorption
Correction Program, Bruker AXS Inc., Madison, WI, 1998.
[11] A.S. Amarasekara, D. Green, E. McMillan, Catal. Commun. 9 (2008) 286.
[12] A.S. Amarasekara, D. Green, L.D. Williams, Eur. Polym. J. 45 (2009) 595.
[13] O. Kamm, Org. Syn. Coll. 1 (1941) 445.
[17] G.M. Sheldrick, Acta Cryst. A64 (2008) 112.
[18] J. Cioslowski, J. Am. Chem. Soc. 111 (1989) 8333.
[19] G.J.M. Velders, D. Feil, Theor. Chim. Acta 84 (1992) 195.
[14] M.J. Frisch, G.W. Trucks, H. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman,
J.J. Montgomery, 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,
[20] J.F. Chiang, J.J. Song, J. Mol. Struct. 96 (1982) 151.
[21] R.M. Aminova, E. Ermakova, Chem. Phys. Lett. 359 (2002) 184.