(Z)-3-(4-(Dimethylamino)benzylidene)thiochroman-4-one: Synthesis, Crystal Structure and Molecular Modelling Using Density Functional Theory

Article abstract of DOI:10.1007/s10870-019-00779-4

Abstract: The title compound C₁₈H₁₇NOS was synthesized by condensation of thiochroman-4-one with 4-dimethylaminobenzaldehyde. The possible existence of the 3-(4-(Dimethylamino)benzylidene)thiochroman-4-one (E)- and (Z)-geometrical is

Full text of DOI:10.1007/s10870-019-00779-4

Journal of Chemical Crystallography
https://doi.org/10.1007/s10870-019-00779-4
ORIGINAL PAPER
(Z)‑3‑(4‑(Dimethylamino)benzylidene)thiochroman‑4‑one: Synthesis,
Crystal Structure and Molecular Modelling Using Density Functional
Theory
Neudo Urdaneta1 · Jesús Núñez2 · Gustavo Liendo3 · Carlos Rivas4 · Teresa González5 · Alexander Briceño5
Received: 17 December 2018 / Accepted: 2 April 2019
© Springer Science+Business Media, LLC, part of Springer Nature 2019
Abstract
The title compound C18H17NOS was synthesized by condensation of thiochroman-4-one with 4-dimethylaminobenza-
ldehyde. The possible existence of the 3-(4-(Dimethylamino)benzylidene)thiochroman-4-one (E)- and (Z)-geometrical
isomers was investigated using B3LYP/6-31G**. Crystallographic and vibrational data are compared with the results
of Density Functional Theory (DFT) B3LYP/6-31G** level. Bond orders were estimated using the NBO calculation
method correlating with changes in the bond length. To explain deviation of an ideal molecular geometry, the concept of
non-equivalent hybrid orbitals was invoked. The studies of X-ray difraction revealed strong intermolecular interactions
between dimers of the compound connected to each other through dipole–dipole interactions S–C, N–C y O–C in the
absence of hydrogen bonding.
* Neudo Urdaneta
urdanet@usb.ve
* Alexander Briceño
abriceno@ivic.gob.ve
1
Departamento de Química, Universidad Simón Bolívar
(USB), Apartado 89000, Caracas 1080-A, Venezuela
2
Departamento de Biología, Universidad Politécnica
Territorial del Oeste de Sucre “Clodosbaldo Russian”,
Carretera Cumaná-Cumanacoa, km. 4, Cumaná 6101,
Venezuela
3
Departamento de Química, Núcleo de Sucre, Universidad de
Oriente, Apartado 90, Cumaná 6101, Venezuela
4
Departamento de Ciencias, Núcleo de Monagas, Universidad
de Oriente, Maturín 6201, Venezuela
5
Centro de Química, Instituto Venezolano de Investigaciones
Científcas (IVIC), Apartado 21827, Caracas 1020-A,
Venezuela
Vol.:(0123456789)
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Journal of Chemical Crystallography
Graphical Abstract
In this report, a study of the structural characterization of (Z)-3-(4-(dimethylamino)benzyliden)thiochroman-4-one is carried
out by spectroscopic methods, single-crystal X-ray difraction and functional density calculations.
Keywords (Z)-3-(4-(Dimethylamino)benzylidene)thiochroman-4-one · Crystal structure · DFT calculations
Introduction
The 3-benzylidenethiochroman-4-ones are compounds
structurally analogous to the homoisofavonoids with a sul-
fur atom in the pyran-4-one ring. There are examples in the
Scheme 1 Synthesis of (Z)-3-(4-(Dimethylamino)benzylidene)thi-
literature of some homoisofavonoids with several biological
ochroman-4-one (I)
properties of interest such as: anti-infammatory [1], anti-
fungal [2], antioxidant [3] and anticancer [4]. Compounds
such as isothiochroman-4-ones analogous to compound (I)
reported several isomerizations of 3-benzylidenechroman-
4-onas and 3-benzylidenethiochroman-4-ones, succeeding
in diferentiating by NMR 1H the isomers E and Z, locat-
ing the signal of the vinyl proton in the spectrum with the
value of it respective coupling constant [10, 11]. The efect
of the substituent attached to the phenyl of the benzylidene
moiety play an important role in the crystalline packing of
(Scheme 1), have been employed as intermediates in the
synthesis of isoxazolines [5].
Some 3-benzylidenechroman-4-ones have been synthe-
sized through a condensation reaction from a chroman-
4-one substituted with an aromatic aldehyde in presence
of an acid [6]. Others have been obtained using bases such
as NaOH, KOH and piperidine [7–9]. Some papers have
1 3

Journal of Chemical Crystallography
these aromatic compounds. Depending on the electronic
nature of the substituents, face-to-face π–π interactions,
face-to-face π-stacking and syn-HH (syn-head-to-head)
products have been found in [2+ 2] photocycloaddition of
(E)-3-benzylidenechroman-4-ones in crystalline state [12].
In this work, we report the synthesis, crystal structure and
density functional calculations of (I).
X‑Ray Crystallography
The X-ray crystallographic data were recorded on a Rigaku
AFC-7S Mercury difractometer with graphite monochro-
mated Mo-Kα radiation. SHELXTL was the program used
to solve and refne the structure [14]. DIAMOND was the
program used to graphics molecular [15]. A summary of the
crystal data and other details concerning data collection and
structure refnement are given in Table 1.
Experimental
Computational Calculation
Synthesis of (I)
The DFT based method was most suitable for the calcula-
tions for molecules of our interest in gas phase. All the
calculations were performed using B3LYP Density Func-
tional Theory method, a hybrid version of DFT and Har-
tree–Fock (HF) methods, in which the exchange energy
from Becke’s exchange functional is combined with the
exact energy from Hartree–Fock theory. Along with the
component exchange and correlation functional, Becke’s
three parameters defne the hybrid functional, specifying
the extent of the exact exchange mixed in. The geometry
of the compounds were optimized at the semi-empirical
method AM1, HF and B3LYP/6-31G** level of theory.
All the calculations were carried out using Gaussian 09
suits of program with its graphical interface Gaussview
5.0.8 [16].
The title compound (I) was synthesized by a
Claisen–Schmidt condensation from commercially avail-
able thiochroman-4-one (332 mg, 2 mmol) (Sigma-Aldrich)
and 4-dimethylaminobenzaldehyde (355 mg, 2.4 mmol)
(Riedel-de Haën) were dissolved in abs. ethanol (6 mL),
and 40% aqueous KOH solution (4 mL) was slowly added
at 0 °C [13]. The reaction mixture was stirred overnight; the
course of the reaction was followed by TLC. After reaction
completion, the mixture was poured into iced water and the
orange precipitate formed, was fltered and dried (yield,
70%). Crystals of (I) suitable for X-ray difraction analysis
were grown by slow evaporation of a saturated chloroform
solution of (Z)-3-(4-(Dimethylamino)benzylidene)thiochro-
man-4-one at room temperature (mp: 431–432 K). 1H NMR
(δ ppm, CDCl3, 400 MHz): 8.16 (d, 1H, Ar, J = 8.1 Hz),
7.76 (s,1H, H–C=), 7.37 (d, 2H, Ar, J =8.8 Hz), 7.21–7.30
(m, 3H, Ar), 6.73 (d, 2H, Ar, J=7.3 Hz), 4.22 (s, 2H, CH2),
3.08 (s, 6H, CH3). 13C NMR (δ ppm, CDCl3, 125 MHz):
185.79 (C8), 151 (C14), 140.63 (C2), 138.81 (C10), 132.88
(C4), 132.51 (C7), 131.8 (C9), 130.25 (C12 and C16),
128.38 (C6), 127.73 (C3), 125.64 (C5 and C11), 112.01
(C13 and C15), 40.29 (C17 and C18), 29.59 (C1). GC–MS
[EI 70 eV, m/z (abundance %)]: 295 (100), 278 (15), 158
(62), 147 (25), 134 (79), 115 (15).
Results and Discussion
Molecular Structure and DFT Calculations
Valkonen et al. synthesized and characterized by X-ray
difraction a series of 3-benzylidenechroman-4-ones and
3-benzylidenethiochroman-4-ones [17]. These studies
revealed strong intermolecular interactions in the absence
of hydrogen bonding. Several weak interactions of the types
C–H···O, C–H···π, and π···π determine the crystal packing
and defne the spatial orientation of the benzylidene group
in (I), with torsion angles of 46–55° conformed by the planes
formed by C2–C7 and C11–C16 atoms of phenyl rings, as
shown in Fig. 1.
Physical Measurements
Melting point was determined using a Fisher-Johns Mel-
Temp melting point apparatus. Proton and carbon nuclear
magnetic resonance spectra (1H NMR and 13C NMR)
were obtained in a JEOL Eclipse Plus 400 spectrometer
(400 MHz), using tetramethylsilane as the reference and
deuterated chloroform as the solvent for 13C NMR and
1H NMR. The F-IR analysis was carried out in a Bruker
Optik GmbH Model No-Tensor 27. Electron ionization
(EI) mass spectra were recorded using a Hewlett-Packard
5917 A mass spectrometer (electron impact ionization
70 eV) linked to a Hewlett-Packard Series II 5890 gas
chromatograph.
In Table 2 are summarized some representative bond
lengths and angles calculated with B3LYP/6-31G**
and the corresponding experimental values, observing
agreement between both sets of data. Differences did
not exceed 0.030 Å for bond lengths (e.g., S1–C1) and
2° for the bond angles (C8–C9–C10). The six-membered
thiopyran-4-one ring, conformed by S1/C1/C9//C8/C7/C2
atoms is not planar due to the fexibility of the C1 atom
and the intermolecular forces, where the torsion angle
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Journal of Chemical Crystallography
Table 1 Crystal data and
structure refnement for
compound (I)
Molecular formula
Molecular weight
Temperature (K)
Wavelength (Å)
Crystal system
Space group
C18H17NOS
295.380
293(2)
0.7107
Triclinic
P1
Unit cell dimensions
a (Å)
8.019(3)
b (Å)
c (Å)
9.701(4)
9.802(5)
α (°)
β (°)
85.400(4)
86.710(3)
γ (°)
77.420(3)
741.190(545)
Volume (Å3)
Z
2
Density (Mg/m3)
Absorption coefcient (mm−1
F(000)
1.324
0.216
312
)
Crystal size (mm3)
Theta range for data collection (°)
Index ranges
0.150×0.275×0.350
2.09–27.71
−9≤h≤9, −12≤k≤12, −12≤l≤12
8661
Refections collected
Independent refections
Completeness to theta=25.25°
Absorption correction
Max. and min. transmission
Refnement method
2736 [Rint =0.0340]
94.5%
Semi-empirical from equivalents
Full-matrix least-squares on F2
2.736/0/259
1.092
R1 =0.0523, wR2 =0.1289
R1 =0.0682, wR2 =0.1387
0.172 and −0.258
Data/restraints/parameters
Goodness-of-ft on F2
Final R indices [I>2σ(I)]
R indices (all data)
Largest dif. peak and hole (e Å−3
)
side of the molecule where the C8=O2 carbonyl group is
1
found, in agreement with the H-NMR characterization:
its resonance signal appeared at 7.76 ppm, indicating the
formation of only the Z-isomer during the synthesis, sup-
porting the results from Table 4. The nitrogen atom from
the dimethylamino- group in the p-position on the phenyl
ring of the benzylidene moiety showed slight sp2-hybrid-
ization, according to its molecular geometry and bond
angles 121.0° (C14–N1–C17), 120.8° (C14–N1–C18),
and 116.7° (C17–N1–C18) shown in Table 2. The planes
containing the dimethylamino group and the adjacent phe-
nyl group are almost coplanar, their torsion angles: 12.4°
and 177.6° for C17–N1–C14–C13 and C18–N1–C14–C13,
respectively (Table 2). The N-atom hybridization and the
near coplanarity of dimethylamino and adjacent phenyl
groups, indicate delocalization of the lone electrons on the
Fig. 1 Molecular structure of compound (I). Ellipsoids have been
draw at 50% probability; hydrogen atoms shown as small spheres
with arbitrary radii
C2–S1–C1–C9 is 54° (Table 2), being consistent with
the calculated value and previous structure reports [12,
17, 19, 20]. The hydrogen atom H10A lies on the same
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Journal of Chemical Crystallography
Table 2 Comparison of geometrical data for compound (I), (Å, °),
from X-ray with those calculated from DFT for Z-isomer and Wiberg
bond order (WBO) obtained by means of NBO method [18]
antibonding orbital of the C14 atom, resulting in a slight
increase in the N1–C14 bond order; (ii) a shift of the
C9=C10 double bond electrons toward the antibonding
orbital of the C8–C9 single bond, resulting in a decrease
in the C9=C10 bond order, and (iii) the electrons of
double bond C2=O8 are shifted to the oxygen atom pπ
orbital, resulting in a large partial negative charge of
− 0.572 a.u. (Table 3) in O2 together with a decrease in
its bond order. The electrostatic potential surfaces show
the mayor negative charge at the oxygen atom position
(red color in Fig. 3).
X-ray
DFT
WBO
S1–C1
S1–C2
O2–C8
N1–C14
C9–C10
C12–C13
1.809
1.749
1.239
1.374
1.345
1.840
1.776
1.233
1.382
1.360
1.387
0.986
1.075
1.690
1.151
1.688
1.497
1.377
C1–S1–C2
S1–C1–C9
C14–N1–C17
C14–N1–C18
C17–N1–C18
O2–C8–C7
98.631
98.364
112.826
119.815
119.878
118.494
119.137
121.269
124.965
116.619
131.221
53.308
−48.707
178.378
−178.765
3.907
29.009
−7.952
−172.390
112.318
120.991
120.750
116.682
119.019
121.137
124.402
118.764
129.920
54.033
−53.737
−177.826
178.136
5.038
NBO Atomic Charges
NBO charge distributions were calculated from the NBO
population analysis at the B3LYP/6-31G** level. The com-
puted NBO charge values on each atom of the compound (I)
are listed in Table 3.
O2–C8–C9
C1–C9–C10
C8–C9–C10
C9–C10–C11
C2–S1–C1–C9
S1–C1–C9–C8
S1–C2–C3–C4
S1–C2–C7–C6
C1–C9–C10–C11
C9–C10–C11–C16
C17–N1–C14–C13
C18–N1–C14–C13
The carbon atoms (C8 and C14) attached some more elec-
tronegative atom exhibited positive charges. The electroneg-
ative atoms N1 (−0.430) and O2 (−0.572) exhibited nega-
tive charge, but sulfur atom show positive charge. This is due
to the great ability of the sulfur atom (S1) to accommodate
positive charge (0.296) and have greater dipole polariz-
ability than the carbon atom, since the sulfur atom is more
voluminous and its electron cloud is easier to distort [22].
This is the reason why the carbon atom C1 has the highest
negative charge (− 0.614) among all the carbon atoms in
the compound (I), where the inductive efect is more pre-
dominant than the resonance efect. After the C1 carbon
atom, the carbon atoms of the C17 and C18 methyl group of
the dimethylamino moiety are more negative carbon atoms
and each has a charge of − 0.480 a.u. These carbon atoms
attached to the nitrogen atom N1 with charge −0.430 a.u.,
are those that act as good donor sites of negative charge in
the molecule.
29.728
12.361
177.566
nitrogen atom due to a high conjugation of the structure
of compound (I), as shown in Fig. 2. This characteristic
has also been observed (Fig. 10) in the crystal structure
of another compound analogous to compound (I), such
as (E)-3-(4-(Dimethylamino)benzylidene)chroman-4-one
[17].
Table 4 shows the relative energies (kcal/mole) obtained
from the structures optimized for the 3-(4-(Dimethylamino)
benzylidene)thiochroman-4-one (Z)- and (E)-isomers. It is
observed the energy diference between both isomers in the
gas phase is 2.119 kcal/mole, indicating greater stability for
Table 2 also includes results from NBO for the iso-
lated molecule of compound (I) [21]: It was found a
delocalization pattern, characterized by: (i) delocaliza-
tion of the electron lone pair from N1 atom toward the
Fig. 2 Possible resonance struc-
tures for compound (I)
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Journal of Chemical Crystallography
Table 3 NBO atomic charge of compound (I)
plotted using Gaussview 5.0.8 viewing package. This
spectrum is shown in Fig. 5 along with the corresponding
experimental spectrum for comparison. The major experi-
mental and calculated frequencies are also summarized in
Table 5. There were some deviations in infrared intensi-
ties between the experimental data and the calculated data.
Any discrepancy noted between the observed and calculated
frequencies may be due to two things: one is that the experi-
mental results belong to solid phase whereas theoretical
calculations were performed in gaseous phase; and the other
is that the calculations were actually done on a single mol-
ecule while the experimental values recorded taking into
consideration the inter-molecular interactions. The Peaks
found in the calculated and the experimental spectrums are
important for the presence of the functional groups in com-
pound (I).
Atom
B3LYP/6-31G**
Atom
B3LYP/6-31G**
C1
C2
C3
C4
C5
C6
C7
C8
C9
C10
C11
−0.614
−0.172
−0.251
−0.208
−0.252
−0.175
−0.158
0.534
C12
C13
C14
C15
C16
C17
C18
N1
−0.179
−0.293
0.194
−0.292
−0.197
−0.480
−0.480
−0.430
−0.572
0.296
−0.147
−0.133
−0.124
O2
S1
The medium peaks in the calculated spectra by the
DFT method in compound (I) appeared between 3081
and 2883 cm−1. The peak of 3081 cm−1 corresponds to
the stretching of the bond =C–H of the aryl rings of the
bicycle thiochroman-4-one and the benzylidene moiety,
while in the experimental spectrum this peak appeared at
3050 cm−1. The peaks calculated at 3030 and 2891 cm−1
are due to the stretching symmetric of the C–H bond of the
dimethylamino moiety, which are found experimentally
at 2990 and 2850 cm−1, respectively; and the calculated
asymmetric stretching of the C-H bond appears at 2950
and 2883 cm−1, while in the experimental spectrum these
bands are located at 2900 and 2800 cm−1.
Fig. 3 Electrostatic potential surfaces for compound (I)
The strong peaks in the calculated spectrum of the
carbonyl group stretching C=O and the bending C–H,
are located at 1664 and 1508 cm−1, and in the experi-
mental spectrum at 1650 and 1440 cm−1. The remaining
peaks (very strong) at 1608 and 1570 cm−1 correspond
to the stretching of the group C=C; 1300 cm−1 at the
N–C stretching and 1380 cm−1 at the C–H bending in
the experimental spectrum. While in the calculated spec-
trum the peaks of the stretching C=C appears at 1608 and
1566 cm−1; stretching N–C to 1344 cm−1 and the bending
C–H to 1420 cm−1.
Table 4 Relative energy (kcal/mole) and dipole moment values for E-
and Z-isomers of (I)
Calculation method
Isomer Z
Isomer E
Energy
µ (debyes)
5.642
Energy
µ (debyes)
3.729
B3LYP/6-31G**
0.000
2.119
the (Z)-isomer of compound (I) (geometry of the structure
optimized in Fig. 4a), in agreement with the solid state struc-
ture determined by X-ray difraction in the present study. In
addition, it has been found in the literature the obtaining of
compounds analogous to the (E)-isomer under conditions
diferent from those used in the synthesis in the present work
[10, 11].
Most of the calculated frequency values had a diference
of 0–50 cm−1 with respect to its experimental value, with
the exception of the experimental values of the stretching at
2800 cm−1 and the bending at 1400 cm−1 of the C–H bond,
whose diference of the calculated values was >50 cm−1,
despite having taken into account a correction factor 0.9613
in its frequency values.
Vibrational Analysis
Crystal Packing
The vibrational frequencies of compound (I) were calcu-
lated at the B3LYP/6-31G** level of theory with a scale
factor of 0.9613 (rmsov = 34 cm−1) and IR spectrum was
The structure of the compound is assembled from the
dipole–dipole S–C [C2···S1 = 4.4826(38) Å] interac-
tions of the dimers, as shown in Fig. 6. This interaction
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Journal of Chemical Crystallography
Fig. 4 Optimized geometries of the structures of: a (Z)-3-(4-(Dimethylamino)benzylidene)thiochroman-4-one and b (E)-3-(4-(Dimethylamino)
benzylidene)thiochroman-4-one
Table 5 Assignments of experimental and calculated vibrational
scaled frequencies (cm−1) of the compound (I)
Vibrational
Experimentalb
B3LYP/6-31G**
assignmentsa
ꢀ = C−H
3050 m
2990 m
2900 m
2850 m
2800 m
1650 s
1608 vs
1570 vs
1440 s
3081
3030
2950
2891
2883
1664
1608
1566
1508
1420
1344
ꢀs
C−H
ꢀas
C−H
ꢀs
C−H
ꢀas
C−H
ꢀC = O
ꢀC = C
ꢀC = C
ꢀC−H
ꢀC−H
ꢀN−C
1380 vs
1300 vs
aν stretching, δ bending in plane, as asymmetric, s symmetric
bs strong, ms medium strong, vs very strong, m medium
Fig. 5 Infrared spectrum of the compound (I); experimental (above)
and theoretical (below)
is supported by the calculated values of charge for the
atoms C2(− 0.172) and S1(0.296), which are shown in
Table 3. Table 4 also shows the values of the total
dipole moment of the (Z)-isomer (compound I) and a
Fig. 6 Dipole–dipole interactions S1–C2 of the compound dimers
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Journal of Chemical Crystallography
[C14···N1 = 4.105(43) Å and C14i···N1 = 4.038(45) Å]
and O–C [C8···O2 = 4.739(43) Å], forming a one dimen-
sional network in the absence of hydrogen bonds (Fig. 8).
The values of the atomic charge for these dipoles that
are shown in Table 3 and Fig. 8 (pointing with arrows to
some atoms where the value of the charge belongs) are:
N1(− 0.430)–C14(0.194) and O2(− 0.572)–C8(0.534).
Each of these molecules being joined by the dipolar
interactions S1–C2···S1–C2, N1–C14···N1–C14 and
O2–C8···O2–C8. The extended arrangement of this com-
pound it is shown in Fig. 9 and consists of an arrangement
of dimmer layers through the crystallographic plane ab.
By way of comparison, by exchanging the sulfur atom
in compound (I) for an oxygen atom, the molecules are
arranged face to face through π–π interactions between phe-
nyl rings of the benzylidene group and benzenes adjacent to
the pyran-4-one ring (Fig. 10) [17]. There is no centrosym-
metric inversion point as in the crystal arrangement of com-
pound (I).
Fig. 7 Total dipole moments of the dimers of the compound (I)
hypothetical (E)-isomer formed during the synthesis.
We can see that the (Z)-isomer, in addition to a lower
energy, the magnitude of its total dipole moment of the
calculated molecule (Table 4 and Fig. 7) is always two
or three times greater than the (E)-isomer, which favors
intermolecular interactions. These dimers are connected
to each other through dipole–dipole interactions N–C
Fig. 8 Dashed lines indicate dipole–dipole interactions between pairs of molecules: S1–C2 (green), N1–C14 (blue) and O2–C8 (red) (Color fg-
ure online)
1 3

Journal of Chemical Crystallography
Fig. 9 A view of the two dimensional network of dimers extended in crystallographic plane ab
compound was investigated using the DFT method. The
Conclusion
calculated geometry of the compound is in good agreement
with the X-ray crystallographic data, while the calculated
vibrational spectrum compared with the experimental data
shown some variations. The energy calculations of the
molecular structures and the X-ray diffraction analysis,
revealed the formation of the (Z)-isomer of the compound
instead of the (E)-isomer. The studies of X-ray difraction
revealed strong intermolecular interactions between dimers
of the compound in the absence of hydrogen bonding. These
dimers are connected to each other through dipole–dipole
interactions S–C, N–C y O–C.
Compound (I) is prepared by Claisen–Schmidt condensa-
tion reaction with thiochroman-4-one and 4-dimethylamin-
obenzaldehyde. Slow evaporation solution growth method
at room temperature was employed to grow single crystal
of good quality for X-ray difraction. The formation of the
compound was also confrmed by spectroscopic analysis.
The Wiberg Bond Order analysis determined the delocali-
zation of the pair of solitary electrons of the nitrogen atom
in the conjugation of the double bonds of the compound
(I). Molecular geometries and vibrational spectrum of the
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Journal of Chemical Crystallography
Fig. 10 A view of the two dimensional network of dimers extended in crystallographic plane a for the structures of the compound (E)-3-(4-
(Dimetilamino)benciliden)croman-4-ona [17]
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https://doi.org/10.1016/j.phytochem.2006.06.021
Acknowledgements We would like to thank the Centro de Investi-
gaciones Tecnológico y Humanístico de la Universidad Politécnica
Territorial del Oeste de Sucre “Clodosbaldo Russian”. We thank
FONACIT (Grant No. LAB-97000821) for fnancial support, Con-
sejo de Investigación de la Universidad de Oriente (Project No. CI
02-010201-1898-14), Proyecto Estratégico Misión Ciencia FONACIT
(Grant No. 2007001522). We want to thank the researcher Marielsys
Moya for contacting the crystallographers.
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