Theoretical and experimental comparative study of a derivative from 2-pyridinecarboxaldehyde which exhibits configurational dynamics

Article abstract of DOI:10.1016/j.molstruc.2016.04.055

The (E)-4-nitro-N′-(pyridine-2-ylmethylene)benzohydrazide, a derivative from 2-pyridinecarboxaldehyde, displays E/Z isomerization induced by ultraviolet radiation in which the process of photoisomerization was evidenced and followed by 1D ¹H NM

Full text of DOI:10.1016/j.molstruc.2016.04.055

Journal of Molecular Structure 1119 (2016) 286e295
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: http://www.elsevier.com/locate/molstruc
Theoretical and experimental comparative study of a derivative from
2-pyridinecarboxaldehyde which exhibits configurational dynamics
a
b
c
d
ꢀ
ꢀ
ꢀ
Monica A. Gordillo , Monica Soto-Monsalve , Gustavo Gutierrez , Richard F. D'vries ,
,
*
Manuel N. Chaur a
a Departamento de Química, Universidad del Valle, 760032, Cali, Colombia
Instituto de Química de Sao Carlos, Universidade de Sao Paulo, 13566-590, Sao Carlos, Brazil
Departamento de Ciencias Farmaceuticas, Universidad Icesi, Cali, Colombia
Instituto de Física de Sao Carlos, Universidade de Sao Paulo, 13566-590, Sao Carlos, Brazil
b
~
~
~
c
ꢀ
d
~
~
~
a r t i c l e i n f o
a b s t r a c t
(E)-4-nitro-N0-(pyridine-2-ylmethylene)benzohydrazide,
a
derivative
from
2-
Article history:
The
Received 5 December 2015
Received in revised form
17 April 2016
Accepted 18 April 2016
Available online 21 April 2016
pyridinecarboxaldehyde, displays E/Z isomerization induced by ultraviolet radiation in which the pro-
cess of photoisomerization was evidenced and followed by 1D 1H NMR. The structure of the compound
was determined by FT-IR and NMR techniques and confirmed by single-crystal X-ray diffraction. The
results in terms of bond angle and length, chemical shift (13C and 1H) and vibrational frequencies ob-
tained experimentally were compared to computed values at two levels of theory (Restricted Hartree
eFock and Density Functional Theory) using different basis set. The understanding of spectroscopy and
dynamic properties of these type of compounds is of importance in view of their potential use in mo-
lecular machines and electronic devices.
Keywords:
Acylhydrazone
Photoisomerization
NMR
© 2016 Elsevier B.V. All rights reserved.
DFT
HF
X-ray crystal diffraction
1. Introduction
synthesis, photochemistry and spectroscopy comparison to theo-
retical calculations of an acyl-hydrazone derivative from 2-
Acyl-hydrazones exhibit a large number of applications, among
them, we can highlight the information storage. This functional
group presents a weak double bond that is able to allow constitu-
tional changes, through substituents interchange [1] establishing
itself as a long term information storage strategy. Furthermore,
acyl-hydrazones presents dynamical responses to diverse types of
stimulus, both chemical (pH and conformational changes induced
by metallic ions) and physical (pressure, heat and radiation) [2e5].
From a wide variety of possible stimuli, probably the most
feasible is the luminous radiation (especially in UV spectral region)
since the energy investment is lower compared to heat or pressure
application and it has a higher practical applicability, interruption
of stimulus once the system has reached the expected response
using changes in the pH solution or the addition of metallic ions, is
one of them and systems showing this on/off behavior are
commonly known as molecular switches. Herein we present the
pyridinecarboxaldehyde capable of responding to UV radiation
through a process of reversible E/Z photoisomerization where the
product is stabilized by an intramolecular hydrogen bond.
2. Experimental and computational methods
The starting materials were purchased from SigmaeAldrich and
Alfa Aesar, they were used without any further purification. 1H and
13C NMR spectra were taken in a 400 MHz Bruker Ultra Shield
spectrometer and the FT-IR spectra were recorded on a Shimadzu
FTIR-8400 instrument.
2.1. Synthesis A-1
An equivalent of 2-pyridinecarboxaldehyde was added to an
ethanol solution (7 mL) of 4-nitrobenzohydrazide (1eq) and a trace
amount of glacial acetic acid. This mixture was heated under reflux
for 3 h, the appearance of precipitate was observed after the first
10 min of reaction. The white solid was filtered and recrystallized
* Corresponding author.
E-mail address: manuel.chaur@correounivalle.edu.co (M.N. Chaur).
http://dx.doi.org/10.1016/j.molstruc.2016.04.055
0022-2860/© 2016 Elsevier B.V. All rights reserved.

M.A. Gordillo et al. / Journal of Molecular Structure 1119 (2016) 286e295
287
from ethanol (96% yield). M. p.: 227e228 ꢀC. FT-IR (
(NeH), 1659 (C]O), 1595(C]N). 1H NMR (400 MHz, DMSO-d6),
n
/cm-1): 3221
2.2. Photoisomerization of (E)-4-nitro-N'-(pyridine-2-ylmethylene)
benzohydrazide
d/
ppm: 12.31 (s, 1 H), 8.64 (d, J ¼ 4.29 Hz, 1 H), 8.49 (s, 1 H), 8.39 (d,
J ¼ 8.59 Hz, 2 H), 8.17 (d, J ¼ 8.59 Hz, 2 H), 8.01 (d, J ¼ 7.80 Hz, 1 H)
7.94e7.87 (m, 1 H) 7.47e7.42 (m, 1 H). 13C NMR (100.60 MHz,
A solution of A-1 (in DMSO-d6), in a NMR Quartz tube was placed
20 cm from a mercury-vapor lamp and was irradiated. Spectra were
taken every 10 min starting from zero up to 90 min.
DMSO-d6), d/ppm: 161.76, 152.96, 149.59, 149.38, 149.12, 138.80,
136.95, 129.26, 124.65, 123.69, 120.08. Elemental analysis calcu-
lated. (%) for C13H10N4O3: C, 57.78; H, 3.73; N, 20.73; found: C,
57.86; H, 3.72; N, 20.62.
2.3. Crystallography
Single crystal X-Ray diffraction data were collected at Bruker
Fig. 1. 13C NMR signals assignments of A-1.
Fig. 2. 1H NMR signals assignments of A-1.

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M.A. Gordillo et al. / Journal of Molecular Structure 1119 (2016) 286e295
Table 1
APEX-II CCD diffractometer with MoK
a
radiation (
l
¼ 0.71073 Å)
NMR chemical shifts (13C and 1H): theoretical-experimental comparison.
13C NMR chemical shifts
monochromated by graphite at 298 K. Data integration, cell and cell
parameters determination was carried out using Bruker SAINT [6].
The structure was solved using the software SHELXS-2013, and
then refined using SHELXL-2013, included in WinGX [7] and Olex2-
1.2 [8]. CCDC 1424366 contains the supplementary crystallographic
data for this paper.
Experimental (ppm)
DFT: B3LYP/6-311G (d, p)
RHF/6-311G (d, p)
149.39
124.66
136.96
120.09
152.97
149.13
161.77
138.81
123.70
129.27
149.60
129.27
155.43
126.00
138.90
128.28
157.34
143.39
161.17
144.61
135.85
128.14
155.67
126.92
149.08
112.92
131.93
113.02
144.52
129.67
149.31
134.38
123.80
119.27
138.87
117.80
2.4. Computational details
The theoretical studies were performed employing GaussView
5.0 [9] as graphic interface and Gaussian 09 [10] for computing.
Determination of minimum energy geometry at ground state was
carried out at two levels of theory: Density Functional Theory
(RB3LYP method) and Restricted HatreeeFock testing accuracy of
three basis set: 6-31Gþ, 6-311Gþ and 6-311G (d, p). Cartesian co-
ordinates for optimized structures are provided in supplementary
materials (see Tables S1eS12). As expected, the best theoret-
icaleexperimental correlation was achieved with the DFT: B3LYP
method [11], for this reason DFT: B3LYP/6-311G (d, p) is used
henceforth for frequency calculations where the absence of imag-
inary frequencies confirmed that the minimum energy structure
was achieved [12]. NMR 13C and 1H chemical shift using TMS DFT:
B3LYP/6-311Gþ (d, p) internal Gaussian's 09 reference, additionally,
DFT results was contrasted with RHF/6-311G (d, p) calculations. All
chemical shifts were calculated using gauge-including atomic or-
bitals (GIAO) method without any solvent or solvation effect
considerations.
Linear regression parameters
Intercept
Slope
R2
16.379
0.9032
0.8764
5.7139
0.8978
0.8527
1H-NMR chemical shifts
Experimental (ppm)
DFT: B3LYP/6-311G (d, p)
RHF/6-311G (d, p)
8.64
7.45
7.91
8.01
8.50
12.31
8.17
8.39
8.17
8.39
9.00
7.09
7.66
7.12
7.70
8.20
8.36
8.46
7.52
8.37
8.58
6.80
7.53
6.70
7.14
6.64
8.07
8.40
7.23
8.31
Linear regression parametersa
Intercept
Slope
R2
3. Results and discussion
5.4683
1.6481
0.7377
5.9871
1.6816
0.6814
3.1. Nuclear magnetic resonance studies and computational
chemical shifts calculations
a
Regression data set excluding signals 8 and 9.
Firstly, the molecular structure determination was carried out
by NMR (1H and 13C), the assignments presented in Figs. 1 and 2 are
supported both by literature information about typical chemical
shift as well as experiments of homo and heteronuclear correlation
NMR spectroscopy (COSY, HMBC and HSQC). Theoretical chemical
shift for 13C and 1H was performed at two theory levels (DFT: B3LYP
and RHF) using 6-311G (d, p) basis set with internal Gaussian's
reference TMS DFT: B3LYP/6-311Gþ (d, p) using gauge-including
atomic orbitals (GIAO) method.
Table 2
Crystal data and structure refinement parameters.
Identification code
A-1
Empirical formula
Formula weight
Temperature/K
Crystal system
Space group
a/Å
C13H12N4O4
288.27
296 (2)
Triclinic
Pꢁ1
13C NMR chemical shifts show an acceptable correlation with
experimental data (see Table 1). DFT: B3LYP/6-311G (d, p)
computed values show a clear tendency to overestimate, where
biggest discrepancy comes from signal C-11: experimental spec-
trum presents two equivalent carbons (strong signal at
123.70 ppm) while theoretical spectrum shows two different sig-
nals (126.92 ppm and 135.85 ppm, 2.95 and 9.82% error, respec-
tively). On the other hand, RHF/6-311G (d, p) computed values are
totally underestimated and show significantly lower accuracy.
The biggest difference between experimental and calculated
data was presented in C-7 signal, 149.13 ppm experimental value
respect to 129.67 ppm computed value (13.05 absolute percent
error), even the second greatest deviation in C-5 signal is near to
the biggest deviation in DFT: B3LYP/6-311G (d, p) calculations
(experimental 124.66 ppm respect to 112.92 ppm computed value,
9.42 absolute percent error).
7.3756 (3)
13.1398 (5)
14.7835 (6)
109.128 (2)
90.531 (2)
101.107 (2)
1324.21 (9)
4
b/Å
c/Å
ꢀ
ꢀ
ꢀ
/
a
b
g
/
/
Volume/Å3
Z
rcalc mg/mm3
m/mmꢁ1
F (000)
1.446
0.110
600
Crystal size/mm3
0.242 ꢂ 0.175 ꢂ 0.112
2q range for data collection
1.462e26.494ꢀ.
Index ranges
Reflections collected
ꢁ9 ꢃ h ꢃ 9, ꢁ16 ꢃ k ꢃ 16, ꢁ17 ꢃ l ꢃ 18
16449
Independent reflections
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I > 2sigma(I)]
R indices (all data)
5290 [R (int) ¼ 0.0195]
5290/0/403
1.024
R1 ¼ 0.0465, wR2 ¼ 0.1191
R1 ¼ 0.0724, wR2 ¼ 0.1377
0.213/ꢁ0.204 e Åꢁ3
Largest diff. Peak/hole/e Åꢁ3
Furthermore, prediction capacity of these theory level and basis
set combination for 1H NMR chemical shifts was unacceptable. The
main source of error is signal 8 (see Fig. 2), NeH proton appears at
12.31 ppm in experimental 1H NMR spectrum but at 8.20 ppm
(DFT: B3LYP/6-311G (d, p), 33.41% error) and 6.64 ppm (RHF/6-311G
(d, p), 46.06% error) in computed spectra. This sharp discrepancy
may not be entirely related to the accuracy of the computational
method used, since the variability of this particular signal in 1H

M.A. Gordillo et al. / Journal of Molecular Structure 1119 (2016) 286e295
289
NMR experiments is remarkable, in fact, it is highly dependent on
the solvent in which the spectrum was taken [13], similarly HeC]
N proton (signal 7 in Fig. 2 and 8.50 ppm) shows significant devi-
ation at both levels of theory: 7.70 ppm (DFT: B3LYP/6-311G (d, p),
9.42% error) and 7.14 ppm (RHF/6-311G (d, p), 16.00% error)
explained by a high electronic environment dependence since this
signal can appear from ꢄ 6e9.50 ppm [14]. Finally, another
important source of error, as well as in computed 13C NMR chemical
shifts, was the presence of separate signals for equivalent protons in
the theoretical spectrum: signal 11, a doublet at 8.16e8.18 ppm
appears in theoretical spectra like two signals at 8.36 ppm and
7.52 ppm, 2.33 and 7.96% error, respectively for DFT: B3LYP/6-311G
(d, p) calculations and 8.07 ppm and 7.23 ppm,1.22 and 11.51% error
for RHF/6-311G (d, p).
angles of 1.28 (1)ꢀ and 158.18 (1)ꢀ for C12eC11eN4eO2 and
O1eC7eC8eC9 bonds, respectively; in conformer 1, the same tor-
sion angles are ꢁ14.65 (1)ꢀ and ꢁ172.94 (1)ꢀ for C25eC24eN8eO5
and O4eC20eC21eC26 bonds respectively (see Fig. 3).
The structure exhibits strong hydrogen bonds between A-1 and
water molecules. These interactions enable to join the conformers,
in an alternating way, giving rise to infinite chains along [010].
Hydrogen bonds parameters are listed in Table 3. O2w e H4w $$$
N5, O2w e H3w $$$ O4 with a distance of 2.9848 (1) Å, 2.8289 (1) Å
respectively, form a R22 (10) motif. N3 e H3 $$$ O2w, O1w e H1w
$$$ O1 and N7 e H7A $$$ O1w form graph sets type D2 (4) (see
2
Fig. 4).
pep stacking interactions between pyridinic and benzene rings
are observed among molecules of the same conformer. The first
conformer presents inter-centroid distances (cg1$$$cg2 and
cg2$$$cg1) of 3.7562 (2) Å and 3.7405 (2) Å, while the second
conformer presents distances (cg3$$$cg4 and cg4$$$cg3) of 3.6582
(1) Å and 3.7560 (2) Å, joining the chains along [100] direction (see
Fig. 5).
Very weak CeH $$$ O interactions between C2eH2 $$$ O3 and
C17 e H17 $$$ O6 with a distance of 3.530 (1) Å and 3.492 (1) Å,
respectively, give raise to infinite chains along the [111] direction to
complete the 3D supramolecular crystal packing (see Fig. 6 and 7).
The Hirshfeld surface shows the susceptible areas to strong and
3.2. Crystallographic study and optimized structure
Non-hydrogen atoms of the molecules were clearly resolved,
and the full-matrix least-squares refinements of these atoms with
anisotropic thermal parameters were carried out. All hydrogen
atoms were stereochemically positioned and refined with the
riding model [15]. Hydrogen atoms bonded to N and O atoms were
localized in the density map. ORTEP diagram was prepared with
ORTEP-3 [16]. Mercury [17] software was used to prepare the
artwork. Crystal data, data collection and structure refinement
details are summarized in Table 2.
Table 3
Hydrogen bonds for A-1.
Compound A-1 crystalized in the triclinic space group P-1. There
are two molecules of A-1 per asymmetric unit, where the pyridinic
ring has rotated over the C14eC19 or C5eC6 bond generating two
D e H … A
d (D e H)
d (H … A)
d (D … A)
<(DHA)
different conformers. This behavior generates
a
pseudo-
O (1W)eH (1W) … N (2)a
O (1W)eH (1W) … O (1)a
O (1W)eH (2W) … O (2W)b
N (3)-H (3A) … O (2W)c
N (7)-H (7A) … O (1W)d
0.82 (3)
0.82 (3)
0.79 (4)
0.87 (2)
0.87 (3)
2.59 (3)
2.08 (3)
2.41 (4)
2.02 (2)
2.02 (2)
3.226 (2)
2.842 (2)
3.170 (3)
2.887 (3)
2.860 (3)
135 (3)
155 (3)
161 (4)
173 (2)
164 (2)
translational symmetry element in (0.027, 0.508 and 0.012).
When both conformers are refined in the same configuration, R1
and WR2 values are 0.0915 and 0.2166 respectively, compared with
the values (0.0465 and 0.1191) obtained when the structure was
refined as different conformers. It means that the model of two
different conformers is closer to the experimental data.
a
ꢁx,ꢁy,ꢁzþ1.
b
ꢁxþ1,ꢁyþ1,ꢁzþ1.
c
ꢁx,ꢁyþ1,ꢁzþ1.
d
Nitro and amide groups for the conformer 2 present torsion
1ꢁx,ꢁy,1ꢁz.
Fig. 3. ORTEP representation of asymmetric unit of A-1. Thermal ellipsoids are drawn at 50% probability level.

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M.A. Gordillo et al. / Journal of Molecular Structure 1119 (2016) 286e295
weak contacts (see Fig. 8). The map of shape index on the Hirshfeld
surface can be used to find complementary regions (concave red
curvature for H acceptor groups and convex blue curvature for H
donor groups) where two molecular Hirshfeld surfaces touch each
other [18,19]. In this case, concave red curvature is placed around N
and O atoms with free electron pairs. The convex blue curvature
was found mainly in NeH and CeH region. Over the aromatic rings
are observed blue and red regions that evidence the p-p stacking.
The fingerprints graphic (see Fig. 9) presents a symmetric
behavior where the two sharp peaks projecting downwards of the
fingerprint are due to the strong O/H and N/H interactions [20].
The most important interaction can be evaluated by overlapping
contributions from the hydrogen interactions which includes N/H,
H/N, H/H, O/H and C/H. This analysis shows that the strong
O/H interactions contribute the most to the Hirshfeld Surface with
33.1% followed by H/H with 29.7%. C/C that corresponds to p-p
stacking contributes with 12.1%, while the strong N$$$H in-
teractions contribute with 7.5% and C/H with 8.8%.
Optimization of molecular structure was performed at two
levels of theory: Density Functional Theory (DFT) and Restricted
HartreeeFock (RHF), using different basis sets.
In concordance to experimental results, all computed structures
show a essentially planar configuration in acyl-hydrazone bridge
and pyridine ring while the nitro-benzene moiety shows a notable
deviation respect to the observed planarity in molecular structure
determined via single-crystal X-ray diffraction (see Fig. 10b). These
deviations represent dihedral angles between acyl-hydrazone
bridge and benzene ring (N3eC7eC8eC9) of 152.3ꢀ and
(N3eC7eC8eC13) ꢁ30.4ꢀ (DFT B3LYP/6-311G) in contrast to exper-
imental values ꢁ174.0ꢀ and 6.6ꢀ, respectively. Since this optimized
geometry was calculated for an isolated molecule in gas phase, the
conformational difference is explained by the absence of intermo-
lecular interactions that modifies the orientation in solid state (see
Fig. 10).
Fig. 5.
direction.
p- p stacking between pyridinic and benzene rings and growth along [100]
tautomerization process [21]. This explanation is supported in 1H
NMR subtle evidence, where small shoulders next to almost all
signals, suggest the existence of the tautomer in mention (see
Fig. 2).
Another trend in computed optimized geometries is the pres-
ence of a possible resonant structure evidenced in dotted line over
N3eC7 bond (see Fig. 10a) probably product of a keto-enol
Fig. 4. a) Hydrogen bonds between A-1 and water forming a R2 (10) and two D2 (4) graph sets, b) Hydrogen bonds between A-1 and water forming a D2 (4) graph set and c)
2
2
2
Infinite chain along [010] direction.

M.A. Gordillo et al. / Journal of Molecular Structure 1119 (2016) 286e295
291
Fig. 6. Weak C e H … O interactions and growth along [111] direction.
Fig. 7. Crystal packing representation of A-1 view along [100] direction.
For the purpose of determining the suitability of the application
of computational methods for predicting bond length and angle in
this particular system, two levels of theory and different basis set
were tested and compared with selected crystallographic data
values via linear regression using experimental data as indepen-
dent variable and computed values as response, which are shown
below in Table 4. Selected geometrical parameters used in linear
regressions analysis are detailed in the supplementary information
(see Tables S13 and S14).
Linear regression provides important information for evaluation
of the general accuracy of these calculations because the slope is a
proportion between experimental and theoretical data variation
[22], estimating in this way the difference between both data sets. A
perfect experimental-computed data correlation represents a slope
equal to one, values greater or less than one represent over- or
underestimated response, respectively. On the other hand, the
intercept value provides quantitative information about deviation
because this parameter is defined as the value of the response
when the input variable is zero. In the same vein, the intercept
determines, in average, in what extent the answer is over or
underestimated. Finally, R2 value provides the percentage of
response variability that is explained by the variation of the inde-
pendent variable, an indirect measure of the reliability of the pa-
rameters: slope and intercept [23].
Fig. 8. Hirshfeld surface of A-1 without water.

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M.A. Gordillo et al. / Journal of Molecular Structure 1119 (2016) 286e295
Fig. 9. Bidimensional fingerprint plot for whole molecule and O/H, N/H, N/H, C/C, C/H and H/H close contacts.
Fig. 10. Optimized geometry for conformer 2: DFT: B3LYP 6-311G (d, p). (a) Frontal view; (b) Side view.
For conformer 2, the Restricted HartreeeFock approximation
basis set and levels of theory. These differences between one or
other conformer using the same optimization routine are probably
associated with intrinsic stability of each conformation, this affir-
mation is supported by a subtle minor energy for conformer 2 and
one order of magnitude of difference between RMS gradients (see
Table 5.) pointing out that the structure for this conformer is closer
to an absolute optimization than conformer 1 and therefore has a
greater stability.
showed considerable accuracy using 6-311G (d, p) basis set for
determining the bond length with a lower computational cost,
while DFT showed a high accuracy in the determination of the bond
angles, however, since both of the calculations cannot be carried
out separately, the level of theory with the greatest correlation was
DFT, thus, DFT: B3LYP/6-311G (d, p) is a suitable option for theo-
retical prediction of molecular structure: R2 ¼ 0.99272 and 0.97648
for bond angles and lengths, respectively. The biggest discrepancy
at this level of theory and basis set was the N3eC7 bond length
(calculated length ¼ 0.040 Å longer than X-ray determinated
length, 2.97% error) and N2eN3eC7 bond angle (calculated angle is
3.1ꢀ higher than X-ray determinated angle, 2.63% error).
3.3. FT-IR vibrational analysis
The Infrared spectrum was recorded on a Shimadzu FTIR 8400
spectrophotometer. The condensation reaction between 2-
pyridinecarboxaldehyde and 4-nitrophenylhydrazine was evi-
denced by the disappearance of a 2823e2841 cmꢁ1 band corre-
sponding to aldehyde CeH stretching and occurrence of a weak
signal at 3062 cmꢁ1 corresponding to CeH stretching in the N]CH
On the other hand, structure optimization for conformer 1
showed similar theoreticaleexperimental correlation trends
regarding conformer 2 (see Table 4). Nevertheless, the correlation
for bond angle calculations showed a notably minor quality fit at all

M.A. Gordillo et al. / Journal of Molecular Structure 1119 (2016) 286e295
293
Table 4
Linear regression fit parameters for bond angle and length data correlation.
Theory level/Basis set
Conformer 2
Slope
Conformer 1
Slope
Intercept
R2
Intercept
R2
Bond length linear regressions
RHF/6-31Gþ
0.9258
0.8699
0.9301
0.8910
1.0846
1.0225
0.0898
0.1846
0.9211
0.8607
0.9235
0.8764
1.0829
1.0179
0.0981
0.1992
0.0934
0.1770
0.1305
0.0261
0.99936
0.99275
0.99884
0.99325
0.99462
0.97884
0.99878
0.99065
0.99804
0.99870
0.99332
0.97648
DFT: B3LYP/6-31Gþ
RHF/6-311Gþ
0.0836
DFT: B3LYP/6-311Gþ
RHF/6-311G (d, p)
DFT: B3LYP/6-311G (d, p)
0.1407
ꢁ0.1339
ꢁ0.0336
Bond angle linear regressions
RHF/6-31Gþ
0.7665
0.9189
0.8433
1.0182
1.2378
1.2710
28.247
9.8312
18.944
2.0043
28.218
32.207
0.6880
0.8268
0.6972
0.8190
1.0681
1.1000
37.637
20.934
36.527
21.885
7.7680
11.594
0.75325
0.94956
0.76970
0.94862
0.88256
0.92603
0.90213
0.98534
0.95548
0.99625
0.97241
0.99272
DFT: B3LPY/6-31Gþ
RHF/6-311Gþ
DFT: B3LYP/6-311Gþ
RHF/6-311G (d, p)
DFT:B3LYP/6-311G (d, p)
Table 5
Energy and RMS gradient values for both conformers.
Level of theory/Basis set
Conformer 2
Conformer 1
Energya
RMS gradient
Energya
RMS gradient
RHF/6-311G (d, p)
DFT: B3LYP/6-311G (d, p)
ꢁ940.55564
ꢁ945.88865
0.00001685
0.00000062
ꢁ940.565634
ꢁ946.201184
0.00000861
0.00000851
a
Energy values are provide in Hartrees.
moiety, finally the disappearance of HeNeH symmetrical stretch
signal in 4-nitrophenylhydrazine IR spectra at 3322 cmꢁ1 corre-
sponding to hydrazine also notes that the expected reaction
occurred.
Computed frequencies using DFT: B3LYP/6-311G (d, p) were
scaled (scale factor ¼ 0.9679) [24] and compared with experi-
mental data below in Table 6.
Vibrational frequencies correlation between experimental and
computed frequencies evaluated via linear regression. Experi-
mental data showed good agreement (R2 ¼ 0.99259), with a slight
tendency to underestimate (slope ¼ 0.9812 and intercept ¼ 43.86).
Largest deviation found comparing the two sets of data occurs in
the bond stretching frequency NeN (5.98% error).
3.4. Photoisomerization
A solution of A-1 in DMSO-d6 was irradiated by a mercury vapor
lamp and several spectra were taken trough time (see Fig. 11).
From the figure above we can observe the apparition of new
signals after 5 min of UV light irradiation, which is an indicative of Z
isomer formation. The percentages were calculated using proton 8
(eNHe) as a reference due to the appearance of a new e isolated
and easy to integrate-signal at 15.84 ppm corresponding to the
eNHe proton, which is part of an hydrogen bond in the metastable
Z isomer. From this data and the rate law expression rate ¼ k
[Eisomer]2, we found that the conversion from E to Z isomer follows a
Table 6
Selected signals in experimental IR and correlation with computed frequencies.
Experimental/cmꢁ1
DFT: B3LYP/6-311G (d, p)
second
order
kinetics,
.
having
a
rate
constant
of
Assignments
k ¼ 0.147 Mꢁ1 minꢁ1
O]N]O Asym. Strech
O]N]O Sym. Strech
NeH Strech
CeH stretch (N]CH)
C]O Strech
C]N Strech
CeH stretch (AreH)
NeH Rock
1543.05
1344.38
3221.12
3062.96
1658.78
1595.13
3221.12
1514.12
1056.99
601.79
1598.19
1380.20
3372.56
2909.69
1726.99
1623.50
3123.25
1506.26
1120.17
614.79
Here we cannot say if the photoisomerization proceed via the
rotation or inversion mechanism since it was not performed in a
wide variety of solvents. However, this mechanism has been
extensively studied for imines and hydrazones (lesser extent),
having different results, some suggest that inversion is the mech-
anistic pathway followed for this kind of compounds, others sup-
NeN Strech
Py skeletal vibration
Benzene skeletal vibration
ported
in
theoretical
calculations
have
found
that
photoisomerization may occur by out-of-plane rotation. Therefore,
more experimentation is needed in order to completely understand
661.58
621.97

294
M.A. Gordillo et al. / Journal of Molecular Structure 1119 (2016) 286e295
Fig. 11. 1D 1H NMR results of photoisomerization assay in DMSO-d6.
the E to Z isomerization mechanism in hydrazones [25,26] For
acylhydrazones, nevertheless, it is known that photoisomerization
proceeds through the weakening of the C]N bond because of the
while 1H NMR chemical shift showed unacceptable correlation
with experimental data, probably because solvation effect was not
included in the calculation method.
electronic transition from the
p
orbital of the amine moiety to the
p1
p* orbital of the entire molecule, commonly named, the p2
-
*
Acknowledgments
transition. [27]
M.N.C., M.A.G. and G.A.G. greatly thanks the Universidad del
Valle, COLCIENCIAS and the Banco de la República for their
generous financial support. M.S-M and R. D. Acknowledges FAPESP
4. Conclusions
~
(2009/54011-8) for provide equipment, Coordenaçao de Aperfei-
The structure of A-1 was confirmed by single crystal X-ray dif-
fractions studies, thus showing that isomer E is the most stable
configuration. Additionally, two conformers where found in the
asymmetric unit differing on the rotation of the pyridinic ring over
the C14eC19 or C5eC6 bond. Also, the configurational dynamics of A-
1 was confirmed by the formation of Z isomer when UV light was
applied, which was evidenced for the apparition of new signals in
the spectrum and specially the one, low field shifted, corresponding
to the eNHe being part of a an hydrogen bond. Therefore, it was
found that photoisomerization follows a second order kinetics,
çoamento de Pessoal de Nível Superior and Conselho Nacional de
Desenvolvimento Cientifico e Tecnologico for the CNPq and CAPES/
PNPD scholarships from the Brazilian Ministry of Education.
ꢀ
Appendix A. Supplementary data
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.molstruc.2016.04.055.
References
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.
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6-311G þ level of theory. On the other hand, calculated 13C NMR
chemical shift showed acceptable theoreticaleexperimental cor-
relation (R2 ¼ 0.8764 and 0.8527 for DFT and RHF, respectively)
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