T. Benković et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 190 (2018) 259–267
261
of 0.1 mL of 1.0 M HCl/NaOH and adequate volume of 10−
hydrazone solution was diluted to 10.0 mL.
3
M
calculated and experimental data, considering the band positions and
their relative intensity pattern.
In the high wavenumber region medium bands attributed to NH
−
1
−1
2
.4. Computational Details
The geometry optimizations and single point calculations were per-
stretching were observed at 3476 cm and 3370 cm in the IR spec-
tra of 3 and 4, respectively, whereas the corresponding bands were
missing in the spectra of hydrazones 1 and 2 with two hydroxyl groups
−
1
−1
formed with the Gaussian 09, revision E.01 software package [28] by
using DFT approaches. The hybrid GGA B3LYP and the meta-GGA ex-
change-correlation functionals [29–33] were used in conjunction with
Pople's split-valence 6-311++G(d,p) basis set. Default criteria were
used to define the convergence of both the electronic density and mo-
lecular geometries. Stationary points were characterized by analytical
frequency calculations.
on the phenyl ring. Moderate IR bands at 3288 cm (1), 3313 cm
−
1
−1
(2), 3142 cm
(3) and 3204 cm
(4) were associated with the
stretching of OH groups, involved in intramolecular hydrogen bonding
with the nitrogen atom of the C_N group. The calculations indicated
the main contribution of the carbonyl stretching to the intense IR
−
1
−1
−1
bands at 1668 cm
(1), 1682 cm
(2), 1669 cm
(3) and
−
1
1663 cm (4). Interestingly, these carbonyl bands were accompanied
−
1
−1
Absorption spectra were calculated using the time-dependent DFT
by weak bands at 1631 cm
and 1624 cm
in the IR spectra of 1
(
TD-DFT) methodology [34], implemented in the Gaussian09 package,
and 2, respectively, while by doublets of medium bands in the spectra
−
1
−1
−1
which describes the excited states in terms of all possible single excita-
tions from occupied to virtual orbital. These spectra have been calculat-
ed on optimized structures at B3LYP/6-311++G(d,p) level of theory.
The calculation of NMR chemical shifts for AHZ derivatives was per-
formed using the GIAO (Gauge-Including Atomic Orbital) method
of
3
(1638 cm
and 1618 cm
)
and
4
(1642 cm
and
−
1
1628 cm ). Given that all additional bands were obtained in the car-
bonyl stretching region at wavenumbers lower than those already the-
oretically assigned to the C_O vibration, they most likely originated
from the associated carbonyl groups, which participated in intermolec-
ular hydrogen bonding between the closely packed molecules in the
solid state. Bands between 1605 cm and 1590 cm in the vibrational
spectra of all the studied aroylhydrazones were assigned to coupled
stretching vibrations of the phenyl ring and adjacent double C_N
bond. The vibrational modes of these conjugated electron rich moieties
gave rise to high intensity of the corresponding Raman bands. The bands
[35,36], implemented in the Gaussian09 package, with the B3LYP ex-
−
1
−1
change-correlation functional, in conjunction with 6-311+G(d,p)
basis set. In order to express the chemical shifts in terms of the total
computed NMR shielding tensors, TMS (tetramethylsilane) shielding
tensors were calculated at the same level of theory.
The solvent effects have been considered by using the implicit
Polarizable Continuum Model (PCM) [37] using the integral equation
formalism (IEFPCM) variant [38].
−
1
−1
−1
−1
at 1285 cm (1), 1282 cm (2), 1281 cm (3) and 1300 cm (4),
which also stood out in the Raman spectra, were attributed to the
mixed vibrations of the phenyl ring stretching and OH group bending.
Vibrational modes of the pyridyl moiety produced moderate bands
3
3
3
. Results and Discussion
−
1
around 1240 cm , while stretching of the N\\N bond contributed to
−
1
.1. Solid State
the bands around 1150 cm in both, IR and Raman spectra.
Observed vibrational spectra of 1–4, supported by the theoretical
calculations, clearly indicated presence of the C_O group, double
C_N and single N\\N bond in the molecular structure of the studied
aroylhydrazones, implying that the dominant form in the solid state
was the keto-amine form I.
CP MAS spectra of 1–4 in solid state confirmed the presence of
form I with the intermolecular H-bond between hydroxyl group
and azomethine nitrogen atom. This is in accordance with data
obtained by IR spectroscopy and calculations, as well as literature
data on X-ray structure analysis of 1 [40] and N′-salicylidene-3-
pyridinecarbohydrazide [22]. As an example, the spectrum of com-
pound 4 is given in Fig. S1.
.1.1. Analysis of Tautomeric Forms
The optimized geometries as well as infrared and Raman
spectra have been obtained for tautomeric forms (I–III) for each of the
synthesized compound shown in Scheme 2 in gas phase as well as in
acetonirile, methanol and DMSO. Solvent effects were accounted for
by using the implicit PCM mode [37].
As shown in Table 1, the theoretical results indicated that in gas
phase, the dominant structure of all the studied aroylhydrazones was
the most stable ketoamine form I. Moreover, the energetic order of the
forms I to III does not change when the three forms are optimized in
different solvents.
3
.1.2. Vibrational Spectra of the Solid Samples
3.2. Solution
Experimental vibrational spectra of the solid samples 1–4 included
infrared spectra measured in reflectance (ATR) mode as well as
Raman spectra (Fig. 1a and b). In addition, vibrational spectra of all
the studied aroylhydrazones in gas phase were calculated at B3LYP/6-
3.2.1. NMR Measurements
Proton and carbon NMR chemical shifts assignments of 1–4 in
1
13
DMSO d
6
solution were obtained by the combined use of one ( H,
C
3
11++G(d,p) level of theory, the computed wavenumbers being
APT) and two-dimensional (COSY, HMQC and HMBC) spectra. Finally,
−
1
15
1
scaled by 0.964 in the high-frequency range (N1000 cm ) [39]. Calcu-
lations were done for monomeric species in tautomeric forms I–III as
well as for dimeric species consisting of the molecules in the energeti-
cally most stable form I. The assignment of the characteristic vibrational
bands given in Table 2 was based on the agreement between the
the signals of amide protons were confirmed by acquiring N\\ H
HSQC spectra. The experimental values together with their calculated
counterparts are given in Table 3, and the HMBC spectra are shown in
Figs. S2–S5. Spectral analysis revealed the presence of form I in solution
for all compounds. High chemical shifts of\\NH protons (12.18–12.55)
are consequence of intermolecular hydrogen bonds between amide
proton with other hydrazone molecule, while 2-OH protons (10.97–
Table 1
1
2.29) are involved both in intermolecular and intramolecular hydro-
Relative free energies of four hydrazone derivatives in gas-phase (B3LYP/6-311++G(d,p)
level of theory), at room temperature.
1
gen bonds. That was confirmed by acquiring H NMR spectra of com-
pounds 1–4 in DMSO at different concentrations (Table S1). Chemical
shifts of 3-OH and 4-OH protons in 1 and 2 have lower values 9.29
and 9.00 respectively. Signals of azomethine protons are, as well as
already mentioned signals, singlets characteristic for enolimino tauto-
meric form I. In ketoamino tautomeric form III signal of azomethine pro-
Conformation
Compound
1
2
3
4
I
II
III
0.00
9.72
9.91
0.00
10.00
10.99
0.00
9.44
9.13
0.00
9.10
10.12
13
ton would be doublet. In addition, in C NMR spectrum the typical