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A. Ozek Yıldırım et al. / Journal of Molecular Structure 1127 (2017) 275e282
the RAHB in the molecule decreases the aromaticity of B ring, while
increases the aromaticity of C ring. Also, the effects of the intra-
molecular proton transfer on the molecular geometry can be
monitored by observing the changes in HOMA indexes.
During the proton transfer process, the HOMA indexes of A, B
and C rings were calculated at each step. As can be seen from Fig. 5.,
HOMA value of B ring decreases with the scan coordinate going
from 0.9 to 1.7 Å while HOMA value of quasi C ring first increases up
to 0.61 at 1.3 Å (transition state) and then decreases to a final value
of 0.52. When considering these results, during the proton transfer
process, aromaticity of the B ring is transferred to into C quasi ring,
group in aromatic compounds are expected in the range
3000e2925 cmꢀ1 and 2940-2905 cmꢀ1, respectively [57]. The
bands at 3011 and 2942 cmꢀ1 in the experimental FT-IR spectra are
attributed to the asymmetric and symmetric methyl stretching
vibrations, while these modes have been calculated at 3009, 2971
and 2966 cmꢀ1, respectively. In addition, in plane bending and out
of plane bending vibrations of CH3 groups were observed at
1451 cmꢀ1 and 1373 cmꢀ1 experimentally, and calculated at 1449,
1447, 1445 cmꢀ1 and 1378, 1368 cmꢀ1, respectively.
As found in XRD study, C7]N1 is double bond character.
Considering this result, the sharp absorption band at 1619 cmꢀ1 is
attributed to C7]N1 stretching vibration. This vibration band has
been calculated at 1618 cmꢀ1 and 1616 cmꢀ1 by GTO and STO
calculations.
indicating the p-electron coupling for the molecule [53]. It can be
seen from Fig. 5, HOMA values of A ring do not change with the scan
coordinate and the ring preserves its aromaticity during the proton
transfer process as found in the previous studies [47,54].
In the aromatic nitro compounds, asymmetric and symmetric
stretching vibrations of the nitro group have strong absorptions
bands in the region 1570e1485 cmꢀ1 [58] and 1370e1320 cmꢀ1
,
3.3. Experimental and calculated infrared spectra
respectively [59]. Corresponding vibrations of the nitro group in the
title compound are observed at 1523 cmꢀ1 and 1330 cmꢀ1
,
The experimental and the calculated (by GTO and STO) infrared
vibrations with the Potential Energy Distributions (PED) assign-
ments are listed in Table 1, comparatively.
respectively. The GTO and STO type frequency calculations show
that the nitro group asymmetric stretching vibrations are at
1532 cmꢀ1, 1527 cmꢀ1 and symmetric stretching vibrations are at
1314 cmꢀ1, 1309 cmꢀ1, respectively.
Vibrational frequencies have been assigned using PED values
that were calculated with the aid of VEDA4 program [33]. The
calculated frequencies were scaled by 0.9687, which is scaling
factor for B3LYP [31]. The experimental (in solid state) and calcu-
lated (in vacuum) FT-IR spectrum of the compound are shown in
Fig. 6.
The carbonecarbon stretching vibrations of aromatic com-
pounds are expected to appear as a strong band in the region of
1650e1200 cmꢀ1 [56]. Hence, the peaks in the region 1598-
1381 cmꢀ1 were attributed to the CeC aromatic stretching vibra-
tions and calculated in the region 1611-1324 cmꢀ1
.
Broad absorption band in the 3000-1600 cmꢀ1 is due to pres-
ence of strong intramolecular hydrogen bonding between the N
and O atom. This OH stretching vibration was calculated at
2993 cmꢀ1 (GTO) and 2966 cmꢀ1 (STO).
3.4. Electronic spectra
As mentioned in section 3.2., the potential energy barrier be-
tween the enol and keto form varies with the solvent polarity.
Therefore, one would expect the keto form more dominant than the
enol form in polar solvent. For this reason, UVeVisible spectra of
the compound were measured within 200e800 nm range in ben-
zene, chloroform, ethanol and DMSO solvents. In addition, theo-
retical UVeVis. spectra of the enol and keto structures were
obtained by TD-DFT calculations with PCM solvent modelling
method in the mentioned solvents. The calculated spectra were
explained by means of the Frontier Molecular Orbitals (FMOs) with
the aid of Gausssum software [60] and a comparative table of the
experimental and the calculated absorbance values with their FMO
assignments are given in Table S4. The experimental and the
calculated spectra were given in Fig. S1.
Stretching vibrations of a methine (aliphatic) group usually are
found at below 3000 cmꢀ1 regions [55]. The C7eH7 stretching vi-
bration was located at 2918 cmꢀ1 while calculated at 2955 cmꢀ1
.
The aromatic CeH stretching vibrations were observed in the re-
gion 3101e3039 cmꢀ1 experimentally and calculated in the region
3121e3063 cmꢀ1 [56]. The aromatic CeH in-plane bending vibra-
tions were found in the region 1488e1184 cmꢀ1, while calculated
results are in the range of 1479e1192 cmꢀ1. The peak observed at
906 cmꢀ1 is attributed to the aromatic CeH out-of-plane bending
vibration. This mode was calculated to be 899 cmꢀ1
.
The asymmetric and symmetric stretching vibrations of methyl
In the UVeVisible spectrum of an o-hydroxy Schiff base, two
maxima can be seen in connection with the tautomeric form of the
Schiff base. A maximum less than 400 nm implies that an enol form,
while a second maximum greater than 400 nm indicates that a keto
form [61,62]. There are three maxima in the experimental spectra,
except for benzene. The experimental maximum at about 312 nm
corresponds to HOMO-1/LUMO transition for the calculated
spectra in benzene, while this maximum is formed by H-2/L
transition for the calculated spectra in the other solvents. The
calculated transition between the HOMO and LUMOþ1 in the all
solvents gives a maximum at about 345 nm in the experimental
spectra. These maxima can be attributed to the
p/p* transitions of
eCH]N- group [61]. n/ * transitions of the eC]O group the in
p
keto form are found at 443 nm in CHCl3, 434 nm in ethanol, 426 nm
in DMSO and they are shifted to shorter wavelengths (blue shift)
with increasing solvent polarity. These maxima were calculated at
428 nm in CHCl3, 425 nm in ethanol, 426 nm in DMSO and they are
related to the HOMO/LUMO transitions of the keto structure. Also,
these transitions indicate that the keto structure is found in the
solvent media, as can be seen in Fig. S1, the keto/enol ratio increases
Fig. 5. The graph of HOMA indexes against the O1eH1 bond distance in PES scan
process.
?zek Y?ld?r?m, Arzu
