A.Z. El-Sonbati et al. / Journal of Molecular Liquids 199 (2014) 538–544
541
the conjugation with a shift to a longer wavelength [20]. The substituent
effect is related to the Hammett's constant values [19–21]. For azo ben-
zene and aryl azo benzene derivatives, as the possibilities of the mes-
merism became greater, the stabilization of the excited state is
increased relative to that of the ground state and a bathochromic shift
of the absorption bands follows. One way of explaining this result is
by means of the MO theory [22], which shows that the energy terms
of the molecular orbital, became more closely spaced as the size of the
conjugated system increases. Therefore with every additional conjugat-
ed double bond the energy difference between the highest occupied and
the lowest vacant π-electron level became smaller and the wavelength
of the first absorption band which corresponds to this transition is in-
creased. The azo group can act as a proton acceptor in hydrogen
bonds. The role of hydrogen bonding in azo aggregation has been ac-
cepted for some time.
The infrared spectra of ligands show medium broad band located at
−
1
~3460 cm due the stretching vibration of some sort of hydrogen of
hydrogen bonding. El-Sonbati et al. [18,19,24] made detailed studies
for the different types of hydrogen bonding which are favorable to
exist in the molecule under investigation:
1
) Intramolecular hydrogen bond between the nitrogen atom of
the \N_N\ system and hydrogen atom of the hydroxy hydrogen
atom (Fig. 1-C). This is evident by the presence of a broad band cen-
−
1
tered at 3460 cm
.
2
) Hydrogen bonding of the OH⋯N type between the hydroxy hydrogen
atom and the N-ph group (Fig. 1-C).
) Intermolecular hydrogen bonding is possible forming cyclic dimer
through NH⋯O_C (G), OH⋯N_N (F) or OH⋯OH (E) (Fig. 1).
3
Infrared spectra of the ligands (HL
040 cm region due to asymmetric and symmetric stretching vibra-
n
) give two bands at 3200–
Geometrical structures and electronic properties of the investigated
−
1
3
compounds and their protonated forms were calculated by optimizing
their bond lengths, bond angles and dihedral angles. The calculated
molecular structures with the optimized bond lengths are shown in
Fig. 2.
tions of N–H group and intramolecular hydrogen bonding NH⋯O
systems (Fig. 1-D), respectively. When the OH group (Fig. 1-C) is in-
volved in intramolecular hydrogen bond, the O⋯N and N⋯O bond dis-
tances are the same. But, if such mechanism happened in the case of
intermolecular hydrogen bond, the O⋯O and O⋯N bond distances differ.
The broad absorption of a band located at ~3400 cm is assigned to
νOH. The low frequency bands indicate that the hydroxy hydrogen
atom is involved in keto ⇔ enol (A ⇔ B) tautomerism through hydro-
gen bonding (Fig. 1-C). Bellamy [23] made detailed studies on some car-
bonyl compounds containing \NH group. The ΔνNH values were used
to study the phenomena of association. On the other hand, the OH group
According to the frontier molecular orbital theory, FMO, the chemi-
cal reactivity is a function of interaction between HOMO and LUMO
levels of the reacting species [25]. The EHOMO often is associated with
the electron donating ability of the molecule to donate electrons to ap-
propriated acceptor molecules with low-energy, empty molecular or-
bital. Similarly, ELUMO indicates the ability of the molecule to accept
electrons. The lower value of ELUMO indicates the high ability of the mol-
ecule to accept electrons [26]. While, the higher value of EHOMO of the li-
gand, the easier is its ability to offer electrons. The HOMO and LUMO are
shown in Fig. 3.
The HOMO–LUMO energy gap, ΔE, which is an important stability
index, is applied to develop theoretical models for explaining the struc-
ture and conformation barriers in many molecular systems. The smaller
value of ΔE, the more reactivity of the compound [26,27]. The dipole
moment, μ, the first derivative of the energy with respect to an applied
electric field, was used to discuss and rationalize the structure [28]. The
calculated quantum chemical parameters are collected in Table 2.
−
1
(
Fig. 1-B) exhibits more than one absorption band. The two bands locat-
−
1
ed at 1330 and 1370 cm
that at 1130 cm
are assigned to in-plane deformation and
−
1
−1
is due to νC–OH. However, the band at 860 cm
is probably due to the out-of-plane deformation of the \OH group. On
−
1
the other hand, the two bands located at 650 and 670 cm are identi-
fied as δC_O and NH, respectively. Similar to the other investigated
compounds, the different modes of vibrations of C\H and C\C band
are identified by the presence of characteristic bands in the low fre-
−
1
quency side of the spectrum in 600–200 cm
.
HOMO
LUMO
HL
1
HL
2
HL
3
n
Fig. 3. The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the investigated compounds (HL ).