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CHAPTER – V
EXPERIMENTAL AND THEORETICAL QUANTUM CHEMICAL
INVESTIGATIONS OF 8–HYDROXY–5–NITROQUINOLINE
5.1. Introduction
The compounds of quinoline family are widely used as a parent compound to
make drugs especially anti–malarial medicines, fungicides, biocides, alkaloids, dyes,
rubber chemicals and flavoring agents. They have antiseptic and antipyretic
properties. They are also used as catalyst, corrosion inhibitor, preservative and as
solvent for resins and terpenes. Halqinol (chlorohydroxyquinoline) is used as a Feed
Premix (anti–bacterial, anti–fungal and anti–protozoal) for large animals and poultry.
Hydroxyquinoline is used as anti–fungal agent in feed of animals. Quinoline dyes are
present in photographic sensitisers [1]. Quinoline derivatives are promising
antiphlogistic activity in rats [2], bacterial inhibitors [3] and precursors to a number of
antimalarial and cancer drugs [4]. The derivatives of aminoquinoline are used as
inhibitors of human immuno deficiency virus (HIV) [5]. Quinoline and isoquinoline
are also active against staphylococcus, epidermis, neisseria and gonorrhea etc. [6].
Quinoline derivatives are also used as a local anesthesia [7]. 8–Hydroxyquinoline act
as CNS stimulant and it has antiseptic with mild fungistatic, bacteriostatic,
anthelmintic, and amebicidal action. 8–Hydroxyquinoline is externally used as a
powerful non–toxic, non–caustic antiseptic and for use in treatment of gynecological
infections (meritritis, vaginitis) as it restores the vaginal acid balance; treatment of
infections in the urinary tract; treatment of diarrhea and colitis without harming the
intestinal flora. The fused quinolines are known to bind DNA with high affinity,
inhibit DNA topoisomerase and display cytotoxic and antitumour activities [8–10].
Pyrazolo fused quinoline derivatives exhibit various biological activities such as anti
viral, anti malarial, lowering of serum cholesterol and display pH dependent
fluorescent properties [11–13]. The tetrahydroquinoline derivative oxaminquine [14]
is used to eradicate blood flukes (Schistosome mansoni), which are a major cause of
disease in tropical regions. Quinoline and their derivatives have been extensively
explored for their applications in the field of biological [15–17], anti filarial [18], anti
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bacterial [19,20] and anti malarial activities [21–26]. Quinolium derivatives have
been widely used as novel inhibitors. i.e., DHA topoisomerase II inhibitor [27],
topoisomerase inhibitor [28], lipoxygenase inhibitor [29], kinase inhibitor [30]. The
derivatives of quinolines are also extensively used as receptor agonists [31–35].
Cardiovascular [36] and anti neoplastic [37] activities of quinolin derivatives have
also been studied.
Thus, understanding the structure, molecular properties as well as nature of
reaction mechanisms of quinoline and its derivatives have been much significant. The
theoretical ab initio and normal coordinate analysis give information regarding the
nature of structure, the functional groups, orbital interactions and mixing of skeletal
frequencies. Harmonic force fields of polyatomic molecules play a vital role in the
interpretation of vibrational spectra and in the prediction of other vibrational
properties. Recent studies shown that, it is possible to compute a complete harmonic
force field at a modest ab initio level within the SCF approximation for a known
molecule. Hence, the investigation on the structure and fundamental vibrations of
quinoline and its derivatives are still being carried out, increasingly [38–49]. The
introduction of one or more substituents in quinoline ring leads to the variation of
charge distribution in the molecule and consequently, this greatly affects the
structural, electronic and vibrational parameters. The structural characteristics and
vibrational spectroscopic analysis of the compound under investigation, 8–hydroxy–
5–nitroquinoline (8H5NQ) has not been studied. Thus, owing to the industrial and
biological importance of substituted quinolines, an extensive spectroscopic studies on
8H5NQ were carried out by recording the FTIR and FT–Raman spectra and
subjecting them to normal coordinate analysis, in an effort to provide possible
explanations for the observations and to understand the effect of nitro and hydroxyl
group on the characteristic frequencies of the quinoline moiety and other
fundamentals. The density functional theory (DFT) is a popular post–HF approach
for the calculation of molecular structures, vibrational frequencies and energies of
molecules [50]. Unlike the HF and MP2 theory, DFT recovers electronic correlation
in the self–consistent Kohn–Sham procedure through the functions of electron density
and gives good descriptions for systems which require sophisticated treatments of
electronic correlation in the conventional ab initio approach, so it is considered as an
effective and reliable method. The DFT calculations with the hybrid exchange–
correlation functional B3LYP (Becke’s three parameter (B3) exchange in conjunction
156
with the Lee–Yang–Parr’s (LYP) correlation functional) which are especially
important in systems containing extensive electron conjugation and/or electron lone
pairs [51–54]. The vibrational studies on 8–hydroxyquinoline and its halogen
derivatives were carried out [38–42] and show better agreement with the experimental
values of structural characteristics than the calculations containing the gradient
corrected exchange functional [55].
5.2. Experimental
The compound under investigation 8–hydroxy–5–nitroquinoline (8H5NQ) was
purchased from Aldrich Chemicals, USA and used as such to record the FTIR and
FT–Raman spectra. The FTIR spectrum was recorded by KBr pellet method, on a
Bruker IFS 66V spectrometer equipped with a Globar source, Ge/KBr beam splitter,
and a TGS detector in the range of 4000 to 400 cm–1. The FT–Raman spectra was also
recorded in the range 4000 to 100 cm–1 using the same instrument with FRA 106
Raman module equipped with Nd:YAG laser source with 200mW powers operating at
1.064 m and the spectral resolution is 2 cm–1. A liquid nitrogen cooled–Ge
detector was used. The frequencies of all sharp bands are accurate to 2 cm–1.
5.3. Computational details
The combination of vibrational spectroscopy with ab initio calculations is
considered to be a powerful tool for understanding the fundamental mode of
vibrations of the molecules and to further investigate the structure, stability, the
thermodynamic properties and to determine the energy of the compound 8–hydroxy–
5–nitroquinoline (8H5NQ), the LCAO–MO–SCF restricted Hartree–Fock (HF) and
the gradient corrected density functional theory (DFT) [56] with the three–parameter
hybrid functional (B3) [52] for the exchange part and the Lee–Yang–Parr (LYP)
correlation function [53], level ab initio calculations have been carried out in the
present investigation, using 6–31G(d,p), 6–311++G(d,p) and cc–pVDZ basis set with
Gaussian–03 [57] program package, invoking gradient geometry optimisation [58] on
Intel core i5/3.03 GHz processor. The familiar Hartree–Fock optimised basis sets
such as Pople’s 6–3l1++G(d,p) basis set give reasonable molecular geometries and
vibrational frequencies when used in molecular DFT calculations. To satisfactorily
describe the conformation and orientation of the nitro and hydroxyl groups a fully
polarised 6–31G(d,p), 6–311++G(d,p) and cc–pVDZ basis sets are required and
157
considered to be a complete basis set. The energy minima with respect to the nuclear
coordinates were obtained by the simultaneous relaxation of all the geometric
parameters using the gradient methods without any constraint in the potential energy
surface at DFT and Hartree–Fock levels, adopting the standard 6–31G(d,p),
6–311++G(d,p) and cc–pVDZ basis sets. The optimised structural parameters were
used in the vibrational frequency calculations at the HF and DFT levels to
characterise all stationary points as minima. Then vibrationally averaged nuclear
positions of the compound 8H5NQ were used for harmonic vibrational frequency
calculations resulting in IR and Raman frequencies together with intensities and
Raman depolarisation ratios.
Normal coordinate analysis provides a more quantitative and complete
description of the vibrational modes. Owing to the complexity of the molecule, the
normal coordinate analysis is carried out to obtain complete informations of the
molecular motions involved in the normal modes of 8H5NQ and also to calculate the
potential energy distribution of the vibrational modes of the compound. The force
constants obtained from the ab initio basis set were used in the normal coordinate
analysis utilising Wilson’s FG matrix method [59–61]. The normal coordinate
calculations were performed with the program of Fuhrer et al. [62]. The
experimentally observed FTIR and FT–RAMAN spectral data of the compound
8H5NQ was found to be in good agreement with the spectral data obtained by
quantum chemical calculations. The potential energy distribution corresponding to
each of the observed frequencies shows the reliability and accuracy of the spectral
analysis.
The thermodynamic parameters entropy, heat capacity at constant pressure
and enthalpy change of 8H5NQ at different temperatures ranging from 100–1000 K
were determined to study the dependence of these properties with temperature.
Isoelectronic molecular electrostatic potential surfaces and electron density surfaces
[37] were calculated using 6–311++G(d,p) basis set. The molecular electrostatic
potential (MEP) at a point r in the space around a molecule (in atomic units) can be
expressed as:
rr
drr
rR
ZrV
AA
A
'
')'()(
158
where, ZA is the charge on nucleus A, located at RA and ρ(r′) is the electronic density
function for the molecule. The first and second terms represent the contributions to
the potential due to nuclei and electrons, respectively. V(r) is the resultant at each
point r, which is the net electrostatic effect produced at the point r by both the
electrons and nuclei of the molecule. The molecular electrostatic potential (MEP)
serves as a useful quantity to explain hydrogen bonding, reactivity and structure–
activity relationship of molecules including biomolecules and drugs [38]. Structures
resulting from the plot of electron density surface mapped with electrostatic potential
surface depict the shape, size, charge density distribution and the site of chemical
reactivity of a molecule. Gauss View 5.0.8 visualisation program [39] has been
utilised to construct the MESP surface, the shape of highest occupied molecular
orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) orbitals. The
energy distribution of the molecular orbitals [40], HOMO–LUMO energy gap have
also been measured by B3LYP/6–311++G(d,p) method.
The Raman scattering activities (Si) calculated by Gaussian 03W program
were suitably converted to relative Raman intensities (Ii) using the following
relationship derived from the basic theory of Raman scattering [41].
)]/exp(1[)( 4
0
kThc
SfI
ii
iii
where v0 is the exciting frequency (cm−1), vi is the vibrational wavenumber of the ith
normal mode, h, c and k are universal constants, and f is the suitably chosen common
scaling factor for all the peak intensities.
5.4. Results and discussion
5.4.1. Molecular Geometry
The structure and the scheme of numbering the atoms of 8–hydroxy–5–
nitroquinoline (8H5NQ) is represented in Figure 5.1.
159
Figure 5.1. Structure and atom numbering scheme of
8–hydroxy–5–nitroquinoline (8H5NQ)
The geometry of the molecule 8H5NQ under investigation is considered by
possessing CS point group symmetry. The 54 fundamental modes of vibrations of
each compound are distributed into the irreducible representations under CS symmetry
as 39 in–plane vibrations of A species and 15 out of plane vibrations of A species.
i.e., vib = 39A + 15A
All vibrations are active in both IR and Raman. All the frequencies are
assigned in terms of fundamental, overtone and combination bands.
5.4.2. Structural properties
The interaction of hydroxyl group and nitro group substituents on the ring is of
great importance in determining its structural and vibrational properties. The
optimised structural parameters bond length and bond angle for the
160
thermodynamically preferred geometry of 8H5NQ determined at B3LYP/
6–311++G(d,p), B3LYP/6–31G(d,p), B3LYP/cc–pVDZ and HF/6–311++G(d,p)
levels are presented in Table 5.1 in accordance with the atom numbering scheme of
the molecule shown in Figure 5.1.
From the structural data given in Table 5.1. it is observed that the various
C─C bond distances calculated between the ring carbon atoms and the C─H bond
length are found to be nearly the same at B3LYP/6–311++G(d,p), B3LYP/
6–31G(d,p), B3LYP/cc–pVDZ and HF/6–311++G(d,p) levels. The bond lengths
determined from B3LYP/6–31G(d,p) level of theory are slightly higher than that
obtained from HF/6–311++G(d,p) method but B3LYP method with 6–311++G(d,p)
and cc–pVDZ series sets yields in excellent agreement with the each other. The
influence of the substituent on the molecular parameters, particularly in the C─C bond
distance of ring carbon atoms seems to be negligibly small except that C5─C6 (1.38
Å) and C7─C8 (1.38 Å), where the nitro and hydroxyl group is attached with C5 and
C8. The C─C bond distances calculated by B3LYP/6–311++G(d,p) are in the range
1.38 to 1.43 Å. In the pyridine ring of the compound under investigation the C3─C4
and N1─C2 bond distances are shorter than that of other C─C and C─N bond
lengths. The C─N bond length of C–NH2 is 0.13 Å greater than that of C─O bond
distance of C–OH. From HF and B3LYP methods the O─H bond distance of 8H5NQ
is determined as 0.95 and 0.98 Å, respectively. In the case of 8–hydroxyquinoline
N–oxide the O─H bond length is measured as 1.007 Å by B3LYP/6–31G(d,p) method
and 1.12 Å by X–ray diffraction method. The shorter O─H bond distance in 8H5NQ
clearly indicate the presence of weak hydrogen bond O–H∙∙∙∙N type and it is strong in
the case of 8–hydroxyquinoline N–oxide. The other bond lengths and bond angles of
8H5NQ determined at the HF and DFT level of theory are in good agreement with the
X–ray data of 8–hydroxyquinoline N–oxide [63,64]. The calculated bond angles are
very close to each other. the CCC bond angle at the point of hydroxyl and nitro group
substitution, C6–C5–C9 and C7–C8–C10 does not show any significant difference
The bond angle C10–C8–O14 is 3.10o less than that of C7–C8–O14, that confirm the
orientation of O─H bond towards N1 in the pyridine ring of 8H5NQ. The
thermodynamic parameters of the compound have also been computed at ab initio
B3LYP/6–311++G(d,p), B3LYP/6–31G(d,p), B3LYP/cc–pVDZ and HF/
6–311++G(d,p) methods are presented in Table 5.2. The total thermal energy,
vibrational energy contribution to the total energy, the rotational constants and the
161
dipole moment values obtained from HF method is slightly over estimated than that of
DFT method. Due to the delocalisation of the lone pair of electrons present in the
hydroxyl group into the ring, the C─O bond distance is shorter by 0.13 Å than the
C─N bond length.
The correlation of bond length and bond angle of 8–hydroxy–5–nitroquinoline
is shown in Figure. 5.2 & Figure 5.3.
N
1-C
2C
2-H
16
C2
-C3
C3
-H1
7C
3-C
4C
4-H
18
C4
-C9
C9
-C1
0C
9-C
5C
5-N
11
N1
1-O
12
N1
1-O
13
C5
-C6
C6
-H1
9C
6-C
7C
7-H
20
C7
-C8
C8
-O1
4O
14-H
15
C8
-C1
0C
10
-N1 -- --
1.0
1.1
1.2
1.3
1.4
1.5
B3LYP/6-311++G(d,p)
HF/311++G(d,p)
B3LYP/6-31G(d,p)
B3LYP/cc-pVDZ
Bo
nd
Le
ng
th (
Ao)
Figure 5.2. The correlation of bond lengths of 8–hydroxy–5–nitroquinoline determined by HF/DFT methods
162
N1-C
2-C
3N
1-C
2-H
16
C2-C
3-H
17
C2-C
3-C
4C
3-C
4-H
18
C3-C
4-C
9C
4-C
9-C
5C
4-C
9-C
10
C9-C
5-N
11
C5-N
11-O
12
C5-N
11-O
13
C5-C
6-H
19
C5-C
6-C
7C
6-C
7-H
20
C6-C
7-C
8C
7-C
8-O
14
C8-O
14-H
15
H20-C
7-C
8H
19-C
6-C
7C
8-C
10-C
9C
7-C
8-C
10
C8-C
10-N
1C
10-N
1-C
2O
14-C
8-C
10
C9-C
10-N
1C
10-C
9-C
5C
9-C
5-C
6N
11-C
5-C
6H
16-C
2-C
3O
12-N
11-O
13
H17-C
3-C
4H
18-C
4-C
9 --
104
106
108
110
112
114
116
118
120
122
124
126
128
B3LYP/6-311++G(d,p)
HF/311++G(d,p)
B3LYP/6-31G(d,p)
B3LYP/cc-pVDZ
Bo
nd
an
gle
(D
eg
ree)
Figure. 5.3. The correlation of bond angles of 8–hydroxy–5–nitroquinoline determined by HF/DFT methods
5.4.3. Thermodynamic analysis
The energies and thermodynamic parameters of the compound have also been
computed at B3LYP methods with 6–31G(d,p), 6–311++G(d,p) and cc–pVDZ basis
sets and are presented in Table 5.2. The frequency calculations compute the zero
point energies, thermal correction to internal energy, enthalpy, Gibbs free energy and
entropy as well as the heat capacity for a molecular system. From Table 5.2., it is
observed that the dipole moment of 8H5NQ calculated at HF and B3LYP methods are
higher than the dipole moments of quinoline (2.02 D) and is due to the presence of
hydroxyl and nitro groups.
163
The temperature dependence of the thermodynamic properties heat capacity at
constant pressure (Cp), entropy (S) and enthalpy change (∆H0→T) for 8H5NQ were
also determined by B3LYP/6–311++G(d,p) method and listed in Table 5.3. The
anharmonicity effects have been eliminated by scaling the thermodynamic properties
by a scale factor of 0.96. The Figures 5.4–5.6. depicts the correlation of heat capacity
at constant pressure (Cp), entropy (S) and enthalpy change (∆H0→T) with temperature.
From Table 5.2, one can find that the entropies, heat capacities, and enthalpy changes
are increasing with temperature ranging from 100 to 1000 K due to the fact that the
molecular vibrational intensities increase with temperature [65]. The quadratic
equations shows the observed relations between the thermodynamic functions and
temperatures. The corresponding regression factors (R2) are all not less than 0.9992.
S = 234.7871 + 0.7136 T – 1.6306 x 10–4 T2
Cp = –0.023 + 0.7169 T – 3.3137 x 10–4 T2
ΔH = –8.0263 + 0.081 T + 1.7963 x 10–4 T2
Figure 5.4. Effect of temperature on heat capacity at constant pressure (Cp) of
8–hydroxy–5–nitroquinoline
164
Figure 5.5. Effect of temperature on entropy (S) of 8–hydroxy–5–nitroquinoline
Figure 5.6. Effect of temperature on enthalpy change (∆H0→T) of
8–hydroxy–5–nitroquinoline.
165
5.4. Analysis of frontier molecular orbitals (FMOs) and Molecular electrostatic
potential (MESP)
The molecular electrostatic potential surface (MESP) which is a method of
mapping electrostatic potential onto the iso–electron density surface simultaneously
displays electrostatic potential (electron + nuclei) distribution, molecular shape, size
and dipole moments of the molecule and it provides a visual method to understand the
relative polarity [66]. The total electron density and MESP surfaces of the molecules
under investigation are constructed by using B3LYP/6–311++G(d,p) method. The
total electron density surface of 8H5NQ is shown in Figure 5.7 while the total
electron density surface mapped with the electrostatic potential of 8H5NQ is shown in
Figure 5.8. The electrostatic potential contour maps for positive and negative
potentials is shown in Figure 5.9.
Figure 5.7. Total electron density iso surface of 8–hydroxy–5–nitroquinoline
166
Figure 5.8. Total electron density mapped with electrostatic potential iso surface
of 8–hydroxy–5–nitroquinoline
Figure 5.9. Electrostatic potential surface of 8–hydroxy–5–nitroquinoline
167
The colour scheme for the MESP surface is red, electron rich, partially
negative charge; blue, electron deficient, partially positive charge; light blue, slightly
electron deficient region; yellow, slightly electron rich region; green, neutral;
respectively. It is obviously from the Figures 5.8–5.10. that the region around nitro
and hydroxyl group represents the most negative potential region (red). The
predominance of light green region in the MESP surfaces corresponds to a potential
halfway between the two extremes red and dark blue colour. In both the compounds
the total electron density surface mapped with electrostatic potential clearly reveals
the presence of high negative charge on the nitro and hydroxyl group while more
positive charge around the ring hydrogen atoms.
The electrostatic potential contour surface of 8–hydroxy–5–nitroquinoline
presented in the Figures 5.10.
Figure 5.10. Electrostatic potential contour surface of 8–hydroxy–5nitroquinoline
Highest occupied molecular orbital (HOMO) and lowest unoccupied
molecular orbital (LUMO) are very important parameters for quantum chemistry. The
HOMO is the orbital that primarily acts as an electron donor and the LUMO is the
orbital that largely acts as the electron acceptor. The energies of HOMO LUMO,
168
LUMO+1 and HOMO–1 and their orbital energy gaps are calculated using B3LYP/
6–311++G(d,p) method and the pictorial illustration of the frontier molecular orbitals
and their respective positive and negative regions are shown in Figure 5.11 for
8–hydroxy–5–nitroquinoline.
Figure 5.11. The Frontier Molecular Orbitals of 8–hydroxy–5–nitroquinoline.
169
Molecular orbitals, when viewed in a qualitative graphical representation, can
provide insight into the nature of reactivity, and some of the structural and physical
properties of molecules. Well known concepts such as conjugation, aromaticity and
lone pairs are well illustrated by molecular orbitals. The positive and negative phase is
represented in red and green colour, respectively. From the plots we can see that the
region of HOMO spread over the entire molecule of 8–hydroxy–5–nitroquinoline.
The calculated energy gap of HOMO–LUMO’s explains the ultimate charge transfer
interface within the molecule. The frontier orbital energy gaps in case of 8H5NQ is
found to be 3.8858 eV. A large LUMO–HOMO gap energy has been taken as an
indication of a high stability of 8H5NQ.
5.5. Vibrational Analysis
The observed vibrational assignments and analysis of 8–hydroxy–5–
nitroquinoline (8H5NQ) are discussed in terms of fundamental bands, overtones and
combination bands. The FTIR and FT–Raman spectra of 8H5NQ are shown in
Figures 5.12 and 5.13. The symmetry co–ordinates of 8H5NQ are presented in Table
5.3. The observed and calculated frequencies using ab initio B3LYP/6–311++G(d,p),
B3LYP/6–31G(d,p), B3LYP/cc–pVDZ and HF/6–311++G(d,p) force field along with
their relative intensities, probable assignments and potential energy distribution (PED)
of 8H5NQ are summarized in Table 5.4 and 5.5, respectively.
Figure 5.12. FTIR Spectrum of 8–hydroxy–5–nitroquinoline
170
Figure 5.13. FT–Raman Spectrum of 8–hydroxy–5–nitroquinoline
5.5.1. Carbon vibrations
The carbon–carbon stretching modes of the phenyl group are expected in the
range from 1650 to 1200 cm–1. The actual position of these mode are determined not
so much by the nature of the substituents but by the form of substitution around the
ring [67], although heavy halogens cause diminish the frequency [68]. In 8H5NQ
under CS symmetry the carbon–carbon stretching bands appeared in the infrared
spectrum at 1632, 1578 1564, and 1474, 1384 and1330 cm–1 are assigned to C–C
stretching vibrations. The band occurring at 1423 cm–1 in IR and 1416 cm–1 in Raman
is assigned to C=N stretching vibrations. These are all considered to be absolute
modes according to the normal coordinate analysis. The very strong fundamental
mode observed at 1252 cm–1 in the FTIR spectrum is assigned to the CO stretching
mode of 8H5NQ. The band observed at 1168 and 1158 cm–1 in IR and Raman, is
assigned to C─N stretching band [69,70]. These assignments are confirmed by the
theoretical observation and PED calculation.
The bands occurring at 750, 732 and 566 cm–1 in the infrared and at 568 cm–1
in Raman are assigned to the CCC in–plane bending modes of 8H5NQ. The results of
171
normal coordinate analysis are used to assign CNC and CCN in–plane bending
vibrations of 8H5NQ. The CNC in–plane bending vibrations of 8H5NQ has been
determined from the B3LYP/6–311++G(d,p) method is 680 cm–1. The CCN in–plane
bending vibration is obtained at 652 and 605 cm–1. The CCC out of plane bending
modes of 8H5NQ under CS symmetry is attributed to the IR and Raman wavenumbers
are 447, 425 cm–1 and 432 cm–1, respectively. All these assignments are agree well
with the reported literature [69]. The CCC in–plane bending and out of plane
vibrations are described as mixed modes as there are about 14 to 22% PED
contributions mainly from CH in–plane bending and out of plane bending vibrations,
respectively.
5.5.2. CH vibrations
The aromatic CH stretching vibrations are normally found between 3100 and
3000 cm–1. The CH stretching of 8H5NQ gives bands at 3116, 3094 and 3010 cm–1
in IR and at 3081 and 3060 cm–1 in Raman. The CH in–plane bending modes are
normally observed in the region 13001000 cm–1. These modes are observed in
8H5NQ at 1088, 1023 and 998 cm–1 in the infrared and the corresponding frequencies
are obtained in the Raman at 1117 and 990 cm–1. The CH out of plane bending
modes are observed in the region 1100 to 600 cm–1. The aromatic CH out of plane
bending vibrations of 8H5NQ are assigned to the bands observed at 930 and 847 cm–1
in the infrared spectrum while at 887 and 855 cm–1 in the Raman spectrum [69,70].
The normal co–ordinate analysis reveals that the aromatic CH in–plane and out of
plane bending vibrations have substantial overlapping with the ring CCC in–plane and
out of plane bending modes, respectively.
5.5.3. OH vibrations
Bands due to OH stretching is of medium to strong intensity in the infrared
spectrum, although it may be broad. In Raman spectra the band is generally weak.
Unassociated hydroxyl groups absorbs strongly in the region 3670–3580 cm–1. The
band due to the free hydroxyl group is sharp and its intensity increases. For solids,
liquids and concentrated solutions a broad band of less intensity is normally observed
[71–73]. The compound under investigation 8H5NQ shows the stretching of hydroxyl
group at 3571 cm–1 in the infrared spectrum. The higher stretching frequency
172
observed in comparison with the intra molecular hydrogen bonded
8–hydroxyquinoline N–oxide [63,64] also signifies that there is a weak intramolecular
hydrogen bonding between the hydroxyl group and N1 atom in 8H5NQ. The in–plane
OH bending vibration give rise to the strong band in infrared and weak band in
Raman spectra of 8H5NQ at 1351 and 1355 cm–1, respectively. The out of plane
OH bending vibration give rise to the strong band both infrared and Raman spectra
of 8H5NQ is observed at 800 and 793 cm–1 respectively [40]. The normal coordinate
calculations shows that the hydroxyl stretching vibrational mode is very pure while
the in–plane and out of plane bending of the hydroxyl group mixed about 14 and 21%
with the CO in–plane and out of plane bending vibrations, respectively.
5.5.4. Nitro group vibrations
The asymmetric and symmetric stretching vibrations of the nitro group are
also occur in the same C–C stretching region. These are the more intense bands of the
spectrum. The frequencies observed at 1521 cm–1 in the infrared spectrum are
assigned to the –NO2 asymmetric stretching mode of 8H5NQ. The very strong
symmetric stretching vibrations of nitro group of the compound is assigned to the
wavenumber 1297 cm–1 in the infrared spectrum [71]. The very strong stretching
vibration of C–NO2 is assigned to the wavenumber 1204 cm–1 in IR and 1199 cm–1 in
Raman spectra. The –NO2 stretching vibrations seems to be sensitive to electronic
effects of the substituent methyl groups. The deformation mode of the nitro group is
observed at 818 cm–1 in 8H5NQ.
5.6. Scale factors
The correction factors used to correlate the experimentally observed and
theoretically computed frequencies for each vibrational modes of 8H5NQ under HF
and DFT–B3LYP methods are similar and an explanation of this approach were
discussed previously [74–81]. A better agreement between the computed and
experimental frequencies can be obtained by using different scale factors for different
modes of vibrations. Initially, all scaling factors have been kept fixed at a value of
1.0 to produce the pure ab initio/DFT calculated vibrational frequencies and the
potential energy distributions (PED) which are given in Tables 5.4 and 5.5.
Subsequently, a scaling factor of 0.98 for O–H stretching mode and 0.96 for
all other the fundamental modes except the torsional mode has been utilised to obtain
173
the scaled frequencies of the compound 8H5NQ in DFT method with 6–311++G(d,p)
and cc–pVDZ basis sets. Similarly, a scale factor of 0.98 and 0.95 were used for the
O–H stretching and all other fundamental modes, respectively to compute the
corrected wavenumbers at DFT/6–31G(d,p) level and compared with the
experimentally observed frequencies.
In the case of HF/6–311++G(d,p) level 0.91 is used for the C–H stretching and
0.88 for all other fundamental modes. DFT–B3LYP correction factors are all much
closer to unity than the HF correction factor, which means that the DFT–B3LYP
frequencies are very much closer to the experimental values than the HF frequencies.
Thus, vibrational frequencies calculated by using the B3LYP functional can be
utilised to eliminate the uncertainties in the fundamental assignments in infrared and
Raman vibrational spectra [82]. The correlation between the calculated and the
experimental frequencies of 8–hydroxy–5–nitroquinoline is shown in the Figure 5.14.
0 500 1000 1500 2000 2500 3000 3500 4000
0
500
1000
1500
2000
2500
3000
3500
4000
(a) B3LYP/6-311++G(d,p)
R2=0.999
Th
eo
reti
cal W
aven
um
ber
(Cm
-1)
Experimental Wavenumber (Cm-1)
174
0 500 1000 1500 2000 2500 3000 3500 4000
0
500
1000
1500
2000
2500
3000
3500
4000
(b) B3LYP/cc-pVDZ
R2=0.996
Th
eo
reti
cal
Waven
um
ber
(Cm
-1)
Experimental Wavenumber (Cm-1)
Figure 5.14. Correlation between the calculated and the experimental frequencies of
8–hydroxy–5– nitroquinoline
5.7. Conclusions
The molecular structural parameters, thermodynamic properties and
vibrational frequencies of the fundamental modes of the optimised geometry of
8–hydroxy–5–nitro quinoline (8H5NQ) have been obtained from ab initio and DFT
calculations. The geometry was optimised without any symmetry constrains using the
DFT/B3LYP and HF methods with 6–31G(d,p), 6–311++G(d,p) and cc–pVDZ basis
sets. The theoretical results were compared with the experimental values. The effects
of substituents (nitro and hydroxyl groups) on vibrational frequencies were analysed.
Although both types of calculations are useful to explain vibrational spectra of
8H5NQ, the deviation between the experimental and calculated (both unscaled and
scaled) frequencies was reduced with the use of DFT–6–311++G(d,p) method in
comparison with the HF–6–311++G(d,p) in the whole range of calculations. B3LYP
method seems to be more appropriate than HF method for the calculation of
geometrical parameters of molecules.
175
Table 5.1. Structural parameters calculated for 8–hydroxy–5–nitroquinoline employing B3LYP/6–311++G(d,p),
HF/6–311++G(d,p), B3LYP/6–311G(d,p) and B3LYP/ cc–pVDZ methods.
Structural
Parameters
8–hydroxy–5–nitroquinoline
Experimentala B3LYP/
6–311++ G(d,p)
HF/
6–311++G(d,p)
B3LYP/
6–31G(d,p) B3LYP/cc–pVDZ
Internuclear Distance (Å)
N1–C2 1.31 1.29 1.32 1.32 1.346
C2–H16 1.09 1.08 1.09 1.10
C2–C3 1.41 1.41 1.41 1.41 1.390
C3–H17 1.08 1.07 1.09 1.09
C3–C4 1.38 1.36 1.38 1.38 1.360
C4–H18 1.08 1.07 1.08 1.09
C4–C9 1.42 1.43 1.42 1.43 1.422
C9–C10 1.43 1.40 1.43 1.43 1.409
C9–C5 1.43 1.43 1.44 1.44 1.408
C5–N11 1.47 1.46 1.46 1.47 1.486*
N11–O12 1.23 1.19 1.23 1.23 1.223*
N11–O13 1.23 1.19 1.24 1.23 1.223*
C5–C6 1.38 1.36 1.39 1.39 1.363
176
C6–H19 1.08 1.07 1.08 1.09
C6–C7 1.40 1.40 1.40 1.41 1.393
C7–H20 1.08 1.07 1.08 1.09
C7–C8 1.38 1.36 1.38 1.39 1.355
C8–O14 1.34 1.32 1.34 1.34 1.352
O14–H15 0.98 0.95 0.98 0.99 1.120
C8–C10 1.43 1.43 1.44 1.44 1.410
C10–N1 1.36 1.35 1.36 1.36 1.402
Bond angle (0)
N1–C2–C3 122.3 122.4 122.4 122.5 121.0
N1–C2–H16 116.9 117.3 116.9 116.9
C2–C3–H17 119.7 119.8 119.7 119.7
C2–C3–C4 120.0 119.7 120.0 120.0 120.9
C3–C4–H18 120.7 120.2 121.0 121.2
C3–C4–C9 119.7 119.6 119.7 119.6 119.6
C4–C9–C5 128.2 128.0 128.2 128.2
C4–C9–C10 115.3 115.4 115.4 115.4 118.7
C9–C5–N11 123.0 122.8 123.0 123.0
C5–N11–O12 118.1 117.9 118.0 117.9 117.3
177
C5–N11–O13 118.7 118.6 118.9 118.7 117.3
C5–C6–H19 117.9 118.7 117.6 117.4
C5–C6–C7 122.9 121.9 122.3 122.3 120.5
C6–C7–H20 120.8 120.7 121.0 121.1
C6–C7–C8 119.2 119.3 119.1 119.1 117.7*
C7–C8–O14 121.5 120.8 121.9 122.2
C8–O14–H15 106.0 108.8 105.0 104.3
H20–C7–C8 120.0 120.1 119.9 119.9
H19–C6–C7 119.8 119.3 120.0 120.2
C8–C10–C9 121.2 121.1 121.4 121.5
C7–C8–C10 120.1 120.1 120.0 119.9 123.4
C8–C10–N1 114.6 114.9 114.3 114.1
C10–N1–C2 118.5 119.0 118.3 118.2 120.4
O14–C8–C10 118.4 119.1 118.2 117.9
C9–C10–N1 124.2 124.0 124.3 124.4
C10–C9–C5 116.5 116.6 116.4 116.4
C9–C5–C6 120.8 120.9 120.8 120.8 120.2
N11–C5–C6 116.2 116.3 116.2 116.2
178
a – values taken from Ref. [63]
* – values taken from Ref. [64]
H16–C2–C3 120.8 120.4 120.7 120.7
O12–N11–O13 123.3 123.5 123.1 123.5 125.3*
H17–C3–C4 120.3 120.5 120.3 120.0
H18–C4–C9 119.6 120.2 119.3 119.3
179
Table 5. 2. The calculated thermodynamic parameters of 8–hydroxy–5–nitroquinoline employing B3LYP/
6–311++G(d,p), HF/6–311++G(d,p), B3LYP/6–311G(d,p) and B3LYP/cc–pVDZ methods.
Thermodynamic parameters (298 K)
8–hydroxy–5–nitroquinoline
B3LYP/
6–311++ G(d,p)
HF/
6–311++G(d,p)
B3LYP/
6–31G(d,p)
B3LYP/
cc–pVDZ
SCF Energy (a.u) –681.848 –677.841 –681.670 –681.710
Total Energy (thermal), Etotal (kcal.mol–1) 95.899 101.693 96.404 96.172
Heat Capacity at const. volume, Cv
(cal.mol–1.K–1)
40.203 35.664 40.010 39.938
Entropy, S (cal.mol–1.K–1) 101.868 93.122 101.851 101.242
Vibrational Energy, Evib (kcal.mol–1) 94.121 99.915 94.626 94.395
Zero–point vibrational Energy, E0
(kcal.mol–1)
89.492 96.200 90.023 89.810
Rotational Constants (GHz)
A 1.103 1.104 1.103 1.102
B 0.648 0.648 0.648 0.646
C 0.408 0.409 0.408 0.407
180
Dipolemoment (Debye)
μx 4.034 4.189 –3.861 3.744
μy 0.860 1.085 0.827 0.778
μz 0.0007 0.0009 0.000 0.0005
μtotal 4.125 4.327 3.948 3.825
ELOMO + 1 (eV) –1.9712
ELOMO (eV) –3.0869
EHOMO (eV) –6.9727
EHOMO – 1 (eV) –7.9910
ELOMO – EHOMO (eV) 3.8858
181
Table 5.3. Thermodynamic properties of 8–hydroxy–5–nitroquinoline determined
at different temperatures with B3LYP/6–311++G(d,p) level.
T (K) S (J.mol–1.K–1) Cp (J.mol–1.K–1) ΔH0→T (kJ.mol–1)
100 304.91 72.48 5.17
150 339.06 98.25 9.43
200 371.15 126.53 15.04
250 402.52 155.67 22.09
298.15 432.31 183.25 30.26
300 433.44 184.29 30.6
350 463.91 211.34 40.5
400 493.78 236.17 51.69
450 522.91 258.52 64.07
500 551.2 278.4 77.51
600 605.01 311.5 107.07
700 655.05 337.45 139.57
800 701.51 358.09 174.38
900 744.68 374.78 211.05
1000 784.9 388.49 249.24
182
Table 5.4. The observed FTIR, FT–Raman and calculated frequencies using B3LYP/6–311++G(d,p) and HF/6–311++G(d,p) methods along
with their relative intensities, probable assignments and potential energy distribution (PED) of 8–hydroxy–5–nitroquinolinea.
Spec
ies
Observed
wavenumber(cm–1)
HF/6–311++G(d,p) Calculated
wavenumber
B3LYP/6–31G(d,p) Calculated
Wavenumber
Assignment
%PED FTIR FTR Unscaled
(cm–1)
Scaled
(cm–1)
IR
intensity
Raman
Activity
Unscaled
(cm–1)
Scaled
(cm–1)
IR
intensity
Depolari
zation
A 3571 vw 4057 3570 182.14 0.477 3553 3482 137.85 0.19 νOH 90OH
A 3116 vw 3446 3136 7.34 0.438 3284 3120 7.55 0.21 νCH 92CH
A 3094 vw 3401 3095 3.93 0.683 3248 3086 3.54 0.23 νCH 91CH
A 3081 vw 3368 3065 2.79 0.950 3228 3067 2.19 0.35 νCH 93CH
A 3060 m 3356 3054 11.01 1.426 3212 3051 10.78 0.25 νCH 92CH
A 3010 vw 3325 3026 17.32 0.928 3177 3018 18.97 0.44 νCH 91CH
A 1632 vw 1833 1613 232.95 0.091 1682 1598 61.07 0.34 C=C 93CC
A 1578 w 1801 1585 143.96 0.091 1647 1565 32.64 0.22 C=C 89CC
A 1564 s 1773 1560 115.28 0.518 1624 1543 68.50 0.47 C=C 91CC
A 1521 vs 1751 1541 217.01 0.100 1607 1527 229.66 0.72 νaNO2 92NO2
183
A 1474w 1661 1462 122.63 0.925 1557 1479 113.97 0.70 C=C 90CC
A 1423w 1416 w 1625 1430 16.33 0.369 1509 1434 43.28 0.25 C=N 91CN
A 1590 1399 682.58 1.883 1472 1398 57.85 0.48 C–C 89CC
A 1384 w 1556 1369 80.40 1.065 1439 1367 31.77 0.09 C–C 85CC
A 1351 s 1355 w 1541 1356 35.27 0.138 1420 1349 39.03 0.74 βOH 75OH + 15C–O
A 1330 s 1496 1316 79.89 0.315 1394 1324 23.53 0.29 C–C 91CC
A 1297 vs 1462 1287 169.04 0.333 1372 1303 417.29 0.11 νsNO2 92NO2
A 1252 vs 1412 1243 38.59 2.000 1330 1264 86.52 0.11 C–O 85CO
A 1364 1200 14.84 0.015 1298 1233 20.19 0.75 C–C 88CC
A 1204 vs 1199 w 1326 1167 245.25 0.498 1252 1189 117.00 0.27 νCNO2 89CN
A 1168 vs 1158 w 1282 1128 18.07 0.145 1217 1156 29.96 0.37 νCN 87CN
A 1265 1113 6.46 0.030 1183 1124 17.09 0.38 βCH 72CH + 12CCC
A 1117 s 1213 1067 11.31 0.116 1165 1107 25.22 0.09 βCH 64CH + 20CCC
A 1088 vw 1172 1031 14.21 0.036 1107 1052 10.83 0.17 βCH 70CH + 16CCC
A 1023 vw 1129 994 11.50 0.159 1074 1020 0.40 0.74 βCH 67CH + 18CCN
184
A 998 vw 990 vw 1126 991 0.19 0.013 1016 965 0.39 0.75 βCH 64CH + 21CN
A 1093 962 0.46 0.011 1012 961 17.78 0.75 γCH 60γCH + 16γCCC
A 1086 956 0.16 0.005 991 941 0.27 0.75 γCH 64γCH + 12γCCC
A 930 vw 887 vw 1067 939 22.21 0.097 972 923 0.05 0.21 γCH 56γCH + 22γCCC
A 847 s 855 vw 948 834 45.41 0.026 863 820 17.62 0.10 γCH 54γCH + 24γCCC
A 818 s 940 827 36.19 0.008 853 810 27.56 0.75 δNO2 64δNO2 + 12CH
A 800 s 793 s 897 789 47.92 0.003 826 785 23.51 0.75 γOH 66γOH + 18γCCN
A 874 778 2.35 0.058 815 774 5.37 0.09 γCH 56γCH + 20γCN
A 750 s 848 755 9.97 0.005 787 748 2.83 0.75 βCCC 57CCC + 18CH
A 732 s 806 717 19.52 0.013 745 708 12.32 0.75 βCCC 54CCC + 12CH
A 788 701 30.06 0.194 740 703 19.15 0.06 βCCC 52CCC + 16CH
A 705 627 3.60 0.009 728 692 123.92 0.57 βCNC 60CNC + 12CH
A 652 w 705 627 13.94 0.009 658 625 1.49 0.75 βCCN 52CCN + 16CH
A 605 w 655 583 33.07 0.032 652 619 0.31 0.24 βCCN 56CCN + 20CH
A 625 556 114.19 0.005 607 577 24.38 0.75 βCCC 54CCC + 18CH
185
A 566 vw 568 vw 612 545 4.25 0.000 590 561 0.00 0.74 βCCC 59CCC + 16CH
A 608 541 0.96 0.053 573 544 0.45 0.28 γCCC 53γCCC+15γCH
A 501 w 494 w 533 474 0.37 0.061 503 478 0.29 0.69 βC–O 55CO + 14CH
A 447 vw 505
449 0.00 0.001 477 453 0.00
0.75 γCCC 53γCCC+18γCH
A 425 w 432 vw 457
407 3.25
0.010 427 406 0.29
0.75 γCCC 54γCCC+14γCH
A 367 w 445 396 5.04 0.055 413 392 4.57
0.33 γC–O 59γCO+12γCH
A 375 334 1.41 0.005 350 333 1.72
0.04 βCN 60CN + 12CH
A 352 313 2.14 0.009 329 313 2.24
0.75 γCN 58γCN+14γCH
A 322 287 1.57 0.008 297 282 3.80
0.57 γCCC 56γCCC+12γCH
A 259 231 7.24 0.002 242 230 4.70
0.53 γCNC 52γCNC+18γCH
A 204 182 1.80 0.005 192 182 0.83
0.75 γCCC 53γCCC+20γCH
A 154 137 0.00 0.005 149 142 0.00
0.75 γCCN 51γCCN+22γCH
A 99 88 4.72 0.011 93 88 3.94
0.75 NO2 torsion
A 40 36 0.24 0.005 18 17 0.21 0.75 OH torsion aν–stretching; β–in–plane bending; δ–deformation; γ–out of plane bending and τ–torsion, wavenumbers, (cm–1); IR intensities, (KM/mole);
Raman scattering activities, (Å)4/(a.m.u).
186
Table 5.5. The observed FTIR, FT–Raman and calculated frequencies using B3LYP/6–31G(d,p) and B3LYP/cc–pVDZ force field along with
their relative intensities, probable assignments and potential energy distribution (PED) of 8–hydroxy–5–nitroquinolinea.
Spec
ies
Observed
wavenumber (cm–1)
B3LYP/6–311++G(d,p)
Calculated wavenumber
B3LYP/cc–pVDZ Calculated
wavenumber Assignment %PED
FTIR FTR Unscaled
(cm–1)
Scaled
(cm–1)
IR
intensity
Unscaled
(cm–1)
Scaled
(cm–1)
IR
intensity
A 3571 vw 3584 3512 142.50 3484 3414 152.85 νOH 94OH
A 3116 vw 3260 3130 7.45 3273 3142 7.85 νCH 92CH
A 3094 vw 3224 3095 4.23 3238 3108 4.40 νCH 93CH
A 3081 vw 3205 3077 2.09 3216 3087 2.26 νCH 95CH
A 3060 vw 3193 3065 10.60 3201 3073 9.56 νCH 90CH
A 3010 vw 3159 3033 16.61 3164 3037 16.50 νCH 93CH
A 1632 vw 1669 1602 52.98 1682 1615 63.36 C=C 90CC
A 1578 w 1632 1567 21.67 1643 1577 47.77 C=C 92CC
A 1564 s 1608 1544 62.15 1623 1558 55.87 C=C 94CC
A 1521 vs 1581 1518 149.58 1608 1544 116.52 νaNO2 92NO2
A 1474w 1543 1481 218.44 1552 1490 236.67 C=C 91CC
A 1423w 1416 w 1497 1437 52.90 1502 1442 43.79 C=N 93CN
187
A
1458 1400 58.00 1470 1411 40.77 C–C 90CC
A 1384 w
1428 1371 41.73 1430 1373 26.17 C–C 89CC
A 1351 s 1355 w 1407 1351 24.00 1409 1353 53.13 βOH 72OH + 14CO
A 1330 s
1378 1323 21.97 1394 1338 41.10 C–C 91CC
A 1297 vs
1351 1297 358.06 1370 1315 426.22 νsNO2 93NO2
A
1252 vs 1315 1262 352.36 1322 1269 235.96 C–O 87CO
A
1284 1233 3.34 1294 1242 14.68 C–C 89CC
A 1204 vs 1199 w 1244 1194 142.62 1245 1195 87.79 νCNO2 85CN
A 1168 vs 1158 w 1211 1163 27.59 1208 1160 27.69 νCN 87CN
A 1179 1132 20.51 1170 1123 15.39 βCH 75CH + 15CCC
A 1117 s 1159 1113 30.38 1153 1107 23.08 βCH 68CH + 20CCC
A 1088 vw
1100 1056 12.18 1098 1054 11.84 βCH 76CH + 14CCC
A 1023 vw
1068 1025 0.98 1069 1026 0.56 βCH 72CH + 18CCN
A 998 vw 990 vw 1020 979 0.16 1024 983 0.11 βCH 74CH + 18CN
A 1005 965 23.60 1007 967 21.91 γCH 66γCH + 21γCCC
A
992 952 0.63 999 959 0.01 γCH 64γCH + 18γCCC
A 930 vw 887 vw 974 935 0.03 974 935 0.02 γCH 62γCH + 24γCCC
A 847 s 855 vw 860 826 24.29 870 835 17.08 γCH 60γCH + 22γCCC
188
A 818 s
858 824 30.80 855 821 32.55 δNO2 62δNO2 + 12CH
A 800 s 793 s 821 788 31.54 830 797 12.71 γOH 66γOH + 21γCCN
A
818 785 3.49 816 783 8.78 γCH 61γCH + 25γCN
A 750 s 792 760 7.92 815 782 4.49 βCCC 70CCC + 18CH
A 732 s 738 708 19.31 748 718 2.20 βCCC 74CCC + 12CH
A
732 703 9.10 746 716 114.89 βCCC 72CCC + 16CH
A
708 680 116.39 739 709 18.33 βCNC 74CNC + 12CH
A 652 w 659 633 1.70 661 635 0.01 βCCN 70CCN + 15CH
A 605 w 656 630 0.02 657 631 1.66 βCCN 67CCN + 18CH
A
606 582 24.42 605 581 23.62 βCCC 64CCC + 21CH
A 566 vw 568 vw 575 552 0.04 593 569 0.02 βCCC 69CCC + 14CH
A
572 549 0.42 572 549 0.35 γCCC 63γCCC+18γCH
A 501 w 494 w 502 482 0.25 501 481 0.32 βC–O 65CO + 14CH
A 447 vw 475 456 0.02 481 462 0.00 γCCC 63γCCC+18γCH
A 425 w 432 vw 427 410 1.03 429 412 0.70 γCCC 64γCCC+16γCH
A 367 w 413 396 4.38 412 396 4.43 γC–O 63γCO+12γCH
A 348 334 1.97 348 334 1.66 βCN 66CN + 15CH
A 325 312 2.02 330 317 2.01 γCN 61γCN+16γCH
189
A 297 285 3.44 294 282 4.33 γCCC 64γCCC+12γCH
A 241 231 4.57 239 229 4.34 γCNC 60γCNC+18γCH
A 191 183 1.49 192 184 1.17 γCCC 56γCCC+21γCH
A 147 141 0.00 151 145 0.01 γCCN 54γCCN+22γCH
A 93 93 3.59 95 91 3.18 NO2 torsion
A 19 19 0.20 24 23 0.16 OH torsion aν–stretching; β–in–plane bending; δ–deformation; γ–out of plane bending; ω–wagging and τ–torsion, wavenumbers, (cm–1); IR intensities,
(KM/mole); Raman scattering activities, (Å)4/(a.m.u).
190
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