CHAPTER–IV
VIBRATIONAL, NUCLEAR MAGNETIC RESONANCE AND
ELECTRONIC SPECTRA, QUANTUM CHEMICAL
INVESTIGATIONS OF 2–AMINO–6–
FLUOROBENZOTHIAZOLE
4.1. Introduction
Indene and its heterocyclic analogs have wide and important biological and
industrial applications [1,2] of which benzothiophene and benzothiazole are sulfur–
containing derivatives. Benzothiazoline–2–thione (BTT) is a bicyclic heteroatomic
molecule and capable of bonding with metal ions via donor atoms (N and exocyclic S
atoms) to form heterocyclic ring metal complexes, and widely applied as an important
rubber vulcanisation accelerator [3], corrosion inhibitor [4,5] and effective flotation
reagent in the flotation of minerals [6–8]. The compounds containing a thiazole ring
have shown useful biological properties and have been developed as fungicides,
herbicides, or plant growth regulators [9]. The biological importance of thiazole
derivatives was emphasized during the period 1941–1945, when research on the
structure of the antibiotic penicillin showed the presence of a thiazolidine ring in this
important therapeutic agent [10]. 2–Aminothiazoles form an important class of
chemical species which are involved in numerous applications, including human and
veterinary medicine [11,12]. The 2–aminobenzothiazole molecule is known for its
local anesthetic action and has numerous applications in human and veterinary
medicine [13]. It is a metabolite of methabenzthiazuron [14] and is reported to form
the main fraction of soil–bound residues [15]. Anthelmintics exert their
chemotherapeutic effect by interfering with some biochemical or physiological
processes essential for the survival of the parasite in the host [16]. Several substituted
benzimidazoles and benzothiazoles [17–20] have been identified as potent
authelmintic drugs. Benzothiazoles constitute an important class of compounds with
profound interest to medicinal/industrial chemists as compounds bearing the
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benzothiazolyl moiety. They exhibit diverse biological properties such as antitumour
[21,22], antimicrobial [23], antiglutamate/antiparkinson [24], broad spectrum Ca2+
channel antagonist [25], inhibition of enzymes such as aldose reductase [26],
monoamine oxidase [27], lipoxygenase [28], cyclooxygenase [29], acetylcholine
esterase [30], thrombine [31], proteases [32], H+–K+ ATpase [33], carbonic anhydrase
[34], HCV helicase [35], plant growth regulation [36] and have industrial applications
such as antioxidants [37].The combination of DFT calculations of chemical shifts and
harmonic vibrations with nuclear magnetic resonance (NMR) and IR/Raman
experimental parameters, respectively, has become an accepted technique to gather
insight into the molecular structure, although according to our best knowledge no
evidence of similar studies for the 2–amino–6–fluorobenzothiazole (2A6FBT) have
been reported in the chemical literature to date.
To shed more light on the accurate prediction of the geometry, the
thermodynamical properties and to determine the energy of the compound under
study and IR/Raman and NMR spectroscopic characteristics of 2A6FBT in the solid
phase detailed DFT calculations have been performed. In this work, interest resides in
correlating the theoretically predicted optimised geometrical parameters, harmonic
vibrations, NMR parameters and thermodynamic properties of 2A6FBT revealed by
high level DFT calculations with the accurate experimental Raman and NMR results.
To provide possible explanations for the experimentally observed vibrational
fundamentals and to understand the effect of fluoro and amino group on the
characteristic frequencies of the benzene ring moiety normal coordinate analysis has
been carried out by utilising the force constants obtained from the B3LYP/
6–311++G(d,p) method.
4.2. Experimental
The compound under investigation 2–amino–6–fluorobenzothiazole was
purchased from Aldrich chemicals, U.S.A, and is used as such to record the FTIR,
FT–Raman spectra, 1H and 13C and UV–Visible spectra. The FTIR spectrum of
2A6FBT 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 spectral resolution was 2 cm–1. The FT–Raman spectrum of
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the compound 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
operating at 1.064 m with 200 mW power. A liquid nitrogen cooled–Ge detector was
used. The frequencies of all sharp bands are accurate to 2 cm–1. 1H and 13C nuclear
magnetic resonance (NMR) (400 MHz; CDCl3) spectra were recorded on a Bruker
HC400 instrument. Chemical shifts for protons are reported in parts per million scales
(δ scale) downfield from tetramethylsilane. The absorption spectrum of the compound
was also recorded with the Shimadzu UV/Visible spectrophotometer. The band width
on half height is 4.5 nm.
4.3. Computational methods
To provide complete information regarding to the structural characteristics and
the fundamental vibrational modes of 2A6FBT the LCAO–MO–SCF restricted
Hartree–Fock and DFT–B3LYP correlation functional calculations have been carried
out. The calculations of geometrical parameters in the ground state were performed
using the Gaussian 03 [38] program, invoking gradient geometry optimisation [39] on
Intel core i3/2.93 GHZ processor. The geometry optimisation was carried out at
Hartree–Fock and DFT methods adopting 6–31G(d,p), triple–ζ 6–311++G(d,p) and
Dunning’s cc–pVDZ basis sets to characterise all stationary points as minima. The
optimised structural parameters of 2A6FBT were used for harmonic vibrational
frequency calculations resulting in IR and Raman frequencies together with intensities
and Raman depolarisation ratios. In DFT methods, Becke’s three parameter exact
exchange–functional (B3) [40–42] combined with gradient–corrected correlational
functional of Lee, Yang and Parr (LYP) [43] by implementing the split–valence
polarised 6–31G(d,p), triple–ζ 6–311++G(d,p) and Dunning’s cc–pVDZ basis sets
[44,45] have been utilised for the computation of molecular structure optimisation,
vibrational frequencies, thermodynamic properties and energies of the optimised
structures. The force field obtained from B3LYP/6–311++G(d,p) method has also
been utilised to perform normal coordinate analysis using Wilson’s FG matrix [46–
48] method with the perturbation program written by Fuhrer et al. [49]. The potential
energy distribution corresponding to each of the observed frequencies shows the
reliability and accuracy of the spectral analysis.
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C5
C4
C8
C9
C7
C6
N3
C2
S1
H11
F10
H12
H13
N14
H15
H16
The isotropic chemical shifts are frequently used as an aid in identification of
organic compounds and accurate predictions of molecular geometries are essential for
reliable studies of magnetic properties. The B3LYP method allows calculating the
shielding constants with accuracy and the GIAO method is one of the most common
approaches for calculating nuclear magnetic shielding tensors. The 1H and 13C NMR
isotropic shielding were calculated using the GIAO method [50,51] using the
optimized parameters obtained from B3LYP/6–311++G(d,p) method. The effect of
solvent on the theoretical NMR parameters was included using the default model
IEF–PCM provided by Gaussian 03. The isotropic shielding values were used to
calculate the isotropic chemical shifts δ with respect to tetramethylsilane (TMS).
δiso(X) = σTMS(X) – σiso(X), where δiso – isotropic chemical shift and σiso – isotropic
shielding. UV–vis spectra, electronic transitions, vertical excitation energies and
oscillator strengths were computed with the time–dependent DFT method. The
electronic properties such as HOMO and LUMO energies were determined by time–
dependent DFT (TD–DFT) approach, while taking solvent effect into account
[52–55].
4.4. Results and discussion
4.4.1. Molecular Geometry
The general molecular structure and the scheme of the atom numbering of
2–amino–6–fluorobenzothiazole are represented in Figure 4.1.
Figure 4.1. Molecular structure and atom numbering scheme of 2–amino–6–
fluorobenzothiazole
The geometry of the 2A6FBT molecule is considered by possessing CS point
group symmetry. The 42 fundamental modes of vibrations of 2A6FBT are distributed
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into the irreducible representations as 29 in–plane vibrations of A species and 13 out
of plane vibrations of A species i.e., vib = 29A + 13A.
4.4.2. Structural properties
The optimised structural parameters bond length, bond angle and dihedral
angle for the energetically and thermodynamically preferred geometry of 2A6FBT
determined by B3LYP and HF methods with 6–31G(d,p), 6–311++G(d,p), cc–pVDZ
methods are presented in Table 4.1 in accordance with the atom numbering scheme of
the compound given in the Figure 4.1. Overall, results obtained using B3LYP method
with 6–31G(d,p), 6–311+G(d,p) and cc–pVDZ basis sets are very similar; with only
slight differences. From the structural data given in Table 4.1 it is observed that the
various benzene ring C–C bond distances and the C–H bond lengths of 2A6FBT are
found to be almost same at all levels of calculations. The bond lengths determined
from B3LYP method with 6–31G(d,p) and cc–pVDZ basis sets are slightly higher
than that obtained from other methods but it yields bond angles in excellent
agreement with each other and also with the experimental values.
The influence of the substituents on the skeletal molecular parameters of
2A6FBT seems to be negligibly small. In the benzene ring the C5–C6 bond length is
slightly longer than C6–C7. The longest bond distance observed in the benzene ring is
C8–C9. This is due to the fusion of thiazoline moiety at these carbons. Comparing the
bond distances of the hetero ring, all varies significantly from each other due to the
variation of electro negativities of the atoms. The S1–C2 bond distance is the longest
while the C2–N3 is the shortest. The longest S1–C2 distance attributes the pure single
bond character. Due to ring strain the C2–N3 bond distance is 0.08 Å shorter than
C2–N14. The mean C–H bond length determined by B3LYP/6–311G(d,p) method is
1.083 Å while the mean N–H bond distance is 1.009 Å.
With the electron donating substituents on the benzene ring, the symmetry of
the ring is distorted, yielding ring angles smaller than 120o at the point of substitution
and slightly larger than 120o at the ortho and meta positions [56]. Due to the electron
withdrawing nature of fluorine atom, it is observed that in 2A6FBT molecule the bond
angle at the point of substitution C5–C6–C7 is 123.2o while the bond angles in at
ortho to the substituted carbon, C4–C5–C6 and C6–C7–C9 positions are found to be
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around 119.4 and 116.6 degree, respectively. The meta position angle C5–C4–C8 is
also observed as less than 120o and is 119.7o. The another angle of meta position to
the fluorine attachment C7–C9–C8 is more than 120o because of the fusion of the
hetero ring with the C8–C9 bond. More distortion in bond parameters is observed in
the hetero ring than benzene ring. The variation in bond angle depends on the electro
negativity of the central atom. If the electro negativity of the central atom decreases,
the bond angle decreases. Thus the bond angle C9–S1–C2 is very less (87.9o) than the
bond angle C8–N3–C2 (111.1o) is the reason of the high electro negativity of nitrogen
than sulphur. The computed structural parameters were correlated with the
structurally related molecule benzothiazoline–2–thione [57]. The discrepancies of the
bond parameters are due to the presence of the substituents fluorine atom and the
amino group in the 2A6FBT molecule.
The thermodynamic parameters of the compound total thermal energy,
vibrational energy contribution to the total energy, the rotational constants and the
dipole moment values obtained from HF method is DFT methods with 6–31G(d,p),
6–311++G(d,p) and cc–pVDZ basis sets are presented in Table 4.2. The energy of the
compound 2A6FBT determined by B3LYP/6–311++G(d,p) method is –877.460
Hartrees. From Table 4.2 it is observed that the SCF energy determined by B3LYP/
6–311G++(d,p) method is the least and the corresponding geometry is most stable
than the geometry of 2A6FBT obtained by other methods. Hence for correlation
discussion the parameters determined from the B3LYP/6–311G++(d,p) method is
taken into account and is more reliable. The dipole moment of 2A6FBT is 3.372 D
due to the presence of charge separation. The natural charges determined by natural
bond orbital (NBO) analysis by B3LYP/6–311++G(d,p) method is presented in the
Table 4.3. The more positive charge on C6 carbon atom is due to the attachment of
fluorine atom with that carbon atom. This is caused by the –I effect of bromine atom.
When compared the charges of the aromatic ring carbon atoms, less positive
charge is observed in the C8 carbon atom which is attached to the highly
electronegative nitrogen (N3). The high negative charge at N14 and a positive charge
at the carbon atom C2 reveal the high electron attracting nature of the nitrogen atom.
Molecular electrostatic potential (MESP) mapping is very useful in the
investigation of the molecular structure with its physiochemical property relationships
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[58–61]. Total electron density and molecular electrostatic potential of 2–amino–6–
fluorobenzothiazole are shown in Figure 4.2. The molecular electrostatic potential
surface MESP which is a 3D plot of electrostatic potential mapped onto the iso–
electron density surface simultaneously displays molecular shape, size and
electrostatic potential values and has been plotted for 2A6FBT using DFT method.
The colour scheme for the MSEP surface is red–electron rich or partially negative
charge; blue–electron deficient or partially positive charge; light blue–slightly
electron deficient region; yellow–slightly electron rich region, respectively. The
MESP of 2A6FBT shows clearly the electron rich centers of nitrogen and sulphur.
The predominance of blue region in the total density surface corresponds to a net
positive potential of 2A6FBT.
Figure 4.2. (a) Total electron density and (b) Electrostatic potential of
2–amino–6–fluorobenzothiazole.
4.5. Vibrational Analysis
The observed FTIR and FT–Raman spectra of 2A6FBT along with the
simulated infrared and Raman spectra are shown in Figure 4.3 and 4.4. This reveals
good correspondence between theory and experiment in main spectral features. The
observed and calculated frequencies using HF and B3LYP methods with 6–31G(d,p),
cc–pVDZ and 6–311++G(d,p) basis sets along with their relative intensities, probable
assignments and potential energy distribution (PED) are summarised in
Tables 4.4 and 4.5.
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Normal mode animations were made possible with GaussView 5.0.8 and
Chemcraft programs [62,63]. Proposed assignments of experimental and simulated
vibrational modes in this study are based upon comparisons and matching of
calculated and observed FTIR spectrum, and visual inspection of eigenvectors for
each normal mode.
4.5.1 Carbon–carbon and carbon–nitrogen vibrations
The aromatic ring carbon–carbon stretching modes are expected in the range
from 1650 to 1200 cm–1. Benzene has two degenerate modes at 1596 cm–1 (e2g) and
1485 cm–1 (e1u). Similarly the frequency of two non–degenerate modes observed at
1310 cm–1 (b2u) and 995 cm–1 (a1g) in benzene [64]. The frequency of e2g degenerate
pair in benzene is fairly insensitive to substitution. Similarly the frequency of e1u
vibrations pair is also not very sensitive to substitution, though heavy halogens
diminish the frequency [65]. The spectral assignment was made by Mille et al. [66]
for benzothiophene and, from the same research group, by Panizzi et al. [67] for
benzothiazole. The carbon–carbon stretching modes of the phenyl group are expected
in the range from 1650 to 1200 cm–1. In 2A6FBT, the carbon–carbon stretching bands
are appeared in the infrared spectrum at 1602, 1573, 1459, 1342 and 1300 cm–1. The
corresponding C–C stretching modes are observed in the Raman spectrum at 1610,
1577, 1464, 1421 and 1310 cm–1. The medium band occurring at 1554 cm–1 in Raman
is assigned to the C=N stretching vibration. The band observed in the infrared
spectrum at 1275 and in Raman spectrum at 1271 cm–1 are assigned to the skeletal
C–N stretching vibration. The more electron withdrawing nature of fluorine atom in
the six membered ring lowered the C–C stretching vibrations. The strong band
observed at 1237 cm–1 in infrared is attributed to the C–N(H2) stretching vibration.
The C–N(H2) in–plane and out of plane bending modes are observed at 250 and 115
cm–1 in the Raman spectrum, respectively. The CN in–plane bending significantly
coupled with 15–20 percent of CNC in–plane bending mode and vice versa.
122
Figure 4.3. FTIR spectrum of 2–amino–6–fluorobenzothiazole
The CCC in–plane bending vibrations are obtained from the non–
degenerate band at 1010 cm–1 (b1u) and degenerate modes 606 cm–1 (e2g) of benzene.
Likewise, the CCC out of plane bending modes is defined with reference to 703
cm–1 (b2g) and degenerate 404 cm–1 (e2u) modes of benzene [64]. The bands occurring
at 918 cm–1 in infrared and 909 cm–1 in Raman spectra and the calculated values by
B3LYP/6–311++G(d,p) method 817 and 710 cm–1 are assigned to the CCC
in–plane bending modes of 2A6FBT. The bands occurring at 705, 697 and 528 cm–1
are assigned to the C–N–C and C–S–C in–plane bending modes, respectively. The
results are in good agreement with the literature values [68–70].
123
Normal coordinate analysis shows that significant mixing of C–C–C in–plane
bending with C–H in–plane bending occurs. Similarly the skeletal out of plane
bending modes are overlapped with C–H out of plane bending modes significantly.
Figure 4.4. FT–Raman spectrum of 2–amino–6–fluorobenzothiazole
4.5.2 C–H Vibrations
The aromatic CH stretching vibrations are normally found between 3100 and
3000 cm–1. In this region the bands are not affected appreciably by the nature of
substituents. The aromatic CH stretching frequencies arise from the modes observed
at 3062 (a1g), 3047 (e2g), 3060 (b1u) and 3080 (e1u) cm–1 of benzene and its derivatives
[64]. The aromatic compounds show C–H stretching vibrations around 3100–3000
cm–1 range. In 2A6FBT these modes are observed at 3091, 3070 and 3047 cm–1 in the
Raman spectrum. The aromatic CH in–plane bending modes of benzene and its
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derivatives are observed in the region 13001000 cm–1. Studies on the spectra of
benzene shows that there are two degenerate e2g (1178 cm–1) and e1u (1037 cm–1) and
two non–degenerate b2u (1152 cm–1) and a2g (1340 cm–1) vibrations involving the CH
in–plane bending [64]. The C–H out of plane bending modes [68–71] usually medium
intensity arises in the region 950–600 cm–1.
In the case of 2A6FBT the bands observed at 1188, 1043 cm–1 in IR and at
1182 and 1045 cm–1 in Raman spectrum are assigned to the C–H in–plane bending
vibrations. The CH out of plane bending mode of benzene derivatives are observed
in the region 1000 to 600 cm–1. The CH out of plane bending results from b2g
(985 cm–1), e2u (970 cm–1), e1g (850 cm–1) and a2u (671 cm–1) modes of benzene [64].
The aromatic CH out of plane bending vibrations of 2A6FBT are assigned to
the medium to very strong bands observed at 848 and 771 cm–1 in the infrared
spectrum. 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.
4.5.3 Amino group vibrations
It is stated that in amines, the N–H stretching vibrations occur in the region
3500–3300 cm–1. The asymmetric –NH2 stretching vibration appears from 3500 to
3420 cm–1 and the symmetric –NH2 stretching is observed in the range 3420 to 3340
cm–1. With the above reference, the vibrational frequencies observed at 3382 and
3260 cm–1 in the infrared spectrum are assigned to the –NH2 asymmetric and
symmetric stretching modes, respectively.
For primary amino group the in–plane –NH2 deformation vibration occur in
the short range 1650 to 1580 cm–1 region of the spectrum. Therefore the medium band
observed in IR at 1631 cm–1 in IR is assigned to the deformation mode of the amino
group. Likewise, the in–plane bending –NH2 rocking and the out of plane wagging
vibrations are also assigned in the expected range. The amino in–plane bending
rocking mode normally appears in the range 1150–900 cm–1 while the wagging bands
between 850 and 500 cm–1.
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Therefore, the bands at 1111, 508 and 294 cm–1 are attributed to the amino
rocking, wagging and twisting modes, respectively. The –NH2 deformation vibrations
are not much affected by the hetero ring. These amino vibrations are also in good
agreement with literature values of aniline [71], 4–aminoquinaldine [72] and
5–aminoquinoline [73]. Considerable overlapping between wagging and twisting
–NH2 out of plane bending modes occurs and is confirmed from the PED.
4.5.4. CF vibrations
The CF absorption is observed in the broad region between 680 and
520 cm–1. The strong band in IR at 603 cm–1 having a weak Raman counterpart is
assigned to the CF stretching. The CF in–plane bending and out of plane bending
modes are observed in Raman spectrum at 415 and 231 cm–1, respectively. These
assignments are in good agreement with the literature [65,74,75]. The CF in–plane
mode is significantly overlapped with CH in–plane bending mode and vice versa.
4.6. Scale factors
A better agreement between the computed and experimental frequencies can
be obtained by using different scale factors for different types of fundamental
vibrations. To determine the scale factors, the procedure used previously [76–84]
have been followed that minimises the residual separating experimental and
theoretically predicted vibrational frequencies or thermodynamic quantities. The
optimum scale factors for vibrational frequencies were determined by a least–squares
procedure of the scaled harmonic frequencies to the experimental fundamentals,
minimising the residual
N
i
2Expti
Theori νλωΔ
where, Theoiω and
Exptiν are the i
th theoretical harmonic frequency and ith
experimental fundamental frequency (in cm–1), respectively and N is the number of
frequencies included in the optimisation which leads to
NΔRMS
126
The scale factors used in this study minimised the deviations between the
computed and experimental frequencies at HF, B3LYP and B3PW91 methods of
calculations. A uniform scaling factor of 0.98 is thus recommended for all frequencies
< 1800 cm–1 at the B3LYP method with 6–31G(d,p)/6–311++G(d,p) basis sets and is
adopted in this study.Due to the large anharmonicities of C–H and N–H stretching
frequencies > 2700 cm–1 were scaled by two different scale factors [82,83]. A scale
factor of 0.92 for N–H stretching, 0.96 for C–H stretching and 0.98 for all other
vibrational modes are used in B3LYP/6–311++G(d,p) method. In B3LYP with
6–31G(d,p) and cc–pVDZ basis sets 0.92, 0.95 and 0.98 is used for N–H, C–H
stretching and all other vibrations, respectively.
In HF/6–311++G(d,p) method the scale factor 0.86 for N–H stretching and
0.91 for C–H stretching and all other vibrational modes are used. DFT–B3LYP/
6–311++G(d,p) correction factors are all very much closer to unity and the
frequencies are much closer to the experimental values and more reliable. The
determined RMS deviation for DFT method is 13 while for HF method is 18. The
correlation diagram for the calculated and the experimental frequencies of 2A6FBT
are shown in Figure 4.5.
127
Figure 4.5. Correlation between the calculated and the experimental frequencies of
2–amino–6–fluorobenzothiazole
128
4.7. NMR Spectral studies
The observed 1H and 13C NMR spectra of the compound 2A6FBT are given in
the Figures 4.6 and 4.7, respectively. The 1H and 13C theoretical and experimental
chemical shifts, isotropic shielding tensors and the assignments of 2A6FBT are
presented in Table 4.6.
Figure 4.6. 1H NMR spectrum of 2–amino–6–fluorobenzothiazole
Figure 4.7. 13C NMR spectrum of 2–amino–6–fluorobenzothiazole
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1H atom is mostly localised on periphery of the molecules and their chemical
shifts would be more susceptible to intermolecular interactions in the aqueous
solutions as compared to that for other heavier atoms. Aromatic carbons give signals
in overlapped areas of the spectrum with chemical shift values from 100 to 200 ppm
[85,86]. The cumulative –I effect of nitrogen and sulphur in the hetero ring of
2A6FBT reduces the electron density of the carbon atom C2, thus its NMR signal is
observed in the very downfield at 165.34 ppm. Due to the influence of electronegative
fluorine atom and the deshielding effect, the chemical shift value of C6 attributed to
the downfield NMR signal position 160.24 ppm. The carbon atoms C4, C5 and C7 are
significantly observed in the upfield with chemical shift values 120.06, 113.91 and
107.79 ppm, respectively reveals that the influences of the electronegative nitrogen,
fluorine and sulphur atoms are negligibly small and their signal are observed in the
normal range. The chemical shift positions of other carbons of 2A6FBT lie in the
range 157.85 to 120.06 ppm. Comparing the chemical shift positions of C8 and C9
carbon atoms 157.85 and 148.85 ppm, respectively, the higher chemical shift of C8 is
due to the more electron withdrawing nature of nitrogen than sulphur.
1H chemical shifts of 2A6FBT were obtained by complete analysis of their
NMR spectra and interpreted critically in an attempt to quantify the possible different
effects acting on the shielding constant of protons. The hydrogen atoms H11, H12 and
H13 present in the benzene ring of 2A6FBT shows NMR peaks in the normal range of
aromatic hydrogen atoms and are assigned to 7.29, 7.02 and 7.46 ppm, respectively.
The upfield substituent effect of N3 and S1 atoms is most pronounced on the chemical
shifts of the benzenoid ring protons H11 and H13. The effect on H11 is to be expected
since it is ortho to the electron releasing S1. However, the magnitude of the effect on
the H13 proton is due to the more electronegative nitrogen (N3). The mesomeric (+M)
effect of fluorine is another reason for the difference in chemical shifts of benzenoid
protons. The amino protons are put into the up filed 4.31 ppm shows that these
protons are under high magnetic shielding.
The calculated and experimental chemical shift values are given in Table 4.6
shows a good agreement with each other. The linear regression between the
experimental and theoretical 1H and 13C NMR Chemical shifts are represented in
Figure 4.8 while the linear regression between the experimental chemical shift and
130
theoretical 1H and 13C isotropic shielding are represented in Figure 4.9. The protons
are located on the periphery of the molecule and thus are supposed to be more
susceptible to molecular solute–solvent effects than the carbon atoms and usually the
agreement between the experimental and calculated shifts for 1H is worse than for 13C
[87].
Figure 4.8. The linear regression between the experimental and theoretical (a)
1H and (b) 13C NMR Chemical shifts of 2–amino–6–fluorobenzothiazole
131
Figure 4.9. The linear regression between the experimental chemical shift and
theoretical 1H and 13C isotropic shielding of 2–amino–6–fluorobenzothiazole
4.8. Electronic properties
The energies of four important molecular orbitals of 2A6FBT: the second
highest and highest occupied MO’s (HOMO and HOMO–1), the lowest and the
second lowest unoccupied MO’s (LUMO and LUMO +1) were calculated and are
presented in Table 4.7. The lowest singlet → singlet spin–allowed excited states were
taken into account for the TD–DFT calculation in order to investigate the properties
of electronic absorption. The experimental λmax values are obtained from the
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UV/Visible spectra recorded in CHCl3. The Figure 4.10 depicts the observed and the
theoretical UV–Visible spectra of 2A6FBT.
Figure 4.10. (a) Observed and (b) Theoretical UV–Visible spectra of 2A6FBT
The calculations were also performed with CHCl3 solvent effect. The
calculated absorption wavelengths (λmax) and the experimental wavelengths are also
given in Table 7. The energy gap between HOMO and LUMO is a critical parameter
in determining molecular electrical transport properties [88]. In the electronic
absorption spectrum of 2A6FBT, there are two absorption bands with a maximum
290.8 and 259.2 nm. The strong absorption band at 259.2 nm is caused by the π–π*
transitions and the other weak intense band is due to n–π* transition [89, 90]. The
π–π* transitions are expected to occur relatively at lower wavelength, due to the
consequence of the extended aromaticity of the benzene ring. The 3D plots important
molecular orbitals are shown in Figure 4.11. The energy gap of HOMO–LUMO
explains the eventual charge transfer interaction within the molecule, and the frontier
orbital gap in case of 2A6FBT is found to be 5.275 eV obtained at TD–DFT method
using 6–311++G(d,p) method. Closer examination (Figure 4.11) shows the electron
density in the HOMO mostly centered on the heterocyclic moiety and part of the
benzene ring while in LUMO the electron density predominantly located on the
benzene ring; indicating a charge transfer of the type π–π* and n–π* upon excitation.
133
While agreement between the calculated and experimental max values of 2A6FBT is
evident, the calculated bands are red–shifted by ~ 20 and ~ 6 nm.
Figure 4.11. The HOMO, LUMO, HOMO–1 and LUMO+1 orbital of
2–amino–6–fluorobenzothiazole
134
4.9. Conclusions
The geometry of 2A6FBT was optimised in different levels with DFT–B3LYP
method using 6–31G(d,p), triple–ζ 6–311++G(d,p) and Dunning’s cc–pVDZ basis
sets and HF/6–311++G(d,p) method. The complete molecular structural parameters
and thermodynamic properties of the optimised geometry of the compound have been
obtained from ab initio and DFT calculations. The computed geometries are
benchmarks for predicting crystal structural data of the molecule. The vibrational
frequencies of the fundamental modes of the compound have been precisely assigned
and analysed and the theoretical results were compared with the experimental
vibrations. The observed and B3LYP/6–311++G(d,p), 6–31G(d,p) and cc–pVDZ)
infrared frequencies show similar profiles in both position and intensities making
normal mode assignments with confidence. 1H and 13C NMR spectra were recorded
and the 1H and 13C NMR isotropic chemical shifts were calculated. The assignments
made were compared with the experimental values. The UV–Visible spectrum was
also recoded and the energies of important MO’s and the λmax of the compound were
also determined from TD–DFT method using 6–311++G(d,p) basis set. The infrared
and UV/Visible spectra are analysed in detail, and interpreted with the help of density
functional theory (DFT) calculations. The relative stabilities, HOMO–LUMO gaps
and implications of the electronic properties are examined and discussed. The
electronic transitions of the compound were discussed. Thus the present investigation
provides complete vibrational assignments, structural informations, chemical shifts
and electronic properties of the compound.
135
Table 4.1. Structural parameters calculated for 2–amino–6–fluorobenzothiazole employing B3LYP/6–311++G(d,p),
HF/6–311++G(d,p), B3LYP/6–31G(d,p) and B3LYP/cc–pVDZ methods.
Structural
Parameters
2–amino–6–fluorobenzothiazole
Experimentala B3LYP/
6–31G(d,p)
HF/6–311++
G(d,p)
B3LYP/ 6–
311++G(d,p) B3LYP/cc–pVDZ
Internuclear Distance (Å)
S1–C2 1.794 1.766 1.790 1.796 1.732
C2–N3 1.293 1.267 1.290 1.295 1.353
N3–C8 1.388 1.388 1.387 1.389 1.380
C4–C5 1.392 1.381 1.391 1.395 1.353
C5–C6 1.395 1.384 1.392 1.397 1.384
C6–C7 1.390 1.373 1.386 1.391 1.395
C7–C9 1.393 1.385 1.392 1.396 1.386
C8–C9 1.417 1.392 1.414 1.419 1.387
C4–C8 1.400 1.389 1.398 1.402 1.391
S1–C9 1.761 1.751 1.759 1.764 1.740
C2–N14 1.371 1.364 1.370 1.374
C6–F10 1.352 1.330 1.359 1.354
136
C4–H13 1.085 1.074 1.083 1.091
C5–H12 1.084 1.074 1.083 1.091
C7–H11 1.084 1.074 1.082 1.090
N14–H15 1.010 0.994 1.008 1.015
N14–H16 1.011 0.996 1.010 1.017
Bond angle (Degree)
C9–S1–C2 87.831 88.114 87.908 87.770 92.3
S1–C2–N14 119.268 119.223 119.362 119.202
C3–C2–N14 124.367 124.362 124.464 124.353
C3–C2–S1 116.275 116.357 116.088 116.351 109.2
C8–N3–C2 110.807 111.026 111.112 110.756 116.4
C5–C4–C8 119.694 119.439 119.662 119.753 120.5
C5–C4–H13 121.047 120.883 120.922 121.066
C8–C4–H13 119.260 119.678 119.416 119.181
C4–C5–C6 119.517 119.518 119.368 119.610 118.6
C6–C5–H12 119.023 119.152 119.244 118.950
C4–C5–H12 121.460 121.330 121.388 121.440
C5–C6–C7 122.920 122.877 123.213 122.755 121.6
137
a– values taken from Ref. [57].
C7–C6–F10 118.392 118.585 118.244 118.474
C5–C6–F10 118.688 118.539 118.543 118.771
C6–C7–C9 116.766 116.643 116.582 116.844 120.6
C6–C7–H11 120.149 120.463 120.335 120.107
C9–C7–H11 123.085 122.894 123.083 123.049
C4–C8–C9 118.963 119.216 119.00 118.823 120.9
C4–C8–N3 125.193 125.150 125.381 125.279
C9–C8–N3 115.841 115.632 115.617 115.895 112.0
C7–C9–C8 122.140 122.306 122.174 122.216 117.6
C7–C9–S1 128.620 128.827 128.556 128.561
C8–C9–S1 109.240 108.867 109.270 109.223 110.0
C2–N14–H15 117.521 117.361 118.202 116.542
C2–N14–H16 113.507 114.003 114.510 113.031
H15–N14–H16 114.679 114.591 115.148 113.639
138
Table 4.2. The calculated thermodynamic parameters of 2–amino–6–fluorobenzothiazole employing
B3LYP/6–311++G(d,p), HF/6–311++G(d,p), B3LYP/6–31G(d,p) and B3LYP/cc–pVDZ methods.
Thermodynamic parameters (298 K)
2–amino–6–fluorobenzothiazole
B3LYP/
6–311++ G(d,p)
HF/
6–311++G(d,p)
B3LYP/
6–31G(d,p)
B3LYP/
cc–pVDZ
SCF Energy (Hartree) –877.460 –873.983 –877.305 –877.348
Total Energy (thermal), Etotal (kcal.mol–1) 74.814 79.630 75.225 75.008
Heat Capacity at const. volume, Cv
(cal.mol–1.K–1) 34.827 32.201 34.682 34.553
Entropy, S (cal.mol–1.K–1) 91.424 89.123 91.370 91.258
Vibrational Energy, Evib (kcal.mol–1) 73.037 77.852 73.447 73.230
Zero–point vibrational Energy, E0
(kcal.mol–1) 69.339 74.554 69.766 69.572
Rotational Constants (GHz)
A 2.657 2.721 2.646 2.657
B 0.622 0.631 0.621 0.622
C 0.504 0.512 0.503 0.504
139
Dipolemoment (Debye)
μx 3.207 3.242 2.852 3.260
μy –0.349 –0.334 –0.214 –0.282
μz 0.804 0.867 0.932 0.776
μtotal 3.325 3.372 3.007 3.362
140
Table 4.3. The natural charges determined by natural bond orbital (NBO) analysis of 2–amino–6–fluorobenzothiazole
using B3LYP/6–311++G(d,p) method.
Atom S1 C2 N3 C4 C5 C6 C7 C8 C9
Charge 0.3190 0.2934 –0.5285 –0.1805 –0.2655 0.4069 –0.2789 0.1114 –0.1885
Atom N14 F10 H11 H12 H13 H14 H15
Charge –0.7903 –0.3571 0.2289 0.2235 0.2199 0.3874 0.3989
141
Table 4.4. The observed FTIR, FT–Raman and calculated frequencies using B3LYP/6–311++G(d,p) and HF/6–311++G(d,p) force field
along with their relative intensities, probable assignments and potential energy distribution (PED) of
2–amino–6–fluorobenzothiazolea.
Spec
ies
Observed
wavenumber (cm–1)
HF/6–311++G(d,p) Calculated
wavenumber
B3LYP/6–311++G(d,p)
Calculated wavenumber Depolari
sation
ratio
Assignment %PED
FTIR FTR Unscaled
(cm–1)
Scaled
(cm–1)
IR
intensity
Raman
Activity
Unscaled
(cm–1)
Scaled
(cm–1)
IR
intensity
A 3382 w 3907 3360 50.78 54.50 3681 3387 35.66 0.74 νaNH2 95NH
A 3260 w 3793 3262 78.68 167.74 3559 3274 72.40 0.13 νsNH2 93NH
A 3091 m 3366 3063 3.87 170.08 3206 3078 3.65 0.18 νCH 92CH
A 3060 m 3070 m 3362 3059 1.54 77.83 3205 3077 0.94 0.33 νCH 90CH
A 3037 m 3047 w 3350 3049 3.04 56.19 3192 3064 3.16 0.72 νCH 93CH
A 1631 m 1812 1649 401.10 20.89 1668 1635 368.10 0.34 δNH2 93NH2
A 1602 m 1610 w 1791 1630 32.62 52.73 1638 1605 9.09 0.39 νCC 90CC
A 1573 m 1577 m 1758 1600 31.12 15.86 1609 1577 36.58 0.73 νCC 92CC
A 1554 m 1743 1586 124.44 50.35 1595 1563 91.31 0.23 νC=N 94CN
A 1459 s 1464 m 1629 1482 288.63 22.80 1490 1460 208.42 0.55 νCC 93CC
A 1421 vw 1556 1416 1.70 2.38 1439 1410 3.16 0.26 νCC 87CC
142
A 1342 m 1440 1310 167.99 15.87 1344 1317 14.70 0.10 νCC 89CC
A 1300 m 1310 vw 1407 1280 8.27 51.45 1316 1290 134.45 0.19 νCC 87CC
A 1275 m 1271 vs 1383 1259 79.64 17.19 1293 1267 10.29 0.03 νCN 89CN
A 1237 s 1319 1200 41.19 6.46 1270 1245 42.65 0.72 νCN(H2) 88CN
A 1188 m 1182 m 1271 1157 1.64 40.40 1208 1184 39.23 0.26 βCH 76CH + 14CCC
A 1212 1103 24.81 1.69 1130 1107 2.78 0.48 βCH 75CH + 15CCC
A 1111 m 1111 vw 1196 1088 3.93 8.97 1109 1087 26.40 0.29 ρNH2 77ρNH2 + 12CN
A 1043 m 1045 w 1145 1042 12.12 0.94 1053 1032 12.49 0.57 βCH 68CH + 20CCC
A 1067 971 0.20 0.10 953 934 0.54 0.66 γCH 67γCH + 16γCCC
A 918 m 909 s 997 907 28.97 18.57 926 907 33.11 0.06 βCCC 74CCC + 12CH
A 848 m 972 885 42.68 0.09 853 836 48.29 0.73 γCH 69γCH + 15γCCC
A 805 s 927 844 32.93 0.36 834 817 54.20 0.71 βCCC 70CCC + 18CH
A 771 vs 870 792 56.16 1.89 800 784 15.73 0.75 γCH 66γCH + 21γCCC
A 731 m 795 723 1.13 1.55 724 710 0.05 0.71 βCCC 67CCC + 18CH
A 705 m 697 s 770 701 5.80 19.10 713 699 5.75 0.09 βCNC 69CCC + 15CH
A 650 m 658 w 713 649 17.03 1.78 636 623 10.39 0.32 νCS 85CS
A 639 m 710 646 45.51 1.08 628 615 22.36 0.65 νCS 82CS
A 603 s 603 w 659 600 12.30 2.25 613 601 12.21 0.31 νCF 87CF
143
A 577 m 582 vw 649 591 25.12 1.16 591 579 21.47 0.47 γCCC 69γCCC + 14γCH
A 528 m 528 s 583 531 251.22 5.94 538 527 274.61 0.39 βCSC 62CSC + 22CN
A 508 m 556 506 17.86 2.46 497 487 5.54 0.32 ωNH2 60ωNH2 + 24τNH2
A 457 m 481 438 2.46 3.08 445 436 1.10 0.72 γCCC 68γCCC + 16γCH
A 481 438 3.71 0.87 440 431 3.09 0.74 γCCC 65γCCC + 19γCH
A 419 s 415 s 458 417 4.22 11.50 427 418 4.32 0.16 βCF 72CF + 12CH
A 391 w 435 396 17.61 2.52 398 390 26.57 0.69 γCNC 64γCNC + 14γCH
A 367 w 390 355 0.09 0.84 361 354 1.32 0.70 γCCC 62γCCC + 18γCH
A 294 m 337 307 51.84 0.53 322 316 57.03 0.75 τNH2 62τNH2 + 20ωNH2
A 250 w 257 234 4.60 0.43 241 236 3.48 0.35 βCN(H2) 65CN + 15CNC
A 231 w 254 231 6.72 0.73 235 230 6.21 0.57 γCF 67γCF + 20γCH
A 210 w 206 187 4.02 0.44 190 186 3.00 0.75 γCSC 64γCSC + 18γCN
A 115 w 109 99 1.52 0.23 104 102 1.59 0.75 γCN(H2) 62γCN + 21γCNC aν–stretching; β–in–plane bending; δ–deformation; ρ–rocking; γ–out of plane bending; ω–wagging and τ–twisting, wavenumbers, (cm–1); IR
intensities, (km/mole); Raman scattering activities, (Å)4/(a.m.u).
144
Table 4.5. 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 2–amino–6–fluorobenzothiazolea.
Spec
ies
Observed
wavenumber (cm–1)
B3LYP/cc–pVDZ
Calculated wavenumber
B3LYP/6–31G(d,p)
Calculated wavenumber Assignment %PED
FTIR FTR Unscaled
(cm–1)
Scaled
(cm–1)
IR
intensity
Unscaled
(cm–1)
Scaled
(cm–1)
IR
intensity
A 3382 w 3651 3359 31.95 3699 3403 33.78 νaNH2 93NH
A 3260 w 3537 3254 55.60 3579 3293 64.07 νsNH2 91NH
A 3091 m 3217 3056 3.89 3226 3065 4.74 νCH 89CH
A 3060 m 3070 m 3216 3055 1.21 3225 3064 1.32 νCH 90CH
A 3037 m 3047 w 3202 3042 3.36 3212 3051 3.47 νCH 91CH
A 1631 m 1674 1641 297.43 1681 1647 355.17 δNH2 92NH2
A 1602 m 1610 w 1641 1608 61.45 1652 1619 20.51 νCC 89CC
A 1573 m 1577 m 1617 1585 67.57 1621 1589 47.38 νCC 91CC
A 1554 m 1587 1555 27.20 1606 1574 58.85 νC=N 87CN
A 1459 s 1464 m 1497 1467 209.42 1506 1476 193.62 νCC 90CC
A 1421 vw 1445 1416 3.45 1454 1425 1.97 νCC 87CC
145
A 1342 m 1362 1335 19.49 1361 1334 18.40 νCC 88CC
A 1300 m 1310 vw 1322 1296 102.92 1325 1299 123.84 νCC 85CC
A 1275 m 1271 vs 1311 1285 22.49 1310 1284 12.48 νCN 87CN
A 1257 s 1270 1245 49.74 1288 1262 50.04 νCN(H2) 85CN
A 1188 m 1182 m 1208 1184 16.59 1228 1203 23.62 βCH 72CH + 16CCC
A 1120 1098 0.82 1136 1113 1.84 βCH 71CH + 12CCC
A 1111 m 1111 vw 1114 1092 28.30 1115 1093 23.51 ρNH2 73ρNH2 + 12CN
A 1043 m 1045 w 1049 1028 12.35 1057 1036 11.34 βCH 65CH + 18CCC
A 965 946 0.08 947 928 0.32 γCH 64γCH + 14γCCC
A 918 m 909 s 932 913 29.71 935 916 27.27 βCCC 67CCC + 12CH
A 848 m 872 855 30.38 853 836 34.22 γCH 66γCH + 15γCCC
A 805 s 841 824 17.41 838 821 46.80 βCCC 67CCC + 15CH
A 771 vs 807 791 52.00 829 812 20.30 γCH 66γCH + 21γCCC
A 731 m 745 730 0.10 718 704 0.07 βCCC 65CCC + 18CH
A 705 m 697 s 713 699 5.62 714 700 5.63 βCNC 67CCC + 12CH
A 650 m 658 w 645 632 64.53 639 626 17.79 νCS 83CS
A 639 m 637 624 13.46 635 622 51.57 νCS 82CS
A 603 s 603 w 613 601 6.16 613 601 11.91 νCF 85CF
146
A 577 m 582 vw 600 588 31.20 590 578 21.89 γCCC 65γCCC + 16γCH
A 528 m 528 s 534 523 187.77 539 528 269.72 βCSC 61CSC + 20CN
A 508 m 524 514 77.57 514 504 18.83 ωNH2 60ωNH2 + 27τNH2
A 457 m 443 434 2.95 445 436 0.56 γCCC 65γCCC + 14γCH
A 442 433 0.45 440 431 2.32 γCCC 66γCCC + 16γCH
A 419 s 415 s 425 417 3.67 427 418 3.81 βCF 62CF + 14CH
A 391 w 403 395 18.12 400 392 23.92 γCNC 62γCNC + 12γCH
A 367 w 361 354 0.55 362 355 0.91 γCCC 62γCCC + 16γCH
A 294 m 322 316 38.72 319 313 50.80 τNH2 59τNH2 + 28ωNH2
A 250 w 239 234 3.95 241 236 4.08 βCN(H2) 64CN + 15CNC
A 231 w 235 230 5.17 234 229 6.07 γCF 63γCF + 18γCH
A 210 w 191 187 3.00 191 187 2.51 γCSC 64γCSC + 15γCN
A 115 w 103 101 1.68 102 100 1.69 γCN(H2) 62γCN + 20γCNC aν–stretching; β–in–plane bending; δ–deformation; ρ–rocking; γ–out of plane bending; ω–wagging and τ–twisting, wavenumbers, (cm–1);
IR intensities, (km/mole); Raman scattering activities, (Å)4/(a.m.u).
147
Table 4.6. The Experimental and calculated 1H and 13C isotropic chemical shifts (δiso, ppm) with respect to TMS and
isotropic magnetic shielding tensors (σiso) of 2–amino–6–fluorobenzothiazole.
Assignment σiso (1H) Cal. (δiso) Expt. (δiso) Assignment σiso (13C) Cal. (δiso) Expt. (δiso)
H11 24.26 7.69 7.29 C2 8.49 175.64 165.34
H12 24.62 7.33 7.02 C4 58.10 126.03 120.06
H13 24.21 7.74 7.46 C5 65.07 119.06 113.91
H (amino) 26.99 4.96 5.28 C6 15.30 168.83 160.24
C7 70.65 113.50 107.79
C8 28.31 155.20 157.85
C9 38.33 145.80 148.85
148
Table 4. 7. Experimental and calculated absorption wavelength (λ), excitation energies (E), oscillator strength (f) and frontier orbital energies of
2–amino–6–fluorobenzothiazole by TD–DFT method.
Compound λ (Expt.; nm) λ (Cal.; nm) E (eV) f Assignment EHOMO ELUMO EHOMO−1 ELUMO+1
2–amino–6–
fluorobenzothiazole
290.8 271.4 4.568 0.0358 n → π* −6.1221 eV −0.8471 eV −6.8051 eV −0.7168 eV
259.2 253.5 4.892 0.2692 π → π*
149
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