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.

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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].

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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

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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

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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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