79

CHAPTER – 2

FT-IR AND FT-RAMAN SPECTRA, VIBRATIONAL

ASSIGNMENTS, AB INITIO AND DFT ANALYSIS OF

,-DICHLOROTOLUENE

2.1. INTRODUCTION

Toluene, also known as methylbenzene or phenylmethane, is a clear, water-

insoluble liquid with the typical smell of paint thinners, redolent of the sweet smell of

the related compound benzene. It is an aromatic hydrocarbon that is widely used as an

industrial feedstock and as a solvent. Like other solvents, toluene is also used as an

inhalant drug for its intoxicating properties [1,2]. Toluene reacts as a normal aromatic

hydrocarbon towards electrophilic aromatic substitution [3-5]. The methyl group

makes it around 25 times more reactive than benzene in such reactions. It undergoes

chlorination by Cl2 in the presence of FeCl3 to give ortho and para isomers of

chlorotoluene. It is also used as a carbon source for making Multi-Wall Carbon

Nanotubes. Toluene can be used to break open red blood cells in order to extract

hemoglobin in biochemistry experiments.

2.2. LITERATURE SERVEY

The vibrational spectra of toluene and its derivatives have been extensively

studied and analyzed in the past years by several workers [6-13] but only little effort

has been spent on chlorotoluenes. The microwave spectrum of p-chlorotoluene was

studied by Herberich et al [9] and the NMR spectrum of o-chlorotoluene in a liquid

crystal was analyzed by Diehl et al [11]. The analysis on the basis of quality,

methodology, experimental and theoretical aspects on toluene and its derivatives were

explained by many other workers [12-23].

Syam Sundar [12] studied the vibrational spectra of substituted toluenes (4-

amino-3-bromotoluene and 5-amino-2-bromotoluene) using infrared absorption and

laser Raman spectra by assuming Cs point group symmetry. According to the study,

he concluded that the first molecule comes under ―1,4-dilight-2-heavy‖ and the

second molecules comes under ―1-heavy-2,4-dilight‖classes. The C-H, C-X

stretching, in-plane-bending, out-of-plane bending vibrations were elaborately

80

discussed. Moreover, internal vibrations of the substituent groups like CH3 and NH2

also discussed in detail.

The computed force constants and vibrational spectra of toluene were

extensively studied by Xie et al [13]. The complete harmonic force field and dipole

moment derivatives have been computed for toluene at the Hartree-Fock level using a

4-21G basis set. The six scale factors optimized for benzene were used to scale the

computed harmonic force constants of toluene. The vibrational frequencies of toluene

computed from this scaled quantum mechanical force field were quite good. After a

correction was made to two previously proposed spectral assignments, the mean

deviation from the experimental frequencies is only 7.8 cm−1

except for the

frequencies related to the methyl group. Five more scale factors for the vibrational

modes of the methyl group were reoptimized. The final comparison showed an overall

mean deviation of 7.5 cm−1

between the theoretical spectrum and the experimental

spectrum. Computed intensities were qualitatively in agreement with experiments.

The microwave rotational spectra of orthochlorotoluene, C6H4CH3Cl, have

been measured in the frequency region 8–40 GHz by Nair et al [14]. Spectra due to

both isotopic species 35

Cl and 37

Cl have been observed. The hyperfine structure due

to the chlorine nuclear quadrupole interaction in both isotopic species has been

studied. Analysis of the spectra yields rotational and nuclear quadrupole coupling

constants for both isotopic species. No splitting which could be attributed to the

internal rotation of the methyl group was observed.

The microwave spectrum of an excited vibrational state of orthochlorotoluene

has been identified and analyzed extensively by Rajappan Nair [15]. The rotational

constants and nuclear electric quadrapole hyperfine interaction constants for 35

Cl and

37Cl species were reported. Accordingly, no sign of splitting due to the internal

rotation has been observed and the barrier hindering internal rotation is believed to be

high in this molecule. Furthermore, the study concluded that microwave study is

necessary to obtain an accurate experimental value of the potential barrier in

orthocholorotoluene. The vibrational frequency has been calculated as 163 cm-1

from

the relative intensities of the ground and excited state microwave spectra. The

vibrational state is expected to be the first torsional state of the molecule.

81

A theoretical study on the molecular structures of toluene, para-fluorotoluene,

para-chlorotoluene, and 4-methylpyridine and their sixfold internal rotational barriers

was conducted by Chen et al [16]. In their study, two kinds of sixfold internal

rotational configurations of toluene, para-fluorotoluene, para-chlorotoluene, and 4-

methylpyridine were calculated using Hartree–Fock (HF), second-order Møller–

Plesset (MP2), and Beck's three parameter hybrid functional using the LYP

correlation functional (B3LYP) theory methods with various high-level basis sets.

Structures and energies were compared for different configurations. Calculations

indicated that the orthogonal configuration has a local minimum while the planar

configuration is a transition structure. Furthermore, geometries of the orthogonal and

the planar configurations were quite similar, except for a methyl CH bond. Sixfold

internal rotational barriers were calculated from the energy difference of two different

configurations. From the results, the study concluded that the HF methods

underestimated the rotational barriers, but MP2 calculations overestimated them.

However, the density functional theory (DFT) method was a reliable method since the

calculated internal rotational barriers were similar to the experimental ones.

An investigation of CH stretching vibrations in benzene and toluene in their S1

states has been carried out using UV–IR and stimulated Raman–UV double resonance

spectroscopic methods by Minejima et al [17]. In benzene two CH stretching

vibrations, were observed, and in the case of toluene both aromatic CH and methyl

CH stretching vibrations were observed. The aromatic CH stretching vibrations in

toluene also exhibit strong anharmonic resonance, leading to the appearance of a large

number of bands in the 3000–3100 cm−1

region. The observed frequencies of CH

stretching vibrations in the S1 state in both benzene and toluene were higher than the

corresponding values in the S0 state. On the other hand, methyl CH stretching

vibrations in the S1 state of toluene occur at a lower frequency than those in the S0

state.

An analysis of vibrational spectra of chlorotoluene based on density functional

theory calculations were carried out by Zhou et al [18]. In this study, the

conformational behavior and structural stability of chlorotoluene were investigated

using RHF/631G* basis set and DFT (BLYP,LSDA,BP86,B3LYP,B3P86) levels. By

comparing the experimental values with the theoretical ones, of the five DFT

82

methods, BLYP reproduces the observed fundamental frequencies most satisfactory

with the mean absolute deviation of the non-CH stretching modes less than 10 cm-1

.

Moreover, the study also implies that two hybrid DFT methods were found to yield

frequencies, which were generally higher than the observed fundamental frequencies.

Furthermore, the study also depicted that it was a promising approach for

understanding the observed spectral features.

Gerhard et al [19] studied the internal rotation and chlorine nuclear quadrupole

coupling of o-chlorotoluene studied by microwave spectroscopy and ab initio

calculations. The microwave spectrum of the molecule was taken using molecular

beam Fourier Transform Microwave spectrometers (MB-FTMW) in the frequency

range of 4-23GHz. The objective of this study was to improve the rotational

constants, determine certifugal distortion constants and the complete quadupole

coupling tensor for both chlorine isotopomers. From the torsional fine structure, the

barrier to internal rotation of the methyl group was found as 5.5798 (52) kJ mol-1

.

The molecular constants obtained from the spectral analysis were interpreted in terms

of structural, dynamical and electrical properties of the molecule.

Scaled quantum mechanical reinvestigation of the vibrational spectrum of

toluene has been reported by Baker [20]. In this study, the IR spectrum of liquid

toluene between 400 and 4000 cm−1

by Keefe and coworkers [J.E. Bertie, Y. Apelblat,

C.D. Keefe, J. Mol. Struct. 750 (2005) 78] has been reexamined theoretically using

the scaled quantum mechanical (SQM) force field method. Accordingly, it was

proposed that three bands which were assigned as fundamentals : – a weak, broad

shoulder at 947 cm−1

(combination band), an unassigned feature at 1467 cm−1

and a

medium broad band at 2979 cm−1

, also assigned as a combination band. An average

deviation of just 5.28 cm-1

has been found out between the observed and theoretically

predicted vibrational fundamentals. Moreover, analysis involving free rotation of the

methyl group and interpretation of the vibrational spectrum in terms of C2v symmetry

for the phenyl ring and C3v for the methyl group very likely contributed to the

experimental misassignments.

83

A combined experimental and theoretical study of 2-chlorotoluene ad 2-

bromotoluene was done by Govindarajan et al [21] using FTIR and FT-Raman

spectra. The molecular structure, fundamental vibrational frequencies and

intensity of the vibrational bands were interpreted with the aid of structure

optimizations and normal coordinate force field calculations based on HF and

DFT methods with different basis set combinations. The complete vibrational

assignments were made on the basis of potential energy distribution (PED). In

addition, the effects due to the substitutions of methyl group and halogen bond

were investigated. The results of the calculations were applied to simulated

spectra of the title compounds, which show excellent agreement with observed

spectra.

Molecular structure and vibrational spectra of o-chlorotoluene, m-

chlorotoluene, and p-chlorotoluene by ab initio HF and DFT calculations were

examined by Ren et al [22]. The vibrational frequencies of these compounds were

obtained theoretically by ab initio HF and DFT/B3LYP calculations employing the

standard 6-311++G(d,p) basis set for optimized geometries and were compared with

Fourier transform infrared (FTIR) in the region of 400-4000 cm-1

and with Raman

spectra in the region of 100-4000 cm-1

. Complete vibrational assignment, analysis and

correlation of the fundamental modes for these compounds have been presented in the

work. In order to cope up with the experimental values, the theoretically calculated

harmonic vibrational frequencies were scaled down with the appropriate scaling

factors.

Spectral studies and quantum chemical calculations of 4-chlrotoluene was

done by Anbarasan et al [23]. In this work, the combined experimental and

theoretical study on molecular and vibrational structure of 4-chlorotoluene (4CT) was

studied based on HF and DFT using the hybrid functional B3LYP. The FTIR and

FT-Raman spectra of 4CT were recorded in the solid phase. The optimized geometry

was calculated by HF and B3LYP methods with 6-31G(d,p) and 6-311++G(d,p) basis

sets. The harmonic vibrational frequencies, infrared intensities and Raman scattering

activities of the title compound were performed at same level of theories. The

thermodynamic functions of the title compound was also performed at HF/6-31G(d,p)

and B3LYP/6-311++G(d,p) level of theories. A detailed interpretation of the infrared

84

and Raman spectra of 4CT was reported. The observed and the calculated frequencies

are found to be in good agreement. The experimental spectra also coincide

satisfactorily with those of theoretically constructed spectrograms.

With the aid of above seen literatures, it is clear that there is no quantum

mechanical study on α,α-dichlorotoluene molecule which has motivated to do a

detailed quantum mechanical analysis for understanding the vibrational modes of this

title compound in the present chapter. A complete vibrational analysis of ,-

diichlorotoluene was performed by combining the experimental and theoretical

information using Pulay‘s Density Functional Theory (DFT) based scaled quantum

chemical approach [24]. The vibrational wavenumbers, geometrical parameters,

modes of vibrations, dipole moment, rotational constants, atomic charges and other

thermodynamic parameters of this molecule were investigated by using HF and

B3LYP calculations with 6-311G(d,p), 6-311++G(d,p) basis sets. Specific scale

factors were employed in the predicted frequencies.

2.3. COMPUTATIONAL DETAILS

The primary task for the computational work was to determine the

optimized geometry of the compound. The molecular structure optimization of the

title compound and corresponding vibrational harmonic frequencies were calculated

using HF and B3LYP methods with 6-311G(d,p) and 6-311++G(d,p) basis sets using

GAUSSIAN 03 program package without any constraint on the geometry. The

stability of the optimized geometries was confirmed by wavenumber calculations,

which gave positive values for all the obtained wavenumbers. TED calculations,

which show the relative contributions of the redundant internal coordinates to each

normal vibrational mode of the molecule and thus enable numerically to describe the

character of each mode, were carried out by the Scaled Quantum Mechanical (SQM)

method using PQS program in which the output files created at the end of the

wavenumber calculations. The optimized geometrical parameters, true rotational

constants, fundamental vibrational frequencies, IR and Raman intensity, Raman

activity, atomic charges (Mulliken Population Analysis), dipole moment, and other

thermo dynamical parameters were calculated using the Gaussian 03 package . By

combining the results of the GAUSSVIEW program with symmetry considerations,

vibrational frequency assignments were made with a high degree of accuracy.

85

2.4. RESULTS AND DISCUSSION

2.4.1 Molecular Geometry

The molecular structure along with numbering of atoms of

,-dichlorotoluene was as shown in the Fig.2.1. The global minimum energy was

obtained by HF and DFT methods with different basis sets (such as 6-311G(d,p),

6-311++G(d,p)) as -1187.63821584 a.u., -1187.64469059 a.u., -1190.87281019 a.u.,

-1190.87712158 a.u. respectively. The most optimized structural parameters were

calculated and were depicted in Table 2.1.

From Table 2.1, the C-C bond lengths in the benzene ring obtained from

B3LYP/6-311++G(d,p) ranges from 1.3847 to 1.4988 Å whereas in B3LYP/6-

311G(d,p) it ranges from 1.3886 to 1.4981 Å . Similarly, in HF it ranges from 1.3796

to 1.5044 and 1.3789 – 1.5039 Å for 6-311 G(d,p) and 6-311++G(d,p) respectively.

From Table 1 it is clear that polarized basis set has greater bond length values than the

normal basis sets. Moreover, HF bond lengths are smaller than the bond length

calculated with DFT. The presence of CH-Cl2 group in the benzene ring elongates that

corresponding C-C bond length up to 1.4988 – 1.4981 Å in B3LYP and 1.5044 –

1.5039 Å in HF with different basis sets while the C-Cl bond length in CH-Cl2 varies

from 1.8149 – 1.815 Å (B3LYP/6-311++G(d,p)) as represented in Table 2.1.

Generally, the theoretically calculated optimized bond lengths are comparatively

larger than the experimental values, which confirm that the theoretical calculations

refer to isolated molecules in the gas phase while it is in the solid phase for

experimental results. Figure 2.2 represents the comparative bond length variation in

the molecule which clearly depicts that the C-C bond between CHCL2 and the

aromatic ring is higher than the other aromatic bond lengths in the molecule.

Moreover, the C-Cl bond length also very high due to it higher electronegativity.

2.4.2. Vibrational assignments

The title molecule ,-dichlorotoluene has 15 atoms. It has 39 normal

vibrational modes. Assuming that the ,-dichlorotoluene defines a symmetry plane,

i.e., that this is the non planar molecule with Cs point group which has the lowest

energy at all levels. 26 of these modes should be symmetric, A‘ and 13

86

antisymmetric, A‖, with respect to the reflection on the symmetry plane. For the

atoms located in the plane of molecule, the A‘ vibrational displacements takes place

in the plane of the molecule, the A‖ modes corresponds to the displacement out of the

plane of the molecule.

The 39 normal modes of ,-dichlorotoluene are distributed amongst the

symmetry species as ΓVib = 26 A‘ (in-plane) + 13 A‖ (out-of-plane). In agreement

with Cs symmetry, all the 39 fundamental vibrations are active in both Raman

scattering and Infrared absorption.

The vibrational spectra could be analysed in terms of the fundamental

characteristic of the molecules, overtones and combinations on the basis of Varsanyi‘s

classification of the benzene derivatives. The detailed vibrational assignments of

fundamental modes of ,-dichlorotoluene along with the symmetry, FT-IR and

FT-Raman experimental frequencies, calculated frequencies (unsclaed & scaled), and

vibrational assignment with TED contribution by HF and DFT methods for different

basis sets are reported in Table 2.2 and Table 2.3 respectively. For visual comparison,

the experimental FT IR and FT Raman spectra were reported in the Figs 2.3 and 2.4

respectively.

2.4.2.1. C-H Vibrations

In the aromatic compounds, the C-H stretching vibrations normally occur at

3100 – 3000 cm-1

[25]. These vibrations are not found to be affected due to the nature

and position of the substituent. In infra red spectra, most of the aromatic compounds

have nearly four peaks in the region 3100 - 3000 cm-1

due to ring C-H stretching

bonds [26-27]. Accordingly, in this molecule, the peaks appeared at 3100 cm-1

in

FT-IR is assigned to aromatic symmetrical stretching vibrations whereas the strong

peaks at 3090 cm -1

, 3070 cm -1

and 3030 cm-1

in FT-IR and the peak at 3080 cm-1

in

FT-Raman are assigned to aromatic C-H asymmetric stretching vibrations. In

general, most of the stretching modes are pure stretching modes as is evident from

TED column in Table 2.2 and 2.3; they almost contribute around 100%. One of the

symmetric vibrations of the aromatic ring is greater than the asymmetric vibration.

The C-H in plane bending vibrations usually occurs in the region

1300-1000 cm-1

and is very useful for characterization purposes [28]. It is noted from

87

literature [40] that strong band around 1200 cm-1

appears due to valence oscillations

in toluenes and substituted toluenes which very much coincides with the assignment

in this work where there is a similar strong peak appeared in FTIR. In this study, the

strong peaks at 1200 cm -1

, 1190 cm -1

, 1100 cm -1

, 1080 cm -1

and 1030 cm-1

are

assigned as C-H in plane bending vibrations.

The C-H out of plane bending vibrations are strongly coupled vibrations and

below 1000 cm-1

. These extremely intense absorptions are used to assign the position

of substituent on the aromatic ring [29]. In this molecule, the peaks at 1000, 840 cm-1

in FT-Raman and 960 cm -1

, 910 cm -1

and 790 cm-1

in FT IR are assigned to C-H out

of plane bending vibrations. All the CH vibrations are in the expected range as stated

in the earlier references.

2.4.2.2. C=C and C-C Vibrations

Generally the C=C stretching vibrations in aromatic compounds form the band

in the region of 1430-1650 cm-1

[30]. According to the literatures [31-32], the six

ring carbon atoms undergo coupled vibrations, called skeletal vibrations which

produces four bands in the region 1660 – 1420 cm-1

and also the C-C stretching

vibrations occurs in the range 1300-1400 cm-1

. As pointed out in [31-32], in this

molecule the prominent peaks occurred at 1600 cm -1

, 1590 cm -1

, 1495 cm-1

are due

to C=C stretching. These three peaks confirms that the compound to be aromatic in

nature [33]. The C-C stretching vibrations are assigned to strong bands in 1450 cm -1

,

1330 cm -1

and 1295 cm-1

of FTIR. The peak at 1450 cm-1

is shifted to upper

frequency by 50 cm-1

from the expected range which may be due to the presence of

substitution in that position.

The ring deformation vibration is assigned at 1025 cm-1

(FT-IR) while the CCC

bending vibrations such as in-plane and out-of plane are assigned at 610 cm -1

and

590 cm-1

, 500 cm -1

, 350 cm-1

in FTIR respectively.

2.4.2.3. C-Cl, CHCl2 and C-CHCl2 vibrations

The presence of halogen on alkyl substituted aromatic ring can be detected

indirectly from its electronic impact on the in-plane C-H bending vibrations [34]. The

strong peak in FT IR and FT Raman at 1000 cm-1

due to C-H out-of-plane bending

88

confirms the presence of chlorine atom in this molecule. The C-Cl stretching

vibrations give generally bands in the region 730 – 580 cm-1

. Compounds with more

than one chlorine atom exhibit bands due to asymmetric and symmetric modes [35-

38]. Accordingly, the two medium peaks at 780 cm

-1 and 680 cm

-1 were assigned to

C-Cl stretching vibrations with the TED contributions of 77% as shown in tables 2.2

and 2.3. The strong peaks at 295 cm-1

(FTIR) and 250 cm-1

(in both the spectra) is

assigned to in-plane bending vibration which is in close agreement with the earlier

literature [39]. Moreover, the strong peak at 190 cm-1

and 120 cm-1

are assigned as

C-Cl out of plane bending vibrations.

The CH asymmetric vibration of CHCl2 is assigned to the medium intensity

peak of 3000 cm-1

which appears in both the spectra. The in-plane and out-of-plane

bending vibrations of CH in CHCl2 are noted at 1230 cm-1

, 690 cm-1

respectively. The

presence of strong peak at 690 cm-1

confirms that the compound having

monosubstituted benzene [25, 33].

The C-CHCl2 vibrations such as stretching, in-plane bending, out-of-plane

bending vibrations are also assigned at 1260 cm-1

, 820 cm-1

, 370 cm-1

respectively in

table 2.2 and 2.3.

2.4.3. Mulliken charge population analysis

The presence of atomic charge on individual atom is established by the

calculation of Mulikken charge on individual atom. This population analysis is made

on this molecule, and the corresponding values are tabulated in the Table 2.4. The

comparative graph for mulliken atomic charge on individual atom by different

methods with different basis sets is as shown Fig 2.5. The CH2Cl group is connected

in the C3 of the benzene ring. Because of the presence of high proton acceptor

chlorine atom in the CH2Cl group, more electron deficit takes place and sharing of

bond pair of electron takes place between C3 and C12 which makes the C3 atom more

positive. The charge obtained on C3 atom when calculated theoretically is 1.7751

(HF/6311++G(d,p)) and 1.6471 (B3LYP/6311++G(d,p)) respectively. Besides, it is

still noted from the Table 2.4 and Fig 2.5 that the charge on C12 atom is negative and

the charges of hydrogen in the methyl group have only marginal difference.

89

Moreover, the charge on C2 atom has very low negative value when calculated with

higher basis sets.

2.4.4. Thermodynamical properties

Several calculated thermodynamical parameters, rotational constants,

rotational temperature, vibrational temperature and dipole moment have been

presented in Table 2.5. The Zero-Point Vibration Energies (ZPVE), the entropy,

Svib(T) and the molar capacity at constant volume were calculated. The variations in

the ZPVEs seem to be insignificant. The total energies are found to decrease with the

increase of the basis set dimension. The changes in the total entropy of

α,α-dichlorotoluene at room temperature at different basis sets are only marginal.

Also, the dipole moment of the molecule was computed with HF and DFT methods

using different basis sets.

2.5. CONCLUSION

Complete vibrational analysis of ,-dichlorotoluene was performed on the

basis of HF and DFT calculations with 6-311G(d,p) and 6-311++g(d,p) basis sets.

This analysis also reported the geometrical parameters of the compound. The changes

in the molecular structure and the assignment of vibrational frequencies due to the

presence of CH-Cl2 group in the benzene ring were discussed elaborately. The

assignment of the fundamentals is confirmed by the qualitative agreement between

the calculated and observed frequencies. Some of the noteworthy points observed in

this molecule is as follows:

The global minimum energy obtained by HF is lesser than the DFT (B3LYP)

method. Moreover, the energy increases as the basis set increases.

The bond lengths calculated for CC, CH, C-Cl by HF method are smaller than

the bond length calculated with DFT (B3LYP) due to electron correlation and

basis set deficiencies.

By comparing the bond length occurrence pictorially with different basis sets,

it is inferred that the presence of CH-Cl2 group attached in the benzene ring

90

elongates the bond lengths of that particular C-C and C-Cl moieties than

others

The calculated vibrational frequencies coincide well with the experimental

frequencies by the utilization of different scaling factors for different methods.

The aromatic C-H stretching and bending vibrations are well within the

expected range, which shows that the substitution in the molecule does not

produce any difference in this corresponding frequency range. But when

compared with the stretching vibration of methyl group in the alkyl substituted

benzene which ranges between 2850 and 2925 cm-1

, here the C-H stretching

vibration connected with Cl2 deviates approximately by 75 cm-1

. This may be

due the presence of Cl in this position which shifts the frequency of CH

vibration from the expected range. However, from the TED calculation, it is

concluded that most of the C-H stretching modes are pure stretching modes as

it contributes nearly 100%.

When comparing the aromatic CH symmetric and asymmetric vibrations, it is

noted that one of the CH symmetric vibrational mode is greater than that of the

CH asymmetric vibrational mode.

In the case of skeletal vibrations, the C=C and C-C vibrations are in the

expected range. However, one of the C-C vibrations is shifted to upper

frequency by approximately 50 cm-1

from the expected range.

The occurrence of strong band at 1200 cm-1

is due to the valence oscillations

in toluenes and substituted toluenes which very much coincide with the earlier

literature predictions.

The occurrence of strong peak 1000 cm-1

confirms that the molecule has a

chlorine substituted group. Furthermore, the C-Cl stretching vibrations

assigned at 780 and 680 cm-1

implies that, there is a compound with more than

one chlorine atom in the alkyl substituted ring and also exhibit stretching

bands due to asymmetric and symmetric vibrations.

91

It is evident from the mulliken population analysis is that the chorine atom

makes the charge of the aromatic carbon atom more positive where it is

connected with the CHCl2 group.

The results produced by DFT with higher basis set in all aspects such as in the

prediction of vibrational parameters (vibrational frequencies, IR intensity, Raman

activity etc), structural parameters (bond lengths, bond angle etc) and

thermodynamical parameters (Zero Point Vibrational Energy, entropy, enthalpy,

specific capacity etc) are highly precise than other methods.

92

Fig. 2.1.

Molecular Structure of α,α-dichlorotoluene with numbering of atoms

93

Fig. 2.2. Comparative Graph for C-C and C-Cl bond lengths with HF and DFT

methods of different basis sets

94

Fig. 2.3. Experimental FT-IR spectra of α,α-dichlorotoluene

95

Fig, 2.4. Experimental FT-Raman spectra of α,α-dichlorotoluene

96

Fig. 2.5. Comparative graph for mulikan charge on individual atom of

α,α-dichlorotoluene with HF and DFT for different basis sets

36

Table 2.1

Optimized Geometrical Parameters of α,α-dichlorotoluene

Parameters HF B3LYP

6-311G(d,p) 6-311++G(d,p) 6-311g(d,p) 6-311++g(d,p)

Bond length (in Å)

C1-C2 1.3789 1.3796 1.3886 1.3893

C1-C6 1.389 1.3899 1.3961 1.3967

C1-H7 1.075 1.0751 1.0838 1.0839

C2-C3 1.39 1.3906 1.3983 1.3986

C2-H8 1.0746 1.0748 1.0836 1.0837

C3-C4 1.383 1.3835 1.3956 1.3961

C3-C12 1.5039 1.5044 1.4981 1.4988

C4-C5 1.3878 1.3889 1.3938 1.3847

C4-H9 1.0759 1.0760 1.0849 1.085

C5-C6 1.3802 1.3808 1.391 1.3916

C5-H10 1.0749 1.0751 1.0838 1.0839

C6-H11 1.0752 1.0753 1.084 1.084

C12-H13 1.0726 1.0732 1.0838 1.0842

C12-Cl14 1.7885 1.7883 1.8158 1.8149

C12-Cl15 1.7886 1.7887 1.8158 1.815

Bond angle (in degrees)

C2-C1-C6 120.22 120.23 120.24 120.24

C2-C1-H7 119.75 119.75 119.73 119.73

C6-C1- H7 120.03 120.02 120.03 120.03

C1-C2-C3 120.03 120.06 120.03 120.06

C1-C2-H8 120.08 120.01 120.26 120.19

C3-C2-H8 119.89 119.92 119.71 119.75

37

C2-C3-C4 119.66 119.62 119.62 119.58

C2-C3-C12 121.61 121.60 121.74 121.73

C4-C3- C12 118.72 118.78 118.64 118.69

C3-C4-C5 120.28 120.31 120.24 120.28

C3-C4-H9 120.14 120.14 119.90 119.90

C5-C4-H9 119.59 119.55 119.86 119.82

C4-C5-C6 119.93 119.93 119.96 119.96

C4-C5-H10 119.76 119.75 119.78 119.77

C6-C5-H10 120.31 120.32 120.26 120.27

C1-C6-C5 119.88 119.85 119.90 119.87

C1-C6-H11 119.99 120.01 120.02 120.04

C5-C6-H11 120.13 120.14 120.08 120.09

C3-C12-H13 110.9 110.87 111.65 111.59

C3-C12-Cl14 112.22 112.19 112.16 112.11

C3-C12-Cl15 112.21 112.18 112.16 112.12

H13-C12-Cl14 105.82 105.82 105.50 105.53

H13-C12-Cl15 105.81 105.81 105.50 105.52

Cl14-C12-Cl15 109.49 109.59 109.45 109.57

38

Table 2.2

Experimental and calculated HF level vibrational frequencies (cm-1

) with TED(%) of α,α-dichlorotoluene

Sl.

No.

Symmetry

Species

Experimental

frequency Calculated HF

Vibrational Assignment

(TED>10%) FT - IR

FT-

Raman

6311 G(d,p) 6311++ G(d,p)

Unscaled Scaleda Unsclaed Scaled

b

1. A' 3100 (m)

3358 3050 3356 3038 CH(97)

2. A' 3090 (s)

3351 3044 3349 3031 CH(99)

3. A'

3080 (s) 3348 3041 3343 3026 CH(99)

4. A' 3070 (s)

3339 3034 3338 3021 CH(97)

5. A' 3030 (s)

3328 3024 3327 3012 CH (99)

6. A' 3000 (m) 3000 (m) 3321 3017 3320 3005 CH of CHCl2 (96)

7. A'

1600 (s) 1795 1631 1791 1621 C=C(84)

8. A' 1590 (w) 1590 (w) 1774 1611 1769 1601 C=C(84)

9. A' 1495 (s)

1654 1502 1651 1494 C=C(93)

10. A' 1450 (s) 1450 (s) 1605 1458 1603 1451 C-C (86)

39

11. A' 1330 (s)

1482 1347 1482 1342 C-C

12. A' 1295 (m)

1414 1284 1413 1279 C-C (78)

13. A' 1260 (s)

1376 1250 1375 1244 C- CHCl2

14. A'

1230 (s) 1326 1204 1325 1199 CH of CHCl2

15. A' 1200 (s)

1299 1180 1298 1175 CH

16. A'

1190 (s) 1292 1173 1291 1169 CH

17. A' 1100 (w)

1202 1092 1203 1088 CH

18. A' 1080 (s)

1172 1065 1172 1060 CH(83)

19. A'

1030 (s) 1122 1019 1122 1015 CH(84)

20. A' 1025 (m)

1119 1017 1118 1012 rd

21. A''

1000 (s) 1102 1001 1103 998 CH(84)

22. A'' 960 (w)

1083 984 1082 980 CH(83)

23. A'' 910 (w)

1042 947 1042 943 CH(86)

24. A''

840 (m) 948 861 946 856 CH(99)

25. A' 820 (s)

910 827 910 823 C-CHCl2(59)

26. A'' 790 (m)

879 798 885 801 CH (72)

40

27. A'

780 (m) 800 727 803 726 C-Cl of CHCl2(77)

28. A'' 690 (s)

786 714 786 712 CH of CHCl2(79)

29. A'

680 (m) 765 695 767 694 C-Cl of CHCl2 (77)

30. A' 610 (w)

673 611 672 608 CCC (79)

31. A'' 590 (s)

595 576 595 572 CCC (60)

32. A'' 500 (w)

565 513 566 512 CCC (84)

33. A''

370 (w) 450 409 452 409 C-CHCl2 (99)

34. A'' 350 (w)

392 356 392 355 CCC (74)

35. A' 295 (s)

305 277 306 277 C-Cl of CHCl2(78)

36. A' 250 (s) 250 (s) 262 238 261 236 C-Cl of CHCl2 (94)

37. A'' 190 (s) 190 (s) 192 175 192 174 C-Cl of CHCl2(78)

38. A'' 120 (m) 120 (s) 112 102 113 102 C-Cl of CHCl2 (85)

39. A''

34 31 38 34 τ CCC-Cl (82)+ τ CCC-H (15)

a Scale factor of 0.9085 for calculated wavenumbers ;

b Scale factor of 0.9051 for calculated wavenumbers; w – weak, m – medium, s – strong,

- stretching, - in-plane-bending, - out-of-plane bending, , torsion, r – ring, d- deformation

41

Table 2.3

Experimental and calculated B3LYP level vibrational frequencies (cm-1

) with TED(%) of α,α-dichlorotoluene

Sl.

No.

Symmetry

Species

Experimental

frequency Calculated B3LYP

Vibrational Assignment

(TED>10%)

FT - IR FT-

Raman

6311 G(d,p) 6311++ G(d,p)

Unscaled Scaleda Unsclaed Scaled

b

1. A' 3100 (m)

3197 3090 3195 3072 CH(97)

2. A' 3090 (s)

3190 3084 3189 3066 CH(99)

3. A'

3080 (s) 3181 3075 3180 3057 CH(99)

4. A' 3070 (s)

3171 3066 3171 3048 CH(97)

5. A' 3030 (s)

3166 3061 3164 3042 CH (99)

6. A' 3000 (m) 3000 (m) 3162 3057 3159 3037 CH of CHCl2 (96)

7. A'

1600 (s) 1645 1590 1642 1579 C=C(84)

8. A' 1590 (w) 1590 (w) 1629 1575 1626 1563 C=C(84)

9. A' 1495 (s)

1528 1477 1526 1467 C=C(93)

42

10. A' 1450 (s) 1450 (s) 1487 1438 1485 1427 C-C (86)

11. A' 1330 (s)

1366 1320 1366 1313 C-C

12. A' 1295 (m)

1348 1303 1348 1296 C-C (78)

13. A' 1260 (s)

1270 1228 1269 1220 C- CHCl2

14. A'

1230 (s) 1246 1205 1245 1197 CH of CHCl2

15. A' 1200 (s)

1215 1174 1214 1167 CH

16. A'

1190 (s) 1205 1165 1205 1158 CH

17. A' 1100 (w)

1185 1146 1185 1139 CH

18. A' 1080 (s)

1107 1071 1106 1063 CH(83)

19. A'

1030 (s) 1051 1016 1049 1009 CH(84)

20. A' 1025 (m)

1018 984 1017 978 rd

21. A''

1000 (s) 1010 977 1011 972 CH(84)

22. A'' 960 (w)

985 952 989 951 CH(83)

23. A'' 910 (w)

936 905 936 900 CH(86)

24. A''

840 (m) 854 825 853 820 CH(99)

25. A' 820 (s)

845 817 844 812 C-CHCl2(59)

43

26. A'' 790 (m)

789 763 794 764 CH (72)

27. A'

780 (m) 720 696 721 693 C-Cl of CHCl2(77)

28. A'' 690 (s)

710 686 711 683 CH of CHCl2(79)

29. A'

680 (m) 685 662 688 661 C-Cl of CHCl2 (77)

30. A' 610 (w)

630 609 630 605 ring (79)

31. A'' 590 (s)

595 576 595 572 ring (60)

32. A'' 500 (w)

510 493 511 492 ring (84)

33. A''

370 (w) 412 399 415 399 C-CHCl2 (99)

34. A'' 350 (w)

359 347 359 345 ring (74)

35. A' 295 (s)

278 269 279 268 C-Cl of CHCl2(78)

36. A' 250 (s) 250 (s) 240 232 238 229 C-Cl of CHCl2 (94)

37. A'' 190 (s) 190 (s) 177 171 177 170 C-Cl of CHCl2(78)

38. A'' 120 (m) 120 (s) 102 99 103 99 C-Cl of CHCl2 (85)

39. A'' --- --- 33 32 37 36 τ CCC-Cl (82)+ τ CCC-H (15)

a Scale factor of 0.9668 for calculated wavenumbers ;

b Scale factor of 0.9614 for calculated wavenumbers; w – weak, m – medium, s – strong,

- stretching, - in-plane-bending, - out-of-plane bending, , torsion, r – ring, d- deformation

Table 2.4

Mulliken atomic charges of α,α-dichlorotoluene performed at HF and B3LYP

level with 6-311G(d,p) and 6-311++ G(d,p) basis sets

Atom

Number

Mulliken atomic Charges

HF B3LYP

6-311 G(d,p) 6-311++ G(d,p) 6-311 G(d,p) 6-311++ G(d,p)

C1 -0.08372 -0.45506 -0.03862 -0.43907

C2 -0.02224 -1.34756 -0.05820 -1.14752

C3 -0.05605 1.77510 -0.09213 1.64715

C4 -0.08190 0.12660 -0.07892 -0.02863

C5 -0.08397 -0.26828 -0.09720 -0.23494

C6 -0.09091 -0.40425 0.00459 -0.23357

H7 0.10721 0.21439 0.10522 0.17527

H8 0.12035 0.18381 0.10913 0.14703

H9 0.09770 0.19260 0.10121 0.15219

H10 0.10521 0.21505 0.09461 0.17753

H11 0.10573 0.19138 0.10910 0.16023

C12 -0.21971 -0.11736 -0.33481 -0.30169

H13 0.21095 0.23870 0.23836 0.24564

Cl14 -0.05428 -0.27224 -0.03100 -0.15957

Cl15 -0.05437 -0.27288 -0.03132 -0.16006

Table 2.5

Theoretically computed Zero point vibrational energy (kcal mol-1

), rotational

constants (GHz), thermal energy (kcal mol-1

), molar capacity at constant volume

(cal mol-1

Kelvin-1

) and entropy (cal mol-1

Kelvin-1

)

Parameter

HF B3LYP

6-311G (d,p) 6-311 ++

G(d,p) 6-311 G(d,p)

6-311++

G(d,p)

Zero Point Vibrational

Energy 73.78603 73.76411 68.95665 68.95099

Rotational Constants

2.08115 2.07921 2.03195 2.03102

0.73292 0.73258 0.7259 0.72582

0.67829 0.67787 0.66948 0.66937

Energy 78.509 78.479 73.988 73.972

Molar capacity at constant

volume 27.159 27.146 29.376 29.354

Entropy 89.951 89.717 91.823 91.574

Dipole moment 2.8024 2.6509 2.9524 2.6285