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
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