CHAPTER – 3

A SCALED QUANTUM MECHANICAL APPROACH OF

VIBRATIONAL ANALYSIS OF O-TOLUNITRILE BASED ON

FTIR AND FT RAMAN SPECTRA, AB INITIO,

HARTREE FOCK AND DFT METHODS

3.1. INTRODUCTION

o-tolunitrile or ortho cyanotoluene has the molecular formula C8H7N. The

prefix cyano is used in chemical nomenclature to indicate the presence of the -C≡N

functional group or nitrile group in the molecule. The cyano group (C≡N), which

consists of a carbon atom triple-bonded to a nitrogen atom. o-tolunitrile is used as a

solvent and chemical intermediate for the synthesis of pharmaceuticals, dyestuffs and

rubber chemicals. It is used as pigments. Due to its greater pharmaceutical

importance, o-tolunitrile has been taken for the present study.

3.2. LITERATURE SURVEY

The microwave spectrum of ortho‐fluoro toluene, C6H4CH3F, was studied in

the frequency range 12–38 GHz by Suzzkind [1]. Rotational spectra in four torsional

states were assigned and each state could be fit by an effective rotational Hamiltonian.

The change in rotational constants from one state to another could not be explained

using the standard torsion–rotation coupled Hamiltonian. A simple model was given

which explains the deviations in the rotational constants in the excited A levels from

their expected positions. The true rotational constants for the ground state were

calculated as A  =  3243.08GHz,B  =  2180.44GHz,C  =  1314.36GHz,κ  =  − 0.10234. By

standard methods, the angle between the top axis and the A axis was found as 32°.

The barrier to internal rotation was 649 cal/mol assuming V6  =  0 and Iα  =

 3.237amu⋅Å2. Including the effects of changing average structure, there was an

evidence of a negative V6 and also that the angle mentioned above was somewhat

smaller in the ground state.

N. Abasbegović et al., [2] performed the Vibrational spectra and normal mode

calculations of p-toluidine and p-nitrotoluene molecules using Raman and infrared

spectroscopic studies. In normal mode calculations, the force field has been

constructed by the local interaction approximation.

Absorption and emission characteristics of o-, m- and p-tolunitriles in polar

and non-polar solvents under different conditions have been investigated in detail by

Maiti et al [3]. Solvatochromic shifts of band origin of these molecules in non-polar

solvents show that their dipole moments in the first excited singlet state are almost the

same while its value in the second excited singlet is larger in the meta than in the

para-isomer. Vibronic analyses of the low temperatures absorption, fluorescence and

phosphorescence spectra of all the three molecules have provided evidence that these

molecules are slightly distorted in the first excited singlet state while such distortion

in the phosphorescence emitting triplet state is larger. The data on fluorescence and

phosphorescence quantum yield and phosphorescence lifetime of the tolunitriles are

reasonably interpreted as showing that in these molecules, particularly m-and

p- tolunitriles, the internal conversion rate from the first excited singlet to the ground

state is probably small and that the charge transfer character of the triplet state in the

p-isomer is larger than that in the meta.

Jing Chao et al., [4] studied the chemical thermodynamic properties of

toluene,o-,m- and p-xylenes. They employed recent molecular, spectroscopic and

thermal constants for the determination of the ideal gas thermodynamic properties

[C0

p, S0, H

0 (T) - H

0(0), ΔH

0f and ΔG

0f] of toluene, o-xylene, m-xylene and p-xylene

in the temperature range 0–3000 K using statistical mechanical method. A potential

function, formed by summation of internal rotational energy levels, was used for

evaluating the internal rotational contributions to the thermodynamic properties

caused by the presence of each CH3 rotor in these molecules. The internal rotational

energy levels for each rotor were calculated by solving the wave equation using the

adopted internal rotational constant and potential function for the given rotor. The

heat capacities and entropies obtained agree with the experimental values. The

sources of molecular data and method of calculation are described in detail.

Electronic spectra of o-, m- and p-tolunitrile and its substituent effect on

internal rotation of the methyl group were analysed by Fujii et al [5]. In this study, the

S1 ← S0 fluorescence excitation spectra and the S1→S0 dispersed fluorescence spectra

of o-, m-and p-tolunitrile were measured in supersonic jets. Low-frequency bands due

to internal rotation of the methyl group were observed in m- and p-tolunitrile.

Observed band positions and relative intensities of the internal rotational bands were

reproduced by a calculation using a free rotor basis set. From the analysis, the

potential curve of the internal rotation was determined in both S1 and S0. It was found

that the barrier height increases in going from S0 to S1 in m-tolunitrile, while it

decreases in p-tolunitrile. In contrast, no low-frequency band was found in o-

tolunitrile. It was concluded that the potential curve in o-tolunitrile does not change in

going from S0 to S1. The change of the barrier height by electronic excitation in

tolunitriles differs greatly from that observed in other toluene derivatives. Moreover,

it was suggested that the electronic properties of a substituent were important for the

methyl rotation in the excited state.

In substituted toluenes, the potential energy barrier to internal methyl rotation

and the preferred methyl conformation depend on the position of the fluorine, amino,

or methyl substituents and also on the electronic state, either S0, S1, or ground state

cation were extensively studied by Lu et al [6]. In their study they presented a unified

picture of the electronic factors controlling these effects. In S0 and cation, ab initio

electronic structure calculations of modest scale produce rotor potentials in good

agreement with experiment. The methyl group provides a sensitive probe of local ring

geometry. When the geometry of the ring in the vicinity of the rotor has good local

C2v symmetry, the barrier is invariably small. In S0 ortho‐substituted toluenes, they

use natural steric analysis to show that repulsive steric interactions between the

halogen lone pair and the methyl CH bonds dominate over attractive donor–acceptor

interactions to favor the pseudo‐trans conformation. When steric interactions are

unimportant, the key determinant of rotor barrier height is the difference in π‐bond

order between the two ring CC bonds nearest methyl.

Molecular geometry, vibrational frequencies, infrared intensities and C N

effective bond charges in a series of simple nitrile compounds were evaluated by

HF/6–31+G(d,p) ab initio quantum mechanical calculations by Dudev et al [7]. In this

study, they analysed the effective charge properties of the nitrile C=N bond in a series

of 11 simple nitrile compounds. Due to the scarcity of reliable experimental gas-

phase intensity data on the C-N vibration, infrared intensities and dipole moment

derivatives were evaluated by HF/6-31+G(d,p) ab initio molecular orbital

calculations. The theoretical infrared intensities were transformed into quantities

associated with the charge distribution and dynamics in the molecules following the

formalism of the effective bond charge method. Satisfactory linear relations were

found between the effective bond charges and bond lengths, as well as between the

bond charges and the molecular electrostatic potential at the nitrogen atom. The

results obtained for the series of 11 nitrile compounds reveal significant changes in

the intensity of the C-N stretching vibration, whereas the frequency varies in narrow

limits. These observations, together with some recently derived relationships with

other molecular parameters implied that the effective bond charge may be employed

as an important intramolecular parameter in structural analysis.

Kumar et al [8] reported the vibrational analysis of substituted benzonitriles

using transferability of force constants with some halogeno-, methoxy and nitro-

benzonitriles. A zero-order normal coordinate analysis of both the in-plane and

out-of-plane vibrations was made for 2-chloro, 6-fluorobenzonitrile,

s-trichlorobenzonitrile, p- and m-methoxybenzonitriles and m-nitrobenzonitrile,

transferring the force constants from our earlier work. The observed and calculated

frequencies agree with an average error of 16.8 cm-1

, demonstrating the transferability

of the force constants obtained previously.

Pavan Kumar et al [9] studied the vibrational analysis of substituted

benzonitriles with the help of normal co ordinate analysis and tranferability of force

constants of monohalogenated benzonitriles. The Raman and Fourier—transform

infrared spectra of p- and o-fluorobenzonitriles, p-, m- and o-chlorobenzonitriles and

p-, m- and o-bromobenzonitriles were measured. A normal coordinate analysis was

carried out for both the in-plane and out-of-plane vibrations of these molecules along

with m-fluorobenzonitrile using a 71-parameter modified valence force filed and an

overlay least-squares-technique was employed to refine the force constants using 269

frequencies of nine molecules.

Jaman et al [10] discussed the microwave spectrum and barrier to internal

rotation in ortho-tolunitrile molecule. The microwave rotational spectra of ortho-

tolunitrile have been investigated in the ground state in the frequency ranges of 22.0-

26.0 GHz and 32.0-37.0 GHz. The true rotational constants were determined to be

Ar=2890.98 MHz, Br=1499.75 MHz and Cr= 993.58 MHz. A least square analysis of

the A-E splitting of 16 transitions resulted in the values of V3=533.53 cal/mol and

θa= 54.22o, assuming v6=0 and Iα = 3.2 a.m.u. Å2. The study concluded that from the

value obtained from the threefold potential barrier (V3) of the CH3 top in o-tolunitrile,

the barrier height in ortho derivatives does not change significantly with the electronic

properties of the substituent.

Nakai et al [11] theoretically investigated the internal rotations of the methyl

group in substituted toluenes such as fluorotoluene (-F), toluidine (-NH2), cresol

(-OH), and tolunitrile (-CN) in the ground, excited, and anionic states. An idea of

π∗–σ∗ hyperconjugation was introduced for a comprehensive interpretation of the

barrier variations. The π∗–σ∗ hyperconjugation mechanism clarified the differences

among ortho-, meta-, and para-systems, between π-electron donating and accepting

substituents, and between first and second excited (anionic) states. The study also

applied ab initio MO calculation to the inter rotational motion in o- and m-substituted

toluenes in both S0 and S1 states and found that the change of rotational barrier in

S1-S0 excitation is mainly determined by the stability of the LUMO orbitals along the

methyl rotation. In addition, it was concluded that the origin of this rotational angle

dependence on LUMO is a new type of hyperconjugation (π∗–σ HC) between an

ortho-carbon and the methyl hydrogen atoms. They also extended this analysis to the

cation and proposed that the change of rotational barrier is related to the rotational

angle dependence of the HOMO.

Shaji et al [12] reported the near infrared vibrational overtone absorption

spectra of liquid phase toluidines. The analysis of the observed CH and NH local

mode mechanical frequency values showed that there was the existence of steric and

electronic interaction between the amino and methyl groups in o-toluidine. This

supports the conclusions drawn from structural studies of toluidines by resonance

two-photon ionization (R2PI) spectroscopy, ab initio calculations and laser induced

fluorescence studies reported earlier.

Pulsed field ionization_ZEKE photoelectron spectroscopy has been applied to

o-, m- and p-tolunitrile in a supersonic jet by Suzuki et al [13] . The PFI-ZEKE

photoelectron spectra of m- and p-tolunitrile show well resolved anharmonic

structures in the low-frequency region, which were assigned to bands due to internal

rotational motion of the methyl group on the cation. Level energies and relative

transition intensities were reproduced well by a one-dimensional rotor model with a

three fold axis potential. Potential curves for the internal rotation have been

determined. For o-tolunitrile, no band due to internal rotation was found in PFI-

ZEKE spectrum. It suggested that the o-tolunitrile cation has the high barrier for

internal rotation and the stable conformation that is the same as that in S1 to S0. The

barrier height and the conformation are compared with other toluene derivatives and

the relation between the electronic character of –CN and internal rotational motion

has been discussed. It was strongly suggested that the internal methyl rotation has a

strong dependence not only on the electronic state and substituted positions but also

on the electronic character of the substituent.

Literature survey reveals that there are more works in toluene molecule and in

the internal roations of the methyl group. But, so far there is no complete vibrational

analysis on o-tolunitrile molecule on the basis of Scaled Quantum Mechanical study

using FTIR and FT-Raman Spectra. So, in this present work, the vibrational

frequencies of the title molecule are studied thoroughly, the fundamentals from the

experimental vibrational frequencies, geometrical parameters and thermodynamical

properties at HF and B3LYP levels with 6-31G(d) basis set were calculated and

discussed in an elaborate manner.

3.3. COMPUTATIONAL METHODS

The molecular structure optimization of the title compound and corresponding

vibrational harmonic frequencies were calculated using Hartree Fock and the Density

Functional Theory with Beckee-3-Lee-Yag-Parr (B3LYP) combined with 6-311G(d)

and 6311++G(d) basis sets . Geometries have been first optimized with full relaxation

on the potential energy surfaces at HF/6-311G(d) and HF/6311++G(d) basis sets. The

geometry was then re-optimized at B3LYP level using the same basis sets. The

optimized geometrical parameters, force constants, true rotational constants,

fundamental vibrational frequencies, IR intensity and Raman Activity were calculated

using the Gaussian 03 package. The total energy, zero point energy, thermal energy,

entropy, specific heat capacity at constant volume and dipole moment were also

calculated theoretically 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. There may be some mismatch in defining internal co-ordination. But, the

defined coordinate form complete set and matches quite well with the motions

observed using GAUSVIEW program.

The FT IR and FT Raman spectrum were taken in the range of 3100 – 100

cm-1

in the solid phase to analyse the very low frequency vibrations.

3.4. RESULTS AND DISCUSSION

3.4.1. Molecular Geometry

The molecular stature of o-tolunitrile has one nitrile group (C≡N) in the ortho

position and one methyl group with a benzene ring. It has a plane of symmetry and

the two methyl hydrogen atoms are symmetrically displaced above and below the

plane. As there is no experimental data available for this molecule and to investigate

the performance of HF and DFT methods, full geometry optimizations has been

carried out by with above said methods using 6-311G(d) and 6311++G(d) as basis

sets. The most optimized structural parameters (bond length and bond angle) by HF,

DFT/B3LYP with different basis sets were shown in Table 3.1. The optimized

molecular structure obtained from GaussView program is shown in Fig.3.1.

The optimized bond lengths of C–C in benzene ring fall in the range from

1.3811 to 1.3976 Å for HF and 1.3887 to 1.4116 Å for B3LYP method with different

basis sets. Table 3.1 depicts that the hierarchy of the optimized bond lengths of the

six C–C bonds of the benzene ring is (C1–C6) < (C5–C6) < (C4–C5) < (C3–C4) <

(C1–C2) < (C2–C3). Moreover, the optimized geometry shows that the CH3 group

substituted in the benzene ring namely C11H14, C11H15, C11H16 reduces the bond angle

of C2-C3-C4 while the bond angle of C1-C2-C3 increases due to the substitution of

C≡N in the ortho position of toluene from 1200. Hence, it reveals that from the order

of the bond lengths and the addition of substitutions in the phenyl ring makes it little

distorted form perfect hexagonal structure.

In this molecule, the length of C-C bond connecting the methyl group and the

benzene ring calculated at B3LYP level varies as 1.5064 Å, 1.5062 Å with 6-311G(d,)

and 6-311++G(d) respectively implies that there is an elongation of bond length due

to CH3 point mass. Also, it is noticed that the bond length connected between benzene

and nitrile group varies as 1.0910 Å, 1.0911Å. The comparative bond length details of

skeletal ring, methyl group and C≡N group is represented in Fig. 3.2. It clearly shows

that, C≡N bond length is very less when compared with the other C-C bonds in the

molecule due to the presence of heavy nitrogen atom connected with it. As well, the

bond length between C-C≡N is also comparatively higher with other skeletal bonds.

3.4.2. Vibrational assignments

The o-tolunitrile molecule has 16 atoms with 42 normal modes of vibrations.

Since the molecule do not possess any rotational, reflection or inversion symmetry,

the molecule is considered under Cs point group symmetry with non planar structure.

The entire modes of vibration was divided into two categories: A‘ and A‘‘.

ie., ΓVib = 28 A‘ + 14 A‖. In agreement with Cs symmetry, all the 42 fundamental

vibrations are active in both Raman scattering and Infrared absorption.

The experimental frequencies (FTIR and FT-Raman), calculated frequencies

(unsclaed and scaled) with HF and B3LYP methods using different basis sets, and

vibrational assignments with corresponding TED values are reported in Table 3.2.

Theoretically calculated IR intensities, Raman activities with experimental values are

shown in the Table 3.3. For visual comparison, the experimental FTIR, FT-Raman

spectra are shown in Figures 3.3 and 3.4 respectively. In order to analyse the

deviation of frequencies computed by theoretical methods from with experimental

frequencies, the comparative spectra are included in Figures 3.5 to 3.6 for higher basis

sets.

So as to improve the calculated values in agreement with the experimental

values, it is necessary to scale down the calculated harmonic frequencies. The scaling

factor of 0.9044, 0.9051 and 0.9663,0.9614 are applied to vibrational frequencies

calculated at HF and B3LYP level for 6-311G(d) and 6-311++G(d) respectively. The

assignments are based on the vibrational animations of fundamentals using the

GaussView package programme with above said methods.

3.4.2.1. C-H Vibrations

The aromatic CH stretching vibrations are expected to appear in the 3100–

3000cm−1

frequency ranges, with multiple weak bands [14]. These vibrations are not

found to be affected due to the nature and position of the substituent. Most of the

aromatic compounds have nearly four infra red peaks in the region 3080 - 3010 cm-1

due to ring C-H stretching bonds [15-16]. In this work, the asymmetric stretching is

assigned at 3060 cm -1

, 3030 cm -1

of FTIR and 3070 cm-1

of FT-Raman while the

band at 3080 cm -1

of FT-Raman is assigned to C-H symmetric and asymmetric

stretching vibrations respectively. Here, one the C-H aromatic ring symmetric

stretching vibration 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 [17]. It is noted from literature

[18] that strong band around 1200 cm-1

appears due to valence oscillations in toluenes

and substituted toluenes. As said, there is a strong FT-Raman peak noticed at

1210 cm-1

. In this work, there are sharp and medium band intensity peaks are

identified at 1210 (s), 1120 (s) ,1110 (m), 1040 (m) cm-1

due to the effect of aromatic

C-H in plane bending vibrations. It is also noted that most of the in-plane bending

vibrations are coupled vibrations with CC and CN. These assignments mostly

coincides with the above said literature values.

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

occurs in the region below 1000 cm-1

. These extremely intense absorptions are used to

assign the position of substituent on the aromatic ring [19]. The ring C-H out-of

plane bending frequencies of aromatic molecules depend on the number of adjacent

hydrogen atoms on the ring. In this compound, the sharp and medium intensity peaks

at 860 cm -1

, 750 cm-1

in FTIR and 990 cm-1

, 540 cm-1

in FT Raman confirms the

C-H out of plane bending vibrations. Generally, the ortho substituted benzene show a

strong band between 735 - 770 cm-1

[20]. As pointed in the literature [20], there is a

strong peak noticed at 750 cm-1

confirms the ortho substituted nature of this molecule.

The vibrations due to C-H in plane bending, and out of plane bending of the title

compound is good agreement with the values assigned by Srivastava [21].

3.4.2.2. CH3 and C-CH3 Vibrations

The o-tolunitrile molecule has possessed a methyl group on the benzene ring.

Basically, nine fundamentals are associated to this methyl group, which are the

symmetric and the asymmetric stretching, three in-plane bending and three out-of

plane bending vibrations. The C–H stretching of the methyl group occurs at lower

wavenumbers than those of the aromatic ring ie., less than 3000cm−1

. The asymmetric

stretching vibrations of the methyl group are generally observed around 2980 cm−1

,

while the symmetric stretch is expected around the region of 2870 cm−1

[22–24]. In

ortho substituted compounds, there is the occurrence of field effect in which the lone

pairs of electrons on two atoms influence each other through space interactions and

change the vibrational frequencies of both the groups. Accordingly, in this work, the

experimentally observed FTIR band at 2990 cm-1

and in FT-Raman band at

2970 cm−1

are assigned to the asymmetric stretching vibrations of the methyl group

where there is a deviation of 10 cm-1

from earlier said literature value. However it

shows good agreement with the theoretical values calculated at B3LYP level. The

experimentally observed FT-Raman band at 2930 cm−1

is assigned to the symmetric

stretching vibrations of the methyl group. The increase of vibrational frequency from

the expected ranges is due to field effect of CN group present in the ortho position. It

is noticed from the TED column in table 3.2 that all the methyl stretching modes are

pure stretching modes by its contribution is 100%.

In Table 3.2, the strong intensity FTIR bands at 1450 cm-1

, 1390 cm-1

and the

FT-Raman bands at 1050 cm-1

are assigned to methyl group in–plane bending

vibrations. Also, the medium peak at 950 cm-1

(FTIR) and the medium intensity FT-

Raman peaks at 1020 cm-1

, 820 cm-1

are assigned to CH3 out-of plane bending

vibrations which coincides with the earlier literature [24-25]. Furthermore, the C-CH3

stretching, in plane and out-of plane bending vibrations are assigned at 1205 cm-1

,

350 cm-1

in FT-IR and 110 cm-1

in FT-Raman respectively which favourably agrees

well with the values predicted by Singh and Pandey [26].

3.4.2.3 Skeletal vibrations of the benzene ring

The earlier literature [27] reported that for aromatic six member ring there are

two or three strong intensity bands of aromatic C=C in the range 1625 – 1590 cm-1

and also there may be a sharp and strong band near 1600 cm-1

in Raman spectrum. In

accordance with this literature values, here also there are three very strong bonds

observed at 1600 cm -1

, 1570 cm -1

and 1490 cm-1

which are assigned to C = C

aromatic stretching without any ambiguities. The ring CCC vibrations also noted and

depicted in Table 3.2.

3.4.2.4 C-N and C-(C≡N)vibrations

The electronegative nitrogen atom makes the carbon atom more positive and

the polar –CN group has –I effect on the adjacent bond. The infrared spectra of

various cyanides (nitriles) have shown that the predominant from with triple bond

between the carbon and nitrogen atoms. Thus the infra red absorptions occur in the

triple bond region between 2280 - 2200 cm-1

. The shift in 𝜈𝐶 ≡ 𝑁 stretching

absorption depends upon the electronic effect of atoms or groups attached to 𝐶 ≡ 𝑁

groups. In aromatic nitriles, the 𝜈𝐶 ≡ 𝑁 stretching decreases by about 20 cm-1 but

band intensity increases as compared to the saturated compounds [28].

The characteristic wavenumbers of 𝐶 ≡ 𝑁 stretching vibrations of

benzonitrile [22, 29-30] fall in 2220 – 2240 cm-1

spectral range, with an IR intensity

which varies from medium-weak to strong depending on the substituent. In

benzonitrile, this band has been identified at 2230 cm-1

[31]. Kitson et al [32] reported

that the Raman shift occurs at 2245 cm-1

in aliphatic nitriles, 2229 cm-1

in the case of

aromatic nitriles whereas it occurs at 2225 cm-1

in conjugated nitriles and moreover,

the intensity of nitrile absorption varies considerably. In nitriles containing carbon

and hydrogen in addition to the nitrile group, the band is usually intense.

Accordingly, in this work, the strong 𝐶 ≡ 𝑁 stretching peak appears at 2230 cm-1

confirms the presence of nitrile stretching in this title compound.

The Raman intensity of the 𝐶 ≡ 𝑁 band is enhanced by the conjugation of the

aromatic ring. Nevertheless, the aromatic ring stretching and deformation modes

often exhibit stronger Raman intensity than the C≡N stretching vibration.

The medium peak in FTIR at 590 cm-1

and the strong Raman peak at 150 cm-1

are

assigned to C≡N in-plane and out-of plane bending vibrations as said in the literature

[33]. In addition, Table 3.2 indicates that C-(C≡N) vibrations are also identified in

this molecule at 1170 cm-1

, 550 cm-1

and 140 cm-1

for stretching, bending (in and out

plane) vibrations respectively.

3.4.3. Thermodynamic properties and Rotational constants

Thermodynamaical parameters such as entropy, enthalpy, specific heat

capacity, rotational constants and dipole moment are calculated in Table 3.4. The

rotational constants of o-tolunitrile are calculated from HF and B3LYP methods using

different basis sets. For higher basis sets the rotational constant values decreases than

lower basis sets in both methods. In literature [10], the true rotational constants

calculated as 2.89098, 1.499975 and 0.99358 GHz which very well coincides with the

values obtained by B3LYP method in this molecule. Furthermore, the Zero Point

Vibrational Energy decreases as the basis set increases but it is reverse in the case of

dipole moment.

3.4.4. Mulliken atomic charge Analysis

The mulliken atomic charges a means of estimating partial atomic charges

from calculations carried out by the methods of quantum mechanical calculations on

individual atoms by different methods (HF & DFT) with different basis sets are

tabulated in the Table 3.5. The nitrogen atom in CN group is more electronegative

and it is connected with C2 atom in the benzene ring. So sharing of band pair of

electron takes place between C2 and C11. Hence, electron deficit in C2 atom makes its

charge more positive which is well agree here with the value of C2 atom obtained

from higher basis sets with HF and DFT. Besides, it is still noted from the Table 3.5

that the charge on nitrogen atom is negative and the charges of hydrogen in the

methyl group has only marginal difference. The charges on the corresponding atom is

shown clearly in the Fig. 3.7

3.5. CONCLUSION

The complete vibrational analysis of o-tolunitrile was performed on the basis

of Hartree Fock and, B3LYP methods with 6-311G(d) and 6-311++G(d) basis sets.

This analysis evaluates the geometrical parameters, force constants, true rotational

constants, Total Energy Distribution (TED)and thermodynamical parameters of the

title compound. The influence of methyl group and nitrile group were also discussed

in this molecule. The assignments of the fundamentals are confirmed by the

qualitative agreement between the calculated and observed frequencies. The following

noteworthy points were observed.

The bond lengths of benzene ring changed in the order (C1–C6) < (C5–C6) <

(C4–C5) < (C3–C4) < (C1–C2) < (C2–C3). Moreover, the addition of methyl and

nitrile group in the benzene ring changes its bond angle from 120o exactly in

the substitution position. Hence, it is concluded that from the order of the bond

lengths and the addition of substitutions in the benzene ring makes it little

distorted form perfect hexagonal structure.

The experimentally observed CH, CC and CN vibrations are in the expected

range as reported in the earlier literatures.

The one aromatic ring CH symmetric stretching vibration is greater than the

asymmetric stretching vibration which differs from the expected value.

In ortho substituted compounds, due to the presence of field effect, the lone

pairs of electrons on two atoms influence each other through space

interactions and change the vibrational frequencies of both the groups. This

happened here also. Even though the CN stretching frequency is between the

expected range as reported in earlier literatures, there is a deviation of around

30 cm-1

from its normal range.

The same thing happened in methyl stretching vibrations also. The observed

methyl group vibrations such as stretching and bending are coincides with the

earlier literature data. However, comparing with methyl group vibration in

toluene, it occurs at 2918 cm -1

and 2875 cm-1

which is lesser than the methyl

vibrations that noticed here. This shifting of frequency to higher value of

methyl stretching vibration is mainly due to the presence of nitrile group

adjacent to methyl group which produced the field effect between them.

The occurrence of C=C stretching absorption in this study confirms the

aromaticity of the compound and it should be in the expected range.

The rotational constants calculated experimentally by microwave spectra in

the earlier literatures is exactly agree well with the theoretical calculation

made in this study.

The higher basis sets in HF and B3LYP method produces higher values of

Mullikan charges on the respective atoms.

Fig.3.1. Molecular structure of o-tolunitrile with numbering of atoms

Fig. 3.2. Comparative Graph for C-C and C≡N bond lengths with HF and DFT

methods of different basis sets

Fig. 3.3. Experimental FT-IR Spectrum of o-tolunitrile

Fig. 3.4. Experimental FT-Raman Spectrum of o-tolunitrile

ig. 3.5. Comparative spectra between experimental and

calculated (unscaled) FT-IR with 6311++G(d)

Fig. 3.6. Comparative spectra between experimental and

calculated (unscaled) FT-Raman with 6311++G(d)

Fig. 3.7. Comparative graph for muliken charge on individual atom of

o-tolunitrile with HF and DFT for different basis sets

Table 3.1

Optimized Geometrical Parameters (Bond lengths and Bond Angles)

of o-tolunitrile

Parameters HF B3LYP

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

Bond length (in Å)

C1-C2 1.3892 1.3898 1.4021 1.4026

C1-C6 1.3811 1.3817 1.3887 1.3892

C1-H7 1.0741 1.0743 1.0841 1.0842

C2-C3 1.3974 1.3976 1.4114 1.4116

C2-C11 1.4433 1.4437 1.4312 1.4315

C3-C4 1.3868 1.3877 1.3953 1.3961

C3-C13 1.5086 1.5086 1.5064 1.5062

C4-C5 1.3852 1.3859 1.3932 1.3938

C4-H8 1.3075 1.0752 1.0855 1.0856

C5-C6 1.3839 1.3847 1.3929 1.3936

C5-H9 1.0752 1.0754 1.0852 1.0852

C6-H10 1.0743 1.0744 1.0844 1.0845

C11-N12 1.1309 1.1313 1.1557 1.1561

C13-H14 1.0842 1.0842 1.0938 1.0939

C13-H15 1.0842 1.0842 1.0938 1.0939

C13-H16 1.0820 1.0821 1.0910 1.0911

Bond angle (in degrees)

C2-C1-C6 120.0291 120.0133 120.1060 120.0670

C2-C1-H7 119.4332 119.4677 119.2655 119.3189

C6-C1-H7 120.5377 120.5191 120.6284 120.6140

C1-C2-C3 121.3842 121.4257 120.9766 121.0456

C1-C2-C11 118.6533 118.6230 119.1277 119.0739

C3-C2-C11 119.9626 119.9530 119.8957 119.8805

C2-C3-C4 117.5235 117.4987 117.6016 117.5595

C2-C3-C13 121.2729 121.3222 121.0324 121.1059

C4-C3-C13 121.2036 121.1791 121.3660 121.3345

C3-C4-C5 121.3004 121.3063 121.5113 121.5158

C3-C4-H8 119.2606 119.2626 118.9788 118.9646

C5-C4-H8 119.4390 119.4311 119.5098 119.5196

C4-C5-C6 120.4993 120.4964 120.2922 120.2985

C4-C5-H9 119.5445 119.5510 119.6669 119.6772

C6-C5-H9 119.9561 119.9526 120.0439 120.0243

C1-C6-C5 119.2635 119.2596 119.5123 119.5136

C1-C6-H10 120.1386 120.1359 119.9894 119.9786

C5-C6-H10 120.5979 120.6045 120.4983 120.5079

C3-C13-H14 111.1623 111.1736 111.4146 111.4349

C3-C13-H15 111.1622 111.1737 111.4146 111.4350

C3-C13-H16 110.806 110.7811 111.1218 111.0788

H14-C13-H15 107.2585 107.2544 107.6289 106.638

H14-C13-H16 108.1523 108.1557 108.031 108.028

H15-C13-H16 108.1524 108.1557 108.031 108.028

Table 3.2

Experimental and Theoretical (HF, B3LYP) level vibrational frequencies (cm-1

) with TED (%) of o-tolunitrile

Sl.

No

Mode of

Symmetry

Experimental

Frequency

HF B3LYP

Vibrational Assignment

(TED > 10%)

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

FT-IR FT-

Raman U S

a U S

b U S

c U S

d

1. A'

3080 s 3378 3055 3374 3053 3201 3094 3199 3076 CH (98)

2. A'

3070 s 3365 3044 3361 3042 3189 3082 3187 3064 CH (99)

3. A' 3060 m

3355 3034 3351 3033 3179 3072 3177 3055 CH (99)

4. A' 3030 m

3341 3022 3338 3021 3167 3060 3165 3043 CH (99)

5. A' 2990 m

3272 2959 3270 2960 3118 3013 3117 2997 CH of CH3 (100)

6. A'

2970 m 3250 2939 3249 2940 3088 2983 3086 2967 CH of CH3 (100)

7. A'

2930 s 3195 2890 3194 2891 3039 2936 3037 2920 CH of CH3 (100)

8. A' 2230 s

2580 2334 2573 2329 2333 2254 2326 2236 C≡N (100)

9. A' 1600 s 1600 s 1796 1625 1792 1622 1649 1593 1645 1582 C=C (84)

10. A'

1570 1762 1593 1758 1591 1615 1561 1612 1550 C=C (84)

11. A' 1490 s

1653 1495 1651 1494 1525 1474 1523 1465 C=C (76)

12. A'

1480 s 1628 1472 1627 1472 1511 1460 1509 1451 C-C (83)

13. A' 1450 s

1621 1466 1621 1467 1502 1452 1501 1443 CH of CH3 (90)

14. A'

1395 s 1599 1446 1596 1445 1479 1429 1476 1419 C-C (78)

15. A' 1390 s

1557 1408 1557 1409 1436 1387 1434 1379 CH of CH3 (96)

16. A' 1290 s

1422 1286 1421 1286 1327 1283 1327 1275 C-C (63)

17. A'

1210 s 1334 1206 1333 1206 1314 1270 1314 1263 CH (83)

18. A' 1205 m

1319 1193 1317 1192 1238 1196 1237 1189 C-(CH3)

19. A'

1170 m 1286 1163 1285 1163 1210 1169 1209 1162 C-(C≡N) (87)

20. A'

1120 s 1218 1101 1217 1102 1193 1153 1193 1147 CH (65)

21. A' 1110 m

1203 1088 1203 1089 1135 1096 1134 1090 CH (56)

22. A‘

1050 s 1169 1057 1170 1059 1070 1034 1070 1028 CH of CH3 (96)

23. A' 1040 m

1136 1027 1134 1026 1069 1033 1068 1027 CH (78)

24. A"

1020 m 1114 1007 1117 1011 1018 983 1016 977 CH of CH3 (88)

25. A"

990 m 1090 986 1089 986 991 958 994 955 CH (83)

26. A" 950 m

1074 971 1077 975 952 920 957 920 CH of CH3 (89)

27. A" 860 m

979 886 979 886 882 852 881 847 CH (90)

28. A"

820 m 881 797 880 797 832 804 831 799 CH of CH3 (85)

29. A" 750 s

854 773 853 772 775 749 773 743 CH (78)

30. A‘

720 s 796 720 794 718 734 710 733 704 CCC

31. A‘ 710 s

777 702 775 702 733 709 731 702 CCC

32. A' 590 m

662 599 661 598 611 590 610 586 C≡N (81)

33. A‘ 550 s

640 579 642 581 589 569 588 565 (C≡CN) (89)

34. A"

540 s 591 535 590 534 556 537 555 534 CH (80)

35. A"

460 m 517 467 517 468 472 456 473 455 CCC (80)

36. A" 450 s

490 443 489 443 460 445 460 442 CCC

37. A" 390 s

438 396 437 396 397 384 397 382 CCC

38. A' 350 m

368 332 367 332 345 334 345 332 C-CH3 (82)

39. A"

230 m 237 214 237 214 217 210 216 208 CCC

40. A" 150 s

169 153 170 153 151 146 152 146 C≡N (82)

41. A"

140 s 146 132 147 133 133 128 133 127 (C≡CN) (80)

42. A"

110 w 107 96 114 103 89 86 94 90 C-CH3 (92)

a Scale factor of 0.9044 for HF/6-311G(d);

b Scale factor of 0.9051 for HF/6-311++G(d);

c Scale factor of 0.9663 for B3LYP/6-311G(d) ;

d Scale factor of 0.9614 for B3LYP/6-311++G(d); U-Unscaled theoretical frequency ; S-Scaled theoretical frequency; w – weak, m – medium,

s – strong; - stretching; in-plane-bending; - out-of-plane bending;

Table 3.3

Theoretical (HF, B3LYP) Infrared Intensity and Raman Activity of o-tolunitrile with different basis sets

Sl.

No.

Experimental frequency HF B3LYP

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

FT - IR FT-Raman IIR SRa IIR SRa IIR SRa IIR SRa

1.

3080 s 16 201 13 193 13 211 10 209

2.

3070 s 29 78 24 72 20 101 17 99

3. 3060 m

13 101 11 94 12 111 10 104

4. 3030 m

3 50 2 47 4 57 3 54

5. 2990 m

25 58 24 55 18 57 17 55

6.

2970 m 20 66 16 59 12 69 10 64

7.

2930 s 21 136 21 151 14 159 14 184

8. 2230 s

50 244 64 304 30 308 42 392

9. 1600 s 1600 s 11 49 12 58 8 55 8 66

10.

1570 2 17 2 21 1 14 1 17

11. 1490 s

19 4 17 4 13 9 12 8

12.

1480 s 19 4 17 3 19 4 17 4

13. 1450 s

8 12 9 10 10 12 10 10

14.

1395 s 2 1 2 1 2 0 2 0

15. 1390 s

0 7 0 6 3 14 3 12

16. 1290 s

2 1 2 1 5 3 5 4

17.

1210 s 3 10 3 13 1 0 1 0

18. 1205 m

1 17 1 21 1 43 1 52

19.

1170 m 1 6 1 7 0 4 0 5

20.

1120 s 2 7 3 8 1 6 1 6

21. 1110 m

14 8 15 8 2 2 2 2

22.

1050 s 2 0 2 0 3 0 2 0

23. 1040 m

1 23 2 32 3 22 4 30

24.

1020 m 0 0 0 0 1 2 1 2

25.

990 m 0 2 0 3 0 0 0 0

26. 950 m

1 0 1 0 1 0 1 0

27. 860 m

0 0 0 0 0 1 0 0

28.

820 m 1 4 1 5 0 3 0 4

29. 750 s

56 0 67 2 43 0 54 1

30.

720 s 15 1 14 1 18 0 2 21

31. 710 s

2 17 2 20 2 17 14 0

32. 590 m

1 1 1 1 1 2 1 2

33. 550 s

7 3 6 2 5 2 4 1

34.

540 s 0 11 0 11 0 9 0 9

35.

460 m 12 2 12 1 10 1 9 1

36. 450 s

0 5 0 4 0 5 0 5

37. 390 s

1 2 1 1 1 2 1 1

38. 350 m

1 1 1 1 1 1 1 1

39.

230 m 1 2 1 1 1 2 0 1

40. 150 s

6 3 6 3 5 4 5 4

41.

140 s 2 1 2 1 2 1 2 1

42.

110 v 1 0 1 0 1 0 1 0

w – weak, m – medium, s – strong, IIR – IR intensity; SRa – Raman Activity

Table 3.4

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

), entropy (cal mol-1

Kelvin-1

) and dipole

moment (Debye)

Parameter

HF B3LYP

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

Zero Point Vibrational

Energy 84.87259 84.80999 79.37665 79.32073

Rotational Constants 2.89964 2.89667 2.878 2.87456

1.52197 1.52126 1.50369 1.50341

1.00418 1.00352 0.99368 0.99315

Energy 89.484 89.413 84.29 84.232

Molar capacity at

constant volume 27.199 27.205 29.203 29.222

Entropy 84.985 84.85 87.032 86.961

Dipole moment 4.5729 4.6425 4.3105 4.4989

Table 3.5

Mulliken atomic charges of o-tolunitrile performed at HF and B3LYP methods

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

Atom

Number

Mulliken Atomic Charges

HF B3LYP

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

C1 -0.1795 -0.3850 -0.1607 -0.4001

C2 0.0309 2.6376 0.0202 2.1881

C3 0.0791 0.6671 0.1087 0.5735

C4 -0.2458 -0.9408 -0.2195 -0.8286

C5 -0.1896 -0.4467 -0.1746 -0.3061

C6 -0.2277 -0.2165 -0.1984 -0.1821

H7 0.2449 0.3403 0.2181 0.2796

H8 0.2277 0.3034 0.2010 0.2414

H9 0.2279 0.3178 0.2030 0.2659

H10 0.2292 0.3203 0.2046 0.2659

C11 0.1006 -1.9596 0.0110 -1.5932

N12 -0.3323 -0.2167 -0.2217 -0.1688

C13 -0.6891 -1.2882 -0.6983 -1.1423

H14 0.2482 0.2884 0.2354 0.2790

H15 0.2482 0.2884 0.2499 0.2625

H16 0.2273 0.2903 0.2211 0.2653