Quantum chemical studies and vibrational analysis of 4-acetyl benzonitrile, 4-formyl benzonitrile and 4-hydroxy benzonitrile – A comparative study

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 98 (2012) 156–169

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Quantum chemical studies and vibrational analysis of 4-acetyl benzonitrile, 4-formyl benzonitrile and 4-hydroxy benzonitrile – A comparative study V. Arjunan a,⇑, K. Carthigayan b, S. Periandy c, K. Balamurugan d, S. Mohan e a

Department of Chemistry, Kanchi Mamunivar Centre for Post-Graduate Studies, Puducherry 605 008, India Department of Physics, Dr. B.R. Ambedkar Polytechnic College, Yanam 533 464, U.T. of Puducherry, India c Department of Physics, Tagore Arts College, Puducherry 605 008, India d Department of Chemistry, Arignar Anna Government Arts College, Karaikal 609 602, U.T. of Puducherry, India e Department of Physics, Hawassa University Main Campus, Hawassa, Ethiopia b

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

" The vibrational analysis of 4-actetyl

The complete vibrational assignment and analysis of the fundamental modes of 4-actetyl benzonitrile (4ABN), 4-formyl benzonitrile (4FBN) and 4-hydroxy benzonitrile (4HBN) molecules have assigned and analysed. The experimental parameters were compared with the theoretical parameters of the compounds determined from the DFT-B3LYP gradient calculations employing the standard 6–31G**, high level 6–311++G** and cc-pVDZ basis sets. The potential energy profile and the effect of substituents – COCH3, –CHO and –OH in the benzonitrile moiety have been analysed and compared. The kinetic and thermodynamic stability and chemical hardness of the molecule have been determined.

"

"

"

"

benzonitrile, 4-formyl benzonitrile and 4-hydroxy benzonitrile were performed. The experimental parameters were compared with DFT-B3LYP calculations. The effect of –COCH3, –CHO and –OH in the benzonitrile moiety have been analysed. The kinetic and thermodynamic stability and chemical hardness of the molecules have been determined. The potential energy profile and the molecular electrostatic potential of the molecules were determined.

a r t i c l e

i n f o

Article history: Received 25 June 2012 Received in revised form 5 August 2012 Accepted 21 August 2012 Available online 27 August 2012 Keywords: FTIR FT-Raman 4-Actetyl benzonitrile 4-Formyl benzonitrile 4-Hydroxyl benzonitrile DFT

a b s t r a c t The FTIR and FT-Raman vibrational spectra of 4-actetyl benzonitrile, 4-formyl benzonitrile and 4-hydroxy benzonitrile molecules have been recorded in the range 4000–400 and 4000–100 cm1, respectively. The complete vibrational assignment and analysis of the fundamental modes of the most stable geometry of the compounds were carried out using the experimental FTIR and FT-Raman data on the basis of peak positions, relative intensities and quantum chemical studies. The observed vibrational frequencies were compared with the theoretical wavenumbers of the optimised geometry of the compounds obtained from the DFT-B3LYP gradient calculations employing the standard 6–31G**, high level 6–311++G** and cc-pVDZ basis sets. The structural parameters and vibrational wavenumbers obtained from the DFT methods are in good agreement with the experimental data. With hope of providing more and effective information on the fundamental vibrations, total energy distributions of the fundamental modes have been performed by assuming Cs point group symmetry. The effect of substituents –COCH3, –CHO and – OH in the benzonitrile moiety have been analysed and compared. The kinetic and thermodynamic stability and chemical hardness of the molecule have been determined. Ó 2012 Elsevier B.V. All rights reserved.

⇑ Corresponding author. Tel.: +91 413 2211111, mobile: +91 9442992223; fax: +91 413 2251613. E-mail address: [email protected] (V. Arjunan). 1386-1425/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.saa.2012.08.053

V. Arjunan et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 98 (2012) 156–169

Introduction Benzonitrile is a useful solvent, chemical intermediate for the synthesis of pharmaceuticals, dyestuffs and rubber chemicals through the reactions of alkylation, condensation, esterification, hydrolysis, halogenation or nitration and a versatile precursor to many derivatives [1]. Its derivatives are used in the manufacture of lacquers, polymers and anhydrous metallic salts as well as intermediates for pharmaceuticals, agrochemicals, and other organic chemicals. 4Hydroxybenzonitrile is an insecticidal [2] and antifungal [3] agent and its toxicity has been studied repeatedly [4–6]. 4-Acetylbenzonitrile finds usage in synthetic organic and pharmaceutical industry. 4-Formylbenzonitrile is as an intermediate for organic synthesis, liquid crystals and pharmacy. Benzonitrile can form coordination complexes with transition metals that are both soluble in organic solvents and conveniently liable. The benzonitrile ligands are readily displaced by stronger ligands, making benzonitrile complexes useful synthetic intermediates [7]. The benzonitrile molecule and some of its derivatives have been studied because of their interesting biochemical and physical properties [8–12]. The ab initio force field computations of 4-hydroxybenzonitrile were performed by using the gamess software [13]. Dimitrova has studied the vibrational spectra of benzonitrile and its radical anion [14], the molecular structure and vibrational properties of 2-formylbenzonitrile [15] and 3chloro-4-fluorobnzonitrile [16], 4-bromobenzonitrile [17], 4(dimethylamino)benzonitrile and its isotopomers [18], 4-(1H-pyrrol-1-yl)benzonitrile and 5-cyano-2-(1-pyrrolyl)-pyridine [19] were carried out. From a theoretical point of view, 4-acetylbenzonitrile, 4-formylbenzonitrile and 4-hydroxybenzonitrile molecules are interesting because it contains different binding sites for its interaction with the metal surface and it contains an aromatic ring, the CN group, and isolated pair of electrons on nitrogen and oxygen atoms [20]. Hence in the present study, vibrational spectra and electronic structure properties of 4-acetyl benzonitrile (4ABN), 4-formyl benzonitrile (4FBN) and 4-hydroxy benzonitrile (4HBN) were studied. To the best of our knowledge no detailed DFT calculations have been performed on the above mentioned compounds. Thus, the experimental and theoretical studies of 4ABN, 4FBN and 4HBN molecules, important conjugated compounds of biological and industrial interest, have been reported in this investigation. Density functional theory (DFT) have been employed in the investigation to provide the structural properties of the above mentioned compounds using B3LYP with 6– 311++G**, 6–31G** and cc-pVDZ basis sets. The theoretical results are compared with available experimental data, in order to dig out more information on the depiction of molecular properties. In the present work, the influence of –COCH3, –CHO and –OH in the benzonitrile moiety have been analysed and compared. Experimental The compounds 4-actetyl benzonitrile, 4-formyl benzonitrile and 4-hydroxy benzonitrile were purchased from Aldrich chemicals, USA, and used as such to record the FTIR and FT-Raman spectra. The FTIR spectra were recorded by KBr pellet method on a Bruker IFS 66 V spectrometer equipped with a Globar source, Ge/KBr beam splitter, and a TGS detector in the range of 4000–400 cm1 with the spectral resolution of 2 cm1. The FT-Raman spectrum was also recorded in the range 4000–100 cm1 using the same instrument with FRA106 Raman module equipped with Nd:YAG laser source operating at 1.064 lm with 200 mW power and Ge detector with liquid nitrogen was used. The frequencies of all sharp bands are precise to 2 cm1. Computational methods The different conformations of 4ABN and 4FBN compounds were found by anaysing the potential energy surface profile deter-

157

mined by B3LYP/6–31G** method. The stable molecular structures of 4ABN, 4FBN and 4HBN in the ground state are optimised and the electronic structural parameters have been computed by utilizing Becke’s three parameter hybrid functional (B3) [21,22] combined with gradient corrected correlation functional of Lee–Yang–Parr (LYP) [23] with high level triple zeta 6–311++G**, standard splitvalence polarised 6–31G** [24] and cc-pVDZ basis sets using Gaussian 03W program [25,26] to characterise molecular structure optimisation, vibrational frequencies, thermodynamic properties and energies of the optimised structures. The optimised parameters of the compounds 4ABN, 4FBN and 4HBN were used for harmonic vibrational frequency calculations resulting in IR and Raman frequencies together with intensities and Raman depolarization ratios. The force constants obtained from B3LYP/6–311++G** method have been utilised to calculate the total energy distribution using the Fuhrer et al. program [27]. The total energy distribution corresponding to each of the observed frequencies shows the reliability and accuracy of the spectral analysis. By combining the results of the Gaussview’s program [28] with symmetry considerations, vibrational frequency assignments were made with a high degree of accuracy. The Raman scattering activities (Si) calculated by Gaussian 03W program were suitably converted to relative Raman intensities (Ii) using the following relationship derived from the basic theory of Raman scattering [29].

Ii ¼

f ðm0  mi Þ4 Si mi ½1  expðhcmi =kTÞ

ð1Þ

where v0 is the exciting frequency (cm1), vi is the vibrational wavenumber of the ith normal mode, h, c and k are universal constants, and f is the suitably chosen common scaling factor for all the peak intensities. Isoelectronic molecular electrostatic potential surfaces (MESP) and electron density surfaces [30] were calculated using 6– 311++G(d,p) basis set. The molecular electrostatic potential (MEP) at a point ‘r’ in the space around a molecule (in atomic units) can be expressed as:

VðrÞ ¼

X A

ZA

   ! !  RA  r   

Z

!

0 0 qðr Þdr  

!0 ! r r   

ð2Þ

where, ZA is the charge on nucleus A, located at RA and q(r0 ) is the electronic density function for the molecule. The first and second terms represent the contributions to the potential due to nuclei and electrons, respectively. V(r) is the resultant at each point r, which is the net electrostatic effect produced at the point r by both the electrons and nuclei of the molecule. The molecular electrostatic potential (MESP) serves as a useful quantity to explain hydrogen bonding, reactivity and structure activity relationship of molecules including biomolecules and drugs [31]. Structures resulting from the plot of electron density surface mapped with electrostatic potential surface depict the shape, size, charge density distribution and the site of chemical reactivity of a molecule. Gauss View 5.0.8 visualisation program [28] has been utilized to construct the MESP surface, the shape of highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) orbitals. Results and discussion Conformational analysis The conformational analysis of 4ABN, 4FBN and 4HBN were carried out with B3LYP method using 6–31G** basis set in order to ascertain most stable geometry and the analysis of the potential energy surface (PES) reveals that all the compounds exhibit two

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Fig. 1. Potential energy profile of 4-acetylbenzonitrile, 4-formylbenzonitrile and 4hydroxybenzonitrile.

conformers. During the scan, all the geometrical parameters were simultaneously relaxed while the dihedral angle C3–C4–C9–C11, C3–C4–C9–H11 and C3–C4–O9–C10 of 4ABN, 4FBN and 4HBN, respectively is varied in steps of 15° ranging from 0° to 360°. The conformer (I) is planar and is the most stable. In the conformer (II), the plane of the benzene ring and the plane of the –COCH3, – CHO and –OH groups, respectively in 4ABN, 4FBN and 4HBN are perpendicular to each other. These conformers are less stable.

The equilibrium value of this angle will be a compromise between two main effects: p-electron communication and steric interactions between the benzene ring and the substituent groups. The potential energy barriers obtained by the rotation of the substituent groups are depicted in Fig. 1. The two conformers of 4ABN, 4FBN and 4HBN are depicted in Fig. 2(a–c). The planar conformers (I) of these compounds are more stable by 6.53, 9.29 and 4.83 kcal mol1, respectively than the less stable conformer (II). It is also found that the resonance and conjugated effects play an important role in determining the barrier height of the groups of the studied molecules. The barrier hights are in the order 4FBN > 4ABN > 4HBN. The stable conformers (I) of have the – COCH3, –CHO and –OH groups along the molecular plane while in the other high energetic conformers (II) the –COCH3, –CHO and –OH groups are perpendicular to the plane of the benzene ring. From Fig. 1, it is possible to determine the barrier energies of the internal rotation of the –COCH3, –CHO and –OH groups in 4ABN, 4FBN and 4HBN. It needs 6.53, 9.29 and 4.83 kcal mol1 to enable the planar –COCH3, –CHO and –OH groups, respectively to have internal rotation perpendicular to the phenyl ring. More energy is required in the case of 4FBN to give the conformation with the –CHO plane perpendicular to the benzene ring. Molecular geometry The molecular structure and the scheme of atom numbering of the compounds of 4ABN, 4FBN and 4HBN are represented in Fig. 2. The geometry of the molecules is possessing Cs point group symmetry. The compound 4ABN has 48 fundamental modes of

Fig. 2. The optimised geometries of (a) 4-acetyl benzonitrile (b) 4-formyl benzonitrile and (c) 4-hydroxy benzonitrile with numbering scheme of the atoms.

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Table 1 Structural parameters calculated for 4-actetyl benzonitrile, 4-formyl benzonitrile and 4-hydroxy benzonitrile employing B3LYP method with 6–311++G**, 6–31G** and cc-pVDZ basis sets. Structural parameters

4-Actetyl benzonitrile B3LYP/6– 311++G**

Internuclear distance C1–C2 C2–C3 C3–C4 C4–C5 C5–C6 C6–C1 C1–C7 C7–N8 C4–C9 C4–O9 C9–O10 O9–H10 C9–H11 C9–C11 C–H(ring)a C–H(methyl)a Bond angle (°) C3–C2–C1 C3–C2–H C1–C2–H C2–C3–C4 C2–C3–H C4–C3–H C3–C4–C5 C3–C4–C9 C3–C4–O9 C5–C4–C9 C5–C4–O9 C4–C5–C6 C4–C5–H C6–C5–H C5–C6–C1 C5–C6–H C1–C6–H C2–C1–C6 C2–C1–C7 C6–C1–C7 C4–C9–O10 C4–O9–H10 C4–C9–C11 C4–C9–H11 O10–C9–C11 O10–C9–H11 C9–C11–H H–C11–H (methyl)a

a

Experimentalb

4-Hydroxy benzonitrile

B3LYP/6– 31G**

B3LYP/ccpVDZ

B3LYP/6– 311++G**

B3LYP/6– 31G**

B3LYP/ccpVDZ

B3LYP/6– 311++G**

B3LYP/6– 31G**

B3LYP/ccpVDZ

(Å) 1.404 1.391 1.402 1.404 1.388 1.407 1.434 1.163 1.505

1.401 1.390 1.401 1.402 1.386 1.405 1.432 1.155 1.507

1.406 1.394 1.405 1.406 1.390 1.409 1.437 1.164 1.507

1.405 1.391 1.400 1.403 1.387 1.408 1.435 1.163 1.485

1.402 1.389 1.398 1.401 1.385 1.406 1.432 1.155 1.485

1.407 1.393 1.402 1.405 1.390 1.410 10,437 1.164 1.487

1.408 1.387 1.402 1.401 1.390 1.404 1.431 1.164

1.405 1.385 1.398 1.398 1.388 1.402 1.429 1.156

1.409 1.389 1.404 1.403 1.392 1.406 1.434 1.165

1.394 1.373 1.385 1.388 1.375 1.392 1.427 1.143

1.359

1.361

1.359

1.353

1.220

1.215

1.219

1.215

1.209

1.213 0.967

0.963

0.969

0.90

1.112

1.110

1.120

1.085

1.084

1.092

1.085

1.084

1.092

1.517 1.085 1.094

1.515 1.083 1.092

1.516 1.091 1.100

119.7 120.5 119.6 120.6 120.1 119.7 119.3 122.8

119.7 120.5 119.6 120.7 120.0 119.7 119.2 122.6

119.8 120.5 119.6 120.6 120.0 119.7 119.1 122.9

119.4 120.5 119.6 120.3 120.1 119.7 120.1 119.9

119.4 120.5 119.6 120.4 120.0 119.7 120.0 119.6

119.5 120.5 119.6 120.4 120.0 119.7 120.0 120.1

117.9

118.2

117.9

120.0

120.4

119.9

120.7 119.0 120.8 119.7 120.5 119.6 120.0 119.9 120.1 120.1

120.7 119.1 120.6 119.8 120.4 119.6 120.0 119.9 120.1 120.1

120.8 119.0 120.8 119.7 120.5 119.6 119.9 120.0 120.1 120.0

120.1 119.0 120.8 119.7 120.5 119.6 120.4 119.8 119.8 124.2

120.1 119.1 120.6 119.7 120.4 119.6 120.3 119.8 119.9 124.4

120.2 119.0 120.8 119.7 120.5 119.6 120.3 119.8 119.9 124.1

119.0

118.9

118.9

121.0

121.0

121.1

110.2 108.7

110.2 108.7

110.2 108.8

180.0 0.0 0.0 180.0

180.0 0.0 0.0 180.0

Dihedral angle (°) C3–C4–C9–O10 180.0 C3–C4–C9–C11 0.0 C5–C4–C9–O10 0.0 C5–C4–C9–C11 180.0 C3–C4–C9–H11 C5–C4–C9–H11 C3–C4–O9–H10 C5–C4–O9–H10 b

4-Formyl benzonitrile

114.7

114.8

114.7

121.1

120.8

121.2

180.0

180.0

180.0

0.0

0.0

0.0

0.0 180.0

0.0 180.0

0.0 180.0

120.5 120.5 119.6 119.8 120.1 119.7 120.1

120.5 120.5 119.6 119.8 120.0 119.7 120.2

120.5 120.5 119.6 120.0 120.0 119.7 119.9

119.9

117.2

117.2

117.3

122.7 120.0 119.0 120.8 120.3 120.5 119.6 119.3 120.4 120.3

122.7 120.0 119.1 120.6 120.3 120.4 119.6 119.3 120.4 120.3

122.8 120.1 119.0 120.8 120.3 120.5 119.6 119.2 120.4 120.4

122.4 119.7

109.7

110.3

109.2

115.5

180.0 0.0

180.0 0.0

180.0 0.0

120.2

120.3

120.5

119.6 121.3 119.1

Mean value. Values taken from Ref. [33].

vibrations and distributed into the irreducible representations under Cs symmetry as 33 in-plane vibrations of A0 species and 15 out of plane vibrations of A00 species, i.e., Cvib = 33A0 +15A00 . The compound 4FBN has 39 fundamental modes of vibrations and distributed into the irreducible representations as 27 in-plane vibrations of A0 species and 12 out of plane vibrations of A00 species, i.e., Cvib = 26A0 + 13A00 . The compound 4HBN has 36 fundamental modes of vibrations and span into the irreducible representations under CS symmetry as 25 in-plane vibrations of A0 species and 11

out of plane vibrations of A00 species, i.e., Cvib = 25A0 + 11A00 . The vibrations present in the molecules are all active in both IR and Raman. Structural properties The optimised structural parameters bond lengths, bond angles and dihedral angles for the thermodynamically preferred geometry of 4ABN, 4FBN and 4HBN are determined by B3LYP method with 6–

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Table 2 The calculated thermodynamic parameters of 4-actetyl benzonitrile, 4-formyl benzonitrile and 4-hydroxy benzonitrile employing B3LYP method with 6–311++G**, 6–31G** and cc-pVDZ basis sets. Thermodynamic parameters (298 K)

Total energy (thermal), Etotal (kcal mol1 K1) Heat capacity at const. volume, Cv (kcal mol1 K1) Entropy, S (kcal mol1 K1) Vibrational energy, Evib (kcal mol1 K1) SCF (Hartree) Rotational Constants (GHz) X Y Z Dipole moment (Debye)

lx ly lz ltotal ELUMO+1 (eV) ELUMO (eV) EHOMO (eV) EHOMO1 (eV) ELUMO–HOMO (eV)

4-Actetyl benzonitrile B3LYP/6– 311++G**

B3LYP/6– 31G**

4-Formyl benzonitrile B3LYP/ccpVDZ

B3LYP/6– 311++G**

B3LYP/6– 31G**

4-Hydroxy benzonitrile B3LYP/ccpVDZ

B3LYP/6– 311++G**

B3LYP/6– 31G**

B3LYP/ccpVDZ

91.77

91.27

91.39

73.16

72.79

72.94

69.52

69.14

69.32

35.69

35.78

35.70

29.96

30.05

29.95

27.80

27.99

27.75

96.74 89.98

97.36 89.49

96.50 89.61

89.08 71.39

89.27 71.01

88.98 71.16

83.96 67.74

84.20 67.36

83.91 67.55

477.1480

477.2662

477.1666

437.8217

437.9327

437.8426

399.7220

399.8266

399.7410

3.67 0.57 0.50 2.82 2.19 0.20 3.58 7.8769 7.6255 2.8071 1.4283 4.8184

3.68 0.58 0.50

3.66 0.57 0.50

2.85 2.37 0.00 3.71

2.76 2.13 0.00 3.48

5.03 0.71 0.62 2.35 0.55 0.15 2.42 8.0369 7.8647 3.0499 1.6036 4.8148

5.06 0.71 0.62

5.01 0.71 0.62

2.38 0.71 0.00 2.48

2.30 0.50 0.00 2.35

5.61 0.99 0.84 1.34 5.02 0.14 5.20 7.9632 7.0064 1.5492 1.2713 5.4573

5.63 0.99 0.84

5.59 0.98 0.84

1.38 5.01 0.00 5.20

1.28 4.99 0.00 5.16

Fig. 3. The total electron density isosurface mapped with molecular electrostatic potential of (a) 4-acetyl benzonitrile (b) 4-formyl benzonitrile and (c) 4-hydroxy benzonitrile.

Fig. 4. The contour map of molecular electrostatic potential surface of (a) 4-acetyl benzonitrile (b) 4-formyl benzonitrile and (c) 4-hydroxy benzonitrile.

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Fig. 5. The molecular electrostatic potential surface of (a) 4-acetyl benzonitrile (b) 4-formyl benzonitrile and (c) 4-hydroxy benzonitrile.

31G**, 6–311++G** and cc-pVDZ basis sets are presented in Table 1. The SCF energy determined by B3LYP/6–311++G** method is lower than that of other methods of calculation. Thus the values determined by B3LYP/6–311++G** method is taken for correlation purposes. The SCF energy clearly reveals that 4ABN (477.14803 Hartree) is more stable by 24.68 kcal mol1 than 4FBN (437.82166 Hartree) while in turn 48.59 kcal mol1 more stable than 4HBN (399.7218 Hartree). The 4FBN molecule has 23.91 kcal mol1 more energy than that of 4HBN molecule. All the C–C bond distances lies in the range 1.387–1.408 Å. Thus, the influence of the substituent on the molecular parameters, particularly in the C–C bond distance of ring carbon atoms is not significant. The C1–C7 and C4–C9 bond length of 4ABN, 4FBN are longer, where the cyano and acetyl and formyl groups are attached. This is due to the electron withdrawing nature of the groups. The longer bond length of C1–C2 and C1–C6 indicates that the cyano group exerts larger attraction on valance electron cloud of the ring resulting easy delocalisation of electrons towards the cyano group and thereby decreases in force constant and increase in bond length. The C@O bond length of 4ABN is 0.005 Å longer than 4FBN. The O–H, C–C, C@O and C„N bond lengths determined theoretically are well agreed with the experimental values of p-cyanobenzoic acid [32], p-cyanophenol [33], 2-fluoro-4-hydroxybenzonitrile [34], 3,5-dihalo-4-hydroxybenzonitriles [35] and 2-formylbenzonitrile [36]. With the electron donating and withdrawing substituents on the benzene ring, the symmetry of the ring is distorted, yielding variation in bond angles at the point of substitution and at the ortho and meta positions as well. The angles at the point of substitution C3– C4–C5 of –OH in 4HBN, –COCH3 in 4ABN and –CHO in 4FBN are 120.1, 119.3 and 120.1°, respectively. The bond angle C2–C1–C6 in 4HBN is 119.3° while this is equal to 120.0 and 120.4° in 4ABN and 4FBN, respectively. Similarly the variation in the bond angles in the ortho and meta positions with respect to the substitutions may also vary due to the different electronic effects. The C4–C9– O10 bond angle of 4ABN (120.1°) is less than in 4FBN (124.2°) and is due to the role of steric hindrance of the methyl group. Thermodynamic properties The thermodynamic parameters of the compounds 4ABN, 4FBN and 4HBN are computed and presented in Table 2, in order to get reliable data from which the relations among energetic, structural and reactivity characteristics of the molecules are clarified. More experimental works were performed for the substituents such as methyl, amino, hydroxy and halogens while the studies of the thermochemistry of compounds with other less common substituents such as the cyano group are rare [37–39]. The total thermal energy, vibrational energy contribution to the total energy, the rotational

constants and the dipole moment values of the compounds were determined by B3LYP method. The dipole moment of 4HBN (5.20 D) is greater than that of other two compounds namely 4ABN (3.58 D) and 4FBN (2.42 D). The larger dipole moment of 4HBN is due to the high delocalization of oxygen lone pair of electrons into the benzene ring. Such kind of delocalization is not possible in the case of 4ABN and 4FBN. Among 4ABN and 4FBN molecules, the 4ABN has greater dipole moment than that of 4HBN. This is due to the electron donating methyl group of – COCH3. Analysis of molecular electrostatic potential (MESP) The total electron density and MESP surfaces of the molecules under investigation are constructed by using B3LYP/6– 311++G(d,p) method. Molecular electrostatic potential (MESP) mapping is very useful in the investigation of the molecular structure with its physiochemical property relationships [40,41,30,42]. The total electron density mapped with electrostatic potential surface, the contour map of electrostatic potential and molecular electrostatic potential surface of 4ABN, 4FBN and 4HBN are shown in Figs. 3–5. The colour scheme for the MESP surface is red, electron rich, partially negative charge; blue, electron deficient, partially positive

Table 3 The natural charges of 4-acetyl benzonitrile (4ABN), 4-formyl benzonitrile (4FBN) and 4-hydroxy benzonitrile (4HBN) determined by B3LYP/6–311++G** method. Atom

4ABN

4FBN

4HBN

C1 C2 C3 C4 C5 C6 C7 N8 C9 O9 O10 C11 H10 H11 H12 H13 H14 H15/12/11 H16/13/12 H17/14/13 H18/15/14

0.1588 0.1517 0.1714 0.1272 0.1485 0.1497 0.2848 0.2988 0.5574

0.1525 0.1594 0.1456 0.1456 0.1496 0.1524 0.2827 0.2930 0.4198

0.2107 0.1293 0.2453 0.3375 0.2782 0.1287 0.2897 0.3227

0.5431 0.6708

0.5136

0.6601

0.4712 0.1099 0.2225 0.2225 0.2335 0.2224 0.2153 0.2382 0.2233

0.2242 0.2155 0.2349 0.2246

0.2210 0.2262 0.2088 0.2208

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charge; light blue, slightly electron deficient region; yellow, slightly electron rich region; green, neutral; respectively. The Figs. 3 and 4 indicate that the region around oxygen and nitrogen atoms represents the most negative potential region (red). The predominance of light green region in the MESP surfaces corresponds to a potential halfway between the two extremes red and dark blue colour. The benzene ring of all the compounds possesses slight negative potential due to the electron releasing substituents. The isosurface clearly reveals the presence of high positive charge on the methyl hydrogen atoms. Topological charge stabilisation The atomic charges of 4ABN, 4FBN and 4HBN determined by NBO method is presented in Table 3. Nature prefers to place the atoms of greater electronegativity in those positions where the

topology of the structure tends to pile up an extra charge [43]. However, the first Hohenberg–Kohn theorem demonstrates that the ground state properties of a many-electron system are uniquely determined by the electron density that depends only on the 3 spatial coordinates. Among the nitrogen atoms, the value of negative charge is greater in N8 of 4HBN (0.3227) than 4ABN (0.2988) and 4FBN (0.2930). This is due to the more electron donating nature of the –OH group in 4HBN molecule. Due to the hyper conjugative effect of the methyl group, the oxygen atom present in –COCH3 group posses more negative charge (0.5431) than –CHO group of 4FBN. The carbon atoms C7 and C9 have positive charges due to the high electronegativity of the nitrogen and oxygen, respectively. The ring carbon atoms of all the compounds possess small negative charge. The more negative charges of the carbon atoms of 4HBN reveals that the delocalization of lone pair electron into the ring due to the electron releasing substituent takes place.

Fig. 6. The frontier molecular orbitals of (a) 4-acetyl benzonitrile (b) 4-formyl benzonitrile and (c) 4-hydroxy benzonitrile.

V. Arjunan et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 98 (2012) 156–169

163

Electronic properties

Scale factors

The energies of four important molecular orbitals of 4ABN, 4FBN and 4HBN molecules, the second highest and highest occupied MO’s (HOMO and HOMO1), the lowest and the second lowest unoccupied MO’s (LUMO and LUMO+1) were calculated and are presented in Table 2. The energy gap between HOMO and LUMO is a critical parameter in determining electrical transport properties [44]. The 3D plots of important molecular orbitals are shown in Fig. 6. The energy gap of HOMO–LUMO explains the eventual charge transfer interaction within the molecule, and the frontier orbital gap in the case of 4ABN, 4FBN and 4HBN molecules are found to be 4.8184, 4.8148 and 5.4573 eV, respectively obtained at TD-DFT method using 6–311++G** basis set. The molecular orbitals shows that the electron density in the HOMO mostly centered on the acetyl and formyl groups and part of the benzene ring while in LUMO the electron density predominantly located throughout the molecule; indicating a most intense HOMO ? LUMO charge transfer of the type p ? p*. The second transition also a p ? p* transition (HOMO1 ? LUMO and HOMO ? LUMO+1). Another weak transitions corresponds to a charge transfer from the substituent groups to the benzonitrile group (n ? p*) upon excitation. The absolute chemical hardness has been used as a measure of kinetic stability or the reactivity of the molecule. The chemical hardness of the molecules has been determined as 2.4092, 2.4074 and 2.7286 respectively for 4ABN, 4FBN and 4HBN. This signifies that the 4HBN molecule is less reactive than that of other two molecules. The energy gap values of LUMO–HOMO for 4ABN and 4FBN are not much differed. The energy gap of 4HBN is more than other two compounds.

A better agreement between the computed and experimental frequencies can be obtained by using different scale factors for different types of fundamental vibrations [45–54] that minimises the residual separating experimental and theoretically predicted vibrational frequencies. The optimum scale factors for vibrational frequencies were determined by a least-squares procedure of the scaled harmonic frequencies to the experimental fundamentals, minimising the residual

where, xTheo and mExpt are the ith theoretical harmonic frequency i i and ith experimental fundamental frequency (in cm1), respectively and N is the number of frequencies included in the optimisation which leads to

Fig. 7. FTIR spectrum of a) p-acetyl benzonitrile (b) p-formyl benzonitrile and (c) phydroxy benzonitrile.

Fig. 8. FT-Raman spectrum of (a) p-acetyl benzonitrile (b) p-formyl benzonitrile and (c) p-hydroxy benzonitrile.

D¼

N  2 X kxTheor  mExpt i i

ð3Þ

i

RMS ¼

rffiffiffiffi D N

ð4Þ

Due to the large anharmonicities of C–H, C„N and O–H stretching frequencies >2700 cm1 were scaled by different scale factors [52,53]. In the present investigation for 4HBN a scale factor of 0.86 for O–H stretching, 0.96 for C–H and C„N stretching vibrations and 0.97 for all other vibrational modes has been used. For 4FBN a scale factor of 0.96 for aromatic C–H and C„N stretching vibrations, 0.98 for aldehyde C–H vibrational mode, for C@O group a scale factor of 0.95 and for other fundamental modes 0.97 were used to get the scaled wavenumbers. In the case of 4ABN a scale factor of 0.96 for aromatic C–H and C„N stretching vibrations

Species Observed wavenumber (cm1) FTIR

A0 A0 A00 A00 A00 A00 A00 A00

3075 m 3030 w 2980 vw 2940 vw 2905 vw 2212 vs 1678 vs 1593 s 1556 m 1485 vw 1415 m 1395 vs 1349 s 1285 s 1259 vs 1198 vw 1163 vw 1102 m 1063 w 1002 m 956 s

844 m 829 vs

644 m 590 s 556 m 534 s

Unscaled (cm1)

B3LYP/6–31G** Calculated wavenumber

B3LYP/cc-pVDZ Calculated wavenumber

Depolarization ratio

% PED

0.16 0.17 0.59 0.74 0.55 0.75 0.009 0.32 0.26 0.42 0.28 0.72 0.75 0.75 0.46 0.74 0.32 0.67 0.24 0.25 0.17 0.60 0.16 0.75 0.20 0.75 0.75 0.19 0.75 0.75 0.09 0.75 0.75 0.47 0.75 0.74 0.75 0.73

95mCH 92mCH 91mCH 94mCH 90maCH3 92maCH3 96msCH3 97mCN 95mCO 91mCC 93mCC 90mCC 94mCC 92mCC 80daCH3 + 15bCO 82dsCH3 + 17bCO 78mC–CN + 16mCC 87mCC 80mC–CH3 + 12mCO 84mC–CO + 10mC–CH3 67xCH3 + 16cCO + 12cCC 66bCH + 21bCCC + 11bCCO 62bCH + 25bCCC 68bCH + 20bCCC 60bCCC + 17bCH + 16bCCN 59bCH + 15bCCC + 14bCCN 57qCH3 + 16bCO + 12bCC 47bCCC + 28bCH 52bCCC + 24bCH + 18bCCO 66bCO + 20bCH + 12bCCC 49bCCC + 32bCH 45cCH + 25cCCC + 16cCCN 42cCH + 21cCCC + 24cCCO 47cCH + 29cCCC 59cCO + 21cCH + 20cCCC 55bCN + 18bCC + 12bCH 42cCH + 32cCCC 43bC– CH3 + 22bCO + 15bCC 45bC– CO + 24bCO + 18bCCC 40bC– CN + 28bCCC + 14bCH 39cCCC + 25cCH + 15cCCN 38cCCC + 29cCH + 12cCCO 42cCCC + 35cCH 40cC–CH3 37cCN + 34cCCC + 22cCH 39cC–

Scaled (cm1)

IR intensity

Unscaled (cm1)

Scaled (cm1)

IR intensity

Raman Intensity

Unscaled (cm1)

Scaled (cm1)

IR intensity

3090 m 3207 3074 w 3204 3192 3025 vw 3190 3147 2942 w 3091 3034 2220 vs 2335 1680 s 1753 1603 vs 1645 1593 1482 vw 1531 1430 vw 1480 1408 vw 1470 1433 1390 1335 1270 m 1323 1269 1221 1179 m 1201 1137 1077 m 1090 1045 1034 965 vw 1009 981 932 vw 961 869 816 w 849 794 w 806 731 vw 747 657 653 w 650 604 546 w 576 557 544

3079 3076 3064 3062 2990 2936 2882 2218 1665 1596 1545 1485 1436 1426 1390 1348 1295 1283 1231 1184 1165 1103 1057 1014 1003 979 952 932 843 824 782 725 637 631 586 559 540 528

1.89 3.15 0.57 1.82 9.71 6.25 2.42 28.23 214.33 17.29 7.85 1.00 12.91 12.26 26.73 43.23 5.34 9.81 217.62 2.32 13.01 4.12 2.19 0.54 7.37 0.35 0.05 39.34 28.60 22.69 1.67 0.02 1.06 15.34 22.58 1.68 3.98 13.68

3228 3222 3212 3207 3171 3113 3050 2349 1784 1661 1608 1542 1492 1482 1445 1400 1347 1334 1282 1227 1206 1141 1095 1050 1035 1004 978 966 871 851 809 745 656 651 607 570 555 542

3099 3093 3084 3079 3012 2957 2898 2232 1695 1611 1560 1496 1447 1438 1402 1358 1307 1294 1244 1190 1170 1107 1062 1019 1004 974 949 937 845 825 785 723 636 631 589 553 538 526

2.05 4.41 0.91 3.11 8.90 7.39 2.25 18.93 159.51 13.87 7.27 1.09 10.17 8.34 29.47 42.69 9.04 2.93 214.30 3.01 10.74 4.73 2.41 1.05 5.77 0.38 0.03 33.60 17.92 21.35 1.44 0.002 1.17 13.93 17.24 3.54 3.53 13.30

0.227 0.165 0.118 0.066 0.199 0.087 0.206 1.000 0.127 0.627 0.006 0.000 0.041 0.018 0.007 0.008 0.001 0.004 0.087 0.065 0.087 0.000 0.051 0.004 0.003 0.001 0.003 0.008 0.007 0.004 0.041 0.002 0.009 0.004 0.001 0.002 0.008 0.007

3220 3215 3204 3200 3159 3106 3038 2340 1774 1657 1605 1529 1450 1439 1434 1373 1349 1314 1278 1223 1190 1127 1089 1044 1029 1015 988 960 877 854 808 759 654 652 612 571 556 543

3091 3086 3076 3072 3001 2951 2886 2223 1685 1607 1557 1483 1407 1396 1391 1332 1309 1275 1240 1186 1154 1093 1056 1013 998 985 958 931 851 828 784 736 634 632 594 554 539 527

2.02 3.49 0.74 2.62 8.19 5.90 1.86 17.32 164.10 14.38 7.43 1.39 9.34 8.74 28.07 57.43 8.19 3.09 195.44 2.96 9.60 6.60 2.73 0.84 6.57 0.38 0.003 31.68 18.87 15.48 1.79 0.02 0.21 14.36 16.32 3.35 3.66 13.57

443

430

0.64

443

430

0.61

0.003

443

430

0.69

0.69

411

399

0.00

414

402

0.14

0.001

415

403

0.01

0.75

406 321 230 209 150 133

394 311 223 203 146 129

0.00 0.16 5.39 1.09 0.06 7.47

408 322 230 211 167 132

396 312 223 205 162 128

0.49 0.24 4.80 1.18 0.23 7.20

0.003 0.009 0.000 0.001 0.000 0.005

409 322 230 212 176 132

397 312 223 206 171 128

0.05 0.27 4.38 1.03 0.22 6.25

0.75 0.27 0.73 0.75 0.75 0.75

437 w 408 vw

337 w 227 w 169 m 150 w

V. Arjunan et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 98 (2012) 156–169

A0 A0 A0 A0 A00 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A00 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A00 A00 A00 A00 A0 A00 A0

FTR

B3LYP/6–311++G** Calculated wavenumber

164

Table 4 The observed FTIR, FT-Raman and calculated frequencies using B3LYP method using 6–31G**, 6–311++G** and cc-pVDZ basis sets along with their relative intensities, probable assignments and potential energy distribution (%PED) of 4acetylbenzonitrile.a

V. Arjunan et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 98 (2012) 156–169

165

CO + 29cCCC + 21cCH 37cCCC + 32cCH + 25cCC 35sCH3 + 35cCO

and 0.97 for methyl C–H vibrational modes. The scale factor of 0.96 for keto group and 0.97 for other fundamental modes were used. The determined RMS deviation for DFT methods is only 8–14. The correlation diagram for the calculated and the experimental frequencies of 4ABN, 4FBN and 4HBN are shown in Supplementary Fig. S1. Vibrational analysis

5.49 1.22

0.75 0.75

The combined experimental FTIR and FT-Raman spectra of 4ABN, 4FBN and 4HBN are presented in Figs. 7 and 8, respectively. The observed FTIR, FT-Raman and calculated frequencies using B3LYP method with 6–31G**, 6–311++G** and cc-pVDZ basis sets along with their relative intensities, probable assignments and potential energy distribution (PED) of 4ABN, 4FBN and 4HBN are summarized in Tables 4–6, respectively.

m – stretching; b – in-plane bending; d – deformation; q – rocking; c – out of plane bending; x – wagging and s – torsion. a

A00 A00

108 s

77 56

77 56

5.13 3.35

79 67

77 65

5.99 1.84

0.001 0.002

79 71

77 69

C–C vibrations The carbon–carbon stretching modes of aromatic ring are usually occurred in the range from 1650 to 1200 cm1. The actual positions of these modes determined are not affected much by the nature of the substituents, but by the form of substitution around the ring. In 4ABN, the C–C stretching vibrations are observed at 1593, 1556, 1485, 1415, and 1259 cm1 in IR spectrum and 1603, 1482, 1430, 1408 and 1270 cm1 in Raman spectrum. The IR bands observed at 1593 and 1285 cm1 are strong while the Raman band 1603 cm1 is very strong. In addition, C–C–C in–plane bending vibrations are attributed to 1002 and 844 cm1 in IR spectrum and 794 cm1 in Raman spectrum. The C–C–C out of plane vibrations are observed at 337, 227 and 108 cm1 in Raman spectrum. In 4FBN, the C–C stretching vibrations are observed at 1610, 1575, 1419, and 1313 cm1 in IR spectrum and 1611, 1482, and 1408 cm1 in Raman spectrum. The 1610 and 1313 cm1 are strong bands in IR and the 1611 cm1 is a very strong band in Raman spectrum. The C–C–C in–plane bending modes are assigned to 639 and 579 cm1 band in IR spectrum and 794 cm1 in Raman spectrum. The C–C–C out of plane vibrations are observed at 369, 256 and 233 cm1 in Raman. In 4HBN, the C–C stretching vibrations are observed at 1615, 1587, 1510, 1449, 1326 and 1249 cm1 in IR and 1611, and 1591 cm1 in Raman spectra. The 1615, 1587 and 1510 cm1 are very strong bands in IR and the band 1611 cm1 is very strong in Raman spectrum. No indication of C–C–C in–plane bending in IR spectrum but peak is observed at 441 cm1 in Raman spectrum. In benzene, the ring breathing (a1g) mode and the CCC trigonal bending (b1u) vibrations exhibit the characteristic frequencies at 995 and 1010 cm1, respectively [55]. In 4ABN the ring breathing modes are observed at 932 while the trigonal bending is attributed to the wavenumber 1002 cm1. In the case of 4FBN and 4HBN the ring breathing modes were observed at lower frequencies than that of benzene. This is due the presence of different kinds of substituents in the ring. All these assignments are well agreed with the literature [32,37]. C–H vibrations The C–H stretching vibrations of aromatic compounds are usually occurring between 3100 and 3000 cm1 [56] and the vibrations are not affected appreciably by the presence of substituents [57]. In 4ABN, the C–H stretching peaks are observed in IR at 3075 and 3030 cm1 and in Raman spectrum at 3090, 3074 and 3025 cm1. In 4FBN, the corresponding C–H peaks are observed in IR at 3092 and 3047 cm1 and in Raman medium peak occurs at 3061 cm1. In 4HBN, the peaks observed in IR at 3079 and 3031 cm1 and in Ra-

Species Observed wavenumber (cm1) FTIR

A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A00 A00 A0 A00 A00 A00 A00 A0 A0 A0 A00 A0 A0 A00 A00 A00 A00 A00 A00 a

B3LYP/6–31G** Calculated wavenumber

B3LYP/cc-pVDZ Calculated wavenumber

Depolarization ratio

% PED

91mCH 94mCH 92mCH 90mCH 92mCH(O) 2 x 1389 1419 + 1319 97mCN 96mC@O 93mCC 92mCC 91mCC 90mCC 85bCH(O)+12bCH 89mCC 87mC–CN 85mC–CHO 88mCC 55bCH+18bCCC+14bCCN 52bCH+24bCCC 50bCH+21bCCC+15bCCO 47bCH+28bCCC 42cCH(O)+24cCO+24cCC 45cCH+32cCCC+20cCCO 50bC@O 57cCH+28cCCC 44cCH+28cCCC+18cCCN 56cC@O+24cCH+12cCCC 46cCH+38cCCC 49bCCC+27bCH+22bCCN 45bCCC+37bCH 41bCN+29bCC+24bCH 44cCCC+36cCH 52bC–CHO 54bC–CN 40cCCC+34cCH+20cCCO 42cCCC+36cCH 43cCCC+24cCH+14cCCN 49cCN+27cCC+14cCH 47cC–CHO+37cCCC 40cCCC+32cCCC

Unscaled (cm1)

Scaled (cm1)

IR intensity

Unscaled (cm1)

Scaled (cm1)

IR intensity

Raman Intensity

Unscaled (cm1)

Scaled (cm1)

IR intensity

3204 3201 3191 3169 2904

3076 3073 3063 3042 2846

1.24 1.48 0.18 3.63 110.88

3225 3221 3212 3187 2914

3096 3092 3084 3060 2856

1.56 2.47 0.24 4.84 127.72

0.252 0.150 0.084 0.124 0.261

3215 3211 3202 3178 2892

3086 3083 3074 3051 2834

1.39 1.84 0.19 4.28 131.35

0.17 0.26 0.72 0.37 0.30

2335 1773 1646 1598 1532 1419 m 1408 vw 1442 1389 s 1411 1313 s 1331 1298 s 1325 1205 vs 1197 m 1222 1217 1175 s 1173 s 1188 1117 vw 1110 vw 1131 1014 m 1034 998 vw 1029 971 vw 1003 979 833 vs 828 w 865 797 vw 852 849 735 s 732 w 746 707 w 704 w 726 639 vw 647 m 656 579 vw 586 543 vs 545 w 569 556 416 vw 434 410 369 w 375 256 vw 259 233 vw 230 154 m 148 140 77

2242 1702 1613 1566 1501 1413 1383 1304 1299 1198 1193 1164 1108 1013 988 963 940 830 818 815 716 697 630 563 546 534 417 394 360 249 221 142 134 77

21.74 306.82 31.68 11.17 1.26 6.76 16.62 19.43 9.41 58.67 1.85 37.24 3.76 4.00 2.00 0.04 0.49 11.62 37.51 41.42 25.85 1.90 0.87 2.81 17.99 5.14 2.57 0.01 1.34 8.40 5.67 2.23 10.11 8.67

2350 1803 1662 1614 1543 1455 1427 1347 1331 1231 1223 1193 1135 1035 1035 998 978 867 858 849 747 731 655 584 570 550 437 415 375 263 231 155 138 80

2233 1713 1612 1566 1497 1411 1384 1307 1291 1194 1186 1157 1101 1004 1004 968 949 841 832 824 725 709 635 566 553 534 424 403 364 255 224 150 134 80

14.34 230.50 24.96 10.34 1.07 5.75 21.09 17.97 7.59 56.24 6.90 27.72 4.09 2.37 0.48 0.002 0.15 7.13 36.21 34.18 23.30 1.45 0.80 4.22 14.78 3.94 0.54 0.13 1.03 8.01 5.40 1.65 9.59 7.80

1.000 0.235 0.652 0.012 0.001 0.007 0.006 0.002 0.005 0.063 0.056 0.125 0.002 0.001 0.017 0.001 0.002 0.010 0.040 0.003 0.009 0.001 0.009 0.001 0.008 0.009 0.003 0.001 0.010 0.001 0.002 0.001 0.005 0.001

2341 1793 1657 1610 1529 1439 1407 1349 1310 1227 1217 1177 1120 1039 1028 1010 988 872 856 855 746 742 652 584 572 551 442 415 375 267 231 158 138 81

2247 1703 1607 1562 1483 1396 1365 1309 1271 1190 1180 1142 1086 1008 997 980 958 846 830 829 724 720 632 566 555 534 429 403 364 259 224 153 134 81

13.02 237.56 24.77 10.63 1.12 10.47 16.36 16.92 8.73 49.63 18.25 17.62 5.39 0.29 2.76 0.002 0.23 10.12 35.54 24.90 21.42 0.88 0.71 4.45 15.00 4.03 1.34 0.02 1.00 6.83 5.12 1.47 8.44 6.47

0.32 0.33 0.42 0.31 0.51 0.38 0.75 0.38 0.65 0.25 0.22 0.18 0.16 0.51 0.75 0.75 0.75 0.75 0.12 0.75 0.24 0.75 0.74 0.40 0.75 0.74 0.75 0.75 0.31 0.75 0.52 0.75 0.75 0.75

3092 m 3061 m 3047 2854 2809 2748 2233 1713 1610 1575

w m w w s vs s m

2230 s 1705 m 1611 vs

m – stretching; b – in-plane bending; and c – out of plane bending.

V. Arjunan et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 98 (2012) 156–169

A0 0 A0 A0 A0 A0

FTR

B3LYP/6–311++G** Calculated wavenumber

166

Table 5 The observed FTIR, FT-Raman and calculated frequencies using B3LYP method with 6–31G**, 6–311++G** and cc-pVDZ basis sets along with their relative intensities, probable assignments and potential energy distribution (%PED) of 4formylbenzonitrile.a

Table 6 The observed FTIR, FT-Raman and calculated frequencies using B3LYP method with 6–31G**, 6–311++G** and cc-pVDZ basis sets along with their relative intensities, probable assignments and potential energy distribution (%PED) of 4hydroxybenzonitrile.a B3LYP/6–311++G** Calculated wavenumber

Species Observed wavenumber (cm1) FTIR

Unscaled (cm1)

3289 s 3079 w

A00

820 m

832 w

703 s 673 s

708 vw 639 w 547 vw 441 vw

368 251 145 100

vw vw m s

Depolarization ratio

% PED

0.28 0.18 0.25 0.73 0.35 0.32 0.48 0.62 0.46 0.45 0.59 0.40 0.11 0.25 0.37 0.20 0.64 0.21 0.75 0.75 0.10

98mOH 95mCH 92mCH 96mCH 91mCH 94mCN 93mCC 92mCC 95mCC 90mCC 92mCC 89mC–CN 87mCC 94mCO 85bOH+15bCH 74bCH+12bCCC 70bCH+14bCCC+10bCCN 65bCH+17bCCC+14bCO 62bCH+24bCCC 60cCH+25cCCC 58cCH+20cCCC+12 cCCN 55cCH+22cCCC+14 cCCO 64bCO+25bCCC 74bCCC+20 bCH 62cCH+33cCCC 65cCO+18cOH 59bCN+28bCC 52bCCC+25bCH 50bCCC+28bCH 51bC–CN+30bCCC 49cCCC+28cCH+18cCCN 47cCCC+35cCH+12cCCO 55cOH+30cCO 46cCCC+35cCH 40cCN+37cCC 42cCCC+32cCH

Scaled (cm1)

IR intensity

Unscaled (cm1)

Scaled (cm1)

IR intensity

Raman Intensity

Unscaled (cm1)

Scaled (cm1)

IR intensity

3294 3077 3070 3063 3036 2235 1617 1586 1510 1432 1337 1298 1268 1201 1170 1163 1105 1007 956 926 833

92.11 1.20 1.68 1.28 10.55 69.12 122.45 19.96 98.44 13.73 34.89 0.97 129.30 0.67 0.96 189.56 18.36 0.23 0.01 1.81 7.57

3820 3225 3217 3211 3178 2344 1669 1632 1557 1475 1379 1333 1322 1232 1200 1196 1133 1030 970 950 855

3285 3096 3088 3083 3051 2250 1619 1583 1510 1431 1338 1293 1282 1195 1164 1160 1099 999 941 922 829

68.15 2.17 2.92 1.88 12.65 52.18 117.14 22.85 83.66 16.91 40.56 0.95 111.46 1.13 25.04 169.27 15.15 0.06 0.003 0.54 5.57

0.341 0.359 0.177 0.133 0.215 1.000 0.266 0.012 0.024 0.001 0.007 0.003 0.016 0.088 0.011 0.055 0.001 0.000 0.003 0.005 0.074

3774 3215 3208 3200 3168 2335 1666 1629 1544 1465 1379 1320 1315 1226 1197 1180 1118 1022 983 962 856

3283 3086 3080 3072 3041 2242 1616 1580 1498 1421 1338 1280 1276 1189 1161 1145 1084 991 954 933 830

80.37 1.73 2.20 1.76 12.16 49.60 118.20 23.14 92.22 14.99 40.25 24.77 85.92 0.90 139.34 34.70 13.42 0.07 0.003 0.26 35.80

845

828

44.70

849

824

36.18

0.004

854

828

5.71

0.75

815 710 704 664 567 562 481 418 414 404 347 261 157 105

799 696 690 651 556 551 471 410 406 396 340 256 154 103

17.98 16.75 0.74 0.59 0.76 24.59 1.33 0.49 0.48 10.35 113.44 3.54 5.56 2.15

818 722 712 663 564 558 484 418 418 405 385 266 154 108

793 700 691 643 547 541 469 405 405 393 373 258 149 105

14.97 0.55 13.37 0.63 17.90 0.73 0.00 0.49 0.15 9.96 121.34 2.44 5.28 2.18

0.013 0.001 0.001 0.011 0.008 0.006 0.009 0.000 0.016 0.001 0.007 0.000 0.008 0.000

821 737 711 661 566 559 486 419 417 402 396 267 155 107

796 715 690 641 549 542 471 406 404 390 384 259 150 104

10.56 0.19 14.06 0.64 17.69 0.53 0.08 2.32 0.21 9.27 105.25 1.81 4.29 1.60

0.75 0.75 0.08 0.74 0.75 0.75 0.75 0.75 0.36 0.74 0.75 0.75 0.75 0.75

3830 3205 3198 3062 w 3191 3031 w 3162 2234 vs 2236 vs 2328 1615 vs 1611 vs 1650 1587 vs 1591 m 1618 1510 vs 1541 1449 m 1461 1326 vw 1364 1286 vs 1324 1249 m 1294 1224 s 1211 w 1225 1193 m 1197 m 1194 1167 vs 1172 m 1187 1106 w 1103 vw 1128 1000 vw 1028 961 vw 975 945 840 vs 844 m 850

548 vs

B3LYP/cc–pVDZ Calculated wavenumber

m – stretching; b – in-plane bending; c – out of plane bending.

V. Arjunan et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 98 (2012) 156–169

A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A0 A00 A00

A0 A0 A00 A00 A0 A0 A0 A0 A00 A00 A00 A00 A00 A00 a

FTR

B3LYP/6–31G** Calculated wavenumber

167

168

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man at 3062 cm1 are assigned to C–H stretching vibrations. The C– H stretching vibrations of 4ABN, 4FBN and 4HBN are not much affected by the substitution of –CH3, –CHO and –OH respectively. At the same time the C–H frequencies of 4FBN is observed in the higher range than that of other compounds. The intensities of the C–H stretching of 4HBN are higher than that of other two compounds. The mode observed at 2854 cm1 in 4FBN is attributed to the C–H stretching of the –CHO group. The aromatic C–H in-plane bending modes of benzene and its derivatives are usually observed in the region 1300–1000 cm1 [57]. These peaks are observed in IR at 1198, 1163, and 1102 cm1 and in Raman at 1179 cm1 for the 4ABN. In 4FBN, the peaks observed in IR at 1175, 1117 and 1014 cm1 and in Raman, 1173, 1110 and 998 cm1 are assigned to C–H in–plane bending modes. The peaks 1175 cm1 in IR and at 1173 cm1 in Raman observed in 4FBN are the strong modes. The peaks observed in IR at 1106 and 961 and in Raman at 1103 and 1000 cm1 are the C–H in-plane bending modes of 4HBN. The C–H out of plane bending vibrations is strongly coupled vibrations and occurs in the region 910–660 cm1. These extremely intense absorptions are used to assign the position of substituent on the aromatic ring [58]. A strong and medium peak at 644 and 534 cm1 are observed in IR and weak peaks at 731 and 653 cm1 in Raman occurred in the compound 4ABN. In 4FBN, the weak peaks observed in IR at 707 cm1 and in Raman at 704 cm1 are assigned to C–H out of plane bending vibrations. In 4HBN, the corresponding peaks are observed in IR at 840, 820 and 673 cm1 in Raman at 844 and 832 cm1. Very strong peaks are observed in the IR spectrum of 4HBN.

–C„N group vibrations The –C„N stretching absorption is sharp and observed in the narrow region between 2260 and 2200 cm1 [26,28]. In the case of benzonitrile compounds conjugation occurs and that reduces to the frequency of absorption of –C„N to 2240–2200 cm1. In this region the bands are not affected appreciably by the nature of substituents. The intensity of the –C„N stretching mode of 4ABN and 4HBN is very strong than 4FBN compound. The –C„N stretching band of 4FBN and 4HBN is 21 cm1 higher than that of 4ABN. In 4ABN, the –C„N stretching band occurs at 2212 cm1 in IR and 2220 cm1 in Raman spectra. The in-plane bending peaks observed at 556 cm1 in IR and 546 cm1 in Raman spectra. The out of plane vibrations observed at 169 cm1 in the Raman spectrum. In 4FBN, the –C„N stretching band occurs at 2233 cm1in IR and 2230 cm1 in Raman spectra. The in-plane bending peaks observed at 543 cm1 in IR and 545 cm1 in Raman spectra. The out of plane vibrations observed at 154 cm1 in Raman spectrum. In 4HBN, the –C„N stretching band occurs at 2234 cm1 in IR and 2236 cm1 in Raman spectra. The in-plane bending peak is observed at 547 cm1 in Raman and a very strong peak at 548 cm1 in IR spectrum. The out of plane bending is observed at 145 cm1 in Raman spectrum.

–OCH3 group vibrations The ma(CH3) frequencies are established at 2980 and 2940 cm1 in the infrared and 2942 cm1 in the Raman spectra, respectively while ms(CH3) is assigned at 2905 cm1 in the infrared spectrum of 4ABN. The asymmetric and symmetric methyl deformation modes of –OCH3 group are obtained at 1395 and 1349 cm1 in IR spectrum. The methyl deformational modes mainly coupled with the CH in-plane bending vibrations. The methyl rocking and wagging modes are presented in the Table 3. These assignments are substantiated by the reported literature [37,58].

C@O group vibrations Ketones, and aldehydes generally show IR absorption at 1750– 1650 cm1. Conjugation, ring size, hydrogen bonding, and electronic effects often result in significant shifts in C@O absorption frequencies [59,60]. The C@O stretching vibration of 4FBN is observed in higher frequency than in 4ABN and is due to the steric influence of methyl group. In 4ABN, the stretching vibrations occur at 1678 cm1 in IR and 1680 cm1 in Raman spectra. The in-plane bending vibrations occur at 829 cm1 in IR and at 816 cm1 in Raman spectra. The out of plane bending peak is observed at 829 cm1 in IR and 816 cm1 in Raman spectra. In 4FBN, the C@O stretching vibrations occur at 1713 cm1 in IR and 1705 cm1in Raman spectra. The in–plane bending vibrations occur at 833 cm1 in IR and at 828 cm1 in Raman spectra. The out of plane bending peak observed at 590 cm1 in IR and no peaks observed in Raman spectra.

–O–H group vibrations For solids, liquids and concentrated solutions a broad band of less intensity is normally observed [61,62]. Unassociated hydroxyl groups absorbs strongly in the region 3670–3580 cm1. The compound under investigation 4HBN shows the stretching of hydroxyl group at 3289 cm1 in the infrared spectrum. The lower stretching frequency observed signifies that there is a possibility of intermolecular hydrogen bonding between the hydroxyl group. The inplane –O–H bending vibration give rise to the medium band in infrared at 1193 cm1 and at 1197 cm1 in Raman spectra. The out of plane bending of –O–H is observed at 368 cm1 in the Raman spectrum and peaks not observed in the IR spectrum.

Conclusion A complete structural, thermodynamic, vibrational and electronic investigation by quantum chemical studies along with experimental FTIR and FT-Raman spectra were carried out for the 4-acetyl benzonitrile, 4-formyl benzonitrile and 4-hydroxy benzonitrile. All the properties and the influences of the substituents of the compounds were compared with each other. The SCF energy clearly reveals that 4ABN (477.14803 Hartree) is most stable by 24.68 kcal mol1 than 4FBN (437.82166 Hartree) while in turn 48.59 kcal mol1 more stable than 4HBN (399.7218 Hartree). The 4FBN molecule has 23.91 kcal mol1 more energy than that of 4HBN molecule. The intensity of the –C„N stretching mode of 4ABN and 4HBN is very strong than 4FBN compound. The –C„N stretching band of 4FBN and 4HBN is 21 cm1 higher than that of 4ABN. The geometrical parameters and normal modes of vibration obtained from DFT methods are in good agreement with the experimental data. The relative stabilities, HOMO–LUMO energy gaps and implications of the electronic properties are examined. This indicating a most intense HOMO ? LUMO charge transfer of the type p ? p*. Thus the present investigation provides geometrical parameters, kinetic and thermodynamic stability of the molecule, chemical hardness, the energy gap between the frontier molecular orbitals and the probable electronic transitions of the compounds. The stable conformers (I) of have the –COCH3, –CHO and –OH groups along the molecular plane while in the other high energetic conformers (II) the –COCH3, –CHO and –OH groups are perpendicular to the plane of the benzene ring. The barrier energies of the internal rotation of the –COCH3, –CHO and –OH groups in 4ABN, 4FBN and 4HBN are 6.53, 9.29 and 4.83 kcal mol1, respectively to enable the planar –COCH3, –CHO and –OH groups, respectively to have internal rotation perpendicular to the phenyl ring.

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Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.saa.2012.08.053.

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