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Spectrochimica Acta Part A 92 (2012) 137– 147

Contents lists available at SciVerse ScienceDirect

Spectrochimica Acta Part A: Molecular andBiomolecular Spectroscopy

j ourna l ho me page: www.elsev ier .com/ locate /saa

ibrational spectroscopic studies, molecular orbital calculations and chemicaleactivity of 6-nitro-m-toluic acid

. Balachandrana,∗, T. Karthickb, S. Perumalc, A. Natarajd

Department of Physics, A.A Government Arts College, Musiri, Tiruchirappalli 621201, IndiaDepartment of Physics, Vivekanandha College for Women, Tiruchengode 637205, IndiaDepartment of Physics, S.T. Hindu College, Nagercoil 629002, IndiaDepartment of Physics, Thanthai Hans Roever College, Perambalur 621212, India

r t i c l e i n f o

rticle history:eceived 18 September 2011eceived in revised form 26 January 2012ccepted 14 February 2012

eywords:-Nitro-m-toluic acidibrational spectra

a b s t r a c t

The potential energy surface scan for the selected dihedral angle of 6-nitro-m-toluic acid (NTA) has beenperformed to identify stable conformer. The optimized structure parameters and vibrational wavenum-bers of stable conformer have been predicted by density functional B3LYP method with 6-311++G(d,p)basis set. The formation of dimer species through carboxylic acid group of the title molecule has also beendiscussed. The theoretical dimer geometries have been compared with that of monomer and the varia-tions of bond lengths and bond angles upon dimerization were also discussed. Natural bond orbital (NBO)analysis has been performed on both monomer and dimer geometries. The significant changes in occu-

imerBOrontier molecular orbitalsEP surface

pancies and the energies of bonding and anti-bonding orbitals upon dimerization have been explained indetail. The predicted frontier molecular orbital energies at B3LYP/6-311++G(d,p) method set show thatcharge transfer occurs within the molecule. The nucleophilic and electrophilic sites obtained from themolecular electrostatic potential (MEP) surface were compared with their derived fitting point charges.The vibrational wavenumbers of NTA affected profusely by the nitro group substitution in comparisonto the toluic acid have been interpreted in this work.

Crown Copyright © 2012 Published by Elsevier B.V. All rights reserved.

. Introduction

m-Toluic acid and its derivatives are widely used in theanufacture of polymer stabilizers, pesticides, light sensitive com-

ounds, animal feed supplements and other pharmaceuticals,igments and dyestuff [1]. In addition, m-toluic acid is widelysed as a raw material for producing most chemically impor-ant compounds such as m-toluoyl chloride, m-tolunitrile and,N-diethyl-m-toluamide. In industry, they are being used as aroad-spectrum insect repellent, low fogging thermal stabilizersnd film formers. In particular, the 6-nitro-m-toluic acid (alsoegarded as 5-methyl-2-nitro benzoic acid) is being used as inter-ediate functional dyes and in the synthesis of organic compounds.erivatives of benzoic acid have been the subject of vibrational

nvestigation for many reasons. A derivative of benzoic acid isn essential component of the Vitamin B complex. Benzoic acid

ccurs widely in plants and animal tissues along with Vitamin

complex and is used in miticides, contrast media in urology,

∗ Corresponding author. Tel.: +91 431 2432454; fax: +91 432 6262630.E-mail address: brsbala@rediffmail.com (V. Balachandran).

386-1425/$ – see front matter. Crown Copyright © 2012 Published by Elsevier B.V. All rioi:10.1016/j.saa.2012.02.048

cholocystrographic examinations and in the manufacture of phar-maceuticals [2].

Because of its wide applications, vibrational spectra of benzoicacid and the substituted benzoic acids have been studied by variousspectroscopists [2–14]. IR and Raman spectra of benzoic acid andits deuterated benzoic acids at low temperature have been investi-gated by Furic and Durig [3]. The detailed vibrational assignmentsof ortho-substituted benzoic acid derivatives have been reportedby Sanchez et al. [4]. They have proposed the influence of differentsubstituents on the frequency of ring vibrational modes of ben-zoic acid. Verma et al. recorded the infrared absorption spectrumof m-flurobenzoic acid in the region 250–4000 cm−1 [5]. The com-bined theoretical and experimental FT-IR and FT-Raman spectralstudies of intumescent agent known as 5-amino-2-nitro benzoicacid have been performed by Ramalingam et al. [2]. Sundaragane-san et al. investigated the vibrational features of various aminoand halogenated benzoic acids by FT-IR and FT-Raman spectra[6–10]. Ahamed et al. [11] recorded the laser Raman and FT-IRspectra of 3,5-dinitrobenzoic acid in the region 250–4000 and

50–4000 cm−1, respectively. The infrared and laser Raman spec-tra of solid 2,3,5- and 3,4,5-triiodobenzoic acid in various solventshave been reported by Goel and Gupta [12]. The infrared absorp-tion spectra of 2-chloro-5-nitrobenzoic acid have been studied by

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astogi et al. [13]. Recently, vibrational spectroscopic studies haveeen performed on the solid phase of toluic acid by Babu et al. [14].

In benzoic acid crystals, the dimerization takes place throughhe hydrogen bonds across a center of symmetry. Besides monomereometry, the dimeric species of benzoic acid derivatives have beenxtensively investigated earlier. The infrared spectra of benzoiccid monomer and normal coordinate analysis of both monomernd dimer geometries have been reported by Reva and Stepanian15]. Quantum proton transfer and the interconversion in benzoiccid dimer have been reported by Fillaux et al. [16]. Theoreti-al model of benzoic acid dimer and symmetric proton stretchingibrations in the centrosymmetric dimers through the hydrogenonds have been studied by Flakus and Chelmecki [17]. Boczar et al.eported the combined experimental and calculated vibrationalavenumbers of benzoic acid and salicylic acid dimer [18,19].

xperimental FT-Raman, FT-IR and theoretical dimer conformer of-bromosalicylic acid [20] and 5-fluro, 5-chlorosalicylic acid [21]ave been studied by Karabacak et al.

Literature survey reveals that to the best of our knowledgehe vibrational studies, molecular orbital (MO) calculations andhe chemical reactivity of 6-nitro-m-toluic acid (NTA) have noteen performed so far. Therefore, an attempt has been made inhe present work to study the detailed experimental (FT-IR andT-Raman) and theoretical (DFT) investigation of the vibrationalpectra of NTA. In this study optimized molecular parameters andO calculations (such as NBO and HOMO–LUMO) for the stableonomer and dimer conformer of NTA are predicted. The assign-ents of vibrational wavenumbers of the title molecule are being

eported along with the percentage of potential energy distributionPED) results obtained from MOLVIB program (Version 7.0-G77)ritten by Sundius [22]. The variations in geometrical parameters

nd the electron density of atoms of the molecule upon dimeriza-ion are also interpreted in this work.

. Experimental procedure

The crystalline sample of NTA is purchased from Sigma Aldrichhemical suppliers (India) with the stated purity of 98%. Then,he compound is used for spectral measurements without fur-her purification. In the present study, the Fourier transformnfrared spectrum (FT-IR) of the title compound is recorded in the

avenumber region 400–4000 cm−1 on a NEXUS 670 spectropho-ometer equipped with an MCT detector in a KBr pellet technique.he FT-Raman spectrum is recorded in the wavenumber region00–3500 cm−1 on a NEXUS 670 spectrophotometer equipped withaman module accessory operating at 1.5 W power with Nd:YAG

aser of wavelength 1064 nm is used as an excitation source. Thepectral measurements were carried out at Sree Chitra Tirunalnstitute for Medical Sciences and Technology, Poojappura, Thiru-anathapuram, Kerala, India.

. Computational details

The potential energy surface (PES) scan for the selected dihe-ral angle (C1 C7 O9 H10) of the title molecule is performed at3LYP/6-311++G(d,p) level of density functional theory (DFT) inaussian 03W software package [23]. Then, the optimized geo-etrical parameters of stable monomer and dimer conformer are

omputed at B3LYP/6-311++G(d,p) basis set method. The dimereometry of NTA molecule is formed by an anharmonic cou-

ling model as described by Marechal and Witkowski, and Wojcik24,25]. An anharmonic coupling is the coupling between the highrequency O H stretching and low frequency intermolecular O· · ·Oydrogen bond stretching vibrations. It is an important mechanism

Acta Part A 92 (2012) 137– 147

for shaping the fine structure of stretching bands in the hydrogen-bonded systems.

The theoretical vibrational wavenumbers of the title moleculeare calculated by assuming C1 point group symmetry. The com-puted vibrational wavenumbers at B3LYP/6-311++G(d,p) showthat, there are some disagreements with the experimentalwavenumbers. These discrepancies are mainly due to the neglectof anharmonicity effect at the beginning of harmonic wavenumbercalculation and basis set deficiencies. In the present work, theoret-ical wavenumbers are scaled down by six different scale factors.The MO calculations such as NBO and HOMO–LUMO calculationsare performed on the stable monomer and dimer geometry of NTA.The occupancy numbers, energies and second order perturbationenergies of interacting Lewis base and Lewis acid sites are reportedwith the help of NBO 3.1 program as stored in Gaussian 03W pack-age. Moreover, the molecular electrostatic potential (MEP) surfacemap is plotted over the optimized electronic structure of NTA usingthe same level of DFT theory. In this work, the electrophilic andnucleophilic sites obtained in the MEP surface are being comparedwith derived fitting point charges to the electric potential.

3.1. Prediction of Raman intensities

Initially, the Raman scaterring activities (Si) obtained from theGaussian 03W program are adjusted during the scaling procedurewith MOLVIB. Then, they are subsequently converted to relativeRaman intensities (Ii) using the following relationship derived fromthe intensity theory of Raman scattering [26,27].

Ii = f (v0 − vi)4Si

vi[1 − exp(hcvi/kbT)]

where v0 is the exciting frequency (1064 nm = 9398 cm−1) of laserlight source used while recording Raman Spectra, vi the vibrationalwavenumber of the ith normal mode. h, c and kb fundamental con-stants, and f is a suitably chosen common normalization factor forall peak intensities of the Raman spectrum of the title molecule.

4. Result and discussion

The present compound under investigation has become greatinterest because it contains three different substituents namelymethyl group ( CH3), carboxyl group ( COOH), and nitro group( NO2) connected with the benzene ring. For getting stable con-former of NTA, the relaxed scan is applied to the most interactingdihedral angle (C1 C7 O9 H10) of the carboxyl group. In relaxedscan, the variables corresponding to the selected dihedral angle ofNTA are varied in steps of 10◦ between 0◦ and 350◦. While per-forming the scan in Gaussian 03 W, the program is searching globallocal minimum point for each 10◦. The Eigen values obtained fromthe scan output reveals that, the structure positioning the dihe-dral angle (C1 C7 O9 H10) at 180◦ possesses minimum energyamong others. The optimized geometry of NTA is shown in Fig. 1and potential energy curve between the dihedral angles and theircorresponding energies is given in Fig. 2.

4.1. Geometrical parameters of stable monomer and dimer

The predicted geometrical parameters such as bond lengths andbond angles of NTA calculated at B3LYP method with 6-311++G(d,p)are presented in Table 1 in accordance with the atom numberingscheme as given in Fig. 1. Since the experimental X-ray diffrac-

tion data of NTA molecule is unavailable, the experimental dataof similar kind of molecule is presented in Table 1 for comparativepurpose [28]. When comparing experimental values, the computedbond lengths and bond angles at B3LYP/6-311++G(d,p) method are

V. Balachandran et al. / Spectrochimica Acta Part A 92 (2012) 137– 147 139

Table 1Optimized geometrical parameters of stable monomer and dimer of 6-nitro-m-toluic acid computed at B3LYP/6-311++G(d,p) basis sets.

Bond lengthsa Theoretical bond lengths Experimental datab Bond anglesa Theoretical bond angles Experimetal datab

Monomer Dimer Monomer Dimer

C1 C2 1.3995 1.3995 1.3878 C2 C1 C6 118.06 118.04 119.96C2 C3 1.3874 1.3874 1.3850 C2 C1 C7 124.90 124.93 120.70C3 C4 1.3912 1.3912 1.3860 C6 C1 C7 116.98 116.98 119.34C4 C5 1.3973 1.3973 1.3910 C1 C2 C3 121.33 121.36 119.35C5 C6 1.4001 1.4001 1.3890 C1 C2 N11 121.22 121.18C1 C6 1.3931 1.3931 1.3940 C3 C2 N11 117.30 117.31C1 C7 1.5001 1.5003 1.4780 C2 C3 C4 119.40 119.39 120.18C2 N11 1.4763 1.4763 C2 C3 H14 119.21 119.21 119.9C3 H14 1.0819 1.0819 0.9500 C4 C3 H14 121.38 121.39 119.9C4 H15 1.0842 1.0842 0.9500 C3 C4 C5 121.02 121.01 121.38C5 C16 1.5073 1.5073 1.5060 C3 C4 H15 119.20 119.20 119.30C6 H20 1.0841 1.0841 0.9500 C5 C4 H15 119.77 119.78 119.30C7 O8 1.2041 1.2195 1.2562 C4 C5 C6 118.22 118.24 117.90C7 O9 1.3482 1.3192 1.2909 C4 C5 C16 121.40 121.40 121.23O9 H10 0.9695 0.9969 0.9840 C6 C5 C16 120.38 120.36 120.86N11 O12 1.2233 1.2231 C1 C6 C5 121.94 121.94 121.21N11 O13 1.2238 1.2241 C1 C6 H20 118.02 118.04 119.40C16 H17 1.0947 1.0947 0.980 C5 C6 H20 120.04 120.02 119.40C16 H18 1.0929 1.0923 0.980 C1 C7 O8 123.94 123.96 120.38C16 H19 1.0911 1.0911 0.980 C1 C7 O9 112.12 112.07 116.63O9· · ·O12 2.8191 2.8201 O8 C7 O9 123.78 123.78 122.99O8· · ·H24 1.500 0.9980 C7 O9 H10 107.66 107.63 112.50O9 H10· · ·O22 1.1318 0.9840 C2 N11 O12 117.29 117.30

C2 N11 O13 117.43 117.43O12 N11 O13 125.24 125.23C5 C16 H17 111.43 111.40 109.50C5 C16 H18 111.20 110.15 109.50C5 C16 H19 110.68 110.76 109.50H17 C16 H18 108.24 108.25 109.50H17 C16 H19 107.85 107.80 109.50H18 C16 H19 107.27 107.30 109.50C7 O9· · ·O12 77.11 76.83H10 O9· · ·O12 114.09 102.18N O · · ·O 84.94 85.20

sur

lcot1el

a For atom numbering scheme, see Fig. 1.b Ref. [28]

lightly larger, because the theoretical calculations are performedpon isolated molecule in the gaseous state and the experimentalesults are performed on the solid phase of the molecule [29].

In the case of free (with no substituents) benzene, the bondengths between the carbon atoms held together in the hexagonalhain are almost equal. If so, there must be a change in bond lengthccurs according to the nature of substituents. In the title molecule,

he calculated C C bond lengths of the benzene ring varies from.3874 to 1.4001 A whereas outside the ring, C C bonds lengthsxceeds by an amount ∼0.1–0.12 A. Similarly, aromatic C H bondengths are of the order of 1.08 A and that of methyl 1.09 A. It is

Fig. 1. Optimized molecular structure of 6-nitro-m-toluic acid.

11 12 9

worth mentioning that, the bond lengths of NTA monomer reportedin Table 1 are matched well with that of the optimized electronicstructure of NTA dimer except few. The variations of bond lengthsupon dimerization may due to the delocalization of electron den-sity from electron donor to acceptor atom. For example, the O9 H10bond length of the monomer is reported as 0.9695 A and that ofdimer 0.9969 A. Thus upon dimerization, the O9 H10 bond length isincreased by 0.0274 A may cause the formation of hydrogen bond-ing interactions with the neigbouring molecule. Likewise, the bondlength C7 O8 is increased by 0.0154 A upon dimerization.

The C H bond length such as C3 H14, C4 H15, C6 H20, C6 H17,

C7 H18 and C6 H20 calculated at B3LYP/6-311++G(d,p) are too longincomparion with the experimental values. It is well known thatDFT method predict bond lengths that ate systematically too long,especially in C H bond lengths [30].This may due to that, the low

Fig. 2. Potential energy surface scan for the selected (C1−C7−O9−H10) dihedralangle of 6-nitro-m-toluic acid.

140 V. Balachandran et al. / Spectrochimica Acta Part A 92 (2012) 137– 147

er of

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cattering factor of hydrogen atoms involved in the X-ray diffrac-ion experiment produces large deviation from the theoretical C Hond lengths. The bond lengths of the strong intermolecular inter-ctions viz. O9 H10· · ·O22 and O23 H24· · ·O8 of dimer conformerre calculated as 1.1318 A and 1.5000 A, respectively. The weakntramolecular hydrogen bond between C7 O9 and N11 O12 is cal-ulated as 2.8191 A for the monomer and 2.8201 A for the dimeronformer of NTA.

For considering free benzene, the hexagonal angles such asC C and C C H bond angles corresponding to the stable

onomer and dimer conformer of NTA are slighty deviated by1◦. This is because of the fact that, the C C C and C C H bondngles of the benzene ring are substituent sensitive. Therefore aingle substituent can alters the entire geometry of the molecule.oreover, the bondangles between the inter- and intramolecu-

ar fragments (C7 O9· · ·O12, H10 O9· · ·O12, and N11 O12· · ·O9) arereatly affected by the dimerization process and are also depictedn Table 1.

.2. NBO analysis

In hydrogen bonded systems, the stability of the moleculeay cause several factors; hyperconjugative interactions, inter-,

ntramolecular hydrogen bonding, intermolecular charge trans-er (ICT), electron density transfer (EDT) and cooperative effectue to the delocalization of electron density from the filled

one pairs of Lewis base ‘n(y)’ into the unfilled antibonding ofewis acid ‘�*(x H) or �*(x H)’. It is found from the literaturehat, natural bond orbital (NBO) analysis is an effective tool foretermining the above mentioned factors [31]. In the presentork, NBO analysis has been performed on the monomer andimer with the aid of NBO 3.1 program as implemented in theaussian 03W package. The dimer structure shown in Fig. 3 con-ists of a couple of inter- and intramolecular hydrogen bondingnteractions. In the case of NTA, the intermolecular interactionsre formed by the orbital overlap between n(O) and �*(O H)i.e. n(O8) → �*(O23 H24), n(O22) → �*(O9 H10)}. Similarly, thentramolecular hydrogen bonding is present in between the nitroroup and carboxylic group of NTA dimer. Since intramolecularnteractions are very week, the ICT and EDT between n(O12) and*(O9 H10) or n(O34) and �*(O21 H22) are not appreciable.

Furthermore, the NBO analysis of NTA monomer and dimerlearly gives the evidence for the formation of strong intermolecu-ar and very week intramolecular interactions between oxygen lonelectron pairs and �*(O H) antibonding orbitals. In Table 2, the

6-nitro-m-toluic acid.

occupation numbers along with the energies of interacting NBOsare being presented. The magnitude of charges transferred fromthe lone pairs of n(O8) into the antibonding orbital �*(O9 H10) issignificantly increased by 0.0397 e and 0.0481 e, respectively, upondimerization. The variations in the magnitude of charges providedan unambiguous evidence for the weakening of C7 O8, O9 H10bonds and their elongation. As it can be seen from Table 2 that, themagnitude of charge transferred between n(O13) and �*(O9 H10)is not significant. Therefore a similar conclusion can be obtainedwhile considering the energy of the corresponding orbitals.

The stabilization energy E(2) associated with the hyper-conjugative interactions viz. n1(O8) → �*(O23 H24) andn1(O21) → �*(O9 H10) are obtained 21.45 and 18.24 kcal mol−1,respectively, as shown in Table 3. It is worth mentioning that,the differences in stabilization energies reported in Table 3 arereasonable. In view of the fact that, the accumulation of electrondensity in the antibonding orbital �*(O H) is not only transferredfrom the lone electron pair n(O) but also from the entire molecule.

Unusually, rehybridization plays a negative effect in O9 H10bond. It is observed in Table 4 that, the s-character of O9 H10 hybridorbitals increases (18.03%) from sp3.70 to sp1.54 that leads to a con-spicuous strengthening of O9 H10 bond and its contraction. Thisshows the existence of a mesomeric structure characterized bythe delocalization of electron density from the �*(O9 H10) anti-bonding orbital to the remaining part of the molecule. This is quitepossible because the energy of �*(O9 H10) antibonding orbital(0.4561 a.u.) is higher than the energy of �*(C7 O9) antibondingorbital (0.3633 a.u.) which supports the likelihood of the delocal-ization of ED from O H to C O region. This is clearly reflectedin the dimer geometry of NTA as the bond C7 O9 contracts toan amount of 0.0290 A with respect to the monomer. The natu-ral atomic hybrids corresponding to the H-bonded NBO also showsthat the redistribution of natural charges in O H bond destabilizesthe H-bond. Because both the hyperconjugation and rehybridiza-tion are in opposite directions, the compression and elongation ofthe bond O H is a result of the balance of the two effects. Howeverthe hyperconjugative interaction is dominant and overshadowsthe rehybridization effect resulting a significant elongation in O Hbond (0.0274 A) with respect to the monomer.

The ED of the carbonyl group antibonding orbitals �*(C7 O8)and �*(C7 O8) are increased significantly (0.0303 e and 0.0446 e,

respectively) upon dimerization. This clearly gives the evidence forthe weakening of C7 O8 bond and its elongation (0.0154 A). Thisis also evident that, the stabilization energy E(2) reported to thehyperconjugative intramolecular interactions n1(O9) → �*(C7 O8),

V. Balachandran et al. / Spectrochimica Acta Part A 92 (2012) 137– 147 141

Table 2Occupancies and energies of interacting Lewis base and Lewis acid sites of 6-nitro-m-toluic acid.

Parameters Occupancies (e) �Occ (e) Enegy (a.u) �E (a.u)

Monomer Dimer Monomer Dimer

n1(O8) 1.9788 1.9391 −0.0397 −0.7227 −0.6922 −0.0305n2(O8) 1.8425 1.8602 0.0177 −0.2895 −0.3141 0.0246n1(O9) 1.9762 1.9612 −0.015 −0.6482 −0.5586 −0.0896n2(O9) 1.8177 1.7438 −0.0739 −0.3585 −0.3088 −0.0497n1(O12) 1.9809 1.9807 −0.0002 −0.8218 −0.8080 −0.0138n2(O12) 1.8941 1.8931 −0.0010 −0.3134 −0.2995 −0.0139n1(O13) 1.9816 1.9816 0.0000 −0.8216 −0.8089 −0.0127n2(O13) 1.8981 1.8981 0.0000 −0.3118 −0.2993 −0.0125n3(O13) 1.4373 1.4413 0.0040 −0.2961 −0.2837 −0.0124�* (C1 C2) 0.0302 0.0297 −0.0005 0.5259 0.5357 0.0098�* (C1 C6) 0.0188 0.0194 0.0006 0.5396 0.5501 0.0105�* (C1 C6) 0.3215 0.3200 0.0015 −0.0007 0.0100 0.0093�* (C1 C7) 0.0704 0.0662 −0.0042 0.3734 0.3643 −0.0091�* (C2 C3) 0.0212 0.0209 0.0003 0.5337 0.5446 0.0109�* (C2 C3) 0.3583 0.3545 −0.0038 −0.0081 0.0027 0.0054�* (C2 N11) 0.1032 0.1033 0.0001 0.2479 0.2598 0.0119�* (C7 O8) 0.0241 0.0544 0.0303 0.5804 0.5340 −0.0464�* (C7 O8) 0.2165 0.2611 0.0446 0.0011 0.0632 0.0621�* (C7 O9) 0.0991 0.0818 −0.0173 0.3253 0.3633 0.038�* (O9 H10) 0.0106 0.0587 0.0481 0.3657 0.4561 0.0904�* (N11 O12) 0.0671 0.0656 −0.00�* (N11 O12) 0.6034 0.5972 −0.00�* (N11 O13) 0.0617 0.0620 0.00

Table 3Second order perturbation theory analysis of Fock Matrix in NBO basis.

Donor (i) Acceptor (j) E(2) kcal mol−1 E(j) − E(i) a.u. F(i,j) a.u.

Within unit 1n1(O8) �* (C7 O9) 3.02 1.06 0.051n2(O8) �* (C7 O9) 25.24 0.68 0.118n1(O9) �* (C7 O8) 8.49 1.09 0.086n2(O9) �* (C7 O8) 39.49 0.37 0.108n1(O12) �* (N11 O13) 2.55 1.23 0.051n1(O12) �* (C2 N11) 4.21 1.07 0.061n1(O13) �* (N11 O12) 2.54 1.22 0.050

From unit 1 to unit 3n1(O8) �* (O23 H24) 21.45 1.15 0.140

Within unit 3n1(O22) �* (O9 H10) 18.24 1.02 0.087n1(O22) �* (C21 O23) 23.30 0.61 0.107

Table 4Composition of hydrogen bonded NBOs interms of natural atomic hybrids.

Hydrogen bonded NBOs Monomer Dimer �NBO

spn (O9 H10) sp3.70 sp1.54 +s% of s character 21.26% 39.29% +18.03% of p character of O9 67.68% 74.88% +7.2% of p character of H10 32.32% 25.12% −7.2q (O9)/e −0.6737 −0.6701 +0.0036q (H10)/e 0.4890 0.4653 −0.0237

spn (C7 O8) sp1.97 sp2.48 −s% of s character 33.58% 28.67% −4.91% of p character of C7 35.04% 32.82% −2.22% of p character of O8 64.96% 67.18% +2.22q(C7)/e 0.8019 0.8650 +0.0631q(O8)/e −0.5757 −0.6793 −0.1036

spn (C7 O9) sp2.60 sp2.54 +s% of s character 27.69% 28.16% +0.47% of p character of C7 32.12% 33.37% +1.25% of p character of O9 67.88% 66.63% −1.25q(C7)/e 0.8019 0.8650 +0.0631q(O9)/e −0.6737 −0.6701 +0.0036

spn (N11 O12) sp2.13 Sp2.12 +s% of s character 31.90% 31.99% +0.09% of p character of N11 40.26% 49.00% +8.74% of p character of O12 59.74% 51.00% −8.74q(N11)/e 0.4885 0.4903 +0.0018q(O12)/e −0.3622 −0.3539 +0.0083

15 0.4002 0.4150 0.014862 −0.1384 −0.1252 −0.013203 0.4085 0.4212 0.0127

n2(O9) → �*(C7 O8) and n1(O21) → �*(C22 O23) are obtained as8.49, 39.49 and 23.30 kcal mol−1. In addition, the s character of spn

hybrid orbital for the C7 O8 bond decreases from sp1.97 to sp2.48

upon dimerization substantiates the weakening of C7 O8 bond andits elongation.

4.3. Molecular electrostatic potential (MEP) surface

For investigating chemical reactivity of the molecule, molecularelectrostatic potential (MEP) surface is plotted over the optimizedelectronic structure of NTA using density functional B3LYP methodwith 6-311++G(d,p) basis set. The MEP generated in space around amolecule by the charge distribution is very helpful in understand-ing the reactive sites for nucleophilic and electrophilic attack inhydrogen bonding interactions [32] and in biological recognitionprocess [33]. In hydrogen bonded systems, the study of interactingbehavior of various constituents of the molecule has great impor-tance. Because the computationally or experimentally observedMEP surface is directly provide information about the electrophilic(electronegative charge region) and nucleophilic (most positivecharge region) regions. Fig. 4 shows the computationally observedMEP contour map along with the fitting point charges to the elec-trostatic potential V(�r). The electrostatic potential V(�r) at any pointin space around a molecule by charge distribution is given by

V(�r) =∑

A

ZA∣∣�RA − �r∣∣ −

∫�(�r′)dr′∣∣�r′ − �r

∣∣ (1)

where �(�r′) is the electron density function of the molecule, ZA isthe charge on the nucleus A located at �RA and �r′ is the dummy inte-gration variable. In the contour map, the regions with red coloursare regarded as most electronegative (electrophilic) regions and theregions with blue colours are most positive (nucleophilic) regions,whereas the bluish green colours surrounded by the ring system ofNTA are related to less positive regions.

In Fig. 4, there are several possible sites for the electrophilicattacks over the oxygen atoms; O8, O9, O12 and O13. The aver-

age maximum negative electrostatic potential values for theseelectrophilic sites calculated at B3LYP/6-311++G(d,p) is about−22.2920 a.u. The fitting point charges to those electrostatic poten-tials are calculated as −0.6410 (O9), −0.6398 (O8), −0.4671 (O13)

142 V. Balachandran et al. / Spectrochimica Acta Part A 92 (2012) 137– 147

F poter e.)

aalsc0oN

4

hmiatac

dtotaifwspaapNaLaotHi

ig. 4. Molecular electrostatic potential surface (left) and the point charges, electriceferences to color in the text, the reader is referred to the web version of the articl

nd −0.4485 e (O12). In the title molecule, all the hydrogen atomsnd a methyl group connected to the ring system of benzene havingess positive charges. Besides, the hydrogen atom H10 (0.4129 e) isurrounded by most nucleophilic region. Similarly, the fitting pointharge corresponding to N11 atom is reported as high positive as.7573 e, but MEP map seems yellow colored on N11. This is becausef the fact that, most of the electrophilic regions are surrounded by11 atom.

.4. Frontier molecular orbitals

The most important frontier molecular orbitals (FMOs) such asighest occupied molecular orbital (HOMO) and lowest unoccupiedolecular orbital (LUMO) plays a crucial part in the chemical stabil-

ty of the molecule [34]. The HOMO represents the ability to donaten electron and LUMO as an electron acceptor represents the abilityo accept an electron. The energy gap between HOMO and LUMOlso determines the chemical reactivity, optical polarizability andhemical hardness-softness of a molecule [35].

In the present study, the HOMO and LUMO energies are pre-icted at B3LYP method with 6-311++G(d,p) basis set. Accordingo the results, the NTA molecule contains 47 occupied molecularrbitals and 247 unoccupied virtual molecular orbitals. Fig. 5 showshe distributions and energy levels of HOMO − 1, HOMO, LUMOnd LUMO + 1 orbitals for the stable monomer of NTA moleculen gaseous phase. It is clear from Fig. 5 that the isodensity plotsor the HOMO and HOMO − 1 are well localized within the ringhereas, the orbital overlapping on the ring system of LUMO

urface shows empty. LUMO + 1 orbital of the title molecule is com-letely delocalized the entire part of the molecule. Commonly, thetom occupied by more densities of HOMO should have strongerbility to detach an electron whereas; the atom with more occu-ation of LUMO should have ability to gain an electron [36]. ForTA molecule, the HOMO orbital of � type is lying at −0.2856 a.und HOMO − 1 orbital of � type is lying at −0.2926 a.u. while theUMO and LUMO + 1 orbitals are �* in type and are lying at −0.1063nd 0.0672 a.u., respectively. As a result, a very small energy gap is

bserved between HOMO and LUMO molecular orbitals of NTA andhe energy gap calculated at B3LYP/6-311++G(d,p) is −0.1793 a.u..ence the probability of � → �* proton transition is highly possible

n between HOMO and LUMO orbitals for the NTA.

ntial values (right) on each atom of 6-nitro-m-toluic acid. (For interpretation of the

The chemical hardness and softness of a molecule is a goodindicator of the chemical stability of a molecule. From theHOMO–LUMO energy gap, one can find whether the molecule ishard or soft. The molecules having large energy gap are known ashard and molecules having a small energy gap are known as softmolecules. The soft molecules are more polarizable than the hardones because they need small energy to excitation. The hardnessvalue of a molecule can be determined by the formula [37]:

� = (−εHOMO + εLUMO)2

(2)

where εHOMO and εLUMO are the energies of the HOMO andLUMO orbitals. The value of � in the title molecule is 0.0897 a.u..hence from the calculation, we conclude that the molecule takenfor investigation belongs to soft material and the polarizabil-ity of the title molecule calculated at B3LYP/6-311++G(d,p) is−111.8798 × 10−30 esu.

4.5. Vibrational spectra

The investigation of vibrational wavenumbers of the chemicalcompounds have been put a primary role in the spectral analy-sis. In the spectroscopic field, the vibrational spectra of substitutedbenzene derivatives have been greatly investigated by variousspectrocopists. Because, a single substitution can have a tendencyto put greater changes in the vibrational wavenumbers of benzene.In other words, the molecular system of benzene is greatly affectedby the nature of the substituents.

In order to obtain the spectroscopic signature, the experimen-tal FT-IR and FT-Raman spectra are being recorded on the solidphase of NTA molecule. Moreover, the theoretical frequency calcu-lation is also performed on the gaseous phase of the molecule usingdensity functional B3LYP method with 6-311++G(d,p) basis set. Asa result, the present molecule holds 54 harmonic wavenumbersunder C1 point group symmetry. Due to the neglect of anharmonic-ity effect at the beginning of frequency calculation, the theoreticallycomputed wavenumbers are found to be somewhat in disagree-ment with that of the experimental. To overcome the discrepancies

between observed and calculated wavenumbers, six different scalefactors are introduced. In the present study, the scaled theoreticalwavenumbers are compared with that of experimental wavenum-bers (Table 5). For comparative purpose, the theoretically simulated

V. Balachandran et al. / Spectrochimica Acta Part A 92 (2012) 137– 147 143

Fig. 5. Electron density surface plots for the most important frontier molecular orbitals with their energies (a.u) of 6-nitro-m-toluic acid.

Fig. 6. (a) Experimental FT-IR and (b) simulated infrared spectra of 6-nitro-m-toluic acid.

144 V. Balachandran et al. / Spectrochimica Acta Part A 92 (2012) 137– 147

Table 5Vibrational wavenumbers, IR intensities, Raman intensities and vibrational assignments corresponding to the normal modes associated with 6-nitro-m-toluic acid.

Modes Experimental wavenumbers Theoreticalwavenumbers (cm−1)B3LYP/6-311++G(d,p))

Scale factors IR intensity Ramanintensity

Vibrational assignmentsa (% PED)

FT-IR FT-Raman Unscaled Scaled

�1. 3563 vw – 3759 3571 0.950 105.5 6.1 OH(100)�2. 3077 m 3073 m 3214 3085 0.960 1.1 6.0 ringCHss(97)�3. 3033 vw – 3184 3057 0.960 1.2 4.2 ringCHss(97)�4. 2976 w 2976 w 3178 2987 0.940 6.9 6.7 ringCHass(98)�5. 2934 w 2927 m 3114 2927 0.940 12.8 4.7 methylCHass(96)�6. 2889 m 2880 vw 3084 2899 0.940 10.2 6.6 methylCHass(99)�7. 2804 w 2805 w 3030 2848 0.940 14.1 23.2 methylCHss(100)�8. 1697 vs – 1805 1697 0.940 360.1 11.9 C O(67), �OH(22)�9. 1612 vs – 1644 1611 0.980 13.8 16.9 CC(61), �CN(23)�10. 1591 ms 1596 s 1627 1594 0.980 65.5 13.2 CC(50), �CH(22), �CN(16)�11. – 1510 w 1587 1508 0.950 262.9 4.7 asNO2(56), �CN(16), CC(12)�12. 1477 w 1482 m 1512 1481 0.980 1.3 3.2 CC(42), �CH(27), �CH3(13)�13. – 1462 vw 1496 1466 0.980 6.5 2.0 �CH3(61), �CH(17)�14. 1433 s – 1487 1428 0.960 8.0 3.3 �CH3(77)�15. 1386 m 1389 w 1436 1379 0.960 40.2 2.4 CC(46), �CH(21), �CH3(14)�16. 1348 vs 1346 vs 1416 1345 0.950 2.5 8.7 �CH3(64), �CH(10)�17. 1306 s – 1382 1313 0.950 315.8 61.3 sNO2(58), �CO(18), CN(16)�18. – – 1360 1292 0.950 53.7 2.7 �OH(47), �CH(20),CC(16)�19. – – 1343 1276 0.950 5.3 2.0 CC(51),�CH3(18), �OH(11)�20. 1269 s 1265 vw 1299 1260 0.970 0.8 0.6 �CH(62), �OH(18)�21. – 1198 vw 1245 1183 0.950 38.6 13.0 CC(50), �CH(19), �OH(13)�22. 1161 w 1167 vw 1187 1151 0.970 157.5 1.8 �CH(54), CC(21)�23. 1141 w 1142 ms 1178 1142 0.970 15.3 5.7 �CH(49), CC(25)�24. 1076 w 1080 vw 1142 1084 0.950 90.7 7.4 CO(45), �CH(21), �CO(16)�25. 1041 w 1043 vw 1085 1042 0.960 22.4 10.7 CN(41), �CH(20), �CO(17)�26. – 1025 vw 1062 1020 0.960 8.2 0.3 �CH3(58), �CH(17)�27 977 vw – 1021 980 0.960 4.0 1.0 �CH3(63), �ring(15)�28 – 920 m 985 926 0.940 1.5 0.1 �CH(78)�29 891 ms 895 m 924 896 0.970 4.0 0.4 �CH(65)�30 843 vs – 918 849 0.925 11.2 10.0 �NO2(53), �ring(19), CC(14)�31 – – 856 820 0.960 24.1 2.7 �CH(62), �NO2(19)�32 785 vw 784 w 851 790 0.930 32.0 3.4 CC(48), �CH(12)�33 – 743 m 798 742 0.930 12.8 3.0 CC(48), �ring(15), �OH(12)�34 740 w – 768 737 0.960 40.9 0.8 �NO2(55), �CH(22), �CCO(14)�35 703 w – 733 704 0.960 10.4 14.3 �COOH(51), �ring (24)�36 674 s 673 w 704 676 0.960 4.1 1.6 �CCC(46), �OH(20), �ring(17)�37 616 ms – 648 622 0.960 27.9 2.4 �COH(47), �ring(25)�38 602 w – 633 608 0.960 20.6 4.3 �CCC(40), �ring(19), �CH(12)�39 585 w – 601 583 0.970 70.0 5.5 �OH(67), �ring(18)�40 – 562 w 593 569 0.960 2.8 2.4 �NO2(49), �ring (20), �CCC(13)�41 541 w – 563 540 0.960 21.8 1.1 �COOH(52), �ring(12)�42 – 420 w 440 422 0.960 4.6 1.6 �COH(41), �CH(19), �ring(15)�43 – 377 vw 393 377 0.960 4.7 11.6 �CCC(39), �ring(22), �COOH(18)�44 – – 380 365 0.960 3.0 4.9 �C NO2(45), �ring(17), �COOH(13)�45 – – 350 336 0.960 0.9 1.7 �CCC(42), �COOH(19), �ring(15)�46 – – 329 316 0.960 1.1 5.0 �ring(50)�47 – 242 vw 261 251 0.960 0.7 10.0 �ring(51)�48 – 205 w 207 199 0.960 0.3 8.0 �ring (45), �CH3(18), �NO2(12)�49 – – 178 171 0.960 2.8 14.5 �C NO2(53), �COH(26)�50 – 118 w 134 129 0.960 1.6 26.3 �ring(50), �NO2(19), �COH(17)�51 – – 99 95 0.960 2.0 45.3 �ring(52), �COH(21)�52 – – 93 89 0.960 0.2 60.5 � ring(65)�53 – – 43 41 0.960 0.1 76.7 CH3(90)�54 – – 32 31 0.960 1.7 100.0 NO2(45), �COH(37)

a t-of-pls

satof

4

Hti

s, symmetry stretching; as, asymmetry stretching; �, in-plane bending; �, oucissoring; �, NO2 wagging; �, NO2 rocking; and , torsion.

caled IR and Raman spectra are being presented in Figs. 6 and 7long with their experimental FT-IR, FT-Raman spectrum respec-ively. The detailed descriptions for the vibrational assignmentsf various functional groups of NTA molecule are summarized asollows.

.5.1. Aromatic C H vibrations

In the present study, the three hydrogen atoms (H14, H15 and

20), which are attached to the benzene ring of NTA give rise tohree C H stretching modes (�2 �4), three C H in-plane bend-ng (�20, �22, �23) and three C H out-of-plane bending (�28, �29,

ane bending; �, methyl in-plane ricking; �, methyl out-of plane rocking; �, NO2

�31) modes. Usually the heteroaromatic organic molecule showsthe presence of the C H stretching vibrations in the wavenumberregion 3000–3100 cm−1 and it is the characteristic region for theidentification of C H stretching vibrations [38]. For NTA molecule,the aromatic C H stretching modes are assigned to 3077, 3033 and2976 cm−1 in FT-IR, and 3073 and 2976 cm−1 in FT-Raman. Thescaled theoretical wavenumbers at B3LYP/6-311++G(d,p) method

for C H stretching modes are in agreement with the experimen-tal wavenumbers and potential energy distribution (PED) resultsshowed that, these modes are very pure. For toluic acid (TA), thebands at 3080, 3030 and 3020 cm−1in FT-IR and 3050, 3030 cm−1

V. Balachandran et al. / Spectrochimica Acta Part A 92 (2012) 137– 147 145

imula

ieioa

b7b1Fw1tCi

ambfw

4

mmbt2sts2t

ai

Fig. 7. (a) Experimental FT-Raman and (b) s

n FT-Raman have been assigned as C H stretching modes by Babut al. [14]. In our previous work, we have assigned 3095, 3060 cm−1

n FT-IR and 3054 cm−1in FT-Raman bands as C H stretching modesf 5-nitrosalicylic acid (also known as 2-hydroxy-5-nitro benzoiccid) [39].

In aromatic compounds, the C H in-plane and out-of-planeending vibrations appear in the range 1000–1300 cm−1 and50–1000 cm−1 [40,41], respectively. Hence the C H in-planeending modes of the title molecule are attributed to 1269,161 and 1141 cm−1 in FT-IR and 1265, 1167 and 1142 cm−1 inT-Raman. For TA, the aromatic C H in-plane bending modesere assigned to 1200, 1120 and 1090 cm−1 in FT-IR and 1160,

120 cm−1 in FT-Raman [14]. The percentage of PED depicted inhe last column of Table 5 shows that, O H in-plane bending and

C stretching vibrations are interacting considerably with C Hn-plane bending mode.

The medium bands observed at 920, 895 cm−1 in FT-Raman andt 891 cm−1 in FT-IR are ascribed to C H out-of plane bendingodes of the title molecule. Thus the scaled theoretical wavenum-

ers of C H out-of plane bending modes depicted in Table 5 areound to be in agreement with the experimental data of NTA asell as the similar kind of molecules [2,14].

.5.2. Methyl group vibrationsA single methyl substitution on the benzene ring of the title

olecule consists of nine fundamental vibrational modes. Nor-ally, the C H stretching modes of the methyl group produce

ands in the region 2840–2975 cm−1 [42,43]. For the title molecule,he bands observed at 2934, 2889 and 2804 cm−1 in FT-IR and927, 2880 and 2805 cm−1 in FT-Raman are attributed to C Htretching mode of the methyl group. The percentage of PED showshat, the methyl C H stretching modes are pure like aromatic C Htretching. In the case of TA, the bands present at 2960, 2930 and890 cm−1 in FT-IR have been assigned as C H stretching modes of

he methyl group.

A prominent intensity band at 1348 cm−1in FT-IR and the bandst 1462, 1346 cm−1 in FT-Raman are assigned to in-plane bend-ng modes of the methyl group. The strong band at 1433 cm−1

ted Raman spectra of 6-nitro-m-toluic acid.

in FT-IR corresponding to mode �14 is assigned to out-of planebending mode of the methyl group. Generally aromatic com-pound displays the methyl in-plane (�CH3) and out-of plane(�CH3) rocking vibration bands in the neighborhood of 1045 cm−1

and 970 ± 70 cm−1, respectively [44]. In the present study, theband present at 1025 cm−1 in FT-Raman and the weak band at977 cm−1 in FT-IR are assigned to in-plane and out-of-plane rock-ing vibrations, respectively. The calculated value of 41 cm−1 atB3LYP/6-311++G(d,p) method is assigned to the twisting mode ofthe methyl group (CH3).

4.5.3. Carboxyl group vibrationsThe vibrational assignments of carboxylic acid group of the title

molecule have great deal of interest. In view of the fact that, thedimeric entities of the molecule is possible to exist via carboxylicacid group. Nevertheless, the derivatives of carboxylic acids are bestcharacterized by the carbonyl and hydroxyl groups. In particular,the presence of carbonyl group is most important in the infraredspectrum because of its strong intensity of absorption and high sen-sitivity toward relatively minor changes in its environment. Intra-and intermolecular hydrogen bonding interactions affect the car-bonyl absorptions in common organic compounds due to inductive,mesomeric, field and conjugation effects [45].

The characteristic infrared absorption wavenumber of C O inacids are normally strong in intensity and are found in the region1690–1800 cm−1 [20,38]. In the present study, the strong band at1697 cm−1 in FT-IR is assigned to C O stretching. The calculatedvalue of this mode at B3LYP/6-311++G(d,p) method is found to bein consistent with the experimental data. For TA, the FT-IR bandobserved at 1690 cm−1 has been assigned as C O stretching mode[14]. For 5-NSA, the strong band at 1665 cm−1 in FT-IR and band at1657 cm−1 in FT-Raman were assigned to C O stretching [39].

A band related to C O stretching mode of carboxylic acid groupis highly coupled with the vibrations of adjacent groups or atoms.

Hence the wavenumber region for the existence of C O is basedon the nature of the nearby substitution. In the title molecule, theweak band at1076 cm−1 in FT-IR and 1080 cm−1 in FT-Raman areassigned to C O stretching mode. For TA, the band at 1270 cm−1

1 himica

iC1iiocitfoTmwmipa

4

zain1zsss1ts

tbismawc3aS

4

tibbttaNFCcH1m3

[

46 V. Balachandran et al. / Spectroc

n FT-Raman has been assigned to C O stretching mode [14]. The O stretching mode normally appears in the frequency region200–1300 cm−1. In the present study, the assignment this mode

s somewhat lower than that of the literature [14]. The inter-,ntramolecular hydrogen bonding interactions and delocalizationf ED from O8 to C7 O9 and O23 H24 may cause a change in theharacteristic frequency of C O stretching mode. The O H stretch-ng vibrations are characterized by a very broad band appearing inhe region 3400–3600 cm−1 [20]. In the present study, the bandor this mode is not active in FT-Raman spectrum. Hence the bandbserved at 3563 cm−1 in FT-IR is assigned to O H stretching mode.he scaled theoretical wavenumbers of C O and O H stretchingodes at B3LYP/6-311++G(d,p) in the title molecule are matchedell with the experimental value as well as the similar kind ofolecules. In the case of NTA, the in-plane and out-of-plane bend-

ng vibrations are assigned to the modes �35, �37, �41 and �42. Theredicted wavenumbers related to these modes are found to be ingreement with their experimental observations.

.5.4. NO2 vibrationsUsually, a single nitro group substitution linked with the ben-

ene ring provides us six vibrations viz. symmetry stretching,symmetry stretching, scissoring, wagging, rocking and twist-ng. The asymmetric and symmetric stretching vibrations of NO2ormally produce bands in the regions 1500–1570 cm−1 and300–1370 cm−1 in nitro benzenes and substituted nitro ben-enes [46], respectively. In the title molecule, the NO2 asymmetrictretching is observed at 1510 cm−1 in FT-Raman. The strong inten-ity band observed at 1306 cm−1 in FT-IR is assigned to NO2ymmetry stretching. The theoretically scaled values at 1508 and313 cm−1 by B3LYP/6-311++G(d,p) method are in correlation withhe experimental observations of NO2 asymmetry and symmetrytretching vibrations, respectively.

Nitro substituted aromatic compounds generate a band of weak-o-medium intensity in the low frequency region belongs to NO2ending vibrations [47,48]. It follows from Table 5 that the strong

ntensity band observed at 843 cm−1 in FT-IR is designated as NO2cissoring (˛NO2) to the mode �30. NO2 wagging and rockingodes of the title molecule display the bands at 740 cm−1 in FT-IR

nd 562 cm−1 in FT-Raman. It should be mentioned that, the scaledavenumbers calculated at B3LYP/6-311++G(d,p) method are coin-

ide well with their experimental datas. The scaled wavenumber1 cm−1 assigned to NO2 torsion mode of the title molecule is ingreement with the assignments proposed by Kanna Rao and Syamundar [49].

.5.5. C N vibrationsThe bond between C2 atom of the benzene ring and N11 atom of

he nitro group gives rise to three vibrational modes; C N stretch-ng (CN), C N in-plane bending (�C NO2) and C N out-of planeending (�C NO2) vibrations. Generally, the identification of C Nands in both the FT-IR and FT-Raman spectra are rather a difficultask, since these bands are highly mixed with other vibrations. Inhe present study, the weak intensity band at 1041 cm−1 in FT-IRnd 1043 cm−1 in FT-Raman is assigned to C N stretching mode ofTA. In previous, we have reported C N stretching at 928 cm−1 inT-Raman for 5-nitrosalicylic acid [39]. The peaks corresponding to

N in-plane and out-of-plane bending modes of the investigatedompound are not observed in both FT-IR and FT-Raman spectrum.

ence with the aid of PED results, the calculated values of 365 and71 cm−1 are assigned to C N in-plane and out-of plane bendingodes, respectively. For 5-NSA, these bands have been assigned at

50 (�C NO2) and 158 cm−1 (�C NO2).

[[[[

Acta Part A 92 (2012) 137– 147

4.5.6. Ring vibrationsIn case of NTA, the carbon atoms coupled together in the hexag-

onal chain of benzene possesses six C C stretching vibrations inthe region of 1460–1150 cm−1. According to the PED results, thering C C stretching modes are observed at 1612, 1591, 1477 and1386 cm−1 in FT-IR and 1596, 1482, 1389, and 1198 cm−1 in FT-Raman for NTA. The in-plane and out-of-plane bending vibrationsof benzene ring are generally observed below 1000 cm−1 [44] andthese modes are not pure but they contributes drastically fromother vibrations and are substituent-sensitive. In the title molecule,ring in-plane (�ring) and out-of plane (�ring) bending modes areaffected to a great extent by the substituents and produce bandsbelow 500 cm−1. The weak intensity bands present at 242 and118 cm−1 in FT-Raman spectrum are assigned to �ring and a bandobserved at 205 cm−1 is assigned to �ring. The scaled theoreticalwavenumbers corresponding to ring vibrations are found to have agood correlation with their available experimental observations.

5. Conclusions

In the present study, the potential energy surface of 6-nitro-m-toluic acid was examined. The optimized geometrical parametersof the minimum energy conformer obtained from the relaxedscan were computed by density functional B3LYP method with6-311++G(d,p) basis set. The optimized geometrical parametersof the dimer conformer of NTA were compared with that of themonomer. The significant changes in bond lengths of the monomerupon dimerization were explained with the help of NBO analysis.The delocalization of ED from Lewis base to Lewis acid sites werediscussed with their stabilization energies. The reported naturalatomic hybrid orbitals of interacting NBOs of the molecule allowedus to know about the lengthening and shortening of O H and C Obonds.

The mapped isodensity surfaces for the frontier molec-ular orbitals were plotted. The smallest energy gap(�HOMO–LUMO = −0.1793 a.u.) between HOMO and LUMO orbitalsrevealed that the molecule used in this study belongs to softmaterial and the probability of � → �* proton transition is highlypossible within the molecule. In addition, the nucleophilic andelectrophilic sites on the MEP surface were determined. Thecharges accumulated on various constituents of MEP surfacewere reported along with their electric potential values. Thedetailed descriptions for the vibrational normal modes of NTAmolecule were presented on the basis of combined experimentaland theoretical IR and Raman studies. The scaled theoreticalwavenumbers computed at B3LYP/6-311++G(d,p) method werefound in agreement with their experimental observations.

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