Journal of Molecular Structure 1005 (2011) 192–201

Contents lists available at SciVerse ScienceDirect

Journal of Molecular Structure

journal homepage: www.elsevier .com/locate /molstruc

Spectroscopic studies, HOMO–LUMO and NBO calculations on monomerand dimer conformer of 5-nitrosalicylic acid

T. Karthick a, V. Balachandran b,⇑, S. Perumal c, A. Nataraj d

a Department of Physics, Vivekanandha College for Women, Tiruchengode 637 205, Indiab Department of Physics, AA Government Arts College, Musiri, Tiruchirappalli 621 201, Indiac Department of Physics, S.T. Hindu College, Nagarcoil 629 002, Indiad Department of Physics, Thanthai Hans Roever College, Perambalur 621 212, India

a r t i c l e i n f o a b s t r a c t

Article history:Received 17 July 2011Received in revised form 26 August 2011Accepted 26 August 2011Available online 16 September 2011

Keywords:Vibrational spectra5-Nitrosalicylic acidDFTHOMO–LUMODimerNBO analysis

0022-2860/$ - see front matter Crown Copyright � 2doi:10.1016/j.molstruc.2011.08.050

⇑ Corresponding author. Tel.: +91 431 2432454; faxE-mail address: brsbala66@gmail.com (V. Balachan

In this work, FT-IR and FT-Raman spectra are recorded on the solid phase of 5-nitrosalicylic acid (abbre-viated as 5-NSA). The energies of all possible conformers obtained from DFT theory with 6-311++G(d,p)basis set identified the most stable conformer of 5-NSA as C6 form. Optimized geometrical parameters,vibrational assignments, HOMO–LUMO energies and NBO calculations are performed on the stable mono-mer and dimer structure of 5-NSA using the same level of theory. Second order perturbation energies andelectron density (ED) transfer from filled lone pairs of Lewis base to unfilled Lewis acid sites of 5-NSA arediscussed on the basis of NBO analysis. Inter- and intramolecular hydrogen bonds exist between ACOOHand AOH group, give the evidence for the formation of dimer entities in the title molecule. The variationsin bond lengths, ED and vibrational wavenumbers of 5-NSA dimer are being discussed. The spectroscopicand theoretical results are compared to the corresponding properties for 5-NSA monomer and dimer ofC6 conformer. Reliable vibrational modes associated with 5-NSA are made on the basis of total energydistribution (TED) results obtained from scaled quantum mechanical (SQM) method.

Crown Copyright � 2011 Published by Elsevier B.V. All rights reserved.

1. Introduction

Owing to their dimeric nature, salicylic acid and its derivativeshave recently become attractive to spectroscopic researchers.Boczar et al. have been reported the optimized dimer geometryand vibrational assignments of salicylic acid [1]. ExperimentalFT-Raman, FT-IR and theoretical dimer conformer of 5-bromosali-cylic acid [2] and 5-fluro, 5-chlorosalicylic acid [3] have been stud-ied by Karabacak et al. Apart from the dimeric characteristics, themonomer geometry of salicylic acid and its analogs play a vital rolein the vibrational analysis. Chen and Shyu investigated the con-formers and intramolecular hydrogen bonding of salicylic acidand its anions [4]. Goulet and Aroca [5] presented the infraredand Raman spectra of solid salicylic acid and performed DFT/B3LYP/6-311+G(d,p) calculations. Likewise, the infrared, Ramanspectra, ab initio calculations and the vibrational assignments of4-aminosalicylic acid were studied by Akkaya and Akuyz [6]. Theyobtained seven stable conformers for 4-aminosalicylic acid. Vargheseet al. elucidated the vibrational assignments of 3,5-dinitrosalicylicacid on the basis of experimental IR and Raman spectra [7]. Re-cently FT-Raman, FT-IR spectra and the structure of benzoic and

011 Published by Elsevier B.V. All

: +91 4326 262630.dran).

3,5-dichlorosalicylic acid were discussed by Krishnakumar andMathamal [8].

The simplest aromatic carboxylic acid known as salicylic acid isfound in nature both in the free state (flowers of wild chamomileand a species of the willow) and in the bound state in many essen-tial oils (clove, tuberose, etc.). It is widely used as a plant growthregulator, antimicrobial and antifungal agent in food productindustry [9,10]. In medicine, salicylic acid and its derivatives areemployed as an intestinal antiseptic agent for treating rheumaticfever [11]. Upto now, in the spectroscopic field, salicylic acid andits derivatives have been taken for elucidating their vibrationalassignments and optimized molecular structures. Besides, a fewof inter- and intramolecular hydrogen bonding studies have beenperformed by modern spectroscopists [1,3].

To the extent of our knowledge, neither FT-IR and FT-Ramanspectra nor dimer geometries of 5-nitrosalicylic acid (5-NSA) havebeen performed so far. The main objective of this work is to illus-trate vibrational assignments, optimized geometrical parameters,HOMO–LUMO energies and natural bond orbital analysis of stableconformer of 5-NSA monomer and dimer conformer. In the presentinvestigation, the delocalization of electron density (ED) from thefilled lone pairs of Lewis base to the unfilled Lewis acid sites of5-NSA dimer and the corresponding changes in bond lengths, ED,vibrational wavenumbers upon dimerization are being reported.

rights reserved.

T. Karthick et al. / Journal of Molecular Structure 1005 (2011) 192–201 193

2. Experimental

The commercial crystalline sample of 5-NSA (99% purity) ob-tained from Lancaster Chemical Company (UK) is directly (withoutfurther purification) used for spectral measurements. The Fouriertransform infrared spectrum (FT-IR) of the title compound is re-corded in the frequency region 400–4000 cm�1 on a NEXUS 670spectrophotometer equipped with an MCT detector, a KBr pellettechnique. The FT-Raman spectrum of 5-NSA is recorded in the fre-quency region 0–4000 cm�1 on a NEXUS 670 spectrophotometerequipped with Raman module accessory operating at 1.5 W powerwith Nd:YAG laser of wavelength 1064 nm as an excitation source.

3. Computational details

The entire vibrational assignments and optimized monomerand dimer geometrical parameters of stable conformer of 5-NSAare predicted by means of density functional B3LYP method withinternally stored standard 6-311++G(d,p) basis set level in Gaussian03W software package [12]. B3LYP represents Becke’s three-parameter hybrid functional method [13] with Lee–Yang–Parr’scorrelation functional (LYP) [14,15].

In order to obtain the stable conformer, the self consistent field(SCF) energy calculation is performed on eight possible conformersof 5-NSA using B3LYP/6-311++G(d,p) method. From SCF energycalculation we deduce that, the conformer C6 acquire dominantstability among others as shown in Table 1. The electronic struc-ture of 5-NSA monomer and dimer of C6 conformer in the groundstate were optimized by assuming C1 point group symmetry. Theoptimized geometrical parameters and vibrational wavenumbersof monomer and dimer of C6 conformer are obtained from thesame level of DFT theory. In the calculations, the charge of eachpoint is taken as zero and the spin multiplicity is taken as one.Due to the neglect of anharmonicity effect at the beginning of cal-culation, initially the predicted vibrational wavenumbers byB3LYP/6-311++G(d,p) are found to be in disagreement with exper-imental wavenumbers. Further, these discrepancies are overcomeby applying the selective scaling factors in the wavenumber regionranging from 4000 to 1700 cm�1 and below 1700 cm�1. Vibrationalmode assignments made in this work are performed on the basis oftotal energy distribution (TED) results obtained from MOLVIB pro-gram (version V7.0-G77) written by Sundius [16–18].

The molecular orbital (MO) calculations such as HOMO–LUMOand NBO calculations are also performed on the monomer and di-mer conformer of 5-NSA with the same level of DFT theory.

4. Results and discussion

The present compound under investigation has become agreater interest because it has three different substituents namelyhydroxyl group (AOH), carboxyl group (ACOOH), and Nitro group

Table 1Calculated energies and energy difference for eight conformers of 5-NSA by B3LYP/6-311++G(d,p) method.

Conformers Energy (a.u) Energy differencesa (a.u)

C1 �700.65161 0.01423C2 �700.65617 0.00967C3 �700.65005 0.01579C4 �700.55947 0.10637C5 �700.64835 0.01749C6 �700.66584 0.00000C7 �700.63449 0.03135C8 �700.65287 0.01297

a Energies of the other seven conformers relative to the most stable C6conformer.

(ANO2) that are attached to the benzene ring. The molecular ener-gies of eight possible conformers of the title molecule are calcu-lated using density functional theory with 6-311++G(d,p) basisset. From the calculations, the most stable conformer is identifiedas C6 (EC6 = �700.66584 a.u) and it is also found that, the con-former C4 (EC4 = �700.63449 a.u) is the least stable conformeramong others as shown in Table 1. The stable conformer structureC6 shown in Fig. 1 contains intramolecular (known as hydrogenbond) O� � �H bonding between the hydroxyl group and O@COH.The intramolecular hydrogen bonding arising in the conformerC6 causes dominant stability of molecule among the others.

4.1. Dimer entity

The Fig. 2 presents the geometry of 5-NSA dimer optimized atB3LYP/6-31++G(d,p) level using Gaussian 03W program package.The dimer entities in 5-NSA can be proved by shaping the structureby joining high-frequency OAH stretching and low-frequencyO� � �O stretching mode. The basic mechanism by couplinghigh-frequency OAH and low-frequency O� � �O band is known asanharmonic-type coupling [1]. The dimer structure shown inFig. 2 contains two intermolecular and two intramolecular hydrogenbonds which are similar as in the model given by Boczar [1], Mare-chal and Witkowski [19]. The energy of the dimer structure calcu-lated at B3LYP/6-31+G(d,p) is about (EDim = �1401.18601 a.u) andit is also found that, the energy of 5-NSA dimer is found to be twicethat of its stable monomer structure.

4.2. Geometrical parameters

The predicted geometrical parameters such as, bond lengthsand bond angles for the most stable C6 conformer and the dimerof C6 conformer of 5-NSA calculated at B3LYP/6-311++G(d,p)method are presented in Table 2 in accordance with atom number-ing schemes given in Figs. 1 and 2 respectively. For comparativepurpose, the experimental X-ray diffraction data [20] of 5-NSA isalso presented. The intermolecular interaction through ACOOHgroup of each 5-NSA molecule forms a dimer and intramolecularinteraction through OAH� � �O connecting the adjacent dimersmay lead to infinite chains in random directions.

When comparing experimental values, the computed bondlengths, bond angles and vibrational frequencies are slightly larger,because theoretical calculations are performed upon the isolatedmolecule in the gaseous state and the experimental results are per-formed on the solid phase of the molecule [21]. The experimentalCAC bond lengths of the aromatic ring fall in the range from1.3749 to 1.4059 Å, while the results obtained from B3LYP/6-311++G(d,p) fall in the range 1.3789–1.4063 Å for the monomerstructure and 1.3785–1.4068 Å for the dimer structure. In contrast,carbon atom C7 in carboxylic group attached to ring C1 makes bondlength C1AC7 longer than that of ring CAC. The experimentally ob-served C1AC7 bond length (1.4772 Å) is approximately 0.1 Å greaterthan that of ring CAC which is in good agreement with the calculatedvalue. The CAH bond lengths such as C3AH13, C4AH14, and C6AH18

calculated at B3LYP/6-311++G(d,p) are too long in comparison withthe experimental values. It is well known that DFT method predictbond lengths that are systematically too long, especially in CAHbond lengths [2]. This may be due to the fact that, the low scatteringfactor of hydrogen atoms involved in the X-ray diffraction experi-ment produces large deviation from the theoretical CAH bondlengths.

According to international crystallography table [22] the C@Oand CAO bond lengths in the aromatic carboxylic group conformto an average value of 1.2260 Å and 1.3050 Å respectively. Theexperimental C@O and CAO bond lengths of the title moleculeare 1.2376 and 1.3105 Å [20] respectively. The calculated value

C5 C6 C7 C8

C2 C3 C1 C4

Fig. 1. The possible theoretical conformers of 5-NSA.

Fig. 2. C6 dimer conformer of 5-NSA.

194 T. Karthick et al. / Journal of Molecular Structure 1005 (2011) 192–201

of CAO (1.3135 Å) in 5-NSA dimer reported in Table 2 is in goodagreement with experimental data. In contrast, the calculatedC@O bond length of dimer is slightly larger than that of experimen-tal. This is because of the fact that, upon dimerization one can findthat the electron density of atoms delocalized from a filled lonepair of lewis base to an unfilled lewis acid. The similar effect wasobtained on the angle C7AO8AH9 and C1AC7AO10. The experimen-tal and the calculated values for the nitro group bond lengths suchas N15AO16 and N15AO17 shows considerable double bond charac-ter type.

Inter- (H27� � �O10, O28� � �H9) and intramolecular (O10� � �H12,O28� � �H30) bond distances which causes stabilization of the mono-mer and dimer structure of 5-NSA are also reported in Table 2. Theoptimized hexagonal CACAC bond angles fall in the range119–121� except C2AC1AC7 (124�) and C6AC1AC7 (116�). The hex-agonal ring CACAH angles are found to be 120�. The CACAO bond

angles of 5-NSA are greatly affected by H-bonding interactions asshown in Table 2.

4.3. NBO analysis

In order to explain the hyperconjugative interactions, inter-,intramolecular hydrogen bonding, intermolecular charge transfer(ICT), electron density transfer (EDT) and cooperative effect due todelocalization of electron density from the filled lone pairs n(Y) of‘‘Lewis base’’ Y into the unfilled antibond r⁄(XH) of ‘‘Lewis acid’’XAH in XAH� � �Y hydrogen bonding systems, one can find naturalbond orbital (NBO) analysis as an effective tool [23]. In the presentwork, NBO analysis has been performed on 5-NSA monomer anddimer to study the intermolecular hydrogen bonding, intermolecu-lar charge transfer (ICT), delocalization of electron density andcooperative effect due to n(O) ?r⁄(OAH) using NBO 3.1 program

Table 2Comparison of geometrical parameters, bond lengths (Å), and bond angles (�), for themonomer and dimer C6 conformer of 5-NSA calculated by the B3LYP/6-311++G(d,p)method.

Parametersa Experimental X-rayb B3LYP/6-311++G(d,p)

Monomer Dimer

Bond lengths (Å)C1AC2 1.4059 1.4063 1.4068C1AC6 1.3996 1.3993 1.4009C1AC7 1.4772 1.4695 1.4682C2AC3 1.4052 1.4057 1.4058C2AO11 1.3466 1.3335 1.3340C3AC4 1.3749 1.3789 1.3785C3AH13 0.9500 1.0824 1.0824C4AC5 1.3944 1.4007 1.4010C4AH14 0.9500 1.0816 1.0816C5AC6 1.3794 1.3823 1.3809C5AN15 1.4625 1.4704 1.4716C6AH18 0.9500 1.0806 1.0806C7AO8 1.3105 1.3423 1.3135C7AO10 1.2376 1.2243 1.2454O8AH9 0.8400 0.9692 0.9972O11AH12 0.8400 0.9841 0.9812N15AO16 1.2320 1.2258 1.2257N15AO17 1.2261 1.2256 1.2250

Inter- and intra-molecular H bond lengthsO11AH12� � �O10 1.9100 1.7461 1.7544O26AH27� � �O10 – – 1.6798O8AH9� � �O28 – – 1.6807

Bond angle (�)C2AC1AC6 119.6 119.6 119.5C2AC1AC7 120.5 118.9 120.0C6AC1AC7 119.9 121.5 120.5C1AC2AC3 119.9 119.4 119.5C1AC2AO11 123.6 122.9 123.2C3AC2AO11 116.6 117.7 117.4C2AC3AC4 120.3 120.4 120.5C2AC3AH13 119.9 118.3 118.2C4AC3AH13 119.9 121.3 121.3C3AC4AC5 119.1 119.6 119.5C3AC4AH14 120.4 121.2 121.3C5AC4AH14 120.4 119.3 119.3C4AC5AC6 122.1 121.5 121.5C4AC5AN15 119.3 119.3 119.3C6AC5AN15 118.6 119.2 119.2C1AC6AC5 119.0 119.5 119.6C1AC6AH18 120.5 120.5 120.3C5AC6AH18 120.5 120.0 120.1C1AC7AO8 121.6 121.6 121.8C1AC7AO10 114.6 114.6 115.8O8AC7AO10 123.8 123.8 122.4C7AO8AH9 109.5 107.6 110.7C2AO11AH12 109.5 108.3 108.3C5AN15AO16 118.4 117.5 117.5C5AN15AO17 118.3 117.8 117.8O16AN15AO17 123.4 124.7 124.8

Inter- and intra-molecular H bond anglesO11AH12 � � �O10 144.0 145.1 144.5O29AH30� � �O28 144.0 145.1 144.5O8AH9� � �O28 – – 179.9O26AH27� � �O10 – – 179.9

a For numbering of atom see Fig. 1.b Taken from Ref. [21].

T. Karthick et al. / Journal of Molecular Structure 1005 (2011) 192–201 195

as implemented in Gaussian 03 W package. The intramolecularOAH� � �O hydrogen bonding is formed by the orbital overlap be-tween n(O) and r⁄(OAH) which results ICT causing stabilization ofthe H-bonded systems. Hence hydrogen bonding interaction leadsto an increase in electron density (ED) of OAH antibonding orbital.The increase in population of OAH antibonding orbital weakensthe OAH bond. Thus the nature and strength of the intramolecularhydrogen bonding can be explored by studying the changes inelectron densities in vicinity of O� � �H hydrogen bonds.

The NBO analysis of 5-NSA monomer and dimer clearly give theevidences for the formation of two strong H-bonded interactionsbetween oxygen lone electron pairs and r⁄(OAH) antibondingorbitals. The occupancies and their respective energies of oxygenlone pairs and antibonding orbitals which are responsible for thestabilization of H-bonded monomer and dimer entities of 5-NSAare given in Table 3. The magnitude of charges transferred fromthe lone pairs of Lewis base sites n1(O10), n1(O28) into antibondingLewis acid sites r⁄(O11AH12), r⁄(O29AH30) respectively are signifi-cantly changed upon dimerization as given in Table 3. The changein occupancies and their respective change in energies of Lewisbase and Lewis acid sites upon dimerization directly give the evi-dence for the existence of intramolecular interactions within themolecule.

In addition to occupancy numbers and energies of various Lewisbase-acid sites, the stabilization energies E(2) associated with thehyperconjugative interactions are given in Table 4. The stabiliza-tion energies corresponding to hyperconjugative interactionsn1(O10) ? r⁄(O11AH12), n2(O10) ? r⁄(O11AH12),n1(O28) ? r⁄(O29AH30),n2(O28) ? r⁄(O29AH30) which are responsible for the stabilizationof the molecule are obtained as 5.57, 9.89, 5.54, 9.90 kcal mol�1,respectively. The differences in E(2) energies are reasonably dueto the fact that the accumulation of electron density in the OAHbond is not only drawn from the n(O) of hydrogen-acceptor butalso from the entire molecule.

Furthermore, the composition of H-bonded NBO’s of 5-NSA is pre-sented in Table 5. It is also observed that, the s-character of C7AO10

hybrid orbital decreases (8.46%) from sp2.21 to sp2.11 upon dimeriza-tion. This leads to a conspicuous weakening of C7AO10 bond and itselongation. This shows the existence of a mesomeric structure char-acterized by delocalization of electron density from r⁄(C7AO10) anti-bonding orbital to the remaining part of the molecule. This is quitepossible because the energy of r⁄(C7AO10) antibonding orbital(0.48102 a.u) is higher than the energy of r⁄(O11AH12) antibondingorbital (0.36845 a.u) which supports the likelihood of the delocaliza-tion of ED from C7AO10 to O11AH12 region. This is clearly reflected inthe geometry as bond C7AO10 elongates to an amount of 0.0211 Åwith respect to the theoretical monomer upon dimerization. Further,the second order perturbation theory analysis of Fock matrix in NBObasis shows that n1(O10) can readily interact with r⁄(C7AO8) andr⁄(O11AH12) antibonding orbitals.

It is also observed that, the s-character of C7O8 increases (0.74%)from sp2.56 to sp2.47 upon dimerization. This results in the strength-ening of C7O8 bond and its contraction. On the other hand, thedelocalization of ED from C7O8 to O8H9 increases the bond lengthof C7O8 by an amount 0.0288 Å with respect to the monomer.

4.4. HOMO–LUMO energy gap

An analysis of the electron density of highest occupied molecularorbitals (HOMO) and lowest unoccupied molecular orbitals (LUMO)of 5-NSA can give us some idea about the ground and excited stateproton transfer processes. Both HOMO and LUMO of 5-NSA mono-mer and dimer are of p type, but their phases are quite different asshown in Figs. 3 and 4, respectively. The energies corresponding tovarious HOMO and LUMO levels of 5-NSA are performed by densityfunctional B3LYP/6-311++G(d,p) method. The HOMO–LUMO energycalculation reveals that, there are 47 occupied and 48 unoccupiedmolecular orbitals associated with 5-NSA monomer. The energiescorresponding to the highest occupied and lowest unoccupiedmolecular orbitals of 5-NSA monomer are found to be �0.2835and �0.1125 a.u respectively as shown in Table 6.

The energy gap between various occupied and unoccupiedmolecular orbitals of 5-NSA calculated at B3LYP/6-311++G(d,p) le-vel is given in Table 6 reflects the chemical reactivity of the mole-cule. LUMO as an electron acceptor represents the ability to obtain

Table 4Second order perturbation theory analysis of Fock Matrix in NBO basis.

Donor (i) Acceptor (j) E(2)a (kcal/mol) E(j)–E(i)b (a.u) F(i, j)c (a.u)

Within Unit 1n1(O10) r⁄(C7AO8) 1.89 1.13 0.041n2(O10) r⁄(C7AO8) 21.75 0.71 0.113n1(O10) r⁄(O11AH12) 5.57 1.15 0.072n2(O10) r⁄(O11AH12) 9.89 0.72 0.078

From Unit 1 to Unit 2n1(O10) r⁄(O26AH27) 11.08 1.12 0.100n2(O10) r⁄(O26AH27) 14.77 0.70 0.092

From Unit 2 to Unit 1n1(O28) r⁄(O8AH9) 11.04 1.12 0.100n2(O28) r⁄(O8AH9) 14.73 0.70 0.092

Within Unit 2n1(O28) r⁄(C25AO26) 1.88 1.13 0.041n2(O28) r⁄(C25AO26) 21.73 0.71 0.113n1(O28) r⁄(O29AH30) 5.54 1.15 0.071n2(O28) r⁄(O29AH30) 9.90 0.72 0.078

a E(2) means energy of hyperconjugative interactions.b Energy difference between donor and acceptor i and j NBO orbitals.c F(i, j) is the Fock matrix element between i and j NBO orbitals.

Table 5Composition of hydrogen bonded NBO’s in terms of natural atomic hybrids.

H-bonded NBO’s Monomer Dimer DNBO

spn(C7AO10) sp2.21 sp2.11 �s%s – char. 31.08% 22.62% 8.46%P – char. of C7 37.49% 34.59% �2.9%P – char. of O10 62.51% 65.41% +2.9%q(C7)/e 0.805 0.829 +0.024q(O10)/e �0.678 �0.717 0.039spn(C7AO8) sp2.56 sp2.47 �s%s – char. 28.01% 28.75% +0.74%P – char. of C7 32.27% 33.10% +0.83%P – char. of O8 67.73% 66.90% 0.83%q(C7)/e 0.805 0.829 +0.024q(O8)/e �0.667 �0.663 0.004

Table 3Occupancies and energies of interacting Lewis base and Lewis acid sites.

Parameters Occupancy (e) Energy (a.u)

Monomer Dimer Docc. Monomer Dimer DE

n1(O10) 1.980 1.946 �0.034 �0.817 �0.780 �0.037r⁄(O11AH12) 0.034 0.042 0.008 0.439 0.368 �0.071n1(O28)a 1.980 1.946 �0.034 �0.817 �0.780 �0.037r⁄(O29AH30)a 0.034 0.042 0.008 0.439 0.368 �0.071r⁄(C20AO29)a 0.025 0.019 �0.006 0.232 0.346 0.114r⁄(C25AO26)a 0.063 0.061 �0.002 0.179 0.353 0.174r⁄(C25AO28)a 0.028 0.026 �0.002 0.227 0.482 0.255p⁄(C25AO28)a 0.304 0.350 0.046 �0.117 �0.070 �0.047r⁄(O26AH27)a 0.006 0.063 0.057 0.369 0.342 �0.027r⁄(O29AH30)a 0.034 0.042 0.008 0.439 0.368 �0.071

a Values for monomer are taken from the identical NBO’s of other unit.

196 T. Karthick et al. / Journal of Molecular Structure 1005 (2011) 192–201

an electron. HOMO represents the ability to donate an electron.HOMO orbital on the aromatic ring of 5-NSA (Fig. 3a) is primarilyof anti-bonding character type over C1, C2, C3 atoms, whereas C4,C5, C6 and O11, H12 show bonding character. Both the hydroxyland carbonyl oxygen having bonding character type, with a largerelectron density over hydroxyl oxygen. A HOMO � 1 orbital shownin Fig. 3c has no amplitude [24] over the aromatic ring, whereasthe orbital overlap on the nitro group of 5-NSA shows considerabledouble bond character type. The HOMO � 2 orbital (Fig. 3e) on thearomatic ring show that the atoms C6, C1 having considerablebonding character, whereas the atoms C3, C4, C5 having anti-bond-ing character.

In contrast, all the three LUMO surfaces shown in Fig. 3b, d and fare p⁄ in nature. Because if we look into the electronic distributionof LUMO within the ring, the C1, C2, C3 position have bonding char-acter, whereas the C4, C6 position have anti-bonding character. Theorbital overlapping on the 5-NSA dimer is shown in Fig. 4. It isworth mentioning that, bonding character positions on the left sideof dimer structure is anti-bonding character in nature on its rightside and vice versa.

4.5. Vibrational spectra

In order to obtain the spectroscopic signature of 5-NSA molecule,a frequency calculation is performed on the gaseous phase of themolecule, while experimental FT-IR and FT-Raman are performedon the solid phase of the molecule. Hence there are disagreements

between calculated and observed vibrational wavenumbers. Toovercome discrepancies between observed and calculated wave-numbers, the scale factor of 0.958 and 0.983 are introduced in theregion from 4000 to 1700 cm�1 and lower than 1700 cm�1, respec-tively [25].

The present molecule 5-NSA consists of 18 atoms, so it has 48normal vibrational modes. On the basis of C1 symmetry the 48 fun-damental vibrations of the title molecule can be distributed into 33in-plane and 15 out-of-plane vibrations of same species. Figs. 5 and6 present the experimental and calculated IR and Raman spectrafor comparative purposes, where the calculated IR intensities andRaman activities are plotted against harmonic vibrational wave-numbers. The experimental wavenumbers are depicted in Table.7 together with the calculated wavenumbers of monomer and di-mer C6 conformer of 5-NSA molecule. The resulting vibrationalwavenumbers for the optimized geometry and the proposed vibra-tional assignments as well as IR intensities (IIR) and Raman scatter-ing activities (SRaman) are also given in Table 7. The completevibrational assignments provided in this study are based on the to-tal energy distribution (TED) results obtained from MOLVIB pro-gram [16–18]. It is observed that, in solid 5-NSA the ACOOH andAOH groups are involved in inter and intramolecular hydrogenbonding interactions. In order to simulate hydrogen bondingthrough the ACOOH group, we also calculate vibrational wave-numbers of 5-NSA dimer of C6 conformer.

4.5.1. CAH vibrationsIn the present study, the three adjacent hydrogen atoms left

around the benzene ring of 5-NSA give rise to three CAH stretchingmodes (m44Am46), three CAH in-plane bending (m30, m32, m33) andthree CAH out-of-plane bending (m24, m26, m27) modes. The het-eroaromatic organic molecule shows the presence of the CAHstretching vibrations in the 3000–3100 cm�1 range which is thecharacteristic region for the identification of CAH stretchingvibrations [26]. Accordingly, the CAH stretching modes of 5-NSAis assigned to 3095 and 3060 cm�1 in FT-IR and 3054 cm�1 inFT-Raman. These modes are calculated from 3099 to 3068 cm�1

for the most stable C6 conformer. They are very pure modes sincetheir TED contributions are almost 100%. For 3,5-dinitrosalicylicacid, the CAH stretching modes are assigned to 3106 cm�1 inFT-IR and 3085 cm�1 in FT-Raman [7]. For 5-BrSA, these modesare assigned to 3075 cm�1 in FT-Raman and 3061 cm�1 in FT-IR [2].

In aromatic compounds, the CAH in-plane and out-of-planebending vibrations appear in the range 1000–1300 cm�1 and750–1000 cm�1 [27,28] respectively. Hence the CAH in-planebending modes of 5-NSA are assigned to 1147, 1235 and1300 cm�1 in FT-IR and 1148, 1235 and 1305 cm�1 in FT-Raman.The calculated values of this mode are 1151, 1247 and1311 cm�1 which shows better agreement with the experimentalvalues. For 3,5-dichlorosalicylic acid, these bands are observed at

(c) HOMO−1 (d) LUMO+1

(e) HOMO−2 (f) LUMO+2

(a) HOMO (b) LUMO

Fig. 3. Electron density plot for the frontier molecular orbitals of stable 5-NSA monomer.

T. Karthick et al. / Journal of Molecular Structure 1005 (2011) 192–201 197

1222, 1312 cm�1 in FT-IR and 1152, 1226, 1318 cm�1 in FT-Raman[8]. The TED contribution results at the last column of Table 7 showthat, OAH in-plane bending vibrations interacting considerablywith CAH in-plane bending mode. The CAH out-of-plane bendingvibrations of 5-NSA are attributed to 863, 990 cm�1 in FT-IR. FT-Raman bands corresponding to these vibrations are not observed.

In the present study, the scaled theoretical values of CAH out-of-plane bending modes calculated at B3LYP/6-311++G(d,p) showgood agreement with the experimental values of 5-NSA as wellas with that of similar kind of molecules [2,8]. The TED contribu-tion to these modes indicates that, CAH out-of-plane bendingmodes are also highly pure modes like CAH stretching modes.

(a) HOMO (b) LUMO

(c) HOMO−1 (d) LUMO+1

(e) HOMO−1 (f) LUMO+1

Fig. 4. Electron density plot for the frontier molecular orbitals of stable 5-NSA dimer.

198 T. Karthick et al. / Journal of Molecular Structure 1005 (2011) 192–201

4.5.2. Carboxylic acid group vibrationsGenerally carboxylic acid group containing molecules possesses

dimeric character. The carboxylic acid dimer is formed by stronghydrogen bonding in the solid and liquid state. Hence the deriva-tives of carboxylic acids are best characterized by the carbonyland hydroxyl groups. The presence of carbonyl group is the mostimportant in the infrared spectrum because of its strong intensityof absorption and high sensitivity toward relatively minor changesin its environment. Intra- and intermolecular hydrogen bondingfactors affect the carbonyl and absorptions in common organiccompounds due to inductive, mesomeric, field and conjugation ef-fects [8]. The characteristic infrared absorption wavenumber of

Table 6Selected occupied and unoccupied molecular orbital energies and energy gap of 5-NSA.

Molecularorbitals

MonomerB3LYP/6-311++G(d,p)

Energy E(a.u)

Possible molecular orbital energytransition

Energy gap(a.u)

Occupied HOMO ? LUMO �0.1710HOMO �0.2835 HOMO � 1 ? LUMO �0.1714HOMO � 1 �0.2839 HOMO � 2 ? LUMO �0.2031HOMO � 2 �0.3156 HOMO ? LUMO + 1 �0.1711Unoccupied HOMO � 1 ? LUMO + 1 �0.1715LUMO �0.1125 HOMO � 2 ? LUMO + 1 �0.2032LUMO + 1 �0.1124 HOMO ? LUMO + 2 �0.1764LUMO + 2 �0.1071 HOMO � 1 ? LUMO + 2 �0.1768

HOMO � 2 ? LUMO + 2 �0.2085

C@O in acids are normally strong in intensity and found in the re-gion 1690–1800 cm�1 [26,2]. In the present study, the strong bandat 1665 cm�1 in FT-IR and the band at 1657 cm�1 in FT-Raman areassigned to C@O stretching. The calculated value of C@O stretchingmode at B3LYP/6-311++G(d,p) shows good agreement with theexperimental values.

The CAO stretching of the carboxylic acid group of 5-NSA ishighly coupled with the vibrations of adjacent group i.e OAH in-plane bending. The band observed at 1507 cm�1 in FT-IR and1512 cm�1 in FT-Raman are assigned to CAO stretching (m39)mode. The wavenumber of this mode calculated by DFT is in excel-lent agreement with the experimental FT-IR and FT-Raman

DimerB3LYP/6-311++G(d,p)

DE Energy E(a.u)

Possible molecular orbital energytransition

Energy gap DE(a.u)

HOMO ? LUMO �0.1711�0.2836 HOMO � 1 ? LUMO �0.1715�0.2840 HOMO � 2 ? LUMO �0.2032�0.3157 HOMO ? LUMO + 1 �0.1712

HOMO � 1 ? LUMO + 1 �0.1716�0.1125 HOMO � 2 ? LUMO + 1 �0.2033�0.1124 HOMO ? LUMO + 2 �0.1765�0.1070 HOMO � 1 ? LUMO + 2 �0.1769

HOMO � 2 ? LUMO + 2 �0.2086

Tra

nsm

itta

nce

(%)

4000 3500 3000 2500 2000 1500 1000 500

Wavenumber (cm-1)

100

80

60

40

20

0

Tra

nsm

itta

nce

(%)

Wavenumber (cm-1)

4000 3000 2000 1500 1000 400

Fig. 5. Experimental (FT-IR), calculated IR spectrum of 5-NSA.

T. Karthick et al. / Journal of Molecular Structure 1005 (2011) 192–201 199

wavenumbers. The OAH stretching vibrations are characterized bya very broad band appearing in the region 3400–3600 cm�1 [2]. Inthe present study, the band for this mode is not active in FT-Ramanspectrum. Hence the band observed at 3610 cm�1 in FT-IR is as-signed to OAH stretching of carboxylic acid group of 5-NSA. Thescaled theoretical value 3613 cm�1 by B3LYP/6-311++G(d,p) is ingood agreement with OAH stretching of similar kind of molecules[2,8]. For example in the case of 5-BrSA, the OAH stretching of car-boxylic acid group is assigned to 3551 cm�1 [2]. The scaled theoret-ical value of this mode reported by Mehmet Karabacak et al. is alsoin good agreement with that of this title molecule. In the case ofcarboxylic acid containing dimer structure, the OAH in-planebending and CAO stretching bands involve some interaction be-

4000 3500 3000 2500 20

Tra

nsm

itta

nce

(%)

Wavenum

Wavenum

4000 3000 2000 15

100

80

60

40

20

0

Ram

an I

nten

sity

Fig. 6. Experimental (FT-Raman), calcu

tween them. Hence these are referred to as coupled OAH in-planeand CAO stretching vibrations [29]. This is also confirmed by TEDoutput results. The strong band observed at 1470 cm�1 in FT-IR and1475 cm�1 in FT-Raman are assigned to OAH in-plane bending ofcarboxylic acid group. In this mode, the TED contribution of CAOstretching is significant.

4.5.3. Hydroxyl group vibrationsThe CAO stretching and OAH bending modes of the hydroxyl

group of 5-NSA are coupled with the vibrations of adjacent groups.Intramolecular hydrogen bonding between the hydroxyl and thecarboxylic acid group of the title molecule alters the wavenumbersof CAO stretching and OAH bending vibrations. In the present

00 1500 1000 500 0

ber (cm-1)

ber (cm-1)

00 1000 500 0

lated Raman spectrum of 5-NSA.

Table 7Comparison of the experimental (FT-IR, FT-Raman wavenumbers (cm�1)) and theoretical harmonic scaled wavenumbers (cm�1), infrared intensities (IIR), Raman scatteringactivities (SRaman) of C6 monomer and dimer conformer of 5-NSA calculated by B3LYP/6-311++G(d,p) method.

Mode no Experimental wavenumber Theoretically calculated at B3LYP/6-311++G(d,p) Assignmentsa/TED(%)

Monomer Dimer wavenumber

Wave number IIR SRamanFT-IR FT-Raman

m1 – 45 s 47 0.02 0.85 46, 55 sNO2 (65), cring(22)m2 – 78 vs 77 2.22 0.35 70, 71 cCO(52), cring(19)m3 – 115 ms 126 1.65 0.13 120, 121 cCO(53), cring(22)m4 – 150 vw 145 0.15 0.96 161, 172 cCCC(47), cring(30)m5 – 161 vw 158 3.61 0.69 187, 203 cCANO2(49)m6 – 285 w 276 0.33 1.85 309 cring(41), cCO(17)m7 – 310 w 303 1.99 0.61 315, 317 cring(43), cCO(27), cOH(14)m8 – – 350 1.31 1.81 351, 361 bCANO2(45), bring(12)m9 – 367 vw 358 8.06 1.32 372 bring(42), bCO(22), bOH(18)m10 – – 424 9.50 0.20 413 bCC(56), bOH(24), bring(13)m11 – 450 w 441 2.95 3.61 447, 448 cring(52), cCANO2(14)m12 – 512 w 505 4.75 0.23 524, 523 qNO2(49), bring(26), bCO(18)m13 502 vw – 507 8.99 2.52 529, 525 cCO(45), cCH(24)m14 570 s – 570 4.23 1.20 587 bring(62), bCN(18)m15 – – 572 90.28 2.19 596 cOH(76)m16 – 645 ms 651 20.16 3.04 650 bCO(58), bring(25), bOH(12)m17 – 680 w 678 63.61 3.81 652 cOH(89)m18 696 vs – 686 14.19 0.25 685, 687 bring(52), bOH(17), NO2 sci.(11)m19 720 w 722 23.37 0.99 698, 702 bCCO(71), bOH(12), bring(10)m20 757 w 760 vs 765 70.69 0.53 748, 749 xNO2(48), cCH(26), cCN(11)m21 – 780 s 780 22.65 33.06 787 tCC(65), bring(22)m22 788 w 795 w 791 61.75 0.01 796, 798 bCO(63), bOH(12)m23 835 vs 829 w 839 3.24 0.22 832, 833 dNO2(55), bring(13), tCO(12), tCN(11)m24 863 w – 855 17.91 0.04 863, 865 cCH(78)m25 – 928 vs 932 23.66 10.91 931, 932 tCN(42), bring(22), tCC(18)m26 – – 952 12.96 0.07 970, 971 cCH(88)m27 990 s – 997 0.05 0.02 1001, 1002 cCH(89)m28 1075 w 1077 ms 1079 52.76 5.04 1098, 1099 tCC(52)m29 1135 vw 1130 ms 1134 157.12 41.78 1124, 1126 tCC(41), bOH(35), bCH(15)m30 1147 w 1148 s 1151 17.92 4.40 1147 bCH(47), bOH(25)m31 1195 s 1198 vw 1195 312.98 29.94 1165, 1166 tCC (45), bOH(17), bCH(13)m32 1235 s 1235 ms 1247 39.92 12.48 1239, 1240 bCH(52), bOH(26)m33 1300 w 1305 w 1311 77.28 3.70 1287, 1288 bCH(60), tCC(13)m34 – 1340 vs 1333 76.00 75.73 1321, 1330 tsNO2(41), tCN(32), bCH(11)m35 1355 w – 1364 471.70 323.44 1353, 1354 tCO(48), bOH(30)m36 1412 w – 1405 98.89 27.11 1377, 1378 bOH(48), tCO(18), tCC(12)m37 – – 1427 114.23 28.28 1406, 1412 tCC(43), bCH(22), bOH(13)m38 1470 ms 1475 vw 1472 11.83 21.11 1508, 1514 bOH(47), tCO(18), bCH(10)m39 1507 vw 1512 w 1507 143.12 7.00 1522, 1523 tCO(58), bOH(17), bCH(12)m40 1580 ms 1578 s 1575 136.63 25.81 1596, 1598 tasNO2(58), bOH(15), bCH(10)m41 – – 1592 98.07 71.69 1630, 1631 tCC(50), bCH(18), bCN(10)m42 1622 s 1625 w 1631 144.15 27.70 1671, 1675 tCC(55), bOH(15), bCN(12)m43 1665 s 1657 w 1663 500.61 76.17 1707, 1749 tC@O(72), bOH(19)m44 3060 w 3054 s 3068 0.27 86.55 2979, 3080 tCH(98)m45 – – 3085 3.76 99.12 3224, 3225 tCH(98)m46 3095 w – 3099 10.53 33.11 3248, 3249 tCH(100)m47 – – 3342 400.14 121.10 3256, 3259 tOH(100)m48 3610 vw – 3613 138.29 136.31 3589, 3595 tOH(100)

a ts, symmetry stretching; tas, asymmetry stretching; b, in-plane bending; c, out-of-plane bending; d, scissoring; x, wagging; @, twisting; q, rocking; s, torsion.

200 T. Karthick et al. / Journal of Molecular Structure 1005 (2011) 192–201

study, the OAH in-plane bending mode appears as weak band at1412 cm�1 in FT-IR spectrum. For 5-BrSA, the OAH in-plane bend-ing mode of the hydroxyl group is assigned to 1411 cm�1 in FT-Raman [2]. For 3-, 4-, 5-aminosalicylic acid, the title vibration isobserved at 1461, 1450 and 1454 cm�1, respectively [10]. Theweak band at 1355 cm�1 in FT-IR spectrum of 5-NSA is assignedto CAO stretching mode. The TED contribution for this mode isabout 48%. It is observed that, the OAH in-plane bending mode ishighly coupled with this mode is evident from TED result. The cal-culated value of 1364 cm�1 at B3LYP/6-311++G(d,p) level of theoryis in good agreement with the experimental value of CAO stretch-ing mode.

4.5.4. NO2 vibrationsGenerally the asymmetric and symmetric stretching vibrations

of NO2 give rise to bands in the regions 1500–1570 cm�1 and1300–1370 cm�1 in nitro benzenes and substituted nitro benzenes

[30] respectively. In the title molecule, the medium band at1580 cm�1 in FT-IR and 1578 cm�1 in FT-Raman is assigned toNO2 asymmetric stretching mode. The very strong band at1340 cm�1 in FT-Raman is attributed to NO2 symmetric stretching.The theoretically scaled values at 1575 and 1333 cm�1 by B3LYP/6-311++G(d,p) method exactly correlate with the experimentalobservations.

Aromatic nitro compounds have a band of weak-to-mediumintensity in the region of 500–590 cm�1 belongs to NO2 bendingvibrations [26]. The nitro group of the compound is capable of differ-ent bending vibrations such as scissoring, wagging, rocking andtwisting. These vibrations give rise to several variable intensitybands at lower wavenumber region. In the present study, the prom-inent band at 835 cm�1 in FT-IR and weak band at 829 cm�1 in FT-Raman is assigned to NO2 scissoring (dNO2) mode. In the case of3,5-dinitrosalicylic acid, the bands at 800 and 915 cm�1 in the IRand at 798 cm�1 in Raman spectra are assigned to dNO2 modes.

T. Karthick et al. / Journal of Molecular Structure 1005 (2011) 192–201 201

In aromatic compounds, the wagging modes (xNO2) are ex-pected in the region 690–790 cm�1 with a moderate to strongintensity. In this region, cCH is also active [31]. For 5-NSA, theweak band at 757 cm�1 in FT-IR and a very strong band at760 cm�1 in FT-Raman is assigned to NO2 wagging (xNO2). ThecCH contribution to this mode is about 26%. The NO2 rocking(qNO2) and NO2 twisting modes of the title molecule are not ob-served in FT-IR spectrum. Hence the weak band at 512 cm�1 anda strong band at 45 cm�1 in FT-Raman are assigned to NO2 rockingand twisting modes. The calculated values of all the NO2 bendingmodes at B3LYP/6-311++G(d,p) method are in excellent agreementwith the experimental observations.

4.5.5. CAN vibrationsNO2 group attached to ring carbon atom C5 of 5-NSA gives rise

to three vibrational modes such as CAN stretching (tCN), CAN in-plane bending (bCANO2) and CAN out-of-plane bending (cCANO2).The identification of CAN bands in the vibrational spectrum israther a difficult task since these bands are mixed with the othervibrational modes. Varghese et al. [7], assigned the CAN stretchingmode of 3,5-dinitrosalicylic acid at 939 cm�1 in IR and 936 cm�1 inRaman spectrum. In the present study, the prominent band at928 cm�1 in FT-Raman is assigned to tCN mode. On the other hand,the assignments of in-plane and out-of-plane CAN bending modesare made on the basis of TED results. These bands are calculated at350 and 158 cm�1 respectively.

4.5.6. Ring vibrationsIn case of 5-NSA, the benzene ring possesses six ring carbon–

carbon stretching vibrations in the region 1460–1660 cm�1 and1070–1150 cm�1. As revealed by TED, the ring CAC stretchingmodes are observed at 1622, 1195, 1135 and 1075 cm�1 in FT-IRand 1625, 1198, 1130 and 1077 cm�1 in FT-Raman for 5-NSA. Thein-plane and out-of-plane bending vibrations of the benzene ringare generally observed below 1000 cm�1 [32] and these modes arenot pure but contain a significant contribution from other modesand are substituent-sensitive. In the title molecule, ring in-planeand out-of-plane bending modes are observed at 570, 696 cm�1 inFT-IR and 367 cm�1 in FT-Raman. From TED results, the bands at285, 310, 450 cm�1 in FT-Raman are assigned to ring out-of-planebending vibrations. The peaks for these modes are not observed inFT-IR spectrum since these modes are possible to appear only infar IR spectrum. The scaled theoretical wavenumbers correspondingto ring vibrations are found to have a good correlation with theexperimental observations.

5. Conclusion

In this study, we have recorded FT-IR and FT-Raman spectra onthe solid phase of 5-NSA. The comparative results between theexperimental and theoretical wavenumbers gave us a full descrip-tion of the vibrational properties of this molecule. The most stableconformer of 5-NSA was determined and according to these resultsthe dimer conformations were analyzed with DFT/B3LYP/6-311++G(d,p) level of theory. Inter- and intramolecular hydrogenbonding between the carboxylic acid and hydroxyl group of the ti-tle molecule were discussed. The significant changes in bondlengths and vibrational wavenumbers of OAH group involving

hydrogen bond were discussed. The delocalization of ED and thechanges in energy upon dimerization were analyzed with the aidof NBO analysis. HOMO–LUMO calculations were performed onthe stable monomer and the dimer of 5-NSA. The energy differencebetween various HOMO and LUMO levels were reported. The cal-culated geometrical parameters and vibrational frequencies ob-tained from density functional theory calculations are in goodagreement with the experimental values obtained for the investi-gated molecule.

References

[1] Marek Boczar, Lukasz Boda, Marek J. Wojcik, Spectrochim. Acta A 64 (2006)757.

[2] Mehmet Karabacak, J. Mol. Struct. 919 (2009) 215.[3] Mehmet Karabacak, Etem Kose, Mustafa Kurt, J. Raman Spectrosc. 41 (2010)

1085.[4] C. Chen, S.F. Shyu, J. Mol. Struct. (Theochem) 536 (2001) 25.[5] P.J.G. Goulet, R.F. Aroca, Can. J. Chem. 82 (2004) 987.[6] Y. Akkaya, S. Akuyz, Vib. Spectrosc. 42 (2006) 292.[7] H.T. Varghese, C.Y. Panicker, D. Philip, J. Chowdhury, M. Ghosh, J. Raman

Spectrosc. 38 (2007) 323.[8] V. Krishnakumar, R. Mathamal, J. Raman Spectrosc. 40 (2009) 264.[9] P. Wen, J. Chen, S. Wan, W. Kong, P. Zhang, W. Wang, J. Zhan, Q. Pan, W. Huang,

Plant Growth Regul. 55 (2008) 1.[10] Y.P. Singh, R. Das, R.A. Sigh, Afr. J. Biochem. Res. 1 (2) (2007) 19.[11] M.K. Jain, S.C. Sharma, Organic Chemistry, Shoban Lal Nagin Chand &

Company, Educational Publishers, New Delhi, 1980.[12] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman,

J.A. Montgomery Jr., T. Vreven, K.N. Kudin, J.C. Burant, J.M. Millam, S.S. Iyengar, J.Tomasi, V. Barone, B. Mennucci, M. Cossi, G. Scalmani, N. Rega, G.A. Petersson, H.Nakatsuji, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T.Nakajima, Y. Honda, O. Kitao, H. Nakai, M. Klene, X. Li, J.E. Knox, H.P. Hratchian,J.B. Cross, C. Adamo, J. Jaramillo, R. Gomperts, R.E. Stratmann, O. Yazyev, A.J.Austn, R. Cammi, C. Pomelli, J.W. Ochterski, P.Y. Ayala, K. Morokuma, G.A. Voth, P.Salvador, J.J. Dannenberg, V.G. Zakrzewski, S. Dapprich, A.D. Daniels, M.C. Strain,O. Farkas, D.K. Malick, A.D. Rabuck, K. Raghavachari, J.B. Foresman, J.V. Ortiz, Q.Cui, A.G. Baboul, S. Clifford, J. Cioslowski, B.B. Stefanov, G. Liu, A. Liashenko, P.Piskorz, I. Komaromi, R.L. Martin, D.J. Fox, T. Keith, M.A. Al-Laham, C.Y. Peng, A.Nanayakkara, M. Challacombe, P.M.W. Gill, B. Johnson, W. Chen, M.W. Wong, C.Gonzalez, J.A. Pople, Gaussian, Inc., Gaussian 03, Revision B.01, Pittsburgh, PA,New York, 2003.

[13] A.D. Becke, J. Chem. Phys. 98 (1993) 5648.[14] C. Lee, W. Yang, G.R. Parr, Phys. Rev. B 37 (1998) 785.[15] B. Miehlich, A. Savin, H. Stoll, H. Preuss, Chem. Phys. Lett. 157 (1989) 200.[16] T. Sundius, J. Mol. Struct. 218 (1990) 321.[17] T. Sundius, Vib. Spectrosc. 29 (2002) 89.[18] Molvib (V.7.0), QCPE Program No. 807, 2002.[19] Y. Marechal, A. Witkowski, J. Chem. Phys. 48 (1968) 3697.[20] National Crystallography Service. <http://www.ncs.chem.soton.ac.uk/>.[21] Mehmet Karabacak, Mehmet Cinar, Sahin Ermec, Mustafa Kurt, J. Raman

Spectrosc. 41 (2010) 98.[22] F.H. Allen, Acta Crystallogr. B58 (2002) 380.[23] C. James, C. Ravikumar, Tom Sundius, V. Krishnakumar, R. Kesavamoorthy, V.S.

Jayakumar, I. Hubert Joe, Vib. Spectrosc. 47 (2008) 10.[24] Mama Nsangou, Nejm-Eddine Jaidane, Zohra Ben Lakhdar, Int. Elect. J. Mol.

Des. 5 (2006) 89.[25] N. Sundaraganesan, S. Ilakiamani, H. Saleem, P.M. Wojciechowski, D.

Michalska, Spectrochim. Acta A 61 (2005) 2995.[26] M. Silverstein, G. Clayton Basseler, C. Morill, Spectrometric Identification of

Organic Compounds, Wiley, New York, 1981.[27] G. Socrates, Infrared Characteristic Group of Frequencies, Wiley, New York,

1980.[28] G. Varasanyi, Assignments of Vibrational Spectra of Seven Hundred Benzene

Derivatives, Wiley, New York, 1974.[29] Y.R. Sharma, Elementary Organic Spectroscopy, Shoban Lal Nagin Chand &

Company, Educational Publishers, New Delhi, 1980.[30] D.N. Sathyanarayana, Vibrational Spectroscopy – Theory and Applications,

second ed., New Age International (P) Limited Publishers, New Delhi, 2004.[31] E.F. Mooney, Spectrochim. Acta 20 (1964) 1021.[32] N.P.G. Roeges, A Guide to the Complete Interpretation of Infrared Spectra of

Organic Structures, Wiley, New York, 1994.