Experimental FT-IR, FT-Raman spectra and quantum chemical studies of optimized molecular structures, conformers and vibrational characteristics of nicotinic acid and thio-nicotinic acid Priyanka Singh a , T.K. Yadav a , M. Karabacak b , R.A. Yadav c , N.P. Singh a,⇑ a Department of Physics, U P (PG) Autonomous College, Varanasi-221002, India b Department of Physics, Afyon Kocatepe University, Afyonkarahisar, Turkey c Department of Physics, Banaras Hindu University, Varanasi-221005, India highlights " Nicotinic acid is a water soluble vitamin has good biological activities and versatile bonding modes. " FT-IR and FT-Raman spectra of nicotinic acid have been recorded in the region 4000–400 and 4000– 50 cm 1 respectively. " The magnitude of zero-point vibrational energy decreases by 3.947 (in Kcal/Mol). " The transition energies from HOMO to LUMO of nicotinic and thio- nicotinic acid is 5.401 and 5.133 eV respectively. graphical abstract The compound nicotinic acid was used as such without further purification for recording the FT-IR and laser Raman spectra. The IR spectra have been recorded in KBr pellets using Perkin–Elmer RX-1 spectrom- eter in the spectral range 50–4000 cm 1 with the following experimental parameters: Perkin–Elmer RX-1: scans – 200; resolution – 2 cm 1 ; gain – 50. The recorded IR for the nicotinic acid molecule is reproduced in Fig. 1. article info Article history: Received 25 August 2011 Received in revised form 1 April 2012 Accepted 14 April 2012 Available online 9 May 2012 Keywords: Ab initio and DFT studies Optimized molecular geometries and APT charges Vibrational characteristics HOMO–LUMO Hyperpolarizability Nicotinic acid and thio-nicotinic acid abstract Fourier transform FT-IR and FT-Raman spectra of the nicotinic acid have been recorded in the region 4000–400 cm 1 and 4000–50 cm 1 , respectively. The computations were carried out by employing the RHF and DFT methods to investigate the optimized molecular geometries, atomic charges, thermody- namic properties and harmonic vibrational frequencies along with intensities in IR and Raman spectra and depolarization ratios of the Raman bands for the nicotinic acid and thio-nicotinic acid molecules. Raman activities calculated by DFT method have been converted to the corresponding Raman intensities using Raman scattering theory. All the 36 normal modes of the both molecules have been assigned and discussed in details in the present study. The complete assignments were performed on the basis of the total energy distribution (TED) of the vibrational modes, calculated with scaled quantum mechanics (SQM) method. Significant changes have been found for different characteristics of a number of vibra- tional modes. The magnitude of zero-point vibrational energy (ZPVE) and thermal energy correction decreases by 3.947 and 3.450 (in Kcal/Mol), respectively, due to the replacement of O 13 atom in the nic- otinic acid by an S atom. The calculation results also show that the electric dipole moment d E value is 1386-1425/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.saa.2012.04.063 ⇑ Corresponding author. Mobile: +91 09889142018. E-mail address: [email protected](N.P. Singh). Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 96 (2012) 163–178 Contents lists available at SciVerse ScienceDirect Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
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Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 96 (2012) 163–178
Contents lists available at SciVerse ScienceDirect
Spectrochimica Acta Part A: Molecular andBiomolecular Spectroscopy
journal homepage: www.elsevier .com/locate /saa
Experimental FT-IR, FT-Raman spectra and quantum chemical studies ofoptimized molecular structures, conformers and vibrational characteristics ofnicotinic acid and thio-nicotinic acid
Priyanka Singh a, T.K. Yadav a, M. Karabacak b, R.A. Yadav c, N.P. Singh a,⇑a Department of Physics, U P (PG) Autonomous College, Varanasi-221002, Indiab Department of Physics, Afyon Kocatepe University, Afyonkarahisar, Turkeyc Department of Physics, Banaras Hindu University, Varanasi-221005, India
h i g h l i g h t s
" Nicotinic acid is a water solublevitamin has good biological activitiesand versatile bonding modes.
" FT-IR and FT-Raman spectra ofnicotinic acid have been recorded inthe region 4000–400 and 4000–50 cm�1 respectively.
" The magnitude of zero-pointvibrational energy decreases by3.947 (in Kcal/Mol).
" The transition energies from HOMOto LUMO of nicotinic and thio-nicotinic acid is 5.401 and 5.133 eVrespectively.
1386-1425/$ - see front matter � 2012 Elsevier B.V. Ahttp://dx.doi.org/10.1016/j.saa.2012.04.063
The compound nicotinic acid was used as such without further purification for recording the FT-IR andlaser Raman spectra. The IR spectra have been recorded in KBr pellets using Perkin–Elmer RX-1 spectrom-eter in the spectral range 50–4000 cm�1 with the following experimental parameters: Perkin–ElmerRX-1: scans – 200; resolution – 2 cm�1; gain – 50. The recorded IR for the nicotinic acid molecule isreproduced in Fig. 1.
a r t i c l e i n f o
Article history:Received 25 August 2011Received in revised form 1 April 2012Accepted 14 April 2012Available online 9 May 2012
Keywords:Ab initio and DFT studiesOptimized molecular geometries and APTchargesVibrational characteristicsHOMO–LUMOHyperpolarizabilityNicotinic acid and thio-nicotinic acid
a b s t r a c t
Fourier transform FT-IR and FT-Raman spectra of the nicotinic acid have been recorded in the region4000–400 cm�1 and 4000–50 cm�1, respectively. The computations were carried out by employing theRHF and DFT methods to investigate the optimized molecular geometries, atomic charges, thermody-namic properties and harmonic vibrational frequencies along with intensities in IR and Raman spectraand depolarization ratios of the Raman bands for the nicotinic acid and thio-nicotinic acid molecules.Raman activities calculated by DFT method have been converted to the corresponding Raman intensitiesusing Raman scattering theory. All the 36 normal modes of the both molecules have been assigned anddiscussed in details in the present study. The complete assignments were performed on the basis of thetotal energy distribution (TED) of the vibrational modes, calculated with scaled quantum mechanics(SQM) method. Significant changes have been found for different characteristics of a number of vibra-tional modes. The magnitude of zero-point vibrational energy (ZPVE) and thermal energy correctiondecreases by 3.947 and 3.450 (in Kcal/Mol), respectively, due to the replacement of O13 atom in the nic-otinic acid by an S atom. The calculation results also show that the electric dipole moment dE value is
164 P. Singh et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 96 (2012) 163–178
found to decreases 2.140 Debye in going from nicotinic acid to thio-nicotinic acid molecules. The transi-tion energies from HOMO to LUMO of the nicotinic acid and thio-nicotinic acid molecules are 5.401 and5.133 eV, respectively. The energy gap (DE) of the nicotinic acid molecule is 5.401 eV. The energy gap ofthe thio-nicotinic acid molecule (5.133 eV) is less than the nicotinic acid (5.401 eV) molecule and hence,the thio-nicotinic acid molecule has less kinetic stability and high chemical reactivity as compared to thenicotinic acid molecule.
� 2012 Elsevier B.V. All rights reserved.
Introduction
Nicotinic acid (pyridine-3-carboxylic acid, also known as niacinor vitamin B3) is a water soluble vitamin has good biological activ-ities and versatile bonding modes. The structures of many of thecomplexes that have been reported show nicotinic acid and itsderivatives acting as bridging ligands through the carboxylategroup and the pyridyl N atom [1]. Nicotinic acid plays a nutritionalrole as a vitamin, deficiency of which results into pellagra. It is in-volved in a wide range of biological processes, including productionof energy, synthesis of fatty acids and steroids, signal transduction,regulation of gene expression and maintenance of genomic integ-rity. It serves as precursor to various forms of coenzyme nicotin-amide adenine dinucleotide and a broad spectrum lipid drugwhich is used to lower cholesterol [2]. Nicotinic acid acts as ananti-hyperlipidemic agent; promotes healthy skin, good digestion,and proper circulation, metabolism of carbohydrates, fats and pro-tein and functioning of the nervous system; serves as origin formost of the commercial compounds, from anti bacterial and cancerdrugs in the biomedical industry to pesticides and herbicides in theagrochemical industry and to charge control agent in photocopiertoners [3]. It is also an important raw material and intermediatewidely used in the synthesis of medicines and dyes [4].
Wright and King [5] determined crystal structure of nicotinicacid by two and three dimensional X-ray methods. Inelastic neu-tron scattering spectrum of nicotinic acid has been measured andanalyzed using DFT method by Hudson et al. [6]. Wang and Bergl-und [7] carried out calorimetric study and thermal analysis of crys-talline for the nicotinic acid molecule. Vibrational investigations ofnicotinic acid based on ab initio molecular orbital calculations havebeen carried out by Sala et al. [8]. They have also studied the RamanSpectra of its aqueous solutions in the range 200–1800 cm�1. Koc-zon et al. [9] studied the experimental and theoretical spectra ofnicotinic acid and obtained its two stable structures which differin orientation of the COOH group with respect to the pyridine ring.IR spectra 3 of this compound were investigated by Taylor [10] andWojcik and Stok [11]. Park et al. [12] investigated the adsorption ofpicolinic acid and nicotinic acid molecule by SERS. Kumar et al. [13]investigated the experimental IR, Raman spectra and density func-tional theory studies of the conformers of nicotinic acid and its N-oxide. Singh et al. [14] investigated the experimental IR, Ramanspectra and density functional theory studies of the nicotinic acidand its derivative. The experimental and theoretical UV, NMR, andvibrational features of nicotinic acid N-oxide were studied by Atacet al. [15]. The molecular structure and vibration frequencies of 2-chloronicotinic acid, 6-chloronicotinic acid and 2- and 6-bromoni-cotinic acid molecules with FT-IR and FT-Raman spectroscopy andquantum chemical calculations were investigated [16–18].
To the best of our knowledge, neither quantum chemical calcula-tions nor the vibrational spectra of thio-nicotinic acid have been re-ported up to now. Therefore, in the present article it is planned tocalculate the optimized geometries, APT charges, thermodynamicproperties and fundamental vibrational wavenumbers along withtheir intensities in the IR, Raman activities and depolarization ratiosof the Raman lines using the restricted Hartree–Fock (RHF) and den-sity functional theory (DFT) methods employing different basis sets
for the nicotinic acid and thio-nicotinic acid molecules. The presentstudy aim to complete vibrational spectroscopic research based onthe conformers of the nicotinic acid molecule to give a correctassignment of the fundamental bands in the experimental FT-IRand FT-Raman spectra. Our present investigations also includedwith the study of electronic and electrical properties as well asHOMO, LUMO energies of the nicotinic acid and thio-nicotinic acidmolecules.
Experimental
The compound nicotinic acid was purchased from Sigma–Al-drich Chemical Company (USA) with a purity P99%. This is a whitesolid at room temperature. It was used as such without furtherpurification for recording the FT-IR and laser Raman spectra. TheIR spectra have been recorded in KBr pellets using Perkin–ElmerRX-1 spectrometer in the spectral range 50–4000 cm�1 with thefollowing experimental parameters: Perkin–Elmer RX-1: scans –200; resolution – 2 cm�1; gain – 50. The recorded FT-IR for the nic-otinic acid molecule is reproduced in Fig. 1.
The laser Raman spectrum of nicotinic acid has been recordedusing 488 nm line of an Ar+ laser for excitation in the region 50–4000 cm�1. The Raman spectrum was recorded on the Jobin YvonHORIBA HR800 Raman spectrometer equipped with air cooledCCD and OLYMPUS microscope with the following parameters: laserspot size-1 lm, resolution ca. 5 cm�1, power at the sample <10 mw,integration time – 10 s, accumulation – 5, time constant – 10 s, onewindow covers ca. 800 cm�1, accuracy of measurements - 1 cm�1,slit width fixed at the entrance of laser – 1 cm�1. The recorded FT-Raman for the nicotinic acid molecule is reproduced in Fig. 2.
Theoretical computations
The theoretical calculations presented in this work have beencarried out to calculate the optimized molecular geometries, APTcharges, thermodynamical properties and fundamental vibrationalwavenumbers along with their corresponding intensities in IRspectrum, Raman activities and depolarization ratios of the Ramanbands for the nicotinic acid and thio-nicotinic acid molecules usingGaussian 03 [19] program package. The computations have beenperformed using density functional theory (DFT) [20] at the B3LYPlevel. The B3LYP functional, consists of Becke’s three-parameter(B3) hybrid exchange functional [21] was combined withLee–Yang–Parr correlation functional (LYP) [22] with the standard6-311++G⁄⁄ basis set, accepted as a cost effective approach, for thecomputation of molecular structure, vibrational fundamental fre-quencies and energies of optimized structures. The basis set usedas a Valence triple-zeta with the addition of diffuse and polariza-tion functions for each atom in the molecules. For the nicotinic acidmolecule, initial parameters were taken from the work of Wright[5] and calculations were performed at the B3LYP/6-311++G⁄⁄ leveltaking charge 0 and multiplicity 1. In the optimized geometry atthe B3LYP/6-311++G⁄⁄ level for the nicotinic acid molecule, theO13 atom was replaced by the S13 atom with the SAH bond lengthas 1.3 Å and with this modification the optimized geometry at theB3LYP/6-311++G⁄⁄ level for the nicotinic acid molecule was taken
Fig. 1. Experimental FT-IR spectrum for nicotinic acid.
P. Singh et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 96 (2012) 163–178 165
as the input structure for the neutral thio-nicotinic acid moleculefor the DFT calculation at the B3LYP/6-311++G⁄⁄ level by takingcharge 0 and multiplicity 1. The geometries were optimized byminimizing the energies with respect to all the geometrical param-eters without imposing any molecular symmetry constraints [23].The total energy distribution (TED) was calculated by using thescaled quantum mechanics (SQM) program [24] in order to charac-terize of fundamental vibrational modes. The vibrational wave-number assignments have been carried out by combining theresults of the Gauss View software [25], symmetry considerationsand the PQS program [26]. The observed IR and Raman frequenciescorresponding to the fundamental modes have been correlated tothe calculated fundamental frequencies. Finally, the calculatednormal mode vibrational frequencies provide thermodynamicproperties also through the principle of statistical mechanics.Graphical representations of the highest occupied molecular orbi-tal (HOMO) and lowest unoccupied molecular orbital (LUMO) datain cube files, were created using the Gaussian 03 [19] and theGauss View visualization program [25].
Results and discussions
Energy analysis and stabilities
The present compounds may have four possible structures. Thenicotinic acid and thio-nicotinic acid molecules can exist as fourconformers: Conformation-1, Conformation-2, Conformation-3
Fig. 2. Experimental Raman spectrum for nicotinic acid.
and Conformation-4. The calculated energies and energy differenceof four structures for the nicotinic acid and thio-nicotinic acid mol-ecules determined by B3LYP method with the 6-311++G⁄⁄ basis setare presented in Tables 1 and 2. According to the energy analysisfor nicotinic acid, the present DFT calculations predicted that the(Conformation-1) (�11891.021 eV) is more stable than (Conforma-tion-3) (�11891.010 eV), the (Conformation-3) (�11891.010 eV) ismore stable than (Conformation-2) (�11890.740 eV) and the (Con-formation-2) (�11890.740 eV) is more stable than (Conformation-4) (�11890.695 eV). Similarly, from DFT calculations, the con-former Conformation-1 is predicted to be more stable than otherconformer for the thio-nicotinic acid molecule (see Table 2). Thecalculated dipole moments of the Conformation-1, Conformation-2, Conformation-3 and Conformation-4 conformers differ signifi-cantly in their magnitudes and hence the Conformation-4 speciesis found to be more polar than the other three (Conformation-1,Conformation-2 and Conformation-3) species. Calculations weredone for four conformers’ structure of these compounds in theground state, and tabulated only for the most stable monomer(Conformation-1) conformers.
Molecular structures
The optimized geometrical parameters of the nicotinic acid andthio-nicotinic acid molecules are collected in Table 3 while theirlabelling scheme is shown in Fig. 3. The corresponding values fromX-ray diffraction experiment [9] are also given in the Table 3 forcomparison of the nicotinic acid molecule. We tabulated only Con-formation-1 calculations data because of most stable conformer forboth the molecules. The calculation predicts that nicotinic acid andthio-nicotinic acid molecules have non-planar structures with Cspoint group symmetry. The nicotinic acid and thio-nicotinic acidmolecules consists of 14 atoms and therefore, they have 36 normalmodes of vibrational modes of the 36 fundamental vibrations is di-vided into 25A0 + 11A00. The vibrations of the A0 species are in-planemodes and those of the A00 species are out-of-plane modes. All fun-damental vibrations are active in both IR absorption and Ramanscattering. But if the molecules were of C1 symmetry point group,there would not be any relevant distribution.
The bond lengths of the identical bond pairs (C2AC3, C6AC5) andC6AN1, C6AH11, C5AH10, C4AH9 of the nicotinic acid are almostsimilar to those found for the corresponding bonds of the thio-nic-otinic acid molecule. The calculated bond lengths in the pyridinering r(C2AH7) and r(C4AC3) increase in going from the nicotinicacid to the thio-nicotinic acid molecules by 0.002 Å whereas thebond lengths ring r(C2AN1) and r(C4AC5) have slightly reduced va-lue by 0.001 Å for the thio-nicotinic acid molecule as compared tothe nicotinic acid molecule. Bond length of the bonds C3AC8 is con-siderable increase by 0.007 Å in the thio-nicotinic acid as com-pared to those of the corresponding bond length of the nicotinicacid molecule. In going from the nicotinic acid to the thio-nicotinicacid molecules, there is increases of the bond lengths of the ringr(C8AO13/S13) and r(O13/S13AH14) by 0.608 Å and 0.378 Å and de-creases of the C8AO12 bond by 0.148 Å.
In moving from the nicotinic acid to the thio-nicotinic acid mol-ecules variations of only ca. 1� is found for the bond anglesC2AC3AC8 and O2AC8AO13/S13 while the magnitude of all the otherangles are nearly same. The bond angles C3AC8AO12 increase by10.0� while the bond angle C3AC8AO13/S13 and C8AO13/S13AH14
decreases by 8.8� and 14.3� in going from the nicotinic acid tothe thio-nicotinic acid molecules.
Atomic charges
The atomic charges of the nicotinic acid and thio-nicotinic acidmolecules are compared in Table 4. The maximum positive charge
Table 1Calculateda total energies (TE) and relative energies (Erel.) for all theoretical structuresof nicotinic acid by DFT (B3LYP).
S.No.
Nicotinic acid Total energy(TE)
Relativeenergy(Erel.)b
(eV)
RMSgradientnorm(RMSGN)
Dipolemoment(DM)
1 �436.98679055(�11891.021 eV)
0 0.00001621 0.7526
2 �436.97647547(�11890.740 eV)
0.281 0.00001198 3.3769
3 �436.98638193(�11891.010 eV)
0.011 0.00013374 3.4266
4
)
�436.97482801(�11890.695 eV)
0.326 0.00003235 5.3535
a TE are measured in Hartree and eV, RMSGN are measured in a.u. and DM aremeasured in Debye.
b Energies of the other three conformers relative to the most stable C1 conformer.
Table 2Calculateda total energies (TE) and relative energies (Erel.) for all theoretical structuresof thio-nicotinic acid by DFT (B3LYP).
S.No.
Thio-nicotinic acid Total energy(TE)
Relativeenergy(Erel.)b
(eV)
RMSgradientnorm(RMSGN)
Dipolemoment(DM)
1 �759.93689390(�20678.944)
0 0.00001850 1.3207
2 �759.93172791(�20678.803)
�0.141 0.00004234 2.4043
3 �759.93642794(�20678.931)
�0.013 0.00000795 3.2200
4 �759.93171799(�20678.803)
�0.141 0.00001906 4.4893
a TE are measured in Hartree and eV, RMSGN are measured in a.u. and DM aremeasured in Debye.
b Energies of the other three conformers relative to the most stable (Conforma-tion-1) C1 conformer.
166 P. Singh et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 96 (2012) 163–178
on the atom C8 is due to attachment of the two electronegative Oatoms to the C8 site for the nicotinic acid molecule. In going fromthe nicotinic acid to thio-nicotinic acid molecules, there is smalldecrease (0.1126) of C8 atomic charge due to attachments of theone side electronegative O atom and other side electronegative Satom to the C8 site. In the pyridine ring, the carbon atoms C2, C4
and C6 are possess to be positive charges while C3 and C5 are pos-sess to be negative charge. However, for the pyridine ring, the C3
and C5 atom(s) the values of the negative charges increases by�0.0356 and �0.0009 in the thio-nicotinic acid molecule as com-pared to the nicotinic acid molecule. The APT charges at the sitesC2 and C6 decreases by 0.0135 and 0.0091 in going from the nico-tinic acid to thio-nicotinic acid molecules.
The APT charges at different atomic sites are plotted in Fig. 4 forthe nicotinic acid and thio-nicotinic acid molecules. All the oxygenatoms are negative charges due to their electron-withdrawingnature. However, the O12 atom the value of the negative charges
Table 3Calculated optimized geometrical parametersa for nicotinic acid and thio-nicotinicacid molecules.
Fig. 4. APT atomic charges at various atoms of the nicotinic acid and thio-nicotinicacid molecules.
P. Singh et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 96 (2012) 163–178 167
increases by �0.0612 in the thio-nicotinic acid molecule as com-pared to the nicotinic acid molecule. The charge at the sites O13
and S13 are found to be �0.8123 and 0.3455 respectively, for thenicotinic acid and thio-nicotinic acid molecules. For the pyridinering, the N1 atom the value of the negative charges decreases by
�0.0033 in the thio-nicotinic acid molecule as compared to thenicotinic acid molecule.
The charges at the H7, H10 and H14 atoms are found to decreaseby 0.0164, 0.0020 and 0.2328 whereas APT charges at the sites H9
and H11 are found to increase by 0.0062 and 0.0017 in going fromthe nicotinic acid to thio-nicotinic acid molecules.
Thermodynamics properties
The calculated several thermodynamic parameters such as theconstant volume molar heat capacity (CV), entropy (S), zero-pointvibrational energy (ZPVE), thermal energy correction (TE), totalenergies (E), and rotational constant of the nicotinic acid andthio-nicotinic acid molecules are presented in Table 5. It is calcu-lated that the total energy of the thio-nicotinic acid molecule is lessthan the nicotinic acid molecule by 322.950 Hartree which sug-gests that the thio-nicotinic acid molecule is thermodynamicallymore stable than the nicotinic acid molecule. The magnitude ofzero-point vibrational energy (ZPVE) and thermal energy correc-tion (TE) slightly decrease by 3.947 and 3.450 (in Kcal/Mol),respectively, due to the replacement of O13 atom in the nicotinicacid molecule by an S atom.
The constant volume molar heat capacity of a molecule definedas the amount of heat required to change the temperature of thatmolecule by 1 �C. The molecules with the larger molecular mass
Table 5Calculated thermo-dynamical propertiesd for the nicotinic acid and thio-nicotinic acidmolecules.
Parameters Nicotinicacid
Thio-nicotinicacid
Total energy (E) �436.987 �759.937Zero point vibrational energy (ZPVE) 64.799 60.852Contribution to the thermal energy
d E are measured in Hartrees, ZPVE and TE are measured in Kcal/Mol, CV and S aremeasured in Cal/Mol-Kelvin and Rotational Constant is measured in GHZ.
168 P. Singh et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 96 (2012) 163–178
will need more heat capacity than the smaller one [27]. Themolecular mass of the thio-nicotinic acid molecule is more thanthe nicotinic acid molecule and thus, the heat capacity of thethio-nicotinic acid molecule is calculated to be greater than thenicotinic acid molecule. According to the similarity principle [28],the higher the similarity among the components is, the higherthe value of entropy will be and the higher the stability will be.Replacement of the S13 atom by an O atom in the thio-nicotinicacid molecule decreases the magnitude of entropy suggesting thatthio-nicotinic acid is more stable as compared to the nicotinic acidmolecule. The magnitude of the rotational constant a, b, c is foundto be increases in going from the nicotinic acid to thio-nicotinicacid molecules.
Vibrational assignments
The experimental FT-IR and FT-Raman spectra of nicotinic acidare shown in Figs. 1 and 2. The calculated wavenumbers along withtheir relative intensities, assignments of nicotinic acid and thio-nic-otinic acid are summarized in Table 6–8. It should be noted that thecalculations were made for a free molecule in vacuum, while exper-iments were performed for solid samples. Furthermore, the anhar-monicity is neglected in the real system for the calculatedvibrations. Therefore, there are disagreements between the calcu-lated and observed vibrational wavenumbers, and because of thelow IR and Raman intensities of some modes, it is difficult to ob-serve them in the IR and Raman spectra. In the last column is givena detailed description of the normal modes with the help of TED andGaussView software. The fundamentals modes for the nicotinic acidand thio-nicotinic acid molecules are compared in Table 8. As thereare no experimental vibrational frequencies are available for thethio-nicotinic acid molecule. All the calculated modes are num-bered from the biggest to the smallest wavenumber within eachfundamental vibration in the first column of the Tables 6–8.
The calculated harmonic force constants and wavenumbers areusually higher than the corresponding experimental quantities be-cause of the combination of electron correlation effects and basisset deficiencies. Nevertheless, after applying a uniform scaling fac-tor, the theoretical calculation reproduces the experimental datawell. The observed slight disagreement between theory and exper-iment could be a consequence of the anharmonicity and the gen-eral tendency of the quantum chemical methods to overestimatethe force constants at the exact equilibrium geometry. Therefore,it is customary to scale down the calculated harmonic wavenum-bers in order to improve the agreement with the experiment. Inour study, we have followed two different scaling factors, i.e.0.983 up to 1700 cm�1 and 0.958 for greater than 1700 cm�1 [29].
As the Gaussian 03 program calculates the Raman activities (Si)at different frequency shifts (mi), the Raman intensities (Ii) are cal-culated by using the relation [30,31]
Ii ¼f ðm0 � miÞ4Si
mi½1� expð�hcmi=kTÞ� ð1Þ
where m0 is the exciting laser light frequency (in cm�1); mi is thevibrational frequency (in cm �1) of the ith normal mode; h, c, Tand k are the Planck constant, the speed of light, temperature andthe Boaltzmann constant, respectively and f is some suitably chosenscaling factor common for all the peak intensities. The Raman spec-trum was recorded using Ar+ laser line (k0 = 488 nm). The corre-sponding excitation wavenumber (20492 cm�1) was used incalculations of the stimulated Raman intensity. The theoreticalinfrared and Raman spectra are shown in Figs. 5–8 of the nicotinicacid and thio-nicotinic acid calculated by using DFT (B3LYP) withthe 6-311++G(d,p) basis set.
Ring modes
The heterocyclic ring of nicotinic acid and thio-nicotinic acid issimilar to that of pyridine ring. Therefore, it is expected that thevibrational frequencies corresponding to the nicotinic acid andthio-nicotinic acid rings should match with those of the pyridinerings. The six ring stretching frequencies of the pyridine moleculeare observed in the range 990–1585 cm�1 by Wilmshurst andBernstein [32]. The ring stretching vibrations are very much impor-tant in the spectrum of pyridine and its derivatives, and highlycharacteristic of the aromatic ring itself. The aromatic ring stretch-ing vibrations occur in the region 1055–1630 cm�1 for both mole-cules. The FT-IR and FT-Raman bands observed at –/1590(vs),1540(vs)/–, 1440(vs)/–, 1407(s)/1390(vw), 1250(w)/1243(m) and–/1030(vs) cm�1 corresponding to the m30, m29, m28, m27, m24 andm19 modes, respectively, for the nicotinic acid molecule.
The planar ring deformation modes for pyridine are observed at605, 652 and 1030 cm�1 by Wilmshurst and Bernstein [32]. Theplanar ring deformation mode m10 wavenumber calculated to be660 cm�1 for the nicotinic acid molecule. The planar ring deforma-tion vibration is assigned to the FT-IR band at 635(m) cm�1 and at635(vw) cm�1 FT-Raman band for the nicotinic acid molecule. Thepresent planar ring deformation modes wavenumbers calculate atca. 1040 and ca. 627 cm�1 (m18 and m9) for both the molecules. Theobserved FT-IR bands corresponding to the planar ring deformationmodes m18 and m9 are found to be at 996(m) and 570(vs) cm�1
while the FT-Raman experimental observations at 1002(w) and570(w) cm�1 modes.
Wilmshurst and Bernstein [32] observed three non-planar ringdeformation modes of the pyridine ring at 374, 405 and 749 cm�1
and the corresponding modes for both the molecules are calcu-lated to be at ca. 715 and ca. 426 cm�1 (m11 and m6). The observedFT-IR bands corresponding to the non-planar ring deformationmodes m11 and m6 are found to be at 680(vs) and 418(vw) cm�1
while the FT-Raman experimental observations at 690 (vw) cm�1
for m11 mode. The non-planar ring deformation mode m4 wave-number calculated to be 385 cm�1 for the nicotinic acid moleculeincreases by 15 cm�1 with decreased IR intensity. The non-planarring deformation vibration is assigned to the FT-Raman band at380(m) cm�1.
COOH modesThe COOH group has a modes as: 3 modes due to the OH group
namely, m(OH), a(COH) and s(OH) and 6 modes due to the COOHmoiety, namely, m(C@O), m(CAOH), rocking q(COOH), scissoringr(COOH), wagging x(COOH) and twisting s(COOH). The bandcorresponding to the OAH stretching vibrations is observed in
Table 6Calculatedp and experimental fundamental frequencies for the nicotinic acid molecule.
Mode no. Species Nicotinic acid Assignment TEDs (P10%)
s: strong, m: medium, w: weak, vs: very strong, vvs: very very strong. m: stretching, d: in-plane bending, c: out-of-plane (o.o.p.) bending, s: torsion.p The first and second numbers within each bracket represent IR intensity (km/mol) and Raman activity (Å4/amu) while the number above and below each bracket
represent the corresponding calculated frequency (cm�1) and depolarization ratios of the Raman band respectively.q From solid state FT-IR spectra in KBr pellet.r Observed frequencies in the FT-Raman spectrum taken in solid from.s TED: total energy distribution.
170 P. Singh et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 96 (2012) 163–178
the region 3300–3700 cm�1 [33]. This mode is found to be pureand highly localized mode for the nicotinic acid molecule. For eth-anoic acid and its derivatives, the OAH stretching frequency wasfound at ca. 3760 cm�1 [34]. The present calculation places thismode at nearly same frequency for the nicotinic acid molecule.For the nicotinic acid molecule Koczon et al. [9] observed a med-ium IR intensity at 3447 cm�1, which could be correlated to theabove frequency. The nicotinic acid molecule OH stretching vibra-tions are observed at 3414(s) cm�1 in the FT-IR spectrum. The the-oretically computed values are in very good agreement withexperimental results. As expected this mode is pure stretchingmode as it is evident from TED column, it is almost contributing100%. The COH stretching and the COH angle bending modesstrongly mix with each other and are expected to lie in the region1150–1450 cm�1 [35]. Generally the COH stretching mode appears
at lower frequency than the COH angle bending mode. The d(COH)and c(OH) modes of nicotinic acid gives bands at 1325(vs) and545(vs) cm�1 in the FT-IR spectrum and at 1320(m) cm�1 in theRaman spectrum. The magnitude of the d(COH) and c(OH) modeswavenumbers calculated at 1370 and 576 cm�1 for the nicotinicacid molecule.
The absorptions are sensitive for both the carbon and oxygenatoms of the carbonyl group. Both have the same amplitudes whileit vibrates. Normally carbonyl group vibrations occur in the region1780–1680 cm�1 [36–38]. The C@O stretching mode (asymmetricstretching vibration) is observed at 1681 cm�1 in the FT-IR spec-trum and observed at 1644 cm�1 (symmetric stretching vibration)in the FT-Raman spectrum by Karabacak and Kurt [17]. In thepresent case the C@O stretching mode (m31) is calculated to havestrong IR and medium Raman intensities with the corresponding
Table 7Calculatedp fundamental frequencies for the thio-nicotinic acid molecule.
Mode no. Species Thio-nicotinic acid Assignment TEDs (P10%)
Calculated
Unscaled wavenumbers Scaled wavenumbers
m36 A0 3203(6,135)0.14
3068 mCH (99)
m35 A0 3188(5,125)0.39
3054 mCH (100)
m34 A0 3158(7,59)0.15
3025 mCAH (100)
m33 A0 3151(18,111)0.47
3019 mCH (100)
m32 A0 2688(2,172)0.28
2575 mSH (100)
m31 A0 1745(296,70)0.31
1672 mC@O (92)
m30 A0 1625(66,99)0.48
1597 mCC (49)mCN (16)dCCH (15)
m29 A0 1603(13,4)0.69
1576 mCC (52)mCN (18)dCCH (12)
m28 A0 1506(3,5)0.35
1480 dCCH (47)mCC (18)mCN (10)dCCN (13)
m27 A0 1420(44,8)0.28
1396 mCC (26)mCN (21)dCCH (20)dNCH (17)
m26 A0 1355(54,0.54)0.7493
1332 dCCH (62)dNCH (30)
m25 A0 1293(2,4)0.69
1271 mCN (50)mCC (39)
m24 A0 1233(162,51)0.18
1212 mC8AC (38)mCC (16)dNCH (10)
m23 A0 1177(4,12)0.55
1157 dCCH (42)dNCH (17)mCC (17)mCN (10)
m22 A0 1117(26,12)0.11
1098 mCN (30)mCC (13)dCCH (34)dNCH (10)
m21 A0 1063(9,29)0.08
1045 mCC (44)dCCH (26)mCN (16)
m20 A0 1038(13,22)0.06
1020 mCC (31)dCCN (23)dCCH (19)dCCC (15)
m19 A0 1011(0.29,0.05)0.75
994 cCH (87)
m18 A0 987(0.50,0.02)0.75
970 cCH (87)
m17 A00 974(66,6)0.72
957 dCSH (41)dOCS (15)mCS (10)
m16 A00 943(0.16,0.37)0.75
927 cCH (80)
m15 A00 850(139,0.39)0.10
836 dCSH (48)mCC (17)mCS (10)
(continued on next page)
P. Singh et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 96 (2012) 163–178 171
Table 7 (continued)
Mode no. Species Thio-nicotinic acid Assignment TEDs (P10%)
Calculated
Unscaled wavenumbers Scaled wavenumbers
dOCS (10)m14 A00 827
(22,0.61)0.75
813 cCH (73)
m13 A0 720(49,15)0.15
708 dCCC (24)dCCN (21)mCS (12)mCC (10)dCCO (14)
m12 A00 711(30,0.13)0.75
699 sCCCN (54)sCCCH (27)sCCCC (11)
m11 A00 660(7,0.56)0.75
649 sCCCO (34)sOCSH (24)sCCCS (16)
m10 A0 626(4,4)0.50
615 dCNC (24)dCCC (27)dCCH (15)dCCN (12)
m9 A0 491(4,10)0.11
483 dOCS (33)mCS (22)mCC (11)dCCC (20)
m8 A00 423(4,0.19)0.75
416 sCCCN (20)sCCCC (22)sCCCH (24)sCCSH (15)
m7 A0 421(4,4)0.56
414 mCS (34)mCC (13)dCCO (28)dOCS (14)
m6 A00 396(0.90,0.26)0.75
389 sCCCN (32)sOCSH (22)sCCCC (18)sCCCH (15)
m5 A0 317(22,2)0.75
312 cSH (80)
m4 A00 315(15,4)0.16
310 dCCC (28)dOCS (18)dCCS (14)mCS (15)mCC (17)
m3 A0 171(3,0.49)0.74
168 dCCC (48)dCCS (31)dCCO (14)
m2 A00 149(0.14,2)0.75
146 sCCCC (33)sCCCN (29)sCCCO (12)sCCCH (10)
m1 A00 12(2,0.35)0.75
12 sCCCS (63)sCCCO (36)
p The first and second numbers within each bracket represent IR intensity (km/mol) and Raman activity (Å4/amu) while the number above and below each bracketrepresent the corresponding calculated frequency (cm�1) and depolarization ratios of the Raman band respectively.
s m: stretching, d: in-plane bending, c: out-of-plane (o.o.p.) bending, s: torsion; TED: total energy distribution.
172 P. Singh et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 96 (2012) 163–178
frequency at 1789 cm�1 for nicotinic acid decreases in magnitudeby 44 cm�1 (thio-nicotinic acid) decrease IR intensity and Ramanactivity. The FT-IR (asymmetric stretching vibration)/FT-Raman(symmetric stretching vibration) bands observed at 1711(vs)/1695(s) cm�1 corresponding to the m31 mode for the nicotinic acidmolecule. After scaled this mode is calculated at 1714 and1672 cm�1 for nicotinic acid and thio-nicotinic acid, respectively.According to TED, the C@O stretching mode is pure mode and itscontribution is 83 and 92% nicotinic acid and thio-nicotinic acid,respectively. The m(CAOH) mode is calculate at 1114 cm�1 in thepresent case for the nicotinic acid molecule drastically decreasesin magnitude by 76 cm�1 (thio-nicotinic acid) with decrease IR
intensity and increase in Raman activity whereas the CAOHstretching good agreement with Sala et al. [8] be highly coupledwith CAOH angle bending mode. The observed IR bands corre-sponding to the m(CAOH) mode is found to be at 1035(s) cm�1 whilethe FT-Raman experimental observations at 1044(s) cm�1 m20 mode.
Out of nine modes of the COOH group three modes, x(COOH),q(COOH) and s(COOH) are found to drastically decrease in magni-tude by 97, 179 and 57 cm�1 in going from the nicotinic acid tothio-nicotinic acid molecules. The x(COOH) and q(COOH) of nico-tinic acid gives bands at 720(s) cm�1 in the FT-IR spectrum(725(w) cm�1 in the FT-Raman spectrum) and 471(vw) cm�1 inthe FT-IR spectrum. Yadav et al. [24] found that for COOH group,
Table 8Calculatedp fundamental frequencies for the nicotinic acid and thio-nicotinic acid molecules.
Mode no. Species Nicotinic acid Thio-nicotinic acid Modet
Calculated unscaled wavenumbers
m36 A0 3769(105,138)0.26
3203(6,135)0.14
m(OAH)/m(SAH)
m35 A0 3202(5,135)0.13
3188(5,125)0.39
m(CAH)
m34 A0 3188(3,144)0.35
3158(7,59)0.15
m(CAH)
m33 A0 3185(6,21)0.34
3151(18,111)0.47
m(CAH)
m32 A0 3154(13,114)0.41
2688(2,172)0.28
m(CAH)
m31 A0 1789(398,81)0.27
1745(296,70)0.31
m(C@O)
m30 A0 1631(61,62)0.54
1625(66,99)0.48
m(ring)
m29 A0 1609(11,6)0.51
1603(13,4)0.69
m(ring)
m28 A0 1508(4,3)0.31
1506(3,5)0.35
m(ring)
m27 A0 1450(34,3)0.35
1420(44,8)0.28
m(ring)
m26 A0 1370(108,9)0.23
1355(54,0.54)0.7493
a(CAOAH)/a(CASAH)
m25 A0 1359(21,2)0.40
1293(2,4)0.69
b(CAH)
m24 A0 1292(0.58,4)0.7479
1233(162,51)0.18
m(ring) Kekule
m23 A0 1226(38,7)0.67
1177(4,12)0.55
b(CAH)
m22 A0 1204(142,21)0.21
1117(26,12)0.11
b(CAH)
m21 A0 1133(5,4)0.15
1063(9,29)0.08
b(CAH)
m20 A0 1114(194,0.94)0.45
1038(13,22)0.06
m(CAOH)
m19 A0 1056(5,37)0.05
1011(0.29,0.05)0.75
m(ring)
m18 A0 1040(13,9)0.07
987(0.50,0.02)0.75
a(ring)
m17 A00 1010(0.08,0.03)0.7465
974(66,6)0.72
c(CAH)
m16 A00 991(0.72,0.06)0.7491
943(0.16,0.37)0.75
c(CAH)
m15 A00 957(0.22,0.27)0.75
850(139,0.39)0.10
c(CAH)
m14 A00 843(3,0.50)0.75
827(22,0.61)0.75
c(CAH)
m13 A0 790(10,16)0.11
720(49,15)0.15
m(CACOOH)
m12 A00 757 711 x(COOH)
(continued on next page)
P. Singh et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 96 (2012) 163–178 173
Table 8 (continued)
Mode no. Species Nicotinic acid Thio-nicotinic acid Modet
Calculated unscaled wavenumbers
(77,0.01)0.7499
(30,0.13)0.75
m11 A00 715(15,0.17)0.75
660(7,0.56)0.75
/(ring)
m10 A0 660(40,1)0.15
626(4,4)0.50
a(ring)
m9 A0 627(10,5)0.7465
491(4,10)0.11
a(ring)
m8 A00 576(97,2)0.75
423(4,0.19)0.75
s(OAH)/s(SAH)
m7 A0 494(10,1)0.60
421(4,4)0.56
q(COOH)
m6 A00 426(0.01,0.47)0.75
396(0.90,0.26)0.75
/(ring)
m5 A 386(6,4)0.25
317(22,2)0.75
r(COOH)
m4 A00 385(6,0.14)0.68
315(15,4)0.16
/(ring)
m3 A0 211(3,0.13)0.68
171(3,0.49)0.74
b(CACOOH)
m2 A00 156(0.04,2)0.75
149(0.14,2)0.75
c(CACOOH)
m1 A00 63(3,0.33)0.75
12(2,0.35)0.75
s(COOH)
p The first and second numbers within each bracket represent IR intensity (km/mol) and Raman activity (Å4/amu) while the number above and below each bracketrepresent the corresponding calculated frequency (cm�1) and depolarization ratios of the Raman band respectively.
t m = stretching, x = wagging, s = twisting, q = rocking, r = scissoring, s = deformation, c = out-of-plane deformation, b = in-plane deformation, a = angle bending, a = in-plane ring bending, / = out-of-plane ring bending.
174 P. Singh et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 96 (2012) 163–178
the scissoring frequency is at higher magnitude than its rockingfrequency. But in the present study, it is found that for the nicotinicacid molecule, rocking frequency of the COOH group is at higher
Fig. 5. Theoretical IR spectrum of nicotinic acid.
magnitude than the scissoring frequency. For the nicotinic acidmolecule, assignment of the COOH scissoring mode is in agreementwith Sala et al. [8]. The frequency of the r(COOH) modes shifts
Fig. 6. Theoretical Raman spectrum of nicotinic acid.
Fig. 7. Theoretical IR spectrum of thio-nicotinic acid.
Fig. 8. Theoretical Raman spectrum of thio-nicotinic acid.
P. Singh et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 96 (2012) 163–178 175
towards the higher wavenumber side by ca. 35 cm�1 for thethio-nicotinic acid molecule with very weak IR intensity as wellas Raman activity.
CAH modesThe hetro-aromatic structure shows the presence of CAH
stretching vibration in the region 3100–3000 cm�1 which is thecharacteristic region for the ready identification of CAH stretchingvibration [39]. In this region the bands are not affected appreciablyby the nature of substituent. The CAH stretching frequencies areobserved in the range 3030–3095 cm�1 for the pyridine [40]. TheCAH stretching vibrations are found to be pure and highly local-ized for both the molecules. Accordingly, in the present study,the four adjacent hydrogen atoms left around the ring of nicotinicacid and thio-nicotinic acid molecules gives rise to four CAHstretching (m35/m36, m34, m33 and m32), four CAH in-plane bending(m28, m25/m26, –/m23, and m21) and four CAH out-of-plane bendingdeformation (m17/m19, m15/m18, m17 and m14) vibrations which corre-spond to stretching modes of C2AH7, C4AH9, C5AH10 and C6AH11
units. The aromatic CAH stretching of nicotinic acid gives bands
at 3099(m), 3075(m), 3040(m) and 3013(m) cm�1 in the FT-IRspectrum and at 3089(vs), 3073(vs), 3041(vw) cm�1 in the FT-Ra-man spectrum. The magnitude of the CAH stretching wavenum-bers calculated at 3068, 3054, 3051, and 3022 cm�1 for thenicotinic acid, is found 3068, 3054, 3025, and 3019 cm�1 for thethio-nicotinic acid molecule. They are very pure modes since theirTED contribution are 98–100%. The CAH stretching wavenumber3051 cm�1 for the nicotinic acid is found to have considerably low-er magnitude by ca. 30 cm�1 for the thio-nicotinic acid molecule.
The in-plane CAH bending vibration occurs in the region 1300–1000 cm�1, the bands are sharp but are weak to medium intensity.It is found that the CAH in-plane bending deformation mode (m23)frequency calculated at 1226 cm�1 for the nicotinic acid moleculedecrease in magnitude by 49 cm�1 and drastically 354 cm�1 forthe thio-nicotinic acid molecule with decrease IR intensity andthe Raman band becomes more weakly polarized. The CAH in-plane bending deformation mode m21 is observed in FT-IR spectrumas strong band at 1090(vs) cm�1 (1117(vw) cm�1 in FT-Ramanspectrum) and m23 mode, the band occur at 1185(m) cm�1 in FT-Ra-man spectrum for the nicotinic acid molecule. The CAH in-planebending deformation modes (m25) wavenumber calculated at1359 cm�1 for the nicotinic acid molecule decrease in magnitudeby ca. 5 cm�1 with increase IR intensity. For the nicotinic acid mol-ecule, this band occur at 1303(m) cm�1 in FT-Raman spectrum.
The aromatic compounds the out-of-plane CAH bending vibra-tion occurs in the region 1000–750 cm�1, the bands are sharp butare weak to medium intensity. In the present case, the theoreticallycalculated CAH out-of-plane deformation frequencies fall in therange 820–1015 cm�1 for both the molecules by B3LYP/6-311++G(d,p) method shows excellent agreement with FT-IR bandat 972(vw), 941(vs) and 800(s) cm�1 (m17, m16 and m14) and the ob-served FT-Raman band are at 976(w), 952(w), 918(vw) and811(vs) cm�1 (m17, m16, m15 and m14). According to TED contributionof the in-plane and out-of-plane modes shows that out-of planemodes are also highly pure modes like the in-plane bending funda-mentals. The in-plane and out-of-plane deformation vibrationalfrequencies are found to be well within their characteristic regions.This show that the substitution COOH/COSH do not affect much ofaromatic CAH modes of vibration as they attached to only are car-bon of the benzene ring a-position.
CACOOH, CACOSH and SH modesThe stretching wavenumber is calculated to be 777 cm�1 for the
nicotinic acid molecule which drastically increases in magnitudeby 443 cm�1 due to substitution of the S atom at the O13 site withincreases IR intensity and Raman activity. This above mode is ob-tained in the Raman spectrum at 750(vw) cm�1.
The present in-plane and out-of-plane CACOOH bending defor-mation modes wavenumbers calculated to be 207 cm�1 (A0 spe-cies) and 135 cm�1 (A00 species) for the nicotinic acid decrease inmagnitude by ca. 40 cm�1 and ca. 10 cm�1 for the thio-nicotinicacid molecule. The CACOOH in-plane and out-of-plane bendingdeformation modes of nicotinic acid gives bands at 206(s) cm�1
and 145(w) cm�1 in the FT-Raman spectrum.The magnitude of the wavenumber m(SH), d(CSH), d(OCS), and
c(SAH) calculated at 2575, 957, 836, 483 and 312 cm�1 for thethio-nicotinic acid molecule. The m(SH) mode is very pure sincetheir TED contribution is 100%.
Electrical properties
The first hyperpolarizability (b) of this molecular system, andrelated properties (btotal, dE, a0 and Da) of the nicotinic acid andthio-nicotinic acid molecules are calculated using B3LYP/6-311++G(d,p) basis set, based on the finite-field approach. In the
176 P. Singh et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 96 (2012) 163–178
presence of an applied electric field, the energy of a system is afunction of the electric field. First hyperpolarizability is a third ranktensor that can be described by a 3 � 3 � 3 matrix. The 27 compo-nents of the 3D matrix can be reduced to 10 components due to theKleinman symmetry [41]. It can be given in the lower tetrahedralformat. It is obvious that the lower part of the 3 � 3 � 3 matricesis a tetrahedral. The components of b are defined as the coefficientsin the Taylor series expansion of the energy in the external electricfield. When the external electric field is weak and homogeneous,the expansion is given by
E ¼ E0 � daFa �12aabFaFb �
16
babcFaFbFc þ � � � ð2Þ
where E0 is the energy of the unperturbed molecules, Fa the field atthe origin, da, aab and babc are the components of dipole moment,polarizability and the first hyperpolarizabilities, respectively. Thetotal static dipole moment dE, the mean polarizability a0, the anisot-ropy of the polarizability Da and the mean first hyperpolarizabilityb using the x, y, z components they are defined as
dE¼ d2xþd2
yþd2z
� �1=2ð3Þ
ao¼13ðaxxþayyþazzÞ ð4Þ
Da¼ ðaxx�ayyÞ2þðayy�azzÞ2þðazz�axxÞ2
2
" #1=2
ð5Þ
b¼ ðbxxxþbxyyþbxzzÞ2þðbyyyþbxxyþbyzzÞ
2þðbxxzþbyyzþbzzzÞ2
h i1=2ð6Þ
btotal¼ b2xxxþb2
yyyþb2zzz
� �1=2ð7Þ
The various electrical properties or non linear optical proper-ties, i.e. total dipole moment (dE), mean polarizability (a0), polariz-ability anisotropy invariant (Da) and first hyperpolarizability(btotal) computed at B3LYP/6-311++G⁄⁄ level are collected inTable 9. The dipole moment is most widely used an importantquantity to describe the polarity of a molecule [42]. It is deter-mined by the charges and induced dipole constituent atoms. Onaccount of more electronegative character of oxygen atom, the
Table 9Calculated electrical propertiese of the nicotinic acid and thio-nicotinic acidmolecules.
e a, Da, b and btotal are measured in a.u. and d is measured in Debye. For a andDa, 1 a.u. = 1.6488 � 10�41 C2 m2 J�1 and for b, 1 a.u. = 3.2064 � 10� 53 C3 m3 J�2.
magnitude of the dipole moment of thio-nicotinic acid is less thannicotinic acid molecule suggesting that nicotinic acid to be morepolar in nature as compared to thio-nicotinic acid. Similar to thedipole moment, the magnitude of the mean polarizability (a0)and polarizability anisotropy invariant (Da) decreases due to thesubstitution of O atom at the S13 site of the thio-nicotinic acid mol-ecule, which could be again due to the electronegative character ofO atom. The magnitude of the first hyperpolarizability is calculatedto be 13.869 a.u. for the nicotinic acid molecule which reduces by7.325 a.u. for the thio-nicotinic acid molecule. The total hyperpo-larizability for the thio-nicotinic acid molecule increases its magni-tude by 4.301 a.u. as compared to the nicotinic acid molecule.
HOMO and LUMO analysis
The output files of the GAUSSIAN 03 provide the energies of themolecular orbitals. The HOMO, LUMO and HOMO–LUMO energygap of the nicotinic acid and thio-nicotinic acid molecules com-puted at the B3LYP/6-311++G⁄⁄ level are collected in Table 10.The energy of the HOMO indicates the ability of the molecule todonate electrons while the energy of the LUMO indicates the abil-ity of the molecule to accept electrons. High value of EHOMO is likelyto indicate a tendency of the molecule to donate electrons toappropriate acceptor molecule of low empty molecular orbital en-ergy. Therefore, the energy of the lowest unoccupied molecularorbital, ELUMO, indicates the ability of the molecule to accept elec-trons. Energies of the HOMO and LUMO are essential for governingmany chemical reactions and determining electronic band gaps insolids [43–45]. The energy of the HOMO and LUMO are calculatedto be �7.551 and �2.150 eV for the nicotinic acid molecule. In caseof the thio-nicotinic acid molecule, the energies of the HOMO andLUMO decrease as compared to the nicotinic acid molecule buttheir energies increase for the nicotinic acid molecule. The energygap, DE is a quantity that measures the kinetic stability of the mol-ecule [46,47]. A large value of the energy gap implies high kineticstability and low chemical reactivity because it is energeticallyunfavorable to add electrons to a high lying LUMO, to remove elec-trons from a low lying HOMO and so to form the activated complexof any potential reaction [48]. The present calculations predict thatthe energy gap (DE) of the nicotinic acid molecule is 5.401 eV. Theenergy gap of the thio-nicotinic acid molecule (5.133 eV) is lessthan the nicotinic acid molecule and hence, the thio-nicotinic acidmolecule has less kinetic stability and high chemical reactivity ascompared to the nicotinic acid molecule.
The transition energies from HOMO to LUMO of the nicotinicacid and thio-nicotinic acid molecules are 5.401 and 5.133 eV,respectively. This electronic transition corresponds to the transi-tion from the ground to the first excited state and is mainly de-scribed by an electron excitation from HOMO to LUMO. Theatomic orbital compositions of the frontier molecular orbital aresketched in Figs. 9 and 10. The green and red solid regions in Figs. 9and 10 represent the molecular orbitals with completely oppositephases. It can be seen that the electron clouds of the HOMO andLUMO are concentrated on the pyridine ring.
Table 10HOMO–LUMO energy value calculated by B3LYP/6-311++G(d,p) of the nicotinic acidand thio-nicotinic acid molecules.
Fig. 9. The atomic orbital compositions of the frontier molecular orbital of nicotinicacid.
Fig. 10. The atomic orbital compositions of the frontier molecular orbital of thio-nicotinic acid.
P. Singh et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 96 (2012) 163–178 177
Conclusions
In going from the nicotinic acid to the thio-nicotinic acid mole-cules, there is increases of the bond lengths of the ring r(C8AO13/S13) and r(O13/S13AH14) by 0.608 Å and 0.378 Å and decreases ofthe C8AO12 bond by 0.148 Å. The bond angles C3AC8AO12 increaseby 10.0� while the bond angle C3AC8AO13/S13 and C8AO13/S13AH14
decreases by 8.8� and 14.3� in going from the nicotinic acid to thethio-nicotinic acid molecules. The maximum positive charge on theatom C8 is due to attachment of the two electronegative O atoms tothe C8 site for the nicotinic acid molecule. In going from the nico-tinic acid to thio-nicotinic acid molecules, there is small decrease(0.1126) of C8 atomic charge due to attachments of the one sideelectronegative O atom and other side electronegative S atom tothe C8 site. Significant changes have been found for different char-acteristics of a number of vibrational modes. The magnitude ofzero-point vibrational energy (ZPVE) and thermal energy correc-tion decreases by 3.947 and 3.450 (in Kcal/Mol), respectively, dueto the replacement of O13 atom in the nicotinic acid by an S atom.The calculation results also show that the electric dipole momentdE value is found to decreases 2.140 Debye in going from nicotinicacid to thio-nicotinic acid molecules. The transition energies fromHOMO to LUMO of the nicotinic acid and thio-nicotinic acid mole-cules are 5.401 and 5.133 eV, respectively. The energy gap (DE) ofthe nicotinic acid molecule is 5.401 eV. The energy gap of the thio-nicotinic acid molecule (5.133 eV) is less than the nicotinic acid
(5.401 eV) molecule and hence, the thio-nicotinic acid moleculehas less kinetic stability and high chemical reactivity as comparedto the nicotinic acid molecule.
Acknowledgements
The authors are thankful to Department of Physics, B.H.U., U.P.(India) for giving permission to use the FTIR spectrometer for get-ting recorded the FTIR spectra. Authors also gratefully thank for theRaman measurement were carried out at the UGC-DAE consortiumfor scientific Research Center, Indore (M.P.), India with the help ofDr. Vasant G. Sathe, Incharge Raman spectrometer.
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