Journal of Molecular Structure 1016 (2012) 97–108

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Journal of Molecular Structure

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Synthesis, molecular structure and spectral analysis of ethyl4-formyl-3,5-dimethyl-1H-pyrrole-2-carboxylate thiosemicarbazone:A combined DFT and AIM approach

R.N. Singh ⇑, Amit Kumar, R.K. Tiwari, Poonam Rawat, Divya Verma, Vikas BabooDepartment of Chemistry, University of Lucknow, University Road, Lucknow 226 007, India

a r t i c l e i n f o

Article history:Received 31 October 2011Received in revised form 12 February 2012Accepted 13 February 2012Available online 20 February 2012

Keywords:Density functional theoryVibrational spectrumHydrogen bonded dimerAIMEllipticityReactivity descriptor

0022-2860/$ - see front matter � 2012 Elsevier B.V. Adoi:10.1016/j.molstruc.2012.02.033

⇑ Corresponding author. Tel.: +91 9451308205.E-mail address: rnsvk.chemistry@gmail.com (R.N.

a b s t r a c t

A new ethyl 4-formyl-3,5-dimethyl-1H-pyrrole-2-carboxylate thiosemicarbazone (EFDMPCT) has beensynthesized and characterized by elemental analysis and FT-IR, 1H NMR, UV–Visible, DART-mass spec-troscopy. Quantum chemical calculations have been performed by DFT level of theory using B3LYP func-tional and 6-31G (d,p) as basis set. The calculated 1H NMR chemical shifts using gauge including atomicorbitals (GIAO) approach are in good agreement with the observed chemical shifts. The electronictransitions within molecule have been interpreted using the time dependent density functional theory(TD-DFT). The calculated and experimental wavenumbers analyses confirm the existence of dimer. Topo-logical parameters electron density, Laplacian of electron density, kinetic electron energy density, poten-tial electron density and the total electron energy density at the bond critical points (BCP) analyzed using‘Atoms in Molecules’ AIM theory reveals intra and inter molecular hydrogen bonding other weaker inter-actions in detail. The calculated intermolecular hydrogen bond energy of dimer is �12.2176 kcal/molusing AIM calculation. The results of AIM ellipticity confirm the existence of resonance assisted hydrogenbonds in dimer. The calculated thermodynamic parameters show that reaction is exothermic and non-spontaneous at room temperature. The local reactivity descriptors find the reactive sites within mole-cules have been calculated.

� 2012 Elsevier B.V. All rights reserved.

1. Introduction

Thiosemicarbazones are versatile starting materials for the syn-thesis of a variety of N, S containing heterocycles thiazoles, thiazol-ones, 1,2,4-triazoles and imidazolones [1]. They having anazomethine proton ANHAN@CHA constitute an important classof compound for new drug development. Thiosemicarbazones areof the considerable pharmacological interest since their derivativeshave shown a broad spectrum of chemotherapeutic properties.Thiosemicarbazones and their metal complexes have extensivelystudied owing to their pharmaceutical importance and present awide variety of biological activity such as antimalarial [2–5], anti-cancer [6], antibacterial [7], antiproliferative activity [8] and anti-viral agent [9]. Thiosemicarbazones are versatile ligands, bearingsuitable donor atoms for coordination to metals. They usually actas bidentate ligands through their azomethine nitrogen and thio-carbonyl sulfur atoms. They have strong coordinating ability to-wards different metal ions Co(II), Ni(II), Cu(II) [10], Ru(II) [11,12],Au(II) [13], Zn(II), Cd(II) [14,15], Pd(II) [16]. Their metal complexes

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Singh).

have used for device applications in telecommunications, opticalcomputing, optical storage and optical information processing[17]. They exhibit photochromic properties [18] and used as sensi-tive and selective analytical reagent [19]. In addition, they alsoshow corrosion inhibition behavior towards metal surface [20].

The electron density distribution governs all atomic and molec-ular properties. The AIM method gives the opportunity to have aninsight into a region of a system [21]. The AIM finds a physicallywell-defined surface that divides a system to subsystems fromestablished electron density. Critical point (CP) is a point wherer! qðBCPÞ ¼~0. At bond critical point (BCP) the electron density isa minimum with respect to the direction of a line connectingtwo nuclei and a maximum with respect to all directions perpen-dicular to this line. Bond critical point is used in the recognitionof chemical bonds between atoms and interatomic interaction.The Laplacian of the electron density measures locally concen-trated r2q < 0 and depleted r2q > 0 within a molecular system.The theory of AIM efficiently describes H-bonding and its conceptwithout border. With respect to change of both the method usedand the basis set the reliability and stability of values of AIMparameters have been studied and found that they were almostindependent of basis set in case of used functional B3LYP in DFT

98 R.N. Singh et al. / Journal of Molecular Structure 1016 (2012) 97–108

[22]. One of the advantages of the AIM theory is that one can obtaininformation on changes in the electron density distribution as re-sult of either bond formation or complexes formation.

Thiosemicarbazones of 2-formyl pyrrole and 2-acetyl pyrrolehave also reported in literature [16,23]. The interest in this com-pound arouse from literature survey on ethyl 4-formyl-3,5-dime-tyl-1H-pyrrole-2-carboxylate which thiosemicarbazidederivatives are not reported. In particular, the interest in this com-pound provides opportunity for synthesis of new heterocyclic com-pounds and metal complexes that may have considerablepharmacological activities and material applications and fact thatthe pyrrole fragment is a constituent of many biological systems.Therefore, the title compound was synthesized and characterized.In this paper we report the synthesis, detailed molecular structure,spectroscopic analysis and chemical reactivity of the EFDMPCTusing experimental and quantum chemical calculations.

2. Experimental section

All the chemicals used were of analytical grade. Thiosemicarba-zide was purchased from Himedia. The solvent methanol was driedand distilled before use according to the standard procedure [24].Ethyl 4-formyl-3,5-dimetyl-1H-pyrrole-2-carboxylate was pre-pared by an earlier reported method [25]. The FTIR-spectrum ofthe title compound was recorded in KBr on a Bruker-spectrometer.The 1H NMR spectrum was recorded in DMSO-d6 on Bruker DRX-300 spectrometer using TMS as an internal reference. The UV–Vis-ible absorption spectrum (1 � 10�5 M in DMSO) was recorded onELICO SL-164 spectrophotometer. The DART-mass spectrum ofcompound was recorded on JEOL-Acc TDF JMS-T100L C massspectrometer.

2.1. Synthesis of ethyl 4-formyl-3,5-dimethyl-1H-pyrrole-2-carboxylate thiosemicarbazone

A ice cold mixture of ethyl 4-formyl-3,5-dimethyl-1H-pyrrole-2-carboxylate (0.200 g, 1.0245 mmol), thiosemicarbazide(0.0933 g, 1.0245 mmol) and two drop of HCl as catalyst in 30 mlmethanol was stirred for 40 h. A creamy color precipitate was ob-tained. The precipitate was filtered off, washed with methanol anddried in air. The compound Yielded 40.80%. The compound decom-poses above 230 �C without melting. DART-mass analysis: ob-served m/z 269.22 [M++1] amu, calc. 268.32 amu. The elementalanalysis observation found: C 49.58%, H 6.04%, N 20.38%, S11.72% whereas calculated for C11H16N4O2S are: C 49.12%, H6.01%, N 20.88%, S 11.94%,

3. Computational methods

All the calculations of the title compound are carried out withGaussian 09 program package [26] to predict the molecular struc-ture, 1H NMR chemical shifts, vibrational wave numbers and ener-gies of the optimized structures using density functional theory(DFT), B3LYP functional and 6-31G(d,p) basis set, which invokesBecke’s three parameter (local, non local and Hartree–Fock) hybridexchange functional (B3) [27], with Lee–Yang–Parr correlationalfunctional (LYP) [28]. The basis set 6-31G(d,p) with ‘d’ polarizationfunctions on heavy atoms and ‘p’ polarization functions on hydro-gen atoms are used for better description of polar bonds of mole-cule [29,30]. It should be emphasized that ‘p’ polarizationfunctions on hydrogen atoms are used for reproducing the out ofplane vibrations involving hydrogen atoms. Convergence criterionin which both the maximum force and displacement are smallerthan the cut-off values of 0.000450 and 0.001800 and r.m.s. forceand displacement are less than the cut-off values of 0.000300

and 0.001200 are used in the calculations. The time dependentdensity functional (TD-DFT) is carried out to find the electronictransitions. The optimized geometrical parameters are used inthe vibrational wavenumbers calculations to characterize all sta-tionary points as minima and their harmonic vibrational wave-numbers are positive. The vibrational wavenumbers in theharmonic approximation are calculated using B3LYP/6-31G(d,p).The calculated vibrational wavenumbers at B3LYP/6-31G(d,p) levelare scaled down using wavenumber-linear scaling method (WLS)performed by Yoshida et al. [31]. All the molecular structures arevisualized using software Gauss-view [32]. Potential energy distri-bution along internal coordinates is calculated by Gar2ped soft-ware [33]. Internal coordinate system recommended by Pulayet al. is used for the assignment of vibrational modes [34]. All theaim calculations are performed with the help of AIM2000 programpackage [35].

4. Results and discussion

4.1. Thermodynamic properties and molecular geometry

The optimized geometries of all the reactants and products in-volved in chemical reaction are shown graphically in Fig. 1, calcu-lated at B3LYP/6-31G(d,p) level. For sake of simplicity reactantsethyl 4-formyl-3,5-dimetyl-1H-pyrrole-2-carboxylate and thio-semicarbazide and product EFDMPCT and water are abbreviatedas A, B, C and D respectively. The vibrational frequency calculationsfor all reactants and products were performed to determine thethermodynamic quantities at room temperature and their valuesare listed in Table 1. For overall reaction the enthalpy changeof reaction (DHReaction), Gibbs free energy change of reaction(DGReaction) and entropy change of reaction (DSReaction) are foundto be �5.5221, �4.5808 kcal/mol and �3.0270 cal/mol K respec-tively. The reaction has negative values for DHReaction, DSReactionand DGReaction indicating that the reaction is exothermic andspontaneous at room temperature. The equilibrium constant (Keq)for formation of 3 is calculated at temperature T, using equationKeq = e�DG/RT. Using this equation, the value of Keq is calculated as2352.99, i.e. Keq � 1 at room temperature. Therefore, Keq favorthe formation of product (3) at room temperature.

The calculated ground state syn, anti and tautomer conformersof the product ethyl 4-formyl-3,5-dimethyl-1H-pyrrole-2-carbox-ylate thiosemicarbazone are shown in Fig. 2. They have energy�1196.49624826, �1196.49556740 and �1196.46200368 a.u.respectively at room temperature with energy difference0.427246 and 21.48 kcal/mol indicating to exist in ratio67.3:32.6:10�16 respectively in the gas phase as per Boltzmann dis-tribution. The syn conformer has geometrical suitability for inter-molecular hydrogen bonding by having pyrrolic NH and ester COon same side and their role lie in molecular association resultinginto dimer. Total energy of the dimer is calculated as�2393.01587643 a.u. at B3LYP/6-31G (d,p) level. The binding en-ergy of dimer is computed as the difference between the calculatedtotal energy of the dimer and the energies of the two isolatedmonomers and found to be �14.67112 kcal/mol. The calculatedhydrogen binding energy of dimer formation has been correctedfor the basis set superposition error (BSSE) via the standard coun-terpoise method [36] and found to be �10.47425 kcal/mol. The cal-culated changes in thermodynamic quantities during the dimerformation in gaseous phase have the negative values of DH kcal/mol), DG (kcal/mol) and DS (cal/mol K) indicating that the dimerformation is exothermic and spontaneous. The calculated equilib-rium constant between monomer and dimer is quite high(K = 11.79) indicating that dimer formation is highly preferredand as a result even anti conformer gets converted to syn and finally

Fig. 1. The optimized geometries of the reactants and products involved in chemical reaction calculated at B3LYP/6-31G(d,p) level.

Table 1Calculated enthalpy, Gibbs free energy and entropy of reactants (A, B) and products (C, D) (in a.u.).

Thermodynamic parameters A B C D

Enthalpy, H (a.u.) �669.126528 �603.461449 �1196.202160 �76.394588Gibbs free energy, G (a.u.) �669.185894 �603.497850 �1196.275054 �76.416024Entropy, S (cal/mol K) 124.946 76.613 153.416 45.116

Syn-(thioamino)

Conformer 1

Anti-(thioamino)

Conformer 2

Tautomer (thioimino) Confromer 3

Fig. 2. The optimized geometries of ground state syn, anti and tautomer conformers using B3LYP/6-31G(d,p).

Table 2The thermodynamics quantities (enthalpy, Gibbs free energy, entropy), their change and equilibrium constant of conversion from monomer to dimer.

Parameters Enthalpy, H (a.u.) Gibbs free energy, G (a.u.) Entropy, S (cal/mol K) DH (kcal/mol) DG (kcal/mol) DS (cal/mol K) Keq

2� Monomer �2392.40432 �2392.550108 306.832Dimer �2392.425124 �2392.552441 267.962 �130.522 �1.462 �38.87 11.79

R.N. Singh et al. / Journal of Molecular Structure 1016 (2012) 97–108 99

forms the dimer. The Thermodynamics quantities (Enthalpy, Gibbsfree energy, Entropy), their change and equilibrium constant ofconversion from monomer to dimer are given in Table 2. The se-lected geometrical parameters of dimer including inter molecularhydrogen bonding atoms are given in Table 3 and the optimized

geometry of dimer is shown in Fig. 3, with atomic numbering.The asymmetry of the N1AC2 and N1AC5 bonds can be explainedby electron withdrawing character of the ethoxycarbonyl groupand conjugation of the N1AC5 with C4AC3AC2 skeleton. It leadsto the elongation of N1AC2 bond. These effects are not only in

Table 3Selected geometrical parameters for dimer including inter molecular hydrogenbonding atoms of the title compound using B3LYP/6-31G (d,p).

Bond length (Å) Bond angle (�) Dihedral angle (�)

R(1,2) 1.3879 A(2,1,5) 110.6686 D(5,1,2,3) 0.0001R(1,5) 1.3476 A(2,1,19) 124.9767 D(5,1,2,9) �179.9947R(1,19) 1.0204 A(5,1,19) 124.3546 D(19,1,2,3) 179.9993R(2,3) 1.3975 A(1,2,3) 107.7288 D(19,1,2,9) 0.0045R(2,9) 1.4496 A(1,2,9) 117.9384 D(2,1,5,4) �0.0006R(3,4) 1.427 A(3,2,9) 134.3328 D(2,1,5,6) �179.9999R(3,8) 1.5008 A(2,3,4) 106.4158 D(19,1,5,4) �179.9997R(4,5) 1.4096 A(2,3,8) 128.3868 D(19,1,5,6) 0.001R(4,7) 1.4442 A(4,3,8) 125.1974 D(1,2,3,4) 0.0004R(5,6) 1.4921 A(3,4,5) 107.6759 D(1,2,3,8) �179.9984R(6,20) 1.0952 A(3,4,7) 124.6969 D(9,2,3,4) 179.9939R(6,21) 1.0952 A(5,4,7) 127.6272 D(9,2,3,8) �0.0049R(6,22) 1.0919 A(1,5,4) 107.5108 D(1,2,9,14) �0.0015R(7,10) 1.2922 A(1,5,6) 121.8483 D(1,2,9,15) 179.9961R(7,23) 1.0968 A(4,5,6) 130.6408 D(3,2,9,14) �179.9945R(8,24) 1.0965 A(5,6,20) 110.6552 D(3,2,9,15) 0.0031R(8,25) 1.0965 A(5,6,21) 110.6561 D(2,3,4,5) �0.0007R(8,26) 1.088 A(5,6,22) 110.3271 D(2,3,4,7) �179.9993R(9,14) 1.2324 A(20,6,21) 106.7652 D(8,3,4,5) 179.9981R(9,15) 1.3481 A(20,6,22) 109.1773 D(8,3,4,7) �0.0004R(10,11) 1.364 A(21,6,22) 109.1769 D(2,3,8,24) �120.5476R(11,12) 1.3681 A(4,7,10) 123.5852 D(2,3,8,25) 120.4654R(11,27) 1.0164 A(4,7,23) 116.4102 D(2,3,8,26) �0.0402R(12,13) 1.6812 A(10,7,23) 120.0046 D(4,3,8,24) 59.4538R(12,18) 1.3472 A(3,8,24) 111.057 D(4,3,8,25) �59.5332R(15,16) 1.4462 A(3,8,25) 111.058 D(4,3,8,26) 179.9612R(16,17) 1.5163 A(3,8,26) 111.377 D(3,4,5,1) 0.0008R(16,28) 1.0945 A(24,8,25) 107.0345 D(3,4,5,6) 180R(16,29) 1.0945 A(24,8,26) 108.0709 D(7,4,5,1) 179.9993R(17,30) 1.0942 A(25,8,26) 108.0689 D(7,4,5,6) �0.0015R(17,31) 1.0935 A(2,9,14) 123.9415 D(3,4,7,10) �179.9928R(17,32) 1.0935 A(2,9,15) 114.0353 D(3,4,7,23) 0.0085R(18,33) 1.0094 A(14,9,15) 122.0232 D(5,4,7,10) 0.009R(18,34) 1.0054 A(7,10,11) 116.666 D(5,4,7,23) �179.9897R(19,60) 1.90775 A(10,11,12) 121.9027 D(1,5,6,20) �120.9495R(14,56) 1.90779 A(10,11,27) 121.8067 D(1,5,6,21) 120.9149R(14,59) 2.55657 A(12,11,27) 116.2906 D(1,5,6,22) �0.0173R(22,60) 2.55655 A(11,12,13) 120.3667 D(4,5,6,20) 59.0514R(1,60) 2.88515 A(11,12,18) 115.0724 D(4,5,6,21) �59.0842R(50,14) 2.88519 A(13,12,18) 124.5609 D(4,5,6,22) 179.9836R(6,60) 3.47034 A(9,15,16) 116.6034 D(4,7,10,11) �179.9967R(51,14) 3.47036 A(15,16,17) 107.4785 D(23,7,10,11) 0.002

A(15,16,28) 108.8306 D(2,9,15,16) �179.9966A(15,16,29) 108.8303 D(14,9,15,16) 0.0009A(17,16,28) 111.9126 D(7,10,11,12) �179.9967A(17,16,29) 111.9112 D(7,10,11,27) 0.0084A(28,16,29) 107.805 D(10,11,12,13) �179.9906A(16,17,30) 109.666 D(10,11,12,18) 0.0153A(16,17,31) 111.0123 D(27,11,12,13) 0.0046A(16,17,32) 111.0118 D(27,11,12,18) �179.9895A(30,17,31) 108.2718 D(11,12,18,33) �0.0553A(30,17,32) 108.272 D(11,12,18,34) �179.9483A(31,17,32) 108.521 D(13,12,18,33) 179.9509A(12,18,33) 119.9345 D(13,12,18,34) 0.0579A(12,18,34) 118.1736 D(9,15,16,17) 179.9809A(33,18,34) 121.8919 D(9,15,16,28) 58.5972A(1,19,60) 159.35039 D(9,15,16,29) �58.6373A(50,56,14) 159.35073 D(15,16,17,30) �179.9983A(6,22,60) 140.61276 D(15,16,17,31) 60.402A(51,59,14) 140.61375 D(15,16,17,32) �60.3987

D(28,16,17,30) �60.5649D(28,16,17,31) 179.8354D(28,16,17,32) 59.0347D(29,16,17,30) 60.5696D(29,16,17,31) �59.0301D(29,16,17,32) �179.8309

Fig. 3. The optimized geometry of dimer for lower energy ground state syn-conformer using B3LYP/6-31G(d,p).

100 R.N. Singh et al. / Journal of Molecular Structure 1016 (2012) 97–108

quantum calculation but also reflect in crystal structure of theethyl-3,5-dimethyl-1H-pyrrole-2-carboxylate [37], methyl 4-p-to-lyl-1H-pyrrole-2-carboxylate [38] and other 1H-pyrrole-2-carbox-ylate derivatives [39]. The molecule exist in E-configuration withthe thiosemicarbazide and pyrrole located on the opposite side of

the (C7@N10) bond [15,40]. In dimer, heteronuclear intermolecu-lar hydrogen bonding (NAH� � �O) between pyrrolic (NAH) and car-bonyl (C@O) oxygen of ester form double hydrogen bonded dimer.In cyclic ester dimer both molecule exist in s-cis form, where(NAH) bond as proton donor and (C@O) bond as proton acceptor.According to the Etter terminology [41] the cyclic ester dimer formthe ten membered ring denoted as R2

2(10) or more extended 16membered ring R2

2(16). The superscript designates the number ofacceptor centers and the subscript, the number of donors in themotif. In dimer, due to intermolecular hydrogen-bond formationboth proton donor (NAH bond) and proton acceptor (C@O bond)are elongated by 0.00967 Å and 0.00900 Å respectively.

4.2. Chemical reactivity

4.2.1. Global reactivity descriptorsThe energies of frontier molecular orbitals (eHOMO, eLUMO), band

gap (eLUMO–eHOMO), electronegativity (v), chemical potential (l),chemical hardness (g), global softness (S), global electrophilicityindex (x) [42–46] and additional electronic charge (DNmax) ofreactant A, B and product C (EFDMPCT) have been calculated usingfollowing Eqs. (1)-(5) and listed in Table 4.

v ¼ �1=2ðeLUMO þ eHOMOÞ ð1Þ

l ¼ �v ¼ 1=2ðeLUMO þ eHOMOÞ ð2Þ

g ¼ 1=2ðeLUMO � eHOMOÞ ð3Þ

S ¼ 1=2g ð4Þ

x ¼ l2=2g ð5Þ

DNmax ¼ �l=g ð6ÞAccording to Parr et al., electrophilicity index (x) is as a global

reactivity index similar to the chemical hardness and chemical po-tential. This reactivity index measures the stabilization in energy

Table 4Calculated eHOMO, eLUMO, energy band gap (eL–eH), electronegativity (v), chemical potential (l), global hardness (g), global softness (S), global electrophilicity index (x) andadditional electronic charge (DNmax) (in eV) for reactant A, B and product C (EFDMPCT), using B3LYP/6-31G(d,p).

eH eL eH–eL v l g S x DNmax

Reactant (A) �6.1609 �1.0163 �5.1446 3.5886 �3.5886 2.5723 0.194379 2.5032 1.3950Reactant (B) �5.5928 �0.0329 �5.5598 2.8128 �2.8128 2.7799 0.179863 1.423 1.0118Product (C) �5.3802 �1.2209 �4.1592 3.3006 �3.3006 2.0796 0.240431 2.6192 1.2601

R.N. Singh et al. / Journal of Molecular Structure 1016 (2012) 97–108 101

when the system acquires an additional electronic charge (DNmax)from the environment. The electrophilicity index (x) is positive,definite quantity and the direction of the charge transfer is com-pletely determined by the electronic chemical potential (l) of themolecule because an electrophile is a chemical species capable ofaccepting electrons from the environment and its energy must de-crease upon accepting electronic charge. Therefore, its electronicchemical potential must be negative.

Electrophilicity, i.e. electrophilicity-based charge transfer (ECT)[47] is defined as the difference between the DNmax value of inter-acting molecules, ECT = (DNmax)A � (DNmax)B. When two moleculesA and B approach to each other (i) if ECT > 0, charge flow from B toA and (ii) if ECT < 0, charge flow from A to B.

ECT ¼ ðDNmaxÞA � ðDNmaxÞB ð7ÞIn approach of reactants A and B the calculated ECT is 0.3832

indicating charge flows from B to A. Therefore, A acts as electro-phile and B as nucleophile. In the same way, the high value ofchemical potential and low value of electrophilicity index for B2also favor its nucleophilic behavior, whereas, the low value ofchemical potential and high value of electrophilicity index for A fa-vor its electrophilic behavior. The high value of electrophilicity in-dex x = 2.6192 eV for EFDMPCT shows that it is a strongelectrophile than reactants 1 and 2.

4.2.2. Local reactivity descriptorsUsing Hirshfeld population analysis of neutral, cation and anion

state of molecule, Fukui Functions are calculated using followingequations.

fþk ¼ ½qðN þ 1Þ � qðNÞ� for nucleophilic attack ð8Þ

f�k ¼ ½qðNÞ � qðN � 1Þ� for electrophilic attack ð9Þ

f 0k ¼ 1=2½qðN þ 1Þ � qðN � 1Þ� for radical attack ð10Þwhere N, N � 1, N + 1 are total electrons present in neutral, cationand anion state of molecule respectively.

In addition, local softness ðsþk ; s�k ; s0kÞ and local electrophilicityindices ðxþ

k ;x�k ;x

0kÞ are also used to describe the reactivity of

atoms in molecule. These local reactivity descriptors associatedwith a site k in a molecule are defined with the help of the corre-sponding ‘condensed to atom’ variants of FF, using the followingequations.

sþk ¼ Sfþk ; s�k ¼ Sf�k ; s

0k ¼ Sf 0k ð11Þ

xþk ¼ xfþk ;x�

k ¼ xf�k ;x0k ¼ xf 0k ð12Þ

Table 5Fukui functions, local softness, local electrophilicity indices ðfþk ; Sþk ;x

þk Þ for selected atomi

global nucleophile (reactant B), using Hirshfeld population analysis at B3LYP/6-31G(d,p) l

Reactant (A) fþk sþk xþk

C7 0.1479 0.0287 0.3702C10 0.0921 0.0170 0.2305

where +, �, 0 signs show nucleophilic, electrophilic and radical at-tack respectively. Eqs. (11) and (12) predicts that the most electro-philic (or nucleophilic) site in a molecule is the one providing themaximum value of sþk ðs�k Þ;xþ

k ðx�k Þ respectively.

Fukui functions ðfþk ; f�k ; f 0k Þ, local softness ðsþk ; S�k ; S0kÞ and localelectrophilicity indices ðxþ

k ;x�k ;x

0kÞ [45,46] for selected atomic

sites of reactants and molecule have been listed in Table 5 and Ta-ble 6.

The maximum values of the local reactivity descriptorsðfþk ; sþk ;x

þk Þ at C7 in comparison to C10 indicate that C7 site is more

prone to nucleophilic attack. In the same way, the maximum val-ues of the local reactivity descriptors ðf�k ; s�k ;x

�k Þ at N1 in compar-

ison to N2 indicate that N1 site is more prone to electrophilicattack. Therefore, local reactivity descriptors confirm the formationof product EFDMPCT by nucleophilic attack of N(1) site of reactantA on the C(7) site of reactant (B).

In product EFDMPCT, the relative high value of local reactivitydescriptors ðf�k ; s�k ;x

�k Þ in Table 6 at S13, N11 and N18 indicate that

these sites are more prone to electrophilic whereas the relative va-lue of local reactivity descriptors ðfþk ; sþk ;x

þk Þ at C7 indicate that this

site is more prone to nucleophilic attack. Thus, synthesized mole-cule EFDMPCT may be used as intermediate for the synthesis ofnew heterocyclic compounds such as thiadiazoline, thiazolidinon-es and azetidinones.

4.3. 1H NMR spectroscopy

1H NMR chemical shifts are calculated with GIAO approachusing B3LYP method and 6-31G(d,p) basis set [48]. Chemical shiftof any proton (X-HAN) is calculated as difference between isotropicmagnetic shielding IMS of TMS and proton (X-HAN). It is defined byan equation written as: CS(X-HAN) = IMSTMS � IMS(X-HAN). Theexperimental and calculated values of 1H NMR chemical shifts ofthe title compound are given in Table 7. In order to compare thechemical shifts correlation graphics between the experimentaland calculated chemical shifts are shown in Fig. 4. The correlationgraph follow the linear equation (y= 1.24335x + 0.49622); where yis the experimental 1H NMR chemical shift, x is the calculated 1HNMR chemical shift (d in ppm).The value of correlation coefficient(R2 = 0.97509) shows that there is a good agreement betweenexperimental and calculated results.

4.4. UV–Visible spectroscopy

The nature of the transitions observed in the UV–Visible spec-trum of EFDMPCT has studied by the time dependent density func-tional (TD-DFT) theory. The observed and calculated electronictransitions of high oscillatory strength are given in Table 8. The

c sites of global electrophile (reactant A) and ðf�k ; S�k ;x�k Þ for selected atomic sites of

evel.

Reactant (B) f�k s�k x�k

N1 0.1350 0.0285 0.2253N2 0.0900 0.0216 0.1708

Table 6Atomic charges (in esu), Fukui functions ðfþk ; f�k Þ, Local softness ðsþk ; S�k Þ and local electrophilicity indices ðxþ

k ;x�k Þ in eV for atomic sites of product EFDMPCT, using Hirshfeld

population analysis at B3LYP/6-31G(d,p) level.

Atom no. Hirshfeld atomic charges Fukui functions Local softnesses Local electrophilicity indices

qN qN+1 qN�1 fþk f�k sþk S�k xþk x�

k

1 N 0.09994 0.06125 0.14928 0.0387 0.0493 0.00930 0.01186 0.10132 0.129242 C �0.00224 �0.03277 0.04770 0.0305 0.0499 0.00734 0.01200 0.07996 0.130793 C �0.01934 �0.07647 �0.00110 0.0571 0.0182 0.01373 0.00438 0.14963 0.047774 C �0.06466 �0.08165 �0.02228 0.0170 0.0424 0.00408 0.01019 0.04450 0.111015 C 0.06049 0.03353 0.11413 0.0270 0.0536 0.00648 0.01289 0.07061 0.140506 C 0.04758 �0.00222 0.10868 0.0498 0.0611 0.01197 0.01468 0.13041 0.160027 C 0.07154 �0.06504 0.13914 0.1366 0.0676 0.03283 0.01625 0.35771 0.177068 C 0.03269 �0.04234 0.07534 0.0750 0.0427 0.01803 0.01025 0.19650 0.111719 C 0.19797 0.14870 0.21194 0.0493 0.0140 0.01184 0.00336 0.12905 0.0366010 N �0.10422 �0.18939 �0.06926 0.0852 0.0350 0.02047 0.00840 0.22307 0.0915511 N 0.08567 0.05653 0.17087 0.0291 0.0852 0.00700 0.02048 0.07632 0.2231412 C 0.12547 0.08060 0.14947 0.0449 0.0240 0.01079 0.00576 0.11754 0.0628313 S �0.36500 �0.51443 �0.07185 0.1494 0.2931 0.03592 0.07048 0.39139 0.7678114 O �0.30441 �0.36974 �0.26442 0.0653 0.0400 0.01570 0.00961 0.17111 0.1047415 O �0.13439 �0.15283 �0.12264 0.0184 0.0117 0.00443 0.00282 0.04829 0.0307616 C 0.12927 0.10053 0.15055 0.0287 0.0213 0.00690 0.00511 0.07527 0.0557317 C 0.03397 0.00462 0.05595 0.0294 0.0220 0.00705 0.00528 0.07687 0.0575518 N 0.10965 0.04109 0.17848 0.0686 0.0688 0.01648 0.01655 0.17957 0.18029

0 2 4 6 8 100

2

4

6

8

10

12

Expe

rimen

tal c

hem

ical

shi

ft (in

ppm

)

Theoretical chemical shift (in ppm)

R2 = 0.97509

Fig. 4. Correlation between experimental and calculated 1H NMR chemical shiftsusing B3LYP/6-31G(d,p).

Table 7Calculated and experimental 1H NMR chemical shifts using B3LYP/6-31G(d,p).

Atom No. Calculated chemical shifts Experimental

Chemical shifts Assignment

H 19 9.1445 11.691 Pyrrolic NH (s)H 20 2.4605H 21 2.4604 2.366 a-CH3 (s)H 22 2.2008H 23 7.7715 8.15 ACH@NA (s)H 24 2.0451H 25 2.0455 2.32 b-CH3 (s)H 26 2.9776H 27 8.5438 10.068 @NANHA (s)H 28 4.2923H 29 4.2874 4.188–4.26 Ester CH2 (q)H 30 1.1878H 31 1.4528 1.26–1.305 Ester CH3 (s)H 32 1.4519H 33 6.6736 8.053 Amino-NH (s)H 34 5.9004 7.091 Amino-NH (s)

Abbreviations: s – singlets, q – quartet.

102 R.N. Singh et al. / Journal of Molecular Structure 1016 (2012) 97–108

experimental UV spectrum is given in Fig. 5 of the title compounds.Frontier molecular orbitals (FMOs), HOMO and LUMO plots alongwith and other molecular orbitals plot, active in the electronictransition are present in Fig. 6. The highest occupied molecularorbital HOMO and lowest unoccupied molecular orbital (LUMO)are very popular quantum chemical parameters. These molecularorbitals are also called the frontier molecular orbitals (FMOs),determine the way to molecule interacts with other species. TheFMOs are important in determine molecular reactivity and theability of a molecule to absorb light. The HOMO is the orbital thatcould act as an electron donor, since it is the outermost (higher en-ergy) orbital containing electrons. The LUMO is the orbital thatcould act as the electron accepter, since it is the innermost (lowestenergy) orbital that has room to accept electrons. The vicinal orbi-tals of HOMO and LUMO play the same role of electron donor andelectron acceptor respectively. The energies of HOMO and LUMOand their neighboring orbitals are all negative, which indicate thetitle molecule is stable [49]. The HOMO–LUMO energy gap is animportant stability index. The HOMO–LUMO energy gap of titlemolecule reflects the chemical stability of the molecule.

HOMO energy ¼ �5:3802 eV

LUMO energy ¼ �1:2209 eV

HOMO� LUMO energy gap ¼ 4:1592 eV

TD-DFT/B3LYP/6-31G(d,p) predict two intense electronic transi-tion at kmax = 316.10 nm, f = 1.0245 and kmax = 252.95 nm,f = 0.1123 are in agreement with the experimental electronic tran-sitions at kmax 302 and 232 nm respectively. The calculated kmax

values are also in agreement with the reported kmax values in liter-ature [50,51]. The calculated one intense electronic transition atkmax = 270.70 nm, f = 0.1343 does not appear in the experimentalUV–Visible spectrum. The TD-DFT calculations show that the lon-gest wavelength experimental band at 302 nm and another exper-imental band at 232 nm originate due to the H? L and H-3? Ltransitions respectively. On the basis of calculated result, both ob-served electronic excitations show p? p* transitions. The ob-served wavelength in experimental UV spectrum is lower thancalculated wavelength by 14 and 18 nm. This difference is ob-served due to hypsochromic shift (blue shift). These transitionsare of higher in energy. The hypsochromic shift is occurring dueto hydrogen bonding of the solvent effect.

Table 8Electronic transitions (calculated and experimental) for title compound.

Transitions(S.No.)

Electronic transitions (molecularorbitals involved)

Energy(in eV)

Calculated kmax

(in nm)Oscillatorystrength (f)

Percentage contribution ofprobable transition

Assignments Observed kmax

(in nm)

1 71? 72 3.9223 316.10 1.0245 48 p? p* 3022 69? 72, 71? 73 4.5802 270.70 0.1343 29, 203 68? 72 4.9016 252.95 0.1123 464 69? 73 5.1359 241.41 0.3946 46 n? p* 232

R.N. Singh et al. / Journal of Molecular Structure 1016 (2012) 97–108 103

4.5. Vibrational assignments

The title compound show both blue and red shift. The title com-pound have four C-H� � �O and two N-H� � �O hydrogen bonds. Theblue shift is observed in four C-H� � �O hydrogen bonds and red shiftin N-H� � �O stretching vibrations. The experimental and calculatedvibrational wavenumbers for dimer using B3LYP/6-31G(d,p) andtheir assignments with their PED are given in Table 9. The compar-ison among calculated monomer, dimer and experimental FT-IRspectra of EFDMPCT in the region 4000–400 cm�1 is shown graph-ically in Fig. 7. The total number of atoms in monomer and dimerare 34, 68 respectively. Therefore it gives 96 and 198, (3n � 6)vibrational modes for monomer and dimer respectively. The calcu-lated vibrational wavenumbers are higher than the experimentalvalues for the majority of the normal modes. The vibrational fre-quency usually is overestimated in DFT due to two factors: (1)the overall neglect of anharmonicity, (2) an incomplete descriptionof electron correlation due to the use of an incomplete basis set.Therefore, calculated wavenumbers are scaled using scaling factorto discard the anharmonicity present in real system. The vibra-tional wavenumbers calculated at B3LYP/6-31G (d,p) level arescaled using wavenumber-linear scaling method (WLS) performedby Yoshida [29]. The relationship obtained as, scaling factor = mobs/mcal = 1.0087–0.0000163 � mcalc. The scaled vibrational wavenum-bers with their potential energy distribution (PED) help in assign-ment of vibrational modes obtained from experimental FT-IRspectrum. The calculated vibrational wavenumbers of monomerare given in parenthesis.

4.5.1. NAH vibrationsThe computed two NHAO bonds for the complex are nicely cor-

relates with the observed red shift of the NAH stretch. In the FT-IRspectrum of EFDMPCT, the NAH stretch of pyrrole (mNAH) is ob-served at 3225 cm�1, whereas it is calculated as 3308 (3457)cm�1. Therefore, observed mNAH at 3225 cm�1 is in agreement with

200 300 400 500-0.1

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

abso

rban

ce

Experimental

Theoretical

Wavelength (nm)

Fig. 5. Experimental and calculated UV–Visible spectrum of the title compoundusing TD-DFT/B3LYP/6-31G(d,p).

the calculated wavenumber at 3308 cm�1 in dimer. The observedmNAH is also in good agreement with the earlier reported strongabsorption at 3358 cm�1 for pyrrole-2-carboxylic acid recordedin KBr pellet, but it deviates from the reported free (mNH) band ob-served at higher wavenumber 3465 cm�1, recorded in CCl4 solution[52]. The observed value of mNAH for EFDMPCT also correlates withthe reported strong absorption at 3358 cm�1 for methyl pyrrole-2-carboxylate recorded in KBr pellet [53]. Therefore, solid state spec-trum of EFDMPCT attributed to the vibration of hydrogen bondedNAH group. The observed wagging mode at 772 cm�1 also con-firms the involvement of pyrrolic NAH group in intermolecularattraction. The N11AH27 stretch of thiosemicarbazide part of mol-ecule is observed at 3158 cm�1, whereas it is calculated as 3364

Fig. 6. Molecular orbitals of the title compound.

Table 9IR vibrational wavenumbers (calculated and experimental in cm�1) for dimer and their assignments.

ModeNo.

Calculated Experimental Vibrational assignment (PEDP 5%)

Unscaled Scaled Intensity(Kmmol�1)

197 3760 3562 237.29 3429 [m(N38H42)](36)-[m(N18H34)](35)-[m(N38H41)](14) [m(N18H33)](14)196 3597 3418 84.28 3274 [m(N38H41)](41)-[m(N18H33)](29) [m(N38H42)](17)-[m(N18H34)](12)194 3537 3364 18.66 3158 [m(N37H40)](98)193 3537 3364 12.32 [m(N11H27)](98)192 3475 3308 2711 3225 [m(N50H56)](50)190 3186 3048 2.29 Me2[m(C8H26)](92)189 3186 3048 3.7 [m(C49H55)](92)187 3149 3015 3.68 [m(C51H59)](40)-[s-(C44C43)](15)[q0-Me1](10)-Me1[m(C6H22)](8)-[m(C51H58)](5)-[m(C51H57)](5)

[s-(H20C63)](5)186 3140 3007 59.19 2975 [s-(C17C16)](19)-[m(C17H31)](10) [m(C17H32)](10)-[m(C63H67)](10) [m(C63H68)](10) [q0-Me](10)

[s-(H58C17)](10)-[s-(C63C62)](5)184 3133 3000 45.06 [s-(C63C62)](32) [s-(H20C63)](30) [m(C63H66)](11)-[m(C17H30)](9)182 3105 2975 19.74 [s-(C63C62)](47)-[q0-Me](11) [m(C16H28)](6)-[m(C16H29)](6)-[s-(O61C52)](5)-[s-(O15C9)](5)

[m(C62H64)](5)-[m(C62H65)](5)180 3101 2971 14.4 [(x-(C47C51)](12)-[(x-(C5C6)](11)m(C51H58)](11)-Me1[m(C6H20)](10) Me1[m(C6H21)](10)-

[m(C51H57)](10)-[s-(C5C6)](9)-[s-(C44C43)](8)[s-(H58C17)](8)178 3085 2957 29.89 [m(C49H54)](33)-[m(C49H53)](32)-Me2[m(C8H24)](17) Me2[m(C8H25)](17)176 3062 2936 41.57 2925 [m(C16H29)](20)m(C16H28)](20)-[m(C62H65)](19)-[m(C62H64)](19)-[dsc-(C62C63O61)](9)173 3061 2935 128.78 [m(C43H45)](49)-[m(C7H23)](48)171 3057 2932 30.42 [m(C17H30)](15)-[m(C63H66)](14) [m(C17H31)](14) [m(C17H32)](14)-[m(C63H68)](13)-[m(C63H67)]

(13)-[s-(H20C63)](11)[ds-Me](7)169 3046 2921 31.6 [s-(H20C63)](26)[s-(H20C62)](16)-[m(C51H58)](13)-[m(C51H57)](11) Me1[m(C6H20)](11) e1[m

(C6H21)](11)-[m(C51H59)](5) Me1[m(C6H22)](5)167 3039 2915 36.44 Me2[m(C8H25)](24) Me2[m(C8H24)](24)-[m(C49H53)](22)-[m(C49H54)](22)166 1722 1689 2033.61 1656 [s-(H20C62)](16) [m(C9O14)](11)-[m(C52O60)](11) [q-(C48C52)](11)-[x-CH2](11)-[d-(C62C52O61)]

(10)-R[d-(C48C52)](5)163 1678 1647 96.45 1595 [m(N39C43)](24)-Me1[m(C7N10)](24)-[q-(C43H45)](6) [q-(C7H23)](6)-[m(C43C44)](5) [m(C4C7)](5)161 1620 1591 451.4 1543 [dSC-NH2](18)-[dSC-NH2](18)-[s-(H20C62)](12) [q0-Me1](6) [m(C12N18)](6)-[m(C36N38)](6)-

[s-(H20C63)](6)159 1608 1579 300.97 [s-(H20C62)](21)-[q0-Me1](12)-[x-CH2](8)[s-(H20C63)](6)-[m(C44C47)](3)158 1567 1541 387.28 [s-(H20C63)](26) [s-(H20C62)](21) [d0as-Me1](20) [s-(H58C17)](8)-[q0-Me1](7)155 1552 1526 1100 1512 [s-(H20C62)](17)-[x-CH2](14)-[s-(C63C62)](10) [dsc-CH2](8)-[s-(H20C63)](7) [q-(N11H27](5)

[q-(N37H40](5)153 1533 1508 98.2 [s-(H20C63)](38) [s-(C63C62)](30)-[dsc-CH2](12)-[s-(H20C62)](5)152 1527 1502 98.3 [d0as-Me1](25) [s-(H20C63)](23) [s-(H20C62)](14) [s-(H58C17)](12) [s-(C5C6)](6)-[s-(C44C43)](6)148 1512 1488 6.17 [s-(C17C16)](33) [s-(H58C17)](32)-[s-(H20C63)](18) [d0as-Me2](5)-[q0-Me](5)146 1500 1476 14.14 [s-(H20C63)](42)-[s-(C17C16)](19)-[s-(H58C17)](18) [s-(C63C62)](16) [q0-Me](5)144 1498 1474 107.73 [s-(H20C63)](33) [d0as-Me1](25) [s-(H58C17)](9)-[s-(C44C43)](9) [s-(C5C6)](8) [s-(H20C62)](8)141 1494 1471 169.85 [s-(H20C63)](31) [d0as-Me1](27) [s-(C5C6)](10)-[s-(C44C43)](10) [s-(H58C17)](8) [s-(H20C62)](7)140 1492 1469 9.67 [das-Me1](35)-[d0as-Me1](35) [s-(C5C6)](11) [s-(C44C43)](11)137 1473 1450 410.09 1440 [s-(H20C63)](34) [d0as-Me1](19) [s-(H58C17)](17) [s-(C5C6)](7)-[s-(C44C43)](6)[s-(H20C62)](6)135 1461 1439 77.49 [s-(H20C63)](30) [d0as-Me1](25) [s-(C5C6)](11)-[s-(C44C43)](10) [s-(H58C17)](9)[s-(H20C62)](6)133 1438 1417 8.47 [s-(H20C63)](57) [s-(H20C62)](15)-[dS-Me](12)-[x-CH2](7)132 1428 1408 23.35 [s-(H20C63)](58) [s-(H20C62)](15) [s-(H58C17)](12) [ds-Me1](6)130 1426 1405 140.7 [dS-Me](33) [s-(H20C62)](25) [ds-Me1](16) [s-(H58C17)](12)-[ds-Me1](10)128 1417 1397 257.11 [s-(H20C63)](50)[d0as-Me1](10)[s-(H58C17)](9)-[dS-Me](8)[s-(C5C6)](7)-[s-(C44C43)](6)125 1396 1376 89.35 1377 [x-CH2](47)-[s-(H20C62)](32)-[dS-Me](9)123 1362 1344 112.39 [x-CH2](34)-[s-(H20C62)](31)-[d0as-Me1](8)-[s-(C5C6)](5) [s-(C44C43)](5)121 1322 1305 976.89 [x-CH2](42)-[s-(H20C62)](30) [d0as-Me1](6) [s-(C5C6)](5)-[s-(C44C43)](5)120 1295 1279 1.37 1278 [t-CH2](43)-[s-(H20C62)](38)-[q0-Me](9)117 1284 1269 801.83 1188 [s-(H20C62)](13)-[q-(N11H27](10)-[q-(N37H40](10)-[x-CH2](9)[q-NH2](8)-[q-NH2](8)-[m(C12S13)]

(6) [m(C36S35)](6)(6)116 1264 1249 150.87 [s-(H20C63)](27)[s-(H20C62)](21)-[x-CH2](9)[q0-Me1](9)[s-(C5C6)](6)-[s-(C44C43)](6)113 1224 1211 534.15 [x-CH2](46)-[s-(H20C62)](13)-[s-(C63C62)](5) [q0-Me1](4)112 1184 1172 6.75 [s-(C63C62)](40)-[q0-Me](28) [s-(H20C62)](12)-[s-(H58C17)](6)110 1145 1133 487.61 1090 [s-(H20C63)](51) [s-(C63C62)](26)-[q-Me](11)-[dsc-(C62C63O61)](5)108 1141 1130 103.83 [s-(H20C63)](56) [s-(C63C62)](28)-[q-Me](11)105 1127 1116 70.51 [s-(H20C63)](50) [s-(C63C62)](32)-[q-Me](12)103 1083 1073 124.7 1029 [q0-Me1](19)-[s-(H20C63)](13)[s-(C5C6)](12)-[s-(C44C43)](11)-[s-(C63C62)](9)[d0as-Me1](5)102 1069 1060 6.12 [s-(H20C63)](38) [s-(H20C62)](13) [s-(H58C17)](11) [(x-(C47C51)](8)-[(x-(C5C6)](7)-[s-(C5C6)](6)-

[s-(C44C43)](5)-[q0-Me1](5)98 1053 1044 38.54 [q0-Me1](32) [s-(C5C6)](20)-[s-(C44C43)](19) [d0as-Me1](8) [q-Me1](5)96 1047 1038 43.19 953 [s-(C63C62)](33) [s-(H20C63)](25)-[s-(H20C62)](15)-[q-Me](14)93 1015 1007 4.47 [q0-Me1](34) [s-(C5C6)](21)-[s-(C44C43)](20) [d0as-Me1](8) [q-Me1](5)92 963 956 10.62 [(x-(C43H45)](19) [(x-(C7H23)](19) [s-(H20C63)](10)-[(x-(C47C51)](8)[(x-(C5C6)](7) [s-(C5C6)](5)

[s-(C44C43)](5)89 905 899 19.41 [s-(H20C63)](59) [s-(C63C62)](18)-[q-Me](10)-[s-(H20C62)](6) [x-CH2](5)88 878 874 20.24 841 [s-(H20C63)](56) [s-(C63C62)](18)-[q-Me](10)-[s-(H20C62)](6)85 860 855 37.35 796 [s-(H20C63)](49) [s-(C63C62)](18)-[q-Me](10)-[s-(H20C62)](9)84 820 816 119 772 [(x-N50H56)](39) [s-(O14H56)](36) [(x-N1H19)](6) [s-(O60H19)](6)82 818 814 2.26 [q0-Me](44) [s-(C17C16)](15) [t-CH2](10) [s-(H20C63)](10) [s-(H58C17)](9)81 816 812 129.07 [q0-Me1] (23) [s-(H20C63)](11) [s-(C5C6)](11)-[s-(C44C43)](10)-[s-(H20C62)](7)[s-(C63C62)](6)

104 R.N. Singh et al. / Journal of Molecular Structure 1016 (2012) 97–108

Table 9 (continued)

ModeNo.

Calculated Experimental Vibrational assignment (PEDP 5%)

Unscaled Scaled Intensity(Kmmol�1)

78 764 761 14.95 [s-(H20C63)](26)[q-(C48C52)](16)-[d-(C62C52O61)](14)[d-(C16C9O15)](5)-[d-(O14O15C2)](5)76 748 746 11.83 729 [(x-(C52C48)](38) [(x-(C2C9)]29) [s0-R](8)-[s-(O61C52)](7) [s0-R](6)-[s-(O15C9)](6)74 708 706 0.81 [(x-(C47C51)](28)-[(x-(C5C6)](25)-[s-(H20C63)](9) [s0-R](8)-[s-(C5C6)](7)-[s-(C44C43)](6)72 669 667 20.89 622 [s-(H20C63)](57) [s-(C63C62)](9) [dsc-(C62C63O61)](8)-[q-Me](5)70 644 642 5.93 [s0-R](21) [(x-(C52C48)](19) [s0-R](17) [(x-(C2C9)](16)68 637 636 13.69 [s0-R](29)[s0-R](21) [(x-(C52C48)](13) [(x-(C47C51)](10) [(x-(C2C9)]10)-[(x-(C5C6)](9)66 591 590 28.56 532 [(x-(C12N18)](18)-[(x-(C36N38)](17)-[s-(C36N38)](11)-[s-(N11C12)](7)-[s-(N37C36)](6)-

[(x-(N11H27)](6)[(x-(N37H40)](5)-[s-(C12N18)](5)64 575 575 0.62 [s-(H20C63)](40) [dsc-(C62C63O61)](9) [s-(C63C62)](7)-[q0-Me1](6)62 533 533 31.46 [s-(H20C63)](66) [dsc-(C62C63O61)](11) [s-(C63C62)](6)60 519 519 166.36 487 [(x-(N11H27)](34)-[(x-(N37H40)](33)-[s-(C12N18)](15)-[s-(C36N38)](7)57 461 462 35.08 [s-(H20C63)](41) [s-(H20C62)](26)-[q0-Me1](9)56 432 433 9.6 429 [s-(H20C63)](69) [dsc-(C62C63O61)](17) [s-(C63C62)](7)54 404 405 3.91 [(x-(C47C51)](27)-[(x-(C5C6)](24)-[(x-(C52C48)](11)-[s-(H20C63)](11)-[(x-(C2C9)]9)

Proposed assignment and potential energy distribution (PED) for vibrational modes: types of vibrations: m – stretching, dsc – scissoring, q – rocking,x – wagging, t – twisting,d and d0 – deformation, d0as – asymmetric deformation, s – torsion.

500 1000 1500 3000 3500 4000

Abs

orba

nce

(arb

itrar

y un

it)

Wavenumber cm-1

Experimental Monomer Dimer

Experimental

Monomer

Dimer

Fig. 7. FT-IR spectrum (experimental and simulated) of the title compound.

R.N. Singh et al. / Journal of Molecular Structure 1016 (2012) 97–108 105

(3363) cm�1. The observed N11AH27 wagging mode at 487 cm�1

corresponds to the calculated wavenumber at 519 (517) cm�1.The NH2 asymmetric stretching vibrations give rise to a strongband in the region 3330–3450 cm�1, whereas symmetricstretching vibrations give rise to a weak band in the region3150–3270 cm�1 and the NH2 bending vibration is reported inthe region 1580–1640 cm�1 [54]. In the experimental FT-IR spec-trum, the asymmetric and symmetric stretches for NH2 areobserved at 3429 cm�1, 3274 cm�1, whereas these are calculatedas 3562 (3562) cm�1, 3418 (3419) cm�1 respectively. The observedasymmetric and symmetric stretches of NH2 also correlate with thereported strong absorption band at 3447 and 3274 cm�1 respec-tively for 5-methyl 2-furaldehyde thiosemicarbazone [55]. Theobserved NH2 scissoring mode at 1543 cm�1 corresponds to thecalculated wavenumber at 1591 (1591) cm�1. The observed NH2

rocking mode at 1188 cm�1 is in agreement with the calculatedwavenumber at 1269 cm�1 in dimer.

4.5.2. CAH vibrationsThree methyl groups are present in the molecule. Two of them

are directly attached to the pyrrole ring and the third is attached tothe CH2 of ester group. The methyl groups directly attached to thepyrrole ring are abbreviated as Me1 and Me2 and methyl group at-

tached to the CH2 of ester as Me. The stretching vibration of Me isobserved at 2975 cm�1, whereas it is calculated as 3007 (3005)cm�1. The observed rocking mode of Me at 953 cm�1 correspondsto the calculated wavenumber at 1038 cm�1 in dimer. The sym-metric deformation of Me is observed at the same wavenumber1405 cm�1 as in the calculated spectrum of monomer and dimer.In PED, the stretching vibrational mode of Me1 and Me2 are as-signed at 2921 (2920) cm�1 and 2915 (2914) cm�1 respectively.The observed rocking mode of Me1 at 1029 cm�1 corresponds tothe calculated wavenumber at 1073 cm�1 in dimer. The observedasymmetric deformation of Me1 at 1440 cm�1 agrees well withthe calculated wavenumber at 1450 (1446) cm�1.

According to Internal coordinate system recommended by Pu-lay et al. [34], CH2 group associate six stretching and bendingvibrational frequencies namely: symmetric stretch, asymmetricstretch, scissoring, rocking, wagging and twisting. The scissoringand rocking deformations belong to polarized in-plane vibrationwhereas wagging and twisting deformations belong to depolarizedout-of-plane vibration. The symmetric CH2 stretching vibrationalband observed at 2925 cm�1 is in an excellent agreement withthe calculated wavenumber at 2936 (2940) cm�1. The asymmetricCH2 stretching vibrational band is assigned at 2975 (2977) cm�1

but does not observed in experimental FT-IR spectrum. The scissor-

Fig. 8. Molecular graph of the dimer: bond critical points (small red spheres), ringcritical points (small yellow sphere), bond paths (pink lines). (For interpretation ofthe references to colour in this figure legend, the reader is referred to the webversion of this article.)

106 R.N. Singh et al. / Journal of Molecular Structure 1016 (2012) 97–108

ing, wagging and twisting vibrational mode of CH2 group are ob-served at 1512, 1377, 1278 cm�1, whereas these are calculated as1508 (1507), 1376 (1375), 1279 (1279) cm�1 respectively.

4.5.3. CAC vibrationsThe C4AC7 stretching vibration is observed at 1595 cm�1,

whereas it is calculated as 1647 cm�1 in dimer. The observed wag-ging vibrational mode of C48AC52 at 729 cm�1 corresponds to thecalculated wavenumber at 746 (751) cm�1. The CAC stretches ofpyrrole ring do not appear in dimer PEDP 5% contribution.

4.5.4. CAO vibrationsThe observed stretching mode of carbonyl group (mC@O) at

1656 cm�1 corresponds to the calculated wavenumber at 1689(1725) cm�1. Therefore, the observed mC@O absorption band at1656 cm�1 correlates well with the calculated wavenumber at1689 cm�1 in dimer. The observed mC@O is also in agreement withthe reported strong absorption band at 1665 cm�1 for dimer ofsyn-pyrrole-2-carboxylic acid [43]. Therefore, mC@O in EFDMPCTconfirms the involvement of C@O group in intermolecular attrac-tion. The mC16AO15 stretching mode does not appear in the dimerPEDP 5% contribution. The observed mC16AO15 stretching mode atwavenumber 982 cm�1 corresponds to the calculated wavenumberat 1037 cm�1 in monomer. It is in good agreement with the re-

ported CAO stretching of methyl pyrrole-2-carboxylate at wave-number 988 cm�1[53].

4.5.5. CAS vibrationsThe C@S stretching vibrations are observed in the range 680–

930 cm�1, with moderate intensity [54]. The C12@S13 stretchingvibration is assigned at 1269 cm�1 with 6% contribution in dimerPED. The C12@S13 stretching vibration is observed at 796 cm�1,whereas it is calculated as 812 cm�1 in monomer. The downshif-ting of C@S stretching wavenumbers can be attributed to the great-er contribution of resonance form >N+@CAS–. These vibrations arealso reported in literature at 846 cm�1 [16] and 851 cm�1 [55].

4.5.6. CAN vibrationsA combination band of the C7@N10 stretching and C7AH23

rocking is observed at 1595 cm�1, whereas it is calculated as1647 (1648) cm�1. The observed band at 1595 cm�1 correlateswith the reported wavenumber at 1602 cm�1 [16], 1655 cm�1

[55]. A combination band of the NH2 scissoring and CANH2 stretch-ing vibration is observed at 1543 cm�1, whereas it is calculated as1591 (1591) cm�1. The observed CANH2 wagging mode at532 cm�1 corresponds to the calculated wavenumber at 590(587) cm�1. The wagging of CANH2 is also reported in literatureat 483 cm�1 for ethyl methyl ketone thiosemicarbazone [55].

4.5.7. NAN vibrationsThe NAN stretching vibrations are reported in the literature at

1121 cm�1 [56]. The N10AN11 stretching vibration does not ap-pear in the dimer PED with contributionP5%. The observedN10AN11 stretching vibration at 1090 cm�1 corresponds to thecalculated wavenumber at 1133 cm�1 in monomer. The NANstretching vibration is also reported in literature observed at1107 cm�1 [55], 1108 cm�1 [16], 930 cm�1 [37].

4.6. AIM calculations

4.6.1. p-Delocalization effect in resonance assisted hydrogen bonds(RAHB) and topological parameters

The AIM method gives the opportunity to have an insight into aregion of a system. Geometrical, topological and energy parame-ters are useful tool to characterize the strength of hydrogen bond.Bond critical point is used in the recognition of chemical bonds andstrength between atoms. The Laplacian of the electron densitymeasures local electron concentration 2q < 0 and depletion 2q > 0within a molecular system. Laplacian of electron density (2q(rBCP))is related to the energy of bond interaction by the local expressionof the viral theorem [57] and additionally to total electron energydensity H(rBCP) [58].

1/4r2q(rBCP) = 2G(rBCP) + Vq(rBCP) = H(rBCP) and bond degree (B.D.) [59] is expressed as:

B:D: ¼ HðrBCPÞ=qðrBCPÞwhere G(rBCP) is the kinetic electron energy density, V(rBCP) poten-tial electron energy density and q(rBCP) the electron density at bondcritical point BCP.

The ellipticity, e = (k1/k2) – 1is a sensitive index to monitor thep-character of bond.

The geometrical criteria for the existence of hydrogen bond areas follows. (i) The distance between proton and acceptor is lessthan the sum of their van der Waal’s radii of these atoms (ii) The‘‘donor (D)–proton (H)� � �acceptor (A)” angle is greater than 90�(iii) The elongation of ‘‘donor (D)–proton (H)” bond length isobserved.

The above criteria are frequently considered as insufficient, theexistence of hydrogen bond could be supported further by Kochand Popelier criteria [57] based on ‘‘Atoms in Molecules”.

Table 10Geometrical parameters (bond length) and topological parameters: electron density (qBCP), Laplacian of electron density(2q(rBCP)), electron kinetic energy density (GBCP), electronpotential energy density (VBCP), total electron energy density (HBCP), ellipticity (e), bond degree at bond critical point (BCP) and estimated interaction energy (Eint) for bonds ofinteracting atoms in dimer; using B3LYP/6-31G(d,p) in AIM calculations.

Atoms involved ininteractions

Bond distance(Å)

qBCP

(a.u.)r2q(BCP)(a.u.)

GBCP (a.u.) VBCP (a.u.) HBCP(a.u.) e Bond degree Interaction energy(kcal/mol)

O60� � �H19 1.90775 0.02557 �0.08196 0.01998 �0.01947 0.000508 0.020584 0.019867 �6.1088O14� � �H56 1.90779 0.02557 �0.08196 0.01998 �0.01947 0.000508 0.020583 0.019867 �6.1088N10� � �H33 2.26305 0.01793 �0.07611 0.01621 �0.01340 0.002810 0.698159 0.156721 �4.20431N39� � �H41 2.26305 0.01793 �0.07611 0.01621 �0.01340 0.002810 0.698171 0.156721 �4.20431H26� � �O15 2.30416 0.01443 �0.04871 0.01137 �0.01057 0.000802 0.053236 0.055579 �3.31639H55� � �O61 2.30416 0.01443 �0.04871 0.01137 �0.01057 0.000802 0.053233 0.055579 �3.31639O14� � �H59 2.55657 0.00797 �0.02776 0.00610 �0.00527 0.000834 0.150648 0.104642 �1.65349O60� � �H22 2.55655 0.00797 �0.02776 0.00610 �0.00527 0.000834 0.150646 0.104642 �1.65349C6� � �N10 3.10333 0.00782 �0.02971 0.00616 �0.00490 0.00126 0.144367 0.161125 �1.5374C51� � �N39 3.10334 0.00782 �0.02971 0.00616 �0.00490 0.00126 0.144291 0.161125 �1.5374

Table 11Calculated ellipticity parameter for bonds involved in 16 membered pseudo ring of dimer using AIM calculation.

Bond Ellipticity Bond Ellipticity

Ring (C9AC2AC3AC4AC5AN1AH19� � �O60AC52AC48AC46AC44AC47AN50AH56� � �O14)O14AC9 0.1007 O60AC52 0.1007C9AC2 0.2252 C52AC48 0.2252C2AC3 0.2885 C48AC46 0.2885C3AC4 0.2174 C46-C44 0.2174C4AC5 0.2551 C44AC47 0.2551C5AN1 0.1820 C47AN50 0.1820N1AH19 0.0308 N50AH56 0.0308H19� � �O60 0.0205 H56� � �O14 0.0205

R.N. Singh et al. / Journal of Molecular Structure 1016 (2012) 97–108 107

These include: (1) correct topological pattern (BCP and gradientpath), (2) appropriate values of electron density at BCP, (3) propervalue of Laplacian of electron density at the BCP, and (4) mutualpenetration of hydrogen and acceptor atoms. Criteria pertainingto integrated properties of hydrogen atoms involved in the H-bonding include (5) an increase of net charge, (6) an energeticdestabilization, (7) a decrease in dipolar polarization, and finally(8) a decrease in atomic volume.

Molecular graph of the dimer at the level of B3LYP/6-31G(d,p)using AIM program is shown in Fig. 8. Geometrical, topologicaland energy parameters for bonds of interacting atoms in dimerare given in Table 10. The dimer has four inter molecular H-bond-ing, four intra molecular H-bonding and two weaker C� � �N interac-tions. The calculated interaction energy at BCP indicates that twointer molecular H-bonding O60� � �H19, O14� � �H56 and two intramolecular H-bonding N10� � �H33, N39� � �H41are moderate in nat-ure, whereas rests H26� � �O15, O61� � �H55, O14� � �H59, O60� � �H22,C6� � �N10, C51� � �N39 are weaker interactions. The two inter molec-ular H-bonding have red shift whereas two have blue shift. On thebasis of electron density, bond degree and estimated interactionenergy, their strength is in the following order (O60� � �H19 �O14� � �H56) > (N10� � �H33� N39� � �H41) > (H26� � �O15� O61� � �H55)> (O14� � �H59 � O60� � �H22) > (C6� � �N10, C51� � �N39).

The e values for bonds involved in 16-membered pseudo ring ofdimer are given in Table 11. In this ring, the values of e for bondsO14AC9, C9AC2, C2AC3, C3AC4, C4AC5, C5AN1 associated withthe N1AH19� � �O60 and for bonds O60AC52, C52AC48, C48AC46,C46AC44, C44AC47, C47AN50 associated with the(N50AH56� � �O60) are in the range 0.1007–0.2885. These valuesof e are corresponds to the aromatic bonds reported in literature[57,58]. This confirms the presence of resonance assisted hydrogenbonds because both N and O atoms are interconnected by a systemof p-conjugated double bonds. According to AIM calculations, thebinding energy of dimer is sum of the energies of all four intermo-lecular interactions and is calculated as �15.52 kcal/mol. The inter-

molecular hydrogen bond energy of dimer as result of only twoheteronuclear intermolecular hydrogen bond (NAH� � �O@C) is cal-culated as �12.2176 kcal/mol as per AIM, whereas as per DFT cor-rection for the basis set superposition error (BSSE) via the standardcounterpoise method [36] is found to be �10.47425 kcal/mol.

5. Conclusions

A new pyrrole containing thiosemicarbazone (EFDMPCT) is syn-thesized and characterized by FT-IR, 1H NMR, UV–Visible, DART-Mass, and elemental analysis. The observed wavelength absorptionmaxima are blue shifted than the calculated wavelength absorp-tion maxima. The molecular orbital coefficients analyses suggestthat the electronic transitions are assigned to p? p*. In the pres-ent study, experimental and calculated vibrational wavenumbersanalyses confirm the existence of dimer by involvement of hetero-nuclear association through pyrrolic (NAH) and carbonyl (C@O)oxygen of ester. The binding energy of dimer using both DFT andAIM theories are in good agreement and found as �14.6711,�15.5246 kcal/mol respectively. The results of AIM ellipticity con-firm the existence of resonance assisted hydrogen bonds in dimer.The calculated local reactivity descriptors of reactants are in com-plete agreement with the formation of product EFDMPCT by reac-tion at appropriate reactive sites C7 in reactant A and N(1) inreactant B. The local reactivity descriptors of EFDMPCT show thatC7 is most reactive site for nucleophilic attack. Hence, title mole-cule may be used as precursor for the synthesis of new heterocycliccompounds such as thiadiazoline, thiazolidinones andazetidinones.

Acknowledgment

The authors thankfully acknowledge the DST New Delhi forfinancial supports.

108 R.N. Singh et al. / Journal of Molecular Structure 1016 (2012) 97–108

Appendix A. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.molstruc.2012.02.033.

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