Journal of Molecular Structure 1076 (2014) 664–672

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Journal of Molecular Structure

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Structural and spectroscopic analysis of 3-[(4-phenylpiperazin-1-yl)methyl]-5-(thiophen-2-yl)-2,3-dihydro-1,3,4-oxadiazole-2-thionewith experimental (FT-IR, Laser-Raman) techniques and ab initiocalculations

http://dx.doi.org/10.1016/j.molstruc.2014.08.0350022-2860/� 2014 Elsevier B.V. All rights reserved.

⇑ Corresponding author. Tel.: +90 368 2715516; fax: +90 368 2714152.E-mail address: mkarakaya@sinop.edu.tr (M. Karakaya).

Fatmah A.M. Al-Omary a, Mustafa Karakaya b,⇑, Yusuf Sert c,d, Nadia G. Haress a, Ali A. El-Emam a,e,Çagrı Çırak f

a Department of Pharmaceutical Chemistry, College of Pharmacy, King Saud University, Riyadh 11451, Saudi Arabiab Department of Energy Systems, Faculty of Engineering & Architecture, Sinop University, Sinop 57000, Turkeyc Department of Physics, Faculty of Art & Sciences, Bozok University, Yozgat 66100, Turkeyd Sorgun Vocational School, Bozok University, Yozgat 66100, Turkeye King Abdullah Institute of Nanotechnology (KAIN), King Saud University, Riyadh 11451, Saudi Arabiaf Department of Physics, Faculty of Art & Sciences, Erzincan University, Erzincan 24100, Turkey

h i g h l i g h t s

� The FT-IR and Laser-Raman spectra ofthe title compound were recorded insolid phase.� The optimized geometry and

vibrational frequencies werecalculated for the first time.� The HOMO–LUMO energies and

related molecular properties wereevaluated.

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

a r t i c l e i n f o

Article history:Received 30 June 2014Received in revised form 18 August 2014Accepted 19 August 2014Available online 26 August 2014

Keywords:FT-IR spectraLaser-Raman spectraHartree–FockDensity functional theoryPiperazine1,3,4-Oxadiazole

a b s t r a c t

Experimental and theoretical harmonic vibrational frequencies of 3-[(4-phenylpiperazin-1-yl)methyl]-5-(thiophen-2-yl)-2,3-dihydro-1,3,4-oxadiazole-2-thione have been investigated in this paper.Experimental FT-IR (400–4000 cm�1) and Laser-Raman spectra (100–4000 cm�1) of title compound insolid phase have been recorded. Theoretical vibrational frequencies and geometric parameters (bondlengths and bond angles) have been also calculated using ab initio Hartree Fock (HF), density functionaltheory (B3LYP hybrid functional) methods with 6-311++G(d,p) basis set, for the first time. Assignments ofvibrational frequencies have been performed by potential energy distribution (PED) analysis. Total den-sity of state (TDOS) diagrams analysis has been also presented for title compound. Theoretical optimizedgeometric parameters and vibrational frequencies have been compared with the correspondingexperimental data, and they have been shown to be in a good agreement with each other. Besides, highestoccupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energies have beenfound.

� 2014 Elsevier B.V. All rights reserved.

F.A.M. Al-Omary et al. / Journal of Molecular Structure 1076 (2014) 664–672 665

Introduction

1,3,4-Oxadiazole heterocycle represents the key pharmaco-phore in various biologically-active compounds including antibac-terial [1-5], antifungal [6,7], antiviral against HIV [8], herbicidal[9,10], anti-inflammatory [11–13], analgesic [13], muscle relaxant[14] and central nervous depressant activities [15]. In addition,1,3,4-oxadiazole derivatives have been reported as photosensitizer[16] and essential components of liquid crystals [17]. Meanwhile,piperazine was early known as uricosuric agent [18]. Moreover,several substituted piperazine derivatives have been known asimportant therapies for various pathological conditions [19–22].In the present study, we present a comprehensive investigationon the molecular structure, electronic properties and vibrationalspectra of the title compound (molecular formula C17H18N4OS2),which have been proved to possess potent and broad-spectrumantibacterial activity [5,23], with the hope that the results of pres-ent study may be helpful in the prediction of its mechanism of bio-logical activity.

According to our literature searches, results based on quantumchemical calculations and FT-IR, Laser-Raman spectral studies andHOMO–LUMO analysis on the title compound have not beenreported elsewhere. Herewith, theoretical and experimental stud-ies have been performed to give a detailed description of themolecular structure and vibrational harmonic spectra of the titlecompound and its related analogues. Density functional theory(DFT) is an approach that the electronic structure and all theground state properties of a system or molecule are defined asfunctions of the charge density [24]. Briefly, DFT is a method fre-quently used in theoretical spectroscopy studies to provide suc-cessful outcomes in identification vibrational and electronicstructure properties of systems in ground state. In our study, thegeometrical parameters computed by using DFT and HF methodhave been also compared with the X-ray analysis results of the titlecompound for discussing the agreement [23]. Furthermore, thefrontier orbitals and energy band gaps have been interpreted forthis compound in different methods and set levels.

Experimental details

FT-IR spectrum (4000–400 cm�1) of the title molecule has beenrecorded by Perkin–Elmer Spectrum Two FT-IR Spectrometer witha resolution of 4 cm�1 in solid phase at room temperature. TheRaman spectrum has been recorded on Renishaw Invia Ramanmicroscope spectrophotometer in the 4000–100 cm�1 region. Theexcitation line at 785 nm has been taken from a diode laser. Itsscan number is 100, the resolution is 1 cm�1, and the sample isin solid phase.

Computational details

In our study, initial atomic coordinates for geometry optimiza-tion has been taken from Gauss View software database [25].Molecular structure of the title compound in ground state andgas phase has been optimized by HF and DFT/B3LYP methods with6-311++G(d,p) basis set level. Calculated harmonic vibrationalfrequencies have been scaled by 0.9051 and 0.9614 for HF andDFT (B3LYP functional; Becke, 3-parameter, Lee–Yang-Parr),6-311++G(d,p) level, respectively [26]. All the calculations(vibrational wavenumbers, geometric parameters and molecularorbital) have been performed by using Gaussian 09 W programpackage on a computing system [27]. Vibrational assignments withthe help of normal coordinate analysis (NCA) have been made onthe basis of the potential energy distribution (PED) calculated byusing Vibrational Energy Distribution Analysis (VEDA 4) program

[28] used in previous studies by many researchers [29–31]. Contri-butions of the molecular orbitals as total density of state (TDOS)spectral analysis have been performed in GaussSum 3.0 program[32].

Results and discussion

Geometric structure

Single crystal X-ray analysis data of the title compound, C17H18-

N4OS2, show that its crystal possesses space group C2/c, monoclinicsystem, with the following cell dimensions: a = 26.1721 Å,b = 5.7253 Å, c = 23.7008 Å and b = 97.802�, V = 3518.5 Å3 and mea-sured density of the molecule is 1.353 Mg/m3 [23]. Theoretical andexperimental structure parameters (bond lengths and angles) [23]of the title compound are shown in Table 1, in accordance with theatom numbering scheme given in Fig. 1. As seen from the opti-mized structure in Fig. 1, the title molecule has C1 symmetry andcomposed of four rings, aromatic, piperazine moiety, oxadiazoleand thiophene with the methylene bridge. Besides, three weakCAH� � �S interactions are noteworthy in optimized structure. Theseresults support the experimental studies have been performed byEl-Emam et al. [23]. C8A� � �S2, C8B� � �S2 and C7B� � �S2 are calculated3.53, 3.73, 2.84 Å (B3LYP) and 3.59, 3.75, 2.84 Å (HF), respectively.The largest differences of bond lengths (in C14AH14 andC16AH16) between the calculated and experimental [23] geome-tries are 0.155 Å (B3LYP) and 0.146 Å (HF) in aromatic ring. Thelargest differences of bond angles are 6.73� (B3LYP) and 7.00�(HF) in C2AC3AC4 (thiophene ring).

Mean values of the CAC bond distances are calculated 1.399 Å(B3LYP), 1.389 Å (HF) and CAH are 1.083 Å (B3LYP), 1.075 Å (HF)in aromatic ring. These mean values are observed in X-ray studyas 1.382 and 0.93 Å for CAC and CAH bond lengths, respectively.In the 1,3,4-oxadiazole ring, N1AN2, N2AC6, C6AO1, O1AC5 andC5AN1 bond lengths have been computed as 1.379(1.372),1.358(1.327), 1.386(1.344), 1.336(1.341) and 1.293(1.260) Å, atDFT (HF) methods, respectively. These bond lengths are respec-tively reported 1.386, 1.338, 1.377, 1.367 and 1.288 Å as theexperimental data for the title compound [23]. For the related1,3,4-oxadiazole derivative, 3-[(N-Methylanilino)methyl]-5-(thio-phen-2-yl)-1,3,4-oxadiazole-2(3H)-thione, the same bond lengthsare respectively reported 1.385, 1.339, 1.382, 1.364 and 1.283 Åby El-Emam et al. [33]. To compare all the calculated values withthe experimental data we have obtained the linear correlationcoefficients (R2) with linear regression analysis of the theoreticaland experimental bond lengths and bond angles. As seen fromTable 1, it can be concluded that the calculated values at DFT levelcorrelate well with the experimental ones for bond lengths and HFmethod also yielded more consistent results for bond angles. Fromthe theoretical geometry, we have found that most of the opti-mized bond lengths are slightly deviating from the experimentalvalues at the DFT/B3LYP and HF method. This situation has beenattributed that the theoretical calculations belong to isolatedmolecule in gaseous phase, while the experimental results belongto one in solid state.

Vibrational analysis

Experimental FT-IR and Laser-Raman spectra of the title com-pound are shown in Figs. 2 and 3, respectively, together with the-oretical IR and Raman spectra computed by using scale factors forcomparative purpose. The scaled calculated harmonic vibrationalfrequencies at HF and B3LYP levels, observed vibrational frequen-cies, and detailed PED assignments are tabulated in Table 2. Also,harmonic frequencies are calculated for gaseous phase of the iso-lated title compound, while experimental spectra are obtained

Table 1Selected structural parameters for the optimized title compound.

Geometric parameters Experimental Ref. [23] Calculated

DFT-B3LYP method 6-311++G(d,p) (gas phase) HF method 6-311++G(d,p) (gas phase)

Bond lengths (Å)S1AC4 1.708 1.745 1.731S1AC1 1.738 1.729 1.720S2AC6 1.636 1.647 1.653O1AC5 1.367 1.366 1.341O1AC6 1.377 1.386 1.344N1AC5 1.288 1.293 1.260N1AN2 1.386 1.379 1.372N2AC6 1.338 1.358 1.327N2AC7 1.485 1.492 1.477N3AC7 1.425 1.423 1.417N3AC8 1.447 1.464 1.453N3AC11 1.449 1.474 1.463N4AC12 1.411 1.387 1.384N4AC10 1.458 1.460 1.449N4AC9 1.463 1.460 1.453C1AC2 1.349 1.368 1.347C1AH1 0.930 1.079 1.071C2AC3 1.342 1.419 1.430C2AH2 0.930 1.081 1.073C3AC4 1.379 1.376 1.351C3AH3 0.930 1.081 1.073C4AC5 1.441 1.438 1.450C7AH7A 0.970 1.090 1.081C7AH7B 0.970 1.091 1.079C8AC9 1.505 1.525 1.521C8AH8A 0.970 1.101 1.091C8AH8B 0.970 1.094 1.085C9AH9A 0.970 1.101 1.090C9AH9B 0.970 1.093 1.083C10AC11 1.512 1.530 1.524C10AH10A 0.970 1.093 1.084C10AH10B 0.970 1.100 1.091C11AH11A 0.970 1.091 1.082C11AH11B 0.970 1.097 1.087C12AC13 1.395 1.412 1.401C12AC17 1.396 1.412 1.400C13AC14 1.374 1.391 1.382C13AH13 0.930 1.082 1.073C14AC15 1.375 1.394 1.385C14AH14 0.930 1.085 1.076C15AC16 1.370 1.394 1.384C15AH15 0.930 1.083 1.075C16AC17 1.379 1.391 1.384C16AH16 0.930 1.085 1.076C17AH17 0.930 1.082 1.073

R2 0.9843 0.9775

Bond angles (�)C4AS1AC1 90.4 90.91 90.69C5AO1AC6 106.05 106.68 107.08C5AN1AN2 103.39 104.18 104.24C6AN2AN1 112.28 112.13 111.10C6AN2AC7 126.94 127.52 128.58N1AN2AC7 120.78 120.35 120.27C7AN3AC8 115.60 115.64 115.71C7AN3AC11 115.92 118.24 118.38C8AN3AC11 109.70 114.61 114.82C12AN4AC10 116.68 121.66 121.19C12AN4AC9 114.66 121.33 121.05C10AN4AC9 110.64 117.00 116.99C2AC1AS1 114.1 112.33 112.63N2AC7AH7A 108.3 105.92 106.03N3AC7AH7B 108.3 110.36 110.31N2AC7AH7B 108.3 104.04 104.80H7AAC7AH7B 107.4 109.65 109.39N3AC8AC9 109.90 110.66 110.64N3AC8AH8A 109.7 112.12 112.13C9AC8AH8A 109.7 108.57 108.58N3AC8AH8B 109.7 108.97 109.10C9AC8AH8B 109.7 109.44 109.23H8AAC8AH8B 108.2 107.00 107.06N4AC9AC8 111.87 110.52 110.69

666 F.A.M. Al-Omary et al. / Journal of Molecular Structure 1076 (2014) 664–672

Table 1 (continued)

Geometric parameters Experimental Ref. [23] Calculated

DFT-B3LYP method 6-311++G(d,p) (gas phase) HF method 6-311++G(d,p) (gas phase)

N4AC9AH9A 109.2 111.86 111.56C8AC9AH9A 109.2 109.72 109.53N4AC9AH9B 109.2 109.23 109.56C2AC1AH1 122.9 128.27 127.58S1AC1AH1 122.9 119.40 119.78C3AC2AC1 108.2 112.64 112.25C3AC2AH2 125.9 123.92 123.81C2AC3AC4 119.4 112.67 112.40C2AC3AH3 120.3 124.60 124.32C4AC3AH3 120.3 122.73 123.28C3AC4AS1 107.6 111.46 112.03C5AC4AAS1 121.91 121.52 121.83N1AC5AO1 113.36 112.90 112.67N1AC5AC4 127.95 128.41 128.63O1AC5AC4 118.66 118.68 118.70N2AC6AO1 104.91 104.11 104.89N2AC6AS2 131.15 131.79 131.55O1AC6AS2 123.94 124.10 123.55N3AC7AN2 116.10 116.41 115.97N3AC7AH7A 108.3 110.14 110.04C8AC9AH9B 109.2 107.94 107.75H9AAC9AH9B 107.9 107.44 107.63N4AC10AC11 110.80 110.28 109.96N4AC10AH10A 109.5 109.16 109.29C11AC10AH10A 109.5 107.83 107.81

R2 0.8863 0.8906

Fig. 1. Optimized geometric structure of the title compound.

F.A.M. Al-Omary et al. / Journal of Molecular Structure 1076 (2014) 664–672 667

from the solid phase of the title compound. The title moleculewhich is C1 symmetry has 120 vibrational modes. All the calculatedmodes are numbered from the largest to the smallest frequencywithin each fundamental wavenumber. Vibrational assignmentsthat combined with the PED analysis are made via B3LYP/6-311++G(d,p) basis set level which the molecular structure of thecompound is more stable. Thus, very small portion of assignmentsmay correspond to its previous or next vibrational frequency valueat HF method, 6-311++G(d,p) level. Considering the correlationcoefficients in Table 2, linear correlation coefficients (R2) areobtained in high values, the DFT level has a higher correlation withthe experimental ones than HF level for vibration wavenumbers.The disagreement between observed and the calculated

frequencies in some modes may be attributed to the intermolecu-lar hydrogen bonds in solid phase [23].

Phenyl-piperazine group vibrations

Aromatic and piperazine ring includes CAH, CAC and NACstretching mode, CAH and CAC in-plane bending mode and outof plane deformations. Aromatic CAH stretching vibrations arecomputed at 3077–3037 cm�1 (B3LYP) and 3045–3000 cm�1

(HF). These modes are supported by the PED contribution of almost89–100%. The CAH stretching modes are commonly observed inthe region 3100–3000 cm�1 in heterocyclic organic molecules[34]. From the experimental FT-IR and Laser-Raman spectra given

Fig. 2. Comparison of calculated and observed infrared spectra of the titlecompound.

Fig. 3. Comparison of calculated and observed Raman spectra of the titlecompound.

668 F.A.M. Al-Omary et al. / Journal of Molecular Structure 1076 (2014) 664–672

in Figs. 2 and 3, the observed peaks at 3089–3023 cm�1 (IR) and3087–3069 cm�1 (Raman) are assigned to CAH stretching modesfor the title compound.

The piperazine CAH vibrations are computed at 2975–2839 cm�1 (B3LYP) and 2944–2825 cm�1 (HF) with the PED contri-bution of almost 76–97%. Computed wavenumbers of piperazinemoiety are 1066–906 cm�1 (B3LYP)/1088–942 cm�1 (HF), and1332–707 cm�1 (B3LYP)/1354–734 cm�1 (HF) for CAC and NACstretching mode, respectively. The aromatic CAC vibrations arecomputed at 1580–906 cm�1 (B3LYP) and 1612–942 cm�1 (HF).The CAH in plane bending vibrations normally are shown as anumber of strong to weak sharp bands in the region 1300–1000 cm�1 [35,36]. The CAH in plane bending vibrations for thearomatic ring are computed at 1580–1068 cm�1 (B3LYP) and1612–1090 cm�1 (HF) with PED contribution of in the range of

11–70%. These frequencies have been attributed to 1599–1065 cm�1 (IR), 1600–1083 cm�1 (Laser-Raman) in experimentalspectra. For a 1,2,4-triazole-based derivatives, the CAH in planebending vibrations are similarly identified in the range of 1574–1043 cm�1 and 1575–1048 cm�1 in FT-IR and Laser-Raman spec-tra, respectively, in the aromatic ring [37]. In addition, computedwavenumbers of the piperazine moiety and the methylene bridgeare 1464–1136 cm�1 (B3LYP) and 1495–1105 cm�1 (HF) for CAHin plane bending modes. The CAC in plane bending are calculatedin the region 1545–596 cm�1 and 1585–604 cm�1 by DFT and HFapproach, respectively, for phenylpiperazine group. The wavenum-bers observed at 1577, 968, 719, 582 cm�1 in FT-IR spectrum and1578,704, 583 cm�1 in Laser-Raman spectrum are assigned to theCAC in plane bending for the phenylpiperazine group. The CANin plane bending is calculated in the region 1364–132 cm�1 and1406–134 cm�1 by DFT and HF approach, respectively, for the phe-nylpiperazine group.

1,3,4-Oxadiazole ring vibrations

The C@N stretching vibrations are calculated 1583(B3LYP)/1692(HF), 1488(B3LYP)/1559 (HF) and 1387(B3LYP)/1440(HF)cm�1 and observed at 1609(IR)/1610(Raman), 1500(IR)/1503(Raman) and 1385(IR)/1383(Raman) cm�1. In a similar study,the C@N stretching vibrations bands of 1,3,4 oxadiazole ring areobserved at 1553, 1538, 1497 cm�1 in IR spectrum [38] and the fre-quencies assigned as C@N stretching vibrations are computed at1572, 1550, 1492 and 1490 cm�1 [39]. The NAN stretching vibra-tion is assigned at 1125 cm�1 (DFT method) [39] and peak at1105 cm�1 in experimental IR spectrum [38]. These modes gener-ally appear as medium band at 1100–950 cm�1, for 1,3,4-oxadiaz-ole ring [40]. In our study, the NAN stretching vibrations arecalculated 1060(B3LYP)/1082(HF) cm�1, 968 (B3LYP)/997 (HF)cm�1, 1387 (B3LYP)/1440 (HF) cm�1 and observed at 1060(IR)/1064(Raman) cm�1, 987(IR)/994(Raman) cm�1. The CAO stretch-ing modes have been observed at 1284(IR)/1285(Raman) cm�1,961(IR) cm�1 and calculated at 1297(B3LYP)/1341(HF),961(B3LYP)/992(HF) and 906(B3LYP)/950(HF) cm�1 with minorcontributions according to the PED analysis. The in plane bendingd(C@NAN) vibrations for 1,3,4-oxadiazole ring have been detectedat 1212(IR)/1211(Raman) cm�1, 1197(IR) cm�1, 987(IR)/994(Raman) cm�1, 524(IR)/509(Raman) cm�1 and calculated inthe region 1212–523 cm�1 and 1241–530 cm�1 by DFT and HFapproach, respectively. The CAH out of plane bending and theother torsional modes of the oxadiazole ring are in complex situa-tions with other rings and calculated in the low frequency range.

Thiophene ring vibrations

In this section, the CAH, CAC, SAC stretching mode, in-planebending mode and out of plane deformations are presented andcompared to similar structures, theoretically and experimentally.In the thiophene ring, the CAH stretching vibrations have beenidentified at 3120–3085(B3LYP), 3076–3045(HF) cm�1 andobserved peaks are 3114, 3098, 3089 cm�1 (IR) and 3087 cm�1

(Raman). The CAH stretching and out of plane bending type vibra-tions of the thiophene ring have been recorded as 3084(exp. IR)and 3093(B3LYP) cm�1 [41]. The CAC stretching modes of the thi-ophene ring are observed at 1500, 1411, 1393, 1326, 1284,1036 cm�1 (IR spectrum), 1503, 1419, 1411, 1339, 1285,1035 cm�1 (Raman) and the frequencies assigned as CAC stretch-ing vibrations are computed at 1488–1039(B3LYP), 1559–1071(HF) cm�1 with moderate PED% contributions. For thiophenederivative, the CAC stretching vibration is resulted at 1525 cm�1

(B3LYP functional) with 57% of PED contribution and 1514 cm�1

(exp. IR spectra) and 1515 cm�1 (Raman spectra) by Karabacak

Table 2Comparison of calculated harmonic frequencies and experimental FT-IR and Laser-Raman wavenumbers (cm�1) at DFT-B3LYP and HF method for the title compound.

Modes no. Assignments* Calculated frequencies(cm�1)

Experimental

B3LYP HF FT-IR Laser-Ra.

m1 tCH(99)sym in Thio. Ring 3120 3076 3114m2 tCH(94)asym in Thio. 3099 3059m3 tCH(93)asym in Thio. 3085 3045 3098m4 tCH(89)sym in Ar. 3077 3045 3089 3087m5 tCH(96)asym in Ar. 3075 3040 3069m6 tCH(100)asym in Ar. 3065 3029m7 tCH(100)asym in Ar. 3043 3007 3036m8 tCH(100)asym in Ar. 3037 3000 3023m9 tCH(99)asym in Me. 3007 2997 2993m10 tCH(97)asym in Pip. 2975 2944 2962m11 tCH(97)sym in Me. 2953 2934 2957m12 tCH(86)sym in Pip. 2944 2922 2947m13 tCH(76)asym in Pip. 2942 2915 2907m14 tCH(85)asym in Pip. 2939 2910 2881m15 tCH(99)sym in Pip. 2889 2865 2874 2873m16 tCH(81)asym in Pip. 2857 2842 2830m17 tCH(82)sym in Pip. 2853 2834 2817m18 tCH(86)sym in Pip. 2839 2823 2819m19 tNC(46) in Oxa. + tCC(25) in C4AC5 1583 1692 1609 1610m20 [tCC(47) + dHCC(17)] in Ar. 1580 1612 1599 1600m21 [tCC(58) + dCCC(10)] in Ar. 1545 1585 1577 1578m22 [tCC(51) + dHCC(19)] in Thio. + tNC(18) in Oxa. 1488 1559 1500 1503m23 dHCC(46) in Ar. 1477 1503 1495m24 dHCH(41) in Pip. + dHCH(24) in Pip. and Ar. 1464 1495 1459 1463m25 dHCH(82) in Pip. and Me. 1459 1489m26 dHCH(71) in Pip. and Me. 1449 1482 1451m27 dHCH(75) in Pip. and Me. 1446 1481 1437m28 [tCC(28) + dHCC(47)] in Ar. 1429 1463 1420 1436m29 dHCH(24) in Pip. and Me. + tCC(10) in Thio. 1418 1453 1411 1419m30 tCC(43) in Thio. 1411 1448 1393 1411m31 tNC(22) in Oxa. + sHCNC(12) in Pip. and Me. 1387 1440 1385 1383m32 sHCNC(37) in Pip. and Me. 1373 1428 1371m33 sHCNC(24) in Pip. + dHCN(17) in Pip. and Me. 1364 1406 1362m34 sHCNC(33) in Pip. 1358 1396m35 sHCNC(34) in Pip. and Me. 1342 1386 1362m36 tNC(26) in Pip. + dHCC(11) in Ar. + dHCN(11) in Pip. and Me. 1332 1354 1349m37 [dHCS(21) + dHCC(20) + tCC(11)] in Thio. + tCC(11) in C4AC5 1327 1354 1326 1339m38 sHCNC(37) in Pip. + [dHCC(15)+tCC(11)] in Ar. 1324 1349 1331m39 dHCC(53) in Ar. 1316 1346 1314 1317m40 dHCN(53) in Pip. and Ar. 1303 1343 1297m41 tCC(25) in Thio. and C4AC5 + tOC(10) in Oxa. 1297 1341 1284 1285m42 dHCC(28) in Pip. 1292 1315 1259m43 tCC(52) in Ar. 1281 1294 1245m44 dHCC(26) in Pip. + tCC(10) in Ar. 1238 1278 1234 1229m45 dHCN(12) in Pip. and Ar. 1216 1242 1219 1220m46 [dHCN(29) + tNC(26)] in Pip. and Me. + dCNN(11) in Oxa. 1212 1241 1212 1211m47 dHCC(26) in Thio. + dHCN(24) in Pip. and Me. 1208 1231m48 dCNN(21) in Oxa. + dHCC(18) in Thio. + dHCN(11) in Pip. and Me. 1202 1216 1197m49 dHCN(11) in C11AN3AC7 1174 1190 1170m50 dHCC(11) in Ar. + tCC(20) in Ar. 1168 1183m51 tNC(12) in Pip. and Me. 1152 1177 1152 1169m52 [dHCC(70) + tCC(13)] in Ar. 1137 1171 1129 1160m53 tNC(36) in Pip. and N3AC4 + dHCC(14) in Pip. 1136 1105m54 [dHCS(37) + dHCC(36)] in Thio. 1069 1092 1081 1089m55 [tCC(42) + dHCC(33)] in Ar. 1068 1090 1065 1083m56 tCC(13) in Pip. + sHCNC(15) in Pip. and Me. 1066 1088m57 tNN(22) in Oxa. + dHCN(17) in Pip. and Me. 1060 1082 1060 1064m58 cCCCN(26) in Pip. + sHCNC(17) in Pip. and Me. 1049 1072 1046m59 [tCC(47) + dHCC(15) + dHCS(12)] in Thio. 1039 1071 1036 1035m60 [dCNC(21) + dNCC(21)] in Pip. 1022 1044m61 [tCC(51) + dHCC(19)] in Ar. 1013 1038m62 tCC(24) in Oxa. + [dHCC(16) + dCCC(15)] in Thio. 1001 1017 1007 1013m63 tNC(14) in Pip. and Me. + tCC(12) in Pip. 978 1012 992m64 [tNN(16) + dOCN(15) + dCNN(11)] in Oxa. 968 997 987 994m65 [dCCC(62) + tCC(15)] in Ar. 963 996 968m66 [dOCN(30) + tOC(14) + dCNN(14)] in Oxa. 961 992 961m67 [sHCCC(64) + sCCCC(18)] in Ar. 943 985 929 930m68 [sHCCC + sHCCN](84) in Ar. 927 966 915 918m69 [dC(13) + tOC(12) + dOCN(12)] in Oxa. 906 950m70 tCC(13) in Pip. + tCC(10) in Ar. 906 942m71 sHCNC(14) in Pip. and Me. + dCNN(11) in Pip. 894 913 876m72 [sHCCC + sHCCS + sHCSC](76) in Thio. 890 910 868 864

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F.A.M. Al-Omary et al. / Journal of Molecular Structure 1076 (2014) 664–672 669

Table 2 (continued)

Modes no. Assignments* Calculated frequencies(cm�1)

Experimental

B3LYP HF FT-IR Laser-Ra.

m73 [sHCNC(25) + tNC(11)] in Pip. and Me. 849 899 855 856m74 [sHCCC + sHCCN](68) in Ar. 834 885 839 841m75 [dCCC(40) + tSC(32)] in Thio. 826 863m76 [sHCCC + sHCCS + sHCSC](85) in Thio. 823 830 786m77 [sHCCC + sHCCN](94) in Ar. 789 830 796 754m78 tNC(36) in Pip. and Me. + tNC(12) in Pip. and N2AC7 749 759 784 749m79 [sHCCC + sHCCN](62) in Ar. + [sCCCC + sCCCN](21) in Ar. 729 757 758 738m80 [tSC(41) + dCCC(38)] in Thio. 719 739 738 722m81 tNC(18) in Pip. and Me. + dCCC(11) in Ar. 707 734 719 704m82 dNCN(14) in N3AC7AN2 + sOCNN(13) in Oxa. + tNC(12) in N2AC7 and N3AC7 701 731 702m83 [sHCCC + sHCCS + sHCSC](52) in Thio. 691 728 686 687m84 [sHCCC + sHCCS + sHCSC](31) in Thio. + sOCNN(26) in Oxa. + cCONC(19) in C4AO1AN1AC5 682 710 667 668m85 [sCCCC + sCCCN](45) in Ar. + [sHCCC + sHCCN](44) in Ar. 677 698 649 648m86 [dSCC(41) + tSC(14)] in Thio. + cSNOC(10) in S2AN2AO1AC6 637 668 637 639m87 cSNOC(45) in S2AN2AO1AC6 617 639 618 617m88 dCCC(87) in Ar. 607 610m89 [dCNC + dNCC](20) in Pip. + dCCC(18) in Ar. + sHCNC(17) in Pip. 596 604 582 583m90 tSC(21) in S2AC6 + [sCCCO in C3AC4AC5AO1 + sCCCC in Thio.](14) 555 559 563 547m91 [sCCCO in C3AC4AC5AO1 + sCCCC in Thio.](43) + sSCCC(12) in Thio. 548 552 546 524m92 dCNN(17) in Oxa. + tSC(14) in S2AC6 + tNC(11) in N2AC7 and N3AC11 523 530 524 509m93 [sCCCC + sCCCN](65) in Ar. 502 520 508m94 dCCN(13) in C4AC5AN1 + dSCO(13) in S2AC6AO1 + dCNN(11) in C7AN2AN1 497 503 493m95 [dCNC + dNCC](14) in Pip. + cCCCN(10) in C11AC8AC7AN3 and C10AC9AC12AN4 480 486 486 443m96 sSCCC(47) in Pip. + [sCCCO in C3AC4AC5AO1 + sCCCC in Thio.](12) 470 483 481 438m97 dCCN(16) in C17AC12AN4 + dCNC(14) in C12AN4AC9 437 439 433 434m98 dCCN(15) in C17AC12AN4 420 423 417 413m99 [sCCCC + sCCCN](68) in Ar. + [sHCCC + sHCCN](12) in Ar. 406 416 414 394m100 dCNC(15) in C8AN3AC7 376 383 404 381m101 tCC(15) in C4AC5 and C2AC3 322 330 321m102 cCONC(13) in C4AO1AN1AC5 312 321 307m103 [dCNC + dNCC](14) in Pip. 289 292m104 [sCNCN in C8AN3AC7AN2 + sNCCN in Pip. + sNCNC in N3AC7AN2AC6](21) + sHCNC(12) in

Pip. + [cCCCN in C11AC8AC7AN3 + cCCCN in C10AC9AC12AN4](11)277 281 288

m105 dCCC(26) in C3AC4AC5 and Thio. 258 258 273m106 [sCCCC + sCCCN](13) in Ar. + dCCN(11) in Ar. 244 250 256m107 dSCO(18) in S2AC6AO1 + [sCCCC + sCCCN](14) in Ar. 211 217 217m108 sCCCC(30) in C2AC3AC4AC5 + cSNOC(15) in S2AN2AO1AC6 193 200 203m109 [sCCCC + sCCCN](15) in Ar. + dCNN(11) in C7AN2AN1 + [sCNCC in C12AN4AC9AC8 + sCNNC

in Oxa.](10)177 183 191

m110 cCCCN(18) in C11AC8AC7AN3 and C10AC9AC12AN4 + cCCNN(10) in C7AC6AN1AN2 152 160 157m111 dCNC(25) in C12AN4AC9 + dCCN(13) in Ar. 132 134 132m112 dCCN(18) in C4AC5AN1 + cCCCN(10) in C11AC8AC7AN3 and C10AC9AC12AN4 110 114 112m113 sCCNC(30) in C17AC12AN4AC9 + sCCCC(14) in C2AC3AC4AC5 + cCONC(12) in

C4AO1AN1AC593 94

m114 dCCN(22) in C4AC5AN1 + sCCNC(19) in C17AC12AN4AC9 79 76m115 cCCNN(15) in C7AC6AN1AN2 55 57m116 sCNCN(31) in C8AN3AC7AN2 + sCCNC(14) in C17AC12AN4AC9 + [sCCCO + sCCCC](14) in

Thio. and C3AC4AC5AO147 45

m117 [sCNNC + sCNCC](19) in Oxa. and C12AN4AC9AC8 + sCCNC(27) in C17AC12AN4AC9 andC9AC8AN3AC7

34 33

m118 [sCCCO + sCCCC](30) in Thio. and C3AC4AC5AO1 + [sCNNC + sCNCC](22) in Oxa. andC12AN4AC9AC8 + cCCNN(14) in C7AC6AN1AN2

30 30

m119 [sCNCN in C8AN3AC7AN2 + sNCCN in Pip. + sNCNC in N3AC7AN2AC6](24) + [sCNCN inC8AN3AC7AN2 + sNCCN in Oxa.](19) + [sCNCN in C8AN3AC7AN2 + sNCCN inPip.](16) + cCCNN(10) in C7AC6AN1AN2

13 14

m120 sCCNC(41) in C9AC8AN3AC7 + [sCNNC + sCNCC](15) in Oxa. andC12AN4AC9AC8 + cCCNN(10) in C7AC6AN1AN2

10 10

R2= 0.9998 0.9992

t; stretching, c; out-of plane bending, d; in-plane-bending, s; torsion, sym; symmetric, asym; anti symmetric, Ar; aromatic ring, Pip; piperazine group, Oxa; oxadiazole ring,Thio; thiophene ring, Me; methylene.

* Assignments are made based on the results B3LYP and potential energy distribution (PED), less than 10% are not shown.

670 F.A.M. Al-Omary et al. / Journal of Molecular Structure 1076 (2014) 664–672

et al. [42]. The SAC stretching are also computed as 826, 719,637 cm�1 (B3LYP) and 863, 739, 668 cm�1 (HF) with 32, 38 and14 PED% contributions. This mode (tSC) are recorded at 819, 675,603 cm�1 (FT-IR spectra) and supported with 815, 705, 586 cm�1

frequencies in B3LYP calculation for thiophene ring on tert-ButylN-(thiophen-2yl)carbamate compound [43]. In addition, the inplane CAH bending and out of plane bending modes are computedin the range of 1319–1024 cm�1 and 861–654 cm�1, respectively,with DFT/B3LYP approach for this thiophene derivative [43]. In

the present study, the in plane carbon-hydrogen bending (dHCC)vibrations are detected as six modes (m22, m47, m48, m54, m59 andm62) at 1488–1001(B3LYP) cm�1, 1559–1017(HF) cm�1 andobserved peaks are 1500–1007 cm�1 (IR) and 3087–1013 cm�1

(Raman). The dCCC vibrations are also computed at 1001, 826,719, 258 (B3LYP) cm�1 and 1017, 863, 739, 258 (HF) cm�1 withmoderate PED% contributions. Generally, the out of plane bendingand the other torsional modes (lowest frequencies) on the thio-phene ring are in complex situations with other rings.

Table 3Calculated values of energies, HOMO–LUMO energy gaps and related molecularproperties for the title compound.

Molecular properties B3LYP 6-311++G(d,p) HF 6-311++G(d,p)

Eelectronic + ZPE (Hartree) �1749.174972 �1741.772420EHOMO (eV) �5.3419 �7.6399ELUMO (eV) �2.2398 �0.7981EHOMO–LUMO gap (eV) 3.1021 6.8418Dipole moment (Debye) 4.2825 5.0218Ionization potential (I) 5.3419 7.6399Electron affinity (A) 2.2398 0.7981Chemical hardness (g) 1.5511 3.4209Chemical potential (l) 3.7909 4.2190Electrophilicity index (w) 4.6325 2.6016Softness (f) 0.6447 0.2923

Fig. 5. Calculated total electronic density of states diagram of the title compound.

F.A.M. Al-Omary et al. / Journal of Molecular Structure 1076 (2014) 664–672 671

As shown in Table 2, correlation coefficients (R2) between theexperimental and computed wavenumbers are 0.9998 at DFT/B3LYP and 0.9992 at HF method. Consequently, the performanceof the DFT/B3LYP method with respect to computing of the wave-numbers is better than HF method.

Molecular orbital analysis

Table 3 contains data on sum of electronic and zero pointenergies, dipole moments, HOMO–LUMO energies and the othermolecular properties of the title compound at HF and B3LYPmethod, 6-311++G(d,p) levels. According to the sum of electronicand zero point energies, the structure calculated in B3LYPapproach and 6-311++G(d,p) level can be concluded to be morestable. The energy gap between DFT/B3LYP and HF method is toohigh, about 201.4 eV. The highest occupied molecular orbital(HOMO) energies, the lowest unoccupied molecular orbital(LUMO) energies, and the gap energy values of HOMO–LUMO arecomputed at HF and B3LYP/6-311++G(d,p) level. The HOMO–LUMO gap value is more higher, about 3.74 eV or 86.24 kcal mol�1,at HF/6-311++G(d,p) level than results of B3LYP/6-311++G(d,p)level. The frontier molecular orbitals (FMO) have important rolein electric and optical properties of molecules and its molecularreactivity and the abilities of molecules to absorb light [44–46].The HOMO as donor represents the ability to give electron, LUMOas electron acceptor represents the ability to obtain electron and

Fig. 4. Visuals of frontier molecular orbitals of the title compound.

HOMO–LUMO energy gap allows us to interpret molecular chemi-cal stability [47]. This energy gap can relate the stability of themolecule to hardness, for instance, the system with least HOMO–LUMO gap means that it is more reactive [48]. The magnitude ofdipole moment is inversely related to structural stability, so, thestructural stability is higher in lower values of dipole moment[49]. For the title molecule, magnitude of dipole moment is lower,about 0.74 debye, at B3LYP/6-311++G(d,p) level. Images of HOMOand LUMO are obtained with the aid of B3LYP level, 6-311++G(d,p)basis set and presented in Fig. 4. The HOMO is located on thephenylpiperazine moiety, intensively over the aromatic CAH andthe LUMO is focused on the 1,3,4-oxadiazole and thiophene rings.The positive, negative diffuse and ambient lights are represented inclaret-red and green color, respectively. Fig. 5 shows the TDOS(total density of states) plot provided a diagrammatic view of themolecular orbital contributions, computed by the GaussSum 3.0program [32] at B3LYP/6-31G(d,p). Molecular orbital parameterssuch as electron affinity, ionization potential energy, chemicalhardness and potential, electrophilicity index [50,51] which canbe obtained by HOMO and LUMO orbital energies have been addedto Table 3.

Conclusions

The calculations of the structural geometry, experimental andtheoretical vibrational analysis of the title compound are per-formed in this study. All the calculations have been performed atthe HF and DFT-B3LYP approach. Taking into account the sum ofelectronic energy and ZPE, the optimized structure at B3LYP/6-311++G(d,p) level of theory have fairly lower energy than HFmethod. Parameters of the optimized structure are generally ingood agreement with results of the experimental studies obtainedfrom literature. Meanwhile, the DFT approach has resulted in ahigher correlation for bond length, this situation is the oppositein the calculation of bond angles computing. The scaled harmonicvibrational frequencies are compared with recorded FT-IR andLaser-Raman spectra, and good compliance between theoreticaland experimental frequencies is determined. The B3LYP methodseems to be more appropriate in comparisons of the vibrationalharmonic frequencies. Minor differences between experimentaland theoretical frequencies attributed to the intramolecular hydro-gen bonding interactions and the large anharmonicity in vibrationsfor the solid phase. The gas phase, solvent-free state, of the titlecompound is taken into account in all theoretical studies. The com-

672 F.A.M. Al-Omary et al. / Journal of Molecular Structure 1076 (2014) 664–672

putation of vibration frequencies are supported by means of PED%analysis. The results for HOMO–LUMO energy gaps and dipolemoment promote to stability of structure optimized by using theDFT/B3LYP approach.

Acknowledgements

The authors would like to extend their sincere appreciation tothe Deanship of Scientific Research at King Saud University forfunding this work through the Research Group Project No. RGP-274.

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