study of vibrational spectra and molecular structure of intermolecular hydrogen bonded...

Journal of Molecular Structure 1004 (2011) 237–247

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Journal of Molecular Structure

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Study of vibrational spectra and molecular structure of intermolecularhydrogen bonded 2-thiohydantoin using Density Functional Theory

Anamika Sharma a, Vineet Gupta a,b, Rashmi Mishra a, Poonam Tandon a,⇑, Shiro Maeda c,Ko-Ki Kunimoto d

a Department of Physics, University of Lucknow, Lucknow 226 007, Indiab Institut UTINAM, UMR CNRS 6213, Université de Franche-Comté, 16 route de Gray, 25030 Besançon cedex, Francec Division of Applied Chemistry and Biotechnology, Graduate School of Engineering, University of Fukui, Fukui, Japand Division of Material Sciences, Graduate School of Natural Science and Technology, Kanazawa University, Kakuma-machi, Kanazawa, Japan

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

Article history:Received 25 March 2011Received in revised form 3 August 2011Accepted 3 August 2011Available online 11 August 2011

Keywords:Hydrogen bondingDensity Functional TheoryIR and Raman spectroscopy2-ThiohydantoinVibrational assignmentsSolid state NMR

0022-2860/$ - see front matter � 2011 Elsevier B.V. Adoi:10.1016/j.molstruc.2011.08.011

⇑ Corresponding author. Tel.: +91 522 2782653; faxE-mail address: [email protected] (P. T

In this work, experimental and theoretical UV, NMR and vibrational spectra of 2-thiohydantoin (2-TH)were studied. We have used a combined FT-IR and FT-Raman spectroscopy along with Density FunctionalTheory (DFT) to study the effect of hydrogen bonding on molecular structure. Comparison between thegas phase and the solid phase data were also carried out. Our results support the hydrogen bonding pat-tern proposed by the reported crystal structure and provide valuable information on the structural rela-tionship between the investigated polymorphs. The ultra violet absorption spectra of the compounddissolved in methanol were examined in the range 210–330 nm. The solid state 13C NMR spectra wererecorded. Isotropic chemical shifts were calculated using the gauge-invariant atomic orbital (GIAO)method. Comparison of the calculated NMR chemical shifts and absorption wavelengths with the exper-imental values revealed that DFT method produces good results.

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1. Introduction

Hydantoins and their derivatives have received much attentionin recent years because of their remarkable pharmaceutical, biolog-ical and industrial applications. 2-THs (2-thioxoimidazolidin-4-one, C3N2H4OS) are the 2-thioxo analogues of hydantoins. 2-THand its derivatives find use in a wide variety of applications, mostnotably as antiandrogen [1,2], anticarcinogenic [3], antimutagenic[4,5], antithyroidal [6,7], antiviral, tuberculosis [8], antimicrobial(antifungal and antibacterial), as well as human immunodeficiencyvirus HIV [9,10]. Apart from their pharmaceutical and biologicalsignificances, this group of molecules has widespread applicationsin the field of textile printing, development of dyes [11,12], metalcation complexation and polymerization catalysis [13,14]. Theirmode of action is complex and is not fully understood. 2-TH carrya thioamide and an amide group in a molecule, which provide equalnumber of proton donor (D) and acceptor (A) in a DAAADAA se-quence. Because of this unique structural feature, 2-TH is expectedto form different complex hydrogen bonding networks in crystal.The new form of 2-TH polymorph has been reported recently byOgawa et al. [15]. Hydrogen bonding patterns in hydantoin andits derivatives has been discussed by Yu et al. [16].

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: +91 522 2740840.andon).

In the course of crystallization experiment, a crystal modifica-tion of 2-TH was found which is different from the previouslyknown A form [17,18]. The major difference between the twoforms lies in the pattern of intermolecular hydrogen bond forma-tion as shown in Figs. 1 and 2. In the crystals of the A form, theamide groups are linked through C@O� � �HAN hydrogen bonds toform cyclic dimers, and amide C@O group is hydrogen bonded tothe thioamide NAH of the neighboring molecule. Whereas, in Bform, the amide and thioamide groups of molecule form centro-symmetric cyclic dimers with the amide and thioamide groups,respectively, of the adjacent molecule through intermolecularNAH� � �O and NAH� � �S hydrogen bonds. The hydrogen bonds ofthe neighboring unit cells appear to play an important role in thestabilization of the crystal form and so it becomes mandatory totake into account such hydrogen bonded interactions betweenthe neighbors. Now days, study of H-bonded systems like DNA[19], RNA, supramolecular structures [20], proteins [21] has be-come the major research interest. As a part of N-halamine chemis-try, it was found that imide and amide N-halamine stabilities onhydantoin rings could be reversed [22]. Verdolino et al. [23] haveperformed the DFT calculation of pKa values of nucleobases andthe Guanine oxidation products Guanidinohydantoin andSpiroiminodihydantoin.

Considering the enormous pharmaceutical and biologicalimportance, we present here the detailed experimental and

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Fig. 1. The structure of 2-thiohydantoin A form. Hydrogen bonds are shown bydotted line.

Fig. 2. The structure of 2-thiohydantoin B form. Hydrogen bonds are shown bydotted line.

238 A. Sharma et al. / Journal of Molecular Structure 1004 (2011) 237–247

theoretical FT-IR, FT-Raman, UV and NMR spectra along with ther-modynamic properties of intermolecular hydrogen bonded 2-THpolymorphs. The geometry optimizations of both forms were per-formed, and the differences in the IR and Raman spectra were inter-preted in terms of changes in molecular structure and hydrogenbonding. The calculated vibrational spectra were analyzed on thebasis of potential energy distribution (PED) of each vibrationalmode, which allowed us to interpret the infrared and Ramanspectra.

2. Experimental details

The FT-IR spectra were recorded on a Perkin Elmer 1650 FT-IRspectrometer for KBr disks, Nujol and hexachlorobutadiene mullsby averaging 64 scans with a resolution of 4 cm�1.

The FT-Raman spectra were obtained on a Perkin-Elmer 2000RFT-Raman spectrometer for powder sealed in a capillary tube. The1064 nm line of an Elforlight Model L04-2000S Nd:YAG laser wasused as the exciting source with an output power of about 200Mw at the sample position. All spectra were accumulated for 60scans with a resolution of 4 cm�1.

The UV spectra were recorded on a UV-2500 spectrophotometer(Shimadzu, Kyoto, Japan) equipped with a 10-nm quartz cell. Themeasurement is done for a 5 � 10�5 M methanol solution at20 �C in the range 210–330 nm.

Solid-state NMR spectra were obtained with a ChemagneticsCMX Infinity 300 spectrometer operating at 75.6 MHz for 13C atroom temperature. The samples were contained in a 5 mm diame-ter cylinder type rotor of zirconia ceramic. The rotor was spun at7.0 kHz. CP/MAS spectra were obtained using contact time of

1 ms, repetition period of 2 s and high power proton decoupling.A 62.6 kHz r.f. field strength was used for a proton 908 pulse, across-polarization and the decoupling. The number of accumula-tion was typically 1000. The 13C chemical shifts were externallyreferred to teramethylsilane.

3. Theoretical details

The vibrational frequencies were calculated for the optimizedgeometries of a series of H-bonded structures of both forms of2-TH, with the harmonic approximation. In the process of geome-try optimization for the fully relaxed method, convergence of allthe calculations and the absence of imaginary values in the wave-numbers confirmed the attainment of local minima on the poten-tial energy surface. All calculations were carried out at the densityfunctional level of theory employing the Becke’s three parameter(local, nonlocal, Hartree–Fock) hybrid exchange functionals withLee–Yang–Parr correlation functionals (B3LYP) [24–26] and6-311++G(d,p) basis set augmented by ‘d’ polarization functionson heavy atoms and ‘p’ polarization functions on hydrogen atomsas well as diffuse functions for both hydrogen and heavy atoms[27,28]. The absolute Raman intensities and IR absorption intensi-ties were calculated in the harmonic approximation, at the samelevel of theory as used for optimized geometries, from the deriva-tives of the dipole moment and polarizability of each normal mode,respectively. It has become customary to scale calculated frequen-cies to facilitate comparisons with experiment. The scaling factorsof Andersson et al. [29] were chosen to scale the frequencies and allfrequencies are scaled by 0.9679. Both Raman and IR spectra weresimulated using line shape of Lorentzian curves type and 8 cm�1

FWHM (the full width at half-maximum) for each peak. The inter-action energies may be affected by the basis set superposition error(BSSE), which is usually corrected by the counterpoise (CP) methodof Boys and Bernardi [30]. We are using moderately large basis setsand so do not believe that the BSSE will significantly affect the con-clusions arrived from our study. The normal mode analysis wasperformed, and the PED was calculated for each internal coordi-nates using localized symmetry [31,32]. For this purpose a com-plete set of internal coordinates was defined for both forms usingPulay’s recommendations [31,32]. All calculations were performedwith the Gaussian03 package [33]. The vibrational assignments ofthe normal modes were proposed on the basis of the PED calcu-lated using program GAR2PED [34].

4. Results and discussions

4.1. Geometry optimizations

The geometry generated from standard geometrical parameterswas minimized without any constraint to the potential energy sur-face and the optimized structural parameters were used in thevibrational frequency calculations. The structure of monomer issame in both 2-TH form A and B, while the dimer and tetramerare different by the intermolecular hydrogen bonding. The geome-try optimization of monomer produced structural parameters(bond lengths, bond angles, dihedral angle) which are almost sim-ilar as given by Walker et al. [17] and Devillanopva et al. [18] forthe A form and by Ogawa et al. [15] for the B form. These experi-mental and calculated (monomer) geometrical parameters arelisted in Table S1 of the Supplementary material. The geometricallyoptimized structures of 2-TH (monomer, dimer and tetramer ofboth forms) and the numbering system adopted for this studyare shown in Fig. 3.

Four molecules appear in the unit cell of form A and two mole-cules per unit cell of form B. The crystal data of the unit cell was the

Fig. 3. Geometrically optimized structure of 2-thiohydantoin. (a) Numbering system adopted for monomer, (b) form A dimer (DA), (c) form B dimer (DB), (d) form A tetramer(TA), (e) form B tetramer (TB).

A. Sharma et al. / Journal of Molecular Structure 1004 (2011) 237–247 239

Table 1Comparison of calculated and experimental vibrational wavenumbers obtained in 2-thiohydantoin in monomer and tetramer.

Monomer Tetramer A Tetramer B

Calculated Calculated Experimental Calculated Experimental

Unscaled Scaled PED Scaled Raman IR Scaled Raman IR

3667 3470 m(N4AH9)(99) 3547,3546,3376,3375

3286 3283 3524,3523,3284,3259

3154 –

3640 3445 m(N3AH8)(99) 3526,3527,3273,3243

– 3197 3248,3247,3219,3218

– 3169

1852 1753 m(C@O)(81) + mring(10) + dring(5) 1793,1792,1720,1696

1693 1716 1794,1795,1757,1741

1723 1736

1547 1464 mring(33) + d(N4AH9)(26) + d(N3AH8)(10) + m(C@S)(9) + dring(6) x(CH2)(5) + d(CH2)(4)

1527,1525,1505,1504

1526 1528 1541,1540,1521,1520

1520 1544

1398 1323 mring(50) + d(N3AH8)(19) + x(CH2)(11) + dring(7) + d(CH2)(5) 1351,1350,1293

1388 1381 1401,1390,1383,1381

1405 1395

1367 1294 d(N4AH9)(37) + d(N3AH8)(20) + mring(26) + d(C@S)(4) + d(C@O)(4) 1355,1354

– – 1369,1368,1359,1358

– –

1302 1232 x(CH2)(46) + mring(27) + dring(7) + d(N3AH8)(6) 1293,1265,1264

1295 1298 1279,1278,1261,1260

1284 1296

1199 1135 mring(59) + d(C@O)(12) + x(CH2)(9) + d(N4AH9)(8) + dring(4) 1210,1207,1170,1169

1223 1223 1205,1202,1174,1173

1210 1210

1165 1103 m(C@S)(27) + mring(23) + d(N3AH8)(20) + dring(10) + d(N4AH9)(6) + x(CH2)(5) 1127,1128,1143,1148

1158 1156 1129,1128

1165 –

1061 1004 mring(77) + d(N4AH9)(11) + m(C@S)(4) 1035,1034,1024,1023

1051 1045 1041,1040,1037,1036

1042 1041

1008 954 q(CH2)(68) + oop(C@O)(18) + sring(11) 978, 977,971, 970

– 978 975, 974,972, 971

– –

972 920 dring(36) + mring(42) + m(C@S)(9) + x(CH2)(6) 956, 950,941, 940

969 964 947, 941,940, 939

955 964

872 825 mring(67) + dring(20) + d(N3AH8)(4) 875, 874,845, 848

894 891 868, 867,848, 847

891 887

675 639 dring(66) + mring(19) + m(C@O)(5) + d(N3AH8)(4) 673, 668,655, 654

– 771 666, 665,664, 663

– 779

659 624 oop(C@S)(54) + sring(40) 638, 637,628, 627

680 687 631, 630,628, 627

677 679

620 587 oop(N3AH8)(65) + oop(C@O)(17) + sring(11) 588, 586 – – 585, 584 – –556 526 oop(C@O)(45) + q(CH2)(14) + oop(N3AH8)(13) + oop(C@S)(12) + c(CH2)(8) + sring(6) 553, 547,

528, 527545 631 547, 526,

525,550 632

515 487 dring(31) + d(C@O)(26) + m(C@S)(23) + mring(8) 523, 509 521 530 516, 506 518 532494 468 d(C@O)(42) + d(C@S)(17) + m(C@S)(15) + mring(15) 495, 489,

488, 481505 – 488, 487,

481, 480,499 –

424 401 oop(N4AH9)(85) + oop(N3AH8)(6) + oop(C@S)(4) 436, 335 – – 300, 296 – –278 263 d(C@S)(73) + d(C@O)(10) + mring(13) 295, 294,

274, 273288 – 274, 276,

296289

149 141 sring(62) + oop(N4AH9)(23) + oop(N3AH8)(13) 190, 189,180, 179

– – 178, 182,183

– –

112 106 oop(N4AH9)(38) + sring(51) + oop(N3AH8)(10) – – – – –


starting point used for the simulation of solid form. Moreover, thesolid state was simulated by tetramer, thus improving the obtainedresults. Form A and B constitute entirely two different kinds of di-mers i.e. DA and DB respectively. The H-bond length in DA is 2.022 Åwhile that in DB is 2.402, with rest of the bond lengths, angles andtorsions in accordance with the monomer molecule. The B form

tetramer (TB) is the combination of DB and DA arranged alternatelygiving rise to a sheet like structure with H-bond values as 2.402 Åand 1.899 Å respectively as shown in Fig. 3. In TB, the value of H-bond length for DB remains the same, while for DA, the H-bondlength decreases from 2.022 Å to 1.899 Å i.e. by an amount0.123 Å. The reason being oxygen is more electronegative than


sulfur, and so the H-bond attraction is more in O—H as comparedto S—H. For A form tetramer (TA) the H-bond lengths are 2.022 Åand 1.932 Å.

4.2. Vibrational assignments

The total number of atoms in 2-TH monomer, dimer and tetra-mer are 11, 22 and 44 respectively; hence it gives 27, 60 and 126(3N-6) normal modes. Here N is the number of atoms in the mol-ecule. The equilibrium geometry of the molecule under investiga-tion is planar, and the plane of the ring is the plane of symmetryof the molecule, with the molecule belonging to Cs point group.

Fig. 4. Comparison of experimental infrared absorp

Fig. 5. Comparison of experimental Raman s

As a consequence, all the fundamental vibrations of free moleculeare both IR and Raman active.

DFT calculations yield Raman scattering amplitudes which can-not be taken directly to be the Raman intensities. The Raman scat-tering cross section, or/oX, which are proportional to Ramanintensity may be calculated from Raman scattering amplitudeand predicted wavenumbers for each normal mode using the rela-tionship [35,36]

@rj

@X¼ 24p4

45

!ðm0 � mjÞ4

1� exp �hcmj

kT

h i0@

1A h

8p2cmj

� �Sj ð1Þ

tion spectra of 2-thiohydantoin A and B form.

pectra of 2-thiohydantoin A and B form.

Fig. 6. Experimental and calculated (scaled) infrared absorbance spectra of 2-thiohydantoin A form in the region 500–2000 and 2800–3510 cm�1.

Fig. 7. Experimental and calculated (scaled) Raman spectra of 2-thiohydantoin A form in the region 200–1900 and 2750–3510 cm�1.


where Sj and mj are the scattering activities and the predicted wave-numbers, respectively of the jth normal mode, m0 is the wavenum-ber of Raman excitation line, T is the temperature and h, c and k areuniversal constants. The Raman intensities obtained using this rela-tionship match quite nicely with the experimentally observedintensities.

The calculated Raman and IR intensities were used to convoluteeach predicted vibrational mode with a Lorentzian line shape(FWHM = 8 cm�1) to produce simulated spectra. Assignments havebeen made on the basis of relative intensities, energies, line shapeand PED. All the vibrational bands have been assigned satisfactorily.The computed harmonic vibrational bands sensitive to hydrogenbonding are given in Table 1 for monomer and tetramer, whilethe other vibrational modes are listed in Table S2 of Supplementarymaterial. The first column lists the calculated (unscaled) wavenum-bers (cm�1) for monomer, while the 2nd column collects their

scaled values. The calculated % PED of the different modes of eachvibration appears in the 3rd column. Contributions lower than 3%were not considered. The values calculated (scaled) for TA and TB

are collected in columns 4 and 7 respectively. The correspondingexperimental Raman and IR values for form A are listed in column5 and 6, and that for form B are given in column 8 and 9 respec-tively. Four wavenumbers appear for each vibration correspondingto the four 2-TH molecules of the tetramer. Of these four wavenum-bers the one which matches the best is shown in bold type.

4.3. Vibrational wavenumbers

Comparison of calculated wavenumbers at the B3LYP/6-311++G(d,p) level with experimental values reveals anoverestimation of the wavenumber of the vibrational modes dueto neglect of anharmonicity present in real system. Since the

Fig. 8. Experimental and calculated (scaled) infrared absorbance spectra of 2-thiohydantoin B form in the region 500–2000 and 2700–3510 cm�1.

Fig. 9. Experimental and calculated (scaled) Raman spectra of 2-thiohydantoin B form in the region 200–2000 and 2800–3550 cm�1.

Table 2Comparison between experimental and calculated 13C NMR chemical shifts (ppm) for 2-thiohydantoin polymorphs.

Form A Form B

Atom no. Calculated Expt D Atom no. Calculated Expt D

C (5) 176.12 183.16 7.04 C (4) 176.31 178.51 2.2C (6) 53.51 56.12 2.61 C (5) 53.58 53.66 0.08C (7) 173.98 180.26 6.28 C (6) 174.43 182.84 8.41


Fig. 10. Experimental solid state 13C NMR spectra of 2-thiohydantoin: (a) form Aand (b) form B.

Fig. 11. Plot of the computed versus experimental 13C relative chemical shifts of2-thiohydantoin form A and B at GIAO-B3LYP level with 6-311++G(d,p) basis set.


vibrational wavenumbers obtained from the DFT calculations arehigher than the experimental wavenumbers, they were scaleddown by 0.9679 and a comparison was made with the experimen-tal values. The scaling predicts vibrational wavenumbers with highaccuracy and is applicable to a large number of compounds, exceptfor those where the effect of dispersion forces is significant [37].The vibrational wavenumbers calculated with appropriate func-tional are often in good agreement with the observed wavenum-bers when the calculated wavenumbers are uniformly scaledwith only one scaling factor [38,39].

Figs. 4 and 5 show a comparison between experimental IR andRaman spectra for form A and B respectively. In the IR spectra, Aform shows two NAH stretching frequencies at 3283 cm�1 and3197 cm�1, whereas B form shows only one NAH stretching fre-quency. The two absorptions in the A form comes from N4AH9and N3AH8 stretching vibrations, and we observe these modesseparately in IR because the hydrogen bonding system is not sym-metrical in the A form. In the B form, where the hydrogen bondingis symmetric i.e. those formed by NAH� � �S@C and NAH� � �O@Ccyclic dimers, we observe only one NAH stretching frequency at3169 cm�1. This band is due to out-of-phase stretching of thetwo NAH bonds involved in the dimer formation. We observe itscounterpart in-phase mode in Raman spectra at 3154 cm�1. A formcrystals show the C@O stretching band at 1716 cm�1 in IR and at

1693 cm�1 in Raman spectra. These two bands correspond to theout-of-phase and in-phase modes of the cyclic dimer type C@Ogroups. Similarly, B form crystals show the C@O stretching at1736 cm�1 in IR and at 1722 cm�1 in Raman spectra. These twobands correspond to in-phase and out-of-phase vibrations of thetwo C@O groups involved in dimer formation.

The effect of hydrogen bonding on the vibrational spectra wasexamined by explicitly taking four molecules in crystal structure.The NH stretching modes shift to lower frequencies and the mostintense C@O stretching band in monomer at 1753 cm�1 is shiftedto lower frequency in tetramer on including C@O� � �NAH interac-tions. Vibrational spectra computed for tetramer provided a goodfit to the observed IR and Raman spectra, consequently it is logicalto extend this strategy to include molecular interactions. The cal-culated (scaled) and experimental FT-IR and FT-Raman spectrafor form A and B are compared in Figs. 6–9.

4.3.1. NAH vibrationsStarting from high wavenumbers, the first feature observed in

the vibrational spectra is the stretching vibration of the NAH bondpresent in molecule. This band is intense in infrared spectra butweak in Raman spectrum. An important feature of hydrogen bond-ing is a decrease of the XAH stretching frequency (red shift), whichis usually accompanied by an elongation of the XAH bond and anincrease in the infrared intensity and broadening of the spectralband associated with the XAH stretching frequency (where X isthe element such as N, O or F with an electronegativity greater thanhydrogen). The detection of the red shift in IR spectrum is regardedas the ‘‘signature’’ of the hydrogen bonding [40].

The molecule under investigation possesses two NH groups i.e.N4AH9 and N3AH8. The N4AH9 stretch is calculated to be at3470 cm�1 and 3375 cm�1 for monomer and tetramer TA respec-tively corresponding to the observed modes at 3286 cm�1 and3283 cm�1 in Raman and IR spectra respectively. For tetramer TB

this particular stretching mode is calculated to be at 3259 cm�1

and is observed at 3154 cm�1 in Raman spectrum only. For mono-mer, the N4AH9 bending mode is calculated to be at 1294 cm�1

having contribution of 37% in PED. For TA and TB this bending modeis found to be at 1355 and 1369 cm�1 respectively. The otherN3AH8 bending, ring stretching, C@S bending and C@O bendingmodes are having contribution of 20%, 26%, 4% and 4 % respec-tively. On moving from free to bond state, a shift of 61Acm�1

Table 3Electronic transitions, absorption wavelengths kmax (nm), oscillator strength (f) of 2-thiohydantoin in gas phase and methanol solution.

Excited state Exp. kmax Calculated

Gas phase Methanol

k (nm) Excitation E (ev) f k (nm) Excitation E (ev) f

1 263 329 H ? L 3.7716 0.0001 307 H ? L 4.0447 0.00012 222 247 H-1 ? L 5.0128 0.2396 243 H-1 ? L 5.0920 0.31363 239 H-2 ? L 5.1843 0.0010 237 H-2 ? L 5.2404 0.0012

HOMO

LUMO

Fig. 12. Frontier orbitals of 2-thiohydantoin by DFT/B3LYP/6-31G method.


and 75Acm�1 towards higher wavenumber side is obtained forform A and B respectively. The reason for this shift is that thismode contains a contribution of 20% from d(N3AH8), which is in-volved in hydrogen bond formation.

The stretching mode of N3AH8 is calculated to be at 3445 cm�1

for monomer, 3243 cm�1 for TA and corresponds to observed modeat 3197 cm�1 in IR spectrum. For TB this stretching mode occurs at3218 cm�1 and corresponds to 3169 cm�1 in IR spectrum. Whilemoving from monomer to tetramer a shift of 200 cm�1 and 227cm�1 towards lower wavenumber side is obtained for form A andB respectively. The broadening and shifting of this band towardslower wavenumbers pointed out the involvement of this bond inhydrogen bond formation.

4.3.2. C@O vibrationsThe C@O stretching vibration is calculated to be at 1753 cm�1,

and 1696 cm�1 for monomer and tetramer TA respectively corre-sponding to observed peak at 1693 cm�1 and 1716 cm�1 in Ramanand IR spectra respectively. For TB this particular mode is calcu-lated to be at 1741 cm�1 and corresponds to 1723 cm�1 and1736 cm�1 in Raman and IR spectra respectively. On moving frommonomer to tetramer, we get a shifting of 57 cm�1 and 12 cm�1

respectively towards lower wavenumber side, as expected, thusexhibiting standard behavior. The contribution of C@O stretchingto this mode is 81% in PED. Deformation d(C@O) is calculated to

be at 468 cm�1 and 495 cm�1 for monomer and tetramer TA

respectively and corresponds to 505 cm�1 in Raman spectrum.For TB this mode occurs at 488 cm�1 and corresponds to 499cm�1 in Raman spectrum.

4.3.3. C@S vibrationsThe C@S stretching mode is calculated to be at 1103 cm�1, and

1148 cm�1 for monomer and tetramer TA respectively correspond-ing to the observed peak at 1158 cm�1 and 1156 cm�1 in Ramanand IR spectra respectively. For TB this particular mode is calcu-lated to be at 1129 cm�1 and corresponds to 1165 cm�1 in Ramanspectrum only. On moving from monomer to tetramer, we get ashift of 45 cm�1 and 26 cm�1 towards the higher wavenumber sidefor form A and B respectively. This might be possible as this modealso contains a significant contribution of 20% from bending moded(N3AH8), which takes part in hydrogen bonding. Bending of C@Smode is calculated to be at 263 cm�1, and 274 cm�1 for monomerand tetramer TA respectively and corresponds to 288 cm�1 in Ra-man spectrum. For TB, this band is calculated to be at 276 cm�1

corresponding to observed peak at 289 cm�1 in Raman spectrum.

4.4. NMR spectra

Both the polymorphs of 2-TH gave an identical NMR spectrumin the solution form. So, in order to differentiate between boththe spectra, the 13C NMR spectra of both forms are those basedon the CP/MAS 13C NMR measurements in solid phase. In thesespectra we get the 13C signals which are completely decoupled,so we do not get any splitting of the 13C signal from the attachedprotons. All the CH3, CH2, CH carbons are observed as singlet, sowe have compared the observed 13C chemical shifts with the aver-age of the calculated values.

It is well recognized that accurate predictions of moleculargeometries are essential for reliable calculations of magneticproperties. Therefore, full geometry optimization of both the poly-morphs were performed using B3LYP method with 6-311++G(d,p)basis set using Gaussian 03W program [33] and GIAO (gauge-including atomic orbital) method [41] under the keywords‘nmr = spinspin’. The relative chemical shifts were then estimatedby using corresponding TMS shielding calculated in advance atthe same level as the reference (d/ppm = dTMS � dcalc). The calcu-lated 13C isotropic chemical shielding for TMS was 183.97ppm.The comparisons between experimental and theoretical results,along with the error are presented in Table 2. The experimental so-lid state 13C NMR spectra for both forms are shown in Fig. 10.

The calculated 13C NMR chemical shifts for the ring carbons ofboth forms of 2-TH gave satisfactory agreement with the experi-mental data. The correlation between the experimental and calcu-lated chemical shift values is plotted in Fig. 11, which shows a goodlinear relationship. The following equation was obtained betweencalculated and experimental 13C NMR chemical shiftsy = �0.78741 + 0.96766 x, where x and y are the experimentaland calculated 13C NMR chemical shifts (d/ppm). The chemical shiftis expressed in the amount by which a proton resonance is shifted


from TMS, in parts per million (ppm), of the spectrometer’s basicoperating frequency. The correlation coefficient R2 is 0.9999. Meth-ylene carbon C6 and C5 for form A and B of 2-TH appears at56.12ppm and 53.66 ppm respectively. C6 carbon appears at moredownfield as compared to C5. The higher frequencies signal forC@S and C@O in both forms is due to the attachment of the electro-negative atom. The packing of both the molecules are different. Theshifting was observed in the spectrum due to the intermolecularhydrogen bonding in both A and B form.

4.5. UV–visible spectroscopy and solvent effect

Both forms of 2-TH gave identical UV spectra in the methanolsolution. The electronic absorption spectra computed in gas phaseand in methanol environment are presented in Table 3. The solventeffect was calculated using PCM [42,43]-TD-DFT [44] method and6-31G basis set. The experimental UV–vis spectrum of 2-TH inmethanol shows two transitions at 222 nm and 263 nm (Fig. S6of Supporting material).

Both the highest occupied molecular orbital (HOMO) and thelowest unoccupied molecular orbital (LUMO) are the main orbitalstaking part in the chemical reaction. The HOMO energy character-izes the ability of electron giving while LUMO energy characterizesthe ability of electron accepting and the gap between HOMO andLUMO characterizes the molecular chemical stability [45]. HOMOand LUMO of 2-TH are shown in Fig. 12. The computed results re-veal that the first excited state originates from the HOMO to LUMOtransition that corresponds to kmax absorption band in the UV–visspectrum. From the calculations in the gas phase as well as inmethanol environment, it is assigned that the frontier level HOMOhas the molecular orbital number 30 with ‘A’ symmetry and theLUMO has the molecular orbital number 31 with the same ‘A’ sym-metry. In HOMO the charge density is mainly accumulated on thethio group. However, in case of LUMO, more charge density movesto thiohydantoin ring and is also accumulated on thio group. TheHOMO LUMO shows that a donor-to-acceptor charge transfer(CT) interaction occurs in 2-TH. The energy gap between HOMOand LUMO is a critical parameter in determining molecular electri-cal transport properties because it is a measure of electron conduc-tivity. Comparison between calculated band gap E and total energyof 2-TH in gas phase and methanol is shown in Table S3 of Supple-mentary material. Shifting is observed between calculated (gasphase) and experimental values. The shifting is due to high polarityof methanol and hence there is a high chance of solvent–solutespecific interaction. The specific interaction of solvent soluteH-bonding causes the red shift in kmax.

4.6. Thermodynamic properties

On the basis of vibrational analysis and statistical thermody-namics, the standard thermodynamic functions: heat capacity(C�

p;m), entropy (S�

m) and enthalpy (H�

m) were obtained and listedin Table S4 of Supplementary material. As observed from the table,values of heat capacity, entropy and enthalpy increases with theincrease of temperature from 100 to 500 K, which is attributed tothe enhancement of molecular vibration while the temperature in-creases. Both forms have almost same values of entropy, enthalpyand heat capacity.

The correlation between these thermodynamic properties andtemperatures T are shown in Fig. S7 of Supplementary material. Thecorrelation equations for both forms are similar and are as follows:

C�

p;m ¼ 3:251þ 0:06965T � 1:95642E � 10�5T2 ðR2 ¼ 0:99941Þ

S�

m ¼ 48:8892þ 0:00435T þ 2:96643E � 10�5T2 ðR2 ¼ 0:99998Þ

H�

m ¼ 51:355þ 0:11172T � 4:61643E � 10�5T2 ðR2 ¼ 0:99983Þ

These equations could be used for the further studies on the ti-tle compound. For instance, when the interaction of title com-pound with another compound is studied, these thermodynamicproperties could be obtained from the above equations and thencan be used to calculate the change in Gibbs free energy of thereaction, which will in turn help to judge the spontaneity of thereaction. The total energy, zero-point energy, rotational constants,entropy, dipole moment calculated at room temperature(298.15 K), are shown in Table S5 of Supplementary material.

5. Summary and conclusions

(1) In the present work, a combined experimental (IR, Raman,UV and NMR) and theoretical (DFT(B3LYP)/6-311++G(d,p))approach has been presented for both A and B forms of2-TH. The differences in geometries of both forms were stud-ied in term of intermolecular H-bonding. An effort is made tointerpret the changes that occur in the vibrational spectra onmoving towards a more stable configuration. A detailed nor-mal coordinate analysis of all the normal modes along withPED allows the composition of each normal mode in terms ofinternal coordinates.

(2) On moving from monomer to tetramer, a remarkabledecrease in the stretching frequencies of NAH and C@Obands clearly predicted their involvement in H-bond forma-tion. In case of monomer molecule, the discrepancy betweenthe observed and calculated frequencies is due to the factthat calculations have been actually done on a single mole-cule contrary to the experimental values recorded in thepresence of intermolecular interactions. In the simulationwith tetramer, the scaled wavenumbers appear in accor-dance with the experimental data in solid state.

(3) The UV spectrum was measured in the methanol solution.The solvent effect was also taken into account usingPCM-TD-DFT method and 6-31G basis set. All theoreticalresults were compared with experimental and are found tobe in good agreement.

(4) The CP/MAS 13C NMR spectra of the compound wererecorded and on the basis of the calculated and experimentalresults assignment of the chemical shifts were done. Tem-perature dependent thermodynamic parameters were pre-sented in the temperature range of 100–500 K for both Aand B form of 2-TH, these parameters increases with tem-perature and are quite similar in both forms.

Appendix A. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.molstruc.2011.08.011.

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study of vibrational spectra and molecular structure of intermolecular hydrogen bonded...

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