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Spectrochimica Acta Part A 90 (2012) 141– 151

Contents lists available at SciVerse ScienceDirect

Spectrochimica Acta Part A: Molecular andBiomolecular Spectroscopy

j ourna l ho me page: www.elsev ier .com/ locate /saa

xperimental (FT-IR, FT-Raman, NMR) and theoretical spectroscopic properties ofntermolecular hydrogen bonded 1-acetyl-2-thiohydantoin polymorphs

namika Sharmaa, Vineet Guptab, Poonam Tandona,∗, Poonam Rawatc, Shiro Maedad, Ko-Ki Kunimotoe

Department of Physics, University of Lucknow, Lucknow 226007, IndiaDepartment of Physics, Banaras Hindu University, Varanasi 221005, IndiaDepartment of Chemistry, University of Lucknow, Lucknow 226007, IndiaDivision of Applied Chemistry and Biotechnology, Graduate School of Engineering, University of Fukui, Fukui, JapanDivision of Material Sciences, Graduate School of Natural Science and Technology, Kanazawa University, Kanazawa, Japan

r t i c l e i n f o

rticle history:eceived 19 July 2011eceived in revised form0 December 2011ccepted 16 January 2012

a b s t r a c t

In this work, use of FT-Raman, FT-IR and 13C NMR spectroscopies have been made for the full charac-terization of 1-acetyl-2-thiohydantoin (ACTH). A detailed interpretation of the vibrational spectra wascarried out with the aid of normal coordinate analysis using single scaling factor. Our results support thehydrogen bonding pattern proposed in the reported crystalline structure. Good reproduction of exper-imental values is obtained and % error is small in majority of the cases. Isotropic chemical shifts werecalculated using gauge-invariant atomic orbital (GIAO) along with several thermodynamic parameters.

eywords:T-IRydrogen bonding-Acetyl-2-thiohydantoinolid state NMRFTibrational analysis

© 2012 Elsevier B.V. All rights reserved.

. Introduction

2-Thiohydantoin derivatives provide useful synthetic interme-iates with a wide range of applications such as therapeutics,ungicides and herbicides [1–3], cytotoxic and antiviral agent [4]. Inontinuation of our previous work on 2-thiohydantoin [5], in thisork the attempts have been made to fully characterize its acetylerivative. The title compound 1-acetyl-2-thiohydantoin (ACTH, 1-cetyl-2-sulfanylideneimidazolidin-4-one, C5H6N2O2S) as shownn Fig. 1 is an important heterocyclic compound which is stronglyydrogen bonded in the solid state. It exists in two polymorphic

orms, i.e. the known form (form A) and the newly discoveredorm (form B). The new crystal modification (form B) was acci-entally obtained by slow evaporation of the ethanol solution ofCTH [6], whose crystal parameters entirely differ from the previ-

usly reported form A [7]. Form A crystallizes in monoclinic form,pace group P21/n with a = 8.2968 A, b = 7.7364 A, c = 10.6066 A and

= 93.434◦. Form B belongs to triclinic system, space group PI with

∗ Corresponding author. Tel.: +91 522 2782653; fax: +91 522 2740840.E-mail addresses: poonam tandon@hotmail.com, poonam tandon@yahoo.co.uk

P. Tandon).

386-1425/$ – see front matter © 2012 Elsevier B.V. All rights reserved.oi:10.1016/j.saa.2012.01.033

a = 4.9865 A, b = 5.5716 A, c = 12.544 A, = 74.793◦, = 80.413◦ and� = 85.001◦. The plane of the bulky acetyl group forms an angle of6.7◦ with the essentially planar thiohydantoin ring.

The major difference between the two forms lies in the pattern ofintermolecular hydrogen bond formation as shown in Figs. 2 and 3.In both the polymorphic forms, the neighbouring molecules arelinked through intermolecular H-bonds to form an infinite chainstructure. In Form B, the H-bonds are formed between the acetylC O and the amide NH groups, whereas, in form A, they are formedbetween the amide C O and the amide NH groups of the thiohy-dantoin ring. The hydrogen bonds of the neighbouring unit cellsappear to play an important role in the stabilization of the crys-tal form and so it becomes mandatory to take into account suchhydrogen bonded interactions between the neighbours. Ellis andGriffiths [8] discussed about the hydrogen bonding in some het-erocyclic thioamides. While performing a separate experiment,Taniguchi et al. [6] found that the ground state sample of B formtransformed into form A within a week, showing that form B is ametastable form. Nowadays, study of hydrogen bonded systems has

gained much research interest [9–11]. Recently, H-bond patternswere studied for rac-1-acetyl-5-methyl-2-thioxoimidazolidine-4-one [12], 5-benzyl-2-thiohydantoin and its acetylated derivative[13].

142 A. Sharma et al. / Spectrochimica Ac

eootb

Fig. 1. Schematic of 1-acetyl-2-thiohydantoin.

In order to better correlate vibrational spectra of ACTH, consid-rable research effort has been invested. The polymorphic behavior

f drugs is a major concern due to different packing arrangementsf the same molecule can have significantly different pharmaceu-ical properties. Considering the enormous pharmaceutical andiological importance, we present here the detailed experimental

Fig. 2. The structure of ACTH form A. Non-bonded conta

Fig. 3. The structure of ACTH form B. Non-bonded conta

ta Part A 90 (2012) 141– 151

and theoretical FT-IR, FT-Raman and NMR spectra along with ther-modynamic properties of intermolecular hydrogen bonded ACTHpolymorphs. The aim of this study is to fully determine the molec-ular structure, vibrational modes and wavenumbers, isotropicchemical shifts of the compound and to understand the effect uponvibrational spectra as we move from gas phase to solid phase.Vibrational spectra have been extensively used to characterize thestructure of A and B form of ACTH. The differences in the IR andRaman spectra were interpreted in terms of changes in molecularstructure and hydrogen bonding. Intermolecular hydrogen bondsthat stabilize the molecular structure are missing in monomer.We have selected hydrogen-bonded A and B forms. The completeassignment of the vibrational spectra of in the crystalline state isquite difficult, and in order to assist this difficulty, theoretical cal-culations are used to support a reliable assignment by potentialenergy distribution (PED) of each vibrational mode, which allowedus to interpret the infrared and Raman spectra. The theoreticalcalculations using density functional theory, to assist vibrationalspectroscopic studies has been highlighted. The vibrational andNMR spectroscopic studies performed in the present work providestrong support for the presence of two polymorphs of ACTH.

2. Experimental details

ACTH was prepared by reacting glycine with acetic anhydrideand ammonium thiocyanate [14]. The crude product was washed

cts and hydrogen bonds are shown by dotted line.

cts and hydrogen bonds are shown by dotted line.

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A. Sharma et al. / Spectrochim

ith cold water and purified by repeated crystallization fromethanol solution.The FT-IR spectra were recorded on a PerkinElmer 1650

T-IR spectrometer as KBr disks, and Nujol and hexachlorobu-adiene mulls by averaging 64 scans with a resolution of

cm−1.The FT-Raman spectra were obtained on a PerkinElmer 2000R

pectrometer as powder sealed in a capillary tube. The 1064 nmine of an Elforlight Model L04-2000S Nd:YAG laser was used ashe exciting source with an output power of about 200 mW at theample position. All spectra were accumulated for 60 scans with aesolution of 4 cm−1.

The solid-state NMR spectra were obtained with a Chemagnet-cs CMX Infinity 300 spectrometer operating at 75.6 MHz for 13C atoom temperature. The samples were contained in a 5 mm diam-ter cylinder type rotor of zirconia ceramic. The rotor was spun at.0 kHz. 13C CP/MAS NMR spectra were obtained using contact timef 1 ms, repetition period of 2 s and high power proton decoupling.

62.6 kHz r.f. field strength was used for a proton 908 pulse, a cross-olarization and the decoupling. The number of accumulation wasypically 1000. The 13C chemical shifts were externally referred toeramethylsilane.

Differential scanning calorimetry (DSC) was measured with ahimadzu DSC60 apparatus at a heating rate of 4 ◦C/min under aowing nitrogen gas.

. Computational details

The vibrational frequencies at the harmonic approximation of aeries of H-bonded structures of both forms of ACTH were calcu-ated from their optimized geometries. In the process of geometryptimization for the fully relaxed method, convergence of all thealculations and the absence of imaginary values in the wavenum-ers confirmed the attainment of local minima on the potentialnergy surface. All calculations were carried out at the densityunctional level of theory employing the Becke’s three param-ter (local, nonlocal, Hartree-Fock) hybrid exchange functionalsith Lee–Yang–Parr correlation functionals (B3LYP) [15–17] and

-311++G(d,p) basis set augmented by ‘d’ polarization functionsn heavy atoms and ‘p’ polarization functions on hydrogen atomss well as diffuse functions for both hydrogen and heavy atomsere used [18,19]. The absolute Raman intensities and IR absorp-

ion intensities were calculated in the harmonic approximation, athe same level of theory as used for optimized geometries, from theerivatives of the dipole moment and polarizability of each normalode, respectively. It has become customary to scale calculated

requencies to facilitate comparisons with experiment. The scal-ng factors of Andersson and Uvdal [20] were chosen to scale therequencies and all frequencies are scaled by 0.9679. Raman andR spectra were simulated using line shape of Lorentzian curvesype and the FWHM (the full width at half-maximum) of each peakas 8 cm−1. The interaction energies may be affected by the basis

et superposition error (BSSE), which is usually corrected by theounterpoise (CP) method of Boys and Bernardi [21]. We are usingoderately large basis sets and so do not believe that the BSSEill significantly affect the conclusions derived from our study.

he normal mode analysis was performed, and the PED was cal-ulated for each internal coordinates using localized symmetry

22,23]. For this purpose a complete set of internal coordinates wasefined for both the ACTH polymorphs using Pulay’s recommenda-ions [22,23]. All calculations were performed with the Gaussian03ackage [24]. The vibrational assignments of the normal modesere proposed on the basis of the PED calculated using programAR2PED [25].

ta Part A 90 (2012) 141– 151 143

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 reference geometries for our vibrational frequencycalculations of ACTH A and B forms are the ones that were fullyoptimized by the same DFT methods characterize all stationarypoints as minima. The structure of monomer is same in both ACTHform A and B, while the trimer and tetramer are different by theintermolecular hydrogen bonding. The geometry optimization pro-duced the A and B form of ACTH whose structural parameters(bond lengths, bond angles, dihedral angle) are fairly similar asgiven by Casas et al. [7] and Taniguchi et al. [6], respectively. Theseexperimental and calculated (monomer) geometrical parametersare listed in Table S1 of the supporting material. The thiohydan-toin ring bond lengths in case of ACTH differ from those found in2-thiohydantoin (2-TH) [26], and are listed in Table S2 of the sup-porting material. Four molecules appear in the unit cell for formA and two molecules per unit cell for form B. As a stable sys-tem we chose tetramer for form B and trimer for form A, becausefor a particular molecule all its H-bonding sites are satisfied. In Aform trimer (TA), the non-bonded O· · ·S [27,28] contact is 3.314 Aand the hydrogen bond length in O· · ·H is 2.086 A. In B formtetramer (TB), the non-bonded contact in O· · ·O [27,29] is 2.953 Aand the hydrogen bond length of in O· · ·H is 2.800 A as shownin Fig. 4.

An overall agreement between theoretical and experimentalmeasurements is observed. The most substantial deviation in calcu-lated bond lengths occurred in the increase of C9-O3 bond lengthby 0.01 A in B form, also, the C6 N5 bond in both form of ACTHis slightly shortened including hydrogen bonding in solid stateusing the DFT calculations. The geometry optimization with theinclusion of electron correlation should be important for moreaccurate prediction. We note that intermolecular hydrogen bond-ing is important in the crystalline state of ACTH. The largestdeviations in calculated bond lengths occur in bonds contain-ing atoms involved in multiple hydrogen bonds. The optimizedparameters in both A and B forms of ACTH are in reasonableagreement with the solid-state experimental data. Hence, we mayconclude that the large discrepancy between the gas phase andthat reported from the X-ray crystal structure can be attributed tothe effect of intermolecular interaction in the molecular geome-try. Considering the overall excellent performance of DFT methodsin geometry optimization for similar compounds in this workand in our previous work [5], the calculated structure for ACTHought to be accurate enough to represent the real structure of thecompound.

4.2. Vibrational assignments

The total number of atoms in ACTH monomer, form A trimer(TA) and form B tetramer (TB) are 16, 48 and 64, respectively;hence it gives 42, 138 and 186 (3N − 6) normal modes. Here N isthe number of atoms in the molecule. The molecular conformationobtained from crystalline structure, as well as the one yielded bygeometry optimization, exhibits no special symmetries and hencethe molecule belongs to C1 point group. As a consequence, all thefundamental vibrations of free molecule are both IR and Ramanactive.

The DFT calculations yield Raman scattering amplitudes whichcannot be taken directly to be the Raman intensities. The Ramanscattering cross section, ∂�/∂˝, which are proportional to Ramanintensity may be calculated from Raman scattering amplitude and

144 A. Sharma et al. / Spectrochimica Acta Part A 90 (2012) 141– 151

Fig. 4. Geometrically optimized structure of ACTH. (a) Numbering system adopted for form B. (b) Form A trimer (TA). (c) Form B tetramer (TB).

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A. Sharma et al. / Spectrochim

redicted wavenumbers for each normal mode using the relation-hip [30,31]:

∂�j

∂˝=

(24�4

45

) ((�0 − �j)

4

1 − exp[−hc�j/kT]

) (h

8�2cvj

)Sj

here Sj and �j are the calculated scattering activities and the pre-icted wavenumbers, respectively of the jth normal mode, �0 is theavenumber of Raman excitation line, T is the temperature and h,

and k are universal constants. The Raman intensities obtainedsing this relationship match quite nicely with the experimentallybserved intensities.

The calculated Raman and IR intensities were used to convo-ute each predicted vibrational mode with a Lorentzian line shapeFWHM = 8 cm−1) to produce simulated spectra. Assignments haveeen made on the basis of relative intensities, energies, line shapend PED. All the vibrational bands have been assigned satisfacto-ily. The harmonic vibrational bands computed in monomer and intable system (solid form) are shown in Table 1. The first columnists the calculated (unscaled) wavenumbers (cm−1) for monomer,

hile the 2nd column collects their scaled values. The calculated %ED of the different modes of each vibration appear in the 3rd col-mn. Contributions lower than 3% were not considered. The valuesalculated (scaled) for TA and TB are collected in columns 4 and 7,espectively. The corresponding experimental IR and Raman valuesor form A are listed in column 5 and 6, and that for form B are givenn column 9 and 8 respectively. Four wavenumbers appear for eachibration corresponding to the four ACTH molecules of the tetramern form B whereas three wavenumbers appear for each vibrationorresponding to the three molecules of the A form trimer. Of theseavenumbers the one which matches the best is shown in bold

ype.

.3. Vibrational wavenumbers

Comparison of calculated wavenumbers at the B3LYP/6-11++G(d,p) level with experimental values reveals an overesti-ation of the wavenumber of the vibrational modes due to neglect

f anharmonicity present in real system. Since the vibrationalavenumbers obtained from the DFT calculations are higher than

he experimental wavenumbers, they were scaled down by 0.9679nd a comparison was made with the experimental values. Thecaling predicts vibrational wavenumbers with high accuracy ands applicable to a large number of compounds, except for those

here the effect of dispersion forces is significant [32]. Also, theibrational wavenumbers calculated with appropriate functionalre often in good agreement with the observed wavenumbers whenhe calculated wavenumbers are uniformly scaled with only onecaling factor [33,34]. Figs. 5 and 6 shows a comparison betweenxperimental IR and Raman spectra for form A and B, respectively.

.3.1. N H vibrationsThe N H stretching vibrations of aromatic primary amines give

ise to higher wavenumbers than aliphatic amines. This band isntense in infrared spectra but shows only low intensity in Ramanpectrum. For monomer, the N H stretch is calculated to be at503 cm−1 and at 3294 cm−1 for TA and corresponds to 3102 cm−1

n IR and 3107 cm−1 in Raman experimental spectrum. For TB thisarticular stretching mode is calculated to be at 3256 cm−1 and isbserved at 3117 cm−1 in Raman and 3126 cm−1 in IR spectrum.n moving from monomer to stable system, a shift of 209 cm−1

nd 247 cm−1 towards lower wavenumbers is obtained for form A

nd B, respectively. The broadening band shape and shift towardsower wavenumbers pointed out the involvement of this bandn hydrogen bond formation. The out-of-plane deformation for

onomer is calculated to be at 601 cm−1 and at 616 cm−1 for TA and

ta Part A 90 (2012) 141– 151 145

corresponds to 629 cm−1 in Raman spectrum only. For TB this modeis calculated to be 633 cm−1 and is observed at 608 cm−1 in Ramanspectrum only.

4.3.2. C O vibrationsThe molecule under investigation possesses two C O groups.

One of them is directly connected to the ring R[C O] and theother one is connected to the acetyl group C9 O3. For monomer,R[C O] stretch is calculated to be at 1779 cm−1 and at 1750 cm−1

for TA and corresponds to1740 cm−1 in IR and 1747 cm−1 in Ramanexperimental spectrum. For TB this particular mode is calculatedto be at 1773 cm−1 and corresponds to 1766 cm−1 in Raman and1764 cm−1 in IR spectrum. The contribution of C O stretching tothis mode is 80%. On moving from monomer to stable system, ashift of 29 cm−1 towards lower wavenumbers is obtained for formA as expected, thus exhibiting standard behavior. R[C O] deforma-tion for monomer is calculated to be at 445 cm−1 and at 455 cm−1

for TA and corresponds to 479 cm−1 in Raman spectrum only. ForTB this band is calculated to be at 465 cm−1 and is observed at483 cm−1 in Raman spectrum only. For this particular mode, onmoving from isolated state to solid state a shift of 10 cm−1 and20 cm−1 towards higher wavenumbers is obtained for form A andB respectively. The reason being, as C O takes part in hydrogenbonding. The out-of-plane deformation of R[C O] for monomer iscalculated to be at 529 cm−1, at 532 cm−1 for TA and correspondsto 521 cm−1 in Raman spectrum only. For TB this band is calculatedto be at 543 cm−1 and is observed at 588 cm−1 in Raman spectrum.

For monomer the C9 O3 stretching mode is calculated to be at1704 cm−1 and at 1703 cm−1 for TA and corresponds to 1704 cm−1

in IR and 1702 cm−1 in Raman experimental spectrum. For TB thismode is calculated to be at 1703 cm−1 and is observed at 1702 cm−1

in Raman and 1672 cm−1 in IR spectrum. Figs. 7 and 8 showsthe difference between calculated (scaled) and experimental FT-IRabsorbance and Raman spectra respectively for form A.

4.3.3. C S vibrationsThe C S stretching mode occurs at 1039 cm−1 in IR spectra and

at 1037 cm−1 in Raman spectra for form A. The calculated value formonomer is at 1015 cm−1 and for TA at 1021 cm−1. In form B, thismode occurs at 1033 cm−1 in Raman spectra and at 1040 cm−1 inIR spectra. The calculated value is at 1019 cm−1. The out-of-planedeformation of C S is calculated to be at 625 cm−1 for monomer,at 631 cm−1 for TA, at 640 cm−1 for TB and matches well with theIR and Raman wavenumbers. Figs. 9 and 10 shows the differencebetween calculated (scaled) and experimental FT-IR absorbanceand Raman spectra respectively for form B.

By comparing the rest of the vibrational spectra of ACTH withthe help of PED distribution presented in Table 1, we find a goodoverall agreement. The difference between the observed and scaledwavenumber values of most the fundamentals are quite small.

4.4. NMR spectra

The 13C NMR spectra of ACTH in DMSO-d6 solution shows signalsassignable to the C S, the C O (amide), the C O (acetyl), the CH2and the CH3 carbons at 182.52, 170.37, 169.34, 52.20, 26.62 ppm,respectively, as in Fig. S1 of the supporting material. In order todifferentiate the spectra of the polymorphic forms, the CP/MAS 13CNMR spectra were measured in the solid phase. In these spectrawe get the 13C signals which are completely decoupled, so we donot get any splitting of the 13C signal from the attached protons. Allthe CH3, CH2, CH, C O carbon atoms are observed as singlet, so we

have compared the observed 13C chemical shifts with the averageof the calculated values.

It is well recognized that accurate predictions of molecu-lar geometries are essential for reliable calculations of magnetic

146A

. Sharm

a et

al. /

Spectrochimica

Acta

Part A

90 (2012) 141– 151

Table 1Comparison of calculated and experimental vibrational wavenumbers obtained in ACTH polymorphs in monomer and solid phase.

Isolated state A form trimer (TA) B form tetramer (TB)

Calculated Calculated Experimental Calculated Experimental

Unscaled Scaled aPED Scaled IR Raman Scaled Raman IR

3619 3503 R �(N H)(99) 3568, 3466, 3295 3102 3107 3507, 3505, 3265, 3256 3117 31263152 3051 �asym(CH3)(99) 3071, 3040, 3031 2968 3011 3054, 3051, 3048, 3047 3001 29183122 3022 �asym(CH3)(100) 3028, 3024, 3011 – 2988 3030, 3025, 3024, 3023 2989 –3112 3012 R �asym(CH2)(100) 2982, 2978, 2948 – 2968 3022, 3021, 3016, 3010 2975 –3074 2975 R �sym(CH2)(100) 2946, 2939, 2932 2930 2930 2982, 2977, 2974, 2971 2922 29163058 2960 �sym(CH3)(99) 2925, 2885, 2854 – – 2962, 2961, 2956, 2957 – 18131838 1779 R �(C O)(80) + ıring(7) + �ring(10) 1780, 1734, 1724 1740 1747 1800, 1791, 1779, 1773 1766 17641760 1704 �(C9 O3)(79) + �(C9 C10)(6) + Rı(N4 C9)(4) + �(C9 C10)(4) + ı(O3 C9 N4)(4) 1697, 1682, 1648 1704 1702 1715, 1703, 1678, 1668 1702 16721481 1434 Rı(CH2)(54) + Rı(N H)(10) + �ring(11) + ıasym(CH3)(5) + R �(C S)(5) + Rω(CH2)(4) 1447, 1441, 1440 1470 1463 1441, 1437, 1432, 1431 1484 14661470 1423 Rı(CH2)(33) + ıasym(CH3)(19) + Rı(N H)(15) + �ring(15) + R �(C S)(5) + ıring(5) 1428, 1424 1416 1427 1428, 1425, 1424, 1423 1419 14171467 1420 ısym(CH3)(92) + �(CH3)(7) 1423, 1422, 1421 – 1413 1422, 1421, 1409, 1406 – –1446 1399 ıasym(CH3)(56) + Rı(N H)(13) + �ring(10) + ısym(CH3)(4) + �(CH3)(5) 1407, 1399, 1396 1406 – 1405, 1401 – 14051407 1362 ısym(CH3)(83) + �(C9 C10)(9) 1362, 1361, 1361 1365 1362 1368, 1367, 1364, 1363 1380 13691366 1322 Rω(CH2)(31) + �ring(38) + Rı(CH2)(6) + ıring(9) + R�(N4 C9)(6) + Rı(C S)(4) 1330, 1328, 1324 1350 1353 1336, 1332, 1328, 1325 1336 13291322 1280 �ring(41) + R �(N4 C9)(11) + ıring(15) + Rω(CH2)(7) + Rı(C O)(7) + Rı(N H)(4) + ı(O3 C9 N4)(4) 1304, 1286, 1278 1334 – 1307, 1299, 1290, 1289 1317 –1295 1253 R

�(N4 C9)(19) + �ring(28) + Rı(N H)(9) + ı(O3 C9 N4)(8) + �(C9 C10)(6) + Rı(N4 C9)(5) + Rω(CH2)(5) +Rı(C O)(4)

1266, 1257, 1243 1300 1297 1283, 1273, 1268, 1265 1300 1303

1240 1200 �ring(40) + Rω(CH2)(20) + R �(N4 C9)(10) + R �(C S)(8) + Rı(N H)(4) + Rı(N4 C9)(4) 1206, 1196, 1171 1222 1220 1221, 1210, 1209, 1205 1229 12231193 1155 R�(CH2)(94) 1163, 1160 1189 1200 1170, 1171, 1165, 1155 1200 –1183 1145 �ring(40) + Rı(N H)(16) + Rω(CH2)(10) + ıring(10) + R�(C S)(6) + Rı(C O)(4) 1154, 1147 – 1188 1151, 1149, 1070, 1067 1187 11841097 1062 �ring(46) + �(CH3)(18) + R �(C S)(9) + R �(N4 C9)(5) + �(C9 C10)(5) + �(C9 C10)(4) + ıasym(CH3)(4) 1058, 1054, 1051 1084 1083 1064, 1063 1087 10861060 1026 �(CH3)(73) + ω(C9 C10)(16) + ısym(CH3)(9) 1033, 1030, 1026 – – 1032, 1031, 1029, 1028 – –1049 1015 R �(C S)(21) + �ring(38) + ıring(14) + ı(O3 C9 N4)(7) + Rı(N4 C9)(5) + �(CH3)(5) + �(C9 954C10)(4) 1015, 1010, 997 1039 1037 1019, 1018, 1015, 1014 1033 1041

998 966 R�(CH2)(65) + R[oop(C O)](19) + ring(10) 988, 978, 967 – – 977, 974, 969, 969 982 –977 945 �(C9 C10)(32) + �(CH3)(31) + �ring(12) + ı(O3 C9 N4)(6) + �(C O)(5) + ıasym(CH3)(5) 943, 939, 934 954 973 952, 950, 948, 946 972 968902 873 �ring(71) + ıring(14) + Rı(N H)(5) 892, 881, 868 894 923 887, 880, 879, 875 908 904742 718 ıring(38) + �(C9 C10)(15) + �ring(17) + R �(N4 C9)(10) + ı(O3 C9 N4)(4) 721, 717, 712 760 748 727, 724, 723, 722 748 746646 625 R[oop(C S)](54) + ring(26) + R[oop(N4 C9)](10) + (N4 C9)(6) 628, 626, 625 – 658 640, 638, 637 659 –624 604 ı(O3 C9 N4)(23) + Rı(C O)(17) + Rı(C S)(15) + Rı(N4 C9)(15) + �(C9 C10)(10) + �ring(9) 604, 603, 603 692 – 606, 607, 609, 615 630 –621 601 R[oop(N H)](67) + R[oop(C O)](14) + ring(8) 616 – 629 633, 634 608 –598 578 �(C9 C10)(32) + ıring(16) + Rı(N4 C9)(12) + R

�(N4 C9)(10) + �(CH3)(8) + Rı(C S)(5) + Rı(C O)(4) + �ring(6)583, 582, 581 – 597 591, 590, 583, 580 597 –

587 569 ω(C9 C10)(56) + �(CH3)(24) + R[oop(N4 C9)](8) + R[oop(N H)](5) + (C9 C10)(4) 572, 572, 570 – 577 576, 575, 574, 573 577 –547 529 R[oop(C O)](50) + R�(CH2)(18) + R[oop(N H)](10) + R�(CH2)(9) + R[oop(C S)](5) + ring(7) 544 – 521 543, 542, 540, 538 558 –516 499 R �(C S)(25) + ıring(35) + Rı(C O)(17) + �ring(12) 504, 501, 496 – – 503, 501, 500 521 –460 445 Rı(C O)(28) + ı(O3 C9 N4)(25) + R �(N4 C9)(13) + �ring(16) + ıring(7) + R �(C S)(5) 452, 447, 445 – 479 499, 465, 458, 450 483 –349 338 �(C9 C10)(57) + ıring(7) + R �(N4 C9)(7) + �ring(7) + Rı(C S)(4) + �(CH3)(5) + �(C9 C10)(4) 341, 340, 338 – 358 339, 341, 343, 345 360 –267 258 Rı(C S)(62) + Rı(C O)(8) + ı(O3 C9 N4)(6) + �(C9 C10)(5) + �ring(12) 274, 268, 264 – 256 262, 263, 267, 277 270 –234 227 R[oop(N4 C9)](45) + ring(37) + (N4 C9)(11) 257, 239 – 220 243, 246, 249 230 –213 206 Rı(N4 C9)(79) + ı(O3 C9 N4)(6) + �ring(5) + Rı(C S)(4) 212, 211, 210 – – 210, 209, 217, 216 – –177 171 (C9 C10)(79) + �(CH3)(11) + ısym(CH3)(6) 155, 151, 146 – – 182, 178, 188, 190 – –128 124 ring(48) + R[oop(N H)](35) + (N4 C9)(9) + R[oop(N4 C9)](4) 124, 123, 114 – – 137, 139, 142, 157 – –

99 96 R[oop(N4 C9)](34) + (N4 C9)(44) + ring(13) + R[oop(N H)](7) 98, 89, 76 – – 105, 115, 120, 130 – –54 52 (N4 C9)(39) + ring(27) + (C9 C10)(20) + ω(C9 C10)(7) 68, 60, 51 – – 51, 56, 60, 65 – –

a Proposed assignment and potential energy distribution (PED) for vibrational normal modes. Types of vibration: �, stretching; ı, deformation; oop, out-of-plane bending; ω, wagging; � , twisting; �, rocking; , torsion. Potentialenergy distribution (contribution >3%).

A. Sharma et al. / Spectrochimica Acta Part A 90 (2012) 141– 151 147

Fig. 5. Comparison of experimental infrared absorption spectra of ACTH A and B form.

Fig. 6. Comparison of experimental Raman spectra of ACTH A and B form.

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

Atom no. Form A Form B

Shielding (ppm) Chemical shifts (ppm) Shielding (ppm) Chemical shifts (ppm)

Cal. Exp. � Cal. Exp. �

C-S −6.7334 190.7297 183.0400 7.6897 −10.5242 194.5205 183.0700 11.4505C-H2 126.0806 57.9157 54.0800 3.8357 126.4007 57.5956 54.0500 3.5456C O(amide) 4.0909 179.9054 175.1700 4.7354 7.4511 176.5452 174.5800 1.9652C O(acetyl) 7.8758 176.1205 170.7000 5.4205 3.1732 180.8231 172.1800 8.6431C-H3 154.4260 29.5703 29.3400 0.2303 154.4286 29.5677 29.4700 0.0977

148 A. Sharma et al. / Spectrochimica Acta Part A 90 (2012) 141– 151

spect

pmbs3aTs

Fig. 7. Experimental and calculated (scaled) infrared absorbance

roperties. Therefore, full geometry optimization of both the poly-orphs were performed using B3LYP method with 6-311++G(d,p)

asis set using Gaussian 03W program [24]. The 13C chemicalhifts for both the polymorphs were calculated with the B3LYP/6-

11++G(d,p) optimized geometries by GIAO (gauge-includingtomic orbital) method [35] under the keywords ‘nmr = spinspin’.he relative chemical shifts were then estimated by using corre-ponding TMS shielding calculated in advance at the same level as

Fig. 8. Experimental and calculated (scaled) Raman spectra of A

ra of ACTH A form in the region 610–1870 and 2850–3580 cm−1.

the reference (ı/ppm = ıTMS − ıcalc). Calculated 13C isotropic chem-ical shielding for TMS is 183.996 ppm. The comparisons betweenexperimental and theoretical results, along with the error are pre-sented in Table 2.

Fig. 11 shows the experimental solid state NMR spectra forboth forms. For A form, the chemical shifts of the carbonyl C(9)and C(8) is at 170.70 ppm and 175.17 ppm respectively. The C(9)is directly attached to the electron donating methyl group, hence

CTH A form in the region 200–1890 and 2850–3600 cm−1.

A. Sharma et al. / Spectrochimica Acta Part A 90 (2012) 141– 151 149

spect

tmvctios

Fig. 9. Experimental and calculated (scaled) infrared absorbance

he C(9) is richer in electron density than the C(8). This causesore shielding at the C(9) position and hence its chemical shift

alue is lesser as compared to the C(8). Similar is the case with thearbonyl C(9) and C(8) atoms in B form. The correlation between

he experimental and calculated chemical shift values is plottedn Fig. S2 of the supporting material. The following equation wasbtained between calculated and experimental 13C NMR chemicalhifts y = 1.83417 + 0.98161x, where x and y are the experimental

Fig. 10. Experimental and calculated (scaled) Raman spectra of A

ra of ACTH B form in the region 680–1890 and 2900–3580 cm−1.

and calculated 13C chemical shifts (ı/ppm). There is an excellentlinear relationship between experimental and computed results.

4.5. Thermal analysis

There are differences in the melting temperature and the heat offusion between the a-(stable) and b-(meta-stable) forms of crystals.

CTH B form in the region 190–1900 and 2880–3560 cm−1.

150 A. Sharma et al. / Spectrochimica Acta Part A 90 (2012) 141– 151

Fa

Oacimm

4

d(iAet

title compound. For instance, when the interaction of title com-

TT

ig. 11. Experimental 13C CP/MAS NMR spectra of ACTH (a) form A and (b) form B,cquired with the contact time of 1 ms.

n heating the sample in N2 atmosphere, a sharp endothermic peakt 171.78 ◦C and 169.88 ◦C for A and B forms, respectively, whichorresponds to the onset of the melting, was observed as shownn Fig. 12 The stable form has 2 ◦C higher temp. compared to the

eta-stable form. However, the heats of fusion of the table andeta-stable forms are 93.07 J/g and −98.85 J/g.

.6. Thermodynamic properties

On the basis of vibrational analysis and statistical thermo-ynamics, the standard thermodynamic functions: heat capacityCp,m

◦), entropy (Sm◦) and enthalpy (Hm

◦) were obtained and listedn Table 3. The scale factor used for the frequencies was also 0.9679.

s observed from the table, values of heat capacity, entropy andnthalpy all increases with the increase of temperature from 100o 500 K which is attributed to the enhancement of molecular

able 3hermodynamic properties at different temperatures at B3LYP/6-311++G(d,p).

Temp. (K) Form A

Cp,m◦ (kcal mol−1) Sm

◦ (cal mol−1 K−1) Hm◦ (cal mol−1 K−

100 73.273 15.365 69.200

200 75.282 24.777 84.131

300 78.221 33.933 96.728

400 82.043 42.331 108.238

500 86.644 49.444 118.917

Fig. 12. Differential scanning calorimetry (DSC) curve of ACTH for a heating rate of4 ◦C/min in N2 atmosphere: (a) form A and (b) form B.

vibration while the temperature increases. Both forms have almostsame values of entropy, enthalpy and heat capacity.

The correlation between these thermodynamic properties andtemperatures T are shown in Fig. S3 of the supporting material. Thecorrelation equations for both forms are similar and are as follows:

Cp,m◦ = 72.0752 + 0.0075 T + 4.33357 × 10−5 T2 (R2 = 0.99998)

Sm◦ = 4.7784 + 0.10867 T − 3.82571 × 10−5 T2 (R2 = 0.99986)

Hm◦ = 53.585 + 0.16465 T − 6.85071 × 10−5 T2 (R2 = 0.99984)

These equations could be used for the further studies on the

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 the

Form B

1) Cp,m◦ (kcal mol−1) Sm

◦ (cal mol−1 K−1) Hm◦ (cal mol−1 K−1)

73.272 15.375 69.21275.282 24.782 84.14878.221 33.937 96.74682.044 42.334 108.25786.645 49.447 118.937

ica Ac

rrea

5

bICwswbtptoOtwmmiafciiNtsmc

A

t

R

[[[

[

[

[[

[[[

[[[

[

[

[[[[

[

[

[

A. Sharma et al. / Spectrochim

eaction, which will in turn help to judge the spontaneity of theeaction. The total energy, zero-point energy, rotational constants,ntropy, dipole moment calculated at room temperature (298.15 K)re shown in Table S3 of the supporting material.

. Summary and conclusions

The equilibrium geometries and harmonic wavenumbers ofoth forms of ACTH were calculated at B3LYP/6-311++G(d,p) level.n case of monomer, the scaled vibrational wavenumbers for CH2,H3, N C, C C and ring vibrations appear reasonable when theyere compared with the IR and Raman experimental data. But the

ame went untrue for the bonds involved in H-bond formation. Buthen we moved to the stable configuration, the scaled wavenum-

ers for N H, C O and C S vibrations appear in accordance withhe experimental data in solid state. An effort is made to inter-ret the changes that occur in the vibrational spectra on movingowards a more stable configuration. The differences in geometriesf both forms were studied in terms of intermolecular H-bonding.n moving from gas phase to stable system, a remarkable shift in

he stretching frequencies of N H and C O bands towards loweravenumbers clearly predicted their involvement in H-bond for-ation. A detailed normal coordinate analysis of all the normalodes along with PED allows the composition of each normal mode

n terms of internal coordinates. In case of monomer, the discrep-ncy between the observed and calculated frequencies is due to theact that calculations have been actually done on a single moleculeontrary to the experimental values recorded in the presence ofntermolecular interactions. In case of stable system, our results aren better agreement with the experimental data. The 13C CP/MASMR spectra of the compound were recorded and on the basis of

he calculated and experimental results assignment of the chemicalhifts were done. The thermodynamic properties of ACTH poly-orphs at different temperatures were calculated, revealing the

orrelations between C◦p,m, S◦

m, H◦m and temperatures.

ppendix A. Supplementary data

Supplementary data associated with this article can be found, inhe online version, at doi:10.1016/j.saa.2012.01.033.

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