Raman and polarized infrared spectra of pyridine-2-thione

Specrrochrmlca

Acra

Vol

40A

No

8 pp

757-761

0584-8539/U $300 + 0 00 0 1984Pergamon Press Ltd

1984

Pnnted,n GreatBrltaln

Raman and polarized infrared spectra of pyridine-2-thione R SHUNMUGAM and D N SATHYANARAYANA Department of Inorgamc and PhysIcal Chemistry, Indian Institute of Science, Bangalore 560012, India (Recewed 26 May 1983, m rewed form 7 March 1984) Abstract-Polarized I r spectra of oriented polycrystals and the I r and Raman spectra of polycrystalhne samples of pyrldme-2-throne have been obtained The Raman spectra of a solution of pyndme-2-throne has also been measured and the polarlzatlon of many hnes determmed The I r spectra of S-methyl and Ndeuterated compounds havealso been mvestlgated A complete assignment ofall the observed peaks has been possible The results are correlated with the assignments available for related systems

INTRODUCTION Pyrtdme-2-throne

(2-pyt)

can be consIdered

as a model

such as 6thloguanosme [l] It 1s closely related to nucleic acid constituents, namely, pyrlmldme thlobases such as pynmldmethlone, thlocytosme, thlouracll etc, the spectra of which have not been adequately studied m the past [2] Previous vibrational assignments reported m the literature on 2-pyt itself were concerned with limited assignments of NH, C=S and skeletal stretch bands [3] A detailed mvestlgatlon of 2-pyt 1s reported here based on polarized 1r spectra of oriented polycrystallme films and Raman spectra of powdered samples and solutions The spectra of Ndeuterated and S-methylated compounds as well as the reliable assignments available for related systems [2, 4-71, are taken mto account The present measurements provide a more complete assignment of the vibrational modes of 2-pyt and help m the mterpretatlon of the spectra of pyrlmldme thlobases compound

of

thiolated

nucleosides

EXPERIMENTAL Commercial 2-pyt (Aldrlch Chemicals) was recrystalhzed from methanol The S-methyl derlvatlve of 2-pyt was obtamed by the hterature method [8] N-Deuterated samples were prepared by repeated crystalhzatlons from DZO, deuteratlon was estimated from I r spectra to be over 90% The lr spectra were recorded with a Parkin-Elmer 599 spectrophotometer Polycrystallme samples were measured as NUJO~mulls and CsI pellets The observed frequencies were calibrated with the usual IUPAC standards KBr cells of path length 0 5 mm was employed to obtain the I r spectra of 2-pyt m chloroform Polarized 1r spectra between 4000 and 400 cm-’ of 2-pyt were obtained on a Carl Zeiss URlO

Table Molecular symmetry C,

Site symmetry C,

1 Selection

spectrophotometer equipped with a selenium polarizer Well qrlented polycrystalhne film grown from the melt was sandwlched between two KBr plates of dlmenslon 2G30 mm2 under a suitable temperature gradient m order to enhance orlentatlon effects The Raman spectrum of 2-pyt was obtained with the ald of a SPEX Ramalog 6 Instrument The 4880A lme of the argon Ion laser (Spectra-Physics) with 100 mW power at the sample as polycrystalhne powder m capillary cells was used Raman depolarlzatlon ratios were also evaluated for some bands recorded m chloroform using a He-Ne laser of 50 mW power at the sample RESULTS AND DISCUSSION

2-Pyt exists m the throne form, the planar molecules being associated with centrosymmetrlc pairs through hydrogen-bonding m the solid state[9] (Fig 1) In particular 2-pyt crystals are monoclinic, space group P2, /a, with four molecules m the unit cell Accordmgly, discussion of the crystal spectra should be performed on the basis of a unit cell containing two dlmers of CZhsymmetry The selection rules derived for this model are reported m Table 1 The frequency data are collected m Table 2 InspectIon of the 1r and Raman frequencies of 2-pyt reveals that there are slight differences m the wave number of the correspondmg bands mdlcatmg non-

Fig

1 Intermolecularly

hydrogen-bonded

rules for 2-pyt crystals

Umt cell symmetry C 2h

757

No of vlbratlons Internal External

Actlvlty

2-pyt dlmers

R SHUNMUGAM and D N SATHYANARAYANA

758

Table 2 Vlbratlonal

spectral

data (cm-‘)

of 2-pyt

2-Pyt-d, Raman CHCI,

2-Pyt powder 3170w

1268~ 1253 p 1184~ 1145p

9881,

1610s 1567 ms 1505 s 1448 ms 1424 ms 1388 ms 1265 vs 1245 sh 1190m

1075 w 1OOOms 970 sh

900 dp

910m

753 dp 734p

738 s

452~

450 m 385 w 335 m

(po:der)

(poider)

3160~ 3100w 3060 w 3040 w 2990 mb 1615s 1575 vs 1500ms 1440s 1420 m 1370s 1260 ms 1240 ms 1186s 1140 vs 1llOm 1095 m 990 s 975 ms 950m 905 ms 870~ 750 vs 730 ms 620 ms 490 s 450 s 390w 330s

2200

I,

1

1LCf.l

,

I

1200

I

I

mb 3095w 3060w 3035w 2990mb

NH str

1610ms 1530s 1155ms 1435 ms 1415m 1365 s 1250m 1240m 1180s 1135s 1105s 1080~ 980s 965 ms 945 w 660 ms 870 w 745 vs 730ms 620 w 490 s 450s 380sh 330s

CC str CN str

I

1coO

I

1

em

700

Valvenumber

Fig 2 Polarued

CH str

NH bend CC str CH bend CN str (ring + C = S) str CH bend CH bend CH bend rmg def CH bend (o p) CH bend (o p) NH bend (op) CH bend (o p) CH bend (o p) C = S str + rmg def rmg def rmg bend (o p )

C=S bend rmg bend (o p ) C=S bend (op)

inter-dlmer interaction may be predlcted to be stdl weaker Accordmgly the interaction between the dlmers m the unit cell appears to be negligible Therefore it appears Justtied to mterpret the crystal spectra of 2-pyt on the basis of single dlmers having CZhsymmetry In this case, the 66 fundamental modes of vibration span the symmetry species 23 A, + 10 B, + 22 B, + 11 A,, the g modes are Raman active while the u modes are 1r active Figure 2 shows the polarized light spectra of the fundamental bands of 2-pyt The results are collected

comcldent 1r and Raman bands The mutual exclusion between 1r and Raman active fundamentals 1s sustamed However, the fact that the wave numbers of a large part of the I r bands are only shghtly different (5-lOcm_‘) from the correspondmg Raman shifts may be interpreted as suggestmg that the mtermolecular hydrogen-bondmg scarcely perturbs most of the fundamentals This 1s consistent with the structure of 2-pyt molecules, which are linked m pairs across centres of symmetry by weak hydrogen bonds between nitrogen and sulphur atoms Logically, the

1600

Approx descrlptlon

I r spectra of 2-pyt, (p)

I

600

e---

( cm-l)

a-spectrum

and (----)

&nzctrum

Raman

and

1r spectra of pyndme-2-throne

m Table 3 together with the data on Raman spectra for the polycrystallme sample The method for taking the 1r crystal spectrum m polarized hght has been clearly discussed m the hterature by several mvestlgators [lO_181, and the procedure as detalled by KRAUSE er al [lo] has been followed m the present work As the determination of the precise orientation of the crystals IS not feasible, the angle of orientation relative to the Incident radlatlon could be known by recording a well Isolated band of appreciable mtenslty as a function of the polarizer angle We have selected as polarlzatlon reference the mtenslty of the fundamental at 620 cm- ’ since this band 1s well isolated m the spectrum and IS of appropriate mtenslty In Fig 2, the full hne spectrum, referred to as the a-spectrum, refers to a polarizer-sample orlentatlon corresponding to the highest mtenslty of this band The dotted hne spectrum, indicated as the /?-spectrum, was obtained with a 90” rotation of the polarizer It can be seen from the spectra that most of the bands show defimte polanzatlon characterlstlcs and may be classified mto two groups These clearly correspond to the two 1r-active factor group species No direct mformatlon for estabhshmg the symmetry species can be attamed by the Raman hnes of polycrystalhne samples, the depolanzatlon ratio measured from solution can however give some help The data m chloroform Included m Table 2 are consistent with the results of the polarized I r spectra The vlbratlonal assignments will be briefly dlscussed below Vhutzonal

assignment

The assignment of the spectra may be attempted on Table 3 Infrared

Raman powder

lr orlented crystal

PO1

3160

a

3170

and Raman

Assignment

4

759

the basis of the above criteria and by analogy with the recent assignments avallable for related systems [2, 4-71 The spectra of S-methyl and N-deuterated compounds have been utlhzed The assignments proposed for 2-pyt shown m Table 2 are given m terms of the vlbrahons of the free molecule The weak band at 3160cm-’ which moves to 22OOcm-’ on N-deuteratlon 1s easily interpreted as due to the stretching fundamental of the mtermolecularly hydrogen-bonded N-H group m the sohd In chloroform, the N-H stretching frequency of the monomer 1s found at 3360 cm-’ and of the dlmer at 3160 cm- ’ For cychc thloamldes (thlolactams) [19, 201, the m-plane and out-of-plane N-H deformations have been located m the rather broad regions 150@1300 and 900-700 cm-’ On grounds of sensltlvlty to N-deutenum labelhng, bands at 1500 and 905 cm- I have been ascribed respectively to m-plane and out-of-plane N-H bendmg modes A band of particular significance 1s that of the C=S stretchmg fundamental The mcldence of coupling of C = S stretching mode with other molecular vlbratlons renders the assignment of C=S stretching difficult [21, 223 The higher mass of sulphur and the lower C=S bond order (particularly when the C=S moiety 1s attached to a strongly mesomerlc atom such as mtrogen) shift the C=S stretchmg mode to lower wave numbers which renders It conducive to mixing with other vlbratlons The 1186 and 730 cm-’ bands of 2pyt move to lower wave numbers on S-methylatlon concordant with the decreased C=S bond order, the magmtude of the shift 1s slgmficant but small (15-20 cm- ‘) Slmllar low frequency shifts are seen on frequenaes*

Raman powder

(cm-‘)

for 2-pyt

lr orlented crystal

Pol

1240

a

1195 1145 1105

WI

1245

B”

Assignment

4

B”

1190 3105 3060 3030 2990

B”

1620 1605

tl

1567 1575

a

47

1075

47

1000

B”

1095

B,

990 970

1505 1500

a

1448 1450

a

1420

a

1375 1348

a

1388

B”

4

B”

G(

a

of a and fi see text

B”

B”

4 4

43

915 875 755

4 4

735 618 485

B”

4

4

4

738

B”

B” B” 4

965 910

4

1265

*For the meanmg

B”

4

1424

1260

A,

a a

450 445

4

AU

4

B” B, 4

4

B”

R SHUNMUGAM

760 Table 4 A comparison of

and D N SATHYANARAYANA

I r assignments for six-membered heterocychc molecules* (frequenaes m cm-‘) S-Methyl PYt

CPY

Pymt

CYS

Pymo

PYt

3088 3060 3005

3160 3080 3050

3117

3110 3070 3019

3040 3020 2990

2950 1608 1565 1420 1335 1225 1210 1490 1475 1165 1050 1040

3169 1615 1505 1465 1364 1236

2820 1618 1540 1470 1434 1350

1538

1230 1223 1160 1106 1050

3100 3060 3040 2990 3160 1615 1575 1440 1420 1370 1240 1500 1260 1186 1140 1110 1095 975 905 750 870 730 -

1310 1170 1120

CH bend ring def CH bend

1050 985 960 730 875 710

NH bend (o p) CH bend (o p)

620

615 600 465 400

1580 1574 1462 1432 1248

1155 992 1045 1091 981

1100 994 1277

1182 980 965 795

734 632 553

1198 1005 952 802 866 1648 782 585 581 518 636 513 405

811

750 735 680

1662 793 620

485 470 405 230

600t 422t 442

490 450 330 385

Assignment

CH str NH str

1650 1570 1510 1410 1370 1270

rmg str

NH bend

360

CH bend (op)

C = S/O str rmg def (o p ) rmg def C=S/O C=S/O rmg def

(o p ) bend bend (o p) (o p )

*Cpy = 2-cyanopyndme [S], Pymt = pyrmndme-2-throne [7], Cys = cytosme [6], Pymo = 2-pynrmdme [4], pyt = pyndme-Zthlone (present work), S-Methyl pyt = S-methyl derlvatlve of pyt (present work) t Revised [2]

coordmatlon of 2-pyt to metal Ions For a pure C=S cyclic systems such as 2-substituted pyndme, cytosme, stretchmg mode a much larger shift (over 50 cm- ‘) 2-pyrumdone etc, for which detailed assignments have recently become available As one can see, there IS a may be predlcted The assignment of C=S stretching mode m 2-pyt may be compared with that of fairly good correlation of the assignments hydrazmecarbothloamlde [23] which also has a CIS -CSNH- group The fundamentals of hydrazmeREFERENCES carbothloamlde at 1282 and 801 cm-’ have contn[l] I P EVANSand G WILKINSON,J them Sot Dalton butlons (16 and 32 % respectively) from C=S stretching Trans 946 (1974) mode [23] By analogy with related molecules [21,22, PI C P BEETZand G ASCARELLI,Spectrochzm Acta 36A, 241, the 1186 cm- ’ band of 2-pyt may be assigned to a 299 (1980) and references therem coupled mode of C-N stretching, N-H bendmg, C=S E SPINNER,J them Sot 1237 (1960) [i]E PICQUENARD and A LAUTIE,Spectrochlm Acta 38A, stretching and rmg deformation and the low frequency 641 (1982) 730 cm- 1band to a coupled mode of ring deformation 151 J H S GREENand D HARRISON,Spectrochwn Acta and C=S stretching The 1186 and 730 cm-’ bands are 33A, 75 (1977) polarized m the Raman and exhibit the predicted C61 H Sum, J S ARD and J M PURCELL,Spectrochtm Acta polarization characterrstlcs m the 1r In the light of the 29A, 725 (1973) and D N SATHYANARAYANA, Bull Sot present study, the assignment of the C=S stretching c71 R SHUNMUGAM chum Belg m press (1984) fundamental to a band at 1140 cm- ’ by SPINNER [3] [81 A ALBERTan&G B .BARI~N,J them Sot 2393 (1959), needs to be reconsidered

The present assignment of m-plane and out-of-plane C=S bending vibrations thmcytosme

assigned

of 2-pyt agree with those of 2at

482

and

305 cm-’

respectively [2] In Table 4, the vibrational compared

assignments

for 2-pyt are

with those m related six-membered

hetero-

RENAULT,Annls Chum 10,135 (1955) [91 B R PENFOLD,Acta crystallogr 6, 707 (1953) [lOI P F KRAUSE,B G GLAGOLAand J E KATON,J them Phys 61, 5331 (1974) and M P MARZOCCHI,Chem Phys 6, Cl11 G KERESZTURY 117 (1974) WI M P MARZOCCHI,H BONADEOand G TADDEI, J them Phys 53, 867 (1970)

Raman and

I r spectra of pyndme-2-throne

[13] C W BROWN,R J OBREMSKI,J R ALLKINSand E R LIPPINCOIT,J them Phys 51, 1376 (1969) [14] U ANTHONI,P H NIELSEN,G BORCH~~~P KLABOE, Spectrochlm Acta 34A, 955 (1978) [15] S DOBOS, E MATRAI and E CASTELLUCCI, J Molec Struct 43, 141 (1978) [16] K MACHIDA,Y KURODAand K HANAI, Spectrochnn Acta 35A, 835 (1979) [17] B ZELEI and S DOBOS, Spectrochlm Acta 35A, 915 (1979) [18] T WOLDBAEK, P KLABOE and D H CHRISTENSEN, Acta them stand 30A, 531 (1976)

761

[19] H E HALLAMand C M JONES,Spectrochlm Acta 25A, 1791 (1969) [20] K DWARAKANATH, D N SATHYANARAYANA and A ANAGNOSTOPOULOS, lndlan J Chem 16,834 (1978) [21] K A JENSENand P H NIELSEN,Acta them stand 20, 597 (1966) [22] K R G DEVI, D N SATHYANARAYANAand S MANOGARAN, Spectroctum Acta 37A, 633 (1981) [23] D N SATHYANARAYANA, K VOLKA and K GEETHARANI,Spectrochlm Acta 33A, 517 (1977) [24] K DWARAKANATH and D N SATHYANARAYANA,Bull them Sot Japan 52, 2699 (1979)