Raman spectra and vibrational assignment of thiosemicarbazide-d0 and -d5

Raman

spectra and

vibrational assignment

thiosemicarbaeide-d,

and 4,

of

G. KERES~TURY*and M. P. MARXWCHI Istituto di Chimica Fkica della Universiti degli Studi, Via Gino Capponi, 9-50121 Firer120 (Re.ceived2 May 1974) &&a&-The Raman spectra of polycrystalline thiosemicarbazide-cZ,, and -a, have been mesaured at room temperature. Utilizing all data available on infraredand Raman spectra for these molecul&, a complete assignmentis proposed for internal and Raman aotive external vibrations.

IN A previous work [lJ we reported the infkared spectra of oriented polyorystalline films of thiosemicarbazide (N”H, -N”H-CS-NdH,, hereafter: TSC) measured in p&rized light. The ,experimentr&y determined transition moment directions allowed a certain clssigamsnt for a number of bands, while for a detailed description of some other bands they did not yield enough information. In order to complete the d&a we measured the Raman spectra of polycryatalline TSC and TSCd,. Utilizing all the data at our disposal, we propose here an improved and complete assignment for the internal and the Roman active external modes of TSC and TSCM,. EXPERIMENTAL The thiosemicarbazide used was a reagent-grade compound purified by recrystallization from ethanol.? The Raman spectra of crystalline TSC-d,, and -d, were measured at room temperature with the aid of a Gary 81 spectrometer equipped with an Argon ion CRL model 62 laser source. The 4880 A exciting line was used for the 4000-2400 cm-l and 2OOO-40cm-l spectral regions, The spectrum in the region between 2400 and 2000 cm-l, where a broad and strong ghost occurs with this radiation, wss recorded with the 6146 A line. The spectral slit width was changed between 1 and 3 cm-l depending upon the band intensities. The deuterated compound ws,s protected from water v&pour by putting a drop of silicon oil on the sample surface. For the sake of comparison the TSC spectrum was rerun at the same conditions: no additional bands due to silicon oil appeared in the spectrum. RESULTSAND ~~I~CXJSSION (a) Internal & The Ramen spectra of polycrystalliue TSC and TSC-d, are shown in Figs. 1 and 2 respectively. The experimental results for the internal vibrations of both isotopic * Present address: Central Rssesrch Institute for Chemistry, 1025 Budapest, Pusztaazeri ut 69/s?, Hungary. t We thank Dr. T. KOMIVEB who supplied ua with the deuterated compound. 1

275

276

0. KEBESZTIJRYand M. P. BURZOCOEI

I 2

L LJ3350

Fig.

3100

1600

1350 cm.,1100

650

600

350

1. Raman spectrum of thiosemicarbazidein the internal modes region.

species together with the infrared data [I, 31 and a complete assignment, are aolleoted in Table 1. The vibrational selection rules for TSC orystal have been discussed in detail in our previous paper [I] on the basis of the crystal and moleauk structure determined by ANDREETTI et al. [Z]. According to the PT = C,f space group with two moIecules in general positions, one expects all the 24 internal vibrations of the molecule to be active both in i.r. and Raman spectra, but owing to intermolecular coupling of vibrations they may occur at slightly different wavenumbers. Since the molecules in the crystal possess an approximate plane of symmetry, we assumed a classification of vibrations based on the Cak pseudo-symmetry which gives better distinction of vibrational modes. The internal crystal vibrations are

r

-1

2700

2450

Fig. 2. Raman

2225

1500

1250 cm'

1000

750

500

spectnunof thiosemioerbtide-d6 in the internal modes region.

[l] a. KE~BZTWY and M. P. MAEZOCCHI, C~~VB.PIga. 6, 117 (1974). [2] G. D. ANDRBUCITI, P. DOBZIUO,C. G~~PABBI FAVA, M. NABDELLIand P. SGARABOITO, Actu cqpt. BM, 1005 (1970). [a] P. VIXKLEE,prkb~CO~~~tiOn.

Raman ape&a and vibrational assignment of thiosemiwbrtzide-d,, and 4, Table 1. Vibrational awignment of thiosemimtide-do rnfmred mc-a, TSC-d5 [Ref. l]

[Ref. 3, 41

3380 II (I)

2646

TSC-&, !W3C-d, Thin work 2647 2437

3260 _L (((1

2440 3180

2383 2364

3160 II 2320 1646 II 1640 1620 II

1143

1634 I(

1640

1167 1167

1630 1499 1480 I/

1391

1400

1318 11 1290 1286 1288 11

1186 1272

1198

1240 11 1073 1162 II

1080 966 863 860

100s J_

@@S II

706 992

788

1010

924

92s 766

760 868

II ‘35II 804

SMI amJ_

698

710

726 646

1

%Pm-l-va~,)

1

~P~d+va,N~,)

I 1 1 1 1 1 1 1 1 I I I 1 I 1 1

410

373

360 _L

296

Frequenaies in em-‘.

11and 1.

V2+-, ~3% BNOH, BNdH* vflH

skel. vibr. msinly flN BNEfVCN 806 + SO6 =

1312

926 + 373 E= 1298

vCN +vNN 640 638+690=1228 + 620 = 1260

VNN+VCN 710+

268 = 978

496 + 372 = 866

-=,w* m=,mok+vCS

486 + 268 E 763 VCS c?N”H, wag 327 + 268 I g2

640

;$

aI * BN*H, rook

620

327 II

*

Approximate desoription

V,, pseudo symmetry)

440 I

666 608

466

407

372

288

268

603

and -ds*

tignment

638

660 I

277

v=

1 I 8cNx skeletel taBion 8CS $8 288X

2=676

6NGNfvCS

polarized parallel end peqmndioular to the molmn&r phane respectively.

then distributed among the symmetry ape&es in the following way:

IsA, + 8B, f 88, + 16B, A, and B, originate from A’ (in-plane), A, and B, from A” (out-of-plane) vibrations of the is&ted molecule. Nine of the in-plane and six of the out-of-plane vibrations are mainly due to different N-H vibrations, while the others (6 A’ and 2 A”) are skeletal motions. The most striking features of the Raman spectra are that: (a) the strongest band in the TSC spectrum occurs at 808 om-1. (b) Three very intense bmds of TSC-dE,at 485, 710 and 924 cm-l correspond to the three most intense bands of TSC-d, at 508,808 and 1010 cm-l with &pproximately unchanged relative intensity ratios. The C-S bond direction is clearly that of maximum polarisability. So, the 808 cm-l band of TSC can safely be ascribed to a vibration which is mostly v(CS), ooniirming the assignment of the i.r. sctive counterpart of this band to Y(CS) proposed by MASHIMA [4). It is expected to shift slightly on deuteration. The band s,t 788 cm-l of TSC-d,, however, is much less intense. Therefore, assuming that the C-S stretching in TSC-cZE, is distributed between the infrared bands at 928 and 710 om-l, the assignment proposed by Ma&ma, is supported. We believe that even the strong band at 508 cm-l (and that at 485 cm-1 in the TSC-d, spectrum} hsa a significant contribution of Y(CS). In spite of the faot that its i.r. counterpart at 503 cm-1 shows the polarization character of the A, modes [I], it can be rassignedto a plenar vibration. In fact the band is quite strong in KBr disc rend very weak in light polarized parallel to Face I. Being in the moleoular plane perpendicular to the face, this vibration band is expected to disappear for any orientation of the electric vector, if it is due to a planar mode with transition moment dire&d along the propagation direction. The occurrence of a very weak band with an out-of-plane polarization character can be ascribed to coupling with the out-of-plane N-H modes, being Cl, exactly the molecular symmetry. In conclusion, we assign the intense Reman bands at 508 and 485 cm-l, for TSC and TSC-d, respectively, to the N-C-N bending in-plane mode coupled with CS &retching. In the Raman spectrum of TSC a band occurs at 288 cm-l. The corresponding band of TSC-G?~ is found at 268 cm-l with somewhat higher intensity. Though these bands do not appear in the i.r., severe1wes,k Raman bands of TSC-dScan be explained as combinations with this vibration. Being the lowest frequency internal mode, it is certainly an out-of-plane skeletal vibration. This conG.rmsthe above assignment of the 508 cm-1 band, since otherwise there would be more out-of-plane vibrations than we should expect. A notices,ble feature of the TSC-d, Raman spectrum is the intense band at 1391 om-l, since we hsve no corresponding band with similar intensity in the ape&urn of TEE. Probably it is related to the 1499 cm-l vibration of TSC!, the vibrational mode changing considerably on deuteration. [4] M. MASEI~KA, Bull. C%twn.Sot. Japan 87, 974 (1964).

Raman spectra end vibrationel assignment of thiosemicarbazide-&,, and -d,

279

Due to the resoution of all the five bands expected in the Raman spectrum of TSC-d5, a more detailed assignment is possible for the N-D and N-E stretching modes than was given before. The two weakest bands at 2647 and 2354 cm-l are assigned to the a&symmetric and symmetric stretching vibrations of the N6D, group. Due to an asymmetrio hydrogen bond, the separation between these two vibrations may begreater than the usual. The very broad and intense i.r. band at 2360 cm-l corresponds to the 2354 cm- l Raman band, and it presumably oovers the two other wesk bands, which are the i.r. active counterprcrt of the 2383 and 2320 cm-l

320 Fig.

3. Raman

250

150

50cd320

250

150

50

zpe&rmn of tbiosemiw~ide (a) and t&G-tide-d6 (b) in the lattice modes region.

Raman bands. The sharp end strong Raman band at 2437 om-l is assigned to the Y,~D, and accepting the normal separation between the symmetxio and asymmetric stretching vibrations of ND, groups to be about 100 cm-l, we assign the 2320 cm-l band to Y;NOD,. The remaining 2383 cm-l band is therefore due to vNbD. Now we suppose that a similar sequence of bsnds can be found in the case of TSC-o?~as in TSC-o?~. This means that the bands at 3386 and 3267 om-l are due to ~~3%~ and Y>H~ coupled with each other through the hydrogen bond, as it is shown by polariz&ion of the corresponding i.r. bands. The other three strongly overlapping bands give rise to a broad Raman band with a maximum at 3178 cm-l. The assignment of the other bands given in Table 1 does not deserve special comment or was discussed in our previous work. tb) External modea All the six Raman aotive external modes predicted by the selection rules appear in the spectra of both TSC-I, end -dl (Fig. 3). The wavenumbers and the proposed assignment of bands are reported in Table 2. Retaining the 0, pseudo-symmetry of the two molecules in the unit cell, the structure of the represent&ions of the Reman active l&ice modes is the following:

Q. -Y

280

Table 2. Raman-active

and M. P. MARZO~OFR external modez of thioaemicarbazide-d,, and -ds*

TSC-$

TSC-&,

R

210 w 162 m 134 m 122 8 112 vs 99 s 65s

198 w 145 w 126 w 117m 108 8 97 s 65m

1.048 1.063 1.042 1.037 1.021 1.000

Assignment Combination Y(T&v(N-H.. v(%) v(%) VW,) VU’,) v(TZy)

. N)

* Frequencies in cm-l; R, ratio between the frequencies of TSC-d,, and TSC-d&E,; &l~ and MI, are the molecular masses, IH and ID the principal momenta of inertia, Z, y, and z the axes of the moments of inertia (see Fig. 4); w, weak; m, medium; 8, strong; vs, very strong.

y

s

x

I

H\ \

\vH I

d

aP;

H

Fig. 4. Moleculax structure and prinoipal inertia 8x1~ of thiosemicarbazide. (MD/&fZ)l’B = 1.027;

(lD/ZH)‘,l” = 1.043;

(ID/l&*

= 1.101;

(ID/l&;‘*

= 1.063.

The correspondence between the bands of the two isotopic species can be established without di&ulty, since the pattern of the two spectra is quite similar. In order to achieve a deilnite assignment, we calculated the ratios of the molecular masses and of the principal moments of inertia. As may be seen in Table 2, the agreement between these and the frequency ratios is good, even if not perfect owing to the coupling between different lattice modes. In addition the frequencies of the librational modes are expected to follow the magnitude order of the inertia moments. Accordingly, the bands at 134, 122 and 112 cm-l, in the spectrum of TSC, are ltssigned to the lib&ions around the y, 2 and z axes (Fig. 4) respectively. The other three bands are due to translstional modes; the highest frequency band among them at 152 cm-l is the stretching of the hydrogen bond between the two molecules of the dimer. The frequency ratio for this vibration is higher than expected because. of the coupling with librational modes. Finally, we assign the lowest frequency band at 66 cm-l to the in-plane translational vibration, which is perpendicular to the hydrogen bond direction. This motion corresponds roughly to a libration of the dimer around the out-of-plane direction, which has the greatest moment of inertia. A&wwle&e~~nM-Th work was supported bs the Italian Conziglio Nazionale delle Rioerohe. G. K. wishes to thank the Italian Foreign Mini&y for a research fellowship and is greatly indebted to Prof. S. CALIFANOfor his kind hozpitality and helpful advice.