Vibrational spectra and structures of S2Cl2, S2Br2, Se2Cl2 and Se2Br2

Journal of Molecular S:ructare

Elsevier Publishing

Compzky,

VIBRATIONAL AND Se,Br,

STEPHEN

Amsierdam.

SPECTRA

AND

Printed in the Netherlands

STRUCTURES

271

OF S,Cl,,- S,Br2, Se&I,

G. FRANJCISS

Department of Chemistry, University College, London (Great Britain) and Mellon Institute,_ Pittsburgh, Pa. X5213 (U.S.A.)

(Receiwd

April Sth, 1968)

SUMMARY

The IR spectra between 33 and 600 cm-’ and the Raman spectra of S,Ci,, S2Br2, Se2C12 and Se,Br, are reported. Complete assignments for all four molecules are presented. The spectra show that all four molecules have non-planar structures with C, symmetry. 1. INTRODUCTION

Molecules of the type XYYX, where Y is a Group-VI element, may have C,,, C,, or C, symmetry depending on whether the azimuthal angle 8 for the rotation of one XY group relative to the other, ‘is O”, 180” or some intermediate value, respectively. SzClz and SzBrz in the vapour state have been reported, from an electron diffraction study’, to have C, symmetry with 6 NN83”. This conformation has been attributed to the interaction between the 3p7relectrons of adjacent S atoms. The only structural investigation of Se& and SezBr, has been-a study of their Raman spectra2, which have been reported to be consistent with either C,, or C, symmetry. In view of this uncertainty it seemed worthwhile to examine the vibrational spectra of these two molecules in more detail in order to determine their structures more definitively, particularly as their structures should give some indication of the steric significance of the 4~7~electrons of adjacent Se atoms. During -the course of- this work it became clear that the fundamentals of Se,Cl, and Se,Br, should be reassigned, and this suggested that a reassignment of the fundamentals of &Cl2 and S2Br2 may also be necessary. The vibrational spectra of all four molecules have therefore been investigated. New assignments are proposed, and the spectra-shoti that all four molecules have C, symmetry. 2. EXPJZRMENTAL S2Br2 was-prepared by. a standard procedure*, while. S&l2 was optameo J. Mol. Structar& 2 (1968) 271-279

Present work

102 206 240 4?6 4@ 540

5,b 4 2 3,b

IO,& 1

dp? :p P p dp

83 105 97 208 28 242, ~430 $I 448’. loo 15 540

p P

dp’! dp? p p.

.. s ‘= str&ng, m = .medium, w =Gweak, v = very, sh = shoulder, p = polarized, dp = deposal + = not examined in this region, b .= broad. & The combination tones in the region 75~1350 cm-~ are not included here. b From dilute tilutioa in’ CS,. c From pure liquid.’ * Fram dilute solution in benzene.

KR

solid cm”

66’ 172 200 302 356 529

7,b 5 3 f

dp? :p p

XO,b p 1 dp

67 170 201 302’

80 F 66 p 24 dp? (1)

50 75 88 X80 211 299

357 534

tO0 ,p dp? 7

353 350 358 534

1 s,sh e> 10 7 2 10 10 10 10 I

(&quid)

Ass~~an~~at

Keteluar et al.?-8

Bradley et a!.6

Present

ems1

cm-l

cm-% I

176@ 19@

175 198 302

354b

355

531c

531

173c .202c

work

w m

iatiice I modes pa % % impurity

% I 3W vs Ys 3654 vs, sh l% 536d m

Abb~~~atio~s as for Table 1. a The weak bands reported m the region 70%t100 cm-z are not iacludtid here. b From. s~ur~~~~ tra&tion spectnrm of 50 % ‘V/Y solution in XX,. C From pure Ii&id. a From dilute. sol&ions in benzene.

SPECTRA TABLE

AND STRUCTURES

OF

S AND. Se HALOGENIDES

213

3

IR AND RAhiANSPECTRA

OF se&~,

Roman

Assignmen@

SRb

Stammreich Fornerisz+

and

Present

work

Present

work

30 % v/v soln.

Pure iiquid

in benzene cm-’ I 87

8

130 146

7 3

288

2

367

10,b

418

1

P

.cm-l I

cm+

I

p

dp?

91 133 148

5 18 7

a3 131 147

14 19 7

p

289

6

291

4

p

357

50

366

50

p

p

p

-420

cm-l

I

93

w

150 271 292 359

s w s vs

409

w, sh

436 581

w vw

V4

1

VS v6

impurity? Vl’ % VZ’ 3 x 133 = 133+288 150-l-292 2x292 =

2 x 133 = 2661

3997 =421 =442 584

Abbreviations as for Table 1. a From dilute solutions in benzene or CSI. b From pure liquid, except * from dilute solution in benzene. d The numbering of the fundamentals v; and va’ in Se&l, is reversed from the usual convention so that v1 is the YY stretch and vZis the u XY stretch in all four XYYX molecules studied here.

TABLE IR AND

4 RAMAN

SPECTRA

OF

Se,Br,

RamarP~s

Ii?

Stamreich and Forneris

Present

cm-l

I

CM-’

50 94 106

10 3 7

204 265 292

2 10 1

63c

Asstgnment work I w

105C 1I9b

w, sh m

265b 29Ob

vs m

*4 2x50 %

= lOO? impurity7

?x 106 = 212? impurity? %t v6 *1

Abbreviations as for Table 1. B From diltite solutions in benzene, CC& and &. b From dilute solutions in benzene. 0 From. pure liquid. J. Mol. Structure, 2 (1968) 2712279

274

S. G. FRANKESS

from Eastman Kodak and was purged by redistillation. &&!I, and Se,Br, were obtained from R and K laboratories, and they were not further purified. AlI compounds were handled in a do-nitrogen Glled dry-box, and the two sulphur compounds were also transferred using a glass vacuum system. IR spectra were recorded. using Beckman IR-I I and IR-9 spectrophotometers. Ah four compounds were studied as pure liquids and as dilute solutions in benzene, held in polythene or polypropylene cells which were usually 0.3 mm in path length. In addition, the ER spectrum of S&l, vapour was recorded using a Beckman variable path length 10 m cell. Raman spectra were recorded with a Gary. 81 spectrophotometer fitted with a He-Ne gas laser using 6328 ir, excitation. Satisfactory spectra of SJ&, S2Br, and Se,C& were obtained with the samples held in sealed melting-point tubes. The spectrum of soIid S,BrZ at about 140°K was obtained using a speciahy constructed, evacuated cell which is described eIsewhere4. Attempts to record the Raman spectrum of solid S2C12, which condensed as a glassy solid, were unsuccessful. Depolarization measurements on the liquids were made using a halfwave plate between the laser beam and the sample, and a Polaroid film between the sample and the monochromator.

The IR spectra in the range 33-600 cm- ’ and the Raman spectra of the four title compounds are iisted in Tables f-4. The frequencies of typical bands reported in the present work should be accurate to 41 cm-’ in the IR, and to 22 cm-’ in the Raman spectrum.

The IR spectrum of S,CI, vapour and the far-m spectrum of Liquid S,C12 are reported for the first time. The IR spectrum of liquid SzCIz above 400 cm-’ has -been thoroughly examined5, and so this region was not further investigated here. The Raman spectrum has been reexamined, and there is good agreement with the data reported by Stammreich and I?ornerisz except in three important respects (Table 1). The bands 105 and 540 cm” are polarized, and the shoulder near 430 cm-l appears to be denolarized.

The‘= and Raman spectra of liquid S,Br, and the Raman spectrum of solid S,Brz obtained in the present study, along with the earlier-Raman data of Stammreich and Forneris’; the IR data reported by Bradley et a.L6, and the previ1. i&&d.Sfriaure, 2 (1968)‘27X-2?9

SPECTRA

AND

STRUCTURES

OF

S

AND

Se HALOGENIDES

275

ously reported’ simultaneous transition spectrum of S2Br2 in CS, are given in Table 2. The frequencies of bands obtained from this work agree well with the earlier measurements, but the relative intensities of the IR bands differ significantly from those reported by Bradley et al. ‘j_ The present results differ in two important respects from the earlier data. The Raman band at 67 cm-’ in the liquid,

which has been reported to be probably depolarized’, is found to be polarized. Secondly, the weak band at 302 cm-’ which has been reported in the Raman’ and IRK spectra as a fundamental, is certainly due to a impurity. We did not observe a band near 302 cm-’ in the IR spectrum (as was reported by Bradley et al.). In the Raman spectrum of the liquid a very weak band was observed near 302 cm-‘, but its relative intensity varied with different samples. In the Raman spectrum of the solid a relatively stronger band 299 cm- ’ was observed, but the relative intensity of this band decreased when the temperature of the sample was allowed to increase. Clearly the band near 300 cm- ’ is due to an impurity, and it can probably be assigned to the fundamental of bromine which has been reported to lie at 306 cm-’ in the liquid’. The relative intensity of a very weak IR band 281 cm-l appeared to vary with different samples, and so it too is attributed to an impurity.

The IR spectrum of Se,Cl, is reported for the first time, and the Raman spectrnm has been reexamined (Table 3). The Raman data agree well with the earlier results2. Additional observations reported here are that the band at 289 cm-l is polarized, and the frequency of the lowest band (the torsion) is very sensitive to the molecular environment. Se,CI, appears to react slowly with polythene, giving a weak IR band at 248 cm” which is not included in Table 3. Se,Br, The IR spectrum of Se,Br, is given in Table 4, but a satisfactory Raman spectrum was not obtained. Like Se2C12, the frequency of the lowest band (the torsion) shows a large dilution shift. A weak IR band at 197 cm-’ is attributed to reaction between SezBr, and polythene, and so it is not included in Table 4.

4.

DI!KXJSSION

a) Structures The XYYX molecules considered here may have a planar-cis, planar-trans or non-planar structure, with ‘C,,, C2, or C, symmetry, respectively. The activity of the fundamentals of each of these-structnral.models is given in Table 5. The J. Mol. Structure, 2 (1968) ill-279

276

S.,GiFRANKXSS

planar-trans structure is centro-sy~etric, and so the rule of mutual exclusion should be operative for this model. Several coineidenees are observed., however, between XR and Raman bar& in each of the four moIecuIes, showing that none of them has a planar-trans structure in the liquid state. The planar-cis and nonplanar structures can be distin~ished by their vibrational spectra only in the activity and state of polarization of the torsion. For the &-planar structure the torsion isexpected to be inactive in the IR and depolarized in the Raman spectrum while, tbr the non-planar structure, it should be active in the IR and polari&d in the Raman spectrum. The torsion can’ be confidently assigned to the lowest band in the liquid-state spectrum of each moiecuie. The torsion is found to be polarized in the Raman spectrum of S&1, (liquid) and S2Br2 (liquid), and it was observed in the IR ‘spectra of S&l2 (liquid and vapour), Se&Y, (liquid) and Se2Brz squid). These observations are inconsistent with C,, structures and they .provide positive evidence that ail four molecures in the liquid state, and SzC!iz vapour, have non-planer C, structures. The similarity between the Raman spectra of liquid and solid SJ3r2 indicates that S2Br, in the solid state also has a C, structure. The C, structures obtained here for S&I, and S,Br, are consistent with the reported. electron diffraction structures of gaseous S,CI, and S2Br,‘. The C, structures of Se&f, ana Se2Br2 indicate that in these moXecuIes (like the analogous sulphur &ofecules) the interaction between the vafeaeepn electrons of the adjacent Group-VI atoms is an important factor in determining the azimuthal angle of rotation of one SeX group relative to the other.

Since earlier assignments of S.&f2 and SzBrz have been discussed in detaailby Stammreich and Forneris* and by Bradley et ah6, respectively, further discussion of this-work is not given.’ The spectra reported here can be readiIy assigned 011the

SPECTRA AND STRUCTURES

OF s AND Se WALOGENIDES

277

TABLE 6

G

Schematic descf iptzon

Freqwzcy ctssa

B?SS&

tYsesect

%!ksQ&r

1 2 3 4

YY stretch XY stretch XYY bend torsion

540. 448 208 105

534 357 170 67

289 357 i33 91

29oc 2650

5 6

XY stretch XYY bend

3stP 202

349 150

26SC 1190

No.=

specips a

b

430 242

(cnrl)b

105

63

= See footnote d in Table 3. @From pure Iiqukls except c from d&ate solutions in bemene.

basis of C, symetry, and so only a brief discussion of the assignmentsof the four moIecules is given. The six fundamentals of each molecule are divided into four of species a and two of species6, and the fundamentaisare summarized in Table & There is a marked sim&rity between the spectra of the four compounds, which provides support for the validity of the assignments,particularly in the case of Se,&, for which no Raman polarization data are available.

The four polarized Raman bands at MU, 448,208 and 105 cm-’ in liquid S&I, (Table I) must be taken as the four a fundamentals,leaving the apparently depolarized bands at 430 and 242 cm” as the two b fundamentals.The higher of the two b fundamentzds430 cm --I is taken as the SC1stretch (v5). leaving the lower 242 cm-’ as fhe ClSS bend (it(i)_ By analogy, we can confidently describe the nearby IR bands at 448 and 208 cm-’ as the SC1 stretch (vz) and the CISS bend (vs), respectively. Ttis leaves 540 cm-r as the SS stretch (vr) and 105cm-l as the torsion (vq). The assignmentof the spectrum of S,C!l, vapour @a& 1) follow; that of the Eouid.

The three poiar&ed Raman lines at 357, 170 and 67 cm-r in liquid §&a (Table 2) m&t &early be +Ggned to three of the four 4 fun~eu~, feaving. those at 534,356 and 202 cm-r to be qssi~ed to the remaining a and the two b fundarucntals.The two lower bar&is(3% and 202 cm-*) are favoured aa the two

278

S. G. FRANKfSS

-b. fundamentals ‘since they are near the two a fundamentals 357 and 170 .cm-‘, respectively. The higher pair (357 and 356 cm-‘) are conveniently assigned to the tie SBr stretches .(v2 and v5, respectively), while the lower pair (170 and 202 cm-“) can be described as the two BrSS bends (vf and v6, respectively). The lowest a fundamental at 67 cm-‘-_*IS clearly the torsion. This leaves 534 cm-r as the SS stretch vl, which is very close to the SS stretch in S&l2 (540 cm-‘). The assignment of the Raman spectrum of solid &Br, (Table 2) follows that of the liquid. The splitting of vs into two components at 350 and 353 cm-’ is probably due to 7pBr-81Br isotope splitting, though it may be crysta1 state splitting. The low-lying bands at SO, 75 and 88 cm-’ can be assigned to lattice modes but, since the torsion in the liquid gives an intense Raman band at 67 cm:‘, this suggests that the equally intense 88 cm-l band in the solid is also the torsion v,.

The three polarized Raman lines at 357,289, and 133 cm-l are three of the four a fundamentals. The fourth a fundamental can be confidently assigned to the lowest band in the spectrum (91 cm-‘) since this is surely the torsion v4. The strong IR band at I Sd cm-’ must be one of the two b fundamentals. The second b fun$amental appears to be near 350 cm -l. In dilute solutions of Se&l2 a Raman band is observed at 367 cm-‘, while the apparent IR counterpart is centered at 359 cm- I. This separation of 8 cm-’ is significantly larger than the experimental errors of these frequencies, which indicates that there are two separate bands in this region;centered at about 367 and 359 cm-’ in the dilute solutions. 367 cm-’ is taken as the a fundamental since it is polarized, leaving 359 cm-’ as the second b fundamental. In the pure liquid the a fundamental shifts to 357 cm-l, which suggests that the nearby b fundamental is close to 349 cm-l. We therefore have 357,289, 133 and 91 cm-’ as a fund~en~s and 349 and 150 cm-’ as b fundamentals in pure liquid Se2Cl,. Since the two b fundamentals 349 and 150cm”’ must be assigned to the SeCI stretch (v5) and the ClSeSe bend (Q), respectively, we can take the nearby a fundamentals at 357 and 133 cm-l as the SeCi stretch (vi) and the ClSeSe bend (v&, respectively- This leaves 289 cm-* as the SeSe stretch (vi), and the lowest band 91 cm-’ as the torsion (v&

Polarization data are lacking for Se,Br,, and so the vibrationalassignment must be based largely on comparisons with the other three halides; The five bands that were observed in the IR spectrum can be confidently attributed to fundamentafs. Comparison .with the related three molecules suggest that&e lowest. three bands at 63, 105 and 119. cm’ I, should be assigned,.to the torsion (VA the a J. Mid.J'iructure, 2 (1968)~27G279

SPECTRA

AND

STRUCTURES

OF s AND

St! HALOGENIDES

279

BrSeSe btXXi (vg) and the b i3rSeSe~bend (vg), respectively. The highest band 290 cm-1 is an obvious choice for the S&e stretch (Ye) since it is very close to the SeSe stretch in Se&l, (289 cm-‘). This leaves the two SeBr stretches unassigned. Comparison with the other three halides indicates that they are likely to be close together, and they can be expected to give strong IR and Raman bands. They are therefore both assigned to the remaining strong IR band 265 CM-~ which has a strong Raman counterpart. Two Raman bands (94 and 204 cm-r) have been attributed by Stammreieh and Forneris to fundamentah?. The present analysis indicates, however, that this is unlikely. They may be overtones (Table 41, but it is possible that, since very long exposure times were required to record the spectrum2, they arise from decomposition products.

ACKNOWLEDGEMENTS

I am very grateful to Prof. F. A. Miller for his interest in this work, and for granting me use of the spectroscopic facilities at Mellon Institute during the tenure of a Visiting Research Feliowship in the summer of 1967. I am also indebted to Dr. G. L. Carlson for recording the Raman spectrum of solid S2Brz_This work was supported by the U.S. Army Research Office - Durham under grant AROD-31-124-G-735. I gratefully acknowledge an LCJ. Research Fellowship at University College, London.

I E. HIROTA, Bull. Chem. Sot. Japan, 31 (1958) 130. 2 H. STAMkfREiCE AND R. FORNERIS, S&wm.w&m. Acra, 8 (1956) 46. 3 G, BRAUER, Handbook of Preparatiue Imrganic Chemistry, Vol. 1, Academic Press, New Yctrk ~ 1963, D. 377. 4 G. L. cAlU.SON, Specfmciiim. Actq in press. 5 H. J. BI%~N.%ELN AND J. Powmtx, J, C&m. Phys+, 18 (1950) lOIS. 6 E. B. BRADLEY, C. R. BE~EZT m E. A. JONES,Spectrochim. Act& 21 U965) 1505. 7 J. A. A, 7KmzLAAR, F. N. Hoam AND G. BUSS%, Rec. Trav. Chfm., 75 (1956) 220. 8 H. STAhfhf~EIcH AND R. 3%mmrs, J_ Chew. Phys., 22 (1954) 1624. J. Mof. Stmcture, 2 (1968) 271-279