Vibrational spectra and conformation of cyclic methylsiloxanes

SpectrochtmicaActa, Vol. 30A, pp. 629 to 639. PergamonPress 1974. Printed in Northern Ireland

Vibrational spectra and conformation of cyclic methylsfloxanes G. I~OOARASI L. EStvSs University, Budapest, Hungary* H . HACKER Universitiit Miinchen, Germany V. HOFFMANN UniversitRt Ulm, Germany

and S. DOBOS Hungarian A c a d e m y of Sciences, Budapest, H u n g a r y (Received 19 September 1972; revised 10 February 1973) Abstract--Complete vibrational spectra (including far-i.r.) are given for the cyclic methylsiloxanes from the trimer up to the hexamer. The assignment, given for hexamethylcyclotrisiloxane, is based on a planar structure, b u t some slight deviations from the strict Daa selection rules can be observed. The conformation of the higher members of the series is investigated in the light of flexibility of the rings. INTRODUCTION

THERE have been several investigations on the vibrational spectra of cyclic methylsiloxanes, --(Si0Me2).--. (In the following the usual notation D will be used for the siloxane unit --SiOMe~--). The molecular conformation was first discussed by KRIEGSMA)r~ for D a [1] and for D 4 [2] and later, with very similar conclusions, by LAZAREV[3] and by LAZ~EV and Tv.~rrSHEVA[4]. The latter authors also discussed the higher rings from D 5 up to D~ [5]. In all these investigations the experimental data were still restricted to incomplete i.r. spectra and to Raman spectra without depolarization values. Without analyzing the molecular conformation, KovAL~.vet al. [6] reported depolarization values for Da up to D~, measured by a classical mercury lamp instrument. Very recently ALvrK and DALE [7] reported i.r. spectra and laser Raman spectra without depolarization values for the oligomers from D 4 up to DI,, giving a brief classification of the observed bands. As the experimental evidence for previous analysis of the molecular conformation was grossly incomplete, it seemed worthwhile to complete the vibrational spectra on the level of present experimental technique and reinvestigate earlier results. We * During this work p a r t l y at the Universit~t Ulm, Germany.

[I] H. KaIEOS~N~, Z. Anorg. Allgem. Chem. 298, 223 (1959). [2] [3] [4] [5] [6]

H. K R I E O S ~ r , Z. Anorg. AUgem. Chem. 298, 232 (1959). A. N. L~Z~R~.V, Opt. Spektrosk. 18, 792 (1965). A. N. LAZA~.V and T. F . TENISH~VA, IZV. Akad. N a u k S S S R . Ser. Khim. 1168 (1964). A. N. LAZA~EV and T. F . TV.~ISHEVA, IZV. Akad. 1Vauk. S S S R , Ser. Khim. 983 (1966). I. F. KOV~LEV, L. A. OzoLxN and hi. G. VO~O~rKOV, Dokl. Akad. Nauk. S S S R , 181, 577 (1968). [7] T. ALVI• and J. DALE, Acta Chem. Scand. 25, 2142 (1971). 629

630

G. F o o x ~ s I

et al.

present here our laser R a m a n polarization data and vapour-phase i.r. and far-i.r. data for the molecules D3, D4, D 5 and D e. A detailed assignment for D~, and, in the light of flexibility of the rings, some critical comments on earlier assignments for D 4 D 5 and D e will be given. EXPERIMENTAL

The purity of the samples used was checked by gas-chromatography and found to be better t h a n 99 per cent. Vapour phase i.r. spectra were recorded in the frequency range 200-4000 cm -1 with a Perkin-Elmer 225 spectrometer. The far i.r. spectra in the region 10-400 cm -1 were obtained on the R I I C FS-720 interferometer coupled with the FTC-100 Fourier transform computer. For comparison and especially in cases of weak absorption solution spectra were also run. The R a m a n spectra were recorded with a laser R a m a n spectrometer. Its main components are a Spectra Physics Ar+ laser Mod. 140 or Mod. 165, respectively, a Jarrel-Ash 1 m double monochromator and a photon counting system. The power of the excitation line (5154/~) was reduced to 100-500 mW, to avoid boiling or destruction of the sample. 90 ° sample illumination system was used. Typical slit width was 3 cm -1, for weak lines 5 cm -1 and for optimal resolution 1 cm -1. Except weak and broad lines the wavenumber error is less t h a n 2 cm -1. For polarized bands where quantitative data are given the error is less t h a n 10 per cent, for well separated depolarized bands p = 0.75 4- 5 per cent. In cases of weak or overlapping bands " d e p " or "pol" is given only if the uncertainty was less t h a n 20 per cent. l~ESULTS

AND

DISCUSSIO~

The frequency data are compiled in Table 1 and the spectra reproduced in Figs. 1 and 2. Every detail of the spectra was also recorded under the best operating conditions, which cannot be reproduced here. The frequency range above 1200 cm -1 is also omitted as this part of the spectrum gives no information on the conformation. Table 1. Vibrational frequencies of cyclic methylsiloxancs up to 1200 cm-I* i.r. (vapour)~

Hexsznethyleyclotrisiloxane Raman s o l u t i o n (p)~

Ootamethylcyelotetrasiloxano Raman Raman i.r. Raman m e l t 90°C t r y s t . (vapour)t l i q u i d (p)~

46 w 157 v w 192vw

221 v w 292 w , sh 308 s 392 s

40 w

ls6/

197~s, d e p ~200)

307~ . 323~vw, a e p --

414 v w , sh 453 vs, pol ( ~ 0-1)

172~ 191J

307~ 32uf r o m 395~ to 410) (vw, pol) 453

lse

198 203

307 323

67 w, d e p 146 m , dep 1 5 4 v w , sh 170 v w , b r

165 w, sh, dep 197 s, p o l (0'65)

227 v w 253 v w 292 v w , sh

257 w,

415 451

342 m , sh 362 s

dep

338 w , pol 341 w , dep 366 v w , pol

Vibrational Table

1

spectra

and conformation

of cyclic methylsiloxanes

(contd.)

i.r. (vapour)t

H e x a m e t h y l c y c l o t risiloxane Raman solution (p)~

R a m a n R~mAu m e l t 90°0 cryst.

490 v w ( i m p u r i t y ? ) ~580 609 650 687 ~720 745 760

631

383 s 524 585 608 ~650 688 724

v w , sh s vw m v w , sh vw w

v w , pol vs, pol ( < 0 ' 1 ) w, dep w, pol w, dep vs, pol (0'2)

7 9 3 m , dep

~525 585 610 654 689 724

525 582 605 645 685 720

785~ 792)

8 7 4 w , pol (0.4)

475 548 618 638 658 694

w m vw vw w m

7 4 6 v w , sh 796m

875~ 877)

865 870

920w

v w , pol m , pol (0-1 / vs, pol (0.02) v w , pol

632 658 689 712

v w , dep w, dep w, sh, dep s, pol (0.041

954 w 1025 v w br, pol

1034 vs, b r 1 0 6 0 m , sh 1110) 1130~ TM 1179 v w

793 w, dep 808 v w , s h , -

815 v s 870w, br

--

386 451 477 548

788

817 v s 850 w, sh 877m

Oct a m o t h y l e y c l o t e t r a s i l o x a n e i.r. Raman (vapourlt liquid (p)~

806 w, pol (0"2) 882 w, dop

1002 v w , sh 1028 w, s h , - 1073 v w , br, pol 1091 v s 1153 w, b r

Decamethylcyclopentasiloxane i.r. Raman (vapour)'[" (liquid) (p):~ 40 w 158 v w 220 240 261 290

v w , sh v w , sh vw w

352 m 386 s 483 w 525 w 612 w

669 w

38 w 155 s, dep 191 s, dop 240 w, sh, pol 288 307 339 350 388 405 485 527 592 613 654

w. pol v w , sh, dep w. pol v w , sh, dep w, dep w, pol vs, pol (0"011 vw, sh,-v w , pol v w , pol vw,--

073 w, sh, dop 688 w, sh, dop

698 m 710 vs, pol (0-01) 746 790 81I 859 880 1010

w, sh w, sh vs m, sh w, sh vw, br

Dodecamethyleyclohexasiloxane i.r. Raman (vapourlt (liquid (p):~

793 m , d e p 806 v w , sh, dep 8 6 6 m , pol 877 w, sh, dep

155 w 260 w 290 w, sh 340-380 m , b r 390 s 490 v w 517 w 586 w 058 w 685 w 700 m

45? 157 s, dep 188 s, dop 259 w, dep 296 w, pol 319 w. pol 390 v w , pol? 493 vs, pol (0"02) 583 v w , - 032 v w , pol 686 m , dep 708 s, pol (0"11

700 788 804 820 860 912

w, sh, b r w, sh vs w, sh m

792 m , dep

864 m , pol (0"3)

vw

1080 vs. b r

1004 w. pol 1076 v w , - -

1075 w, br, d e p 1095 vs, b r * w, m , s, ~ w e a k , m e d i u m , strong; v ~ v e r y ; sh ~ shoulder; b r ~ b r o a d . t F o r s o m e v e r y w e a k b a n d s t h e a b s o r p t i o n in t h e v a p o u r p h a s e was n o t sufficient in this case t h e f r e q u e n c y d a t a r e f e r to solution spectra. D e p o l a r i z a t i o n v a l u e s g i v e n in b r a c k e t s .

G. FOGXRASI et al.

632

IOC

oo

'

fi~ .~

40 t'

~

2O

II II

"

I

II ~

,

•

ii

J

i III ; ~j i

n 'V

l; 11 i I II

II

600

400

lOCi

4O 20 I00 so

i

II ~II I

II I

60

I

Jl

40

, I

l i

II

I/l/Ii '~l]

i

r

,

I

.

v

]~/

ill

,I !~.Jv

II

,,

I

~.

I00

o.

so

r! ' II

40

,~!~ /V

'

I

i

I

g

-

~,

il t,II LZ II I I

I100

900

I~

700

500

300

I00

1200

I000

SO0

200

cm-I

Fig. l . R a m a n s p e c t r a (left,) a n d i.r. s p e c t r a (right) of cyclic m e t h y l s i l o x a n e s , D n. T h e R a m a n s p e c t r u m of D s w a s i n v e s t i g a t e d o n t h e m e l t a t 90°C a n d for c o m p a r i s o n o n a p o l y c r y s t a l l i n e s a m p l e ( b r o k e n line). T h e o t h e r R a m a n s p e c t r a were o b t a i n e d o n liquids a t r o o m t e m p e r a t u r e . T h e i.r. s p e c t r a were r e c o r d e d i n a 10 c m gas cell f i t t e d w i t h C s I w i n d o w s u n d e r full v a p o u r p r e s s u r e a t o p e r a t i n g t e m p e r a t u r e (ca. 35°C). D 3 w a s also i n v e s t i g a t e d i n a 1 m cell ( b r o k e n line).

V i b r a t i o n a l spectra a n d c o n f o r m a t i o n o f cyclic m e t h y l s l l o x a n e s

633

100

6O 40 D3 2O

0 I00

I

I

I

I

I

I

I

I

i

t

I

!

1

I

I

I

I

I eO

I I~:~'

I 160

I 200

I 240

I

80 60 40

[34

20

ioo°

I

I

I

80 60 40

D5

2O 0 I00

I

I

I

8O 6O 40

De

2O Q 2O

I 40

I 60

I 80

I00 40

280

c m -I

Fig. 2. Far-i.r. spectra of cyclic methylsiloxanes, I) n Left---vapour, 1 m cell, ca. 80°C; right---solution (10% in cyclohexane), 4 mm cell. The sharp, weak bands in the vapour spectra are due to traces of water.

T h e v a p o u r phase i.r. d a t a , where t h e y can be c o m p a r e d , agree w i t h earlier solution spectra. Thus, we feel t h a t it will be justified in the following t o c o m p a r e i.r. v a p o u r spectra with R a m a n spectra o b t a i n e d on liquids. T h e only considerable f r e q u e n c y shift was d e t e c t e d for t h e v e r y strong, b r o a d b a n d b e t w e e n 1000 and 1100 cm -z, whose m a x i m u m i n t e n s i t y position is 10--15 cm -1 higher in the gas t h a n in solution or p u r e liquid. Due t o t h e b e t t e r resolution some weak shoulders could be observed which are o f g r e a t i m p o r t a n c e concerning c o n f o r m a t i o n and were n o t y e t reported. I t is w o r t h m e n t i o n i n g t h a t e v e n u n d e r the best resolution, c h a r a c t e r istic b a n d c o n t o u r s or fine s t r u c t u r e s could n o t be detected.

634

G. FoG~.AsI et al.

In the far-i.r, spectrum special attention was paid to the low-frequency band at about 45 cm -1 in each molecule. As it can be observed not only in solution b u t very clearly in the vapour, too, (Fig. 2), it is surely an internal vibration. I t is to be mentioned that its overtones could not be detected. Among the R a m a n spectra our spectrum for D 3 is the first one published* which contains depolarizations. For the other rings the agreement with KOVAL~V'S results [6] is good, b u t our spectra, especially the polarizations, are more complete. With the results b y ALVIK and DALE [7] only a qualitative comparison could be made because they did not tabulate the frequencies. In the following we discuss each molecule separately.

Hexamethylcyclotrisiloxane, D a A planar structure (D3h point group) has been suggested for D 3 both b y vibrational spectroscopy [1] and b y an early electron diffraction study [8]. We confirmed this in a recent work on the basis of i.r. dichroism of oriented crystalline films [9]. However, no R a m a n polarization data have been reported as yet, although they can obviously deliver the most direct information on the molecular symmetry. I f one considers the two most probable molecular structures, namely Dab and Cag, respectively, the representation formed b y the skeletalvibrationsis the following: F(Dah ) -= 4AI'(R,o~) + 2A2' ( - ) q- 6E'(ir. Rdep) -~-AI~(--) -~- 3A2"(i.r. ) A- 4E"(Rdep) F(C3~) = 7Al(ir. R~ol) -b 3A2(--) + 10E(i.r., Rde,) Important is the remarkable difference in the number of expected polarized R a m a n fines. (In addition, for any other symmetry the number of polarized R a m a n lines is also higher than four.) The completed vibrational data strongly support the planar structure of Dab symmetry. The assignment, as given in Table 2 can be made in a relatively straightforward way, the few uncertainties being of minor importance for the conformation. Three of the four totally symmetric modes can easily be found. The fourth (SIC2 scissoring) mode should certainly lie below 400 cm -1. Actually we have found no polarized R a m a n band in this region. However, as it will be seen later, there is in all other rings a weak polarized band at about 300 cm -x which can relatively well be assigned to the SiC2 scissoring. Thus, we tentatively assign the 322 cm -1 R a m a n line to this mode, assuming an accidentally high depolarization value. Most of the i.r. active, A2" and E', modes have already been assigned in our previous work on oriented films [9]. Now, in addition we have found the third As" mode (ring out-of-plane) in the very far i.r. (46 cm-1). The fact that this band dearly appears also in the vapour phase spectrum proves that it belongs to an internal mode. I t is to be pointed out that the frequency value found is very low. This indicates that even D a has a relatively high flexibility which is expected to be of more importance * When writing the manuscript Dr. D. M. Adams informed us that they had also carried out a detailed vibrational study on D 3, submitted for publication in J. Chem. Soc. (Da/ton). [8] E. H. W. AOOA~WALand S. H. BAUEr, J. Chem. Phys. 18, 42 (1950). [9l S. DOBOS, O. FOGARASI and E. CASTEI~UOCI, Sl~-~trochim. Acta ~SA, 877 (1972).

Vibrational spectra and conformation of cyclic methylsiloxanes

635

Table 2. Assignment of the skeletal modes and methyl rooking modes of hexamethyleyelotrisiloxane Species A I ' ( R , pol)

A,~ (i.r.)

E ' ( i . r . , R)

E~(R)

AI'-]- E' A i M-~- E H A i r _~ ~ t -41~ ~- E ~

A p p r o x . description

Frequency

SiO s t r e t c h i n g SiC s t r e t c h i n g ring d e f o r m a t i o n SiC s scissoring

585 724 454 (322)~

SiC s t r e t c h i n g SiC 2 rocking ring out-of-plane

817 392 46

SiO s t r e t c h i n g (as.) SiO s t r e t c h i n g ( s y m ) SiC s t c t c h i n g ring stretch.-def. SiC I scissoring SiC a w a g g i n g SiC s t c t e h i n g SiC 2 twisting~ SiC I r o c k i n g ) r i n g out-of-plane CH s rocking CH s rocking CH s rocking CH 8 rocking

Pl Ps Ps p~

1034 609 687 414 308 192 793 197~ 200) -877 687 (overlap) 817 (overlap) (785)~

A p p e a r a n c e in t h e s p e c t r u m i.r. P~man vw vvw ---

vs s w vs s m v w , sh s vw

vs, pol s, pol vs, pol vw,

dep ----

-w, d e p m, dep -vw,-s, dep

-----

m , dep ~m, dep }

m m vs --

w, pol w, d e p -m, dep

in the higher rings. The other out-of-plane mode (E ") which should be active in the Raman, was not observed. A rather uncertain point in the assignment is the E ' ring stretching-deformation mode. K~EGS~A~N reported a weak absorption in solution at 450 cm -1 and assigned it to this vibration, assuming an accidental degeneracy with the ring A 1' mode. In S~rTH'S work [10], in his Fig. 5 there seems also to be a weak absorption at about this frequency. We also reported a weak absorption in the solid [9]. I n the present work, in our 10 cm gas cell we could not detect this band, although we were able to observe some very weak bands not reported previously. We tried to find it in a 1 m cell, b u t it seems that one can definitely exclude any absorption (in the vapour) at this frequency (Fig. 3). (It is to be mentioned, however, that in a cyclohexane solution we have also found a weak absorption.) We tentatively assign the E ' ring stretching-deformation mode to the weak band found at 415 cm -1. Obviously the experimental evidence for this is rather vague because no R a m a n counterpart was found. The other uncertain point is the assignment of the weak band at 650 cm -x. Considering its position and low intensity and the fact that it appears both in the R a m a n spectrum (polarized!) and in the i.r., we think that it can hardly be a fundamental. The obvious solution would be to look for a combination level of the type E ' × E ' or E " × E ~, which giving AI' -~ A2' ~- E ' representation, would account for the experiment. However, we have found no such combination and the assignment remains here open. For some weak bands in the i.r. spectrum the experimental data are not enough to give a satisfactory assignment. The higher frequency bands are considered as [10] A. L. SJarx~, ,~pe~roehim. Ac~a 19, 849 (1963).

G., FOGAa~SI et al.

636 I00

80

c o

E c

40

I-

20

I 650

600

I 5~

I 5~

I 4~

400

c m- I

Fig. 3. Part of the vapour phase i.r. spectrum of I) 3, indicating the absence of any absorption at 450 cm -1, but showing clearly an absorption at 580 cm -1. 1 m cell, KBr windows; broken line--zero absorption level.

various combinations, some bands in the far-i.r, region can eventually be methyl torsional modes. For the methyl rocking vibrations we assume that they split into the following four components:

pl(Al' + E'), p2(A2"--]- .~"), p3(A2' -~ E ' ) , p4(AI" Jr- E") i.e. they split within one Si(CH3) 2 grouping, according to the local C2, symmetry, but do not couple with methyl groups on another silicon atom. Three of these four rocking modes were found at 877, 687 and 814 cm -1, respectively, in our work on infrared dichroism [9]. There, we were able only to establish that the 687 cm -1 band is the one with A2" component and the other two are those with E' component. The fact that the i.r. band at 877 em -x has a polarized Raman counterpart (depolarization ratio ---- 0.4) proves now, in addition, that this is the pl(Al' --FE') mode, thus leaving the 814 em -1 i.r. band (overlap with the SiC2 As" stretching) for P3. To the fourth methyl rocking mode (active only in the Raman) can eventually correspond the 785 cm -1 band, observed only in the melt, in solution perfectly overlapped by the band at 793 cm -x. Concerning molecular conformation, a very important fact is that we have found two weak shoulders in the i.r. spectrum at about 580 em -1 and 720 em -1. (The latter cannot be quite surely observed, but the first one is clearly present in the spectrum, see Fig. 3). It seems obvious that they are the i.r. counterparts of the corresponding strong, polarized Raman lines at 585 and 724 em -1, respectively, which means slight deviations from the strict Daa selection rules. Vice versa, the Raman counterpart of one of the A ~" i.r. active bands was also observed in the best quality spectrum of the melt (395 cm -1, pol. In solution this very weak band is overlapped by the

Vibrational

Table

3.

Assignment

spectra

of the totally

and

symmetric

conformation

modes

and

of cyclic

the Pz modes

D8

(A~o)

Ai ~ (Al")

SiC str. SiC s t r . r i n g def. S i C a sciss.

585 724 453 322

i.r. vs, p vs, p vs, p vw, dep

SiC s t r . -SiC l r o c k . ,~395 vw, p out of-plane --

in the

Da

R

A x"

methylsfloxanes

~580 ~720

R

vw, sh vvw ~ --

817 v s 392 s 46 w

477 712 451 338

vs, p s, p m. p w, p

-386 v w , p --

637

approximate

D,~

D5 i.r. 475 w ---815 v s 383 s 40 w

R 485 710 405 339

vs, p vs, p w, p w, p

----

description De

i.r. 483 v w ---811 v s 386 s 40 w

R 493 708 390 319

i.r.

v~, p 490 v w s, p -vw, p -w, p --

~ ---

804 v s 390 s 38 w

453 cm -1 band). We interpret the appearance of these forbidden bands by the existence of states where the very low-frequency A~" out-of-plane mode is thermally excited, thus lowering the s y m m e t r y to C3~. This again points out the flexibility of the ring.

Octamethylcyclotetrasiloxane, D 4 The molecular conformation of D 4 has been discussed by K R I E G S ~ [2, 11] and b y LAZARv,V and TE~ISH~VA [4]. They both interpreted the vibrational spectra on the basis of C4~ symmetry, with slight deviations from each other in the assignment. I f one compares these assignments (Table 3 in Ref. [2] and also Table 3 in Ref. [4]) with our R a m a n data, one sees clearly that the lack of R a m a n depolarization values inevitably led to serious errors. Nevertheless, as we feel, the important question is not the assignment in detail but rather the conformation, namely, whether the C4~ symmetry can be confirmed on the basis of completed experimental data. Going from D4a point group down to its subgroups we have systematically investigated all possible conformations. As to specifically the Ca~ symmetry, this can be relatively simply checked and actually, excluded. Let us start as a rough first approximation with D4a symmetry. Considering depolarization values, band positions and intensities and by (cautious!) comparison with hexamethylcyclotrisiloxane, one can relatively uniquely identify the four Ala modes and three A~, modes, as given in Table 3. When going from D4a to C4, symmetry, as a consequence of the Alg(R ) -~Al(ir, R) and A~,(ir)-~ Al(ir, R) correlations t h e y should mutually become active both in the R a m a n and in the i.r. spectrum. Actually only two coincidences are found and even here the counterparts are of very low intensity. (An eventual third coincidence could be the R a m a n band at 338 cm -1 if one considered this and not the 341 cm -1 band as counterpart for the i.r. band at 342 cm -1, see Table 1.) One should realize that our Rayleigh line was sharp enough to make possible the observation even of the counterpart of the i.r. band at 40 cm -1, if this existed. One can, of course, suppose that some allowed bands actually do not appear in the spectrum, but their number seems to be too high. Indeed, the coincidences are not at all more expressed than in the case of 1)3 which is certainly practically planar. Furthermore, while here m a n y bands are lacking the [11] H. JA_~CKE, G. 25A, 85 (1969).

ENGELHA~DT,

R. RADEOLIA and H. KR~GS~rN, Spec~roch~,n. Acta

638

G. F o G ~ u ~ s i et a/.

polarized R a m a n lines at 197, 366, 548 and 1073 cm -1 cannot be accounted for on the basis of C4, symmetry. (The polarized bands at 866 cm -1 is considered as a methyl rocking mode.) In addition, according to C4~ selection rules, there should be four very low frequency out-of-plane modes (A x, B 1, B 2 and E), actually only two are found as it is allowed b y D4a. With respect to conformation it is to be mentioned that we have found a polarized R a m a n band at about 1073 cm -1. If, as it seems very probable, this is considered as an antisymmetric SiO stretching, among the higher symmetries it is only ~q4, where the totally symmetric species does contain such a vibrational mode. However the spectrum as a whole (number and position of polarized bands, i.r.-Raman coincidences, etc.) excludes the possibility of a fixed ~q4 conformation, too. Our attempts with any other conformation led to the same result that a unique conformation actually does not exist. The failure of previous attempts lies just in the fact that this was not clearly realized. I f a unique conformation does not exist, one has a choice to select a convenient conformation, about which one wants to describe the internal motions of the molecule. The most convenient choice is obviously D4h, taking into account that the planar structure is permanently distorted b y thermally excited low-frequency out-of-plane vibrations. Without attempting a detailed discussion on the basis of non-rigid molecular group, we have compiled in Table 3 those vibrational modes which can be relatively uniquely assigned. These results also suggest that the distortion of the planar structures cannot be extreme. In this model, the polarized R a m a n line at 1073 cm -1, discussed above, is considered as the A~o antisymmetric Si0 stretching mode which appears due to the thermal excitation of the BI~ and B2, out-of-plane vibrations. D 5 and D e The molecular conformation was discussed b y T~AZA~,:EVand TElqISHEVA [5] on the basis of i.r. and R a m a n frequencies. They proposed C 5 and C 6 symmetry respectively, which would mean distorted C ~ structures, where the methyl groups are twisted out of the original ~ plane. Their main reason was that there are two i.r. bands observed in the antisymmetric SiO stretching region. Our vapour phase d a t a do not confirm this. The band at 1095 cm -1 for D~ and that at 1080 cm -1 f o r / ) 6 , respectively, is very broad, as it is characteristic of this band in all siloxanes, b u t it cannot be resolved into two components. In addition, we think that there is no significant steric effect that would twist the methyl groups out of the ~ plane. Elementary calculations show that the shortest C---C distance in a plausible G ~ structure is much longer than the C--C distance within the SiMe 2 grouping. Even more important are the following arguments against a possible Cno structure which exclude at the same time the Cn symmetry, too. As it was done for D 4, let us consider again in a D~h model the totally symmetric modes (A 1' for D 5 and Alg for De) and the /z~ modes (A2" and A2~, respectively). According to the Cn, selection rules they should become mutually active in the i.r. and R a m a n spectrum. As can be seen from Table 3, among the totally symmetric vibrations only the SiO stretching appears in the i.r. with very low intensity. I n addition, even this can very well be considered as not the i.r. counterpart, but, due to a possible accidental coincidence, as the other SiO symmetric stretching, that of species E 1' for Da and

Vibrational spectra and conformation of cyclic methylsiloxanes

639

EI~ for De, which otherwise cannot be found. At least two of the three A2* (A~u) modes remain inactive in the R a m a n spectrum. (For the low-frequency vibration this cannot be definitely stated, as our Rayleigh line was here not sharp enough.) I t is to be mentioned that the band at 388 cm -1 in the R a m a n spectrum of D 5 cannot correspond to the 386 cm -1 i.r. band, as in this case it should be polarized. (If only not an accidentally high depolarization value is assumed.) Furthermore, the i.r. and R a m a n bands at 390 cm -1 in the D e spectrum are considered as accidental coincidence, otherwise either the ring deformation or the SiC2 rocking vibration is not found. For D 5 the only possible higher symmetry is Use and this is thus excluded. For D e we have also examined the two D3d structures and the S e structure, with the same negative result. The conclusion is the same as it was given and discussed in more detail for D 4, namely, that the rings have a high flexibility and the spectra cannot be interpreted b y any single conformation. GENERAL CONCLUSION On the basis of the completed vibrational spectra the following conclusion can be drawn on the molecular structure of cyclic methylsiloxanes. D s is practically planar. I t means, its vibrational spectrum can well be interpreted b y Dsh symmetry, but, as consequence of the thermally excited low-frequency out-of-plane vibrations, one cannot establish, whether the potential energy has a single (fiat) minimum at D3h or double-minimum at slightly puckered U3~ conformations. The higher rings, D 4, D 5 and D e have no unique conformation. I n this respect the following arguments are of primary importance. 1. The out-of-plane vibrations are found at very low frequency values (40-70 cm-1), which means that t h e y are thermally highly excited at room temperature. 2. F o r each molecule only two out-of-plane vibrations are found, although their number should be higher in any conformation, except D~h. 3. Overtones of the out-of-plane vibrations cannot be detected which indicates that although the corresponding potential function is certainly anharmonic, the anharmonlcity cannot be extremely gross. 4. Although a full discussion would be rather complicated, the totally symmetric modes and t h e / ~ modes can fairly well be interpreted on the basis of D,~ symmetries. The low intensity of the corresponding counterparts indicates t h a t the displacements from the planar structure are not very great, keeping the rings in a fairly fiat geometry. Ac~ow~geme~s~One of us (G. F.) wishes to thank Professor W. ZEvr. for his kind hospitality during his stay at the Universit~t Ulm in 1969/70 and for the kind invitation for the summer 1971. He also wishes to ~cknowledge the Deutschen Akademischen Austausehdienst for a fellowship during his first stay in Germany. This research was supported in part by the Deutsche Forsehungsgemeinschaft.