Spectrochimica Acta, 1967, Voi. 25A, pp. 1075 to 1087. Pergamon Press Ltd. Printed in Northern ~ d

Vibrational spectra and assignments for the phenyl chlorosflanes A. L. Sm-r~,

Spectroscopy Laboratory, Dew Coming Corporation, Midland, Michigan

(Received II May; revised II August 1966)

Abstract--Infrared spectra have been recorded for the phenyl chlorosilanes from 35 to 3800 em -1, and R-rnan shifts and polarisation data obtained. The characteristic phenyI-silicon absorption bands are identified, and internally consistent assignments are made for nearly all fundamental and observed combination bands (except torsional modes). Low frequency absorp- tions are assigned as phenyl ring modes and vibrations of the Ph,SiC14_ . skeleton; the two types of vibrations axe separated and identified by comparison with several related series of silanes.

INTRODUCTION

MUCH of the usefulness of inf rared spect roscopy derives f rom the existence of group frequencies. Whereas mos t group frequencies show var ia t ions in posi t ion wi th mass, induct ive effects, or o ther less easily recognised a t t r ibu tes of the a t t ached atoms, the pheny l der ivat ives of the metalloids, par t icu lar ly those of group IV-A, show an amazing s imilar i ty of the i r inf rared absorpt ion pa t t e rns [1, 2]. I t is of in teres t to iden t i fy these cons tan t absorpt ions wi th the normal v ibra t ions of the phenyl group, and of even more in teres t to ident ify, characterise, and explain the var iable ab- sorptions. Compounds containing phenyl groups a t t ached to carbon, however , cons t i tu te an except ion to the general pa t t e rn and give bands differing in b o th f requency and in tens i ty f rom o ther members of the series.

Phenyl -s i l i con containing compounds were ear ly observed to have character is t ic spectra, regardless of the s t ruc tu re of the pa ren t molecule [3]. Subsequent studies of such compounds have resul ted in a few assignments of bands to specific v ibra t ional modes [4-17] b u t a n u m b e r of assignments conflict wi th one ano the r and wi th those of o ther monosubs t i tu t ed benzenes. I n the present work, a sys temat ic invest igat ion of the molecular spectra of the phenyl chlorosilanes has been carr ied ou t and the

[1] L. A. HxlmA~r, M. T. RYAN and C. TAMBORSKI, Spectrochim. Acra 18, 21 (1962). [2] J. G. NOLTES, M. C. Hm~RY and M. J. JA2cssm% Chem. and Ind. (London) 298 (1959). [3] 1~. WmOHT and M. ft. HUI~TER, J. Am. Chem. Soc. 69, 803 (1947). [4] F. W. B~.m~-KE and C. TA~mORSXI, A.S.D. Technical Report ASD-TDR-62-224 (Feb. 1962). [5] D. H. BROWl% A. M O ~ E D and I). W. A. SHARP, SpecCroehi/m. Acts ~1, 659 (1965). [6] C. C. C~I~TO, J. L. LAUER, and H. C. BEACH~Im, J, Chem. Phys. 22, 1 (1954). [7] M. C. H~vJZY and W. H. I~.BEROArJT., AppZ. Sper2ry 16, 12 (1962). [8] YlI. P. EGOROV and E. A. CHERI~YSa~.V, Fiz. $5. L'vovsk. Goa. Univ. 1957, 390, C.A. 55,

16147e. [9] M. HoRAx, B. ScmC~ID~.I~ and V. BAza_~, CoUer2ion Czech. Chem. Commun. 24, 3381 (1959).

[10] A. MARO~a~rD, M. T. FOREL, F. METRAS and J. VA.LADE, J. Chim. Phys. 81, 343 (1964). [11] H. OIX~WA, 1Vippon Kagaleu Zaashi 84, 1, 11 (1963). [12] 1%. E. R I C ~ D S and H. W. THOMPSON, J. Chem. Soe. 124 (1949). [13] V. A. KOLESOVA and M. G. VOROI~KOV, Coll, eaion Czech. Chem. Commun. 22, 851 (1957). [14] M. E. GRENOBLE and P. J. LAUNEI% AppL Spectry 14, 85 (1960). [15] H. KRIEOSMA~¢I~ and K. H. SCHOWTKA, Z. Physik. Chem. 209, 261 (1958). [16] A. L. S~,'~'n, SperArochim. Acts 19, 849 (1963). [17] L. SI'IALTm% I). C. PRIEST and C. W. HARRIS, J. Am. Ohem. Soc. 77, 6227 (1955).

1075

1076 A . L . S ~ . ~ .

o

~D

A

I

~ ~ ~°°~o~o ~

oo~ooo~ ~ ~

A ~

V i b r a t i o n a l s p e c t r a a n d a s s i g n m e n t s for t h e p h e n y l e ldoros i lanes 1077

f ~

q ~ q ~

A O

O~

A

I

!

II iI zl II

II II II II II II II

1078 A . L . Sm'r~

observed bands have been assigned in such a way as to be consistent throughout the series. A number of other phenyl derivatives of carbon, silicon and tin were studied concurrently, and the results of that work will be reported elsewhere.

EXPERIMENTAL

The pheuyl trichlorosilane and diphenyl diehlorosilane were redistilled products from special preparations, and are believed to be at least 99 percent pure. The triphenyl chlorosilane and tetraphenyl silane were recrystallised, the former in the absence of moisture, and no impurities were detected spectroscopically in either material.

Infrared spectra were determined using a Perkin-Elmer Model 521 grating spectrometer from 3800 to 250 cm -1, and a Beckman IR-11 spectrometer from 250 to 35 cm -1. Calibration of the Perkin-Elmer spectrometer was checked frequently using indene [18]. The far infrared spectrometer was calibrated with water vapor. Samples were run as 10 percent solutions in CC14 (3800 to 1350 cm -1 and 650 to 250 em -1, 0.1 mm.), CS9 (1350-650 em -1, 0.1 ram.) and cyclohexane (250-40 cm -1, 1 ram.) except for very weak bands, which were studied using undiluted samples. Raman spectra were obtained using a Hilger Model E-612 Raman spectrometer, and the method of polarised incident light was used to obtain polarisation data.

Infrared frequencies are believed accurate at least to + 2 cm -1 below 2000 em -1 and ~-4 cm -1 above 2000 cm-1; Raman shifts should be reliable to d-3 cm -1.

CHARACTERISTIC FREQUENCIES OF THE Ph-Si GROUP

In this section we will consider some of the strong absorptions which distinguish the Ph-Si group from Ph-C compounds. Mono-, di-, tri-, or tetrasubstitution of phenyl groups on Si will not be differentiated, since most of the higher frequency bands change only a few cm -1, if at all, throughout the series. Pu t another way, the ring vibrations in compounds containing more than one phenyl group are accidentally degenerate. This observation implies very little coupling between phenyl groups, which are chemically and electrically equivalent. This situation is in contrast to phenyl ether [19], for example, in which nonequivalence of the phenyl groups causes separation of the frequencies.

A complete assignment for all the phenyl chiorosilanes is shown in Table 1. Although the overall symmetries of the molecules vary, the phenyl groups usually act as if their symmetry were C~. Vibrations will therefore be designated by the symmetry symbol appropriate to the group being considered. Above 600 cm -1, assignments are reasonably straightforward. They were made by comparison with other monosubstituted phenyl compounds [20-24] and with the help of the Raman

[18] R. N. JONES and A. N~-UEAU, ~l~ctrochim. Acta 20, 1175 (1964). [19] J .E. KATON, W. R. FEAIRHELLER, JR. and E. R. LII~I'INCO~, J. MoZ. ~'pect~J 18, 72 (1964). [20] J. R. SOIKERER, P ~ 7 ~ V~bT"a,~oft8 of C h ~ o f ~ Bengef~8o The Dov7 Chemical Company"

(1963). [21] J. R. SO~RER and J° C. EVANS, S ~ r o ~ h ~ . A ~ 19, 1739 (1963). [22] C. V. 8TEPHE~SO~, W. C, C O ~ N and W. S. W ~ o o x , S ~ h i ~ . ~ 17, 933 (1961). [23] R. R. R~'VDL~ and D. H. W ~ N , M o / ~ r S ~ r o a ~ (Edited by E. THO~O~ and

H, W. T~oM~soN) p. 111, Inst. of Petroleum, London (1955). [24] D. H. W ~ E N , J. O t ~ . So~, 1350 (1956).

Vibrational speotra and assignments for the phenyl chlorosilamm 1079

data, and will not be discussed in detail. I t was helpful to note the similarity of the absorption frequency patterns in PhSiC18 and PhC1, for which the coordinates of the normal vibrations have been determined [20, 21]. Bands reputed to be especially characteristic of the PhSi group [25] are assigned as follows, using the W ~ numbering system [24]: 1430 cm -1, n, B 1 in-plane hydrogen bending with some C--C stretch; 1121, q, A 1 in-plane hydrogen bending with some Ph-Si stretch; 1030, b, an A 1 hydrogen in-plane bending vibration involving ring distortion; 998, 10, an A 1 ring distortion mode; 740, f, B 2 out-of-plane hydrogen bending; and 692, v, B 2 out-of-plane bending (puckering). These absorptions are clearly evident in the spectrum of Ph3SiC1 (Fig. 1). The low frequency region will be discussed in detail later.

The reason that most of these frequencies do not change appreciably with the number of phenyl substituents, or indeed, with the metal to which they are attached [2], has been pointed out by WHn~EN [24]: they involve almost no change in length or direction of the Ph - -X bond. Thus, the only truly characteristic Ph--Si frequency in the higher frequency range is the "X-sensitive" vibration at 1121 cm -1 which has a component of Si--C stretching. This band shows a regular variation in frequency with the electronegativity of the ring substituent [26]. I t is also sensitive to the degree of phenyl substitution, showing splitting when two or three phenyl groups are attached to the same silicon atom.

Assignment of the overtone a n d combination bands furnishes an interesting exercise in support of the assignment of fundamentals. In general, phenyl ring vibrations were not combined with skeletal frequencies to give summation bands, and in most cases it was possible to arrive at assignments that gave reasonable anharmonic constants by considering only fundamentals arising from either the ring or the skeleton of the molecules. Also, it was almost always possible to assign corresponding combinations and overtones to the same origin throughout the whole series of molecules. The assignments for these absorptions are shown in Table 2.

A check on the correctness of the assignment for some of the weakly absorbing out-of-plane hydrogen fundamentals was made by calculating the positions of their characteristic combination bands [24, 27, 28]. Results of this calculation are shown in Table 2. Assignment of the very weak 990-995 cm -1 band to j is not well sub- stantiated; however, the other possibility for this assignment, which lies about 6-9 cm -1 lower, would require an unpalatable large negative anharmonicity in order to account for some of the combination bands; the assignment given seems more reasonable.

SKELETAL AND LOW FREQV~NCY R~G MoDEs

We now consider the most interesting and at the same time the most difficult region of the spectrum. The principal problems, aside from experimental ones, are the large range of intensities and half-widths of the fundamental bands. Such factors make it difficult to be sure that one has, in fact, found all the fundamentals.

[25] A. L. SMrrH, ,~pec~rochim. Acta 16, 87 (1960). [26] R. D. KRoss and V. A. FASS~.L, J. Am. Chem. Soc. 77, 5858 (1955). [27] Y. KA~,~TI, J. Ghe~n. Phys. 25, 777 (1956). [28] C. W. YOUNO, R. DvVAT.T. and N. WRIG~rr, Anal. Chem. 23, 709 (1951).

1080 A . L . S~,-z'H

$

! g

1 '

8

o

g

_ g

r~ ~ Ln ¢Dr,-~0 o Ln d 0 d 0 0 ; ¢ ; o - - : -"

e : ,uoq4osq V

co

~ ° ~

o ° ~o

i 0

0

o o .

Vibrational spectra and assignments for the phenyl chlorosilanes 1081

Numerous complications arise in the assignment of the low frequency absorptions. The SiC1 deformations are easily confused with some of the low-lying phenyl ring modes. Further, the skeletal modes all involve compression or bending of the Si Ph bond as do the low-lying X-sensitive ring vibrations, so mixed vibrations involving the entire molecule might be expected. When more than one phenyl group is a t tached to the same silicon, the X-sensitive modes may split and their frequencies undergo a migration from their unperturbed positions. These complications, taken together, make it unlikely that one can arrive at a correct assignment by considering only one molecule, or indeed, one series of molecules. In the following discussion, we therefore invoke data from a number of related structures in the hope that with the emergence of consistent patterns in related species, the individual vibrational modes will be more readily recognised.

PhSiC1 s

Taken as a whole, the PhSiC1 s molecule belongs to point group Co and has 39 fundamental frequencies. Phenyl ring modes are conveniently assigned as Cso vibrations, however, and skeletal modes closely approximate those of a Cso molecule.

The SiC] stretch vibrations have been identified previously [16]; the anti- symmetric stretch falls at 590 cm -1 and the symmetric stretch at 514 cm -1. The 590 cm -1 mode has a shoulder at ~583 cm -1. A similar asymmetry is noted in the corresponding absorptions of PhSiH s and PhSiFs; the effect may arise from the removal of degeneracy from this vibration by the overall molecular symmetry.

The phenyl ring modes may be sorted out from the skeletal modes by comparing the spectrum of BrSiC1 s. (If we consider the phenyl group as a point mass, it is closely approximated b y the Br a tom both in mass and electrical characteristics.) Assignment of the bromo-chlorosilanes has been made unambiguously on the basis of Raman polarisation data [30, 31]; examination of Fig. 2 shows a stril~ing parallel between the vibrational patterns of BrSiC1 s and PhSiCI s, with bands of the latter compound falling at slightly lower frequencies. Also shown in Fig. 2 are the most probable ranges for the skeletal and phenyl ring modes, as well as the positions of the latter for chlorobenzene.

The polarised Raman line at 347 em -1 in PhSiC1 s arises from a symmetric vibration which can be considered as either a skeletal vibration or an A1 ring mode, t, the "Ph - -S i stretch" vibration. The 461 cm -1 infrared band corresponds to the out-of- plane bending mode y, which falls at 467 cm -1 in chlorobenzene. The species A s vibration w (not X-sensitive), predicted to be infrared inactive, absorbs very weakly at 389 cm-L

Skeletal modes falling below 300 cm -1 are: symmetric and antisymmetric SiC1 s bending; SiC1 s "rock"; and torsion of the phenyl group around the SiC axis. These descriptions are, of course, very approximate, and it should not be inferred tha t other parts of the molecule are unaffected b y these motions.

Unfortunately, Raman polarisation data are of no help in distinguishing the

[29] G. J. J~rz and Y. Mr~AwA, Bul l Ghem. Soc. Japan 34, 1495 (1981). [30] M. L. DELWAUr.T.~, M. B. BUISSET and M. D~rr.~rAyE, J. Arf~. Chem. Soc. 74, 5768 (1952). [31] M. L. D~.LW~.UI~E and F. FRancois, Oom~. Re~d. 219, 335 (1944).

108~ A . L . SZ~t-~H

-I--~--I--~- -I--{- -I-

|

o

o

o

¢ )

,.,.i , .-i

÷ - I - - l - ÷ ~ ° ' ~ ÷~ ÷÷~÷÷÷÷.+÷÷o÷÷ -~

Vibrational speotra and assignments for the phenyl chlorodlanes 1083

. ~÷÷÷÷÷÷÷ ÷÷ ÷÷÷ ÷ ÷ ÷ ÷ ÷ ÷

+ ÷°~÷÷÷÷÷÷÷ ÷÷ ÷ ÷÷ ~ ÷ ÷ ÷

@~ , ~ = ~ = ~ ~ = ~ . ~

÷ ÷ ÷ ~ . . ~ ÷ ÷ ÷ ÷ ÷ ÷ ÷ ÷ ÷ ÷ ÷ ÷÷ ÷÷÷• ~

÷ ÷ . . , ÷ ÷ ÷ ÷ ÷ ÷ ÷ ÷ ÷ ÷ ÷ ÷ ÷ ÷ ÷ ÷ , ÷ ÷ ~ ' ~

~=~ ,=~

1084 A.L. S ~

symmetric and antisymmetric S i C ] a deformation modes. In making this assignment, we will find it helpful to visua]ise the normal coordinates for the vibration. For a molecule of the type GSiC]a, the symmetric SiC] a deformation should be essentially a S i - -G stretch combined with SiC1 bending. Since the mass of the silicon is less than that of the chlorines, most of the motion is executed by the silicon and the substituent Q. We therefore expect the frequency to be a function of the mass of Q.

CLPh

s , , s_ , . . - _ -_ - - - - - - - - -

~o . . . . . . . . . 20' x B2

14' u S~

x I u I

S~CL PSii8 } e " sl I ' I 8~sIcL~

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19' y Bz

Wl tl Yl

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SiBr4 [ 8, I Br I

I I I 0 I00 200

I u, SiBtz I ~' SiBr3

v~ SiBrzl v~ SiCL~I lUa SiBr] u SiCtj

~ ° 1

I 300

Ct~-J

I L 400 500 600

PhSiCl a

Ph2SiCL z

Ph~SiCL

Ph4Si

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I ,

^A

^

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/

i I

2 0 0 300

cm-I

t w Y v~ / ~ A A / r \

-"" ....... h AA,, ; A, A ) i L

'A ~^^

400 500 600

Fig. 2. Schematic representation of the low frequency spectra for the pheny| chlorosflanes. Triangles indicate infrared absorption; lines portray Raman shifts. Polarised Raman lines are indicated by cross bars. Heights are roughly propor- tional to intensities. The scale changes at about 300 cm -1, below which bands are

much weaker than shown.

Indeed, for a large number of substituted trichlorosilanes [29] a plot of the sym- metrical deformation frequency shows a good correlation with the mass of group G (Fig. 3), allowing us to assign the 189 cm -1 band to this mode with confidence. The 169 cm -1 band is assigned as SiC1 a antisymmetric deformation. This assignment is supported by the intensities of the corresponding Raman lines.

The Raman band at 85 cm -1 is attributed to an SiC13 "rocking" mode. The weak infrared band at 110 cm -1 is probably due to the other component of the SiC1 s rocking. For 03, molecules this is a doubly degenerate vibration, but for PhSiCla, which has G, symmetry, two components appear. A possible reason for the com- paratively large separation of components will become apparent shortly.

Vibrational spectra and assignments for the phenyl ohlorosilanes 1085

Two ring vibrations remain to be assigned. They are x, the out-of-plane bending, and u, the in-plane rotation of the ring against the substituent X. For a group such as --SiCI s, the former vibration is expected near 180 and the latter near 260 cm -1. I t is suggested tha t the bands observed at 244 and 284 cm -1 are x and u, respectively, for the following reasons: The normal coordinates of mode x resemble those of the out-of-plane SiOl s rocking mode, which is expected near 135 cm -1. These two very similar vibrations, p± and x, may mix and interact to drive the former to a lower

400

Eo 200

I

~ . _ ~ E ~ cl oE~

IO0 I ] I I I I 0 20 40 60 80 I00 120

MASS

Fig. 3. Dependence of the SiC1 s deformation frequency o n t h e m a ~ o f group G in GSiCI$ molecules.

(85 cm -1) and the latter to a higher (244 cm -1) position than expected. Also, in PhGeCls, the 214 cm -1 Raman band shifts to 206 cm -1 in the pentadeuterophenyl compound; no other band below 300 cm -1 shows any shift [32]. This band is unmistakably the counterpart of the 244 cm -1 PhSiCI 8 band, which would presumably show analogous behavior. Thus 244 cm -1 must arise from a ring vibration; since z is usually lower than u, the assignment is made as indicated (it is possible tha t u may also be raised by interaction with the parallel component of the SiC1 s rocking mode).

Possible alternative assignments are somewhat less attractive. I f we put z coincident with the SiC] s deformations near 180 cm -1 and set u at 244 cm -1, we have no satisfactory explanation for the 284 cm -1 band. I t is equally unlikely tha t x is the 110 cm -1 absorption; cyclohexyl SiC1 s also shows a band (attributable to SiCI s rocking) in this region. The compounds PhSiH s, PhSi(CHs) 3, and PhSi(OCH3)s all have a band near 180-230 cm -1 which is reasonably assigned to x.

The Ph--SiCI s torsion mode is expected to have a six-fold barrier and to be near 10 em -1. I t is left unassigned.

PhsSiCl~

This molecule has 69 normal vibrations, of which nearly half are accidentally degenerate or almost degenerate. I f the phenyl groups are considered as point masses, the symmetry of the framework is C~.

Assignment of frequencies above 600 cm -1 follows the same pattern as in PhSiC18. Below 600 em -1, we find the SiCI= stretching modes at 540 and 576 cm -1; the Si - -Ph symmetric stretch must be the polarised Raman line at 319 cm -1. The antisymmetrie

[32] ft. R. Dingo, privat~ communication.

1086 A . L . Smith

SiPh stretch is at 444 cm -1 (the resemblance of the pat tern to tha t of Cl~SiBr~ should be noted).

Assignment of the remaining bands is somewhat more tentative, and is based on t h e following considerations. In parallel to our experience with PhSiC1 a (and later with Ph4Si ), the skeletal bands are assumed to fall near to, but slightly lower than, the corresponding frequencies in BraSiC1 ~. Using this reasoning, we put the sym- metric SiCI~ deformation and the SiC] z wagging at 166, the SiC] z rocking at ~136, SiC12 twisting at 132, and the symmetric SiPh 2 bending at 105 cm -1. Ring mode x is assigned to be the band at 211 cm -1. Mode u apparently splits int~ symmetric (9.46 cm -1) and antisymmetric (283 cm -1) parts. The SiPh symmetric stretch, mode t, gives the polarised Raman line a t 319 cm -I. Its antisymmetric counterpart is found at 434 cm -1. The remaining assignments are self-evident.

The twisting of the phenyl group around the C--Si axis can take two possible forms; one with the two groups rotating in the same direction; the other, rotating in opposite directions. Those modes are probably very low in frequency and are left unassigned.

Ph3SiC1 Although this molecule is calculated to have 99 normal vibrations, most of the

internal modes of the phenyl ring are again accidentally degenerate and almost coincident with those assigned for PhSiCI s. I f we approximate the phenyl groups by point masses, the symmetry is Us,. I t is more probable, however, tha t the phenyl groups are twisted so tha t the molecule is propeller-shaped (point group Us).

Below 600 em -1, we anticipate finding five ring modes (some possibly split), and six skeletal frequencies. The SiC1 stretch is obviously 552 cm-1; the antisymmetric S i - -Ph stretch falls at 434 cm -1 with the symmetric stretch at 306 cm -1. Again by comparison with BrsSiC1, the deformation modes are tentatively assigned as follows: SiC1 bend, 120; SiPh s symmetric deformation, 109 (this band appears in the spectrum of solid PhsSiC1 but not in solution, see Fig. 4), and SiPh s antisymmetric deformation, not observed, probably near 80-90 cm -1.

0

03

0.2

0.3

0.4

0.5

0.6 O.7 0.8 1.0

1,5 400 350

[

V

/

/ \ /.

v \ / A

250 200 70 60 :3.00 150 140 130 i20 rio I00 90 80

F requency , cm -L

Fig. 4. Original recording of Che far infrared spectrum of PhsSiC1; solution in oyclohexane, 3 mm cell l~t~h.

,0

:0-!

3.2

0"3

0"4

0'5

0'6 0"7 0"8 PO

I-5 50

13 percent

Vibrational spectra and assignments for the phenyl chlorosilanes 1087

Ring mode V is found at 504 cm-1; other assignments are less certain. We suggest tha t ring rotation ~, splits into antisymmetric (250) and symmetric (240 cm -1) parts. Similarly, x is probably found at 212 and 170 cm -1.

Ph4Si

Of the 129 normal modes expected for Ph4Si, only about { of this number appear in the infrared and Raman spectra since many of the pheny] modes are accidentally degenerate, as observed for other members of the series. The phenyl ring vibrations are assigned as before.

The Ph - -S i stretch modes t and t' correspond closely to the SiBr stretches in SiBr 4 (Fig. 2). The symmetric stretch is clearly the polarised Raman line at 236 cm -z, and the antisymmetric stretch is the 426 cm -1 band. The wide separation of these two frequencies is worth noting.

Skeletal deformations of this molecule, considering the phenyl group as point masses, should consist of one doubly degenerate and one triply degenerate bending mode. The former is assigned as a weak band near 57 cm-Z; the latter as the 100 cm -1 absorption. Many of the bands in solid Ph4Si show splitting, presumably because of crystalline field effects or removal of degeneracy b y configuration isom- erism. Again, we have no evidence to permit assignment of the Ph - -S i twisting modes.

The spectra of Ph4M compounds will be considered more fully elsewhere [33].

CONCLUSIONS

Above 600 cm -1, the absorption frequencies of the phenyl chlorosilanes are quite constant, and parallel the corresponding absorptions in other monosubsti tuted phenyl compounds. Between 600 and 300 cm -1, the spectra are unique bu t not difficult to interpret. Below 300 cm -z, however, mixing of ring and skeletal modes, combined with complications arising from splitting of the ring modes where multiple phenyI substitution occurs on the same central atom, leads to rather tenuous assignments. The nature of some of these modes is not well understood; their clarification is likely to require considerably more effort and possibly a normal coordinate treatment.

Ac/cnow/edgements~Thanks are due H. SLOb_WE of Beckman Instruments and R. A. l~TYqUIST of The Dew Chemical Company for far-infrared spectra, to J. EVAl~S and Y--S. Lo of The Dew Chemical Company for Raman data, and to J. R. I)umo of the University of South Carolina for making available his unpublished data on phenyl germanium compounds. Valuable discussions were held with F. A. MILLER, W. G. FATELEY and J. EVANS.

[33] A. L. Sm'rH, to be published.