Infrared spectra of organosilicon compounds in the CsBr region

Spectrochimica Aeta, 1963, Vol. 19, pp. 849 to 862. Pergamon Press Ltd. Prmted in Northern Ireland

Infrared spectra of organosilicon compounds in the CsBr region A. Spectroscopy

Laboratory,

LEE

SMITH

The Dow Corning Corporation, Midland, Michigan (Received

8 October 1962)

number of organosilicon compounds show characteristic bands in the 13-38 p range. These absorptions arise from SiCl, SiBr and SiI stretching, SiOSi symmetrical stretching, SiCH=CH, hydrogen deformation, ring vibrations in Si-aromatic compounds, SiOC bending, Some of these absorptions fall within SiH, and SiH, rocking, and other skeletal vibrations. narrow wavelength intervals and some are good group frequencies which correlate well with other group parameters. In many cases frequencies can be sorted out and assigned to discrete vibrational modes. Using these concepts it is possible to make assignments with reasonable confidence and correct some misassignments which have appeared in the literature.

Abstract-A

INTRODUCTION

It is well known that organic groups attached to larger and heavier elements such as silicon tend to vibrate in a more localized manner than in the corresponding carbon compounds [l]. Consequently, for vibrations within these attached groups, the term “group frequency” is particularly appropriate. As we probe the longer wavelength regions of the spectrum, however, we find absorptions which involve larger portions of the framework of the molecule. Such absorptions may vary in position and intensity in a seemingly random manner among related molecules, but a pattern usually emerges once the actual forms of the vibrations have been derived. Detailed theoretical analyses of a large number of related polyatomic molecules require a major effort, however, and while more and more such molecules are being vigorously studied, it is valuable in the meantime to use such information as can be derived empirically from simple correlations with mass, electronic effects and other well known parameters. This is not to say that group frequencies do not exist at longer wavelengths. Indeed, on an energy (wavenumber) basis, the frequency range covered by a characteristic group motion is usually much smaller than in the NaCl region. Although in some cases low-lying vibrational modes are mixed because of strong coupling between vibrations, other groups (such as phenyl) have relatively invariant abSome of these will be considered later. sorptions which are true group frequencies. The division of the vibrational spectrum into regions corresponding to the limitations of particular optical materials is, of course, completely artificial. It is legitimate, however, to examine fundamental vibrations corresponding to certain group motions which may fall within well-defined wavelength limits. We have, * Presented at the International Ohio, August 1962. [l]

A. L. SMITH, Spectrochim.

Symposium

Acta 16, 87 (1960). 841)

on Far Infrared

Spectroscopy,

Cincinnati,

850

A. LEE SMITH

for example, in earlier papers [l-3] considered SiH, SiO-alkyl, and other groups which show characteristic absorptions in the 2-16 p range. Some longer wavelength correlation studies on aliphatic and aromatic hydrocarbons [4, 51 as well as inorganic materials [6] have recently appeared in the literature. In the present paper we will examine some of the characteristic absorptions of organosilicon compounds which are found in the 15-38 ,u region. Detailed vibrational assignments have been made for all the bands discussed, because only in this way can the resulting correlations be used intelligently. Such assignments, of course, are also a necessary step in the direction of understanding the spectra and ultimately the bonding forces involved in complex molecules. EXPERIMENTAL Spectra were recorded using a Perkin-Elmer Model-221 spectrometer equipped with a CsBr prism and a Reeder thermocouple. Wherever possible, samples were run in 2,2,4 tri-methylpentane solution at a concentration of 50 mg/cms using O-4-mm thick cells. The scanning rate was approximately 1 min/p, which was sufficiently slow to give good response throughout the entire region. SPECIFIC GROUP CORRELATIONS Sic’1 The SiCl stretching modes give rise to absorption in the 420-620 cm-l (16-24 p) region of the spectrum. This assignment can be verified by reference to simple molecules such as SiCl,, for which assignments [7] are unambiguous. In the case of phenyl chlorosilanes, which show an aromatic ring absorption in the same region, the assignment is not quite so clear, but it can be resolved by comparison with other phenyl-substituted silanes and by reference to Raman polarization data. When three chlorines on silicon are present, the asymmetric stretch falls between 570 and 625 cm-l (16-O and 17.6 ,u), and the symmetric stretch lies in the range 450-535 cm-l (18.7-22.4 ,u). The stretching modes of dichlorosilanes also fall in the rather broad range of 535-595 cm-l (16.8-18.7 ,u) and 460-540 cm-l (18.6-21.7 ,u). Monochlorosilanes absorb between 470 and 550 cm-l (18 and 21.4 ,u), with the exact position of the band apparently governed by the inductive effect of the other three substituents as shown in Fig. 1. Here the Sic1 frequency correlates well with the Taft sum of the c* parameters [S] for the other substituents, except in cases of substituents such as OMe, OEt, and ClCH,, where dative dn--pr bonding to the silicon is known to take place [g-11]. In such cases we would [a] A. L. SMITH and N. C. ANGELOTTI, S~ectrochim. Acta 15, 412 (1959). [3] A. L. SMITH and N. C. ANGELOTTI. To be published. [4] F. F. BENTLEY and E. F. WOLFARTH, Spectrochinz. Acta 15, 165 (1959). [?I] W. R. POWELL,WADC Technical Kate 57-413 (May 1958). ASTIA Document No. 151189. [6] F. A. MILLER, G. L. CARLSON, F. F. BENTLEY

and W. N. JONES, i!$ectrochin&. Acta 16,

135 (1960). [7] A. L. SMITH, J. Chem. Phys. 21, 1997 (1953). [8] R. W. TAFT. JR., Steric Effects in Organic Chemistry

(Edited by M. S. SEWMAN) Wiley, New York (1956). [9] F. G. A. STONE and D. SEYFERTH, J. Inorg. Xucl. Chem. 1, 112 (1955). [lo] D. W. J. CRUICKSHANK, J. Chew. Sot. 5486 (1961) [ll] 0. W. STEWARDand 0. R. PIERCE, J. Am. Chem. Sot. 83, 4932 (1961).

Chap. 13.

Infrared spectra of organosilicon compounds in the CsBr region

851

Fe/_,,,,,j F3CiCH,),hIe,t$VI

0

$;

PhMeEt 0 MePrH

0 /

4

0Et2H

0

Atlyl

Me2

Me3

C-Hex

Me

0

CI(CH,),Me*

0

0

L

-5

450

0 h4epr 0 EtMePr EIhk2

550

500

B,

CM-’

Fig. 1. Correlation of the Sic1 stretching frequency in monochlorosilanes with the sum of Taft’s u* for the other substituents.

expect the Sic1 bond to be weakened and the frequency to be lower than predicted. This is indeed the case. In all honesty, however, it must be admitted that phenyl groups, which also rr-bond to silicon, show no such effect. Nor is it clear why another parameter [12] which combines a measure of both mesomeric and inductive effects in organosilicon compounds and has been correlated successfully with other organosilicon stretching frequencies [13] does not correlate as well in this case as does the Taft parameter. Nevertheless, the relationship given in Fig. 1 is a useful one. OVEREND and SCHERER [14] have shown that in the series H,SiX (where X = halogen) the Urey-Bradley force constants for the Six bonds correlate well with the Pauling electronegativity of X. SiOSi Most compounds containing the SiO bond absorb in the CsBr region. The SiOSi symmetric stretch vibration lies at 600 cm-l in disiloxane [15], but in substituted disiloxanes [lS, 171 it falls at lower frequencies. Since the SiOSi bond is non-linear, the symmetric stretch vibration has a dipole moment change along the [12] [13] [14] [VI] [16] [17]

A. A. J. D. H. H.

L. SMITH and N. C. ANGELOTTI, Spectrochim. Acta 15, 412 (1959). L. SMITH and N. C. ANGELOTTI. To be published. OVEREND and J. R. SCHERER, J. ClLem. Phys. 34, 574 (1961). C. MCKEAN, Spectrochim. dcta 13, 38 (1968). KRIEGSMANN, 2. anorg. U. allgem. Chem. 299, 78 (1959). KRIEGSMANX, 2. Elektrochem. 61, 1088 (1957).

852

A. LEE SMMX

This band two-fold symmetry axis which gives rise to a weak infrared absorption. is usually rather broad, and ordinarily lies in the 480-625 cm-l (16-21 p) range for both disiloxanes and linear siloxane polymers. For example, Pig. 2 shows the SiOSi stretch [18] absorption for Me,SiOSiMe, at 520 cm-1 (19.2 ,u), for Me,SiOSiPh, at 570 cm- l (17.5 p) and for Ph,MeSiOSiMePh, at 590 cm-l (16.9 ,u).

Fig. 2. Spectra of some phenyl methyl substituted disiloxenes.

The band splits in an interesting manner in the short chain dimethyl siloxanes, analogous to the behaviour of the asymmetric SiOSi stretch vibration in the lOOO1100 cm-l (9-10 p) range. As shown in Fig. 3, both the methyl and the chlorine end-blocked polymers give one band between 500 and 625 em-i for each SiOSi bond in the molecule. In higher polymers, the bands coalesce into a broad asymmetrical absorption centered at 500 cm-l. One can infer from this that the skeletal vibrations of trimethyl end-blocked siloxanes are coherent up to about five adjoining siloxane units, but probably not much beyond that. [18j II. W. SCOTT, J. F. MESSERLY, 8. ii.TODD, a. B. GUTHRIE, I. A. ROSSENLOPP, R. T. MOORE,ANNOYBORN,W.T.BERG~~~J.P.MCCVLLOVGH, J. Phys.Chem. 65,1320(1961).

Infrared

spectra Me

Cl SiO [ Me

of organosilicon

compounds

in the CsBr region

853

1 Me

Sic.3 Me

m

m:3

Fig. 3. The SiOSi symmetric

stretch vibration

for short-chain

linear polymers.

Methyl substituted siloxanes show an intense absorption in the 300-400 cm-l (25-O-33.0 p) range. In Me,SiOSiMe, (Fig. 2), this band is found at 330 cm-l and is attributed to an asymmetric Sic, deformation [lS]. In the dimethyl siloxanes (Fig. 4), it is reasonable to assign this absorption to the Sic, scissors or wagging mode. If we assume the same ratio of frequencies for the Sic, bending to stretching modes as holds for the CH, modes in the analogous polyoxymethylene [19], we calculate the position of the Sic, scissors mode to be approximately 380 cm-l, the wagging modes at approximately 370 cm-l, and the Sic, rocking frequency at approximately 240 cm-l. Infrared bands are, in fact, found at 393 cm-l (25.5 ,u) and 288 cm-l (34.7 p) in dimethyl polysiloxanes. Other methyl-alkyl and methylaryl substituted siloxanes show absorptions in this region, but (Ph,SiO), does not (Fig. 4). Cyclic siloxanes show one or more characteristic SiOSi symmetric stretch modes, depending on the size and geometry of the siloxane ring (Fig. 5). The position of the Sic, deformation absorption also varies with ring size (Fig. 5). A partial assignment has been carried out by KRIEGSZUNN for lower frequencies in the spectra of (Me,SiO), [SO], (Me,SiO), [21] and higher cyclics [21]. It is obvious from a chart of band positions in substituted cyclic siloxanes (Fig. 6) that one cannot generalize about the position of the SiOSi symmetric [19] [20] [Zl]

T. MIYAZAWA, J. Chem. Phyys. 35, 693 (1961). H. KRIEGSMANN, 2. alborg. u. allgem. Chem. 298, H. KRIEGSMANN, 2. axorg. u. allgem. Chern. 298,

283 (1959). 233 (1959).

854

A.LEE SMITH

Infrared

spectra

of organosilicon

compounds

Vertical Fig. 6. Bands in substituted cyclic siloxanes. band centers: horizontal lines indicate

855

in the CsBr region

lines indicate half-nidt,h.

intensities

at

stretch frequency for a given ring size as is true of the asymmetric stretch [22]. This uniqueness is a distinct advantage in the characterization of cyclics containing four or more siloxane units and makes possible characterization of molecular species not feasible in the NaCl region. Silsesquioxanes (mono-substituted siloxane polymers) give a silica-like spectrum; i.e., a strong broad band in the vicinity of 500 cm-l (20 ,LL). Silsesquioxane (Fig. 7) as well as the methyl and phenyl substituted cubical octamers (RSi01J8 [23] all show this absorption, which has been attributed to vibrations involving Octamers reportedly show three or the SiO skeleton by SPRVTRGand &ESTHER. four prominent bands in the 360-660 cm-l (15-28 ,M) region, whereas hexamers give six to eight well defined maxima [24]. Silica itself shows one or more bands from 100-600 cm-l (16.7-25 ,u), depending on the crystalline form. These bands are assigned to SiO bending modes [25]. C’oesite and cc-quartz also have a lower frequency absorption at approximately 350 cm-l (28.6 ,u) classified as a “distortion” mode [25]. In silica, the SiOSi symmetric stretch falls at approximately 800 cm-l (13.5 1~). SiOC Methoxy substituted met,hyl silanes give one or more broad absorption bands in the 285-480 cm-l (21-35 ,u) region, ascribed by various authors to an asymmetric I221 [23j [2-k] [25]

N. WRIGHT and M.J. HUNTER,J. _4m. Chem.Soc. 69, 803 (1917). I(. OLSSON,A&iv Kemi 13, 367 (1958). M. M. SPRUNG and F. 0. GUENTHER, J. Polymer Sci. 28, 17 (1958). E.R. LIPPINCOTT,A. VANVBLGENBURG, C'.E. \IT~l~ and E. S. BPSTING,J. Bur. Standards 61, 61 (1958).

Research

h’at.

856

A. LEE SMITH

Fig. 7.

SiOC deformation [26-281, SiO bending [29], or SiOC torsion [30]. None of these workers had available the infrared spectra of the compounds beyond 400cm-l, so that in many cases strong fundamentals were missed entirely and consequently their assignments are incomplete or incorrect. Spectra of some tetra alkoxy silanes are shown in Fig. 8, and the methyl methoxy-silanes in Fig. 9. The strong band at 285-480 cm-l (21-35 p) is ascribed to SiOC asymmetric deformation. The position and intensity of the band varies depending on the number of methoxy groups involved. Silanes with only a single methoxy group may not give any recognizable SiOC absorption in the CsBr region. [26] A. N. LAZARET, Op~ika i Spektroskopiyn [27] [2!3] [29] [30]

8, 511 (1960); Optics and Spectroscopy (Translation). H. KRIEGSMANN and K. LICHT. 2. Elektrochem. 62, 1163 (1958). H. MURATA, J. Chem. E%ys. 20, 347 (1952). R. FORNERIS and E. FUNCIC, 2. Elektrochem. 62, 1130 (1958). T. TANAKA, Bull. Chem. Sot. Japan 33, 446 (1960).

8, 270 (1960)

Infrared

spectra

of organosilicon

compounds

in the C’sRr region

857

838

A. LEE SMITH

Bs might be expected, other alkoxy silanes show absorption of similar character, presumably originating from the same type of vibration. Ethoxy silanes absorb between 415 and 500 cm-l (20 and 24 p) as do n- and i-propoxy silanes. Molecules containing Si-acetoxy groups also show absorption in the CsBr region, but are not covered in this report because insufficient data are available. SiPh Interpretation of the spectra of SiPh compounds may be expedited by reference to the spectrum of PhSiCl, which has been completely assigned [31] and to other monosubstit,uted benzenes [32, 331. Certain ring vibrations which do not involve the substituent on the phenyl ring would be expected to be essentially invariant. Other modes, designated by RANULE and WIIIFPEN [32, 341 as “X-sensitive,” might be expected to fall within rather restricted wavelength ranges since in all cases the phenyl group is attached to silicon and any other influence would be secondary. A careful study of a large number of substituted silanes containing one, two, and three phenyl groups, plus tetraphenyl silane, has led to assignment of the following group frequencies : . . R&Ph. The vibration vi9 (RANDLE and WHIFFEN symbol y),* which is an out-of-plane ring bending involving some Si-C-C bending motion, gives an intense absorption which falls in the range 445-490 cm-l (20.5-22 p). This frequency range is consistent with that observed for other monosubstituted benzenes [33]. . . The vibration yi8 (RANDLE and WHIFFEN symbol t) covers a wider range of 345-405 cm-l (24.8-29 ,u). This is an in-plane ring deformation and Si-C” stretch mode which gives a polarized Raman line in PhSiCl,. The y14 mode (RA~DLE and WHIFFEN symbol U) is a SiPh in-plane deformation and falls in the vicinity of 290 cm-l (35 ,u). R,SiPh,. In diphenyl substituted silanes, the v19, vibration occurs at 470495 cm-l (20.2-21.2 ,u). The vls, mode splits into symmetric and antisymmetric SiC stretch modes which fall at 305-380 cm-l (26.2-32.8 ,D) and 400-445 cm-l (22.5-25 ,u) respectively. The symmetric mode is strong in the Raman and weak in the infrared spectrum, whereas the reverse is true for the antisymmetric mode. This intensity difference, plus the fact that far-infrared data were not available to them, no doubt accounts for the misassignment of vis, and vi9, by KRIEQ~MANN and SCHOWTKA[35]. RSiPh,. The frequency of the vi9, vibration in triphenyl silanes is slightly higher than in mono and diphenyl silanes, and now gives an intense band in the range 485-515 cm-l (19*4-20.6 ,u). The SiPh asymmetric stretch v18, is found at 420-445 cm-l (22. 5-23.8 ,u) and the weakly absorbing symmetric stretch may be * The vibrations are numbered using the Herzberg system. the different numbering systems. [31] [32] [33] [34]

A. D. C. R. of (351 H.

See Ref. [25] for correlation of

L. SMITH. To be published. H. WHIFFEN,J. Chem. Sot. 1350 (1956). V. STEPHENSON,W. C. COLBURN, JR. and TV. S. WILCOX, Spectrochim. Acta 17,933 (1961). R. RANDLE and D. H. WHIFFEN, Molecular ,Spectroscom (Edited by G. SELL) Institute Petroleum, London (1955). KRIEGSMANN and K. H. SCHOWTKA, 2. physik. Chem. (Leipzig) 299, 261 (1958).

Infrared spectra of organosilicon compounds

in the C’sBr region

s59

found in the 330 cm-l (30 p) region. It is interesting to observe the decrease in intensity of the symmetric stretch as the number of phenyl groups attached to silicon increases, until for Ph,Si, the vibration is silent in the infrared, as predicted by selection rules. Spectra of the phenyl chlorosilanes are shown in Fig. 10.

Fig. 10. Spectra of phengl chlorosilanes.

On the basis of the conclusions reached above, it is clear that the 515 cm-l band in Ph,Si assigned by HARVEY and NEBERGALL [36] to the silicon-phenyl asymmetric stretch vibration is in fact Q,, the ring-silicon bending mode. The Si-Ph asymmetric stretch actually falls at 434 cm-l. It is not surprising that the higher frequency shows a dependence on the mass of the central atom in tetraphenyl metal derivatives [36, 371, since y19 is also an X-sensitive frequency. This interpretation is in agreement with a previous assignment of tetraphenyl metal compounds [38]. Where several phenyl groups are present on a silicon, certain of t,he [36] M. C. HARVEY and TV. H. NEBERGALL, AppZ. Spectrosc.

16, 12 (1962). [37] L. A. HARRAH, M. T. RYAN and C. TAMBORSKI, Spectrochim. dcta 18, 21 (1962). [38] W. J. RATSEK. M.S. THESIS, University of Wisconsin (1960).

A. LEE SBIITH

860

above vibrations may themselves show splitting, particularly for materials in the solid state. In the spectrum of solid Ph,Si, for example, Q,, is a doublet [36], whereas in solution only a single band is present. The vibration yls (RANDLE and WHIFPEN symbol s) is an in-plane ring bending mode, not X-sensitive. It is found in all phenyl silanes (as well as other monosubstituted phenyl derivatives) at about 620 cm-l (16-l p). This is the weak sharp band noted by GRENOBLE and LAUNER in the phenyl chlorosilanes [39]. The other low-lying phenyl frequencies yzO and yzO, (RANDLE and WHIFFEN symbols UTand x) were not observed, the former because it is either inactive or only weakly absorbing, and the latter because it falls near 190 cm-r (52.5 ,u), beyond the range of the CsBr prism. Si Pinyl An out-of-plane hydrogen deformation gives rise to an absorption (17.2 p). It is claimed that this is not a good group frequency [40] vinyl group is attached to silicon, the band has a moderate intensity over which the absorption is found is limited enough, 515-580 cm-l to be useful.

near 580 cm-l but when the and the range (17.3-19.5 ,u),

Si-Me, A broad, weak absorption band attributed to -Si(CH,), deformation is found around 290 cm-l (34.5 ,u). The range of frequencies (240-330 cm-l) is so great and the intensity usually so low that the band is of little use in structural analysis. SiH, The spectra of a number of compounds of general formula (CH,SiH,),X have been studied by EBSWORTH et aE. [41], who find the SiH, rocking mode in the range 460-520 cm-l (19.2-21.8 ,u). Consideration of the other disubstituted silanes necessitates extending this range to 460-600 cm-l (16.7-21.8 ,u). The band is usually sharp and of medium intensity. SiH, A strong band attributed to SiH, rocking ordinarily falls at 540-680 cm-l (14.7-18.5 ,u) but lies at 728 cm-l (13.7 p) for HaSiF [42]. At the low frequency limit of 540 cm-l (18.5 p) H,SiMe is an example of a compound in which the rocking and deformation modes are mixed [43]. SiBr Because of the paucity of data on the spectra of bromosilanes, any conclusions drawn at this time regarding the SiBr stretch vibrations must be considered tentative. The literature covers only the bromosilanes [44], methyl bromosilanes [41, 451, [39] [40] [41] [49] [43]

M. E. GRENOBLE and P. J. LAUNER,a~$. Spectrosc. 14, 85 (1960). W. J. POTTSand R. A. NTQUIST,Spectrochim. Acta 15, 679 (1959). E. A. V. EBSWORTH, M. ONJISZCH~JK and N. SHEPPARD, J. Chem. Sot. 1453 (1958). C. NE~I\IAN, J. K. O’LOANE,S. R. POLO and M. K. WILSON, J. Chem. Phys. 25, 855 (1956). M. RANDIC,Spectrochim. Acta 18, 115 (1962).

[44] H. MURATA and K. KAWAI, J. Chem. PhyVs. 23, 2451 (1955).

561

Infrared spectra of organosilicon compounds in the CsBr region

and mixed chloro-bromo-iodo silanes [46, 471. Our own files contribute only a few additional spectra. From this limited number of compounds we provisionally propose the following ranges: SiBr, asymmetric stretch, 450-480 cm-l (20*822.2 p); SiBr, symmetric stretch, 300-360 cm-l (27.7-33.4 p); SiBr, asymmetric stretch, 425-460 cm-l (21.6-23.5 ,u); SiBr, symmetric stretch, 330-395 cm-’ (25-30 ,D); SiBr stretch, 360-430 cm-l (23.3-28 ,u). SiI Iodosilanes are unstable to light and extremely susceptible to hydrolysis. Molecular spectra of only a few examples of these compounds are available, including normal [48] and deuterated [49] iodosilane, trimethyl iodosilane [50] and mixed halosilanes [46, 471. From consideration of these compounds together with Sir,, and by comparison with the frequency spread in substituted chloro- and bromosilanes, we suggest tentative assignment of the following ranges to the SiI stretch vibrations: SiI,, 365-410 and 220-280 cm-l (24.5-27.5 and 36-45.5 p);

Fig. 11. Structure-spectra

correlation chart for organosilicon compounds CsBr region.

in the

[45] H. MURATA and S. HAYASHI,J.C~~~. Phys. 19, 1217 (1951). [46] M. L. DELWAULLE, M. B. BUISSET and M. DELHAYE,J. Am.Chem.Soc. 74, 5768 (1952). [47] M. L. DELWAULLE and M. B. DELHAYE-BUISSET, Colloq. Section Inorg. Chem. Id. Un. Pure ,4ppl. Chem., Mtinster (ll’estfalen), Z-6 September, 1954. pp. 31-37. Verlag Chemie, GmbH, Weinheim Bergstr. (1955). [48] R. N. DIXON and N. SEEPPARD, Trans. Faraday Sot. 53, 282 (1957). [49] H. R. LINTON and E. R. NIXON, Spectrochim. Acta 12, 41 (1958). [50] J. GOUBEAU and H. SOMMER, 2. anorg.U. allgem.Chem. 289, 1 (1957).

862

Sir,, 330-390 and 275-325 cm-l

A.

LEE SMITH

(25.5-30.5 and 31-36.5 ,u); and SiI. 280-365 cm-l

(27.5-34 /A).

CoNCLUsIoNs This study has re-emphasized the value of the 15-38 ,u region both for qualitative work and as an aid to making vibrational assignments. As has been pointed out ~~BEHNKE and TAMBORSKI [5l],spectra oforganic derivativesof heavy atoms may be almost indistinguishable in the NaCl region, but show striking differences Further, certain frequencies of attached groups may at longer wavelengths. remain relatively constant and constitute a useful diagnostic tool. Some of the correlations which we have discussed are summarized in Fig. 11. Wavelength ranges cover compounds available and run by us, except where it is stated that literature values have been used. It is possible that unusual structures may occasionally fall outside these ranges. Fluorine or oxygen, for example, usually tend to increase stretching frequencies of adjacent groups. In general, however, the span given should hold for the majority of compounds encountered. It becomes obvious from a study of the spectra given here and of vibrational assignments on these molecules given in the literature that a complete assignment of vibrational frequencies is virtually impossible without far infrared data. Important frequencies are frequently weak or forbidden in the Raman spectra because of symmetry, and a far-infrared study is the only reliable means of obtaining them. It might be noted parenthetically that Raman polarization data is of much more value than Raman shifts alone and should be a routine part of every Raman study of a new molecule. It is also valuable when making vibrational assignments to study a series of By doing this, one can often avoid errors and arrive at a similar molecules. reasonable interpretation more quickly. Acknowledgement-The assistance of Miss in this study is gratefully acknowledged.

WANDA RATSEK in running many of the spectra used

[51] F. W. I)EHNKE and C. TAMBORSKI,ASD-TDR-62-224 (February Commerce, Washington 25, D.C.

1962).

U.S.

Dept.

of