Spectrochimica Acta, 19&l, pp. 412 to 420. Pecgamon Press Ltd. Printed in Northern Treland

Ciirrelation 6f the SiH stretching frequency with miilecular structure

A. L. SMITH and N. C. ANGELOTTI Spectroscopy Laboratory,

Dow Corning Corporation, Midland, Michigan

(Received 2 February 1959)

Abstract-The frequency of the SiH stretching absorption varies with the other substituents on the .silicon in a manner which roughly parallels the sum of their electronegativities. Using a few selected compounds as standards, one can derive a characteristic constant for each sub- stituent group such that the SiH frequency of other substituted silanes and siloxanes can be predicted to an accuracy of *2 cm- l. The relationship of these constants to the Gordy electro- negativities, and their use in characterizing silanes of umlmown structure are discussed.

Introduction

IT has been observed, both in our laboratory and in the literature [l, 2-J that the frequency of the SiH stretching absorption is not constant from one compound to another, but varies over the range 44-4,~ ~1 as the substituents on the silicon are changed. For example, the frequency of the SiH band seems to increase more or less regularly with the number of Si-Cl bonds. HARVEY et al. attribute this shift to the inductive effect of the halogen atom [2]. We have undertaken an investi- gation of this phenomenon with a view toward establishing useful correlations between the SiH absorption frequency and the structure of the molecule, and have found that the SiH frequency can be predicted accurately from a table of constants characteristic of the other substituents.

Several authors have discussed the intramolecular factors which cause shifts in group frequencies [3, 41. They are:

(a) Changes in atomic mass. (b) Coupling of vibrations. (c) Electronegativity of adjacent atoms or groups (inductive effect). (d) Conjugation. (e) Hydrogen bonding. (f) Bond-angle strain. (g) Mesomerism. With a group such as SiH, which from a mechanical viewpoint can be con-

sidered as a single-bonded diatomic molecule of reduced mass approximately equal to 1, we are not concerned with effects (a), (d), (f) or (g). The SiH frequency lies in a spectral region sparsely populated by other fundamentals, and we have never encountered any appreciable coupling effects. Under certain circumstances, however, some interactions may conceivably occur. Hydrogen bonding in SiH

[l] M. I. BATUEV, A. D. PETROV, V. A. PONOMARENKO and A. D. MATVEEVA, Bull. acad. Sci. U.S.S.R. Div. C%ena. Sci. 1243 (1956); p. 1269 of English Translation.

[2] M. C. HARVEY, W. H. NEBERGALL and J. S. PE~KE, J. Am. Chem. Sot. ‘76,4555 (1954). [3] R. C. LORD and I?. A. MJXLER, Appl. Speclroscopy 10, 115 (1956). [a] L. J. BELLAXY, Spectrochint. Acta 13, 60 (1958).

412

Ccrrelation of the SiH stretching frequency with molecular structure

compounds has never been found [5], and if it exists, the effect on the frequency must be small. Electronegativity of adjacent groups, however, has been shown to influence the phosphoryl [6] and carbonyl [3, 7, 81 stretching frequencies as well as other molecular vibrations [9, 10, 111. Indeed, vibrational frequencies have been used to predict electronegativity values [6, 12, 13, 14, 151. It therefore seemed reasonable to correlate the SiH frequency with the electronegativities of the substituents on the silicon.

Effect of electronegativity KAGARISE [7] found a linear relationship between the stretching frequency of

the C=O band and the sum of the electronegativities of substituents X and Y for structures of the type X.CO+Y. Therefore, we attempted to correlate the frequency of the SiH stretching absorption and the sum of the electronegativities of the silicon substituents. That is, if v is the SiH frequency,

v=m172xi+b (1)

where CX( is the sum of the electronegativities of the other three substituents, and nz and b are constants.

A plot of the sum of Gordy electronegativity values [7, 161 for the substituents on the silicon against the SiH stretching frequency was reasonably successful for molecules of the type Ph,Me,Cl,SiH (see Fig. 1) where x + ZJ + z = 3. For mole- cules containing SiH,, SiH, or SiBr, however, large deviations appeared, and it was obvious that such a straightforward correlation was of very limited applica- bility.

The next step was to devise a new “electronegativity” scale which would permit the calculation of SiH frequencies using equation (1).

Trivial solutions result if one attempts to substitute data for several molecules into equation (1) and solve simultaneously for the constants. To obtain unique solutions, one must arbitrarily assign two constants, or one electronegativity value and one constant. The former alternative was chosen, with b set equal to zero and m set equal to unity for ease of calculation. This procedure is equivalent to saying that the SiH frequency is equal to the sum of characteristic numbers assigned to the other three substituents on the silicon. These numbers are used instead of Gordy electronegativities in equation (1). We shall call them E-values, and show that they are extremely useful constants. Their relationship to Gordy electronegativities will be discussed later. It should be pointed out that numerical

[5] Unpublished results, Dow Corning Corporation. r6] J. BELL, J. HEISLER, H. TANNENBAUM and J. GOLDENSON,J. Am.Chen~Soc. 76, 5185-(1954). [7] R. E. I<A~ARISE, J. Ant. Chem. Sot. 77, 1377 (1955). 181 G.RAPPAPORT,M.HAUPTSCEIEIN, J.F. O’BRIEN~~~R.FILLER,J.A~~LC~L~~~.SOC. 75, 2695 (1953). [SJ R.D. KROSS and V. A. FASSEL,J.A~.C~L~~.SOC. 77,%X8 (1966).

[lo] N. SHEPPARD, Tram Faruda~Soc. 51, 1465 (1955). [ll] S. C. WNT, JR. and G. J. JANZ,J. Chem. [12] W. GORDY, J. Chevn. Phys. 14, 305 (1946).

Phys. 26, 1554 (1957).

[13] A. D. WALSH, Proc. Roy. Sot. (London) A 207, 13 (1951). [14] J. K. WILMSHURST, J. Chena. Phys. 26, 426 (1957). [15] J. K. WILMSECURST, J. Chew Plhya. 28, 733 (1958). [16] W. GORDY and W. J. 0. THOP+IAS, J. Chem. Phyys. 24, 439 (1956).

413

,A. L. SWTE and N. C. ANQELOTTI

values given in Table 1. are significant‘ only in relation to each other. The actual numbers depend on the choice of constants in equation (1).

It is interesting that the halogen-substituted methanes do not show the same trend in their C-H stretching frequencies with increasingly electronegative substituents as do the silanes [l’ir].

Fig. 1. SiH stretching frequency &s a function of the electronegativity sum of the silicon substituents.

Experimental results All frequency values, except as noted, are for dilute solutions in Ccl,. A

Perk&Elmer Model 112 spectrometer equipped with a CaF, prism was used for the measurements. The instrument was calibrated in the range 2050 to 2250 cm-l using carbon monoxide gas. The organ0 silicon compounds were as pure as could be obtained, in most cases of 99 + per cent purity. Wave number measurements were reproducible to within about fl cm-l.

Whenever possible, E-values were derived from y(SiH) values of trisubstituted silanes with identical substituents. Thus, E(C,H,) is from HSi(C,H,),; E(Br) from HSiBr,, etc., as shown in Table 1. In some cases it was necessary to use E-values obtained from silanes with mixed substituents.

[15] R. N. JONES and C. SANDOBFY, Chemical Applicationa of Speotiooscopy p. 414 Interscience Publishers, New York (1960).

414

Correlation of the SiH stretching frequency with molecular structure

Table 1. Calculation of E-values from selected compounds

Compound Obs. Group

HSiMeF, 2227.6 HSiCl, 2268.3 HSiBr$ 2236.0 HSi(OMe), 2203.2, HSi(OEt), 21960 HSi(B-EtBuO), 2191.2 HSi(Oi-Pr), 2190.8 H,SiMeCH,Cl 2156.0 H,SiPh 2158.3 H,Si(pC,H,Cl)Me 2144.7 H,Si(mC,H,Cl)Me 2142.0 HSi(CH:CHJMePh 2123.8 HSiPh, 2126.0 HSi(CH&H:CH2)C12 2212.0 HSiPhMe, 2120.5 HSiPrCl, 2205-7

HSi(n-C,H,,)Cl,

HSiEt, HSi(iPr)Clz H,Si(cycZo-CsHll)Ph

2205.7

2097.2 2199.9 21250

F Cl Br OMe OEt 2-EtBuO Oi-Pr CH,Cl H PWW &,H,Cl CH=CH, Ph CH,CH=CH, Me Pr Bu n-%% n-C,% Et i-Pr cycle-C,H1,

E (cm-l)

760.8 762.8 745.3 134.4 732.0 730.4 730.3 725.3 724-8 714.0 711.3 709.2 708.7 706.4 706.9 700-l

(700.1) 700-l (700-l) 699.1 694.3 691.5

-

-

- *a: Infrared data, this laboratory. b: Interpolated values. R: Raman shift for liquid.

Numbers in square brackets refer to bibliography.

Once we know E for a group, we can substitute its value into equation (2) and predict the SiH stretching frequency for any given structure.

This calculation has been carried out for a large number of molecules, and the results checked by experiment. Predicted values are compared with the observed frequencies in Table 2. Siloxane (Si-0-Si) compounds form a special class which will be discussed later. In order to simplify calculations as much as possible, we have chosen to sum E’s for three rather than four substituents on silicon in equation (2). This means that the E-value for the fourth substituent (hydrogen) G implicit in the numbers used for the other substituents.

Agreement of the calculated and observed values is seen to be excellent, in most cases within 2 cm-l. The largest error which we have found is 4.7 cm-l for MeHSi(OEt) 2. The standard deviation of the error for the eighteen compounds we have measured is 2-O cm-r. Some frequencies taken from the literature are

[18] K. W. F. KOELFGAUSCH, Ramanqektren. Becker und Erler, Leipzig (1943).

415

.A. L. SNITH and N. C. ANQELOTTI

Table 2. Calculation of frequencies from E-values

Compouncl

--_

A. il!lea.sured in our laboratory

H,SiCl, MeHSiCl, Me,HSiCl

Me$iH, PhHSiCl, PhMeHSiC:

PhsMeSiH PhMeSiH,

Ph,SiH, PhsHSiCl

Ph,(CH,:CH)SiH Ph(CH,:CH)SiHCl Ete(CH2:CH)SiH

Et,HSiCl EtHSiCl,

Et,MeSiH MeHSi(OEt), H,Si(mC,H,Cl)

Standard deviation

3. Literature dues

?I-C,H,,SiHs Pr,SiH, PhHSiBr,

PhH,SiBr PhEtSiH, PhPrSiH, PhBuSiHs

Ph(tt-CSH,,)SiHz Ph(i-Pr)SiH,

Ph(n-CsH&SiH, SiH, HsSiBr,

Me,SiH

--

-

2229.3* 2230.4 1.1

2213.5 2211.5 2.0

2168.2 2164.6 3.6

2135.2 2136.6 1.4

2212.3 2214.3 2.0

2170.2 2167.4 2.8

2123.6 2123.3 0.3

2140.7 2139.4 1.3 2142.7 2142.2 0.5

2168.2 2170.2 2.0

2128.0 2126.6 1.4

2169.5 2170.7 1.2

2108.3 2107.4 0.9 2152.7 2151.0 1.7

2205.8 2204.7 1.1 2103.4 2104.1 0.7

2165.2 2169.9 4.7 2162.6 2160.9 1.7

2.0

2152 2149.7 2.3

2127 2125.0 2.0 2193 2199.3 6.3

2177 2178.8 1.8 2132 2132.6 0.6 2132 2133.6 1.6 2132 2133.6 1.6 2132 2133.6 1.6 2129 2127,s 1.2 2129 2133.6 4.6 2175 2174.4 0.6 2219* 2215.5 3.5 2118 2117.7 0.3

%alc A

(cmT1) -

PI R PI R

;;;

PI PI PI PI PI 121

WI R P91 R W’l R

-

Source?

* Average of symmetric and antisymmetric stretching frequencies. t R: Raman shift for liquid.

Numbers in square brackets refer to bibliography.

also tabulated. Literature values of questionable accuracy have not been included. It is therefore possible, provided E-values are known for aI.l the silicon sub-

stituents, to forecast accurately the SiH frequency for silanes of any given structure.

[19] F. FRANCOIS and RI. BIJISSET, Comph rend. 230, 1946 (1950). [ZO] V. A. KOLESOVA, 2. V. KTJJGTWI~AYA and D. N. ANDREEV, Izvest. Alcad. Nauk S.S.S.R., Otdel.

Iihim. Nuuk 294 (1953).

416

Correlation of the SiH stretching frequency with molecular structure

Effect of strained bonds

To evaluate the effect of strained silicon bonds on the SiH frequency, we have studied the cyclic series (MeHSiO),-,. The difference in bond strain energy be- tween trimer and tetramer must be appreciable, since in the analogous compounds (Me,SiO), and (Me,SiO),, the difference in the ring energy is about 3 kcal/mole [5]. Furthermore, the siloxane stretching frequency, which falls at about 1085 cm-l for the unstrained cyclics, is displaced to about 1020 cm-l in the cyclic trimers [5, 211. The shift in the SiH frequency, however, is only about 4 cm-l (Table 3),

Table 3. Observed SiH frequencies in methyl hydrogen siloxane cyclics

Compound I

“ohs

(MeHSiO), 2177.0 (MeHSiO), 2173.0

(MeHSiO), 2170.5 (MeHSiO), 2170.5

(MeHSiO), 2167.7

which is negligible compared with the shifts caused by electronegative groups. It is therefore apparent that the change in the SiH stretching frequency caused by strain on the other Si bonds is either very small, or is offset by some unrecog- nized factor.

Effect of adjacent oxygen

Experimental results on a number of substituted disiloxanes show that the SiH frequency is affected relatively little by the presence of a single adjacent oxygen. Apparently the electronegative character of the oxygen is largely neutral- ized by the more electropositive silicons. Further, the transmission coefficient of the SiO linkage for the R,Si substituents must be near unity, since the effect of the OSiR, group on the SiH frequency is approximately equal to the average E for the three substituents on the second silicon.

Multiple oxygen substitution, however, has a pronounced effect on the SiH frequency. For example, in (Me,SiO),SiH the SiH frequency is raised by about 80 cm-l. The stretching frequency for any given structure may still be calculated, although not as accurately as for the silanes, by the use of the empirically derived equation (3) :

v = 5 Ei + 9n2 (3) i

where n is the number of siloxane oxygens attached directly to the hydrogen- bearing silicon, and the E’s are average values for the terminally substituted silicons, ignoring the fact that one or more siloxane linkages may intervene.

1211 N. WRIGFIT nncl M. J. HUNTER, J. Am. Chem. Sot. 69, 803 (1947).

417

A. L. SMITH and N. C. ANGELOTTI

For example, in the compound (Me,SiO),SiH, Y (talc.) = 3[(3 x 705*9)/3] + 9(3)2 = 2198.7 cm-l. The observed frequency is 2200-S cm-l. For the compound (MeHSiO),, ?z = 2 and Y (talc.) =.705-9 + 2[(7059 + 724*8)/2] + 9(2)2 = 2172.6 whereas v (obs.) = 2173-O. Calculated and observed results are compared for a number of siloxane compounds in Table 4.

Table 4., Calculated frequencies from equation (3)

Compound

HMe(EtO)SiOSiMe, HMe,SiOSiMe,H HMe,SiOSiMe, HCl,SiOSiCl,H HPh,SiOSiPh,H HPhMeSiOSiMePhH HPhEtSiOSiEtPhH (HMe,SiO),SiH

(Me,SiO),SiH (MeHSiO), (HMe,SiO),Si Standard deviation

-

-

-

VObS

2151.8 -2152G3

2126.0 2133.0 2119.6 2126.7

2263.5 2258.1 2138.5 2140.5 2132.5 2136.7

2126.0 2127.7

2133.8 2137.2

2222,5 2217.6 2200.8 2198.7

2173.0 2172.6 2132.0 2133.0

Vcalc

Discussion

-

A

(cm-‘)

1.0 8.0 7.2

5.4 2.0 4,2

1.7 3.4

4.9 2.1

0.4 1.0 4.4

Several points are worth noting. First, equation (2) applies only to infrared frequencies of SiH compounds in Ccl, solution. Infrared and Raman frequencies for pure liquids will probably not be much different from those of the same materials in solution. Vapor state frequencies, however, may be either higher or lower by as much as 30 cm-l, and cannot be used in this correlation.

Relation of E to electronegativities

It is of interest to compare E-values from Table 1 with the Gordy electro- negativity scale as shown in Fig. 2. The relationship is not a linear one, but the two scales correlate reasonably well except for one or two points.

It has been emphasized [16, 221 that although one of the most attractive features of the electronegativity concept is its universal applicability, individual electronegativity values depend on the method by which they were derived as well as on the molecular environment of the atom, and tabulated figures represent the most probable of many possible choices. may vary from 2.3 to 2.8 [16].

For example, the value for carbon The question then arises as to whether the E-values

quoted in Table 1 represent an electronegativity scale for substituents in silicon compounds.

The frequency increase observed upon substituting a more electronegative group on the silicon is evidently caused by a withdrawing of electrons from the

[22] H. 0. PRITCHARD and H. A. S KINNER, Chem. Rem. 55, 745 (1955).

418

Correlation of the SiH stretching frequency with molecular structure

WI 0 I 700 710 720 730 E 740 750 7e

Fig. 2. Relation between E-values and electronegativities.

Table 6.

I Compound I v(s(~~-~lc. r(SiH)

I

Reference 7-G

H&F, / 2282 1.455 * 0.01 r241 7028

H,SiF, 2246 1.471 f 0.007 [251 7149

MeHSiF, 2228 1.474 * 0.007 WI 7136

MeH,SiF / 2191 l-473 f o-005 WI 7002

MeSiH, , 2155 1.485 f 0.005 i 1281 7055

Average 7074

silicon. This redistribution of electrons presumably increases the degree of s- character of the hydrogen-directed silicon orbital. A shortening of the SiH-bonds results, and as observed for other systems [23], the vibrational frequency increases. lkcrowave values for some SiH bond distances are compared with their predicted stretching frequencies in Table 5. The SiH frequency shows the same inverse

[23] E. M. LAYTON, JR., R. D. KROSS and V. A. FASSEL,J. Chem. Phys. 25, 135 (1956); R. C. LORD and R. E.MERRIFIELD,J.C~~~. Phys. 21,166 (1953).

[!?4] G. A. HEATH, L. F. THOMU and J. SHERIDAN, Trans. Faraday Sot. 59,779 (1954). [25] V. W. LAURIE, J. Chem. Phys. 26, 1359 (1957). [26] J. D. SWALEN and B. P. STOIOEEFF, J. Chem. Phys. 28, 671 (1958). [27] L. PIERCE, J. Chem. Phys. 29, 383 (1958). [28] R. W. KILB a.nd L. PIERCE, J. Chem. Phys. 27, 108 (1957).

419

A. L. Sram and N. C. iiNQELOTT1

proportionality to the cube of the interatomic distance (last column of Table 5) as has been observed for carbon-hydrogen frequencies [29].

We can therefore conclude that although the E-values listed in Table 1 are related to electronegativities, the quantitative relationship between the electron- withdrawing power of a substituent on Si and the change in the force constant of the SiH bond is not yet clear. It is possible that additional data on electro- negativities of groups would allow us to deduce such a relationship.

Conclusions Substituents on silicon affect the SiH vibrational frequency in a precise and

reproducible manner. Since with the exception of oxygen substitution these effects are additive, vibrational, steric, and electronic interactions between groups must be small or nonexistent. Siloxane oxygens, on the other hand, apparently do interact in a non-linear yet predictable manner.

We’have shown that, by use of a table of constants, it is possible to predict accurately the SiH stretching frequency. Such a procedure should prove valuable in the cha.racterization of unknown structures.

Aclcnowledgement--We wish to thank Mr. D. H. THOMSON for preparing and purifying some of the compounds used in this study, and Dr. W. J. POTTS for helpful suggestions regarding bonding mechanisms.

[29] H. FEILCHENFELD,S~~C~~OCA~~,. Acla 12, 2SO (195S).

120