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Correlation of the SiH stretching frequency with molecular structure
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 substituent 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 electronegativities, 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,~ ~1as 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 investigation 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 considered 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 molecules containing SiH,, SiH, or SiBr, however, large deviations appeared, and it was obvious that such a straightforward correlation was of very limited applicability. 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] r6] [7] 181 [SJ [lo] [ll] [12] [13] [14] [15] [16]
Unpublished results, Dow Corning Corporation. J. BELL, J. HEISLER, H. TANNENBAUM and J. GOLDENSON,J. Am.Chen~Soc. R. E. I
413
76, 5185-(1954).
75,
2695
(1953).
,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, New York (1960).
Chemical
Applicationa
414
of Speotiooscopy
p. 414 Interscience
Publishers,
Correlation of
the SiH
stretching frequency with molecular structure
Table 1. Calculation of E-values Compound
Obs.
HSiMeF, HSiCl, HSiBr$ HSi(OMe), HSi(OEt), HSi(B-EtBuO), HSi(Oi-Pr), H,SiMeCH,Cl H,SiPh H,Si(pC,H,Cl)Me H,Si(mC,H,Cl)Me HSi(CH:CHJMePh HSiPh, HSi(CH&H:CH2)C12 HSiPhMe, HSiPrCl,
2227.6 2268.3 2236.0 2203.2, 21960 2191.2 2190.8 2156.0 2158.3 2144.7 2142.0 2123.8 2126.0 2212.0 2120.5 2205-7
HSi(n-C,H,,)Cl,
2205.7
HSiEt, HSi(iPr)Clz H,Si(cycZo-CsHll)Ph
2097.2 2199.9 21250
from selectedcompounds Group
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 predict the SiH stretching frequency for any given structure.
(2) and
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
415
und Erler, Leipzig (1943).
.A. L. SNITH Table
2. Calculation
and N. C. ANQELOTTI of frequencies
from
Compouncl
3.
--
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
A (cmT1)
%alc --_
A.
E-values
2229.3* 2213.5 2168.2 2135.2 2212.3 2170.2 2123.6 2140.7 2142.7 2168.2 2128.0 2169.5 2108.3 2152.7 2205.8 2103.4 2165.2 2162.6
Source?
-
1.1 2.0 3.6 1.4 2.0 2.8 0.3 1.3 0.5 2.0 1.4 1.2 0.9 1.7 1.1 0.7 4.7 1.7
2230.4 2211.5 2164.6 2136.6 2214.3 2167.4 2123.3 2139.4 2142.2 2170.2 2126.6 2170.7 2107.4 2151.0 2204.7 2104.1 2169.9 2160.9
deviation
2.0
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
2152 2127 2193 2177 2132 2132 2132 2132 2129 2129 2175 2219* 2118
2.3 2.0 6.3 1.8 0.6 1.6 1.6 1.6 1.2 4.6 0.6 3.5 0.3
2149.7 2125.0 2199.3 2178.8 2132.6 2133.6 2133.6 2133.6 2127,s 2133.6 2174.4 2215.5 2117.7
-
PI R PI R ;;; PI PI PI PI PI 121 WI R P91 R W’l R -
* Average of symmetric and antisymmetric stretching t R: Raman shift for liquid. Numbers in square brackets refer to bibliography.
frequencies.
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 substituents, 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, Iihim. Nuuk 294 (1953).
416
Izvest.
Alcad.
Nauk
S.S.S.R.,
Otdel.
Correlation
Effect of strained
of the SiH stretching
frequency
with
molecular
structure
bonds
To evaluate the effect of strained silicon bonds on the SiH frequency, we have The difference in bond strain energy bestudied the cyclic series (MeHSiO),-,. 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 hydrogen
SiH frequencies siloxane cyclics
Compound
I
(MeHSiO), (MeHSiO), (MeHSiO), (MeHSiO), (MeHSiO),
in methyl
“ohs
2177.0 2173.0 2170.5 2170.5 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 unrecognized 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 neutralized 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 hydrogenbearing 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.
417
69, 803 (1947).
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
-
Compound
from equation
(3) A
VObS
Vcalc
(cm-‘)
2151.8 2126.0 2119.6 2263.5 2138.5 2132.5 2126.0 2133.8 2222,5 2200.8 2173.0 2132.0
-2152G3 2133.0 2126.7 2258.1 2140.5 2136.7 2127.7 2137.2 2217.6 2198.7 2172.6 2133.0
1.0 8.0
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
-
7.2 5.4 2.0 4,2 1.7 3.4 4.9 2.1 0.4 1.0 4.4
-
Discussion 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 electronegativity 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. For example, the value for carbon may vary from 2.3 to 2.8 [16]. 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. SKINNER,
Chem.
Rem.
418
55, 745 (1955).
Correlation
of the SiH
WI Fig.
0 700 2. Relation
stretching
frequency
I 720 E 730
710 between
Compound
H&F,
I I I /
H,SiF, MeHSiF, MeH,SiF MeSiH,
/ ,
v(s(~~-~lc.
2282 2246 2228 2191 2155
740
E-values Table
with
and
750
* f * f f
structure
7e
electronegativities.
6.
r(SiH)
1.455 1.471 1.474 l-473 1.485
molecular
Reference
0.01 0.007 0.007 o-005 0.005
7-G
r241
7028 7149 7136 7002 7055
[251
i
WI WI 1281 Average
This redistribution of silicon. character of the hydrogen-directed results, and as observed for other lkcrowave values for some SiH stretching frequencies in Table
7074
electrons presumably increases the degree of ssilicon orbital. A shortening of the SiH-bonds systems [23], the vibrational frequency increases. bond distances are compared with their predicted 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); 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
R. C. LORD
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 electronwithdrawing 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 electronegativities of groups would allow us to deduce such a relationship.
Conclusions Substituents on silicon affect the SiH vibrational frequency in a precise and Since with the exception of oxygen substitution these reproducible manner. 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
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 substituent 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 electronegativities, 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,~ ~1as 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 investigation 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 considered 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 molecules containing SiH,, SiH, or SiBr, however, large deviations appeared, and it was obvious that such a straightforward correlation was of very limited applicability. 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] r6] [7] 181 [SJ [lo] [ll] [12] [13] [14] [15] [16]
Unpublished results, Dow Corning Corporation. J. BELL, J. HEISLER, H. TANNENBAUM and J. GOLDENSON,J. Am.Chen~Soc. R. E. I
413
76, 5185-(1954).
75,
2695
(1953).
,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, New York (1960).
Chemical
Applicationa
414
of Speotiooscopy
p. 414 Interscience
Publishers,
Correlation of
the SiH
stretching frequency with molecular structure
Table 1. Calculation of E-values Compound
Obs.
HSiMeF, HSiCl, HSiBr$ HSi(OMe), HSi(OEt), HSi(B-EtBuO), HSi(Oi-Pr), H,SiMeCH,Cl H,SiPh H,Si(pC,H,Cl)Me H,Si(mC,H,Cl)Me HSi(CH:CHJMePh HSiPh, HSi(CH&H:CH2)C12 HSiPhMe, HSiPrCl,
2227.6 2268.3 2236.0 2203.2, 21960 2191.2 2190.8 2156.0 2158.3 2144.7 2142.0 2123.8 2126.0 2212.0 2120.5 2205-7
HSi(n-C,H,,)Cl,
2205.7
HSiEt, HSi(iPr)Clz H,Si(cycZo-CsHll)Ph
2097.2 2199.9 21250
from selectedcompounds Group
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 predict the SiH stretching frequency for any given structure.
(2) and
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
415
und Erler, Leipzig (1943).
.A. L. SNITH Table
2. Calculation
and N. C. ANQELOTTI of frequencies
from
Compouncl
3.
--
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
A (cmT1)
%alc --_
A.
E-values
2229.3* 2213.5 2168.2 2135.2 2212.3 2170.2 2123.6 2140.7 2142.7 2168.2 2128.0 2169.5 2108.3 2152.7 2205.8 2103.4 2165.2 2162.6
Source?
-
1.1 2.0 3.6 1.4 2.0 2.8 0.3 1.3 0.5 2.0 1.4 1.2 0.9 1.7 1.1 0.7 4.7 1.7
2230.4 2211.5 2164.6 2136.6 2214.3 2167.4 2123.3 2139.4 2142.2 2170.2 2126.6 2170.7 2107.4 2151.0 2204.7 2104.1 2169.9 2160.9
deviation
2.0
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
2152 2127 2193 2177 2132 2132 2132 2132 2129 2129 2175 2219* 2118
2.3 2.0 6.3 1.8 0.6 1.6 1.6 1.6 1.2 4.6 0.6 3.5 0.3
2149.7 2125.0 2199.3 2178.8 2132.6 2133.6 2133.6 2133.6 2127,s 2133.6 2174.4 2215.5 2117.7
-
PI R PI R ;;; PI PI PI PI PI 121 WI R P91 R W’l R -
* Average of symmetric and antisymmetric stretching t R: Raman shift for liquid. Numbers in square brackets refer to bibliography.
frequencies.
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 substituents, 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, Iihim. Nuuk 294 (1953).
416
Izvest.
Alcad.
Nauk
S.S.S.R.,
Otdel.
Correlation
Effect of strained
of the SiH stretching
frequency
with
molecular
structure
bonds
To evaluate the effect of strained silicon bonds on the SiH frequency, we have The difference in bond strain energy bestudied the cyclic series (MeHSiO),-,. 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 hydrogen
SiH frequencies siloxane cyclics
Compound
I
(MeHSiO), (MeHSiO), (MeHSiO), (MeHSiO), (MeHSiO),
in methyl
“ohs
2177.0 2173.0 2170.5 2170.5 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 unrecognized 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 neutralized 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 hydrogenbearing 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.
417
69, 803 (1947).
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
-
Compound
from equation
(3) A
VObS
Vcalc
(cm-‘)
2151.8 2126.0 2119.6 2263.5 2138.5 2132.5 2126.0 2133.8 2222,5 2200.8 2173.0 2132.0
-2152G3 2133.0 2126.7 2258.1 2140.5 2136.7 2127.7 2137.2 2217.6 2198.7 2172.6 2133.0
1.0 8.0
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
-
7.2 5.4 2.0 4,2 1.7 3.4 4.9 2.1 0.4 1.0 4.4
-
Discussion 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 electronegativity 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. For example, the value for carbon may vary from 2.3 to 2.8 [16]. 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. SKINNER,
Chem.
Rem.
418
55, 745 (1955).
Correlation
of the SiH
WI Fig.
0 700 2. Relation
stretching
frequency
I 720 E 730
710 between
Compound
H&F,
I I I /
H,SiF, MeHSiF, MeH,SiF MeSiH,
/ ,
v(s(~~-~lc.
2282 2246 2228 2191 2155
740
E-values Table
with
and
750
* f * f f
structure
7e
electronegativities.
6.
r(SiH)
1.455 1.471 1.474 l-473 1.485
molecular
Reference
0.01 0.007 0.007 o-005 0.005
7-G
r241
7028 7149 7136 7002 7055
[251
i
WI WI 1281 Average
This redistribution of silicon. character of the hydrogen-directed results, and as observed for other lkcrowave values for some SiH stretching frequencies in Table
7074
electrons presumably increases the degree of ssilicon orbital. A shortening of the SiH-bonds systems [23], the vibrational frequency increases. bond distances are compared with their predicted 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); 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
R. C. LORD
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 electronwithdrawing 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 electronegativities of groups would allow us to deduce such a relationship.
Conclusions Substituents on silicon affect the SiH vibrational frequency in a precise and Since with the exception of oxygen substitution these reproducible manner. 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