AND Af. T. ROGERS R. SUMMITT, J. J. EISCH,J. J. TRAINOR,
2362
is invariably greater for H-bonding with ethers (for example, by over 100 cm.-l with phenol as the Hdonor). The latter is taken t o mean that ethers are more powerful electron donors than esters with respect to H-bond formation.13 The present results appear to concur with this view. The broad bound OH peak observed a t about 3420 cm.-l with PLMA shifts only about 17 cm.-l lower with PAM-PG; however, the peak broadens considerably (80 to 105 cm.-l a t the halfintensity width) on the low frequency side. (13) W.Gordy, J. Chem. Phys., 7, 93 (1939); 9, 215 (1941).
Vol. 67
The data presented here, support the view that increased film thickness occurs with polymeric dispersants because of the preferential adsorption of the more polar segments. This results in a configuration more extended away from the surface than with a monophyletic polymer. This is in accord with the consequent enhancement of the effectiveness as colloidal dispersants. Acknowledgment.-The author is obligated to Dr. J. R. Thomas for past contributions in our Laboratory to the study of colloid stabilization.
PROTON MAGNETIC RESONANCE SPECTRA OF VINYLSILANES BY
ROBERTSUYMITT,~ JOHN J. EISCH,~ JAMES T. TRAINOR,3
AND &[AX
T. R O G E R S 4
Corniny Glass Works, Corning, New York, Kedzie Chemical Laboratory, Michigan State University, East Lansing, Michigan, and the Department of Chemistry, University of Michigan, Ann Arbor, Michigan Received M a y 11, 1963 The proton magnetic resonance spectra of a series of vinylsilanes, RaSiCH=CH2 (where R = chloro, alkyl, or a substituted phenyl group), have been analyzed to obtain the spin coupling constants and chemical shift parameters for the vinylic protons. The small variation in the coupling constant sum, ZJi,, supports the view that the silicon atom effectively shields the vinyl group from inductive electronic effects exerted by the substituent R. On the other hand, the approximate correlation noted between the electronic nature of R and the trans proton chemical shift is consistent with a variable d,-p, resonance effect between the silicon atom and the vinyl group. Possible reasons for the failure of 6-cis and a-protons to show a similar trend are discussed.
Introduction Considerable attention has focused on the question of bond overlap between the n-cloud of vinyl and propargyl groups and available d-orbitals of elements from the second or higher rows of the periodic table (d,-p, ~ v e r l a p ) . ~ -Studies ~ of acid and base strengthsa and the addition of hydrogen halides to vinylsilanes9 have been interpreted in terms of such d,-p, bonding, and measurements of dipole moments10 and infrared1’-1s and proton magnetic resonance (p.m.r.)14s‘5 spectra have been used to investigate this problem. P.m.r. spectra of a few vinylsilanes, R,Si(CH= C H Z ) ~where - ~ R = CH3 or C&, have been analyzed and the results discussed with reference to possible d,-p, bonding.l4~l6We have synthesized a series of vinylsilanes, R3SiCH=CH2, in which the group R can be varied to achieve differences in the electronic environment at the silicon atom.13 Examination of the infrared spectra of these compounds has revealed a (1) Research and Development Division, Corning Glass Works, Corning, N. Y. (2) Department of Chemistry, Catholic University of America, Washington, D. C. (3) Research Laboratories, Raybestos Division, Raybestos-Manhattan, Inc., Stratford, Conn. (4) Kedaie Chemical Laboratory, Michigan State University, East Lansing, Mich. ( 5 ) P. D. George, M. Prober, and 3.R. Elliot, Chem. Rev., 66,1065 (1856). (6) C. Eaborn, “Organosilicon Compounds,” Academic Press, Inc., New York. N. Y.,1960,pp. 91-103. (7) D. Seyferth,”Progress in Inorganlo Chemistry,” Vol. 3, F. A. Cotton, Ed., Interscience Publishers, New York, N. Y.,1962,p. 129 ff. (8) R. A. Benkeserand H. R. Krysiak, J . A m . Chem. Soc., 75,2421 (1953). (9) L. H. Sommer, D. L. Bailey, G. M. Goldberg, C. E. Buck, T. S. Bye, J. F. Evans, and F. G. Whltmore, zbad., 78, 1613 (1954). (IO) H. Soffer with T. DeVnes, %bid.,78,5817 (1951). (11) W. J. Potts and R. A. Nyquist, Spectrochzm. Acta, 15,679 (1959). (12) H. Buchert and W. Zeil, ebid., 18, 1043 (1862). (13) J. J. Eisch and J. T. Trainor, J. Orp. Chem., 28,487 (1963). (14) R. T. Hobgood, Jr., J. H. Goldstein, and G. S. Reddy, J. Chem. Phys., 85,2038 (1961). (15) R. T.Hobgood, Jr., and J. H. Goldstein, Spectrochim. Acta, 19, 321 (1963).
correlation between the CH2 “wag” frequency of the vinyl group and the electron-withdrawing power of the substituent R.13 The p.m.r. spectra of these vinylsilanes have now been obtained and analyzed to learn whether they reflect a similar sensitivity to the electronic nature of R. This paper presents the results and discusses the vinylic spin coupling constants and chemical shift parameters in relation to possible substituent effects. Experimental The preparation and purification of the vinylsilanes empIoyed in this study (Table I ) have been given e1se~here.l~ P.m.r. spectra of the vinylsilanes listed in Table I were recorded on a Varian A-60 analytical spectrometer a t 25”. Chemical shifts of spectral lines were measured relative to tetramethylsilane (TMS) as an internal standard (ca. 5% concentration) using a 500-C.P.S. sweep width. These measurements and the chemical shifts relative to TMS computed from them are accurate to fl C.P.S. Higher resolution spectra of the vinylic region were recorded using a 100-c.p.s. sweep width. Reproducibility in the measurement of relative chemical shifts of these lines was f0.05 c.P.s., and relative chemical shifts and spin coupling constants appear to be accurate to about f0.3 c.p.s. All samples were 15% concentration by weight in carbon tetrachloride, because small concentration shifts were noticed.lB
Results With one exception, spectra in the vinylic region were of the ABC type.17 The spectra of (C6Hs)aSiCH=CH2, (CBH6)2C1SiCN=CH2,and (CsH6CHz)aSiCH=CH2 are reproduced in Fig. 1. These examples serve to illustrate the’forms of the spectra and the assignment used in computation. The spectra of (CaH6)a(16) Strictly speaking, one would prefer data from spectra at several concentrations which could be extrapolated to obtain the chemical shifts a t infinite dilution. However, the observed solvent effects were small (for affected lines, differences of less than 0.5 0.p.s. between a dilute sample, w., Ei%, and pure liquid (CHs)rSiCH=CH*), and since these effects are not understood well, no effort was made t o obtain these data. (17) H. J. Bernstein, J. A. Pople, and W. G. Schneider, Can. J . Chsm., 85, 65 (1957).
Nov., 1963
P.M.R. SPECTRA OF VINYLSILAXES
2363
TABLE I Hc
\
/HB
R3Sl/"="\ -VOsA
Chemical shiftsa uOsE3
-
HA
_____--
- vosc 3%. 8 384.2 364.9 397.7 394.2 395.1 396.9 395.2
Coupling constantab Jhn.
J W Z
Z7ijc
JCi.
332.2 355.9 3.1 21.0 15.1 39.10 353.9 370.7 3.2 20.1 13.9 37.83 336.5 351.5 3.6 20.2 15.0 38.53 345.0 374.1 3.6 20.4 14.5 38.40 343.5 375.9 3.4 20.5 14.5 38.33 341.9 370.9 3.7 20.1 14.4 38.25 344.3 373.5 3.8 20.3 14.3 38.40 347.0 380.0 3.0 20.1 14.2 37.30 374, 8d a Relative to tetramethylsilane (5%) as internal standard, in C.P.S. a t 60 Me. Error in calculated values f l C.P.S. Error in chemical shift differences, L e . , V O ~ A- v&, etc., 1 0 . 3 C.P.S. Calculated coupling constants in C.P.S. Error, f 0 . 3 C.P.S. Sum of the repeated spacings observed in the Eipectra. Only one line observed, with SiZ9side bands.
SiCH=CH2, (p-CB:,CsH,)BiCH=CH,, andT(p-CH3OCsH4)3SiCH==CH2were remarkably similar, differing only by slight chemical shifts of the entire spectrum and partia,l overlap by the phenyl proton resonances. Those of (CsH&C1SiCH=CH2, (CHa)3SiCH=CH2, (CeHsCH2)3SiCH=C€12,and C13SiCH= CH2showed the greatest multiplet collapse. Hobgood, Goldstejn, and Reddy14 examined C13SiCH-CH2 a t 40 Mc. and reported its spectrum to consist of a single slightly broadened line at -248 C.P.S. (-372, converted t o 60 Mc.), with a suggestion of structure. Our spectrum of this substance a t 60 Mc. showed a single sha,rp line a t -374.8 c.P.s., with no suggestion of structure save for the side bands presumably due to the Siz9 isotope (spin l / 2 , natural abundance 4.7%). The separation of these side bands was 4.0 C.P.S. The chemical shift of the signal is almost exactly the same as that of Cl3SiH.I8 It is puzzling that all three prot,ons apparently are magnetically equivalent to both the external field and the Siz9nucleus, Chemical shifts and coupling constants mere calculated from the spectxa by the method of Castellano and Waugh.19 This is an exact method in which parameters are computed directly from measured frequencies rabher than by the numerical fitting of the observed spectra by successive approximations. The details of the computation can be found in ref. 19. For most of the vinylsilanes assignment of lines to the A, B, and C quartets was unambiguous. In the remaining cases assignment was made by comparison with simpler spectra (see Fig. 1) and with the spectra of other vinyl derivatives, making use of the fact that Jtmm > J e i s> Jgem.2"--22Calculated values for chemical shifts and coupling constants, together with experimental values for thle coupling constant sum, are listed in Table I. These were computed from the eigenvalues of the Hamiltonian matrix, obtained as differences between observed lime frequencies. Although these latter were measureable t o =tO.O5 c.P.s., the resulting error in derived eigenvalues mas large enough to (18) D. E. Webster, J . Chsm. Soc., 5132 (1960). (19) S.Castellano and J. 8. Waugh, J . Chem. Phys., 34,295 (1961). ( 2 0 ) N.Banwell and N. Sheppard, Mol. Phys., 3,351 (1960). (21) W. Brkgel, Th. Ankel, and F. Kruckeberg, 2.Elebtrochem., 64,1121 (1960). (22) J. A. Pople, N. G. Sobneider, and H. J. Bernstein, "High Resolution
Nuclear Magnetic liesonance," McGraw-HI11 Book Co., Inc., New York, N. Y.. 1960.
\-,
/
I
-420
t
/
-400
,
1
-380
1
8
-360
,
I
I
-340
I
I
-320c,p,s,
region of the high resolution p.m.r. spectra of ( CsH6CH2)3SiCH=CH2. Scale is in C.P.S. relative to tetramethyleilane a t 60 Me. Letters A, B, and C indicate nucleus t o which transition has been "assigned," and X indicates combination transition. Fig. 1.-Vinyl
( CGH&S~CH=CHZ,( CsH6)2CISiCH=CH2, and
produce small discrepancies between calculated and observed values for the coupling constant sums. Analyses have been given previously for trimethylvinylsilane14and triphenylvinylsilane, l6 using a numerical method of fitting the spectra. The agreement with the present results is quite satisfactory, in that the values calculated for BJij are identical, and both sets differ from the experimental value by less than 1%. In all cases studied here the signs of Jets,Jtlans,and J,,, were the same and presumably all positive.
Discussion The experimental values of the coupling constant sum lie in the narrow range between 37.3 and 39.1 C.P.S. This seems surprising a t first sight. Waugh and CastellanoZ3have noted that a rough correlation exists between this quantity and the electronegativity of the vinyl substituent. While electronegativities here do not span a tremendous range, one might expect, for example, different electronegativities for the triphenylsilyl and the diphenylchlorosilyl groups. If the differences in the electronegativities of the (23) J. 9. Waugh and S. Castellano, J . Chsm. Phys., 36,1900 (1961).
2364
R. SUMMITT, J. J. EISCH,J. J. TRAIKOR, AKD M.
chlorine and phenyl groups are transmitted through the silicon atom as an inductive effect, these variations in the group electronegativity should be apparent in ZJi,. For a comparison, the values for styrene and ~ vinyl chloride are 29.5 and 20.7 C . P . S . , ~ respectively. However, a situation similar to that of the vinylsilanes has been noted1*in the chemical shifts of the single proton in trisubstituted silanes, R3SiH, where substituent effects would probably be transmitted through the single u-bond, and the observed differences in the chemical shift would reflect the different inductive effects of the substituents. I n two series of compounds where three methyl groups were successively replaced by chlorine and by phenyl groups, the proton chemical shift of R3SiH was found to move downfield by approximately one-third the downfield shift observed in the corresponding methane series, R&H. Clearly the inductive electronic effect of substituents bonded directly to silicon is attenuated markedly in comparison with that observed for carbon analogs. A greater attenuation of such purely inductive effects should be expected a t atoms one or two bonds further removed from the silicon atom. Consequently, the small variation in BJ,, for the rinylsilanes is consonant with the view that silicon shields the vinyl protons from the inductive electronic effect of the substituent R. Banwell and SheppardZ0have shown that an approximately linear relationship exists between JtTans and the substituent’s electronegativity (x),while Joe, and Jcis are similarly related to it. If their plot of Jgm,, us. x is extended to the value xsl = 1.8, the value J t r a n s = 19.8 C.P.S.is obtained; values calculated from the spectra of the vinylsilanes lie between 20.1 and 21.0 C.P.S. From this and the previous considerations it is apparent that the ~-aluesof vinyl proton spin coupling constants in vinylsilanes are in agreement with the interpretation of Banwell and Sheppard : namely, that the variation in vinylic coupling constants from one compound to another is primarily a function of inductive effects with relatively minor contributions by other factors. T’alues of J,,, for the vinylsilanes vary between 3.0 and 3.8 C.P.S. The geminal coupling in the HCH group has been related theoretically t o the HCH bond angle.24 By utilizing the curve obtained by Gutowsky and coworkersz4one obtains values of the bond angle HACHB (notation of Table I) of 119-120’ in the vinylsilanes, in good agreement with the value reported25 from microwave spectroscopy for H3SiCH=CH2 itself. In view of the known dependence of Jgem on the electronegativity of the substituent, however, this agreement is probably fortuitous. Chemical shifts of the vinylic protons in the vinylsilanes have two noteworthy features: first, the resonance of the trans proton is a t a lower field than that of the cis proton; and second, the chemical shifts, v06czs-v06trans and v~dgrans-vo6geml are approximately equal (except in (C6H5CH2),SiCH=CH2). These characteristics contrast sharply with those of typical vinylic shifts in which the trans proton occurs a t highest field and the chemical shift difference, Vo8eis-~odtrans, is 5 to 50 times greater than v06trans - v06gem.20-22 Closer examination of the p.m.r. spectra of vjnylsilanes shows (24) H. S. Gutowsky, M. Karplus, and D. M. Grant, J. Chem. Phys.,31, 1278 (1959). ( 2 5 ) J. M. O’Reilly and L. Pierce, zbzd., 34,1176 (1961).
Vol. 67
T.ROGERS
that the resonances of both /?-protons are a t lower field and, in general, the center of gravity in these spectra is at a lower field than in most vinyl spectra. Banwell and SheppardZ0have noted a correlation between the internal chemical shift of the or-proton and the mean shift of the @-protons (their quantity A, = 6 A - (aB 8c)/2, where 6 is the chemical shift in p.p.m.) and UR the resonance contribution to the Hammett u - c o n ~ t a n t s . ~This ~ ~ ~correlation ~ appears to reflect the extent to which the electrons are supplied to or removed from the vinyl group by resonance (I).
+
-
R-CH=CHz
+
+
-
++R-CH-CHZ
++R=CH-CHz
(1) Such a description as I implies that the electron density a t the two @-protonswould be affected equally and their chemical shifts should move in unison as the nature of the vinyl substituent changes, which is the observed result for most vinyl compounds. However, when the vinyl substituent is changed to an atom of the second (or higher) period, as in the vinylsilanes studied here and the series (CH2=CH).1\1, where M = As, Pb, Sb, Bi, etc.,21 the resonance of the @-proton trans to the substituent suffers a much larger downfield shift than the P-cis proton, and the trans proton resonance now occurs a t lower field than the cis. The /?-protons clearly do not shift in unison if second or higher row substituents are considered. A comparison between the @-trans proton chemical shifts and the u-parameters for two series of vinylsilanes, (R-CaH5)3SiCH=CH2 and R3SiCH=CH2 (Table 11), reveals the following points of interest. First, with the exception of tris-p-anisylvinylsilane, the triarylvinylsilanes, (RC6H&SiCH=CHz, tend to display increasingly deshielded @-trans proton resonances as the u,-value of the para substituent becomes more positive. This implies the operation of the electron-withdrawing power of the para-substituted phenyl group through the Si-C-C chain upon the /?-trans proton. Secondly, with the exception of chlorodiphenylvinylsilane (the only entry with nonidentical Si substituents and, hence subject to conformational considerations), the vinylsilanes of the type R3SiCH= CH2 tend to exhibit increasingly deshielded @-trans proton resonances as the group R withdraws electrons more strongly from silicon by the inductive effect ~ the small variation (increasing value of u * ) . ~Since in the value of BJij for the series of vinylsilanes doesnot suggest any significant inductive electronic effect, this trend in chemical shifts is ascribed to a variable contribution in the d,-p, resonance effect (11). ElecR J3 R RI R; H R’! H /H Si- C
R/
R/Si-C
e/
L-H I IIa
(26) (27)
-
H
‘(3
C--H
-
-%e
Si=C
\
n
H
IIb
/
\@H
IIC
R. W. Taft, Jr., J . Am. Chem. Soc., 79,1045 (1957). R.W. T a f t , Jr., “Sterio Effects in Organic Chemistry,” M. S. New-
man, Ed., John Wiley and Sons, Inc., New York, N. Y . , 1956,pp. 594-597. (28) Similar t o the findings in the infrared study of vinylsilanes,’a 0,values may correlate better with the obserred chemical shifts of the @-trans protons t h a n the u*-values. Unfortunately, not all the requisite u,-values are available for examination (Table 11). However, with both types of vinylsilanesit is apparent t h a t the voS tians-u correlationsare not cleanly linear in any event.
P.M.R.SPECTRA OF VIKYLSILANES
Sov., 1963
2365
TABLE I1 COMPARISON OF P-trans PROTON CHEMICAL SHIFTSWITH U-PARAMETERS p-Substituted phenyl%i nylsilanes, (R-CaHa)zSi-CH=CHr
- vostranIB 370.9 373.5 374.1
R
CHI CH@ H
"P
-0.170 .268
,--
- "la -0.13 - .50 .00
Trisubstituted vinylsilanes, RzSi-CH=CHz
R
up
-vobonsb
up
u*c
- "'a
"P
CH3 351.5 0.00 -0.13 -0.17 355.9 C&sCHz .22 ? ? .ooo 364.4 CH%=CH~ .1-0.5" ? ? (C6H,)2ClY 370.7 1 38 F 375.9 .062 - .44 C6H6 374.1 0.6 0.00 0.00 CF3 380.0 f .551 .14 Cl 374.8 2.94 .24 .23 a up - u' = crR, resonance contribution of the substituent. Cj. ref. 26. Chemical shift in c.p.n. a t 60 Mc. relative to tetramethylU* = net polar contribution of the substituent. Cj. ref. 27, p. 587. silane. Reference 15. e Values estimated to be from 0.1 to 0.5 (ref. 18). Weighted average of U*of 2C6H5 1C1atom. (May be subject to conformationalvariations.)
-
+ +
+
-
+
+
f
/
c1
'C-H
I
H
tron withdrawal by R would be expected to enhance the importance of canonical forms I I b and IIc and hence to cause incrleased deshielding of the P-trans proton in both series of vinylsilanes. It seems unlikely that the downfield shift of the trans proton could arise entirely from a neighbor anisotropy effect. On the other hand, no equally satisfactory correlation can be discerned bettween the chemical shifts of the pcis and a-protons and any reasonable a-parameters ~ t ~ (Tables 111 and IV). The failure of ~ 0 8 and to parallel any definite trend in the electronic character of the substituent R is open to at least two interpretations. In tlhe first place, trans-delocalization of the rr-cloud toward silicon (IIIa) (and hence, selective deshielding) nnay be favored preferentially over cisdelocalization (IIIb). Such preferential truns-delocali-
the P-cis and a-protons to anisotropic affects arising from the neighboring groups on silicon (IIa). The projection of the P-trans-proton away from such R groups on silicon would make it less susceptible to these field effects. TABLE 111 COMPARISON OF 8-cis PR.OTON CHEMICAL SHIFTSWITH U-PARAMETERS p-Substituted phenylvinylsilanes, (R-CsH&Si-CH=CHz R u08~i~" up q3 c'
-
-
CH3 341.9 -0.17 F 343.5 $0.06 CHaO 344.3 -0.27 H 345.0 0.00 CFa 347.0 $0.55 a Chemical shift in c.p.8. silane. Reference 15.
Trisubstituted vinylsilanes, RaSi-CH=CHz R -u&iea u*
-0.13 CeHbCHz -0.44 CH3 -0.50 CeHa 0.00 CH2=CHb 4 0 . 1 4 C1 at 60 Mc. relative
332.2 0.22 336.5 0.00 345.0 0.60 346.2 0.1-0.5 374.8 2.94 to tetramethyl-
TABLE IV COMPARISON OF a-PROTON CHEMICAL SHIFTSWITH U-PARAMETERS IIIa
IIIb
zation would be consistent with the hypothesis of the minimum bending of inolecular orbitals.29 The smadler curvature of the hypothetical molecular orbital encompassjng the silicon atom and the P-trans proton (IIIa) would presuimably make it more important in the ground state than IIIb. However, an alternative explanation may lie in the greater sensitivity of
a.
H. Stewart, and R. F. Smith, Proc. Natl. Acad. (29) (a) H. Eyring, Sei. U.S.,44, 269 (1958); (b) G.H. Stewart and H. Eyring, J. C h e n . Edue., 36, 550 (1958).
p-Substituted phenylvinylsilanes, (R-CeHc)aSi-CH=CHz
R
-vo8,,rna
up
pp
-
u'
Trisubstituted vinylsilanes, RsSi-CH=CHn R -uo8pcm4 s*
F 394.2 C0.06 -0.414 CeHbCHz 356.8 $0.22 CHt 395.1 -0.17 -0.13 CH3 364.9 0.0 CF3 395.2 +0.55 +0.14 CH2=CHb 366.8 $0.1-0.5 CHDO 396.9 - 0 . 2 7 -0.50 C1 374.8 2.94 H 397.7 0.00 0.00 CaHj 397.7 0.60 a Chemical shift in c.p.5. at 60 Mc. relative to tetramethylsilane. Reference 15.
Acknowledgment.-We gratefully acknowledge a grant from the Atomic Energy Commission which supported part of this work.
