STEPHEN G. FRANKISS
3418
Fluorine Magnetic Resonance Spectra of Methylfluorosilanes
by Stephen G. Frankiss William Ramaay and Ralph Forster Laboratories, University College, London, W.C.1, England Accepted and Travsmitted by The Faraday Society
(February 8, 1967)
The fluorine chemical shifts, (29Si-28Si)isotope shifts, and (H,F) and (Si,F) coupling constants in six methylfluorosilanes are reported. The vicinal (H,F) coupling constants in CH,SiFXY increase linearly with the vicinal (H,F) coupling constants in CH3CFXY. Both sets of coupling constants are closely dependent on the electronegativities of the substituent groups. The geminal (H,F) coupling constants in SiHFXY show a general increase with the geminal (H,F) coupling constants in CHFXY, and they also increase with increasing substituent electronegativity. The directly bound (Si,F) coupling constants in methylfluorosilanes lie in the narrow range 250 15 cps. The reduced directly bound (Si,F) coupling constants in a series of SiF3Xcompounds become less negative with increasing electronegativity of X. The fluorine chemical shifts in methylfluorosilanes lie in a narrower region (ca. 100 ppm) than the fluorine chemical shifts in fluoroalkanes (ca. 200 PPm).
*
The fluorine magnetic resonance spectra of methylfluorosilanes that are reported here form part of a study of the nmr spectra of derivatives of silane. The fluorine spectra of only a few fluorosilanes have been previously reported,l-1° and so it seemed worthwhile to report the spectra of a series of methylfluorosilanes, particularly as they have several similarities with the spectra of analogous fluoroalkanes.
Experimental Section Di- and i,rimethylfluorosilane were prepared by previously reported methods.11,12 The reaction between lead fluoride and dimethyliodosilane gave a 10% yield (based on the amount of iodide taken) of dimethyldifluorosilane Methyltrifluorosilane and methyldifluorosilane were prepared by the fluorination of methyldiiodosilane with lead fluoride and antimony trifluoride, respectively. The yields (based on the amount of iodide taken) were 93 and 5Oa/o, respectively. A small quantity of 1,l’-dimethyl-1,l’-difluorodisiloxanewas obtained (in 15% yield) as a by-product in the preparation of methyldifluorosilane. The compounds were purified by low-temperature bulb-to-bulb distillation, their purity being checked by measurements of vapor pressure and vapor density. They were :studied as solutions in trichlorofluoromethane, which was also used as an internal standard. The Journal of Physical Chemistry
For each compound at least two solutions of known concentration (ca. 10 and 90% by liquid volume) were studied, but only a limited amount of 1,l’-dimethyl1,l’-difluorodisiloxane was available, and so only two solutions of concentration (ca. 10 and 30y0) of this compound were studied. The samples were held in 5-nim 0.d. Pyrex tubing, and the spectra were recorded using a Varian Associates (1) H. S. Gutowsky and C. J. Hoffman, J . Chem. Phys., 19, 1259 (1951). (2) E. Schnell and E. G. Rochow, J . A m . Chem. Soc., 7 8 , 4178 (1956); J . Inorg. J’t~cl. Chem., 6, 303 (1958).
(3) E. L. Muetterties and W. D. Phillips, J . Am. Chem. Soc., 81,1084 (1959). (4) G. V. D. Tiers, J . Inorg. .Vucl. Chem., 16,363 (1961). (5) E. A. V. Ebsworth and J. J. Turner, J . Chem. Phys., 36, 2628 (1962). (6) E . A. V. Ebsworth and S. G. Frankiss, Trans. Faraday SOC.,59, 1518 (1963). (7) E. A. 1‘. Ebsworth and J. J. Turner, J . Phys. Chem., 67, 805 (1963). (8) S. S. Danyluk, J . Am. Chem. Soc., 86,4504 (1964). (9) P. L. Timms, R. A. Kent, T . C. Ehlert, and J. L. Margrave, ibid., 87, 2824 (1965). (10) R. B. Johannesen, T. C. Farrar, F. E. Brinckman, and T. D. Coyle, J . Chem. Phys., 44, 962 (1966). (11) H. J. Emelbus and h l . Onyszchuk, J . Chem. Soc., 604 (1958). (12) H. S. Booth and J. F. Suttle, J . Am. Chem. Soc., 68, 2658 (1946).
FLUORINE AqAGNETIC RESONANCE SPECTRA OF RfETHYLFLUOROSILANES
3419
Table I: Fluorine Chemical Shifts, (2@Si-Wi)Isotope Shifts, and (H,F) and (Si,F) Coupling Constants in Some MethylRuorosilanes Molecule
CH3SiFa (CHa)2SiF~ CHaSiHFz (CHI)3SiHF (CH&SiHF (CH3SiHF)20 a
6
135 f.1 132 f. 1 138 It 2 159 f.1 173 A 3 138 f.1
267.9 zt 0 . 6 291 f:3 293.4 f.0 . 6 274 f.2 278 f:3 a
4 . 1 7 zt 0 . 1 0 6.13i.0.06 6 . 6 3 f.0 . 1 1 7 . 1 5 f.0 . 0 8 7 . 6 3 f.0.05 6 . 6 1 f.0.15
0.009&0.007 0.010 & 0.015 0.007&0.008 0.005 =k 0.040 a
67.5 =k 0 . 2 52.1 zt 0 . 3 6 8 . 2 f.0 . 4
a
Not measured.
V4300B spectrometer operating at 40 Mclsec with sample spinning, flux stabilization and a K-3519 field homogeneity control system. Measurements were made using side bands generated by a Nuirhead Wigan D695A decade oscillator.
Table 11: Vicinal (H,F) Coupling Constants in Methylfluorosilanes and Some Related Fluoroalkanes
I JvicHFl Molecule
The spectra were analyzed by a first-order treatment, and the results are given in Table I. Each measurement is the average of a t least ten separate determinations, the error quoted being the calculated mean error. The directly bound (Sj,F) coupling constants and the (*9Si-28Si)isotope shifts were measured only in the 90% solutions. The (H,F) coupling constants and fluorine chemical shifts were measured in all solutions. They showed a marginally significant concentration dependence, and so values of these parameters given in Table I arc taken from measurements on the dilute (ca. 10%) solutions.
Discussion Vicinal ( H , F ) Coupling Constants. For the limited (CH3SiFXY) increases data in Table 11we find lJvIcHFl linearly with increasing lJvccHFl (CH&FXY), and the plot passes through the origin. This suggests that there are similar contributions to the vicinal (H,F) coupling constants in these two series of compounds; it would therefore be interesting to see if this relationship holds for other methylfluorosilanes and fluoroalkanes. The vicinal (H,F) coupling constants in CH3SiFXY vary approximately linearly with the sun1 of the Huggins electr~negrttivities'~ of the atoms in X and Y which are bound to the silicon atom (Figure 1). Similar relationships hare been reported for the vicinal (H,F) coupling constants in f l u ~ r o a l k a n e and s ~ ~ for the vicinal (H,H) coupling constants in substituted methylsilanese and alkanes.l5-l7 The vicinal (H,F) coupling constants in CHaSiFXY are, to a good approximation, an additive property of
a
(CHsCFXY),
CPS
CPS
4.2 6.1 6.6 6.6 7.1 7.6 8.3"
CHaMFa (CH3)2MB CHaMHFz (CH&lHF)20 (CHahMF (CH3)zMHF CHalllHzF
Results
I JvicHFI
(CHJSiFXY),
12. 71°
a 20.7'9 a a a 25. 7lS
Not known.
methyl substituents, but deviations from additivity are observed for successive fluorine substitution (Table 11). A similar effect is observed for the vicinal (H,H) coupling constants in CH&3iHXY.6t18 Geminal ( H , F ) Coupling Constants. The geminal (H,F) coupling constant in SiHFXY (where X, Y = H, F, CH3) increases with 1JgemHFI(CHFXY) in analogous compounds (Table 111). Since values of JpamHF (CHFXY) in these compounds19 are almost certainly positive, 2o this regular increase strongly suggests that the values of JQemHF(SiHFXY)are also positive. [JBemHFi (SiHFXY) generally increases with (EX EY),where EX and EY are the Huggins electronegativi-
+
~
~
~~
(13) 11,L. Huggins, J. Am. Chem. Soc., 75, 4123 (1953). (14) R. J. Abraham and L. Cavalli, .Mol. Phys., 9, 67 (1965). (15) R. E. Gliclc and A . A. Bothner-By, J . Chem. Phys., 25, 362 (1956). (16) C. N. Banwell and N. Sheppard, Discusswns Faraday Soc., 34, 115 (1962). (17) R. J. Abraham and I(.G. R. Pachler, Mol. Phus., 7, 165 (1963). (18) S. G. Frankiss, Ph.D. Thesis, Cambridge University, 1963. (19) D. D. Elleman, L. C. Brown, and D. Williams, J . M o l . Spectry., 7, 307 (1961). (20) D. F. Evans, S. L. Xanatt, and D. D. Elleman, J . Am. Chem. SOC.,85, 238 (1963).
Volume 7 1 , h'umber 11 October 1967
STEPHEN G. FRANKISS
3420
SiHF< and SiH2< being dependent on the s character of the silicon hybrid orbital^.^ Directly Bound ( S i , F ) Coupling Constants. The directly bound (Si,F) coupling constants in fluorosilanes do not appear to be additive properties of the substituents. Marked deviations from additivity are observed for fluorine substitution since progressive substitution of H for F at Si in SiF4 and CH3SiF3increases and then decreases IJs~FI. Substitution of CH, for H at Si in SiHFXY decreases ~ J s ~ by F ] 1-7 cps (Table IV).
Table IV : Directly Bound (Si,F) Coupling Constants in Fluorosilanes
1 JSiF I
1 JSiFl, 4
5
6 (Ex+Ey)
7
8
+
Figure 1. Plot of IJ&F1(CH8SiFXY) VS. (Ex EY), where Ex and Ey are the Huggins electronegativities18 of the atoms in X and Y, respectively, which are bound to Si.
Molecule
SiF8SiFa SiH2F2 CHaSiHFz (CH&SiF* SiH8F CH8SiH2F (CH&SiHF
CPS
3221° 2986 293 29 1 2815 2808 278
Molecule
SiHFa (CH8)&F CHaSiFs SiF4 (SiFdzO (NH&SiFe
I
CPS
275s 274 268 17OZ3 1681° 10S4
ties’s of X and Y. A similar relationship has been reported for lJ,,HFI(CHFzX).21 I n spite of these regularities, neither (SiHFXY) nor IJUemHFI The directly bound (Si,F) coupling constants in (CHFXY)2zis an additive property of CH, or F subSiFXYZ, SiFzXY, and S i F S (where X, Y, Z = H, stituenl s. CH,) lie in the narrow range 268-298 cps, but the more symmetrical and heavily fluorinated species SiF, (170 cps23)and SiFe2- (108 cps4)have much smaller coupling Table 111: Geminal (H,F) Coupling Constants in constants. For the limited number of SiFpX comFluorosilanes and Some Analogous Fluoroalkanes, and pounds that have been studied (Table IV) it appears Directly Bound (Si,H) Coupling Constants in Fluorosilanes that IJs~FI is much smaller when X is a strongly electronegative group (e.g., F or OSiF,) than when X is a I JoemHFI IJSiHi I JoanHF! (SIHFXY), (CHFXY), (SiHFXY), more electropositive group (e.y., CH,, H, or SiFJ. The Molecule CPS CPS CPS values of the reduced (Si,F) coupling constantsz4K S ~inF MH3F 45.85 46. 422 229.0‘ these molecules, however, are almost certainly nega47.5’9 222,36 CHaMHzF 48.86 tive.* Thus K S ~inF SiF3Xprobably becomes less negaa 215.818 (CH8)zMHE’ 52.1 tive with increasing electronegativity of X, which may 50,2z2 2825 MH2F2 60. 55 be related, in part, to decreasing Si-F bond ionicity CHaMHFz 67.5 57. ll0 273. 118 a 261. 11* (CHaMHF)iO 68.2 along this series. MHF3 96. 35 79.722 381. 76 Fluorine Chemical Shifts. S o simple correlations are observed between +(SiFXYZ) and I J s i ~(SiFXYZ) l like ‘ Not known. the reported increase of +(CFXYZ) with decreasing IJcFI(CFXYZ).~~There is, however, a general in-
It is interesting to note (Table 111) that IJUemHFI (21) B. H. Arison, T. Y. Shen, and N. R. Trenner, J . Chem. SOC., (SiHFXY) increases linearly with I J s i ~ (SiHFXY) l for 3828 (1962). the simple fluorosilanes (though not for the methyl(22) S. G. Frankiss, J . Phys. Chem., 67, 752 (1963). fluorosilanes) since a similar relationship has been re(23) T. D. Coyle. R. B. Johannesen, F. E. Brinckman, and T. C. Farrar, ibid., 70, 1682 (1966). ported between the geminal (H,H) and directly bound (24) J. A. Pople and D. P. Santry, Mol. Phys., 8, 1 (1964); K s i ~= (Si,H) coupling constants in SiH2XY.5 These rela( 2 7 r / h ~ s i i ~ ) Jwhere ~ i ~ , yai and YF are the magnetogyric ratios of tionships appear t o be consistent with the directly Si and F, respectively. bound (Si,H) and the geminal coupling constants in (25) N. Muller and D. T. Carr, J . Phys. Chem., 67, 112 (1963). The Journal of Physical Chemistry
FLUORINE MAGNETIC RESONANCE SPECTRA OF METHYLFLUOROSILANES
3421
Table V : Fluorine Chemical Shifts in Fluorosilanes and Rlethylfluorosilanes Molecule
SiF4 SiHFa SiH2F2 SiH3F
0
1632!7 109' 1517 2177
ilIolecule
CH3SiF3 CHaSiHF2 CH3SiHZF
+ 135 138 1926
crease in +(SiFXYZ) with increasing 4(CFXYZ) in analogous molecules,'* though SiF4 and CF, form a significant exception. The range of fluorine chemical shifts in simple fluorosilanes and methylfluorosilanes (Table V) is about 100 ppm, which is significantly smaller than the range of fluorine chemical shifts in analogous fluoroalkanes (about 200 ppm) . 2 2 , 2 5 A similar effect has heen noted for the proton chemical shifts in substituted silanes which are about half as sensitive to change of substituents as are the proton chemical shifts in analogous substituted alkanes.6Js The fluorine chemical shifts in methylfluorosilanes and simple fluorosilanes cannot be described by additive substituent parameters. Substitution of CH, or F for H at Si in SiFHXY, however, generally results in a low-field shift, though substitution in SiHF8 is against
Molecule
(CHd2SiF2 (CH3&3iHF
8
Molecule
+
132 173
(CH3)aSiF
159
this trend (Table V). This effect is consistent with the fluorine shielding being determined by local paramagnetic contributions, which are negative and increase in magnitude when a neighboring group is replaced by one having more relatively low-lying states than the group it Acknowledgment. The author is grateful to Dr. E. A. V. Ebsworth for his interest in this work. He is indebted to I.C.I. for the award of a research fellowship (University College) and to the D.S.I.R. for a maintenance grant (Cambridge University), during the tenure of which part of this work was done. (26) E. Pitcher, A. D. Buckingham, and F. G. A. Stone, Phys., 36, 124 (1962).
J. Chem.
Volume 71, Number 11 October 1967
