5375
Silicon-29 Nuclear Magnetic Resonance. Substituent Effects’ Carl R.
Chemical Shift
Leonard Spialter,” Glen R. Buell, and Douglas L. Wilhite2
Contribution from the Aerospace Research Laboratories, Wright-Patterson Air Force Base, Ohio 45433. Received October 8, 1973 Abstract: Silicon-29 nmr chemical shifts (6Z9Si)are reported for a number of aryltrifluoro- (1-F), aryltrichloro(1-Cl), aryltriethoxy- (1-OEt), and aryltrihydrosilanes (1-H). 629Sifor these series, with the exception of 143, exhibited good linear correlations with Hammett’s c constants. In contrast to the chemical shift trends in 1-H and in aryltrimethylsilanes (l-CH3),silanes 1-F, 1-Cl, and 1-OEt gave downfield shifts with increasing electron donation by meta and para substituents on phenyl. Semiempirical quantum chemical (CNDO/2) calculations of electron density at silicon for silanes l-F5 1-H, and 1-CH3indicated no unusual reversal in the density trends to account for the observations. An additive although nonlinear relationship between 6Z9Siand the summed electronegativities of the groups bonded to silicon was found. and its applicability was demonstrated for the estimation of 6Z9Si for grossly different silanes.
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nly recently have experimental and instrumental techniques become available that permit facile nmr studies on nuclei of low natural abundance. The advent of pulsed Fourier transform spectrometry (FT), a computer signal averaging, and relaxation reagents4 has greatly reduced experimental difficulties, particularly in obtaining silicon-29 ( 29Si)nmr spectra. F o r this nucleus, they have allowed spectral measurements despite preexisting problems of low natural abundance (4.7 %),5 relatively long relaxation times (Tl 2 20 ~ e c ) , ~low - ~signal sensitivity (0.008 relative to l H = 1.0),5band the negative nuclear Overhauser effect [ p = -0.555)5a that are characteristic of the magnetically active silicon isotope. Previous studies on monotonic series of silanes, (CH,)4-,SiX,, where X = F , C1, OAc, O R , CsHj, CzH3,6and O2CR,lo (CH3)3-,Si(X),0Ac where X = O A c a n d OC2H3,11and F.l-,SiY,, where Y = CI, Br, and CHs,12 have established general trends of 29Si chemical shifts. An investigation of the effect of structural, electronic, and solvent perturbations in various a1kylsilanes provided a detailed description of the more subtle effects on the 29Siresonance for this particular class of silanes. Compilations of chem( I ) Presented in parts at the 1973 Organosilicon Symposium, North Texas State University, Denton, Texas, April 1973, and the 166th American Chemical Society Meeting, Chicago, Ill., Aug 1973. (2) National Research Council Associate 1972-1973. (3) For a comprehensive treatment of application to nmr cy, T. C. Farrar and E. D. Becker, “Pulse and Fourier Transform NMR. Introduction to Theory and Methods,” Academic Press, New York, N. Y., 1971. (4) 0. A. Gansow, A. R. Burke, and G. N. LaMar,J. Chem. Soc., Chem. Commrin., 456 (1972). (5) (a) J. A. Pople, W. G. Schneider, and H. J. Bernstein, “High Resolution Nuclear Magnetic Resonance,” McGraw-Hill, New York, N. Y., 1959, p 480; (b) J. Shoolery, private communication, Varian Associates, Palo Alto, Calif. (6) B. I
