29Si chemical shifts solvent effect

The unexpected downfield shift in 66a,b, in connection with strong temperature and solvent effects on the 29Si chemical shift (Table XXI), indicates that the dibromo complexes 66, unlike the difluoro- (64) and dichloro- (65) analogs, undergo ionization to form the bromide salts (68) already at room temperature (Eq. 27).20,52... 

The obtained l3C and 29Si NMR data do not vary significantly with the solvent as long as aromatic hydrocarbons are used. That is the maximum solvent effect on l3C NMR chemical shift, AS l3C w/v, for cation 9a is AS I3C w/v = 0.5 when the solvent is changed from benzene to toluene and the position of the 29Si resonance remains even unchanged. This indicates negligible interaction between the cation and solvent molecules, in particular no Wheland-type intermediates are formed. (38) Solvents other than aromatic hydrocarbons are however reactive towards vinyl cations 8-10 (see below). 

In the case of silylium ions the formation of Wheland-type intermediates is shown by appreciable solvent effects on the 29Si NMR chemical shifts. 

Although observations are scattered throughout the literature, solvent effects on 29Si NMR parameters have been studied systematically only to a slight extent. Attention has been focussed mainly on effects on chemical shifts despite the fact that all parameters are affected by solvent, concentration and temperature. 

Fig. 20. Solvent effect on the 29Si NMR chemical shift of 34a in CD2C12 and CDC13 solutions at various temperatures.66 Reproduced with permission from the American Chemical Society.

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