29Si NMR chemical shift

Silanethione 38 was characterized by H, 13C, and 29Si NMR, Raman, and UV-vis spectroscopic methods. The 29Si NMR chemical shift of 38 (8Si 166.56/C6D6) for the silathiocarbonyl unit is much downfield shifted from those of the thermodynamically stabilized silanethiones, 31, 34, 35,28 and 36,29 mentioned in the previous sections, clearly indicating a genuine Si=S double bond in 38 without any intra- or intermolecular coordination. The molecular structure of 38 was successfully established by X-ray crystallographic analysis, and the detailed structural parameters are discussed in the following section. 

Calculation of the 29Si NMR Chemical Shifts in Molecules with SiN, SiCl, and SiSi Bonds... 

DL-AcNHCHRSnBu3 13C and 29Si NMR. Chemical shift and coupling constants depend on R, coordination number of Sn and solvent. 190... 

The results indicate that the formation of long-lived trimethyl substituted silyl cations, in the presence of aromatic solvents, as claimed by Lambert et al.95 is not feasible under these conditions. Persistent silicenium ions require sterically more shielding substituents at silicon or hypercoordinative stabilization.96 98 13C and 29Si NMR chemical shifts were calculated for a series of disilylated arenium ions 85 using density functional theory (DFT). The calculations predict consistently the unsaturated carbon atoms to be too deshielded by 8-15 ppm. Applying an empirical correction, the deviation between experiment and theory was reduced to -0.4 to 9 ppm, and the 13C NMR chemical shift of the highly diagnostic cipso is reproduced by the calculations (Ad = -3.8 to 2.7 ppm).99... 

Figure 9.29Si NMR chemical shifts at various temperatures in polar solvents.

The 29Si-NMR chemical shift of 12 (8 = 66.34 ppm) is considerably deshielded as compared to the progenitor alkene 13 (8 = 0.14 ppm), which is in accord with substantial P-Si-C-bond hyperconjugative delocalization of positive charge to silicon. 

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

TABLE 7. Selected l3C and 29Si NMR chemical shifts" for 20a and analogues25... 

It is known that the 29Si NMR chemical shift in zeolites is sensitive to the type of the exchangeable cation (56), which indicates the presence of interactions between cations and the framework. In particular, the substitution of Na+ by Li+ in zeolite A and in synthetic faujasite moves the 29Si resonances ca. 4 ppm downfield in both cases. Melchior et al. (235) have used this effect to study the location of cations in a series of partially exchanged zeolites (Li,Na)-A. They found that the average 29Si chemical shift is not a... 

Even non-nucleophilic media, such as nobel gases, were shown to influence the 29Si NMR chemical shifts of silicenium ions to a remarkable extent6, e.g. in Me3Si+-Ng complexes, where Ng = Fie (28), Ne (29) and Ar (30) (Figure 15), compared to the... 

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