Silicon-29 chemical shifts

Magi, M., E. Lippmaa, A. Samoson, G. Engelhardt, and A. R. Grimmer (1984). Solid-state high resolution silicon-29 chemical shifts in silicates. J. Phys. Chem. 88, 1518-22. 

Silicon- 29 Chemical Shifts (Rel. % Intensities) S i/Al Chem. Line- Al(IV)... 

FIGURE 6. Silicon-29 chemical shift ranges for various organosilanes... 

NMR spectroscopy of organosilicon compounds TABLE 16. Silicon-29 chemical shifts of some silaethylenes31... 

TABLE 19. Silicon-29 chemical shifts for ring silicon atoms in some representative silacyclopropanes and disilacyclopropanes... 

The presently known silicon chemical shift range is 990 ppm. This includes the Dsd form of decamethylsilicocene 28 (5 Si = —423 (solid state)), which is the most shielded resonance reported to date and the alkyl-substituted silylene 45, which presently defines the high-frequency end of the spectrum at 5 Si = 567. Most silicon chemical shifts occur, however, in a much smaller range from 5 Si = +50 to —190. This includes hexa-, penta- and tetracoordinated silicon compounds and for trivalent, positively charged silicon a significant low-field shift compared to comparable tetravalent silicon species is expected. 

The analysis of the spectra is aided by the fact that the silicon chemical shifts are, to some extent, dependent on the number, n = 0, 1, 2, 3, of the nearest-neighbour tetrahedrons, Q (or Q A, where A represent a tricyclic arrangement). As an example, the connectivity b-n-j observed in the INADEQUATE spectrum of Figure 23 identifies a species that corresponds to the connectivity Q2A-Q3A-Q2 or Q2A-Q3A —Q3A. Each of these can be represented by several different structures. The candidate structures were computed using silicon anions containing up to 11 Si atoms and are, for this particular species, shown in Figure 24 as structures 7a-7e. 

The 7-sila-7-norbomadienylcation congener, 4, has a bent structure similar to the 7-norbomadienyl cation (Fig. 19). The silicon chemical shift is strongly shielded in the equilibrium C, structure, but it is deshielded in the Qv transition state 5 (while the chemical shifts of transition structures cannot be observed, the calculated values provide useful interpretive information [49]). The relative stability of 4 vs the 7-norbomadienyl cation is 15.1 kcal mof at MP2-FC/6-31G according to Eq. 3. 

Magic angle spinning Si, Al and NMR spectra were recorded on a variety of spectrometers. The data reported here are from results obtained at 200 MHz proton. Thus, the Si, Al and frequencies are 39.7 MHz, 52.15 MHz and 81.0 MHz, respectively. Chemical shifts for aluminum are reported relative to A1(N03>3 in aqueous solution at infinite dilution and are not corrected for second-order quadrupole effects. The phosphorus and silicon chemical shifts are reported relative to 85 wt% H3P0 and Me Si, respectively. 

Much larger effects, of course, are observed if the substituent Y is directly attached to silicon. In a recent paper (53) silicon chemical shifts of several iV-silylated triorganophospWimines are reported. A comparison similar to those shown above is made between derivatives for X = Cl, OMe (Table II). The results obtained in this investigation are explained preferentially on the basis of hyperconjugation rather than (p d)n bonding. If this is assumed to be the dominant... 

Accordingly, the approach of Grant and Paul (61) was taken to evaluate the additivity of alkyl substituent effects in these silanes. The silicon chemical shift is thought to be represented by ... 

Schraml et al.(137) found that the Si resonances of a series of silyl ester, alkoxy-silyl, and amino-silyl derivatives appear in different regions of the spectrum. On the basis of this information they suggested that Si NMR can be used for the structure elucidation of silylated hydroxy- or aminoacids. The silicon chemical shifts for DL-serine [55] and DL-threonine [56 ] are shown in ppm. The variation in the shielding for the alkoxy-silyl group demonstrates the previously noted sensitivity of Si NMR to the nature of R in Mc3SiOR. (140)... 

PDHS Structures in Solution. The determination of the chain conformation of polysilylenes in solution, particularly the conformations at temperatures just above or below the low-temperature thermochromic transition, is of great interest. NMR spectroscopy is one of the most useful techniques for probing chain conformation in solution (2i), and NMR is especially effective because of the large sensitivity of the carbon chemical shift to bond conformation (22). Silicon nuclei are also very sensitive to chain conformation, but a good correlation between silicon chemical shift and bond conformation has not been established yet. Unfortunately, both of these nuclei suffer from low sensitivity, primarily because of their low natural abundance. In contrast, protons have an essentially 100% natural abundance, but compared with the carbon or silicon chemical shift, the proton chemical shift is not very sensitive to bond conformation. Efforts to use NMR to probe the low-temperature dilute-solution conformation of the polysilylenes have been unsuccessful thus far. The diflSculty is that PDBS and PDHS precipitate from solution in 20-30 min after cooling through the thermochromic tran-... 

Tables. Carbon and silicon chemical shifts <5 [ppm] and coupling constants /(CSi) [Hz] in. (CH2) ...

In order to consider the relationship between the silicon chemical shift and silicon bond conformation, the Si NMR spectra with CP/MAS and GHD/MAS of PDFS at various temperatures are measured (Figs. 17.14 and... 

FIGURE 10. Relationship between silicon chemical shift and Hammett a values for aryltriethoxy-silanes and arylsilanes59,60... 

Pronounced shifts to lower frequency occur in the 29Si resonance when silicon compounds with coordination numbers higher than four are obtained. Several studies have now been conducted in which intramolecular coordination of nitrogen to silicon to form a pentacoordinate bond has been shown to produce a strong shielding effect of the silicon chemical shift. Specifically, the silatranes (15) are compounds in which a transan-... 

An empirical relationship has also been developed by Hahn118 for the silicon chemical shift of linear and branched polysilanes (Table 26) ... 

The alleged preparation of triphenylsilicenium (sityl) ions in different solvents, e.g. in sulpholane or dichloromethane , was reported by Lambert and coworkers in the mid-1980s. Based on conductance measurements and Cl/ Cl NMR spectra of the sityl perchlorate solutions, the authors concluded that the species which they observed were free siUcenium ions °. Unfortunately, Lambert and his coworkers did not report the Si NMR spectra of the sUyl compounds. Lambert s claims were challenged by Olah and coworkers and by Eaborn based on experimental data on related compounds. In particular, the reported silicon chemical shift of 3.0 ppm ° was that of a normal covalently bound, i.e. non-dissociated, triphenylsilyl perchlorate. In the case of MesSi-OClOs (529si = 43.4-47.0 ppm) and (i-PrS)3Si-0CI03 = 18 ppm) i,... 

There have been several proposals to rationalize silicon chemical shifts. Most of them centre on the charge on the silicon atom. Ernst and co-workers observed that the chemical shifts follow a parabolic curve if plotted against the sum of the electronegativities of the substituents of the silicon. Thus, above the sum of electronegativities of 9.5, a further increase of the electronegativity leads to an increase of the shielding. However, a decrease of shielding is observed if this sum is below 9.5. 

The a values so calculated do not give chemical shifts directly but have to be fitted empirically. Nevertheless, it is remarkable that the shape of the curve of Si chemical shifts as a function of the number of substituents (Figure 3) is reproduced by such calculations. The literature and a discussion of this procedure and other aspects of silicon chemical shift interpretation has been given by Marsmann (see Further reading section). Empirical aspects of chemical shifts of different classes of some silicon compounds only are given briefly below. 

Exceptionally high-frequency shifts are found for tin bound to one or more first row transition metals, and these have been attributed to a reduction of JE [equation (10), Chapters] resulting from dn-dn overlap between tin and the transition metal. Such interaction is less favorable for silicon, and silicon chemical shifts apparently do not show such large effects, although XIV provides an exception to this with 5( Si)= -1-173 ppm. Similarly, dn-dn overlap is less efficient between tin and a second or third row transition metal, with the result that shifts to relatively low frequency are found. ... 

Note for highly substituted allenes containing silicon, gernanium or tin we observed deviations of the calculated chemical shifts from the measured values, and one should therefore treat these particular cases with special care. 

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