Organosilicon compounds shifts

Table 4. IGLO/II computed Si NMR relative chemical shifts of several organosilicon compounds vs TMS...

Fig. 2. Dependence of the activation energies for reaction of electrophilic substitution of proton in surface silanol groups vs. the proton-accepting properties of atoms bound with silicon (a) and the values of 3750 cm" shifts (b) after adsorption of organosilicon compounds 1 - SitCHj) 2 - (CH3)3SiCl 3 - (CH3>3SiOCH3 4 -(CH3)3SiN(CH3)j.

Figure 2. Dependencies of shift of valence vibration band of silica surface hydroxyl groups at adsorption of small organic bases (a) and organosilicon compounds (b) on the maximum negative charge of the bases (a) and on the ionization potential of organosilicon compounds (b).

In one detailed review206 it was pointed out that the concept of group frequencies is particularly useful in the interpretation of the solution IR spectrum of organosilicon compounds. This is due to the insulating effect of the silicon atom which causes the spectra of these compounds to be approximately the same as the sum of independent vibrations of the substituent groups. Small shifts from these characteristic frequencies are to some extent predictable, so the assignment of molecular structure is in many cases comparatively simple. Such perturbations of group frequencies as do occur usually arise from ... 

FIGURE 4. General chemical shift ranges for protons in organosilicon compounds. Vi = vinyl... 

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

NMR spectroscopy of organosilicon compounds TABLE 18. 29Si chemical shifts of cyclic di- and polysilanes"... 

If organosilicon compounds have an acetoxy group on the carbon adjacent to the silicon atom, a facile 1,2-shift of the acetoxy group from carbon to silicon has been observed1 s, in addition to capture of the initially formed silyl radical by abstraction of halogen from the CC14 solvent (equation 3). Similar behavior was shown by a related benzyldimethylsilyl... 

The IR and Raman spectra of organosilicon compounds have been thoroughly studied and largely systematised. Shifts in the valence vibrations (symmetrical) and Vas (asymmetrical) yield essential information. If one or two methyl groups in tetramethylsilane (Vj = 598 cm", = 696 cm" ) are replaced by carbofunctional... 

The established standard compound to calibrate and spectra is tetramethylsilane (CH3)4 i (TMS). As this substance contains silicon also, it is natural to use it as standard for Si NMR. Although TMS is an inert substance, has a low boiling point and a rather short relaxation time, its chemical shift is in the middle of the shift range of other organosilicon compounds and peak misassignments are possible. Two strategies to circumvent this problem have been developed. The first is the use of secondary standards. Table 2 lists a number of those used in the literature. 

Figure 5 Shift ranges of building units in polysiloxanes. Reproduced with permission of John Wiley Sons from Williams EA (1989) NMR spectroscopy of organosilicon compounds. In Patai S and Rappoport Z (eds) The Chemistry of Organic Siiicon Compounds. New York John Wiley Sons.

Several reference compounds have been suggested for Si NMR. Early studies reported Si chemical shifts for organosilicone... 

The spectral dispersion for organosilicones may be considerable for certain families of compounds. This is reflected in the Si chemical shifts of siloxanes, -(SiRR 0) -, an important class of compounds which includes resins, fluids, room-temperature vulcanized and heat-cured rubber consumer products. The first, Si NMR results (5,78) reported on polydimethylsiloxanes showed that individual resonance... 

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