29Si NMR study

Rate enhancement and an improved stereoselectivity was also found for higher-order cuprates when chlorotrimethylsilane was addedl9. H- and 29Si-NMR studies revealed that higher-order... 

IR and Raman spectroscopic studies on films and powders of PDHS indicate that the hexyl side chains are crystallizing into a hydrocarbon type matrix (40). This is indicated by the presence of a number of sharp characteristic alkane bands which become dramatically broadened above the transition temperature. Similar changes are observed for n-hexane below and above the melting point. CPMAS 29Si NMR studies on PDHS also show that the rotational freedom of the side chains increases markedly above the transition temperature (41,42). All of the spectral evidence... 

IGLO-HF 29Si NMR studies of silylated arenium ions 83 and 84 and comparison with experimental data have been reported.93,94... 

GC-MS (electron impact, El), 1H and 13C-NMR analysis indicated the cyclic trimer with all the EO chains cleaved [8 m = 3 R = H] as the major component of the heptane-soluble fraction of acid-degraded M2D-C3-0-(E0)n-CH3. The 1H and 13C spectra of the water-soluble fraction were indicative of linear disilanol structures, i.e. D(OH)2-C3-0-(E0)n-R, 6, or other linear disilanols (7 R1, R2 = H), and 29Si-NMR studies indicated formation of cyclic and linear structures [10]. 

Si NMR studies of solutions are difficult because of the long spin-lattice relaxation times of the nucleus and its negative nuclear Overhauser enhancement. The 29Si-1H dipole-dipole relaxation is inefficient because in most compounds the intemuclear distance is large. Fortunately, the problem of relaxation can often be overcome by resorting to cross-polarization (see Section II,E). 

The last ten years have experienced a great advance in the synthesis of novel and unusual organosilicon compounds which are interesting in view of their structure or relevance to theory. 29Si NMR study of such interesting and unusual compounds will be treated in this section. 

In recent years the literature has furnished a wealth of information concerning chemical structures from solid-state 29Si NMR studies of crystalline and noncrystalline silicates and aluminosilicate, polysiloxanes, polysilanes and other organosilicon compounds. 

Various solvents have been used in 29Si NMR studies and applications. It appears that the choice of solvent and concentration has been dictated more by the tradition in the particular field of organosilicon chemistry or laboratory than by the more general consideration of obtaining comparable and reproducible results. The solvents used include chloroform, benzene, tetrahydrofuran, pyridine, dioxane, dimethyl sulfoxide etc., usually in their fully deuteriated isotopomers the effects of these solvents on common secondary references were summarized earlier in Table 2. 

In solution, as expected, 29Si NMR studies revealed the presence of two signals for all noncentrosymmetric heterodimers, whereas for the centrosymmetric koilands a unique signal was observed. 

Ag and 29Si NMR study of cyclo-tetrakis-[tri-f-butoxy]... 

Recently, high resolution solution and solid state 29Si NMR studies have been conducted on the polysilanes (RR Si)w124 127. It is obvious that this technique will be a powerful method for determining the microstructure of these polymers. Figure 12 shows a comparison between the 29SiNMR spectrum of poly(methyl-n-hexylsilane) and poly(di-n-hexylsilane)124. In the former case the effects of tacticity can be seen in the fine structure of the spectrum whereas in the latter case for which this is impossible only a single peak is observed. 

Stock Solution. The stock solution is prepared by the acid-catalyzed hydrolysis of TEOS with sub-stoichiometric water (r = 1). 29si NMR studies of the fresh stock solution indicate that weakly-branched oligomeric species (Q and Q ) are the initial products of condensation. With aging, further condensation occurs, as is evident from the significant reduction in all hydrolyzed species and the appearance of branched (Q ) species. Whereas the concentration of unhydrolyzed dimers remains virtually constant witii aging, the concentration of the singly hydrolyzed dimer (-86.5 ppm) decreases dramatically. The concomitant increase in the unhydrolyzed cyclic tetramer suggests the reaction sequence ... 

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