29Si-enrichment

Si NMR from enriched material began in 1954 when Williams, McCall and Gutowsky369 used 29Si-enriched (70%) SiF4 to determine the spin and estimate the... 

Many of the individual lines observed in spectra such as those shown in Figure 1 can be assigned to specific silicate structures based on spectra of well defined silicate species and detailed NMR studies of the homonuclear spin-spin coupling of Si nuclei in 29si-enriched silicate solutions r6-81. Such studies have revealed that the chemical shift of a given silicon atom depends on its connectivity, the length of the Si-O bonds, and the angle of Si-O-Si bonds, as well as the pH of the medium and the cation type. 

Two novel species, XII and XIII, have also been identified in the 29si-enriched potassium silicate solution, as described in ref. (X). The chemical shifts of these species are unusual, probably indicating structural strains. 

Use of 29Si NMR can generally be avoided in favour of 1II NMR. For example, Narayan et al. [253] have used proton NMR for the detection of a HALS stabiliser based on silicon technology, after extraction from a PP matrix. 170 1-NMR and s-NMR have been used for the study of PP, EPDM, PIP and NR oxidatively degraded with enriched 02 [204,254]. 

For investigations with nuclei with low natural abundance, such as 29Si (4.7%), isotopic enrichment is often applied. This is accompanied with difficulties in the analysis of connectivities between selected pairs, because multiple site interactions become more pronounced. Cadars et al. have suggested incorporation of a z-filter that results in a robust method to select local site connectivities and remove complications from multiple site interactions [122]. 

The chemical state of dissolved silica (if Si02 is supplied as silica sol) must influence the nucleation and growth of crystalline silicates. Cary et al. (213) dissolved isotopically enriched silica in H20/D20 and, using 29Si NMR, concluded that tetrahedral dimers corresponding to pyrosilicic acid, H6Si207, built of two Q1 units were present in addition to monomers (Q°). The former species resonated at —9.26 ppm from TMS and accounted for up to 6 % of total spectral intensity. 

Beginning at 850 K and higher, heating of a methoxylated silica sample is accompanied by the sequential formation of the carbon-centered =Si- CH2 (900 K), =Si-HC -Si= (1100K), and (=Si-)3C (1300 K) radicals. With a rise in pyrolysis temperature, the concentration of radicals increases. Their EPR spectra parameters are reported in Table 7.15. The answer to the question of the chemical structure of these radicals was obtained in the experiments on samples enriched with magnetic isotopes of the H(D), C(13C), and Si(29Si) nuclei. The (=Si-)3C radicals EPR spectra are shown in Figure 7.30. 

The traditional aid in measurements and assignments of NMR spectra, i.e. selective isotopic substitution, has also been used in 29Si NMR spectroscopy. In the case of the 29Si isotope it has been mainly total 29Si substitution while selective isotopic enrichment in other isotopes (e.g. 2H) has been used to facilitate some types of measurements such as assignments. 

FIGURE 53. Expanded part of 29Si—117Sn NMR heteronuclear 2D spectrum of stannosilicate solution (enriched in both 29Si and 117Sn). The circles represent octahedral stannate centres, filled circles represent tetrahedral silicate centres and lines represent shared oxygen links. Reprinted with permission from Reference 384. Copyright 1996 American Chemical Society... 

Absolute rate constants and Arrhenius parameters have been determined for the thermal E,Z-isomerization of the stable disilene derivatives 92-96 in deuteriated aromatic solvents or THF-ds solution by XH or 29Si NMR spectroscopy133-136. With 1,2-dialkyl- and 1,2-diamino-l,2-dimesityldisilenes such as 92a-94, the (E)-isomers are considerably more stable than the (Z)-isomers, and so rate constants for E,Z-isomerization were determined after first generating mixtures enriched in the (Z)-isomer by UV-irradiation of the (El-isomer, and then monitoring the recovery of the solution to its equilibrium composition. On the other hand, little difference in thermodynamic stability is observed between the (Eland (Z)-isomers of tetraaryldisilenes such as 95a,b, and E,Z-isomerization kinetics were hence determined starting from solutions prepared from the individual, pure (or almost... 

The depolymerization of silicate species under the influence of dilution or increased alkali seems to be unaffected by the presence of aluminate. However, even with enriched 29Si, the spectra take time to measure and so information about kinetics is lost and the spectra relate to the solutions some time after mixing. 

Composition and structure of synthesized oligomers is proved on the basis of the ultimate analysis and by IR, H and 29Si NMR spectral data. Spectral data (oligomers No 1-3, Table 2.18) indicate that the presence of cis-trans, trans-trans and trans-cis combinations of rings in the polymer chain is si-milar to polydecaorganocyclohexasiloxanes [26] in the case of trans-dihydroxyorganocyclohexasi-loxane, czs-czs-combination in the polymer chain is absent. They display atactic structure of cycloli-near chain, enriched with trans-trans-sequences. 

---