Silicon-29 NMR data

Early reports of silicon NMR data employ the magnetic field definition of chemical shifts instead of... 

Although apparently no Si NMR data have yet been published for MesSiOH 4, several silicon-oxygen compounds, for example TMSOTf 20 [11], hexamethyldisil-... 

When phenyl (Ph) groups replaced both Me3Si groups, again a rather unstable 1,2-disilacyclobutane dimer appeared to be formed,90 as shown by NMR data but when f-butyl replaced a Me3Si group, the silene failed to dimerize.87 Thus, it is evident that whether or not head-to-head [2 + 2] cyclodimerization occurs depends on the bulk of the substituents on both sp2-hybridized silicon and carbon. 

Since both analyses assumed random occupancy of T sites, the NMR data substantiate the model of random distribution of silicon and aluminum in ZSM-4. 

Classical shielding arguments indicate an electron-rich phosphorus atom, or equally, an increase in coordination number. The silicon atom seems also to be electron-rich, while the carbon has a chemical shift in the range expected for a multiply bonded species. The coupling constant data are difficult to rationalize, as it is not possible to predict the influence of orbital, spin-dipolar, Fermi contact, or higher-order quantum mechanical contributions to the magnitude of the coupling constants. However, classical interpretation of the NMR data indicates that the (phosphino)(silyl)carbenes have a P-C multiple bond character. 

The reported Si NMR data for silyl cation 3 demonstrate its homoaromatic character and characterize 3 as a free silyl cation in solution. The four-membered ring in 3 is identified by three Si NMR signals 8 Si = 77.3 (Si Si ), 315.7 (Si ) and 34.3 (Si ). Remarkably, the most deshielded silicon atom in 3 is the central tricoordinated silicon. This is in agreement with some homoaromatic nature of the cation 3 with charge localization at Si (see Fig. 2a) and it discards the possibility of a classical... 

Solid-state MAS 29Si NMR data for the pentacoordinate bis(siliconates) 64 and 65 and mono(siliconate) [PhSiF4], K+ 18-crown-6 are also summarized in Table 15 for comparison. The solid-state spectrum of 64 shows an uncharacterizable multiplet, possibly due to the slightly unsymmetrical structure. In contrast to the absence of peaks in solution,... 

The H NMR spectrum shows resonances for (5.95 ppm), Hg/H g/ exo (4.01 ppm), H /Hg/ endo (3.12 ppm) and Hy (0.30 ppm). The NMR data are in accord with a bridged, puckered bicyclobutonium ion structure that is static on the NMR time scale. The 29Si NMR chemical shift of 43.1 ppm for ion 428 indicates that the silicon is involved in stabilization of the positive charge. The stabilization occurs by shifting electron density from the Cy—Si cr-bond across the bridging bond to the formal carbenium carbon Ca. This y-silyl- type of interaction may be termed silicon homohyperconjugation. 

TABLE 24. Comparison of 29Si NMR data for hexa- and tetracoordinate silicon compounds ... 

TABLE 27. X-ray crystallographic and 29Si NMR data for hepta- and octacoordinate silicon complexes and tetravalent model compounds... 

One further finding with the —lIRNMeR — pyrolysis studies comes from Raman studies of the ceramic product. The NMR data show no traces of Si—C bonds, nor does the Raman spectrum. However, the Raman spectrum does show the presence of C—N bonds up to 1200 °C (the highest temperature studied). This suggests that the silicon nitride nanoparticles interact with the carbon matrix through C—N bonds. One might speculate that the interface looks somewhat like C3N4. 

To study the possible stabilizing effect of [3-silyl cations, Olah and co-workers334 prepared the 2- [(1 -trimethylsilyl)vinyl]-2-adamantyl cation 132 [Eq. (3.43)] as well as the parent silicon-free carbocation. In contrast to the above observations, NMR data [the (Cl ), (C2), and (C2 ) carbons are more deshielded in 132 than in the parent ion] showed that cation 132 is destabilized compared with the silicon-free analog. Furthermore, at — 100°C the C(l) and C(3) carbons were found to be equivalent, whereas in the parent ion they were nonequivalent. This indicates a rapid rotation about the C(l)-C(3) bond in 132, which can be rationalized by assuming the intermediacy of the [3-silyl-stabilized cation 133. The difference between cation 132 and those having [3-silyl-stabilization discussed above may be the orthogonal arrangement of the [3-C-Si bond and the p-orbital of the carbocation center. 

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