29Si-NMR

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... 

Fig. 4. 29Si-NMR data of selected silylene complexes (ref. to table 1)...

Fig. 3a and b. Fast exchange-slow exchange NMR transition for the conformational interconversion of octamethyltetrasiloxane. a, MAS 13C-NMR solid state spectra on the left side in comparison to solution spectra in propane-di on the right side (at 75.47 MHz), b. MAS 29Si-NMR spectra at 59.63 MHz. Temperatures are indicated in K, shift positions refer to TMS = 0 ppm and correspond to the scale at the bottom. (Ref. I0))... 

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]. 

Closely related studies by Markl using a 2-diazo-l-silacyclohexadiene such as 20 led in some cases to highly substituted silabenzenes,108 109 one of which, 21, was stable at -100°C, based on its 29Si NMR spectrum (Eq. 16). 

Trimethylenemethane is a special type of alkene that does not exist as the free compound. Various synthetic equivalents to the synthon 43 shown below have been reported. Trost, in particular, has exploited these compounds in 1,3-dipolar cycloaddition reactions.138 139 A metal-bound, isolated trimethylenemethane species was recently reported by Ando (Scheme 6). It resulted from the complexation of an ero-methylenesila-cyclopropene with group 8 carbonyls (Fe, Ru).140,140a The structure was proved by X-ray crystal structure analysis.29Si NMR data were consistent with the -structure shown. 

The 31P- and 29Si-NMR specta of 22 confirmed that the exo configuration is preferred at 25°C.14 At higher temperatures (above 38°C), inversion of the configuration of the peripheral phosphorus atom was observed in the 3IP-NMR spectrum that is, 22 rearranges into the endo isomer 22 and vice versa (Scheme 5). 

Disilenes exhibit the relatively low-field (8 = 49-155) 29Si chemical shifts characteristic of low-coordinate silicon compounds (Table I) thus 29Si NMR spectra are very important in their characterization. This deshielding is similar to that observed in the 13C chemical shifts of doubly bonded carbons relative to those of their saturated counterparts. 

The unusually low-field 29Si NMR resonances (S 142-155) observed for tetrakis(trialkylsilyl)disilenes 22-24 are interesting,21 but whether these effects result from the trialkylsilyl substituents is open to question in view of their similarity to the chemical shifts between diarylbis(trimethylsilyl)-disilene 9 and dialkyldiaryldisilene 10 ((Z)-9 (8 97.68) vs. (Z)-10 (8 96.93)). 

More recently, a new mode of cis-trans isomerization of a disilene has been suggested for the extremely hindered disilene 27. As will be detailed in Section VIII. B, 27 undergoes thermal dissociation into the corresponding silylenes. Monitoring the thermolysis of (Z)-27 at 50°C by H and 29Si NMR reveals a competitive formation of the isomerized ( >27 and benzosilacyclobutene 37, which is most likely formed by intramolecular insertion of silylene 36 into the C—H bond of the o-bis(trimethylsilyl)-methyl group (Scheme 3).22,59 This suggests the possible occurrence of cis-trans isomerization via a dissociation-association mechanism. 

Reaction of 1 with nitrobenzene gives dioxazadisilolidine 55. Since an intermediate having a single 29Si NMR peak appears at low temperature, the reaction most likely proceeds via the initial formation of [3 + 2] cycloadduct 54 (Eq. 25).88... 

The only reported, l9Sn NMR spectrum of a stannanimine (entry 5, Table VI) shows a single signal at the surprisingly high field of 8 = -3.5, for which there is no obvious explanation. The other spectroscopic data, H, 13C, and 29Si NMR spectra, are consistent with the stannanimine structure.88... 

Proton, 13C and 29Si NMR spectra for polysilanes have been recorded.(32) The proton NMR provide little structural information, but integration of areas under the proton resonances is quite useful for determining the composition of copolymers. 

The 29Si NMR spectra are of particular interest because they reflect the configuration of the polymer chain.(33) Some 29Si spectra of alkylpolysilanes are shown in Figure 5. Symmetrically-substituted polymers such as (n-hexyl2Si)a have no chirality since there can be a plane of symmetry through each silicon atom. 

For arylpolysilanes the results are quite different.(34) The 29Si NMR for (PhSiMe)n is shown in Figure 6 it consists of three broad lines with relative intensity 3 3 4, each line evidently containing a cluster of resonances. The patterns for other aryl-alkylpolysilanes differ, but in general two or three broad resonances are found none of the aryl compounds studied so far has given a symmetrical pattern like those observed for the alkylpolysilanes of Figure 5. 

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... 

Figure 4. Variable temperature CPMAS 29Si NMR spectra of PDHS (left) and PDPS (right).

Solid state MASS 29Si NMR spectra of dried gels prepared with 4 equivalents of water show a similar trend. QJ-Q4 species are evident in gels prepared under neutral conditions (41), whereas under more basic conditions, Q1 species are absent and Q2 species are greatly reduced relative to Q4 (see NMR results presented below). Thus the effects of restructuring are preserved in the fully dried gel. 

According to the 29Si NMR spectral data (vide infra), the silicon center of 36 is assumed to be pentacoordinated due to its intramolecular interaction with both terminal amino groups attached to the substituents. This kind of two-fold coordination of the nucleophilic side arm of the 2-(dimethylaminomethyl)phenyl substituent to... 

Silanethione 38 was characterized by H, 13C, and 29Si NMR, Raman, and UV-vis spectroscopic methods. The 29Si NMR chemical shift of 38 (8Si 166.56/C6D6) for the silathiocarbonyl unit is much downfield shifted from those of the thermodynamically stabilized silanethiones, 31, 34, 35,28 and 36,29 mentioned in the previous sections, clearly indicating a genuine Si=S double bond in 38 without any intra- or intermolecular coordination. The molecular structure of 38 was successfully established by X-ray crystallographic analysis, and the detailed structural parameters are discussed in the following section. 

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