Silicon-29 NMR

Silyl cations like 3 and 7 in which the positively charged silicon is part of a n-conjugated system attracted particular interest. The marginally stable silatropylium ion 7, is characterized by a Si NMR resonance at 8 Si = 149 in CD2CI2 at —50 °C, downfield-shifted by 192 ppm compared to the precursor silane.This experimental value is in fair agreement with the calculated silicon NMR chemical shift for the optimized gas phase structure of 7 (8 Si = 159.9, at GIAO/HF/6-311 + G(2df,p)(Si), 6-31G(d) (C,H)). This indicates only small interactions between the cation and dichloromethane, the solvent used for the NMR investigations. 

Silylium ions, which are not protected sterically or are not stabilized either electronically or by intramolecular interaction with a remote substituent do interact strongly with the solvent and/or the counteranion. The reaction of the transient silylium ion with solvents like ethers, nitriles and even aromatic hydrocarbons lead to oxonium, nitrilium and arenium ions with a tetrahedral environment for the silicon atom. These new cationic species can be clearly identified by their characteristic Si NMR chemical shifts. That is, the oxonium salt [Me3SiOEt2] TFPB is characterized by S Si = 66.9 in CD2CI2 solution at —70°C. " Similar chemical shifts are found for related silylated oxonium ions. Nitrilium ions formed by the reaction of intermediate trialkyl silylium ions with nitriles are identified by Si NMR chemical shifts S Si = 30—40 (see also Table VI for some examples). Trialkyl-substituted silylium ions generated in benzene solution yield silylated benzenium ions, which can be easily detected by a silicon NMR resonance at 8 Si = 90—100 (see Table VI). ... 

NMR spectra are drawn such that the X-axis uses a dimensionless quantity called the chemical shift. The shift is the relative frequency variation divided by the Larmor frequency of the reference and expressed in ppm. For silicon NMR, the reference is usually neat tetramethylsilane (TMS) which is assigned 0 ppm. In the current work, the silicon reference is chosen as the NO silicons in neat APS liquid which is assigned 0 ppm. Chemical shifts can be converted to the TMS... 

Species (IV) will be referred to ammoniated secondary species . This species would absorb in the neighbourhood of band 2 (2240 cm"1 species (II)), but due to the absence of a Cl surface group, presumably at the low wavenumber side of this band. This hypothesis is proved, using infrared curve fitting and silicon NMR. 

The intermediate and key species proposed for the reaction in Scheme 3.2c are hypervalent silicates based on the silicon NMR spectra of (Z)-crotyltrichlorosilane in DMF. This hypervalent silicate has sufficient Lewis acidity based on the electron-withdrawing chlorine groups as well as nucleophilicity due to electron donation from the hypervalent silicon atom to the allyl systems, which enables the reaction to proceed smoothly. Thus, the high levels of diastereoselectivity can be explained by a six-membered cyclic transition state (Scheme 3.2d). 

In order to further extend our understanding of the origin of chemical shift variations in zeolites, we have examined the effects of gallium substitution for aluminum in the framework on the silicon NMR spectra of sodalite and faujasite. Gallium substitution in zeolites is well known though most preparations... 

The gallium containing zeolites analclme and nepheline hydrate II have also been described (18). Only the silicon NMR of gallium thomsonite (-82.9 ppm (7) c.f. thomsonite, -83.5 ppm (3)) has, to our knowledge, been previously described. 

Silicon NMR measurements were made at 39.5 MHz on a Jeol FX200WB spectrometer using the combined techniques of magic-angle sample spinning and proton dipolar decoupling. All samples were examined in the fully hydrated state. Chemical shifts were referenced to sodium 4,4-dimethyl-4 silapentane sulfonate taken to... 

A Comparison of the Chemical Shifts and Relative Intensities in the Silicon NMR Spectra of ZK-4, Na-X and Gallium Sodalite... 

Tossell, J. A., and P. Lazzeretti (1987a). Ab initio calculation of oxygen nuclear quadrupole coupling constants and oxygen and silicon NMR shielding constants in molecules containing Si-O bonds. Chem. Phys. 112, 205-12. 

Clearly these features make silicon NMR spectroscopy a valuable technique in examining the microstructure of silica and its derivatives (36-40). However, almost no difference in chemical shift is seen between a silicon atom that has an alkoxy and one that has a hydroxy group bonded to it. Carbon solid-state NMR spectroscopy is useful to determine the presence of alkoxy groups and to detect the carbon atoms of a coupling agent if present. 

The resulting complex mixture is well illustrated by the silicon NMR spectrum (Fig. 1, bottom). 

High-resoIution proton and silicon NMR has been used to study structure formation in solution mixtures of ethylene oxide/propylene oxide triblock copolymers and methyl silsesquioxane. These mixtures are precursors to ultra low dielectric constant filim used in the fabrication of integrated circuits. The solution NMR results show that micelle formation is suppressed during solvent casting and curing of the films, and that miscibility is enhanced by the interactions of both the ethylene oxide and propylene oxide blocks of the triblock copolymer with the methyl silsesquioxane matrix. 

Figure 2. The solution (left) and solid-state (right) silicon NMR spectra of methyl silsesquioxane before and after heating to 12(fCfor 1 hour.

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