29Si NMR parameters

Although observations are scattered throughout the literature, solvent effects on 29Si NMR parameters have been studied systematically only to a slight extent. Attention has been focussed mainly on effects on chemical shifts despite the fact that all parameters are affected by solvent, concentration and temperature. 

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. 

Workers in a field usually know the values of relevant parameters in the classes of compounds they study, and, if not, they can use abstracting/indexing services to localize the source of the data for the compound of interest. The traditional sources of such information (Chemical Abstracts, Beilstein Data) have been supplemented by sources dedicated to 29Si NMR data collection2. 

Parameters of the 29Si nucleus have been given in all previous reviews of 29Si NMR, and detailed comparison with other nuclei can be found in reviews of Harris and coauthors6,10 and Brevard and Granger11, which also give clear definitions of the terms (e.g. relative receptivity) used in the discussion. 

Triple resonance experiments are used when double resonance experiments (either ID or 2D) fail to provide the needed answer, whether it be assignment of 29Si lines, their resolution, or separation of NMR parameters. A number of determinations, however, can be carried out using pseudo-triple resonance that is less demanding on the spectrometer hardware as described in Sections V.B, VI.A, and VI.C.2. 

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 main properties of these materials were characterized by means of x-ray diffraction (Siemens D-500 with A.Cu radiation of 1.54 A), Transmission Electron Microscopy (Phillips-CM200) and N2 adsorption (Micromeritics ASAP-2000), 29Si-NMR(MAS). As the textural properties of the catalytic materials, for example the inner pore structure, is a key parameter for their performance, in the present work the N2 adsorption isotherms of the calcined mesoporous Si02-based solids were determined. The solids were prepared using different CTAB surfactant and some co-surfactants based in the light alcohols, i.e. MeOH, EtOH and PrOH. Thus, Figure 15.5 shows the isotherms of the mesoporous solids prepared with MeOH (co-surfactant). In all... 

Single-crystal X-ray results (9) point to strict alternation of Si and Al (the 4 0 ordering scheme) in accordance with Loewenstein s rules (10), but this model was recently challenged on the basis of 29si NMR measurements (1, 2) and our discovery that the sodium derivative can be rhombohedral (11,12). However, the controversy has now been resolved and the correctness of the X-ray model reaffirmed (13,14). A consequence of the Si, Al ordering is that the lattice parameter of the cubic cell is ca. 24.6 A, not 12.3 A as reported in earlier work, a feature reflected in weak superlattice reflections. The space group is Fm3c. 

G and 9.8 G respectively satisfying the Hartmann-Hahn condition. A single contact sequence comprised 5.0 rrjs contact time and a recycle time of 4.0 s. The corresponding parameters for the 29Si-NMR cross-polarization experiments are 29Si (39.7 MHz) and (200.0 MHz) rf fields were 39.3 and 9.8 G respectively, with a contact time of 10.0 ms and a recyle time of 3.0 s. 

The samples used for NMR spectroscopy were freeze-dried to prevent any reaction during the drying process. Adamantane and the trimethylsilyl ester of double four-ring octameric silicate, Q8M8 were used to optimize experimental parameters and as external secondary (relative to TMS) chemical-shift references for 13C and 29Si, respectively. Both the Ti measurements and a discussion on the use of 29Si NMR spectroscopy for quantitative measurements will be described elsewhere (47). 

TABLE 7. Microstructure parameters of poly(dimethy co-methylphenyl)siloxane determined by 29Si NMR. Average composition M2D58DJ2... 

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