Tetramethylsilane, effect

Deshielding (Section 13.2) An effect observed in NMR that causes a nucleus to absorb downfield (to the left) of tetramethylsilane (TMS) standard. Deshielding is caused by a withdrawal of electron density from the nucleus. 

Sc, carbon chemical shift, referred to tetramethylsilane (8 = 0) (cf. Sect. I) SCS, substituent-induced chemical shift, or substituent effect difference between S s of a given carbon atom in a monosubstituted and the respective unsubstituted parent molecule (cf. Sect. Ill) NAE, nonadditivity effect nonadditivity of individual SCSs in disubstituted molecules (cf. Sect. IV) ICS, intramolecular-interaction chemical shift = NAE (cf. Sect. IV) A, polarization effect difference in S s of sp2 carbon atoms in a double bond (cf. Sect. IV-C) LEF, linear electric field (cf. Sect. II-B-3) SEF, square electric field (cf. Sect. II-B-3). 

Si NMR chemical shifts were calculated for each molecule relative to the theoretical shielding for tetramethylsilane (TMS), at the HF/6-311+G(2d,p)86 level using the GIAO method,94 as implemented in Gaussian 94 and Gaussian 98. Shifts for gas-phase molecules are reported because the inclusion of solvation via the SCRF method was found to have little effect on the predicted shifts.83 Comparison of calculated shifts with experimental values for compounds with well-known structures yielded an error estimate of about 1 to 8% for quadra-coordinated silicon and 2 to 9% for penta-coordinated silicon. 

Whereas the solvent influence on the and chemical shifts of the apolar tetramethylsilane is comparatively small (Ad ca. 0.5... 1.5 ppm), much greater effects are observed in the case of dipolar molecules such as 4-fluoro-nitrosobenzene and triethyl-phosphane oxide (Ad ca. 3. .. 25 ppm) as well as for the thallium(I) ion (Ad > 2000 ppm ). 

With tetramethylsilane the only organic reaction product was the trimethylsilyl methyl mercaptan, (CHj)3SiCH2SH, as expected for a C—H bond insertion. Further evidence that the product arose from S( Z)) insertion was provided by the suppressing effect of CO2 on the reaction. A novel feature of this reaction is the large damping effect of (CH3)4Si on the CO rate, as shown by the product rate vs. substrate pressure plot in Figure 12. From simple stoichiometry the following relation should obtain for paraffinic insertion processes ... 

Some condensed papers [16, 17, 211] review the fundamentals, the applications, and the limitations of aqueous-phase homogeneous catalysts and the special role of water [21, 167, 201, 204, 212]. Various papers substantiate the advantages of aqueous-biphasic versus purely homogeneous techniques, the effectiveness of water-soluble over organic-soluble ligands for special substrates (e. g., [67, 213]), or tbe role of counter-ions within the ligands [215 a, b, 218 h, 244 k] or of co-additives [215 c, d]. The overall solvatation capability (solvation power, t ) of various solvents from nonpolar, aprotic tetramethylsilane (TMS) to water, which influences the reactivity considerably, is shown in Figure 3 [213 b]. Special... 

Nuclear spins are sensitive to the chemical environment of a nucleus. Electrons moving near the nucleus establish an internal magnetic field that modifies the local effective field felt by each proton to a value different from that of the externally applied field. The resulting chemical shift causes protons within different structural units of the molecule to show NMR peaks at different values of magnetic field. All protons in chemically equivalent environments will contribute to a single absorption peak in the spectrum. The relative area under each absorption peak is proportional to the number of protons within each equivalent group. In order to standardize procedures, chemical shift values are recorded relative to the selected reference compound tetramethylsilane (TMS) by adding a very small amount of... 

Marks has examined the reactivity of thorium metallacycles with hydrocarbons, where ring strain is used to provide the thermodynamic driving force for alkane activation in a reaction with methane (Eq. 17). Reaction with CD4 shows a dramatic kinetic isotope effect, with kH/kD=6, which is typical of the four-centered electrophilic transition state hydrocarbon activations [76]. The metallacy-cle is formed by the elimination of neopentane from the bis-neopentyl derivative [77]. Reaction with cyclopropane and tetramethylsilane gave the bis-cyclopropyl product Cp 2Th(c-propyl)2 and the bis-TMS product Cp 2Th(CH2SiMe3)2, respectively [78]. 

The derivation of Eq. (5) assumed that the molecular geometry is independent of orientation with respect to the nematic optic axis. If that is not true, the expectation value of Eq. (2) may be non-zero even if the motional constants are zero. Evidence for some distortion has been found in the spectra of tetramethylsilane and neopentane,s tetrahedral molecules for which zero motional constants are expected. The observed Dq h of about -7 Hz and Z>hh of 2 Hz can be interpreted in terms of a variation of the Si-C-H or C-C-H bond angles of about 0.1° during tumbling. Since the H-H interactions within methyl groups are of order 1000 Hz for oriented molecules, this effect is apparently relatively unimportant for geometry determinations.5... 

The metalation of tetramethylsilane is another dramatic example of the effect of chelation on reactivity since both n-BuLi and sec-BuLi are nearly inert under the same conditions. Chelated sec-BuLi is the most reactive soluble metalating agent we have found. TMED Li-sec-Bu reacts with tetramethylsilane about 1000 times faster than TMED Li-n-Bu and yields purer product (47). Broaddus (48, 49) has discussed kinetic metalation of olefins and alkyl aromatic compounds using TMED LiBu, and he also observed the slow equilibration to the thermodynamically favored isomers. The extent of ring metalation in toluene and the conditions for isomerization to benzyllithium are discussed in Chapter 2, Smith. 

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