29Si chemical shift

B. Calculations of31P and 29Si Chemical Shifts of Parent Phosphasilenes... 

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. 

Principal Values of 29Si Chemical Shift Tensors for Disilenes (ppm)... 

The formation of silaneselone 57 was evidenced by the trapping reaction with mesitonitrile oxide leading to the corresponding cycloadduct 58 and was also supported by the observation of a remarkably downfield 29Si chemical shift (8Si = 174) indicative of the Si=Se double bond of 57. Although this direct selenation of silylene 55 with an equimolar amount of selenium was not reproducible, the use of excess amount of elemental selenium resulted in the formation of a new cyclic diselenide, diselenasilirane 59, as a stable compound (8Si = -44 and... 

Table 1 summarizes the 29Si chemical shifts and 29Si29Si coupling constants of all compounds, prepared during this work. There are several interesting features. 

Table 1 29Si chemical shifts [ppm, TMS] and SiSi coupling constants [Hz] of bromo- and iododisilanes and -trisilanes... 

The bulky, stable silenes of Brook et al. (104,122-124,168) and Wiberg et al. (166,167) have been the only systems capable of being studied by nuclear magnetic resonance (NMR) spectroscopy to date. Table III lists the 13C and 29Si chemical shifts and the relevant coupling constants of these compounds. 

The ZSM-4 sample was prepared following the previously described procedures.[20] The elemental analysis showed that the Si/Al ratio was 3.0. 29Si MAS NMR spectra were recorded at 11.7 T MHz on a Varian InfinityPlus 500 spectrometer on a sample loaded in a 7.5 mm MAS rotor spinning at 4 kHz using a rc/2 rad pulse length and a recycle delay of 360 s. The 29Si chemical shifts are referenced with respect to an external solution of TMS (5Si = 0.0 ppm). 

Figure 14. The five possible local environments of a Si atom tetrahedron together with their characteristic 29Si-chemical shift ranges (boxed areas). Si(aAl) represents Si bonded, through oxygen bridges, to nAl atoms where n ranges from 0 to 4. The chemical shifts of the five resonances of zeolite ZK-4 are shown in broken vertical lines. (Figures adapted from References 25 and 35).

In addition to the influence of neighbors on 29Si chemical shifts, the geometrical effects (such as Si-O-T angles) already described above are also evident of mixed frameworks with elements other than Si on tetrahedral positions. This is reflected by the broadness of the bars shown in Fig. 1. Multinuclear NMR investigations on a large set of sodalite structures with various framework compositions show that T-O-T bond angle (T = Si, Al, Ga) and dTT distance chemical shift dependences exist, and mutual correlations between chemical shift of these NMR nuclei can be observed [68],... 

These shifts are similar to the 29Si chemical shifts in the spectra of silyl derivatives of analogous oximes. This is evidence for the absence of essential additional coordination of silicon in SENAs. The qualitative and quantitative analyses of the 29Si NMR signal can be considered as a simple method of NMR monitoring of SENAs in solution. 

Si Chemical Shifts for Compounds 4, 6-8, 21, and 22 in Solution (CD3CN) and/or... 

NMR studies (XH, 13C, 29Si) have shown that the zwitterions 3 and 27-38 also exist in solution (3, 27-29, 31-38, [D6]DMSO 30, CDC13). The NMR data, in context with the results of ab initio studies of 39, can be interpreted in terms of a rapid Berry-type pseudorotation at room temperature, leading to an interconversion of the (A)- and (A)-enantiomers. The 29Si chemical shifts of the zwitterions 3 and 27-38 are listed in Table IV. In most cases, these values are very similar to the isotropic 29Si chemical shifts obtained for the respective crystalline compounds by solid-state 29Si CP/MAS experiments. However, the 29Si chemical shifts observed for 31 (8 -131.9) and 32 (8 -123.0) in solution differ substantially from those determined for... 

CH3CN (8 -85.1) and 32 CH3CN (8 -84.8) by the solid-state NMR experiments. Furthermore, the solution-state NMR data of 31 and 32 differ significantly from all the other 29Si chemical shifts listed in Table IV. These results are indicative of special structural features of the zwitterions 31 and... 

The 29Si chemical shifts of 47 and 48 are as follows 47, S -87.6 (solid state) 48, S -89.6 ([D6]DMSO), S -85.6 (solid state). These data are quite similar to those obtained for the related compounds described in Section III,A (see Table IV). 

NMR studies ([D6]DMSO XH, 13C, 29Si) demonstrated that the zwitter-ions 50-52 also exist in solution. The 29Si chemical shifts of these compounds are as follows 50 iHOCH2CH2OH, 8 -91.8 ([D6]DMSO), 8 -91.3 (solid state) 51, -93.0 (CDC13), -92.7 (solid state) 52, 8 -85.7 ([D6]DMSO). 

Si Chemical Shifts for Compounds 81-84 in Solution ([D6]DMSO) and/or in... 

The isotropic 29Si chemical shifts of 88 and 89 in the solid state are as follows 88, <5 -69.8 89, S 98.8. As would be expected from the different electronegativities of sulfur and oxygen, these chemical shifts differ significantly from those determined for the corresponding oxygen analogs 53 (<5 -88.6) and 77a (8 -120.0). 

Solution-state NMR studies (XH, 13C, 19F, 29Si) at room temperature have shown that the zwitterions 94-104 are also present in solution. The 29Si chemical shifts were found to be very similar to the isotropic 29Si chemical... 

Si Chemical Shifts and Vsif Coupling Constants for Compounds 94, 95 CH3CN, 96-105, IO6 5CH3CN. and 107 in Solution and in the Crystal"... 

The samples have also been characterised by means of solid state NMR. 27A1 chemical shifts have been shown to be sensitive to the coordination number and 29Si chemical shifts are strongly influenced by nearest neighbour effects. 

Zeolite Idealized unit cell composition Si/Al ratio 29Si chemical shifts (ppm from TMS) ... 

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