Chemical shielding tensors, calculate

Several calculations of the chemical shielding tensor have been carried out by different methods for simple model disilenes. For H2Si=SiH2, calculated values of 300,44 291,43 and 195 ppm46 have been obtained for Act, the CSA spread. A value of 255 ppm has been calculated for Me2 Si=SiMe2,46 and a value of 120 ppm was obtained for (CH2=CH)2Si=Si... 

TABLE 8. Calculated diamagnetic, paramagnetic and total 29Si chemical shielding tensors a (in ppm, gauge origin at Si) compared with experimental chemical shifts, r... 

Calculated Chemical Shielding Tensors as an Aid to Elucidating the Method of Attachment of Alkoxysilanes to Magnesium Chloride... 

Ab initio calculations of 29Si and 13C NMR chemical shielding tensors have been used as an aid in the interpretation of experimental results aimed at elucidating the role of alkoxyalkylsilanes as external electron donors in Ziegler-Natta catalysis of propene. The results support a model in which dialkoxydialkylsilanes are loosely attached to two adjacent magnesium ions on the surface of magnesium chloride, but trialkoxyalkylsilanes are attached primarily by only one magnesium ion. 

Figure 1. Calculated absorption lineshapes and representative C-13 spectra for polycrystalline samples (a) an axially symmetric chemical shielding tensor (o. and

Fig. 6, Dependence of the chemical shielding on the dihedral angles (, for C" carbons of L-alanine in peptides. The chemical shieldings are calculated with the Gl AO-CHE method using a 4-3IG basis set. The geometry of TV-acetyl-TV -methyl L-alanine amide was optimized on the 4-31G HE level. The different panels show (a) the isotropic value, as well as the tensor components, (b) cth, (c) 022, and (d) 0 33 (adapted from Asakawa et al. with permission of the authors).

Table 21 Comparison Between Experimental and Calculated Absolute C and 0 Chemical Shielding Tensor Components (ppm) for the CO Molecule and Group 6 Transition Metal Carbonyls... 

To give a further insight into the C chemical-shift behavior, principal values of the chemical shielding tensor were also calculated [33]. From Fig. 

In order to calculate the spectrum, the chemical-shielding tensors of all spins, as well as their relative orientation and distances, have to be known. The assumption of a quasi-equilibrium state simplifies Equation (4.28) considerably. For long mixing times t , all elements in exp(Wr ,) have equal intensity. This leads to the following signal function for a single-crystallite orientation ... 

Accurate calculation of chemical shielding tensors is being actively pursued because it facilitates the use of NMR chemical shift data in the studies of structures and bonding. The chemical shifts of hexacoordinated Co(III)... 

Table 6. Summary of the calculated and experimental Co chemical shielding tensors"...

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