Sensitivity of chemical shifts

Another serious problem in applying these methods is that unambiguous assignments of N VIR signals to monomer sequences are, as yet, only available for a few systems. Moreover, assignments are complicated by the fact that the sensitivity of chemical shifts to tacticity may be equal or greater than their... 

Sensitivity of chemical shift analysis is determined by the spectral resolution of the XPS system. The resolution of a typical XPS system without a monochromator is 1.0 eV. This corresponds to the intrinsic line width of Al Ka or Mg Ka radiation. The analysing system contributes only little to the overall resolution. This resolution is sufficient to determine the binding energies of most core levels within 0.1 eV. Considerable improvement of the resolution down to 0.3 eV can be achieved by use of a monochromator. The higher resolution has to be paid for by a loss in intensity which, however, is no problem in modern instruments. 

There is no doubt that the most powerful tool for studying molecular structures in solution at the present time is NMR. Given the high sensitivity of chemical shifts and coupling constants to conformation and steric environment, an obvious expedient is to use MM-... 

We have prepared a number of acylium ions on metal halide powders and measured the principal components of their chemical shift tensors (43-45). Most of these cations have isotropic l3C shifts of 154 1 ppm. Often insensitivity to substituents results from opposite and offsetting variations in the principal components. The acetylium ion has an axially symmetric chemical shift tensor because of its C3 rotation axis. When the symmetry is reduced from C3v to C2v or lower, a nonzero 27 value may be observed. The sensitivity of chemical shift tensors to symmetry is a powerful means of probing molecular structure and temperature-dependent molecular dynamics. Multiple orders of spinning sidebands may offend those who seek solution-like NMR spectra of solids, but discarding most of the information inherent in the chemical shift is a considerable concession to aesthetics. 

The high structural sensitivity of chemical shifts makes 29 Si NMR a powerful tool for determination of the structure of oligomeric and polymeric siloxanes. Detailed information can be obtained for the characterization of the different structural units, for the... 

The stereospecific polymerization of ot-olefins is one of the best examples which illustrate the possible applications of high resolution NMR to the determination of reaction mechanisms. As a matter of fact, the stereoregular structure of the reaction products and the sensitivity of chemical shifts to stereochemical environments make it possible to obtain considerable and important information about very subtle details of the reaction mechanism. This is especially true when NMR is used in conjunction with isotopic substitution. 

All the NMR data are strongly supported by combining a theoretical approach by performing qnantum-chemical calculations to determine the structure and NMR chemical shifts of the host-guest complex. The approach presented by these authors clearly exploits the sensitivity of chemical shift to aromatic ring currents and can be in principle applied to crystalline and amorphous systems. 

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