Anisotropic shielding

Anisotropic shielding in unsaturated groups. (Shielding zones + deshielding zones -). (a) Ethyne. (b) Ethene. (c) Ethanal. (d) Benzene. 

Anisotropic shielding of a given nucleus remains essentially constant (in ppm) no matter what nucleus is being considered (84). Since... 

In order to discuss the origin of these terms we need to allow the spins to have anisotropic shielding tensors. Molecular tumbling in solution makes the chemical shielding in the direction of the external magnetic field a stochastic function of time and acts therefore as a relaxation mechanism, called the chemical shielding anisotropy (CSA) mechanism. The Hamiltonian for each of the two spins, analogous to Eq. (5), contains therefore two... 

The indices k in the Ihs above denote a pair of basis operators, coupled by the element Rk. - The indices n and /i denote individual interactions (dipole-dipole, anisotropic shielding etc) the double sum over /x and /x indicates the possible occurrence of interference terms between different interactions [9]. The spectral density functions are in turn related to the time-correlation functions (TCFs), the fundamental quantities in non-equilibrium statistical mechanics. The time-correlation functions depend on the strength of the interactions involved and on their modulation by stochastic processes. The TCFs provide the fundamental link between the spin relaxation and molecular dynamics in condensed matter. In many common cases, the TCFs and the spectral density functions can, to a good approximation, be... 

Here, a denotes the shielding constant caused by the anisotropic shielding of the external magnetic field by the electron shell around the resonating nuclei. The so-called isotropic chemical shift, 5, is defined as <5 = cr ref—<7, where cr ref is the shielding constant of the nuclei in a reference material, and <5 is defined by the following ... 

The chemical shift may reveal the position of one atomic grouping with respect to another by anisotropic shielding. This is an effect likely to be sensitive to temperature and solvent changes as seen in the valine amide-proton resonance in Figure 3. 

Double bonds contain one a bond and one it bond, which results in anisotropic shielding, as shown in Figure 11.9. There is a conical shielding region normal to file molecular plane and a deshielding region in file molecular plane. This is... 

Scheme 4. Anisotropic shielding surface surrounding the nitrate anion.

Observed and Calculated [IGLO Method (130)] Components of the Anisotropic Shielding Tensors of Phosphorus Oxides... 

The same paper deals with the diamagnetic anisotropic shielding of a proton by a carbon-carbon bond. This was demonstrated from a comparison of the chemical shifts of 12 and 13 which are displayed underneath the structures in ppm. In the latter compound... 

As a continuation of studies of anisotropic shielding effects brought about by a cyclopropane ring, the large high-field shifts within the series 30-34 were examined by Tori and coworkers. In 3 1, Q is shielded by 11.1 ppm with respect to the corresponding... 

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