Anisotropy magnetic shielding

A demonstration of the efficacy of MBER spectroscopy is the recent experiments on HF carried out by Bass, DeLeon, and Muenter [14]. In an effort to obtain Stark, Zeeman, and hyperfine properties, measurements were made that gave accurate values for both the ground and first excited vibrational levels of HF. Conventional resonance experiments can be done if the D = 1 state can be sufficiently populated. Using a color center IR laser to excite HF to u = 1, J = 1 levels, all the properties measured for the u = 0 and V = 1 states had essentially identical precision. The results included dipole moments, magnetic shielding anisotropies, rotational magnetic moments, magnetic susceptibilities, transition moments, and first and second derivatives with respect to internuclear separation of the properties. 

Phosphorus nuclear magnetic shielding anisotropy in (1-hydroxyalkyl)dimethyl-phosphine sulfides has been studied using the IGLO method, and the tautomeric stability, molecular structure, and internal rotation of methylphosphonic dicyanide MeP(0)(CN)2, dicyanomethoxyphosphine MeOP(CN)2, and their isocyano analogues have been extensively followed using ab initio calculations. ... 

TABLE 6 Magnetic-Shielding Anisotropies for First-Row Atoms (ppm)... 

NMR spectra of solid samples since its NMR signal is usually spread over several thousand ppm, a consequence of the large magnetic shielding anisotropy typical for Pt. For example, the span for K2PtCl4 is greater than lOOOOppm. On the other hand, techniques such as cross-polarization (CP vide infra) facilitate the acquisition of NMR spectra for apparently challenging nuclei such as N. 

When comparing the magnetic shielding tensors in Cgo and it is observed that the six extra electrons in the latter yield an important increase in the anisotropy and a strong shielding effect in the isotropic part of the nuclear shielding. 

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 complications that arise in solid state NMR spectra as compared to NMR spectra in solution are the consequence of the fixed orientation of the sample relative to the external magnetic field Bq. Mainly three interactions are responsible for the enormous linebroadening that can be observed for solid powder samples. These are (1) the shielding or chemical shift, including the chemical shift or shielding anisotropy CSA or Acr Hqsa) (2) homo- and/or heteronuclear dipole-dipole coupling (ffoo) and (3) in addition, for nuclei with spin >1/2, the quadrupolar interactions (Hq). 

The shielding factor is a property of the molecule, but as we see in later examples, the ability of the magnetic field to influence the motion of electrons depends on the orientation of the molecule relative to B0. Hence, O is a second-rank tensor, not a simple scalar quantity. It is always possible to define three mutually orthogonal axes within a molecule such that o may be expressed in terms of three principal components, on, molecular symmetry requires that two of the components of o be equal (and in other instances it is possible to assume approximate equality), so that the components may be expressed relative to the symmetry axis as chemical shielding anisotropy defined (ct — cr,). 

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