Nuclear magnetic shielding anisotropy

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. ... 

Wolff R, Vogel C, Radeglia R (1993) IGLO calculations of Si NMR chemical shift anisotropies in silicate models. In Nuclear Magnetic Shieldings and Molecular Stracture, Tossell JA (ed), p 385-399... 

Spin-rotation and spin-spin interaction constants, rotational -values, susceptibility anisotropies, nuclear magnetic shielding, and related parameters... 

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. 

A second important mechanism for fluorine spin-lattice and spin-spin relaxation is produced by the chemical shielding anisotropy (CSA) [13, 14, 21, 71]. The magnetic field experienced by a nuclear spin depends on both the electronic structure of the molecule and how easily the electrons can move in the molecular orientations. In addition, the CSA depends on how the molecule is oriented in the magnetic field. Like spin-spin and dipole-dipole interactions, the CSA of small, rapidly tumbling molecules will be an averaged value (the chemical shift). However, these tumbling motions cause fluctuations of the local magnetic field that lead to relaxation. Also slower reorientation, or an environment that restricts the molecular motion, will result in broader lines due to CSA. 

A 13C nuclear magnetic resonance (NMR) study in solution and in the solid state has been reported for three thermochromic spirooxazines, 29-31.32 From a quantitative analysis of I3C cross-polarization/magic-angle-spinning (CP-MAS) NMR spectra shielding anisotropies were estimated and a correlation of these characteristics with thermochromic activity was suggested. 

Finally, [Eq. (6)] represents the interaction between the field and the nuclear magnetic moment. g is also a second-rank tensor that reflects anisotropies in the currents experienced by the shielded nuclei. However, the nuclear Zeeman interaction is, when included at all, usually treated as a scalar quantity since the magnitudes of these anisotropies are small compared to the frequencies and widths of the magnetic resonance spectra of triplet states. 

The aim of this section is to provide an overview about the development and application of solid state NMR spectroscopic techniques for the study of molecular structures and dynamics on the molecular and intermolecular length scale (lA-lOA). In particular, anisotropic magnetic nuclear spin interactions like chemical shielding anisotropy (CSA), magnetic dipolar interaction and quadrupolar interaction are used as probes for interatomic distances and orientations of molecular groups, i.e. structures, and changes of these interactions are monitored and used as a measure of dynamic processes inside the system. 

---