Chemical shift anisotropies definition

J-splitting, when it exists, imposes the definition of new spin quantities. These quantities also evolve according to relaxation phenomena and may interfere (by relaxation) with the usual magnetization components. This latter interference stems precisely from cross-correlation rates, i.e., relaxation parameters which involve two different mechanisms, for instance the dipolar interaction and the so-called Chemical Shift Anisotropy (27,28) (csa)... 

Over the last decade, developments in high-resolution NMR techniques for solids have been extended to mercury nuclei. Solid-state NMR studies provide more definitive characterization of mercury complexes since their interpretation is not compromised by exchange processes or solvent coordination. While medium effects on spectra are not absent in the solid-state, they are generally more defined and therefore studied more readily. Furthermore, the chemical shift anisotropy, coupling constant anisotropy, and dipolar coupling constants obtained by solid-state NMR provide an additional probes into structure and bonding. Comparison of solid-state NMR spectra of structurally characterized complexes with solution-state NMR spectra promises to reveal significant differences between the solution and solid-state structures. 

Solid-state 207Pb NMR spectra can be obtained for static samples or by MAS and CP MAS techniques,19 and for organolead compounds CP MAS techniques are definitely preferred (see Fig. 3). The huge chemical shift anisotropy is a drawback for all techniques, although, on the other hand, this parameter contains valuable additional information. 

Although only the trace of the chemical shift tensor is measured in isotropic solution, in general F nuclei have large chemical shift anisotropies and in anisotropic media such as solids or liquid crystal solutions, which can include biological matrices such as membranes, the spectrum can be severely dependent on such parameters in that the observed chemical shift is dependent on the orientation of the molecule relative to the magnetic field. The definition of the chemical shift anisotropy in terms of molecule-based axes depends on the symmetry of the molecule under study but it can easily have a magnitude of tens or hundreds of ppm, for example, being 158 ppm in hexafluoro-benzene, which has an isotropic shift of — 163 ppm. 

The conventional, and very convenient, index to describe the random motion associated with thermal processes is the correlation time, r. This index measures the time scale over which noticeable motion occurs. In the limit of fast motion, i.e., short correlation times, such as occur in normal motionally averaged liquids, the well known theory of Bloembergen, Purcell and Pound (BPP) allows calculation of the correlation time when a minimum is observed in a plot of relaxation time (inverse) temperature. However, the motions relevant to the region of a glass-to-rubber transition are definitely not of the fast or motionally averaged variety, so that BPP-type theories are not applicable. Recently, Lee and Tang developed an analytical theory for the slow orientational dynamic behavior of anisotropic ESR hyperfine and fine-structure centers. The theory holds for slow correlation times and is therefore applicable to the onset of polymer chain motions. Lee s theory was generalized to enable calculation of slow motion orientational correlation times from resolved NMR quadrupole spectra, as reported by Lee and Shet and it has now been expressed in terms of resolved NMR chemical shift anisotropy. It is this latter formulation of Lee s theory that shall be used to analyze our experimental results in what follows. The results of the theory are summarized below for the case of axially symmetric chemical shift anisotropy. 

The shielded chemical shift is in turn due to the strong magnetic anisotropy of the sp -hybridized carbons. All the above dications are definitively classical, nonbridged cations, as also shown by the chemical shift additivity criterion of Schleyer and coworkers. 

Alternatively, in the so-called Haeberlen notation, the convention is <5zz- isol > l xx- isol > YY- iso / where the capital letters refer to the principal components and the isotropic chemical shift has a definition similar to Equation 1. The anisotropy (A5) and asymmetry ( ) are then defined as ... 

The traditional definitions of the three principal value components (5i 1, 22, and 533), isotropy 5iso, anisotropy and asymmetry of the chemical shift are equivalent to those of the magnetic shielding. This traditional scheme is indefinite. It leads to a loss of physical and chemical generality which is disadvantageous. Therefore, besides the isotropy two new parameters, span Q and skew k, are often introduced and used instead of the anisotropy A3 and... 

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