Fluorine tensors

In the area of fluorine shielding, in contrast to that of carbon-13 shielding, there has been a great deal of conflicting experimental data and interpretation of results. The most successful theoretical models, moreover, have not yet been applied to several of the most controversial fluorine compounds. The task of comparing fluorine tensors and anisotropies is complicated by the fact that there are usually several sets of axes for which these quantities can be defined, since the fluorine nuclei often do not lie on the molecular symmetry axis.20... 

Proton and fluorine are the most frequently studied nuclei in the solid state NMR of polymers. However, useful information can be obtained from deuterium resonance, (2H I = 1). In the presence of nuclei with spin I > 1/2, which possess a quadrupolar moment, the most dominant interaction occurs between the quadrupolar moment and an electric field gradient tensor V, generated by the C—D bonding electrons. Consequently, the only dominant interaction, besides Hz (Eq. (1)), will be described by the quadrupolar Hamiltonian, which for a single spin has the form ... 

The potential of other nuclei for the study of surfaces is yet to be explored. Gottlieb and Luz (388) measured 2H spectra of a number of perdeuterated molecules adsorbed on active alumina and interpreted the results in terms of quadrupolar tensors. Yesinowski and Mobley (369) have shown that 19F MAS NMR can provide useful information about fluorinated surfaces of calcium hydroxyapatite, Cas(0H)(P04)3. In particular, l9F, 2H, and H MAS NMR may become powerful techniques for the study of interface systems in general. 

Optimized Geometries. As the elements of the NMR shielding tensor for a nucleus represent local properties, the most important geometrical parameter influencing the value of the 19F shielding constant can be expected to be the length of the bond which attaches the fluorine to the benzene ring. The optimized values of the carbon-fluorine... 

Here we have introduced three second-rank tensors, the rotational magnetic moment tensor T2(G) and the proton and fluorine screening tensors, T2(rotational magnetic moment and chemical shift (shielding) tensors, a further term which should be included in the Zeeman Hamiltonian describes the diamagnetic susceptibility. This term takes the form... 

The numerical values of the chemical shift tensor elements depend, of course, on the axis system used to define the tensor. This fact has led to considerable difficulties in the analysis and intercomparison of the experimental data. The transformation of tensor elements from one axis system to another may require assumptions as to bond symmetry or geometry that are not necessarily justified. The axis system that yields tensor elements that are easiest to compare from one molecule to another has an axis parallel to the bond axis of the nucleus under study and an axis perpendicular to the bond. Unfortunately, in many highly symmetric molecules the tensor experimentally determined in the molecular symmetry axis system cannot be transformed to the bond axis system unambiguously and some hard-to-justify assumptions frequently must be made. Some of the difficulties of this sort that arise from an intercomparison of shielding data from liquid crystals and the coherent averaging of solids for aromatic fluorine compounds have been discussed by Mehring et al.1... 

The scalar interaction, like the quadrupole coupling and the shift tensor, depends on the atomic number. There is a complete set of isotropic scalar coupling data from high resolution solution NMR. Typical proton coupling may be 10 s of Hz while fluorine couplings can be as big as a few kHz, for example, in TeFg. 

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