Fluorine shielding anisotropy

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. 

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... 

An enhanced endohedral shielding is apparent for the fluorinated species as compared to their hydrogenated counterparts, which is to some extent probably due to anisotropy effects of the CF vs the CH o bonds The endohedral shielding in fully saturated CeoFeo is computed to be ca. 3 ppm larger than that... 

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