Origin of the E.S.R. hyperfine anisotropy and the cos2θ rule of the β-fluorine couplings

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Abstract
The origin of the hyperfine couplings of β-fluorine nuclei in free radicals has been studied using the hyperfine tensors determined for and in single crystals. The direction of the maximum principal element of the β-fluorine coupling tensor in was found to be very close to the direction of the maximum overlap of the fluorine 2p π orbital with the half-filled carbon 2p π orbital. This means that the spin density in the fluorine 2p π orbital is mainly due to the direct overlap with the carbon 2p π orbital and the contributions from the hyperconjugation and the spin polarization mechanisms are less important. The cos2θ rule, B 0 + B cos2θ, for the β-fluorine coupling tensor was also derived from the conformational dependence of the overlap integral. Using the ratio of B/B 0 calculated from the overlap integral together with the observed hyperfine tensor for , the absolute values of B 2 and B were determined and results were applied to the β-fluorine coupling tensor of the rotating CF3 group in . The calculated tensor is in very good agreement with the one observed.