Orientation Dependence of Magnetic Hyperfine Structure in Free Radicals

The anisotropic part (B term) of the spin Hamiltonian for a free radical is averaged to zero when there is rapid molecular tumbling. Otherwise, it combines with the isotropic part (A term) to produce orientation‐dependent hyperfine splittings in ESR spectra. In the high‐field case, the electron is strongly coupled to the external field. The nucleus, however, is effectively coupled to the electron's tensor field. Consequently, the two spins have, in general, different axes of quantization. Applying first‐order perturbation theory, expressions are derived for the orientation dependence of the resonance in a single crystal and for the line shape in a powder.