Isotropic Hyperfine Interactions in Aromatic Free Radicals

It is shown, by a procedure similar to one used by Abragam, Horowitz, and Pryce in their studies of hyperfine structure in paramagnetic ions and atoms, that configurational interaction may contribute to the isotropic hyperfine interaction in free radicals. For a configurational admixture of the type (σB)2(π)+λ(σB) (σA) (π), the hyperfine interaction is 32π/3√6μημeλσB(rη)σA(rη) where μe and μη are magnetic moments of the electron and the nucleus η, and σ(rη) is the magnitude of the orbital σ at the position of the nucleus. Since the hyperfine coupling is linear in λ, only small admixtures of excited configurations are required to account for the proton hyperfine splittings observed in aromatic free radicals.