Isotropic Hyperfine Interactions in Aromatic Free Radicals

Abstract

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 (σ_B)²(π)+λ(σ_B) (σ_A) (π), the hyperfine interaction is 32π/3√6μ_ημ_eλσ_B(r_η)σ_A(r_η) where μ_e and μ_η are magnetic moments of the electron and the nucleus η, and σ(r_η) 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.