Hyperfine splitting in the ESR spectra of random oriented alkali earth biradical chelates

The ESR spectra of rigid glass solutions of biradicals of glyoxal diimine anions and alkali earth cations were recorded. The spectra could be computer simulated only by adding to the spin hamiltonian

= gβH·S + D[S_Z²?$\text{1}\text{2}$S(S+1)] a hyperfine term due to the anisotropic interaction of the nitrogen nuclei. Two different models were tested: the first one with the two radicals in the same plane and the second one with the two radicals perpendicular; agreement with the experiment was found for the latter.