Simulation of ENDOR spectra of radicals with anisotropic hyperfine and nuclear quadrupolar interactions in disordered solids Original Research Article

A general theory to calculate ENDOR spectra of S = 12 radicals in single crystal or disordered solids is presented. The theory is suitable for paramagnetic systems with n interacting nuclei which can be described by the spin-Hamiltonian H= βBgS + Σi=1nIiAiS + IiQiIi − giβNBIi. There are restrictions on the relative magnitude of the hyperfine, quadrupolar and nuclear Zeeman interactions, nor on the relative orientation of principal axes of the tensors. ENDOR transition frequencies and intensities are calculated with perturbation theory under the assumption that the electron Zeeman interaction is dominating. A simple formula for first order transition moments, including the effect of the quadrupolar coupling, is presented. The impact on the transition moment of the orientational distribution of the radio-frequency field in single crystal and disordered systems is taken into account. The method is tested against experimental proton- and 14N-ENDOR data of neutral and ionic radicals in disordered solids.