The role of the multiconfigurational character of nitronyl-nitroxide in the singlet-triplet energy gap of its diradicals

While technological applications demand the development of reliable computational techniques and accurate experiments for the characterization of diradicals, these species are still challenging systems for both theory and experiments. The singlet-triplet energy gap, the J-term of the Heisenberg-Dirac-van Vleck spin Hamiltonian, is the most significant quantity; its measurement and computational evaluation may serve for understanding and controlling magnetism at the molecular scale. In this framework, we report a study of three diradicals containing one or two nitronyl-nitroxide species. Using Difference Dedicated Configuration Interaction (DDCI) calculations, we investigate the multiconfigurational character of the O-N-C-N-O fragment of this unit. We find that a computational scheme that takes this nature into account is necessary to confidently obtain reliable values of the spin-spin coupling J. In addition, we show that the reduced DDCI2 scheme with a CAS(2,2) reference, which can reproduce experimental data in some cases, provides quite poor results in the present context.