Magnetic interactions in phenyl-bridged nitroxide diradicals: conformational effects by multireference and broken symmetry DFT approaches.

In the present paper we report the key results of a comprehensive computational study aimed at investigating the dependence of the singlet-triplet energy gap in phenyl-bridged bis-nitroxide diradicals, on the basis set and on soft structural parameters like torsion and pyramidalization. We have compared the BS-DFT technique with the post-Hartree-Fock DDCI2 multireference approach. With this latter method we have also studied the different role that sigma and pi core and virtual orbitals have in the resulting singlet-triplet energy gap.The results obtained represent one step forward in the definition of a protocol for an efficient and reliable computation of spin-spin coupling in diradical systems.