Intramolecular exchange interactions in non-aromatic bis-nitronyl-nitroxides

Two bis-nitronyl-nitroxides linked by unsaturated non-aromatic spacers have been synthesized and investigated: 3D-NIT2 and DTD-NIT2. The magnetic properties of these diradicals have been investigated in the crystalline state by SQUID magnetometry and in dilute solution by EPR spectroscopy. According to magnetic measurements in the crystalline state both compounds experience strong antiferromagnetic exchange interactions. The singlet-triplet splitting has been estimated as DeltaE(ST) = 95 K for 3D-NIT2, and DeltaE(ST) = 75 K for DTD-NIT2. Solid-state MAS NMR spectroscopy has been used to determine the spin densities at the H-1 and C-13 nuclei of DTD-NIT2, thereby establishing spin polarization as the origin of the magnetic properties. The synthesis of an analogous mono-nitronyl-nitroxide has been undertaken in order to investigate the large intermolecular magnetic interactions observed in the crystalline state for 3D-NIT2.