Intramolecular antiferromagnetic vs ferromagnetic spin coupling through the biphenyl unit

Two isomeric triarylmethyl-based diradicals, with 3,3'-and 3,4'-biphenyl as the spin coupling units, were prepared and studied in frozen solutions by ESR spectroscopy and SQUID magnetometry. The 3,3'-isomer was found to be the singlet ground state, with a singlet-triplet energy gap of -0.1 kcal/mol. The 3,4'-isomer was found to be the triplet ground state. This dependence of intramolecular spin coupling (antiferromagnetic vs ferromagnetic) on topology is in agreement with simple theoretical models. The singlet ground state for the 3,3'-isomer, which has two half-filled nonbonding orbitals, is in formal violation of Hund's rule, as applied to molecules.