Mechanisms of Exchange Modulation in Trimethylenemethane-type Biradicals: The Roles of Conformation and Spin Density

The molecular structures and magnetic properties of six dinitroxide biradicals are described. Five of the dinitroxides are trimethylenemethane-type (TMM-type) biradicals; that is, the intramolecular exchange parameter, J, is modulated by a carbon−carbon double bond. However, the efficacy of the carbon−carbon double bond as an exchange coupler is determined by the molecular conformation. Our results show that the exchange parameters correlate with phenyl-ring torsion angles (φ) via a simple Karplus−Conroy-type relation: J = 44 cos2 φ − 17. Comparison of these results to those obtained for our isostructural series of bis(semiquinone) biradicals shows that both the magnitude of J and the resistance of ferromagnetic J to bond torsions is proportional to the spin density adjacent to the exchange coupler.