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  • Title: Mechanisms of exchange modulation in trimethylenemethane-type biradicals: the roles of conformation and spin density.
    Author: Shultz DA, Fico RM, Lee H, Kampf JW, Kirschbaum K, Pinkerton AA, Boyle PD.
    Journal: J Am Chem Soc; 2003 Dec 17; 125(50):15426-32. PubMed ID: 14664588.
    Abstract:
    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 (phi) via a simple Karplus-Conroy-type relation: J = 44 cos(2) phi - 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.
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