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

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posted on 17.12.2003, 00:00 by David A. Shultz, Rosario M. Fico,, Hyoyoung Lee, Jeff W. Kampf, Kristin Kirschbaum, A. Alan Pinkerton, Paul D. Boyle
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.