title: Mechanisms of exchange modulation in trimethylenemethane-type biradicals: the roles of conformation and spin density.
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.
authors.affiliation: Department of Chemistry, North Carolina State University, Raleigh, NC 27695-8204, USA. david_shultz@ncsu.edu
authors.forename: David A
authors.initials: DA
authors.lastname: Shultz
authors.forename: Rosario M
authors.initials: RM
authors.lastname: Fico
authors.forename: Hyoyoung
authors.initials: H
authors.lastname: Lee
authors.forename: Jeff W
authors.initials: JW
authors.lastname: Kampf
authors.forename: Kristin
authors.initials: K
authors.lastname: Kirschbaum
authors.forename: A Alan
authors.initials: AA
authors.lastname: Pinkerton
authors.forename: Paul D
authors.initials: PD
authors.lastname: Boyle