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 (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.
Shultz
David A
DA
Department of Chemistry, North Carolina State University, Raleigh, NC 27695-8204, USA. david_shultz@ncsu.edu
Fico
Rosario M
RM
Lee
Hyoyoung
H
Kampf
Jeff W
JW
Kirschbaum
Kristin
K
Pinkerton
A Alan
AA
Boyle
Paul D
PD