In agreement with the experimental results of the groups of Rassat and Iwamura on respectively m-phenylene
bis(tert-butyl nitroxides) 2 and 3, ab initio calculations on m-benzoquinodimethane (la) and m-phenylene bis(nitroxide)
(la) find that near dihedral angles of 4 = 90" between the benzene ring and the radical-bearing groups, the singlet
falls below the triplet in energy. The change in m-phenylene from a ferromagnetic coupler at angles around 4 = 0" to an antiferromagnetic coupler around 4 = 90" is found to be largely due to selective destabilization of the
antisymmetric (A) combination of the singly-occupied orbitals on each of the radical centers by a u orbital of this symmetry on the m-phenylene coupler. The asymmetry about 4 = 90" in the singlet-triplet energy difference that
is calculated for Id is shown to be due to additional interactions of the oxygens of the singly-occupied nitroxyl
orbitals with p-n AOs of the benzene ring.
Shu Fang,Ming-Shi Lee,David A. Hrovat,et al. Ab Initio Calculations Show Why m-Phenylene Is Not Always a Ferromagnetic Coupler[J]. Journal of the American Chemical Society,1995(117):6727-6731.
Shu Fang,Ming-Shi Lee,David A. Hrovat,&Weston Thatcher Borden.(1995).Ab Initio Calculations Show Why m-Phenylene Is Not Always a Ferromagnetic Coupler.Journal of the American Chemical Society(117),6727-6731.
Shu Fang,et al."Ab Initio Calculations Show Why m-Phenylene Is Not Always a Ferromagnetic Coupler".Journal of the American Chemical Society .117(1995):6727-6731.