In agreement with the experimental results of the groups of Rassat and Iwamura on respectively m-phenylene(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 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 (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 calculated for Id is shown to be due to additional interactions of the oxygens of the singly-occupied nitroxyl with p-n AOs of the benzene ring.
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Shu et al. (1995) studied this question.