Dependence of multiplicity on conformation: ground states of o-, m- and p-phenylenediamine dications

The restricted open-shell Hartree–Fock (ROHF)/general valence bond (GVB) and complete active space self-consistent field (CASSCF) calculations have been performed on o-, m- and p-phenylenediamine dication diradicals. The results indicate that the energy differences between the singlet and the triplet states depend on the conformations of these compounds. The origin of the energy difference can be rationalized in terms of through-space and/or through-bond interaction between the two radical centers of a given compound.