The effect of the conformation on the spin multiplicity of the ground state and the stability of the ground state were investigated for m-phenylene type of biradicals by means of comparative study with DFT, CASSCF and AM1-CI approaches. It was found that AM1-CI approach is reliable in dealing with the stability of the high-spin ground state with the change of conformation; DFT method can give the reasonable results of the spin density of the high-spin state. Furthermore, when one or two radical centers are twisted sufficiently out of the conjugation with the benzene ring, m-phenylene turns into weak ferromagnetic and weak antiferromagnetic coupling units, respectively.
Computational studies of a class of potentially stable high-spin biradicals that two-atom-three-electron spin centers SC units connected by heterocycles FC and phenyl EG were described. The geometry and character of the spin exchange interaction were obtained by means of UB3LYP/6-31G*. The results show that the molecules possessing three different arranged fashions are designed with -·N-S as SC fragment, pyridine as FC and phenyl as EG, the spin densities on the two atoms of the SC fragment are different from the delocalization results in the specific stability of-·N-S. In these molecules, the stabilities of the triplet states decrease when the distance between the atoms of central SC (-N-) increases. Molecules with -·N-S as SC fragment, pyridine, pyrazine and triazine as FC and phenyl as EG are designed, the stability of triplet states for the molecule with pyridine as FC is the highest, and that for the molecule with pyrazine as FC is the lowest. Molecules with -·N-S, -·N-O and -·N-NH as SC fragment, pyridine as FC and phenyl as EG are designed, the stability of triplet states for the molecule with -·N-S as SC is the highest, that for the molecule with -·N-NH as SC is the lowest.
Novel stable high spin molecules possessing three different arranged fashions are designed with - · N-N< as a spin-containing(SC) fragment, phenylene as an end group and various aromatic molecules, such as benzene(1), 2,6-pyridine(2), 3,5-pyridine(3), pyridazine(4), 4,6-pyrimidine(5), 2,6-pyrimidine(6), pyrazine(7) and triazine(8), as a ferromagnetic coupling(FC) unit. The effects of the different coupling units on the spin multiplicities of the ground states and their stabilities were investigated by means of AM1-CI approach. It has been found that the spin densities on the two atoms of the SC fragment are different from delocalization results in the specific stability of - · N-N<. In these molecules, the stabilities of the triplet states decrease when the distance between the atoms of central SC(-N-) increases. It is shown that the heterocycles as the coupling units have influence on the stabilities of the high-spin ground states. That the heteroatom lying in m -phenyl can improve ferromagnetic coupling, while the heteroatom lying in o -phenyl or p -phenyl is not in favor of the ferromagnetic coupling.
