Electronic structures of aromatic hydrocarbons with high spin multiplicities in the electronic ground state

Abstract

-It has been established experimentally that there are alternant hydrocarbons (AH) with high spin multiplicities in the electr~pic ground state, the highest multiplicity known up to date being septet. Such electronic structures are rationalized by utilizing the degeneracy of non-bonding pi molecular orbitals (NBMO) predictable from the theory of Longuet-Higgins in conjunction with the non-bonding orbitals of divalent carbon atoms which are perpendicular to pn orbitals and are nearly degenerate with NBMO. The degeneracy of NBMO is determined by the topological nature of molecules which permits the ground state with high spin multiplicities according to Hund's rule in spite of their low molecular symmetries. The nature of AH having high spin multiplicities in the ground state reported so far is understood in this manner.It was found, however, that for some AH the high spin multiplet states are unexpectedly not the ground states although they lie very close to the singlet ground state. The difference between these two types of AH with high spin multiplicities is associated with the fact that the spin distribution of NBMO is alternant in the former AH, but not in the latter. To the latter AH the weak interaction model was found to apply in which a molecule is divided into two units weakly interacting with each other.

Photochemical generation of triplet—triplet nitrene pairs in aromatic diazide crystals

“…According to the semiempirical Itoh model, 30 the pa rameters of the D tensor of the quintet state formed by a cluster of two triplet centers are determined by the expression containing a simple relationship between the D q and E q parameters of the quintet and the triplets forming the quintet D ≅ 1/6(D t (1) + D t (2)).…”

Benzoic acid and phenol derivatives of nitronyl nitroxide biradical as building blocks of organic molecule-based ferrimagnets

“…1) as precursors of anionic components in the above strategy referred to as "organic salt ferrimagnets ". Metaor 1,3,5-benzene linkage of unpaired electrons in it-aromatic homoatomic systems such as hydrocarbons of it conjugation usually affords a high-spin ground state [7], or a triplet ground state for 1,3-disubstituted benzene derivatives [8]. Only a few biradicals with functional groups, e.g., an ionizable group, have been found so far, which are chemically stable enough to investigate magnetic properties in crystalline solid states.…”

Ferromagnetic Interactions in Non-Kekulé Polymers. II

“…Mataga theoretically proposed possible organic ferromagnets based on molecular orbital methods. 2 The theory is based on topological degeneracy 3 of non-bonding molecular orbitals (NBMOs) characteristic of non-Kekulé systems. In particular, m-phenylene (1) in Figure 1 has been regarded as a robust ferromagnetic coupler in organic ferromagnets.…”

“…2, 3 The carbon atomic sites can be divided into two groups, that is, so-called starred atoms and unstarred atoms. 4 They can be chosen so as to be not adjacent each other.…”